Bulletin of the American Physical Society
APS March Meeting 2012
Volume 57, Number 1
Monday–Friday, February 27–March 2 2012; Boston, Massachusetts
Session V1: Focus Session: Advances in Scanned Probe Microscopy III: SPM of Single Atoms & Molecules
Sponsoring Units: GIMSChair: Frances Niestemski, Stanford University
Room: 203
Thursday, March 1, 2012 8:00AM - 8:12AM |
V1.00001: Non-contact friction and the relaxational dynamics of surface defects Jian-Huang She, Alexander V. Balatsky When a cantilever oscillates near a sample surface, it experiences a dissipative force. Such non-contact friction is of great practical importance to the ultrasensitive force detection measurements. Previous theories predict the friction coefficient to be six orders of magnitude smaller than the experimental value. Here, inspired by the recent experiments reported in Phys. Rev. Lett. 105, 236103 (2010), we propose two new mechanisms to explain the magnitude, as well as the distance, temperature and frequency dependence of the friction. We assume that the surface defects couple to the cantilever tip either in the spin or charge channel, and their relaxational dynamics gives rise to the observed behavior of both the friction coefficient and the induced spring constant. Nice agreement is found with the experiments. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V1.00002: Dissecting an organic-metal bond by molecular manipulation Christian Wagner, Norman Fournier, Christian Weiss, Ruslan Temirov, F. Stefan Tautz \\ Scanning probe microscopy with a dynamic AFM has been able to answer some fundamental questions of surface science, like the force necessary to move an atom[1]. Recently, we demonstrated the gradual removal of a single 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) molecule from Ag(111) using a dynamic AFM [2]. The continuous force gradient measurement allowed the structural control over the junction. Here, we show how to extract details of the molecule-surface bonding (physisorption, chemisorption, bonding via functional groups) from such an experiment. The importance of a full-fledged simulation of the lifting -including tip oscillation- is emphasized. We point out the necessity of, and fundamental problems related to, a curved tip trajectory. We study PTCDA on Au(111) and Ag(111) and find qualitatively and quantitatively different binding potentials and adsorption energies. The data represents an ideal benchmark for existing and future ab-initio calculations on these systems. Our method should be applicable to various substrate-adsorbate systems and hence has the potential to answer many open questions in the field.\\{[}1] M. Ternes et al., \textit{Science} \textbf{319}, 1066 (2008)\\{[}2] N. Fournier et al., \textit{Phys. Rev. B} \textbf{84}, 035435 (2011) [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V1.00003: Multi-frequency amplitude modulated non-contact atomic force microscopy for nanoscale dielectric measurements Bharat Kumar, Joseph Bonvallet, Scott Crittenden Multi-frequency non-contact atomic force microscopy with amplitude feedback in air was used to obtain the dielectric constant of ultra-thin films on metallic substrates. The cantilever was excited at its second bending mode by applying an AC electric field between the substrate and cantilever. The capacitance gradient between the cantilever tip and sample substrate was obtained by measuring the capacitive force driving the cantilever at its second bending mode. An analytic expression relating capacitance and dielectric constant of thin film was then used to fit the experimental data and the dielectric constant was obtained from the fit parameters. The method was validated by obtaining the dielectric constants of self-assembled monolayers of thiol molecules ($2.0 \pm 0.1$) on gold substrate, and sputtered SiO$_2$ ($3.6 \pm 0.07$) thin film. The high Q-factor of the second bending mode of the cantilever increases the accuracy of capacitive measurements while the low applied potentials minimize the likelihood of variation of dielectric constants at high field strength and of damage from dielectric breakdown of air. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V1.00004: Anomalously large $g$-factor of single atoms adsorbed on a metal substrate Jens Wiebe, Alexander A. Khajetoorians, Bruno Chilian, Roland Wiesendanger, Samir Lounis, Antonio T. Costa, Douglas L. Mills We performed magnetic field dependent inelastic scanning tunneling spectroscopy (ISTS) on individual Fe atoms adsorbed on different metal surfaces. ISTS reveals a magnetization excitation which is shifting linearly to higher energies in the magnetic field. The data is used to extract the magnetic anisotropies and the $g$-factors of the Fe atoms, as well as the lifetimes of the excitations. We find lifetimes of hundreds of femtoseconds limited by coupling to electron-hole pairs in the substrate and decreasing linearly upon application of the magnetic field. As expected, the magnetic anisotropy strongly depends on the substrate. Astoundingly, we find that the $g$-factor is $g \approx 3.1$ for Ag(111) instead of the regular value of 2 which is observed for the Cu(111) substrate [1]. This very large $g$-shift can be understood when considering the complete electronic structure of both the Ag(111) surface state and the Fe atom, as shown by \textit{ab initio} calculations of the magnetic susceptibility. \\[4pt] [1] A. A. Khajetoorians et al., Phys. Rev. Lett. \textbf{106}, 037205 (2011). [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V1.00005: Spin Parity effects in STM single magnetic atom manipulation Fernando Delgado, Joaqu\'In Fern\'andez-Rossier Recent experimental work shows that a spin polarized scanning tunneling microscopy tip can be used both to read and write the spin orientation of a single magnetic spin [1]. Inelastic electron tunneling spectroscopy (IETS) shows that spin of the magnetic atom is quantized [2], like the spin of a molecular magnet. Here we discuss two fundamental problems that arise when a bit of classical information is stored on a quantized spin: quantum spin tunneling and back-action of the readout process. Quantum tunneling is responsible of the loss of information due to the relaxation of the spin coupled to the environment, while the detection induced back-action leads to an unwanted modification of the spin state. We find that fundamental differences exist between integer and semi-integer spins when it comes to both, read and write classical information in a quantized spin.\\[4pt] [1] S. Loth et al, Nature Physics 6, 340 (2010).\\[0pt] [2] C. Hirjibehedin et al, Science 317, 1199 (2007). [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V1.00006: Magnetic Particle Imagining with a Cantilever Detector Jacob Alldredge, John Moreland We present a novel method for the measurement of the magnetic moment of single micrometer scale particles as a function of the applied field. Our technique is based on magnetic force microscopy (MFM) with a hard magnetic tip in a uniform opposing field. By moving the tip position and using a simple model we can extract the magnetic properties of isolated particles and precisely measure subsurface particle locations in 3D. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V1.00007: Scanning Probe Microscopy and Spectroscopy of Molecules on Thin Insulating Films Invited Speaker: Jascha Repp Ultrathin insulating films on metal substrates facilitate the use of the scanning tunneling microscope (STM) to study the electronic properties of single atoms and molecules, which are electronically decoupled from the metallic substrate. The ionic relaxations in a polar insulator lead to a charge bi-stability in some adsorbed atoms and molecules. It is shown that control over the charge-state of individual molecules in such systems can be obtained by choosing a substrate system with an appropriate work function. The distribution of the additional charge is studied using difference images. These images show marked intra-molecular contrast [1]. We discuss how atomic-force-microscopy (AFM) in a combined STM/AFM based on the qPlus-sensor [2] reveals additional information that is truly complementary to the STM data set. In the case of a non-planar molecule that shows two different adsorption geometries, only the AFM channel provides reliable information on the conformation of the molecule. In another example of an artificially formed molecule-metal-molecule complex, the AFM channel provides information on the bonding that is important to understand the STM results. \\[4pt] [1] I. Swart, T. Sonnleitner, and J. Repp, Nano Letters 11, 1580 (2011).\\[0pt] [2] F. J. Giessibl, Appl. Phys. Lett. 76, 1470 (2000). [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V1.00008: Tailoring Dirac Fermions in Molecular Graphene Kenjiro K. Gomes, Warren Mar, Wonhee Ko, Charlie D. Camp, Dominik K. Rastawicki, Francisco Guinea, Hari C. Manoharan The dynamics of electrons in solids is tied to the band structure created by a periodic atomic potential. The design of artificial lattices, assembled through atomic manipulation, opens the door to engineer electronic band structure and to create novel quantum states. We present scanning tunneling spectroscopic measurements of a nanoassembled honeycomb lattice displaying a Dirac fermion band structure. The artificial lattice is created by atomic manipulation of single CO molecules with the scanning tunneling microscope on the surface of Cu(111). The periodic potential generated by the assembled CO molecules reshapes the band structure of the two-dimensional electron gas, present as a surface state of Cu(111), into a ``molecular graphene'' system. We create local defects in the lattice to observe the quasiparticle interference patterns that unveil the underlying band structure. We present direct comparison between the tunneling data, first-principles calculations of the band structure, and tight-binding models. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V1.00009: Synthesizing and Observing Electric and Magnetic Gauge Fields in Molecular Graphene Dominik Rastawicki, Charlie D. Camp, Kenjiro K. Gomes, Ming Rue D. Thian, Alex W. Contryman, Hari C. Manoharan, Wonhee Ko, Warren Mar Molecular graphene is an artificial analogue of graphene that can be built using a scanning tunneling microscope (STM). It is realized by constraining surface-state electrons to a honeycomb lattice, which we have shown reproduces the Hamiltonian of natural graphene. We experimentally demonstrate that creating strains within the honeycomb lattice modifies the Hamiltonian in the same way that true laboratory-frame electric and magnetic fields do. In our experiments we have created artificial magnetic fields that can reach values as high as 60 T, and which can change their magnitude significantly over a distance as short as 2 nm. Some of the new physical phenomena we have been able to observe include: gauge invariance for lattices with very different real-space strains but the same artificial fields, emergence of Landau levels due to constant artificial magnetic fields, occurrence of a confined state at an interface straddling a sign-inversion of the effective magnetic field, and effective chemical doping and potential changes associated with applied electric gauge fields. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V1.00010: Topologically protected chiral edge state realized on molecular graphene Wonhee Ko, Warren Mar, Kenjiro K. Gomes, Dominik K. Rastawicki, Charlie D. Camp, Hari C. Manoharan Graphene has many interesting topological properties arising from the hexagonal lattice shape and sublattice symmetry. Although its edge state is known to be extremely sensitive to the shape of the edge, and is non-topological, it has been shown that graphene with an energy gap produced by broken sublattice symmetry can possess different topologies due to different sign of the carrier mass. By making a junction of two gapped graphene regions with opposite mass, we can realize a topologically protected edge state at the mass domain wall junction, with chirality emerging from the valley degeneracy. Molecular graphene is an artificial honeycomb lattice built by atom manipulation, and due to its extreme tunability, we have realized a topological edge state between gapped molecular graphene regions with opposite signs of Dirac fermion mass. Enhanced density of states restricted only to the junction clearly shows the existence of the edge state. Also, its robustness to the geometrical detail is observed from its persistence over various edge structures, and chirality is revealed by selective scattering at the intersection of edges. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V1.00011: Measuring Electronic Properties of Topological Defects in Molecular Graphene Charlie Camp, Dominik Rastawicki, Kenjiro Gomes, Wonhee Ko, Warren Mar, Ming Rue Thian, Francis Niestemski, Alex Contryman, Carolina Gonzalez, Hari Manoharan With the development of artificial ``molecular'' graphene, it is possible to create a two-dimensional electron system very similar to graphene by assembling molecules in the appropriate geometry on surface states using a scanning tunneling microscope (STM) tip. Using this same system, we recreate many lattice defects that occur naturally in graphene. Such defects have a significant effect on the electronic and transport properties of natural graphene, and are thus of notable interest in the development of nanoelectronics. In particular, we study rotational grain boundaries, which are formed by the rotation of a region of graphene with respect to the rest of the lattice. These include the Stone-Wales defect, the simplest with two adjacent carbon sites rotated by 90 degrees, as well as a common larger topological defect recently identified as the flower defect. Using STM, we examine the electronic properties of these defects in molecular graphene, paying particular attention to the emergence of new states close to the Dirac point and the quasiparticle scattering. These geometries are also studied in hole- and electron-doped variants. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V1.00012: Scanning Vibronic Spectroscopy of Single Molecules Ming Rue D. Thian, Warren Mar, Wonhee Ko, Kenjiro K. Gomes, Dominik K. Rastawicki, Charlie D. Camp, Alex W. Contryman, Hari C. Manoharan The dynamics of a single molecule within a quantum point contact or tunnel junction can be quite complex, exhibiting motion over frequency scales ranging from kHz to THz. In this work, we examine in detail the dynamic motion of single molecules by using low-temperature scanning inelastic tunneling, isotopic sorting, and isotopic labeling. Carbon-based molecules such as carbon monoxide, C$_{60}$, and diamondoids function as prototype homonuclear and heteronuclear molecules. Target structures are assembled on surfaces via atomic manipulation or self-assembly in order to tailor local THz-scale vibrational modes and to control kHz-scale molecular motion. In addition we functionalize a scanning tunneling microscope tip and use its own THz-scale vibrational modes as a probe and a tunable perturbation. These techniques reveal structure not visible in traditional STM data. We compare this data to expected local quantum forces. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V1.00013: Pitfalls in normalizing scanning tunneling spectroscopy and how to do it ``right'' (from an experimentalist perspective) Michael Dreyer, Rami Dana, Anita Roychowdhury Scanning tunneling spectroscopy is widely used to probe the local density of states (LDOS) by measuring $\mathrm{d}I/\mathrm{d}V$ typically using a lock in amplifier. According to theory, the data has to be normalized by $(I/V)$ to reveal the true LDOS - of cause still convoluted with the tip density of states. This process can induce its own artifacts and has to be done carefully. In our case the algorithm includes finding the true bias offset (if any), correct any offset in the measured current, rescale the measured $\mathrm{d}I/\mathrm{d}V$ curves using the calculated version $(\Delta I/\Delta V)$, replacing $I(V)$ around $V_{\mathrm{bias}}=0$ by a polynomial and finally performing the normalization. The motivation and consequences of each step will be discussed using example data. [Preview Abstract] |
Session V2: Invited Session: Spin Caloritronics of Magnetic Structures and Devices
Sponsoring Units: GMAGChair: Gerrit E. W. Bauer, Delft University of Technology
Room: 204AB
Thursday, March 1, 2012 8:00AM - 8:36AM |
V2.00001: Thermal creation of stronger spin-transfer torque in oscillators and memories Invited Speaker: John Slonczewski Oscillators and magnetic random-access memories (MRAMs) investigated today rely on spin-transfer torque (STT) carried by an electric current flowing through a magnetic tunnel junction (MTJ) having barrier composition MgO.\footnote{See the STT review by D. Ralph and M. Stiles, J. Magn. Magn. Mater. \textbf{320, }1190 (2008).} Experiments confirm the theoretical upper bound \textit{$\tau $}$_{e}$=1/2 on the torque yield (defined as dimensionless torque per unit supplied electric current). This bound limits the performance potential of STT-MRAM in which current supplied by one transistor within each cell switches the information bit. Replacement of electric current with heat flow (supplied by a Joule heater) carried by magnons may provide a greater torque yield \textit{$\tau $}$_{h}$.\footnote{J. Slonczewski, Phys. Rev. B \textbf{82}, 054403 (2010).} The essential structure for this \textit{thermagnonic} spin transfer (TMST) comprises a stack of three nano layers: a spontaneously magnetized insulator (\textit{ferrite} for brevity), a non-magnetic metallic spacer, and the free metallic magnet responding to the transferred torque. Phonons carry most of the heat flowing through the ferrite. But spin-1 magnons also carry a portion of it and deposit pure spin polarization into the spacer whose free electrons transport it to the free magnet. Ferrite-metal interfaces also occur in a spin-Seebeck effect.\footnote{The talk by E. Saitoh in this Symposium} Principles of spin relaxation provide estimates of \textit{$\tau $}$_{h}$ based on existing data for sd-exchange, superexchange, and non-magnetic interfacial thermal resistance; \textit{$\tau $}$_{h}$ may exceed \textit{$\tau $}$_{e }$by one order of magnitude.\footnote{J. Slonczewski, Phys. Rev. B \textbf{82}, 054403 (2010).} Related results of an FMR spin-pumping experiment\footnote{B. Heinrich et al, Phys. Rev. Letts. \textbf{107}, 066604 (2011).} and DFT computations\footnote{The talk by K. Xia in this Symposium.} support the potential of TMST-MRAM. In the case of an oscillator, TMST could increase its efficiency and enable largely independent controls of frequency and output voltage. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 9:12AM |
V2.00002: Spin pumping and spin Seebeck effect Invited Speaker: Eiji Saitoh Utilization of a spin current, a flow of electrons' spins in a solid, is the key technology in spintronics that will allow the achievement of efficient magnetic memories and computing devices. In this technology, generation and detection of spin currents are necessary. Here, we review inverse spin-Hall effect and spin-current-generation phenomena recently discovered both in metals and insulators: inverse spin-Hall effect, spin pumping, and spin Seebeck effect. (1)Spin pumping and spin torque in a Mott insulator system We found that spin pumping and spin torque effects appear also at an interface between Pt and an insulator YIG.. This means that we can connect a spin current carried by conduction electrons and a spin-wave spin current flowing in insulators. We demonstrate electric signal transmission by using these effects and interconversion of the spin currents [1]. (2) Spin Seebeck effect We have observed, by using the inverse spin-Hall effect [2], spin voltage generation from a heat current in a NiFe, named the spin-Seebeck effect [3]. Surprisingly, spin-Seebeck effect was found to appear even in insulators [4], a situation completely different from conventional charge Seebeck effect. The result implies an important role of elementary excitation in solids beside charge in the spin Seebeck effect. In the talk, we review the recent progress of the research on this effect. This research is collaboration with K. Ando, K. Uchida, Y. Kajiwara, S. Maekawa, G. E. W. Bauer, S. Takahashi, and J. Ieda. \\[4pt] [1] Y. Kajiwara and E. Saitoh et al. Nature 464 (2010) 262. \\[0pt] [2] E. Saitoh et al., Appl. Phys. Lett. 88 (2006) 182509. \\[0pt] [3] K. Uchida and E. Saitoh et al., Nature 455 (2008)778. \\[0pt] [4] K. Uchida and E. Saitoh et al.,Nature materials 9 (2010) 894 - 897. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V2.00003: Computational spin caloritronics Invited Speaker: Ke Xia Recent experimental and theoretical studies focused on spin-mediated heat currents in at interfaces between metals and insulators, where the latter can be either the barrier in a magnetic tunnel junction or a ferromagnetic insulator. A crucial parameter is the efficiency of spin injection and spin-transfer torque. In this talk, we will report realistic electronic structure calculations for two material systems. 1) The pertinent material parameter governing spin transfer and spin Seebeck effect is the spin mixing conductance that we calculate for the Silver-YIG (Yttrium-Iron-Garnett) interface. This turns out to be much larger than expected from the Stoner model. We find mixing conductance comparable to intermetallic interfaces, a surprising result that can be rationalized in terms of magnetic local moments at the interface. These results imply that the spin-mediated energy and information transmissivity of magnetic insulators is potentially much better than has been measured in early experiment, a result that has been experimentally corroborated very recently by several groups. 2). We demonstrate that the thermal spin-transfer torque (TST) in a junction Fe-MgO-Fe tunnel junctions with ultra thin barriers can amount to 10$^{-7}$J/m$^{2}$/K at room temperature, which is estimated to cause magnetization reversal for temperature differences over the barrier of the order of 10 K. The large TST for ultrathin barriers can be explained by multiple scattering due to interface states. Direct evidence for the existence of these states can be obtained by comparing shot noise calculations with recent experiments for high-quality junctions [Arakawa et al., Appl. Phys. Lett. 98, 202103(2011)]. *This work was carried out in collaboration with Xingtao Jia, Kai Liu and Gerrit E.W. Bauer. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:24AM |
V2.00004: Magneto-Seebeck effect and thermal torques in magnetic tunnel junctions Invited Speaker: Markus Muenzenberg Creating temperature gradients in magnetic nanostructures has resulted in a new research direction, i.e., the combination of magneto- and thermoelectric effects. Magnetic tunnel devices, known for application as magnetic sensor in hard disc drives or magnetic random access memories (MRAM) show large magnetoresistance. We show that in nanoscale magnetic tunnel junctions, the Seebeck voltage in a heat gradient can be controlled via the magnetization. The Seebeck coefficient changes during the transition from a parallel to an antiparallel magnetic configuration in a tunnel junction -- the magneto-Seebeck effect. In that respect, it is the analog to the tunneling magnetoresistance and thus is called tunneling magneto-Seebeck effect (or tunneling magnetothermopower). The change in Seebeck coefficients is in the order of the voltages known from the charge-Seebeck effect in semiconductors (up to 100 $\mu $V/K). Their size and sign can be delicately controlled by the composition of the electrodes' atomic layers adjacent to the barrier and the temperature and we observe a characteristic sign change from positive to negative magneto-Seebeck effects as theoretically predicted. It is known that generally strong electronic asymmetry at around the Fermi level results in a large Seebeck effect. Here the magnetization dependence of the charge-Seebeck coefficients varying up to $>$100{\%} for the parallel and the antiparallel originates from the half-metallic like transmission of the tunnel junction. Using heating with ultrafast laser pulses, these thermal gradients can be of up to 20 K across the tunnel barrier. We demonstrate that we can achieve the parameters predicted, where by thermal torques magnetization switching is expected. This allows to conceptually think of MRAM's driven by heat gradients only. \\[4pt] [1] M. Walter, et al. Nature Mater. 10, 742 (2011). [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 11:00AM |
V2.00005: Thermomagnonic spin transfer and Peltier effects in insulating magnets Invited Speaker: Alexey A. Kovalev The recent discovery of the spin Seebeck effect [1] in metals, insulators and semiconductors stimulated development of spincaloritronics [2]. The possibility of measuring the Onsager reciprocal spin Peltier effect has been investigated recently as well. In our theoretical work [3], we study the fictitious electromagnetic fields induced by magnetic textures which may offer an alternative route for observing the spin Peltier effect. Particularly, in an insulating ferromagnet a moving magnetic texture should effectively drive the spin (wave) current which in turn should lead to the heat current by the spin Peltier effect. We further study the coupled magnon energy transport and collective magnetization dynamics in ferromagnets with magnetic textures. We conclude that the analogy between the fictitious electromagnetic fields and real fields should lead to magnonic counterparts of such effects as the Hall effect, the Ettingshausen effect, the Nernst effect, and the Righi-Leduc effect. By constructing a phenomenological theory based on irreversible thermodynamics, we describe motion of domain walls by thermal gradients and generation of heat flows by magnetization dynamics. From microscopic description based on magnon kinetics, we estimate the transport coefficients and analyze the feasibility of energy-related applications (e.g. nanoscale heat pumps [4]) in insulating ferromagnets, such as yttrium iron garnet and europium oxide. Our estimates show that the viscous coupling effects between magnetization dynamics and magnon flows can be strong in materials with low spin densities (e.g. dilute magnetic systems) and narrow domain walls, which can allow the magnonic manipulation of magnetization dynamics and heat pumping.\\[4pt] [1] K. Uchida et al. Nature 455, 778 (2008).\\[0pt] [2] G. E. W. Bauer, A. H. MacDonald, S. Maekawa, Solid State Commun. 150, 459 (2010).\\[0pt] [3] A. A. Kovalev and Y. Tserkovnayk, arXiv:1106.3135.\\[0pt] [4] A. A. Kovalev and Y. Tserkovnyak, Solid State Commun. 150, 500 (2010). [Preview Abstract] |
Session V3: Invited Session: Spin Fluctuations and Cooper Pairing in Unconventional Superconductors
Sponsoring Units: DCMPChair: Assa Auerbach, Technion
Room: 205AB
Thursday, March 1, 2012 8:00AM - 8:36AM |
V3.00001: Linear-T scattering and pairing from spin fluctuations in organic superconductors Invited Speaker: Nicolas Doiron-Leyraud The (TMTSF)$_2$X series of organic superconductors, with X=PF$_6$ or ClO$_4$, are clean single-band metals that exhibit unconventional superconductivity in the vicinity of a pressure-induced spin-density wave (SDW) quantum critical point. As such, they epitomize the interplay between magnetism and superconductivity observed in heavy fermion, cuprate, and iron-pnictide superconductors. We have recently examined the electrical resistivity $\rho(T)$ of (TMTSF)$_2$X materials as a function of temperature and pressure. At the SDW quantum critical point, we observed a strictly linear temperature dependence of the resistivity over two decades in temperature [1,2]. Moving away from SDW order with increasing pressure, this linear resistivity was found to decrease in parallel with the weakening superconductivity, such that $A$, the coefficient of the linear contribution to $\rho(T)$, directly correlates with the superconducting $T_c$ [1,2]. This shows that linear-$T$ scattering and superconducting pairing share a common origin. A similar correlation was also found between $A$ and the spin fluctuations seen by NMR experiments [2]. Owing to the quasi-1D nature of the (TMTSF)$_2$X system, this connection between spin fluctuations, scattering, and superconductivity is well described theoretically by a model that considers the hitherto overlooked mutual reinforcement of SDW and pairing correlations [3]. In particular, the feedback of pairing correlations on SDW fluctuations appears to be decisive for the strength of the linear resistivity and its extent in temperature and pressure. The fact that the same empirical correlation between linear-$T$ resistivity and $T_c$ is observed in the hole-doped [4,5] and electron-doped [6] cuprates, as well as in iron-pnictides [1,4], shows that the same mechanism is at play in these materials. This points to a common, magnetic origin to the superconducting pairing. Work done in collaboration with S. Ren\'e de Cotret, P. Auban-Senzier, D. J\'er\^ome, C. Bourbonnais, K. Bechgaard, and L. Taillefer. \\[4pt] [1] N. Doiron-Leyraud et al., Phys. Rev. B 80, 214531 (2009).\\[0pt] [2] N. Doiron-Leyraud et al., Eur. Phys. J. B 78, 23 (2010).\\[0pt] [3] C. Bourbonnais and A. Sedeki, Phys. Rev. B 80, 085105 (2009).\\[0pt] [4] N. Doiron-Leyraud et al., arXiv:0905.0964.\\[0pt] [5] L. Taillefer, Annu. Rev. Condens. Matter Phys. 1, 51 (2010).\\[0pt] [6] K. Jin et al., Nature 476, 73 (2011). [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 9:12AM |
V3.00002: Link between spin fluctuations and Cooper pairing in copper oxide superconductors Invited Speaker: Kui Jin Although it is generally accepted that superconductivity is unconventional in the high-\textit{T}$_{c}$ cuprates, the relative importance of phenomena such as spin and charge (strip) order, superconductivity fluctuations, proximity to Mott insulator, a pseudogap phase and quantum criticality are still a matter of debate. In electron-doped cuprates, the absence of $``$anomalous\textquotedblright\ pseudogap phase in the underdoped region of the phase diagram and weaker electron correlations suggest that Mott physics and other unidentified competing orders are less relevant and that antiferromagnetic (AFM) spin fluctuations are the dominant feature. In this talk, I will report results of low temperature magnetotransport experiments in optimal to overdoped (non-superconducting) thin films of the electron-doped cuprate La$_{2-x}$Ce$_{x}$CuO$_{4}$ (LCCO). We find that a linear-in-\textit{T} scattering rate is correlated with the superconductivity (\textit{T}$_{c}$). Our results show that an envelope of such scattering surrounds the superconducting phase, surviving to 20 mK (the limit of our experiments) when superconductivity is suppressed by magnetic fields [1]. Comparison with similar behavior found in organic superconductors [2] strongly suggests that the linear-in-\textit{T} resistivity in the electron-doped cuprates is caused by spin-fluctuation scattering. Because linear-in-T scattering has also been linked to \textit{T}% $_{c}$ in some hole-doped cuprates [2], our results suggest a fundamental connection between AFM spin fluctuations and the pairing mechanism of high temperature superconductivity in all cuprates. In addition, I will discuss how quantum criticality plays a significant role in shaping the anomalous properties of the electron-doped cuprate phase diagram. We identify quantum critical scaling in LCCO with a line of quantum critical points that surrounds the superconducting phase as a function of magnetic field and charge doping [3]. \\[4pt] [1] K. Jin, N.P. Butch, K. Kirshenbaum, J. Paglione, and R.L. Greene, Nature 476, 73 (2011).\\[0pt] [2] L. Taillefer, Annu. Rev. Cond. Matter Phys. 1, 51 (2010). \\[0pt] [3] N.P. Butch, K. Jin, K. Kirshenbaum, R.L. Greene, and J. Paglione, submitted. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V3.00003: Intense paramagnon excitations in a large family of high-temperature superconductors Invited Speaker: Mathieu Le Tacon Motivated by the search for the mechanism of high-temperature superconductivity, an intense research effort has been focused on the evolution of the spin excitation spectrum upon doping from the antiferromagnetic insulating to the superconducting states of the cuprates. Because of technical limitations, however, the experimental investigation of doped cuprates has been largely focused on excitations with energies $\bf \leq 100$ meV in a small range of momentum space~[1]. Here we take advantage of the recent developments of high-resolution resonant inelastic x-ray scattering~[2,3] to show that a large family of superconductors, encompassing the model compounds YBa$_2$Cu$_4$O$_8$ and YBa$_2$Cu$_3$O$_{7}$, exhibits damped spin excitations - or paramagnons - with dispersions and spectral weights closely similar to those of magnons in undoped, antiferromagnetically ordered cuprates over much of the Brillouin zone. The results are in excellent agreement with the spin excitations obtained by exact diagonalization of the $\bf t-J$ Hamiltonian on finite-sized clusters. A numerical solution of the Eliashberg equations based on the experimental spin excitation spectrum of YBa$_2$Cu$_3$O$_{7}$ reproduces its superconducting transition temperature $\bf T_c$ within a factor of two. The discovery of a well-defined, surprisingly simple spin excitation branch over a wide range of doping levels thus strongly supports magnetic Cooper pairing models for the cuprates~[4]. \\[4pt] [1] M. Fujita \textit{et al.} arXiv/condmat:1108.4431\\[0pt] [2] G. Ghiringhelli \textit{et al.}, Review of Scientific Instruments, \textbf{77}, (2006).\\[0pt] [3] L. Braicovich \textit{et al.}, Phys. Rev. Lett., \textbf{104}, 077002 (2010).\\[0pt] [4] M. Le Tacon \textit{et al.}, Nature Physics \textbf{7}, 725 (2011). [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:24AM |
V3.00004: Magnetically driven superconductivity in CeCu$_2$Si$_2$ Invited Speaker: Oliver Stockert The origin of unconventional superconductivity, including high-temperature and heavy-fermion superconductivity, is still discussed controversially. Spin excitations instead of phonons are thought to be responsible for the formation of Cooper pairs. Unconventional superconductivity is quite often observed in the vicinity of a magnetic quantum critical point (QCP), i.e., a continuous magnetic phase transitions occurring at $T = 0$. Such a QCP can be approached when tuning a continuous finite temperature phase transition to $T = 0$ by means of a non-thermal control parameter like doping, pressure or magnetic field. As a result of the quantum-critical spin fluctuations unusual low-temperature properties are observed. The heavy-fermion compound CeCu$_2$Si$_2$ displays unconventional superconductivity and is already at ambient pressure located in the vicinity of a QCP where long-range antiferromagnetism vanishes. Using elastic and inelastic neutron scattering we studied in detail the antiferromagnetic order and the spin excitations spectrum around the QCP. Antiferromagetism and superconductivity exclude each other on a microscopic scale. While for magnetically ordered samples critical slowing down of the spin fluctuations above $T_{\rm N}$ is observed, shows the normal state response of superconducting CeCu$_2$Si$_2$ an almost critical slowing down for $T \rightarrow 0$. Its temperature dependence and scaling behavior are in line with the expectations for an itinerant spin-density-wave QCP. This interpretation is substantiated by an analysis of specific heat data and the momentum dependence of the magnetic excitation spectrum. The magnetic response in the superconducting state is characterized by a transfer of spectral weight to energies above a spin excitation gap. Compared to the condensation energy there is a larger saving of magnetic exchange energy as the system condenses into a superconducting state. Our results strongly imply that the coupling of Cooper pairs in CeCu$_2$Si$_2$ is mediated by overdamped spin fluctuations. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 11:00AM |
V3.00005: 2012 IUPAP C10 Young Scientist Prize on the Structure and Dynamics of Condensed Matter Lecture: Spin Fluctuations and Pairing in Fe-based Superconductors Invited Speaker: A.D. Christianson The origin of superconductivity in the Fe-based superconductors, like that in other unconventional superconductors, remains shrouded in mystery. How the pairing bosons emerge either due to or in spite of the strong magnetic interactions found in the Fe-based superconductors is one of the most thoroughly investigated questions in the field. A prominent example of the interplay of superconductivity and magnetism is the dramatic shift of spectral weight from the low energy spin excitations to an energy which is related to the superconducting gap resulting in a peak in the spin excitation spectrum localized in both momentum and energy which occurs at the onset of superconductivity. The appearance of the new peak in the spin excitation spectrum below the superconducting transition temperature is referred to as s spin resonance and is most commonly interpreted as indicating a sign change of the superconducting order parameter on different portions of the Fermi surface and thus is consistent with an extended s-wave or s$\pm $ pairing symmetry in many Fe-based superconductors. We will review the observations and implications of the spin resonance across the Fe-based superconductors. In particular we will examine the relationship between the resonance energy and the superconducting transition temperature as a function of chemical doping and pressure. While the spin resonance provides important information about pairing symmetry, there does not appear to be sufficient spectral to explain the pairing strength. Thus the remainder of the spin excitation spectrum must be examined to determine if spin fluctuations are ultimately responsible for pairing in the Fe-based materials. Consequently, we will discuss in detail the way in which the spin excitations evolve from the nonsuperconducting compounds to their superconducting relatives as a function of chemical doping. [Preview Abstract] |
Session V4: Bosons and Fermions in Optical Lattices
Sponsoring Units: DAMOPChair: Emanuele Dalla Torre, Harvard University
Room: 205C
Thursday, March 1, 2012 8:00AM - 8:12AM |
V4.00001: Doublon production rate by optical lattice modulation for strongly correlated Fermionic atoms Akiyuki Tokuno, Eugene Demler, Thierry Giamarchi Currently lattice modulation spectroscopy technique is applied to experiments. [1] In this spectroscopy, the number of doubly occupying atom (doublon) produced by amplitude modulation of an optical lattice potential is probed. Theoretically, it allows us to access a kinetic energy correlation function. [2] We discuss doublon excitations of strongly correlated fermionic atoms in a high-temperature regime relevant to current experiments of fermionic atoms in an optical lattice. [3] We employ a slave particle representation, and the self-energy is estimated by using non-crossing approximation based on a spin-incoherent assumption. Furthermore, this formalism is applied to calculation of the doublon production rate as a function of the lattice modulation frequency, chemical potential and temperature. Using parameters given in the experiment [1], a fit to the experimental data is implemented, and quantitatively good agreement is obtained. \\[4pt] [1] D. Greif, L. Tarruell, T. Uehlinger, R. J\"ordens, and T. Esslinger, Phys. Rev. Lett. 106, 145302 (2011).\\[0pt] [2] C. Kollath, A. Iucci, I. P. McCulloch, and T. Giamarchi, Phys. Rev. A 74, 041604(R) (2006).\\[0pt] [3] A. Tokuno, E. Demler, and T. G.iamarhi, arXiv:1106.1333. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V4.00002: Superfluid to normal phase transition in strongly correlated bosons in two and three dimensions Juan Carrasquilla, Marcos Rigol Using quantum Monte Carlo simulations, we investigate the finite temperature phase diagrams of hardcore bosons in two- and three-dimensional lattices. To determine the phase boundaries, we perform a finite-size-scaling analysis of the condensate fraction and/or the superfluid stiffness. We then discuss how these phase diagrams can be measured in experiments with trapped ultracold gases, where the systems are inhomogeneous. For that, we introduce a method based on the measurement of the zero-momentum occupation, which is adequate for experiments dealing with both homogeneous and trapped systems, and compare it with previously proposed approaches. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V4.00003: Double occupancy as a universal probe for antiferromagnetic correlations and entropy in cold fermions on optical lattices Nils Bluemer, Elena Gorelik, Daniel Rost, Thereza Paiva, Richard Scalettar, Andreas Kluemper We study antiferromagnetic (AF) order and correlations in the half-filled Hubbard model using dynamical mean-field theory, determinantal quantum Monte Carlo (in dimensions $d=2,3$), and Bethe Ansatz (in $d=1$). We establish a low-temperature enhancement of the double occupancy $D$ at stromg coupling as a local probe of strong AF correlations accessible in cold-atom experiments [1]. As a function of entropy $s=S/(N k_{\rm B})$, $D$ is nearly universal with respect to dimensionality, with a minimum in $D(s)$ at $s\approx \log(2)$ [2]. The quantum AF Heisenberg regime at $s\la \log(2)$, driven by an abrupt gain in kinetic energy and with clear signatures also in the next-nearest neighbor correlation function, should be in immediate experimental reach. Long-range order appears hardly relevant for the current search of AF signatures in cold fermions. Thus, experimentalists need not achieve $s<\log(2)/2$ (on a cubic lattice) and should consider lower dimensions, for which the AF effects are larger, or even use dimensionality as a tunable parameter.\\[4pt] [1] E. V. Gorelik, I. Titvinidze, W. Hofstetter, M. Snoek, and N. Bl\"umer, Phys. Rev. Lett. {\bf 105}, 065301 (2010).\\[0pt] [2] E. V. Gorelik, T. Paiva, R. Scalettar, A. Kl\"umper, and N. Bl\"umer, arXiv:1105.3356v1. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V4.00004: Tunable Phases of Fermionic Cold Atoms Systems in Mixed Dimensions Kyle Irwin, Shan-Wen Tsai We investigate a system with two species of fermions. One species, f-fermions, moves on a two-dimensional square lattice. Another species, c-fermions, is constrained to move on a one-dimensional lattice embedded in the square lattice of f-fermions. The phases of the effective one-dimensional system whose interactions are mediated by the two-dimensional system can be tuned by manipulating the two-dimensional density. We explore effective theories, quantum phases, correlations, and relevant energy scales for various fillings of the mixed dimensional system using a functional renormalization group approach. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V4.00005: Pairing and Density-Wave Phases of Population Imbalanced Fermi-Fermi Mixture on Optical Lattice Chen-Yen Lai, Chuntai Shi, Shan-Wen Tsai We study a two species fermion mixture with different populations on a square lattice, which can be modeled by a Hubbard Hamiltonian with on-site inter-species interaction. Such a model can be realized in a cold atom system with fermionic atoms in two different hyperfine states loaded on an optical lattice, and with interaction strength that can be tuned by an external magnetic field. When one of the fermion species is close to half-filling, the system is highly affected by lattice effects. We find several correlated phases for this system, including spin density wave state, d-wave charge density wave state, and p-wave superfluid state for the minority species. We study this system using a functional renormalization group method, determining its phase diagram and providing an estimate for the critical temperature of each phase. These phases emerge from a combination of interaction, population imbalance, and lattice effects. Lattice effects in particular lead to a much richer phase diagram than that of a imbalanced mixture of fermionic gas. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V4.00006: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V4.00007: Finite-temperature Properties of the Fermi-Hubbard Model on the Honeycomb Lattice Baoming Tang, Ehsan Khatami, Thereza Paiva, Marcos Rigol We study thermodynamic properties of the Fermi-Hubbard model on the honeycomb lattice utilizing the numerical linked-cluster expansion, which is exact in the thermodynamic limit, and quantum Monte Carlo simulations. We obtain the equation of state, double occupancy, entropy and spin correlations for a wide range of temperatures, chemical potentials, and interaction strengths. Employing a local density approximation, we study properties of the system in the presence of a harmonic trapping potential and compare the efficiency of various adiabatic cooling schemes to those obtained for such model on the square lattice. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V4.00008: Realizing a fermionic superfluid state from a band insulator in an optical lattice Yogeshwar P. Saraswat, Amal Medhi, Vijay B. Shenoy We propose a route to realizing a fermionic superfluid state in an optical lattice starting from a band insulator. We show that by increasing the strength of attractive interaction between the fermions in the singlet channel, a band insulator can be driven to a superfluid state in an optical lattice. The band structure is suitably designed to avoid other competing states. We estimate the Kosterlitz-Thouless transition temperature of such a superfluid. The proposal could help the realization of a superfluid state of fermions in an optical lattice circumventing the cooling problem. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V4.00009: A Boson Core Compressibility Measure for Optical Lattices Yasamin Khorramzadeh, Fei Lin, V.W. Scarola Trapping in experiments on cold atomic gases in optical lattices leads to in homogeneity and different phases within the trap. We model a global measure, the boson core compressibility, that can be used to access local properties of a single phase at the center of the trap using observations of double occupancy. We test this measure on the trapped Bose-Hubbard model using mean field theory and quantum Monte Carlo. We find that the boson core compressibility focuses on the core region of the system and eliminates edge effects. We use the core compressibility to identify the transition from Mott insulator to superfluid and show that it is essentially the same as local compressibility in the core region when the system has doubly occupied sites. We generalize the definition of core compressibility to study systems with large densities at the trap center. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V4.00010: Numerical studies of exotic paired states in optical lattices Simone Chiesa, Shiwei Zhang, George Batrouni Ultracold atoms are a unique tool that allows the exploration of phases of matter not easily accessible in condensed matter systems. Two interesting possibilities are spin-imbalanced systems and spin dependent optical lattices with Fermi surfaces that differ for the two hyperfine species. We use mean-field theory and quantum Monte Carlo simulations of Hubbard-like models with an attractive contact interaction to study the FFLO state and, by rotating the two Fermi surfaces by 90 degrees with respect to each other, a recently proposed exotic paired state [Feiguin and Fisher, 2009]. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V4.00011: Competing instabilities in a two band Hubbard model on a square lattice Chuntai Shi, Shan-Wen Tsai We study a two band Hubbard model on a two dimensional square lattice. In particular, we focus on the cases wherein one band is doped to have a small electron pocket while the other band is doped to have a hole pocket and the Fermi lines of these two pockets are nearly nested. Similar models have been studied extensively in the context related to the Iron-based material where the interactions between electrons are always repulsive. Here we investigate the generalized cases that the interactions between the fermions within the same band $U_1$ and $U_2$ and the interactions between electrons in different bands $U_{12}$ can be tuned independently. Such models can potentially be realized in a cold atom system where the manipulation of the interaction is possible by taking advantage of the Feshbach resonance. The freedom of tuning the strength and the sign (repulsive or attractive) of the interactions, combined with the nearly nested Fermi lines, allows both the density wave phases and the pairing phases to be potential candidates for the ground state. We employ the functional renormalization group approach so that we can investigate the competition between these possible instabilities on an equal footing. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V4.00012: Condensate Properties for Strongly Repulsive Bosons in a Double Well Joel Corbo, Jonathan DuBois, Birgitta Whaley We present path integral ground state (PIGS) quantum Monte Carlo calculations for the ground state (T = 0) properties of repulsively interacting bosons in a three-dimensional external double well potential over a range of interaction strengths and potential parameters. We focus on calculation of ground state number statistics in order to understand the level of squeezing that the system may exhibit as a function of interaction strength. For weak interactions (i.e. where the standard two-mode model of a BEC in a double well is applicable) we produce results consistent with the two-more model. However, for stronger interactions, we find a novel and somewhat surprising relationship between squeezing and interaction strength. We find that these new features are qualitatively consistent with squeezing calculations carried out using an improved, recently-proposed eight-mode model, although this model is insufficient to quantititively predict the results of the full quantum Monte Carlo simulation. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V4.00013: A Quantum Plasmonic Circuit for Cold Atoms Michael Gullans, Darrick Chang, Johannes Feist, Tobias Tiecke, Jeff Thompson, Ignacio Cirac, Peter Zoller, Mikhail Lukin We propose a new architecture for quantum simulation with atoms using a two dimensional lattice of plasmonic nanoparticles to both trap the atoms and mediate interactions between them. This proposal combines existing technologies from ultracold atoms and plasmonics to exploit the unique coherence properties of atoms and the strong light-matter interaction and subwavelength confinement provided by plasmonic systems. We first show that this system allows to increase the energy scales of Hubbard models by two orders of magnitude compared to optical lattices. We then show how this system can realize a dissipative quantum simulator to prepare a wide range of many-body entangled states. [Preview Abstract] |
Session V5: Surface Electronic & Lattice Properties: Structure, Surface States, & Adsorption
Sponsoring Units: DCMPChair: Ted Einstein, U. of Maryland, College Park
Room: 206A
Thursday, March 1, 2012 8:00AM - 8:12AM |
V5.00001: Surface energy anisotropy for FCC metals: Functional forms Yan-Jiun Chen, Mihir Khadilkar, James P. Sethna The energy of a crystalline surface depends on the angle of the surface normal with respect to the crystalline axes (i.e.~the Miller index). We show that the surface energy as a function of angle is surprisingly easy to describe with a simplistic broken bond model including only a few parameters. In particular, this model is successful at capturing the cusps at high symmetry surfaces. We will use our fitted functional form as a characteristic continuum description of material surface energies for FCC metals. The anisotropy of these surface energies can then be utilized in the study of many material properties- equilibrium shapes (Wolff plots) of crystals and voids, fracture mechanics, cleavage and faceting. We calculate the surface energies using ab initio calculations and various interatomic potentials, providing a measure of the fitness of the potentials for studying physical systems with surfaces. Furthermore, we assemble systematic tables of these results, available to the community through the Knowledgebase of Interatomic Models (KIM, https://openkim.org/). [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V5.00002: Explicit consideration of surface structures for work function calculations Seungchul Kim, Andrew M. Rappe We investigate the effects of atomic structure of surfaces on work functions, using density functional theory (DFT) calculations. Despite the well-known orientation dependence of work functions, the effect of geometric structure of surfaces on work functions is rarely considered. To examine whether the atomic structure of a surface is important to the work function, we perform DFT calculations of thick slabs in the periodic supercell. We then extract the energy levels of the occupied states relative to the vacuum level. Surfaces of oxides, such as BaTiO$_3$ (001), SrTiO$_3$ (001), and TiO$_2$ (110), are especially well-tested because of their various reconstructed structures. Our calculations show that, even for the same surface orientation, the difference in stoichiometry and geometry of the surface gives rise to substantially different work functions. This finding strongly implies that the atomic structures of surfaces must be explicitly taken into account in predicting work functions of materials. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V5.00003: The role of surface states in inelastic electron tunneling into metal surfaces Minghu Pan, Qing Li, Peter Maksymovych Inelastic electron tunneling spectroscopy is a highly capable technique to explore dynamic properties of material surfaces, particularly the phonon spectrum. Presently, many of the so-called ``propensity'' rules that determine the strength of the measured inelastic loss in STM experiments are still unknown or controversial. We have carried out systematic IETS of surface phonons on a Au(111) surface, and spatially-resolved variations in the vibrational spectrum of an adsorbate on the STM tip. In both cases, the IETS intensity markedly dropped at the step edges. At the same time, the IETS intensity exhibits long-range oscillations, the wavelength of which coincided with the Friedel oscillations of the surface state in the vicinity of the defects. All the observations combined, attest to the important role of the surface state in electron-phonon coupling. We will rationalize the observed effects by invoking the symmetry of the tunneling states, with particular emphasis on the effect of the projected band-gap of the gold surface. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V5.00004: Anisotropic Surface State Mediated RKKY Interaction Between Adatoms on a Hexagonal Lattice Theodore Einstein, Paul Patrone Motivated by recent numerical studies of Ag on Pt(111), we derive a far-field expression for the RKKY interaction mediated by surface states on a (111) FCC surface, considering the effect of anisotropy in the Fermi edge. The main contribution to the interaction comes from electrons whose Fermi velocity {\bf v}$_F$ is parallel to the vector {\bf R} connecting the interacting adatoms; we show that in general, the corresponding Fermi wave-vector {\bf k}$_F$ is not parallel to {\bf R}. The interaction is oscillatory; the amplitude and wavelength of oscillations have angular dependence arising from the anisotropy of the surface state band structure. The wavelength, in particular, is determined by the component of the aforementioned {\bf k}$_F$ that is parallel to {\bf R}. Our analysis is easily generalized to other systems. For Ag on Pt(111), our results indicate that the RKKY interaction between pairs of adatoms should be nearly isotropic and so cannot account for the anisotropy found in the studies motivating our work. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V5.00005: Inverse photoemission and LEED investigation of the ion-bombarded Ni(110) surface Benjamin Young, Jim Warner, David Heskett Ion bombardment of the clean Ni(110) surface is investigated by a combination of Inverse Photoemission Spectroscopy (IPES) with a Geiger-Muller detector and Low Energy Electron Diffraction (LEED) with a homebuilt video capture system. Disorder of the surface is induced by argon ion bombardment with various combinations of argon pressure and sputtering time. The intensity of the unoccupied surface state at $\sim$ 2eV above the Fermi level at the \={Y} point of the surface Brillouin zone decreases with increasing surface bombardment. Simultaneously, intensity profiles of diffraction spots in LEED exhibit broadening and a rising background level for increasing surface disorder. Multiple attempts at correlation between the results of the two techniques are presented. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V5.00006: Electronic structure of the indium-adsorbed Au/Si(111)-$\sqrt{3}\times\sqrt{3}$ surface: a first-principles study Feng-Chuan Chuang, Chia-Hsiu Hsu, Wen-Huan Lin, Vidvuds Ozolins Electronic structures of the indium-adsorbed Au/Si(111)-$\sqrt{3}\times\sqrt{3}$ surface were examined using first-principles calculations at In coverages range from 0.15 ML to 1 ML. The band structures of the various proposed models were analyzed in detail. Our results show that the calculated bands for the identified atomic models for indium-adsorbed on conjugate honeycomb-chained-trimer model at In coverages of 1/3 ML and 2/3 ML are in agreement with the identified bands in the angle-resolved photoemission study [Phys. Rev. B \textbf{80} 075312 (2009)]. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V5.00007: Lattice relaxation as the origin of the insulating nature of the alkali/Si(111):B surface Laurent Chaput, Cedric Tournier-Colletta, Louis Cardenas, Bertrand Kierren, Yannick Fagot-Revurat, Daniel Malterre, Patrick Lefevre, Francois Bertran, Antonio Tejeda, Amina Taleb \emph{Ab initio} density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the $2\sqrt{3} \times2\sqrt{3}R30$ surface reconstruction observed previously by LEED on 0.5 ML K/Si:B. It is found that the large K-induced vertical lattice relaxation obtain in the calculations and occurring only for $3/4$ of Si adatoms is shown to quantitatively explain both the chemical shift of $1.14$ eV and the ratio $1/3$ measured on the two distinct B 1s core levels. A gap is observed between valence and conduction surface bands by ARPES and STS which is shown to have mainly a Si-B character using the ab initio calculations. Finally, the calculated STM images agree with our experimental results. Therefore, the insulating character of alkali/Si:B interfaces has been captured to an excellent accuracy, from the low lying 1s state of boron, to the unoccupied states above the gap, within a one-electron approach. This work solves the controversy about the origin of the insulating ground state of alkali-metal/Si(111):B semiconducting interfaces which were believed previously to be related to many-body effects [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V5.00008: First-Principles Study of One-Dimensional Metal-Molecule Hybrid Chains Self-Assembled on Ag Substrate Bon-Gil Koo, Young-Kyun Kwon We explore the formation mechanism and the structural and electronic properties of one-dimensional metal-molecule hybrid chain self-assembled on Ag(111) substrate using {\em{ab initio}} density functional theory. It is observed that such a hybrid chain is formed by spontaneous transformation from 4,4"-dibromo-{\em{p}}-terphenyl (DBTP) molecules when deposited on Ag(111) substrate at room temperature. We find that the chain is composed of {\em{p}}-terphenyl (TP) connected through an Ag atom to form (TP-Ag)$_n$. Our study shows that Ag(111) surface plays a catalytic role removing two Br atoms (DB) from DBTP and connecting the remaining TP through an Ag atom, which is a similar phenomenon as the Ullmann cross coupling reaction occurred on Cu surface. We find that those Br atoms detached from DBTP play important roles for spontaneous formation of well-aligned pattern. Our calculated electronic structures and simulated scanning tunneling microscopy (STM) images of (TP-Ag)$_n$ hybrid chain exhibit remarkably different charge distributions depending on the energy values, which are related to the tip voltage in STM experiments. We also investigate the end state of the hybrid chain, which can be spatially resolved. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V5.00009: Quantum Monte Carlo studies of surface adsorptions Ching-Ming Wei, Cheng-Rong Hsing Surface adsorption is the first step to the study of surface catalytic reaction. The most common used tool is the Density Functional Theory (DFT) based on exchange-correlation approximations and the accuracy usually has not been checked carefully by highly accurate quantum many-body approaches. We have performed calculations of the surface adsorptions using the state-of-the-art diffusion quantum Monte Carlo (QMC) method to examine the accuracy of LDA and GGA (PBE) functionals in the study of surface adsorptions. The systems examined include the H$_{2}$O and OH adsorptions on various types of surfaces such as NaCl(100), MgO(100), TiO$_{2}$(110), graphene, Si(100)-(2x2) and Al(100). By comparing GGA (PBE) results with DMC, our results indicate that (i) for the H$_{2}$O adsorption, PBE predicts the correct adsorption energies; (ii) for the OH adsorption, PBE has predicted a large over-binding effect except on graphene and Si(100) surfaces. This fact indicates that one needs to be cautious when using DFT to study the surface adsorptions of OH free radical. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V5.00010: Resistivity of thin gold films on mica induced by electron-surface scattering from a self-affine fractal surface Raul C. Munoz, Claudio Gonzalez-Fuentes, Ricardo Henriquez, Marcos Flores We present a rigorous comparison between resistivity data and theoretical predictions involving the theory of Palasantzas [G. Palasantzas et al., \textit{Phys. Rev. }\textbf{B 56 }7726 (1997)], and the mSXW-fractal theory [R. C. Munoz et al., \textit{Phys. Rev. }\textbf{B 66 }205401 (2002)], regarding the resistivity arising from electron scattering by a self affine fractal surface on gold films \textit{using no adjustable parameters}. We find that both theories lead to an approximate description of the temperature dependence of the resistivity data. However, the description of charge transport based upon fractal scaling seems oversimplified, and the predicted increase in resistivity arising from electron-surface scattering seems at variance with other experimental results. If the samples are made up of grains such that the mean grain diameter D $>$ L(300), the electronic mean free path in the bulk at 300 K, then the predicted increase in resistivity at 4 K is of the order of a few percent. This contradicts published measurements of magnetomorphic effects arising from size effects where \textit{electron-surface scattering} \textit{is the dominant electron scattering mechanism at 4 K }. On the contrary, if the samples are made out of grains such that D $<$ L(300), then \textit{the dominant electron scattering mechanism controlling the resistivity is not electron-surface scattering but rather electron-grain boundary scattering}, \textit{and the latter electron scattering mechanism is not included in either theory.} [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V5.00011: Electronic confinement imposed by a nanoporous network: Band formation from coupled quantum dots Kathrin Mueller, Meike Stoehr, Manfred Matena, Jorge Lobo-Checa, Thomas Jung, Lutz Gade The electronic and optical properties of crystalline solids exhibit characteristics that derive to a large extent from the periodic arrangement and interactions of their component quantum systems, such as atoms or molecules. Quantum effects due to confinement of electronic states have been extensively studied for surface states of noble metals which are characterized by a quasi 2D electron gas. The design of such periodic 2D structures on surfaces is more readily achieved using molecular self-assembly from building blocks instead of atom by atom manipulation. We chose a perylene derivative (DPDI) as organic building block which is known to form a highly ordered porous network on Cu(111) upon thermal dehydrogenation. To study the interaction between the electronic surface state and our porous network structure, scanning tunneling spectroscopy (STS) and angle-resolved photoemission (ARPES) was used. Each pore of our porous network confines the surface state of the Cu substrate what can be described as a 0D quantum dot. This work can lead to artifically created electronic structures by modification of the dimensions of the molecular network periodicities together with the appropriate choice of the substrate and the building block. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V5.00012: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V5.00013: First-Principles Investigations of Oxygen Vacancies on SnO2 Nanofilms Daniel Cellucci, Steven Lewis In recent years multiple critical advances in nanofabrication have allowed for the well-controlled formation of nanocrystals of the n-type semiconductor tin oxide ($SnO_2$). Because gas sensing in $SnO_2$ involves changes in surface resistivity as a function of gas concentration, the high surface-to volume ratio of $SnO_2$ nanocrystals could be leveraged to produce a gas sensor with significantly enhanced sensitivity. A key feature of the sensing mechanism is the facile formation and destruction of oxygen vacancies at (or near) the surface. In this talk I will discuss our ongoing first-principles investigations of surface oxygen vacancies in $SnO_2$ nanofilms. We have focused on vacancy formation among the so-called bridging oxygen atoms on the (110) surface of rutile $SnO_2$ as a function of vacancy concentration and film thickness and have studied the effect on local atomic and electronic structure. From a set of first-principles Density Functional Theory calculations on ordered vacancy structures, we have parametrized and tested a lattice-gas model describing vacancy-vacancy interactions. Using this model we have conducted extensive Monte Carlo simulations to investigate the oxygen vacancy phases on $SnO_2$ (110) as a function of temperature and oxygen vapor pressure. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V5.00014: Orbital tomography: Deconvoluting photoemission spectra of organic molecules Peter Puschnig, Eva-Maria Reinisch, Thomas Ules, Georg Koller, Sergey Soubatch, Markus Ostler, Lorenz Romaner, F. Stefan Tautz, Claudia Ambrosch-Draxl, Michael G. Ramsey We study the interface of an organic monolayer with a metallic surface, \emph{i.~e.}, PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) on Ag(110), by means of angle-resolved photoemission spectroscopy (ARPES) and \emph{ab initio} electronic structure calculations. We present a tomographic method which uses the energy and momentum dependence of ARPES data to deconvolute spectra into individual orbital contributions beyond the limits of energy resolution. This provides an orbital-by-orbital characterization of large adsorbate systems without the need to invoke sophisticated theory of photoemission, allowing us to directly estimate the effects of bonding on individual orbitals. Moreover, this experimental data serves as a most stringent test necessary for the further development of \emph{ab initio} electronic structure theory. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V5.00015: Two-Dimensional Band Structure Study of Bi$_{1-x}$Sbx Thin Films Shuang Tang, Mildred Dresselhaus Alloys of Bi$_{1-x}$Sb$_{x}$, are considered as one of the best thermoelectric materials for low temperature applications below 200 K. The band structure of Bi$_{1-x}$Sb$_{x}$ varies as a function of stoichiometry. At a temperature below 77 K, it does not change with temperature. At a certain Sb composition (x=0.04), the conduction band and the valence band touch each other at the $L$ point, and the band-crossing occurs. The electronic dispersion relation becomes linear at the $L$ point, which implies that a Dirac cone is formed at each of the three $L$ points. By making the alloys of Bi$_{1-x}$Sb$_{x}$ into thin films, we have two more parameters to vary the band structure, namely film thickness and growth orientation. In our present work, the rich variety of band structure configurations, as well as various phases, of Bi$_{1-x}$Sb$_{x}$ thin films has been revealed. [Preview Abstract] |
Session V6: Focus Session: Carbon Nanotube Optics I: Spectroscopy and Excitons
Sponsoring Units: DMPChair: Philip Collins, University of California, Irvine
Room: 206B
Thursday, March 1, 2012 8:00AM - 8:36AM |
V6.00001: High Throughput Optical Imaging and Spectroscopy of Individual Carbon Nanotubes Invited Speaker: Jiwoong Park Laser-based imaging and characterization of individual carbon nanotubes provides a number of significant advantages over other imaging techniques, including its high throughput, spectral characterization capability, and relatively simple sample preparation. We recently reported a novel on-chip Rayleigh imaging technique using widefield laser illumination to measure optical scattering from individual single-walled carbon nanotubes (SWNTs) on a solid substrate with high spatial and spectral resolution. This method accurately measures the resonance energies and diameters for a large number of SWNTs in parallel. This technique can be used for fast mapping of key SWNT parameters, including the electronic-types and chiral indices for individual SWNTs, position and frequency of chirality-changing events, and intertube interactions in both bundled and distant SWNTs. Further Rayleigh characterization showed that SWNTs can form ideal optical wires. Interestingly, the spatial distribution of the radiation scattered by the nanotubes is determined by their shape, but the intensity and spectrum of the scattered radiation are determined by exciton dynamics and other intrinsic properties. Moreover, the nanotubes display a uniform peak optical conductivity, suggesting universal behavior similar to that observed in nanotube conductance. Finally, two other high-throughput optical imaging techniques, widefield Raman imaging and confocal absorption microscopy, and their applications in nanotube imaging will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V6.00002: Plasmon Nanooptics with Pristine and Hybrid Nanotube Systems Igor Bondarev, Maxim Gelin, Wolfgang Domcke In general, plasmons cannot be excited by light in optical absorption since they are longitudinal excitations while photons are transverse. In small-diameter ($\sim $1 nm) semiconducting carbon nanotubes (CNs), light polarized along the CN axis excites excitons which, in turn, can couple to the nearest (same-band) interband plasmons [1,2]. Both of these collective excitations originate from the same electronic transitions and, therefore, occur at the same (low) energies $\sim $1 eV, as opposed to bulk semiconductors where they are separated by tens of eVs. They do have different physical nature and properties. Their coexistence at the same energies in CNs is a unique feature of confined quasi-1D systems where the transverse electronic motion is quantized to form 1D bands and the longitudinal one is continuous. We discuss how low-energy interband plasmon excitations can efficiently mediate enhanced electromagnetic absorption in pristine semiconducting CNs and bipartite entanglement in hybrid metallic CN systems. We develop a theory for (non-linear) optical monitoring and control of the phenomena above.\\[4pt] [1] I.V.Bondarev, JCTN7, 1673(2010).\\[0pt] [2] I.V.Bondarev, L.M. Woods, and K. Tatur, PRB80, 085407(2009). [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V6.00003: Photoluminescence from suspended individual $^{13}$C-enriched nanotubes T. Shimada, A. Yokoyama, A. Ishii, J. Shiomi, S. Maruyama, Y.K. Kato We investigate isotope effects on the electronic structure of single-walled carbon nanotubes by photoluminescence microscopy. In order to suspend nanotubes for luminescence measurements, trenches are formed on SiO$_2$/Si substrates by electron beam lithography and dry etching processes. No-flow chemical vapor deposition is used to grow carbon nanotubes with small amounts of isotopically enriched ethanol with 99\% $^{13}$C. Optical measurements are done in air at room temperature using a laser scanning confocal microscope with a wavelength tunable Ti:sapphire laser as an excitation source. We have successfully identified suspended $^{13}$C-enriched nanotubes by photoluminescence imaging and assigned their chirality with excitation spectroscopy. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V6.00004: Circular dichroism in air-suspended single-walled carbon nanotubes A. Yokoyama, A. Ishii, M. Yoshida, T. shimada, Y.K. Kato Chiral carbon nanotubes lack inversion symmetry and are expected to show circular dichroism. We have investigated the helicity dependence of absorption in air-suspended single-walled carbon nanotubes by using photoluminescence intensity for detection. Patterned chemical vapor deposition is used to grow carbon nanotubes over trenches etched on SiO$_{2}$/Si substrates. We identified suspended carbon nanotubes by taking photoluminescence images with a home-built laser-scanning confocal microscope and characterized them by excitation spectroscopy. A quarter-wave plate is inserted in the excitation laser path, and the photoluminescence intensity is measured as a function of the angle of the wave plate. We observe differences in the luminescence intensity between left- and right-circularly polarized light. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V6.00005: Visible fluorescence from 5-Angstrom single-wall carbon nanotubes Yasumitsu Miyata, Toshiya Nakamura, Miho Fujihara, Hong En Lim, Ryo Kitaura, Hisanori Shinohara We report the observation of visible fluorescence from the ultrathin single-wall carbon nanotubes (SWCNTs) with diameters of less than 5-Angstrom. Such ultrathin nanotubes were prepared by extracting the inner shells of double-wall carbon nanotubes using ultrasonication [1]. The extracted sample shows two visible photoluminescence (PL) peaks at 700 and 720 nm under light excitation at 410 and 540 nm, respectively. These peaks can be assigned, respectively, as the PL of (4,3) and (5,3) SWCNTs by comparison with the experimental Kataura plot proposed by Weisman et al. [2]. The present findings provide an important insight for the studies of the structural stability and electric structure of ultrathin SWCNTs. [1] Y. Miyata et al. ACS Nano. 4, 5807 (2010), [2] R. Weisman et. al., Nano Lett. 3, 1235 (2003). [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V6.00006: Symmetry-breaking of carbon nanotubes vibrational modes induced by transversal deformations Newton M.B. Neto, Helio Chacham, Ado Jorio, Sabrina S. Carara, Jaqueline Soares, Ronaldo J.C. Batista, Ernesto Joselevich, Mildred Dresselhaus, Paulo Araujo In this work we combine an atomic force microscope (AFM) with a setup to perform confocal Raman spectroscopy to follow, \textit{in situ}, the evolution of the G-band feature of a Single Wall Carbon Nanotube (SWNT) with transversal pressure applied to the nanotube via the AFM probe. We observe a previously elusive and fundamental symmetry-breaking effect of the totally symmetric tangential optical TO modes in the G-band feature which exhibits two distinct Raman active modes with an anomalous frequency behavior with increasing applied pressure, while the totally symmetric longitudinal optical LO component remains unaltered. We propose a simple analytical model based on a mass-spring ring system, which satisfactory explains the main observed effects and shows that the pressure effects change with tube flattening. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V6.00007: Non-Radiative Exciton Decay in Single-Walled Carbon Nanotubes Mark Harrah, Jude Schneck, Alexander Green, Mark Hersam, Lawrence Ziegler, Anna Swan We report on the exciton dynamics for an ensemble of individual, suspended (6,5) single-walled carbon nanotubes via single color $E_{22}$ pump-probe spectroscopy for a wide range of pump fluences. The calculated initial exciton population ranges from $\sim$ 5 to 120 excitons per $\sim$ 725 nm long nanotube, putting the high fluence experiment well into the nonlinear regime. The pump-probe data is not well described by multi-exponential decay or by power law behavior for all fluences. We have developed a single model that describes all data, ranging over two decades of pump fluence and three decades of delay times. The signal decay at low fluence is dominated by a stretched exponential that is consistent with the distribution of relaxation rates resulting from diffusion-limited contact quenching for a nanotube ensemble. The change in dynamics as a function of increasing pump intensity is attributed to exciton-exciton Auger de-excitation in the $E_{11}$ subband and, to a lesser extent, in the $E_{22}$ subband. The initial sub-picosecond decay of the observed response is attributed to $E_{22}$ excitons rapidly acquiring non-zero momentum while remaining in the $E_{22}$ subband. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V6.00008: Fine structure of exciton levels in carbon nanotubes: A semianalytical approach Serguei Goupalov We propose a new approach [1] toward excitons in carbon nanotubes whereby the matrix elements of the electron-hole Coulomb interaction are expanded into a series over the nanotube's one-dimensional reciprocal lattice vectors. We show that only a few terms of this expansion give a non-vanishing contribution to the Coulomb matrix elements. The proposed approach allows one to single out Fourier components of the Coulomb potential responsible for the intervalley coupling and formation of the exciton fine structure for each particular nanotube chirality. \\[4pt] [1] S.V. Goupalov, Phys. Rev. B 84, 125407 (2011). [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V6.00009: Dynamical crossovers, universality and long-range interactions of excitons on carbon nanotubes Jeremy Allam, Tariq Sajjad, Richard Sutton, Sophia Siddique, Zhongyang Wang, Konstantin Litvinenko, Quan-Hong Yang, Tom Brown, Wei Loh Simple microscopic interactions in non-equilibrium systems give rise to complex emergent macroscopic phenomena. There has been much theoretical work to understand dynamics of different systems, and equilibrium concepts of scaling, criticality and universality have proved useful. However there is a noted lack of experimental studies. Here we show that exciton reactions on carbon nanotubes display the rich kinetics of the prototypical 1D coalescence reaction A+A-$>$A. An Auger-like exciton interaction\footnote{Y.-Z. Maet al, Phys Rev Lett 94, 157402 (2005).} and anomalous kinetics\footnote{R. M. Russo et al. Phys Rev B 74, 041405 (2006).} have already been reported. Here we demonstrate the existence of four distinct dynamical regimes: (1) early dynamics determined by spatial ordering of excitons due to Pauli repulsion at high concentrations; (2) a classical mean-field region with exciton population n decaying as $t^{-1}$; (3) a self-organized critical state with anomalous reaction kinetics limited by diffusion and characterized by $n\sim t^{-1/2}$, which we show to be universal with respect to the initial population; and (4) an exponential approach towards an absorbing state corresponding to one exciton per nanotube. The abrupt crossover between regimes indicates a long-range exciton interaction, which introduces a non-scaling dimension that breaks universality at intermediate length-scales. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V6.00010: Exciton transport and exchange self-energy in semiconducting carbon nanotubes Jared Crochet, Jay Sau, Marvin Cohen, Juan Duque, Laurent Cognet, Stephen Doorn We present direct measurements of $S_1$ exciton transport in (6,5) carbon nanotubes. Exciton diffusion lengths associated with end quenching, photoluminescence lifetimes, and homogeneous emission linewidths provide a basis for determining an intrinsic diffusion constant of 5 cm$^2$s$^{-1}$ within the dispersion of light. Exciton diffusion is modeled in terms of an anomalous dispersion within a marginal Fermi liquid description of the exciton exchange self-energy and acoustic phonon scattering. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V6.00011: Quantum Interference Between the Third and Fourth Excitonic States in Semiconducting Carbon Nanotubes Hagen Telg, Juan G. Duque, Hang Chen, Anna K. Swan, Xiaomin Tu, Ming Zheng, Andrew P. Shreve, Stephen K. Doorn We exploit an energy level cross-over effect\footnote{H\'aroz,~E.~H. \emph{et al.}; \emph{Phys. Rev. B.} \textbf{2008}, \emph{77}, 125405} to probe quantum interference in the resonance Raman response from carbon nanotube samples highly enriched in the single semiconducting chiralities of (8,6), (9,4), and (10,5). UV Raman excitation profiles of G-band spectra reveal unambiguous signatures of interference between the third and fourth excitonic states (E33 and E44). Both constructive and destructive responses are observed and lead to anomalous intensity ratios in the LO and TO modes. Especially large anomalies for the (10,5) structure result from nearly identical energies found for the two Eii transitions. The interference patterns demonstrate that the sign of the exciton-phonon coupling matrix elements changes for the LO mode between the two electronic states, and remains the same for the TO mode. Significant non-Condon contributions to the Raman response are also found.\footnote{Duque,~J.~G. \emph{et al.}; \emph{ACS Nano} \textbf{2011}, \emph{5}, 5233--41} [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V6.00012: New Fundamental Optical Behaviors of Single-Wall Carbon Nanotubes at Cryogenic Temperatures: Closer to their Intrinsic Behavior Juan Duque, Christopher Hamilton, Jared Crochet, Andrew Dattelbaum, Scott Crooker, Stephen Doorn Development of single walled carbon nanotube (SWNT) materials for optoelectronics and nanophotonics has been especially challenging in that SWNT optical properties are highly sensitive to environmental interactions, which can be particularly severe in composite matrices. Here, we present for the first time an innovative approach to obtain highly photoluminescent (PL) solid-state SWNT-nanocomposites, which provides access to novel photophysical properties. Strongly blue-shifted spectral features, and significant increase ($\sim $ 3x) in PL intensities at croyogenic temp in comparison to room tem or previous reports. This difference can be understood as arising from a significantly slower relaxation of excitons from bright to dark states in our SWNTs, as a result of much weaker interaction with the environment. That is, the bright/dark exciton distribution is highly non-thermal, even at the lowest temperatures. In our SWNT-nanocomposites, environmental interactions are minimized, thus bright excitons \textit{cannot} relax efficiently to the dark state, causing a highly non-equilibrium exciton distribution and a correspondingly large PL intensity, even at low temperatures. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V6.00013: Coherent anti-stokes spectroscopy as a probe of chemical disorder in isolated carbon nanotubes Tatyana Sheps, Jordan Brocious, Eric O. Potma, Philip G. Collins We use a third-order coherent anti-stokes (CAS) optical technique to study chemical disorder in individual carbon nanotubes. The CAS response is highly sensitive to this disorder, to the extent that a few chemical defects can appreciably decrease the overall signal. The experiments are performed on individual single- and multi-walled carbon nanotubes (SWNTs and MWNTs) connected in a transistor geometry and subjected to varying degrees of controlled, electrochemical oxidation. The overall CAS intensity can be used to probe the extent of chemical modification, and inhomogeneities along a nanotube resolve local coherent electron density fluctuations. We find that the CAS signal is also strongly affected by substrate interactions: aligned SWNTs grown on single crystal quartz are quenched compared to SWNTs on fused quartz. Finally, CAS spectroscopy on individual SWNTs and MWNTs using picosecond pulses resolves the third-order vibrational signal component at the G-band frequency. The ratio of electronic to vibrational CAS signal components is diameter dependent and in small diameter SWNTs the vibrational component is dominated by the electronic CAS signal. However, in MWNTs, this technique is a first step toward chemically sensitive CARS imaging on a single nanotube level. [Preview Abstract] |
Session V7: Focus Session: Computational Design of Materials - Engineering of Electronic Structure Materials
Sponsoring Units: DMP DCOMPChair: Serdar Ogut, University of Illinois at Chicago
Room: 207
Thursday, March 1, 2012 8:00AM - 8:12AM |
V7.00001: Band-edge engineering of Silicon by Surface Functionalization: a Combined Ab-initio and Photoemission Study Yan Li, Leslie O’Leary, Nathan Lewis, Giulia Galli The electrode material choice is limited in solar to fuel formation devices because of the requirement of band-edge matching to the fixed fuel formation potential. This limitation can be relieved via band-edge engineering. The changes of band-edge positions of Si electrodes induced by the adsorption of H-, Cl-, Br- and short-chain alkyl groups were investigated by combining density functional (DFT), many-body perturbation theory (MBPT), and ultraviolet photoelectron spectroscopy. The band edge shifts are related to the formation of surface dipole moments, and determine the barrier height of electrons and holes in doped silicon surfaces. We find that the trends of the sign and magnitude of the computed surface dipoles as a function of the adsorbate may be explained by simple electronegative rules. We show that quasi-particle energies obtained within MBPT are in good agreement with experiment, while DFT values may exhibit substantial errors. However computed band edge differences are in good agreement with spectroscopic and electrical measurements even at the DFT level of theory. [1] Y. Li and G. Galli, Phys. Rev. B 82, 045321 (2010). [2] Y. Li, L. O'Leary, N. Lewis and G. Galli, to be submitted. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V7.00002: Genetic engineering of band-egde optical absorption in Si/Ge superlattices Mayeul d'Avezac, Jun-Wei Luo, Thomas Chanier, Alex Zunger Integrating optoelectronic functionalities directly into the mature Silicon-Germanium technology base would prove invaluable for many applications. Unfortunately, both Si and Ge display indirect band-gaps unsuitable for optical applications. It was previously shown (Zachai \textit{et al.} PRL \textbf{64} (1990)) that epitaxially grown [(Si)$_n$(Ge)$_m$]$_p$ (i.~e.~ a single repeat unit) grown on Si can form direc-gap heterostructures with weak optical transitions as a result of zone folding and quantum confinement. The much richer space of \emph{multiple-period} superlattices [(Si)$_{n_1}$(Ge)$_{n_2}$(Si)$_{n_3}$(Ge)$_{n_4}$\ldots$Ge_{n_N}$]$_p$ has not been considered. If $M=\sum n_i$ is the total number of monolayers, then there are, roughly, $2^M$ different possible superlattices. To explore this large space, we combine a (i) genetic algorithm for effective configurational search with (ii) empirical pseudopotential designed to accurately reproduce the inter-valley and spin-orbit splittings, as well as hydrostatic and biaxial strains. We will present multiple-period SiGe superlattices with large electric dipole moments and direct gaps at $\Gamma$ yielded by this search. We show this pattern is robust against known difficulties during experimental synthesis. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V7.00003: New mechanism for work-function tuning: ZnO surfaces modified by a strong organic electron acceptor Yong Xu, Oliver T. Hofmann, Rinke Patrick, Matthias Scheffler, Raphael Schlesinger, Norbert Koch, Antje Vollmer A key task for optimizing optoelectronic devices comprising hybrid inorganic/organic systems is to control the energy level alignment at interfaces. The use of interlayers provides a pathway to solve this challenge. To demonstrate the concept we investigated the polar surfaces of ZnO, modified by the prototypical organic electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) theoretically and experimentally. DFT-PBE $\Delta$SCF calculations of the O 1s surface core-level shifts combined with XPS measurements suggest that ZnO(000-1)/ZnO(0001) are H/OH covered. Depositing F4TCNQ on these surfaces considerably increases the work function that is shown to be insensitive to the doping level in PBE+vdW. F4TCNQ on ZnO(0001) exhibits an extraordinary high work function due to the appearance of upright adsorption. The PBE+vdW results are in line with the UPS data that shows work-function increases up to 1.4/2.8 eV on ZnO(000-1)/ZnO(0001). In contrast to F4TCNQ on metals, where pronounced bidirectional charge transfer occurs, the charge transfer from ZnO to F4TCNQ is small, pinning the LUMO close to the Fermi level. The polarization of the system, caused by strong charge rearrangement within the adsorbate, is the main mechanism for the large work-function increase. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V7.00004: Quantum Monte Carlo Characterization of Excited States and Energy-Level Alignment at Oligomer/Quantum-Dot Interfaces Yosuke Kanai, Jonathan L. DuBois, Donghwa Lee Charge separation of excitons in materials is one of the most important physical processes that need to take place in excitonic solar cells and in photocatalytic devices. Heterogeneous interfaces with the so-called type-II character are often employed for inducing the exciton dissociation through interfacial charge transfer. As the simplest criterion for designing such an interface, the energy alignment of the quasi-particle states is often discussed in literature, together with the exciton binding energy of electron-donating materials. Therefore, accurate characterization of the interfacial energy-level alignment and the exciton binding energy using first principles calculations is important for making systematic progresses in designing better materials for solar energy conversion. However, Density Functional Theory calculations need to be employed with caution in this context. First principles calculations such as Many-Body Perturbation Theory and Quantum Monte Carlo are promising alternatives for accurate characterization, but much more work is needed in this area to assess how well these methods perform in practice. In this talk, we will discuss our preliminary results using diffusion Quantum Monte Carlo on calculating the excited states and energy-level alignment of popular Oligomer/Quantum-Dot interfaces. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V7.00005: Shape managing the cross-section of a semiconductor nanowire as a method for fine tuning electro-optical properties Osman Baris Malcioglu, Jean-Yves Raty, Sorin Melinte Geometry design in nanowire and nanopillar arrays is used to create photon managing structures in a number of optical applications ranging from photovoltaics to field emission devices. The diameter dependence of the intrinsic properties of the material at the nanoscale provides further tunability and significantly improved performance. Using ab-initio methods (TDDFT), we explore the additional possibility of tuning and enhancing the optical properties by fine structuring the cross-sectional geometry. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V7.00006: Dimensional Reduction: A design tool for new semiconductor compounds for radiation detection B.W. Wessels, J.A. Peters, Z. Liu, J. Androulakis, M.G. Kanatzidis, H. Jin, A.J. Freeman To address the need for new wide gap semiconductors for efficient radiation detection, we present a new design tool called \textit{dimensional reduction }(DR). The method is based on reducing the dimensionality of highly dense but low bandgap ($<$1 eV) compounds to create comparably dense wide bandgap ($>$1.6 eV) semiconductors without compromises on their mass density. We utilize electronic band structure calculations of bandgap and effective mass to aid in the selection among the wide variety of compounds that can be formed by DR. As a proof of the concept, we report on computational design as well as optical characterization of three such dimensionally reduced materials based on \textit{$\beta $}-HgS, HgSe, and CdTe compounds. These compounds, namely, Cs$_{2}$Hg$_{6}$S$_{7}$, Cs$_{2}$Hg$_{3}$Se$_{4}$, and Cs$_{2}$Cd$_{3}$Te$_{4}$, show great promise as detector materials. Band structure calculations show that they have direct bandgaps of 1.28, 1.97, and 2.35 eV, respectively, in good agreement with experimental values of 1.63, 2.2 and 2.5 eV, respectively. Measured electrical resistivity values of $\sim $10$^{6}$, 10$^{7}$, and 10$^{9} \quad \Omega $-cm, respectively, are high enough for further evaluation of these materials for hard radiation detection. Computational design of other wide gap semiconductors is underway. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V7.00007: First principles study of LaGaO$_3$/MgAl$_2$O$_4$ (001) polar interfaces Alejandro Rebola, Peter Zapol, Jeffrey Eastman, Serdar Ogut Materials with high oxygen ion conductivity have been the center of much attention due to both fundamental interest and technological applications. One of the most remarkable ionic conductors and an excellent candidate for future solid oxide fuel cells is LaGaO$_3$ (LGO), as it exhibits very high ionic conductivity when doped with Sr or Mg. To achieve enhanced ionic transport in this system, where oxygen vacancies (V$_{\rm O}$) are the dominant carriers, we propose a negatively charged interface as a way of inducing a V$_{\rm O}$ enrichment layer. In this study, the interface is comprised of LGO and the spinel MgAl$_2$O$_4$, both of which exhibit nominally charged (001) planes. We consider an interface where the (GaO$_2$)$^{1-}$ layer of LGO is in contact with the (AlO$_2$)$^{1-}$ layer of the spinel. Such negatively charged interfaces require compensating defects, providing a strong driving force for enhancing the V$_{\rm O}$ concentration, and hence, the in-plane ionic conductivity in the space charge region adjacent to the boundary. We report results from first-principles calculations which provide information on the structure and relative stability of these polar interfaces, compensation mechanisms, and defect formation energies as a function of distance from the interface. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V7.00008: Theory-driven design of hole-conducting transparent oxides G. Trimarchi, H. Peng, Im J., A.J. Freeman, V. Cloet, A. Raw, K.R. Poeppelmeier, K. Biswas, S. Lany, A. Zunger The design of {\em p}-type transparent conducting oxides (TCOs) aims at {\em simultaneously} achieving transparency and high hole concentration and hole conductivity in one compound. Such design principles (DPs) define a multi-objective optimization problem that is to be solved by {\em searching} a large set of compounds for optimum ones. Here, we screen a large set of ternary compounds, including Ag and Cu oxides and chalcogenides, by calculating via first-principles methods the design properties of each compound, in order to search for optimum {\em p}-type TCOs. We first select Ag$_{3}$VO$_{4}$ as a case study of the application of {\em ab-initio} methods to assess a compound as a candidate {\em p}-type TCO. We predict Ag$_{3}$VO$_{4}$ (i) to have a hole concentration of $\approx 10^{14}$ $cm^{-3}$ at room temperature, (ii) to be at the verge of transparency, and (iii) to have lower hole effective mass than the prototype {\em p}-type TCO CuAlO$_{2}$. We then map the hole effective mass $vs.$ the band gap in the selected compounds and determine those that best meet the DPs by having simultaneously minimum effective mass and a band gap large enough for transparency. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V7.00009: Screw Dislocated ZnO and Si Nanostructures Studied with Objective Molecular Dynamics Evgeniya Akatyeva, Ilia Nikiforov, Dong-Bo Zhang, Traian Dumitrica Objective molecular dynamics [1] coupled with tight-binding density functional-based models makes it possible to investigate the stability and electronic structure of ZnO and Si nanotubes [2] and nanowires [3] containing axial screw dislocations. The dislocated structures adopt twisted configurations that stabilize the dislocation at the center despite the close vicinity of surfaces, in excellent agreement with Eshelby's elasticity model of cylinders containing an axial screw dislocation. Coupled to this elasticity model, our simulations represent a new efficient method of calculating the core energy of a dislocation and allow to rationalize the stability of chiral hollow nanowires. The uncovered mechanical and electronic behaviors have implications for a broad class of nanomaterials grown by engaging a screw dislocation. 1. T. Dumitrica and R.D. James, J. Mech. Phys. Sol. 55, 2206-2236 (2007). 2. D.-B. Zhang, E. Akatyeva, and T. Dumitrica, Phys. Rev. B 84, 115431 (2011). 3. I. Nikiforov, D.-B. Zhang, and T. Dumitrica, J. Phys. Chem. Lett. 2, 2544 (2011). [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V7.00010: Tunable band gaps in transition metal dichalcogenides Ashwin Ramasubramaniam, Doron Naveh, Elias Towe We investigate band-gap tuning in transition-metal dichalcogenide bilayers by external electric fields applied perpendicular to the layers. Using density functional theory, we show that the fundamental band gap of MoS$_{2}$, MoSe$_{2}$, MoTe$_{2}$, and WS$_{2}$ bilayer structures continuously decreases with increasing applied electric fields, eventually rendering them metallic. We interpret our results in the light of the Giant Stark Effect and obtain a robust relationship, which is essentially characterized by the interlayer spacing, for the rate of change of band gap with applied external field. Our study expands the known space of layered materials with widely tunable band gaps beyond the classic example of bilayer graphene and suggests potential directions for fabrication of novel electronic and photonic devices. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V7.00011: Growth of Monolayer Boron Sheet on Metal and Metal Boride Surface Yuanyue Liu, Qingbo Yan, Hoonkyung Lee, Boris Yakobson Monolayer boron (B) sheet has attracted lots of interests recently due to its metallic conductivity. However, their experimental synthesis has not been achieved so far, which calls for theoretical investigation. Using first principles calculations, we study the possibility of growing monolayer B sheets on metal (Ag, Au) and metal boride (MgB2, TiB2) surface as catalytic substrate. It is shown that after decomposition from precursor, B atoms will aggregate to cluster, then to sheets, while three dimensional bulky B is prohibited due to high nucleation barrier. Charge transfer between substrate and B sheet shifts its stability dependence on hexagon vacancy density. B sheet with specific vacancy density can have cleavage energy as small as graphite thus should be easily peeled off. This work suggests promising approach to synthesize B sheets and would possibly pave the way towards their applications to electronic, optic, and mechanic nano-devices. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V7.00012: Understanding the properties of hexagonal Semiconduncting Nanomembranes Rodrigo Amorim, Xiaoliang Zhong, Saikat Mukhopadhyay, Ravindra Pandey, Shashi Karna Namomembranes are an interesting material with novel applications, such as their integration into electronic devices. We can highlight the high degree of bendability of nanomembranes that can be important to device integration and the possibility of modifying electronic properties by changing the roughness. Using density functional theory (DFT) combined with non-equilibrium green's function (NEGF) theory, we investigate different hexagonal semiconducting nanomembranes (e.g. BN, AlN and GaN). We will show the stability, electronic and transport properties of these nitride membranes and look into their possible integration with graphene. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V7.00013: Single-atom Magnetic Anisotropy on a Surface Chiung-Yuan Lin, Jing-Neng Yao Studying single-atom magnetic anisotropy on surfaces enables the exploration of the smallest magnetic storage bit that can be built. In this work, magnetic anisotropy of a single rare-earth atom on a surface is studied for the first time, both computationally and theoretically. The substrate surface is chosen to be a copper-nitrite surface, where single transition-metal magnetic atoms on the same surface were previously studied one atom at a time by STM.\footnote{C. F. Hirjibehedin, C.-Y. Lin, A. F. Otte, M. Ternes, C. P. Lutz, B. A. Jones, A. J. Heinrich, Science 317, 1199 (2007).} We propose unconventional $f$ and $d$ subshell symmetries so that following first-principles calculations, simple pictorial analyses of such anisotropy can be performed for the first time, independently for both rare-earth and transition-metal adatoms. The analyses explain the spin-density distribution of a single adatom, and derive the spin orientation of its largest spin-orbit coupling. The magnetic anisotropy energy of the present study is calculated to be a factor of five larger than the previous highest one. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V7.00014: Computer Simulated Cold Welding of Metal Nanowires Edison da Silva, Zenner Pereira Metallic contacts are of great importance in electronic devives, the ability of welding them without temperature change is quite remarkable and of interest. Recently cold welding was achieved in gold and silver nanowires (NWs) with diameters in the range of 4 to 10 nm [1]. In the present work we use computer simulations to produce cold welding in gold, silver and silver-gold NWs at room temperature. We used molecular dynamics with many body effective potentials based on the embedded-atom method EAM using the LAMMPS code to simulate first the braking of gold and silver NWs. The two produced NWs are then cold welded and similarly as occurred in the experiments the newly welded NWs showed fcc structures as the printine samples. The structural analysis is done with two independent methods [2] and strain stress curves of the breaking and welding are present. Our computer simulation compare very well with the experiments. This work is supported by CNPq CAPES and FAPESP and FAEPEX. ZSP is supported by CAPES. CENAPAD-SP and IFGW are acknowledged for computer time. \\[4pt] [1] Y. Lu, \textit{et al}. Nature Nanotechnology 5, 218 - 224 (2010)\\[0pt] [2] E. Z. da Silva and Z.S. Pereira, Phys. Rev. B \textbf{81}, 195417 (2010). [Preview Abstract] |
Session V8: Focus Session: Frustrated Magnetism - Kagome II
Sponsoring Units: DMP GMAGChair: Joe Helton, NIST
Room: 208
Thursday, March 1, 2012 8:00AM - 8:12AM |
V8.00001: Monte Carlo simulations of the fcc Kagom\'{e} lattice Vahid Hemmati, Martin Plumer, John Whitehead, Byron Southern For many years, Ir-Mn alloys have been widely used by the magnetic storage industry in thin-film form as the antiferromagnetic pinning layer in GMR and TMR spin valves [1]. Despite the technological importance of this structure, it has not previously been noted that the magnetic Mn-ions of fcc IrMn$_3$ reside on Kagom\'{e} layers ABC stacked along $<$111$>$ axes normal to the film plane [2,3]. Results of Monte Carlo simulations will be reported on the bulk fcc Kagom\'{e} lattice for both XY and Heisenberg models including the eight NN exchange interactions. Degeneracies persist in the 3D case and there is strong evidence for a fluctuation-driven first-order transition to well-defined long-range order characterized as the layered ``$q$=0'' 120-degree spin structure. Effects of varying the inter-layer coupling are also examined. \\[4pt] [1] M. Tsunoda et al, Appl. Phys. Lett. {\bf 97}, 072501 (2010).\\[0pt] [2] I. Tomeno et al, J. Appl. Phys. {\bf 86}, 3853 (1999).\\[0pt] [3] L. Szunyogh et al, Phys. Rev. B {\bf 79}, 020403(R) (2009). [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V8.00002: Phase Control of Magnetic Order in (Y,Lu)BaCo$_{4}$O$_{7}$ John Mitchell, Sevda Avci, Omar Chmaissem, Laurent Chapon, Dmitry Khalyavin The RBaCo$_{4}$O$_{7}$ (R=Ca, Y, Tb-Lu) system provides a novel topology for studying geometric frustration, in which face-sharing tetrahedra of magnetic ions link to form trigonal bipyramids on a Kagom\'{e} lattice. Here we describe the structural and magnetic behavior of the Lu member and the solid solution joining Lu to Y as a chemical means to tune between magnetically ordered and disordered ground states. Mean-field models of the generic magnetic phase diagram of RBaCo$_{4}$O$_{7}$ determined recently by our group (D. D. Khalyavin et al. Physical Review B 82, 094401 (2010)) show a variety of magnetic states as a function of two exchange parameters: J$_{ab}$ and J$_{c}$, where J$_{ab}$ links Co ions in the Kagom\'{e} planes and J$_{c}$ links Co ions from the Kagome plane to the interleaving triangular layer. Experimentally, we find that YBaCo$_{4}$O$_{7}$ has a long-range ordered antiferromagnetic ground state, while LuBaCo$_{4}$O$_{7}$ appears to be disordered above 2 K with very slow dynamics measured by neutron scattering. We use the solid solution to interpolate between these endpoints and discuss these results with respect to the mean-field phase diagram. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V8.00003: Spin dynamics in the extended kagom\'{e} YBaCo$_{4}$O$_{7}$ Michael Hoch, Philip Kuhns, Tiglet Besara, Arneil Reyes, John Mitchell The extended kagom\'{e} systemYBaCo$_{4}$O$_{7}$ consists of antiferromagnetically coupled Co$^{2+}$ and Co$^{3+}$ ions arranged in stacks with alternating kagom\'{e} and triangular layers in ab planes in the orthorhombic lattice. The oxygen-stoichiometric material orders below 110 K. The system exhibits interesting exchange topology with both trigonal bipyramids and triangular kagom\'{e} clusters of Co ions. Model calculations and neutron scattering experiments, made by other workers, have provided considerable information on the magnetic structure. Ordered chains are found for the apical ions along the c-axis with neighbor chains having oppositely directed polarizations perpendicular to c in an antiferromagnetic configuration. Only short range order is present in the kagom\'{e} planes at temperatures as low as 2 K. The present pulsed NMR measurements, made on a single crystal, both in zero magnetic field and in low applied fields, distinguish the Co ion sites and provide information on the evolution of the spin dynamics for the plane and chain sites as a function of temperature in the range 1.7 - 50 K above which signal wipe-out occurs. A dramatic change in the spin dynamics is found below 5 K. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V8.00004: Spectral Signature of Neodymium Dopants in Frustrated Gadolinium Gallium Garnet Lattice Christopher Ferri, Michael Tennenbaum, Sayantani Ghosh We investigate the spectral emission of Nd3+ dopant ions (1{\%} at. wt) in the frustrated magnet Gadolinium gallium Garnet (GGG) as a function of temperature and magnetic field. We concentrate on the low energy excitations centered at 1064 nm and 935 nm, which show a multiplet structure at room temperature. As temperature decreases the emission spectra demonstrate changes in relative intensities that undergo a cross-over at 122 K under zero field cooled conditions. This cross-over is magnetic field dependent and changes as we field-cool the sample. Typically, with decreasing temperature the line widths of the spectral peaks decrease, as is expected. However, when cooled below 10 K selective peaks start exhibiting broadening, even when zero-field cooled. We follow this line broadening as a function of magnetic field and dopant concentration and speculate it is a result of the dopant ions coupling to the internal magnetic fields of the host lattice. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V8.00005: Investigating the nature of magnetic correlations in the anti-ferromagnetic hyper-kagome material, Yb$_3$Ga$_5$O$_{12}$ Kate Ross, Katharina Fritsch, Robert Bewley, Tatiana Guidi, Yiming Qiu, Chris Wiebe, Haidong Zhou, Hanna Dabkowska, Bruce Gaulin The magnetic Yb$^{3+}$ ions in Yb$_3$Ga$_5$O$_{12}$ (YbGG) reside on a hyper-kagome lattice, which has the same connectivity as the planar kagome lattice but in higher dimensions. For anti-ferromagnetically (AFM) coupled spins the hyper-kagome lattice provides a highly-frustrated geometry in three-dimensions. YbGG is isostructural with the well-studied Gd$_3$Ga$_5$O$_{12}$ (GGG), which enjoys an exotic magnetic phase diagram. In GGG, the effects of geometric frustration manifest as a disordered, partial spin-glass ground state down to 25mK in zero-field. The application of an external magnetic field first induces an intermediate spin liquid state, then a long range AFM ordered phase. Much less is known about YbGG, though all experimental evidence indicates a lack of LRO in zero-field down to 30mK. We have recently produced single crystals of YbGG and have performed neutron scattering experiments over a range of temperatures (80mK - 10K) and magnetic field strengths (0T - 8T). The results indicate low-energy, fluctuating spin correlations at 80mK, 0T, as well as a dramatic response to an applied magnetic field. At zero-field, we also observe a low-energy dispersionless spin excitation that softens as the temperature is increased above the Schottky anomaly in the specific heat. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V8.00006: Spatially anisotropic kagome antiferromagnet with Dzyaloshinskii-Moriya interaction Vladimir A. Zyuzin, Gregory A. Fiete We theoretically study the spatially anisotropic spin-1/2 kagome antiferromagnet with Dzyaloshinskii-Moriya (DM) interaction using a renormalization group analysis in the quasi-one-dimensional limit. We identify the various temperature and energy scales for ordering in the system. For very weak DM interaction, we find a low-temperature spiral phase with the plane of the spiral selected by the DM interaction. This phase is similar to a previously identified phase in the absence of the DM interaction. However, above a critical DM interaction strength we find a transition to a phase with coexisting antiferromagnetic and dimer order, reminiscent of one-dimensional antiferromagnetic systems with a uniform DM interaction. Our results help shed light on the fate of two dimensional systems with both strong interactions and significant spin-orbit coupling. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V8.00007: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V8.00008: Dimensional reduction, avalanches and disorder in artificial kagome spin ice Remo V. Hugli, Gerard Duff, Hans-Benjamin Braun In collaboration with an experimental team at the Swiss Light Source we have recently demonstrated that emergent monopoles and associated Dirac strings can directly be observed in real space via x-ray circular dichroism in a kagome lattice geometry. Here we build on the fact that the experimental results are in excellent agreement with MC simulations of a lattice of point dipoles with disorder realized in the form of random switching fields. We demonstrate that within a large range of physical parameters such as interdipolar coupling and randomness, magnetization reversal proceeds via a novel 1D avalanche behaviour whose hallmark is an exponential avalanche size distribution. After presenting simple arguments for the origin of such dimensional reduction we demonstrate that such 1D avalanche behavior also occurs in a model where the dipoles are stretched into magnetic charge dumbbells which provides a more realistic model for nanolithographic islands. Finally we demonstrate how a judicious design of the island anisotropy can be used to achieve controlled switching and avalanche propagation which paves the way for spintronic applications [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V8.00009: Propagation of monopole defects and flux channels in an artificial square spin-ice lattice Yimei Zhu, V.V. Volkov, Shawn Pollard The recent development of artificial lattices of magnetic islands in which competing interactions give rise to macroscopic analogs of atomically frustrated spin ices has opened up a new field of research, in which the interaction, frustration and evolution of individual magnetic elements can be directly observed in real space. We investigate the magnetic reversal along the (11) symmetry axis of permalloy islands in an artificial ``square'' spin-ice geometry with in-situ Lorentz transmission electron microscopy. Novel differential transport-of-intensity allows for the identification of ``monopole''-like defects and flux channels, similar to Dirac strings, that link them. We track the growth and propagation of these defects and flux channels throughout the reversal process. Simulations are used to compare with experiment to show how nucleation and propagation of defects affect the reversal of the lattice as a whole. We find that interactions between defects and flux channels can explain the saturation of defect populations at low net magnetizations. This work was supported by U.S. Department of Energy, Office of Basic Energy Science, Material Sciences and Engineering Division, under Contract No. DE-AC02-98CH10886. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V8.00010: Magnetization dynamics in artificial spin ice lattices Olle Heinonen, Sebastian Gliga Artificial spin ice lattices (ASIL) consist of regular arrays of single-domain nanomagnets displaying ice rule ordering. Frustration is introduced through shape anisotropy. ASILs have been shown to exhibit complex behavior, with rich phase diagrams and quasi-static magnetization reversal. In particular, topological defects, such as Dirac monopoles and Dirac strings, play a fundamental role in the quasi-static behavior of ASILs. In this work, we use micromagnetic simulations to investigate the resonant frequencies of square lattice ASILs consisting of stadium-shaped nanomagnets. We calculate the evolution of the fundamental modes of a single element when elements are combined in four-stadia configurations and large lattices. In a cross-shaped four-stadium configuration for example, the Dirac monopole splits the frequencies of the lowest (near)-degenerate symmetric and antisymmetric edge modes of a single stadium. This splitting increases in a 24-stadium system with two monopoles. We also calculate the evolution of the spectral characteristics as the monopoles move farther apart in the lattice, but stay connected through a Dirac string. Our work suggests that these topological defects have distinct spectral signatures that can be detected experimentally. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V8.00011: Study of system-size effects on the emergent magnetic monopoles and Dirac strings in artificial kagome spin ice Alejandro Leon In this work we study the dynamical properties of a finite array of nanomagnets in artificial kagome spin ice at room temperature. The dynamic response of the array of nanomagnets is studied by implementing a ``frustrated celular aut\'{o}mata'' (FCA), based in the charge model. In this model, each dipole is replaced by a dumbbell of two opposite charges, which are situated at the neighbouring vertices of the honeycomb lattice. The FCA simulations, allow us to study in real-time and deterministic way, the dynamic of the system, with minimal computational resource. The update function is defined according to the coordination number of vertices in the system. Our results show that for a set geometric parameters of the array of nanomagnets, the system exhibits high density of Dirac strings and high density emergent magnetic monopoles. A study of the effect of disorder in the arrangement of nanomagnets is incorporated in this work. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V8.00012: Magnetic reversal of an artificial square ice: dipolar correlation and charge ordering Jason Morgan, Aaron Stein, Sean Langridge, Christopher Marrows Artificial spin ices are lithographically patterned arrays of single domain nanomagnets [1-4]. The elongated elements form a 2D system of interlinked vertices at which Ising-like dipole moments meet with incompatible interactions. They are directly analogous to 3D bulk spin ice materials [5]. We report on the magnetic reversal of an athermal artificial square ice pattern subject to a sequence of magnetic fields applied slightly off the diagonal symmetry axis, investigated via magnetic force microscopy of the remanent states that result [1]. From an initial diagonally polarised state, sublattice independent reversal is observed via bulk-nucleated incrementally-pinned flipped moment chains along parallel channels of magnetic elements, as evident from analysis of vertex populations and dipolar correlation functions. Weak dipolar interactions between adjacent chains favour antialignment and give rise to weak charge ordering of ``monopole'' vertices during reversal. \\[4pt] [1] J. P. Morgan, A. Stein, S. Langridge \& C.H. Marrows, New Journal of Physics (2011), 13, 105002.\\[0pt] [2] R. F. Wang et al., Nature (2006), 439, 303-306.\\[0pt] [3] E. Mengotti et al., Nature Physics (2011), 7, 68-74.\\[0pt] [4] J. P. Morgan et al., Nature Physics (2011), 7, 75-79.\\[0pt] [5] M. J. Harris et al., PRL (1997), 79, 2554-255 [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V8.00013: Experimental Realization of Isotropic Ising Spins in Frustrated and Unfrustrated Artificial Spin Ice with Perpendicular Anisotropy Sheng Zhang, Jie Li, Ian Gilbert, Yu Pan, Paul Lammert, Kriti Kohli, Rajiv Misra, Vincent Crespi, Nitin Samarth, Peter Schiffer, Cristiano Nisoli, Mike Erickson, Chris Leighton We have studied lithographically defined arrays of magnetostatically interacting single domain ferromagnetic islands with moments normal to the plane, leading to fully isotropic magnetostatic interactions. Probing both frustrated kagome and unfrustrated honeycomb array geometries, we find that the spin configurations can be reproduced with models based on only nearest-neighbor correlations. While the honeycomb geometry displays ordering of moments in well-defined domains, the kagome geometry has only short range correlations that show striking similarities to those of analogous in-plane systems and are closely comparable to expectations for a simple Ising system. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V8.00014: Hysteresis and Return Point Memory in Artificial Spin Ice Systems Cynthia Reichhardt, Andras Libal, Charles Reichhardt We investigate hysteresis loops and return point memory for artificial square and kagome spin ice systems by cycling an applied bias force and comparing microscopic effective spin configurations throughout the hysteresis cycle. Return point memory loss is caused by motion of individual defects in kagome ice or of grain boundaries in square ice. In successive cycles, return point memory is recovered rapidly in kagome ice. Memory is recovered more gradually in square ice due to the extended nature of the grain boundaries. Increasing the amount of quenched disorder increases the defect density but also enhances the return point memory since the defects become trapped more easily. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V8.00015: Collective Magnetic Behavior of Geometrically Frustrated Arrays with Perpendicular Anisotropy Y. Pan, K.K. Kohli, R. Fraleigh, A.L. Balk, D. Finkel, S. Zhang, J. Li, I. Gilbert, P.E. Lammert, R. Misra, V.H. Crespi, P. Schiffer, N. Samarth, M. Erickson, C. Leighton We use the magneto-optical Kerr effect (MOKE) to study the global and local magnetic behavior of geometrically frustrated arrays of single domain ferromagnetic islands with perpendicular anisotropy. MOKE measurements over macroscopic length scales probe the global properties of arrays with different lattice geometries and island spacings. The variation of switching field as a function of island spacing gives us insight into the influence of local frustration on the collective magnetic response of the arrays. The experimental results are compared with mean field calculations. Finally, we use spatially resolved Kerr microscopy to probe nucleation and domain propagation in the magnetization reversal process. Supported by U.S. Department of Energy Award DE-SC0005313. Lithography performed with the support of the National Nanotechnology Infrastructure Network [Preview Abstract] |
Session V9: Focus Session: Magnetic Oxide Thin Films And Heterostructures - Transport Properties
Sponsoring Units: GMAG DMPChair: Alex de Lozanne, University of Texas at Austin
Room: 209
Thursday, March 1, 2012 8:00AM - 8:12AM |
V9.00001: Scaling of the anomalous Hall effect in SrRuO$_{3}$ Noam Haham, Yishai Shperber, Moty Schultz, Netanel Naftalis, Efrat Shimshoni, James Reiner, Lior Klein Being one of the most intriguing manifestations of a transport phenomenon that is sensitive to spin and topology, the anomalous Hall effect (AHE) is at the focus of considerable theoretical and experimental efforts. SrRuO$_{3}$ has played a pivotal role in the study of the AHE and numerous attempts have been made to elucidate its complicated behavior. By using SrRuO$_{3}$ films with a wide range of thicknesses that vary considerably in the temperature-dependence of their resistivity, we show that the AHE scales with resistivity. The scaling provides a compelling piece of evidence that resistivity, \emph{irrespective of its sources or nature (elastic or inelastic)}, determines the AHE of SrRuO$_{3}$ in the entire ferromagnetic phase. This observation strongly suggests that changes in Berry phase due to assumed temperature-dependent exchange splitting cannot explain the complicated temperature dependence of the AHE. On the other hand side jumps mechanism combined with Karplus-Luttinger (Berry phase) mechanism that takes into account effects of finite scattering time may explain the observed behavior. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V9.00002: Optical spectroscopy of magnetic exchange splitting above the Curie temperature in ferromagnetic SrRuO$_{3}$ thin film D.W. Jeong, C.H. Kim, C.H. Sohn, S.H. Chang, H.J. Park, T.D. Kang, Y.S. Lee, J.J. Yu, T.W. Noh SrRuO$_{3}$ is a representative itinerant ferromagnetic material with the Curie temperature about 150K. To investigate the relation between magnetism and electronic structure, we measured temperature dependent optical spectra of SrRuO$_{3}$ thin film on SrTiO$_{3}$ substrate. With decreasing temperature, spectral weight transfer between 2.5eV and 3.2eV transition was observed. Comparing to the first-principles calculation, we attributed the weight transfer as a magnetic exchange splitting due to the Stoner transition. Interestingly, the exchange splitting remained above the Curie temperature, and destroyed near the room temperature. This observation indicates the long range magnetic ordering of SrRuO$_{3}$ is destroyed at the Curie temperature due to the transverse spin fluctuation not a Stoner transition. Moreover, using the optical sum rule of optical spectra, we systematically and quantitatively studied the ferromagnetic property of SrRuO$_{3}$ thin film. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V9.00003: Electronic and magnetic properties of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ epitaxial thin films Ludi Miao, Wenyong Zhang, Punam Silwal, Xiaolan Zhou, Ilan Stern, Jin Peng, Leonard Spinu, Zhiqiang Mao, Daeho Kim Strongly correlated Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ (CSRO) has attracted much attention for its rich physical properties such as Mott metal-insulator (MI) transition, antiferromagnetism (AFM), and spin-triplet superconductivity. We have grown epitaxial CSRO thin films on LaAlO$_{3}$ (001) substrates using a pulsed laser deposition method and investigated their electronic and magnetic properties. Ca$_{2}$RuO$_{4}$ thin films show strong compressive strain leading to an itinerant ferromagnetic (FM) phase coexisting with insulating AFM phase in the ground state and a suppressed broad and gradual MI transition. This is in sharp contrast to bulk Ca$_{2}$RuO$_{4}$, which exhibits an AFM Mott-insulating ground state and sharp MI transition. While the $x$=0.1 and 0.5 CSRO films also exhibit coherent strain, the MI transition and itinerant ferromagnetism are partially suppressed in the $x$=0.1 film and fully suppressed in the $x$=0.5 film. In contrast, Sr$_{2}$RuO$_{4}$ thin films are not susceptible to strain on any perovskite substrates including LaAlO$_{3}$; superconductivity in these films is suppressed due to disorders resulting from strain relaxation. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V9.00004: Optical properties of ferrimagnetic NiFe$_{2}$O$_{4}$ thin films Dipanjan Mazumdar, Qi Sun, Ken O Neal, Brian Holinsworth, Hunter Sims, Jianxing Ma, Arunava Gupta, William Butler, Janice Musfeldt Magnetic insulators like NiFe$_{2}$O$_{4 }$are attracting attention due to the high Curie temperature (850 K), which is rare among oxides . We recently demonstrated the growth of high-quality NiFe$_{2}$O$_{4}$ thin films practically down to room temperature, which permits investigation of the optical properties over wide growth temperatures. Our spectroscopic work reveals that NiFe$_{2}$O$_{4}$ displays both direct and indirect band gaps. A plot of ($\alpha $E)$^{2}$ vs. energy places the 300 K direct gaps for the highest quality films at 2.77 and 2.36 eV for the majority and minority channels, respectively whereas a plot of ($\alpha $E)$^{0.5}$ vs. energy places the indirect band gap at 1.64 eV. For the indirect case, we extract a coupling phonon energy of $\sim $50 meV (400 cm$^{-1})$, which corresponds an infrared active O-Fe-O bending mode. The difference between the direct and indirect gap energies reveals an opportunity to obtain spin-polarized carriers via optical excitation. These features have strong overlap with the solar spectrum. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V9.00005: Effects of Strain in Highly Ordered Sr$_{2}$CrReO$_{6}$ Epitaxial Films A.J. Hauser, J.R. Soliz, M. Dixit, R.E.A. Williams, M.A. Susner, B. Peters, L.M. Mier, T.L. Gustafson, M.D. Sumption, H.L. Fraser, P.M. Woodward, F.Y. Yang Sr$_{2}$CrReO$_{6}$, a double-perovskite ferrimagnet, has attracted much attention because of its Curie temperature well above room temperature and predictions of half-metallicity. We have deposited pure-phase Sr$_{2}$CrReO$_{6}$ epitaxial films on several (001)-oriented substrates and buffer layers by ultrahigh vacuum off-axis magnetron sputtering, in order to study tensile and compressive strain effects due to lattice mismatches up to 1.5{\%}. Triple-axis x-ray diffractometry was used in tandem with direct observations via HAADF STEM to confirm film epitaxy, phase purity, Cr/Re ordering, and strain via film lattice constants. Magnetic characterization shows a marked effect on the saturation magnetization due to strain, with slight changes in the Curie temperatures. Finally, electrical and optical characterization suggest that Sr$_{2}$CrReO$_{6 }$is a gapped material under both unstrained and strained systems, and will be discussed in detail as well. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V9.00006: Probing of polarization reversal and charge conduction in epitaxial (Ga,Fe)$_{2}$O$_{3}$ thin films on conducting oxide SrRuO$_{3}$ R.H. Shin, S.H. Oh, J.H. Lee, W. Jo, C. Lefevre, A. Tomasson, F. Roulland, C. Meny, N. Viart Ga$_{2-x}$Fe$_{x}$O$_{3}$ (GFO) thin films are the promising room-temperature multiferroics since their magnetic T$_{C}$ has been reported up to 370 K at x=1.4. However, most polarization hysteresis loops of the GFO thin films have been showed lossy behaviors due to the large leakage current. The origin probably lies on charge movement between Fe$^{3+}$ and Fe$^{2+}$ sites which is generated by oxygen vacancy. We report the large reduced leakage current of the GFO thin films by chemical doping to reduce Fe$^{2+}$. The doped GFO thin films were deposited by pulsed laser deposition at 750$^{\circ}$C for 15 min in oxygen partial pressure of 200 mTorr on SrRuO$_{3}$/SrTiO$_{3}$ substrates with various doping concentration. Epitaxy of b-axis orientation in out-of plane was confirmed by x-ray diffraction. The leakage current was reduced up to 5$\sim $6 order of magnitude depending on doping concentration. In order to investigate their conduction mechanism, temperature dependent macroscopic I-V curves were measured. Ferroelectric polarization and switching of the films were acquired over a wide range of temperature as well. Scanning probe microscopy has been used to measure local leakage currents as well as polarization reversal as a mode of conductive atomic force microscopy and piezoelectric microscopy, respectively. Local investigation of their electrical properties alludes to ferroelectricity in GFO. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V9.00007: Manipulating magnetic phase competition in manganites T. Zac Ward, Zheng Gai, Xiaoshan Xu, Hangwen Guo, Wenbin Wang We will discuss recent investigations attempting to isolate single order parameters in the spin-charge-orbital-lattice hierarchy in order to further understand how these complex interactions govern macroscopic electronic and magnetic properties. Specifically, we will present recent findings on strongly correlated thin films of [LaCa]MnO3, [LaSr]MnO3, and [LaPrCa]MnO3. We have developed new experimental methods that allow for both spin engineering and strain engineering at an interface to be tested on these materials; this has allowed us a glimpse at the interplay driving emergent phenomena. We have found that it is possible to exert a measure of control over the electronic phase competition leading to colossal magnetoreistance and the metal-insulator transition in manganites. This work has also been coupled with novel confinement techniques that allow us to observe single domain transitions using basic resistivity measurements which has led to several new discoveries on the formation and dynamics of electronic domains. These studies offer new means to quantitatively investigate the balanced energetics that drive complex materials and promise an ability to tune critical temperatures and desired electronic/magnetic properties. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V9.00008: Role of Interface Transport in the Magnetoresistance of Strained Rare Earth Manganite Thin Films Parul Srivastava, Tyler Goehringer, Cacie Hart, Gilles Dongmo-Momo, Ekembu Kevin Tanyi, Grace Yong, Vera Smolyaninova, David Schaefer, Rajeswari Kolagani We are studying the temperature and field dependence of magnetoresistance in epitaxial thin films of rare earth manganites which are subject to lattice mismatch strain, with the goal of investigating the possible role of transport across strain-induced internal interfaces. We will present our results, comparing La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) thin films grown on by Pulsed laser deposition on different substrates with varying degrees of compressive or tensile lattice mismatch. Strained films of thickness $\sim $ 10 nm, show a large magnetoresistance in fields $<$ 1 Tesla, which continues to increase with decrease in temperature, similar to magnetoresistance associated with grain boundary transport. The temperature dependence of MR in these films does not correlate with the temperature dependence of resistivity and thus seems to originate from effects other than Mn spin alignment. Lattice mismatch strain is known to suppress the insulator-metal transition and enhance charge and orbital ordering, leading to the coexistence of insulating and ferromagnetic metallic phases. Our results suggest possible contributions to the magnetoresistance from transport across the interfaces between the different phases. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V9.00009: Correlation between the transport properties, atomic structure and oxygen vacancies of perovskites thin films Juan Rubio-Zuazo, Pilar Ferrer, Igor Madariaga, Jonander Gallastegui, German R. Castro We present a study of the role of the oxygen vacancies on the atomic structure and the transport properties of a 20 nm thick La0.7Ca0.3MnO3 thin film grown by the pulsed laser deposition method on a SrTiO3 (001) substrate. The results show that the manganite lattice can accommodate up to 13{\%} of oxygen vacancies maintaining high crystalline order. Under such conditions the atomic structure is characterized by the movement of the La/Ca cations to the perovskite regular position, by the absence of the MnO6 basal plane rotation, and by a cooperative tilting of the octahedra along the out-of-plane direction. The metal-to-insulator transition temperature decreases from 265K for the fully oxygenated sample to 28 K for the sample with 13{\%} of oxygen deficiency.. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V9.00010: The metal-insulator transition in a phase-separated manganite studied by in situ STS P.C. Snijders, M. Gao, H. Guo, T.Z. Ward, H.-J. Gao, J. Shen, Z. Gai Electronic phase separation (EPS) is a key feature at the heart of the wide variety of electronic and magnetic properties in complex oxides. One consequence of EPS is that electronic transport experiments in bulk materials or 2D films mostly probe the low resistivity electronic phases due to the percolative path of the current. We study oxygen deficient La$_{5/8-x}$Pr$_{x}$Ca$_{3/8}$M nO$_{3}$ (LPCMO) thin films using both \emph{in situ} scanning tunneling spectroscopy (STS) and \emph{ex situ} transport experiments. The oxygen deficiency is known to decrease the metal-insulator transition (MIT) temperature or even completely suppress the MIT in conventional transport experiments. We show that \emph{in situ} STS is able to detect the MIT even in systems where conventional transport experiments do not show an MIT at zero magnetic field. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V9.00011: Electroresistance and Joule heating effects in manganite thin films Benjamin Martinez, Luis Pena, Regina Galceran, Zorica Konstantinovic, Alberto Pomar, Bernat Bozzo, Lluis Balcells, Felip Sandiumenge Electroresistance (ER), i.e. electric field- and/or current-induced resistance switching, has attracted much attention recently because of the possibility of using it for the implementation of resistance random access memories (ReRAM). Although ER is a quite common phenomenon in transition metal oxides, that has been extensively studied both theoretically and experimentally, the precise mechanism involved is not clear yet. In this work we report on the ER measurements in patterned La$_{2/3}$Sr$_{2/3}$MnO$_{3}$ (LSMO) thin films prepared by sputtering. In order to analyze Joule heating effects we have evaporated a Pt layer on top of the LSMO path to have access to the actual temperature of the sample while measuring resistance of LSMO path or I(V) characteristic curves. I(V) curves have been measured at different temperatures and the corresponding resistance values are compared with that of the R(T) curve taking into account the actual temperature of the sample in order to clarify the role of Joule heating in the observed change of the resistance. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V9.00012: Anisotropic magnetoresistance in thin films of the Mott metal CaVO$_{3 }$ Jiwei Lu, Man Gu Bulk CaVO$_{3}$ (CVO) is a Pauli paramagnetic metal with a singe 3d electron. Some unusual drastic changes in the magneto-resistance, magnetic susceptibility and the Hall effect have been reported in single crystal CVO. We have simultaneously synthesized epitaxial CVO films grown on three differently oriented SrTiO$_{3}$ substrates. Colossal magneto-resistance (MR) as well as large crystalline anisotropic was observed at low temperatures. The maximum MR, defined as (R(7 T)-R(0 T))/R(0 T)*100 {\%}, was over 1,0000 {\%} at 2 K and 35 Tesla (parallel magnetic field) on the CVO films deposited on a (110) SrTiO3 single crystal substrate, and didn't show any sign of saturation. When the magnetic field was perpendicular, MR was dropped to 6,000{\%}. The MR ratio was much larger than that of single crystal CVO. We have also investigated the magneto-transport behaviors of CVO films deposited on (111) and (100) STO and will discussed the dependence of MR in CVO on the crystal orientation as well as the orientation of external magnetic field. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V9.00013: Magnetoelectric effects in Fe3O4 thin films Jared Wong, Adrian Swartz, Renjing Zheng, Roland Kawakami Recently, there has been great interest in voltage-induced manipulation of magnetic properties inside magnetic materials. One attractive avenue is using magnetic oxides, which could be susceptible to manipulation of magnetic properties by electric fields at the interface and/or directly altering the electronic behavior within strongly correlated systems, for example. Magnetite (Fe3O4) is quite interesting because it exhibits many interesting properties such as ferrimagnetism, ferroelectricity (at low temperatures) and a metal-insulator phase transition near 120 K known as the Verwey transition. Single crystal Fe3O4 samples are deposited through reactive molecular beam epitaxy (MBE) on MgO (001) substrates and the structural quality is confirmed through XRD, RHEED, and LEED. Magnetic properties are examined through magneto-optic Kerr effect (MOKE) measurements and 4-probe I-V / Van Der Pauw measurements were used to determine the electronic properties. Samples are incorporated into an electrically gated structure by the addition of a dielectric layer and metallic top electrode and we report our results and observations of voltage-induced manipulation of the magnetic properties inside thin films of Fe3O4. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V9.00014: Dynamics of multiple phases in manganite as revealed by dielectric spectroscopy Zhigao Sheng, Masao Nakamura, Masashi Kawasaki, Yoshinori Tokura Phase separation is a very important feature in correlated electron oxides. The coexistence and competition of multiple phases give rise to gigantic response to tiny stimuli, producing dramatic changes in magnetic, transport, and other properties in these compounds [1]. It is crucial to probe the physical properties of each phase separately for a comprehensive understanding of correlated electron oxide materials and their phase separation, but it is difficult due to their nano-scale distribution. Here we report dynamic properties of multiple phases in manganite thin films by using dielectric spectroscopy with a unique $p-N$ junction configuration. The multiple dielectric relaxations have been detected and we distinguished their corresponding multiple phases. The activation energy and dielectric properties of different phases have been deduced separately. We also elucidated their phase evolution with changing of temperature or applied magnetic field from viewpoint of dielectric response. These results provide a guideline to explore the electronic phase separation phenomena in correlated electron oxides. \\[4pt] [1] Y. Tokura et al. Rep. Prog. Phys. 69, 797 (2006). [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V9.00015: Interface induced giant magnetoelectric coupling in multiferroelectric superlattices Hongwei Wang, Lixin He, Xifan Wu AMnO$_{3}$ (A=Ca, Sr, ...) are good candidates as building blocks for the multiferroic supperlattices, because they have several competing instabilities coupled to the magnetic ordering. The coupling between the spin, the AFD modes and the FE modes depends on the relative energetics of these instabilities. Unfortunately in bulk AMnO3, there is a strong AFD instability associated with a large oxygen octahedral rotation that suppresses the FE mode. As a result, the linear magnetoelectric (spin-ferroelectricity) coupling is usually found to be weak. We take the CaMnO$_{3}$(CMO)/BaTiO$_{3}$(BTO) SLs as our model systems. We find that the MnO$_{6}$ octahedral rotation will be strongly suppressed by the neighboring BaO layers, leading to the enormous enhancement of magnetoelectric coupling and the local electric polarizations in the CMO layers are significant, comparable to that in the BTO layers. This enhancement will be strengthened with the increasing density of interfaces and reaches its maximum at the shortest SL, (i.e., n=1) where one observes a huge change of electric polarization between the antiferromagnetic (AFM) and ferromagnetic (FM) states. [Preview Abstract] |
Session V10: Invited Session: Quantum Entanglement in Many-Body Systems
Sponsoring Units: GQI DAMOPChair: John Preskill, California Institute of Technology
Room: 210A
Thursday, March 1, 2012 8:00AM - 8:36AM |
V10.00001: Entanglement, teleportation and memory in atomic spin ensembles Invited Speaker: Eugene Polzik Recent experimental progress with entanglement generation and processing in macroscopic atomic spin ensembles will be reviewed. It includes atomic entanglement maintained for an unlimited time via engineered collective dissipation mediated by light and teleportation of collective atomic spin states. A proposal for quantum memory assisted detection of strongly coupled systems will be presented. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 9:12AM |
V10.00002: Entanglement and real-space renormalization group methods for quantum field theories Invited Speaker: Frank Verstraete We will demonstrate how the reformulation of the density matrix renormalization group as a variational method within the class of matrix product states has lead to a wide class of novel applications and insights into strongly correlated quantum systems in 1 dimension. The discussion will detail the crucial role of entanglement and area laws, and then focus on the generalization of matrix product state methods to quantum field theories and the prospects of simulating experiments with cold gasses. Joint work with I. Cirac, J. Haegeman, T. Osborne. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V10.00003: Entangled States of Trapped Ions Invited Speaker: Dietrich Leibfried Entangled states of the internal degrees of freedom are an important resource in Quantum Information Processing (QIP) and Quantum Simulation (QS) with trapped ions. Most basic requirements for QIP and QS have been demonstrated for trapped ions, with two big challenges remaining: Improving operation fidelity and scaling up to larger numbers of qubits. In the last few years, steady progress has been achieved with laser-based entanglement schemes with demonstrated fidelities of deterministically produced Bell states of 99.3{\%} and up to 14 ion-qubits entangled in generalized GHZ-states. Scalable architectures have been proposed; one scheme, where ion-qubits are moved through a multi-zone trap array, is studied in several laboratories. Sympathetic cooling with a second ion species, which initializes the motional states for multi-qubit operations, has been demonstrated in an experiment where arbitrary operations on two qubits were implemented. Micro-fabrication approaches to ion-trap-arrays have yielded structures that should be capable of holding and manipulating large numbers of ions. Recently, with the use of microwaves, single-qubit rotations with fidelities of 99.998{\%} per gate operation were demonstrated and two ion-qubit gates have been implemented. Microwave control could potentially be easier to scale by directly integrating microwave-lines on micro-fabricated trap devices. It also eliminates several sources of decoherence that are present in laser-based schemes by exclusively coupling to long lived hyperfine ground states. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:24AM |
V10.00004: Topological order and long range quantum entanglements Invited Speaker: Xiao-Gang Wen What is the origin of fractional charges and fractional statistics in FQH states? What is the origin of light? It turns out that long range entanglement is the reason why fractional charges and fractional statistics can appear FQH state. Long range entanglement is also the reason why waves that satisfy Maxwell equation can appear in some qubit (spin) systems. Long range entanglement also lead to a deeper understanding of gapped quantum phases. It allows us to obtain a classification of interacting topological insulators/superconductors, as well as the much more general symmetry protected topological phases, and intrinsic topological phases. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 11:00AM |
V10.00005: Entangled states as resources in quantum complexity theory Invited Speaker: Scott Aaronson |
Session V11: Focus Session: Graphene Structure, Stacking, Interactions: Edges and Grain Boundaries
Sponsoring Units: DMPChair: Aaron Bostwick, Lawrence Berkeley National Laboratory
Room: 210B
Thursday, March 1, 2012 8:00AM - 8:12AM |
V11.00001: Theory and hierarchical calculations of [0001] tilt grain boundaries in graphene Johan Carlsson, Luca M. Ghiringhelli, A. Fasolino Several experiments have revealed the presence of grain boundaries in graphene that may change its electronic and elastic properties. Here, we present a general theory for the structure of [0001] tilt grain boundaries in graphene based on the coincidence site lattice (CSL) theory. We show that the CSL theory uniquely classifies the grain boundaries in terms of the misorientation angle $\theta$ and periodicity $d$. The structure and formation energy of a large set of grain boundaries generated by the CSL theory for 0$^\circ < \theta < 60^\circ$ (up to 15 608 atoms) were optimized by a hierarchical methodology and validated by density functional calculations. We find that low-energy grain boundaries in graphene can be identified as dislocation arrays. In contrast to three-dimensional materials, the strain created by the grain boundary can be released via out-of-plane distortions that imply to an effective attractive interaction between dislocation cores. This leads to a (secondary) minimum structure at $\theta = 32.2^\circ$, where the grain boundary is made of a flat zigzag array of only 5-- and 7--rings. We discuss the importance of these findings for the interpretation of recent experimental results. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V11.00002: Localized states at grain boundaries in graphite Adina Luican-Mayer, Guohong Li, Gabriel Autes, Oleg Yazyev, Eva Y. Andrei Scanning Tunneling Microscopy and Spectroscopy at low temperature and in magnetic field was used to characterize the electronic states near grain boundaries on the surface of graphite. Topographic surface maps show the grain boundaries as narrow stripes within which the lattice is reconstructed into a periodic pattern whose period is determined by the relative orientation between adjacent grains. In spectroscopy the grain boundaries produce sharp peaks in the density-of-states at energies that are characteristic of the misorientation between adjacent grains. Spatial maps of the density-of-states at these peak energies show that the peaks correspond to electronic states that are localized on the grain boundaries. We will present measurements of these localized electronic states, their evolution with magnetic field and misorientation angle between grains. The experimental results will be compared with theoretical calculations. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V11.00003: Altering graphene line defect properties using chemistry Smitha Vasudevan, Carter White, Daniel Gunlycke First-principles calculations are presented of a fundamental topological line defect in graphene that was observed and reported in Nature Nanotech. 5, 326 (2010). These calculations show that atoms and smaller molecules can bind covalently to the surface in the vicinity of the graphene line defect. It is also shown that the chemistry at the line defect has a strong effect on its electronic and magnetic properties, e.g. the ferromagnetically aligned moments along the line defect can be quenched by some adsorbates. The strong effect of the adsorbates on the line defect properties can be understood by examining how these adsorbates affect the boundary-localized states in the vicinity of the Fermi level. We also expect that the line defect chemistry will significantly affect the scattering properties of incident low-energy particles approaching it from graphene. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 9:12AM |
V11.00004: Imaging the Structure of Grains, Grain Boundaries, and Stacking Sequences in Single and Multi-Layer Graphene Invited Speaker: David Muller Graphene can be produced by chemical vapor deposition (CVD) on copper substrates on up to meter scales [1, 2], making their polycrystallinity [3,4] almost unavoidable. By combining aberration-corrected scanning transmission electron microscopy and dark-field transmission electron microscopy, we image graphene grains and grain boundaries across six orders of magnitude. Atomic-resolution images of graphene grain boundaries reveal that different grains can stitch together via pentagon-heptagon pairs. We use diffraction-filtered electron imaging to map the shape and orientation of several hundred grains and boundaries over fields of view of a hundred microns. Single, double and multilayer graphene can be differentiated, and the stacking sequence and relative abundance of sequences can be directly imaged. These images reveal an intricate patchwork of grains with structural details depending strongly on growth conditions. The imaging techniques enabled studies of the structure, properties, and control of graphene grains and grain boundaries [5]. \\[4pt] [1] X. Li\textit{ et al.}, \textit{Science} \textbf{324}, 1312 (2009).\\[0pt] [2] S. Bae\textit{ et al.}, \textit{Nature Nanotechnol.} \textbf{5}, 574 (2010).\\[0pt] [3] J. M. Wofford,\textit{ et al.}, \textit{Nano Lett.}, (2010).\\[0pt] [4] P. Y. Huang, et al., \textit{Nature} \textbf{469}, 389--392 (2011); \textit{arXiv:1009.4714}, (2010)\\[0pt] [5] In collaboration with Pinshane Y. Huang, C. S. Ruiz-Vargas, A. M. van der Zande, A. W. Tsen, L. Brown, R. Hovden, F. Ghahari, W. S. Whitney, M.P. Levendorf, J. W. Kevek, S. Garg, J. S. Alden, C. J. Hustedt, Y. Zhu, N. Petrone, J. Hone, J. Park, P. L. McEuen [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V11.00005: Experimentally Controlling the Edge Termination of Graphene Nanoribbons Xiaowei Zhang, Oleg Yazyev, Juanjuan Feng, Chenggang Tao, Yen-Chia Chen, Liming Xie, Steven Louie, Alex Zettl, Hongjie Dai, Michael Crommie The edges of graphene exhibit several unique features, such as the presence of localized edge states, and are anticipated to be a powerful means of controlling the electronic properties of this two-dimensional material. Understanding such properties, however, requires a precise knowledge of the atomic-scale structure and chemical composition of the edge. In this work, the edges of graphene nanoribbons (GNRs) are controlled by hydrogen plasma and are investigated through a combination of high-resolution scanning tunneling microscopy (STM) and first-principles calculations. We recover the atomic structure of the edge termination in atom-by-atom fashion and establish the chemical nature of terminating functional groups at graphene edge segments of different orientation -- (i.e., zigzag, armchair and chiral). These results allow us to conclude that the edges of hydrogen-plasma-etched GNRs are generally free of structural reconstructions and are terminated by hydrogen atoms with no rehybridization of the outermost carbon edge atoms. Both zigzag and chiral edges show the presence of edge states. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V11.00006: Tunable magnetism at graphene edges Manuel Schmidt Electron-electron interactions drive clean graphene zigzag edges to a ferromagnetic state, known as edge magnetism. In this state, the spin of specific one-dimensional electronic modes, that are localized at the edge (the so-called edge state), is fully polarized. I will discuss a mechanism by which the edge magnetism can be manipulated via electric fields. One possible realization of this mechanism is based on graphene/graphane interfaces. As the field strength is varied, the graphene edge undergoes two phase transitions: (1) from fully polarized edge magnetism to a ferromagnetic Luttinger liquid (fLL) and (2) from the fLL to an ordinary Luttinger liquid. The intermediate phase (fLL) is a realization of the unusual itinerant one-dimensional ferromagnet and is therefore in seeming contradiction with the Lieb-Mattis theorem, which forbids such one-dimensional magnetism. The resolution of this seeming contradiction is shortly discussed. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V11.00007: Dynamics of Graphene Edges Interaction under Joule-heating Xu Zhang, Xiaoting Jia, Eduardo Cruz-Silva, Lee-Ping Wang, Jessica Campos-Delgado, Paulo Antonio Araujo, Vincent Meunier, Mauricio Terrones, Tomas Palacios, Mildred Dresselhaus The edge physics of graphene plays an essential role in the electronic properties of nanometer scale graphene. Studying the Joule-heating of a graphene sample supported by an integrated TEM-STM instrument is an effective way to sharpen graphene edges, and therefore produce smooth graphene nanoribbons, which will be studied in this work. Through observation of the movement of graphene platelets heat-treated within a crystalline domain of graphene substrate underneath, we advance our understanding about the mechanism of edge reconstruction, edge-edge interaction, in addition to the graphene substrate interaction for single layer graphene. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V11.00008: Structure, Stability and Electronic Properties of Graphene Edges on Co(0001) Deborah Prezzi, Daejin Eom, Kwang T. Rim, Hui Zhou, Michael Lefenfeld, Colin Nuckolls, Tony F. Heinz, George W. Flynn, Mark S. Hybertsen We recently demonstrated the growth of epitaxial graphene flakes on Co(0001) surfaces [1]. Here we combine low-temperature scanning tunneling microscopy (STM) measurements and DFT calculations to study graphene edge stability and electronic properties, as resulting from the coupling with the substrate. Graphene edges display straight well-ordered structure with zigzag orientation. DFT calculations provide insights into their stability by comparing several edge morphologies with both armchair and zigzag orientation. Simulated images indicate that different edge structures can be clearly distinguished in topography at low bias. The calculated electronic properties for the low energy edge structures are consistent with the measured STS tunneling spectra, which show a prominent edge-localized peak at low bias. [1] D. Eom et al., Nano Lett. 9, 2844 (2009). [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V11.00009: Chemical versus Thermal Folding of Graphene Edges Ninghai Su, Miao Liu, Feng Liu Using molecular dynamics (MD) simulations, we have investigated the kinetics of the graphene edge folding process. The lower limit of the energy barrier is found to be $\sim $380 meV/{\AA} (or about 800 meV per edge atom) and $\sim $50 meV/{\AA} (or about 120 meV per edge atom) for folding the edges of intrinsic clean single-layer graphene (SLG) and double-layer graphene (DLG), respectively. However, the edge folding barriers can be substantially reduced by imbalanced chemical adsorption, such as of H atoms, on the two sides of graphene along the edges. Our studies indicate that thermal folding is not feasible at room temperature (RT) for clean SLG and DLG edges and is feasible at high temperature only for DLG edges, whereas chemical folding (with adsorbates) of both SLG and DLG edges can be spontaneous at RT. These findings suggest that the folded edge structures of suspended graphene observed in some experiments are possibly due to the presence of adsorbates at the edges. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V11.00010: Edge effects in Zigzag Graphene Nanoribbons Wen Ying Ruan, Yiyang Sun, Sheng Bai Zhang, Mei-Yin Chou Analytical and numerical results based on the tight binding model are presented for zigzag graphene nanoribbons with z1 and z12$_1$2 edges. We show the crucial importance of the symmetry of the two edges in determining the electronic structures of the system. Examples of significant band gap narrowings due to symmetry breaking are illustrated. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V11.00011: Mapping edge-state wavefunctions in chiral graphene nanoribbons Yen-Chia Chen, Dimas G. de Oteyza, Xiaowei Zhang, David Strubbe, Liying Jiao, Alex Zettl, Hongjie Dai, Steven G. Louie, Michael F. Crommie The electronic behavior of graphene is readily tunable through nanoscale patterning. A particularly important structural motif is the nanoribbon (GNR), a narrow strip of graphene defined by its width, length, and edge properties. GNRs, due to quantum confinement and edge effects, have been predicted to exhibit many novel behaviors, such as tunable energy gaps and the presence of magnetic edge states. Here we report measurement of the local electronic structure of GNRs with highly ordered edges obtained by unzipping carbon nanotubes. Due to variation in the precursor nanotubes, this synthesis method generally produces single- or multi-layered nanoribbons with varying widths, lengths and chiralities. We have combined scanning tunneling microscopy (STM) and spectroscopy (STS) to simultaneously characterize the structural and electronic properties of these GNRs at the atomic scale. In particular, we observe 1D edge states that exhibit an energy gap that is dependent on nanoribbon width and the chirality. We have further spatially mapped patterns in the electronic local density of states associated with different GNR spectroscopic resonances. These patterns are compared with theoretical simulations. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V11.00012: Effects of edge-potential on armchair graphene open boundary and nanoribbon Chi-Hsuan Chiu, Chon-Saar Chu The physics for the edge state formation and gap opening at an armchair graphene open boundary and nanoribbon due to an edge potential are investigated. At an open boundary, the edge-potential $U_0 $ is shown to turn on pseudospin-fipped (intravalley) scattering even though $U_0 $ does not post an apparent breaking of the AB site (basis atoms) symmetry. The interference between the pseudospin conserving (intervalley) and nonconserving (intravalley) processes in the scattering state leads to a finite out-of-plane pseudospin density. Similar two-waves feature in the evanescent regime leads to the formation of the edge state. This physical origin of the edge state is different from that for the Tamm states in semiconductors. For an armchair graphene nanoribbon with gapless energy spectrum, applying $U_0 $ to both edges opens up an energy gap. In addition, dispersive edge state can be found inside the energy gap for the bulk-like states. The$U_0 $-induced out-of-plane pseudospin density vanishes for the armchair graphene nanoribbon, but we expect it to be finite, for more general cases, at an armchair graphene open boundary. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V11.00013: Formation of unconventional standing waves at graphene edges Changwon Park, Heejun Yang, Jisoon Ihm, Gunn Kim The electron scattering properties of graphene edge have been investigated by the interference images using the scanning tunneling microscopy (STM). A conventional metal with a terrace and a step can be modeled as a two-dimensional electron gas with a hard wall and this behavior was directly observed at the steps of Au(111) and Cu(111) surfaces by STM. Now, a question arises as to how two sublattices and two inequivalent valleys in graphene affect the scattering and the standing wave formation. We present how the contributions from two valleys vary in the scattering at different graphene edges. For the zigzag edge, only intravalley scattering is possible due to the different edge-direction crystal momentum of two valleys. For the armchair edge, in contrast, the wave is reflected mostly via intervalley scattering and as a result, an atomic-scale nodelike pattern and beats in the standing wave are generated near the edge. When the incident angle is small, this intervalley scattering process is quite robust in the presence of defects so that we can still observe nodal patterns even for edges of relatively high defect densities. [Preview Abstract] |
Session V12: Graphene: Optical Properties and Responses
Sponsoring Units: DMP DCMPChair: Larry Carr, Brookhaven National Laboratory
Room: 210C
Thursday, March 1, 2012 8:00AM - 8:12AM |
V12.00001: Temperature dependence of hot carrier-assisted photoresponse in graphene Qiong Ma, Nathaniel Gabor, Nityan Nair, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero We report on temperature dependent photocurrent measurements of high-quality dual-gated monolayer graphene p-n junction devices. Over temperatures ranging from 5 K to 300 K, we find that the photocurrent at the p-n interface peaks at an intermediate temperature, and decreases at higher and lower temperatures. Spatial photocurrent microscopy (at wavelength 850 nanometers) shows that the photocurrent measured as a function of distance away from the p-n interface also varies with temperature. We consider various electronic cooling processes in graphene to explain the photocurrent temperature dependence. Our measurements may reveal novel energy loss processes that reduce the electronic temperature of photoexcited charge carriers in graphene, and give additional insight into hot carrier photoresponse. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V12.00002: Hot-Carrier cooling at Graphene-Metal Contact Interface Jason Ross, Grant Aivazian, Dong Sun, Aaron Jones, Wang Yao, David Cobden, Xiaodong Xu There has been a recent surge of interest in using graphene as broadband and ultrafast optoelectronics, however the mechanisms of photodetection are not yet fully understood. Our previous measurements at a top gated graphene pn junction and at a monolayer-bilayer interface have found the dominating mechanism to be photothermoelectric (PTE) in nature, whereas most graphene-metal contact (GM) studies attribute photocurrent to the photovoltaic effect. By performing comprehensive ultrafast optical pump-probe measurements of photocurrent as a function of temperature, Fermi level, and laser power at various GM interfaces, the current work differentiates the contributions of PTE and photovoltaic effects to the photocurrent and identifies a hot-carrier relaxation time of $\sim $2 ps at room temperature and $\sim $7 ps at cryogenic temperatures. This work provides valuable insight to the design of new graphene based optoelectronic devices for sensing and communication. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V12.00003: Time Resolved Carrier Distributions in Graphene Steve Gilbertson, Georgi Dakovski, Tomasz Durakiewicz, Jian-Xin Zhu, Keshav Dani, Aditya Mohite, Andrew Dattelbaum, George Rodriguez Graphene, a recently discovered two-dimensional form of carbon, is a strong candidate for many future electronic devices. A question of central importance in optoelectronics, particularly high-speed applications, is how photoexcited carriers behave on ultrashort time scales. Even though time-resolved studies have provided a wealth of information, fundamental questions concerning the quantum descriptions of the transient electron-hole plasma remain. On one hand, conflicting views on the relaxation dynamics go as far as precluding the possibility of observing some predicted phenomena, such as THz lasing or tunable lasers while the observation of phenomena such as ultrafast photoluminescence and carrier multiplication are already established. Here, by employing the technique of time-resolved photoemission, we directly obtain the evolving Fermi-Dirac distributions of the electrons and holes: on an ultrashort 500 fs time scale the electron and hole populations can be described by two separate Fermi-Dirac distributions, while on longer time scales the populations coalesce to form a single Fermi-Dirac distribution at an elevated temperature. This unusual behavior is a consequence of graphene's unique band structure and has important implications for possible applications. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V12.00004: Gate-tunable nanoplasmonic effects in single- and bi-layer graphene Zhe Fei, Gregory Andreev, Wenzhong Bao, Aleksandr Rodin, Alexander McLeod, Lingfeng Zhang, Zeng Zhao, Gerardo Dominguez, Mark Thiemens, Michael Fogler, Antonio Castro-Neto, Chunning Lau, Fritz Keilmann, Dimitri Basov We employed near-field infrared (IR) nanoscopy and nanoimaging to study mid-IR nanoplasmonic effects of both single-layer graphene (SLG) and bilayer graphene (BLG) on SiO2/Si substrate. In our previous study, we found that SLG enhanced and blueshifted the surface phonon resonance of SiO2 due to plasmon-phonon coupling [Z. Fei et al. Nano. Lett. 2011]. Here we report that both these effects are also observed in BLG. Using back-gate we were able to systematically change the carrier density in both SLG and BLG while monitoring the evolution of the hybrid plasmon-phonon resonance. New data are in accord with our point-dipole modeling results. IR imaging with nanoscale resolution revealed fringe patterns extending along the edges of both SLG and BLG. We ascribe these patterns to the interference of plasmon waves launched by the near-field probe with those reflected from the edges. Detailed analysis allowed us to observe gate-induced changes in the plasmon dispersion of both SLG and BLG, which are consistent with the notion of massless Dirac fermions in SLG and massive carriers in BLG. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V12.00005: Atomically Localized Plasmon Enhancement in Monolayer Graphene Juan Carlos Idrobo, Wu Zhou, Jaekwang Lee, Jagjit Nanda, Sokrates T. Pantelides, Stephen J. Pennycook Graphene has attracted significant attention due to its exceptional properties and very promising applications, including optoelectronics and nanoplasmonics. All localized plasmon resonances observed so far in materials have been limited to the sub-10 nanometer scale, with a reported record of $\lambda/40$, where $\lambda$ is the wavelength of the related plasmon excitation. In this talk, using aberration-corrected scanning trasmission electron microscopy and total energy first-principles calculations, we show that single point defects can enhance the $\pi$ and $\pi + \sigma$ plasmons of monolayer graphene at the atomic level. Our study shows that point defects in monolayer graphene represent a length scale smaller than $\lambda$/200, and suggest that the physical limit for the size of plasmonic and optoelectronic devices can be down to the single atom level. This research was supported by NSF grant No. DMR-0938330 (WZ, J-CI), DOE grant DE- F002-09ER46554 (STP), by the Shared Research Equipment (SHaRE) User Facility, Oak Ridge National Laboratory, which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences (J-CI), and by the Office of Basic Energy Sciences, Materials Sciences and Engineering Division, U.S. Department of Energy (JL, JN, SJP). [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V12.00006: Effect of nonhomogenous dielectric background on the plasmon modes in graphene double-layer structures Samvel Badalyan, Predrag Krstajic, Francois Peeters We have calculated the plasmon modes in graphene double layer structures, taking into account the non-homogeneity of the system dielectric background. The effective dielectric function is obtained from the solution of the Poisson equation in three-layer dielectric medium with the graphene sheets located at the interfaces, separating different materials. Due to the momentum dispersion of the effective dielectric function, the intra- and inter-layer bare Coulomb interactions in graphene double layer systems acquire additional momentum dependence--an effect of the order of inter-layer interaction itself. It has been shown that in the long wavelength limit the energies of optical and acoustical plasmons, respectively, with the square root and linear dispersions are determined by different dielectric permittivities. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V12.00007: Measurement of plasmon dispersion in graphene: tunable graphene plasmonics Ben D. Dawson, Mike S. Lodge, Nima Nader Esfahani, R.E. Peale, M. Ishigami Graphene is an intriguing material for plasmonics. We have measured the transmission spectrum of large area graphene, grown using chemical vapor deposition, using Fourier transform infrared spectroscopy. By varying the Fermi level of graphene, we are able to explore the energy dispersion of plasmons in graphene. Our result lays the foundation for tunable plasmonic devices based on graphene. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V12.00008: Scattering theory for graphene plasmons near edges and interfaces Aleksandr Rodin, Michael Fogler Motivated by recent infrared nano-imaging experiments, we study eigenmodes of graphene plasmons near sample boundaries, corners, and interfaces. Such modes can be understood as standing-wave patters formed by multiple scattering of elementary waves. We derive the rules of the corresponding scattering theory by analyzing the integro-differential equation for the plasmon dynamics. Our analytical results include the solution for the edge reflection problem in uniform graphene and a quasiclassical formalism for graphene of slowly varying density. Numerical simulations are employed for more complicated boundary geometries (wedge, constriction, etc.) and for singular density distributions that exist near the edge of a gated graphene. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V12.00009: Edge Functionalization and Optical Excitations in Graphene Nanoflakes Caterina Cocchi, Deborah Prezzi, Alice Ruini, Marilia J. Caldas, Elisa Molinari We investigate the effects of edge covalent functionalization on the opto-electronic properties of finite elongated graphene nano-flakes (GNFs). Following our previous work on nanojunctions[1], we compute mean-field ground state electronic properties and configuration-interaction UV-vis optical excitations at varying size and functionalization by means of semi-empirical methods. The character of the lowest energy excitations and the influence exerted on them both by length/width modulation and by the specific chemical properties of the terminating groups are analyzed in details. The role of local distortions spontaneously arising upon geometrical optimization is inspected. Nanoplasmonic-like features related to the spectrum of these elongated finite graphene nanostructures are also discussed. [1] C. Cocchi et al. J. Phys. Chem. Lett. 2, 1315 (2011) [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V12.00010: Theory of double-resonant Raman spectra in graphene: intensity and line shape of defect-induced and two-phonon bands Pedro Venezuela, Michele Lazzeri, Francesco Mauri We calculate the double resonant Raman spectrum of graphene associated to both phonon-defect processes (such as the $D$ and $D'$ lines), and two-phonons ones (such as the $2D$, $2D'$ and $D+D''$ lines). For an excitation energy of $2.4$~eV, the agreement with measurements is very good and calculations reproduce the relative intensities among phonon-defect or among two-phonon lines and the measured small widths of the $D$, $D'$, $2D$ and $2D'$ lines. We determine how the spectra depend on the excitation energy, on the light polarization, on the electronic linewidth, on the kind of defects and on their concentration. The intensity ratio between the $D$ and $D'$ lines depends on the kind of model defect, suggesting that this ratio could be used to identify actual defects. The present analysis reveals that, for both $D$ and $2D$ lines, the dominant DR processes are those in which electrons and holes are both involved in the scattering. The most important phonons belong to the {\bf K}$\rightarrow{\mathbf{\Gamma}}$ direction ($inner$ phonons) and not to the {\bf K}$\rightarrow${\bf M} one ($outer$ phonons), as usually assumed. The small $2D$ line width at $\epsilon_L=2.4$~eV is a consequence of the interplay between the opposite trigonal warpings of the electron and phonon dispersions. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V12.00011: A TD-DFT study on the Optical and dielectric properties of graphene nanoflakes Carlos Ramos, Eduardo Cifuentes, Romeo de Coss Optical and dielectric properties of graphene nanostructures are of current interest because the potential applications in electronic and photonics devices. Recently, it has been reported important progress in the synthesis of graphene nanoflakes and their applications as quantum dots. In this work, the size and geometric shape effects on the optical spectrum and dielectric constant of graphene nanoflakes are studied by using an ab-initio approach. The calculations were performed using the Time-Dependent Density Functional Theory (TD-DFT) as implemented in the plane wave method and the results provide insights about controlling the optical properties of graphene using the size and shape of the nanostructure. This research was supported by Conacyt-M\'{e}xico under Grant No. 83604. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V12.00012: Two-photon absorption measurements in graphene fragments: Role of electron-electron interactions A. Sandhu, A. Roberts, K. Aryanpour, A. Shukla, S. Mazumdar Many-body interactions in graphene are an active field of research. There is a clear evidence of strong electron correlation effects in other carbon based materials which have the same sp$^2$ hybridization as graphene. For example, in linear-polyenes, the electron-electron interactions are considered responsible for the occurrence of lowest two-photon state below the optical one-photon state. The electronic correlation in these linear systems is a strong function of the chain length. Thus, it is pertinent to question if the two-dimensional graphene fragments also exhibit strong correlation effects and how these effects scale with fragment size. Using a white light super-continuum source, we perform z-scan measurements to extract frequency-dependent two-photon absorption coefficients in symmetric molecular fragments of graphene, e.g. coronene and hexabenzocoronene. A comparison of one-photon and two-photon absorption coefficients is then used to uncover the extent of correlation effects. In the smallest fragment, coronene, our results indicate a strong signature of the Coulomb interactions. We will discuss how the importance of electron-electron interaction varies with system size and its implication for the correlation effects in graphene. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V12.00013: Pump-probe study of electron dynamics in bilayer graphene Long Ju, Tsung-Ta Tang, Feng Wang Bilayer graphene exhibit many unusual physical properties, including a gate tunable electronic bandgap, and there is great interest in using it for novel electronic and optical devices. Understanding ultrafast dynamics in bilayer graphene is a prerequisite for many of its potential applications. We use ultrafsat pump-probe spectroscopy to investigate such ultrafast electron relaxation behavior in bilayer graphene. In this talk, I will discuss the observed dynamic relaxations taking place in femto- and pico-second time scales.. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V12.00014: Theory of Two-Photon Absorptions in Graphene Fragments K. Aryanpour, A. Shukla, S. Mazumdar, A. Sandhu, A. Roberts Electron-electron correlations in graphene is currently an active field of research [1-3]. The carbon atoms in graphene have the same sp$^2$ hybridization as in strongly correlated $\pi$-conjugated polymer systems. The low energy behavior in graphene however appears to be reasonably described within the one-electron Dirac massless fermions model. Historically, the occurrence of the lowest two-photon state {\it below} the optical one-photon state provided the strongest proof for strong electron correlations in linear polyenes [4]. We systematically study the Coulomb interaction effects on the ground state and nonlinear absorptions in graphene fragments as a function of system size, beginning from the smallest stable fragment coronene. We report high order calculations of one- vs two-photon spin singlet and triplet states, in coronene, hexabenzocoronene and other molecular fragments that clearly indicate the strong role of electron-electron interactions. We will discuss the implications of our work on molecular systems for the thermodynamic limit of graphene. \\[4pt] [1] Siegel David A.; et al., PNAS, v108, 28, 11365-11369 (2011)\\[0pt] [2] Gr\"onqvist J. H.; et al., arXiv: 1107.5653v1\\[0pt] [3] Uchoa B.; et al., arXiv: 1109.1577v1\\[0pt] [4] Ramasesha S.; et al., J. Chem. Phys. 80, 3278 (1984) [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V12.00015: Floquet Spectrum and Transport Through an Irradiated Graphene Ribbon H.A. Fertig, Zhenghao Gu, Daniel Arovas, Assa Auerbach Graphene subject to a spatially uniform, circularly-polarized electric field supports a Floquet spectrum with properties akin to those of a topological insulator, including non-vanishing Chern numbers associated with bulk bands and current-carrying edge states. Transport properties of this system however are complicated by the non-equilibrium occupations of the Floquet states. We address this by considering transport in a two-terminal ribbon geometry for which the leads have well-defined chemical potentials, with an irradiated central scattering region. We demonstrate the presence of edge states, which for infinite mass boundary conditions may be associated with only one of the two valleys. At low frequencies, the bulk DC conductivity near zero energy is shown to be dominated by a series of states with very narrow anticrossings, leading to super-diffusive behavior. For very long ribbons, a ballistic regime emerges in which edge state transport dominates. [Preview Abstract] |
Session V13: Focus Session: Magnetic Nanostructures-Domain Walls
Sponsoring Units: DMP GMAGChair: Laura Lewis, Northeastern University
Room: 211
Thursday, March 1, 2012 8:00AM - 8:36AM |
V13.00001: Domain-wall dynamics in ferromagnetic nanowires Invited Speaker: Oleg Tretiakov Current-induced domain-wall (DW) dynamics is studied in a thin ferromagnetic nanowire. We derive effective equations of motion describing the dynamics of the DW soft modes with or without topological defects. Because the DWs are topological objects with a rigid spin structure, these equations are rather universal. The DW rigidity makes the microscopic details irrelevant, and it allows us to solve the DW dynamics for a very general class of spin Hamiltonians. We show that the DW dynamics is described by simple equations with only four parameters. Based on these equations, we study DW dynamics in a ferromagnetic wire with Dzyaloshinskii-Moriya interaction (DMI). We find spin spiral DW structure and how the critical current required to move the domain wall depends on DMI. We also investigate the DW dynamics driven by time-dependent currents. We find the most efficient (with the lowest Ohmic losses) way to move the DWs by resonant current pulses. In addition, we propose a procedure to unambiguously determine the DW dynamics parameters by all-electric measurements of the time-dependent voltage induced by moving DW. Furthermore, based on the derived DW dynamics equations for the translationally non-invariant nanowires, we show how to make potential magnetic memory nanodevices much more energy efficient. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V13.00002: Reduction in critical current for domain wall injection by ion irradiation of perpendicular magnetic anisotropy nanowires Timothy Phung, Luc Thomas, See-Hun Yang, Charles Rettner, Kwang-Su Ryu, John Baglin, Brian Hughes, Stuart Parkin One of the key problems for realization of domain wall motion devices is the reliable and energy efficient injection of domain walls (DWs) into magnetic nanowires. In this work, we explore the injection of domain walls in perpendicular magnetic anisotropy nanowires (Co/Ni multilayers) which are locally softened by ion irradiation. We observe a minimum in the domain wall injection critical current, which occurs where the anisotropy of the irradiated region transitions from out of plane to in plane anisotropy. Furthermore, we find that the irradiated site acts as a pinning site for the DWs. At the irradiation site, we are able to create localized nanosecond long pulsed magnetic fields used to inject the DWs. By performing DC resistance measurements after each injection event, we are able to probe for the existence of the domain wall, and also find the strength of the ion irradiated pinning site. Using the above technique, we have demonstrated a five fold reduction in the domain wall injection current. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V13.00003: 360 Degree DW formation during vortex to vortex switching in thin ferromagnetic nanorings in an applied circular field Yineng Sun, Abby Goldman, Abigail Licht, Yihan Li, Nihar Pradhan, Tianyu Yang, Mark Tuominen, Katherine Aidala We present simulations of the switching process between clockwise and counterclockwise vortex states in ferromagnetic nanorings in an applied circular field, relevant to potential data storage devices. This circular field can be experimentally generated by passing current through the solid metal tip of an atomic force microscope, which has achieved vortex-to-vortex switching in thicker asymmetric rings [1]. We find that in sufficiently thin rings, the vortex switching process occurs through the nucleation and annihilation of pairs of 360 degree domain walls (DW), with opposite topological indices. The DW with the same circulation as the vortex annihilates first. We can control which DW annihilates first by offsetting the center of our circular field to target a specific DW. Both exchange energy and demagnetization energy must be considered in predicting the energy barrier to DW annihilation. [1] T. Yang, N.R. Pradhan, A. Goldman, A.S. Licht, Y.Li, M. Kemei, M.T. Tuominen, K.E. Aidala. APL, 98, 242505 (2011). [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V13.00004: Individual domain wall manipulation in a local oersted circular field Xiaolin Hu, Huajie Ke, Nihar Pradhan, Yineng Sun, Mark Tuominen, Katherine Aidala Understanding domain wall (DW) motion in nanoscale ferromagnetic structures reveals intriguing physics, with potential applications in nanoscale devices and DW data storage. One challenge is to create and move individual DWs in arbitrary locations. We developed a technique to generate localized circular magnetic field by applying a current through the tip of the atomic force microscope (AFM) and thereby manipulating the state of ferromagnetic rings [1]. Now we extend our ability to control domain walls in various structures, such as straight wires with notches and zigzag wires. By placing the tip near a 180 DW in a vertex of a zigzag wire, we can move the 180 DW along the wire and form a stable 360 DW in nearby vertex. We can also move 360 DWs with the local magnetic field around the AFM tip. We will discuss simulations and experimental implementations. \\[4pt] [1] T. Yang, N. R Pradhan, A Goldman, A. Licht, Y. Li, M T. Tuominen and K. E. Aidala, Applied Physics Letter, http://apl.aip.org/resource/1/applab/v98/i24/p242505\_s1 98, 242505, (2011) [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V13.00005: Probing and manipulating magnetization at the nanoscale Invited Speaker: Nitin Samarth Combining semiconductors with magnetism in hetero- and nano-structured geometries provides a powerful means of exploring the interplay between spin-dependent transport and nanoscale magnetism. We describe two recent studies in this context. First, we use spin-dependent transport in ferromagnetic semiconductor thin films to provide a new window into nanoscale magnetism [1]: here, we exploit the large anomalous Hall effect in a ferromagnetic semiconductor as a nanoscale probe of the reversible elastic behavior of magnetic domain walls and gain insight into regimes of domain wall behavior inaccessible to more conventional optical techniques. Next, we describe novel ways to create self-assembled hybrid semiconductor/ferromagnet core-shell nanowires [2] and show how magnetoresistance measurements in single nanowires, coupled with micromagnetic simulations, can provide detailed insights into the magnetization reversal process in nanoscale ferromagnets [3]. The work described here was carried out in collaboration with Andrew Balk, Jing Liang, Nicholas Dellas, Mark Nowakowski, David Rench, Mark Wilson, Roman Engel-Herbert, Suzanne Mohney, Peter Schiffer and David Awschalom. This work is supported by ONR, NSF and the NSF-MRSEC program.\\[4pt] [1] A. L. Balk et al., Phys. Rev.Lett. {\bf 107}, 077205 (2011).\\[0pt] [2] N. J. Dellas et al., Appl. Phys. Lett. {\bf 97}, 072505 (2010).\\[0pt] [3] J. Liang {\it et al}., in preparation. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V13.00006: Beyond a compact magnetic domain wall Philipp Eib, Carl Zinoni, Antoine Vanhaverbeke, Gian Salis, Reto Schlittler, Andreas Bischof, Rolf Allenspach The generally accepted concept that limits magnetic domain wall velocity is the Walker breakdown. This is the magnetic field at which wall motion becomes oscillatory capping the performance of domain wall-based spintronic devices. To understand the limiting mechanisms, we study vortex walls in Ni$_{80}$Fe$_{20}$ wires with widths between 300 and 900 nm. We detect the walls by time-resolved magneto-optical Kerr effect in a pump-probe technique setup. The wires are fabricated by electron-beam lithography and by a nanostencil tool [1]. We find the dynamics of vortex walls to depart significantly from the current description of a compact entity evolving along the wire. Instead, the wall is composed of several substructures propagating in different dynamic regimes with very different velocities. Wire edges crucially affect this dynamics and can be influenced by variation of growth parameters. Extensive, parallelized micromagnetic simulations reveal the unusual wall structure and complement the experimental findings [2]. Possibilities how to overcome the limits imposed by the Walker breakdown will be discussed.$\\$ [1] L. Gross $\emph{et al., Nanotechnology}$ $\bf{21}$, 325301 (2010)$\\$ [2] C. Zinoni $\emph{et al., Phys. Rev. Lett.}$ $\bf{107}$, 207204 (2011) [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V13.00007: Domain walls in ultrathin magnetic nanowires at finite temperatures Thomas Michaels, Alessandro Vindigni, Danilo Pescia The possibility of inducing domain-wall (DW) motion in magnetic nanowires by means of electric currents has recently renewed theoretical interest in this field. The problem is usually modelled on a micromagnetic approach, but ignoring thermal fluctuations. However, some relevant experimental facts - like the correct order of magnitude of the critical current needed for DW motion - still lack satisfactory explanations. We thus developed a one-dimensional stochastic model for DW dynamics, which allowed us to take into account both thermal fluctuations and external drifts (magnetic field and electric spin-polarized current) simultaneously. We also provided a general theoretical framework, which highlights the crucial role played by thermal fluctuations at the centre of DWs. The latter, for example, qualitatively accounts for the shrinking of magnetic domains observed in Fe films on Cu(001) with increasing temperature and the renormalisation of the critical current for DW motion in magnetic nanowires at finite temperatures. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V13.00008: Antiferromagnetic Domain Wall Engineering in Chromium Films Jonathan Logan, Hyekyung Kim, Eric Isaacs, Oleg Shpyrko, Daniel Rosenmann, Zhonghou Cai, Ralu Divan We have engineered an antiferromagnetic domain wall by utilizing a magnetic frustration effect of a thin iron cap layer deposited on a chromium film. Through lithography and wet etching we selectively removed areas of the Fe cap layer to form a patterned ferromagnetic mask over the Cr film. Removing the Fe locally removes magnetic frustration in user-defined regions of the Cr film. We present x-ray microdiffraction results confirming the formation of an antiferromagnetic spin-density wave propagation domain wall in Cr. This domain wall nucleates at the boundary defined by our Fe mask. We have characterized the region surrounding the domain wall using x-ray microdiffraction and microfluorescence with a resolution of 1 micron. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V13.00009: Formation and structure of 360 and 540 degree domain walls in thin magnetic stripes Mark Mascaro, Youngman Jang, S.R. Bowden, J. Unguris, C.A. Ross A method is presented for forming a 360$^{\circ}$ domain wall (DW) and more complex structures such as a 540$^{\circ}$ DW in a wire attached to an injection pad by applying an alternating in-plane field perpendicular to the wire. SEMPA, MFM measurements and OOMMF micromagnetic simulations give a consistent picture of the magnetic structure and stray field distribution of the 360$^{\circ}$ DW. Equilibrium 360$^{\circ}$ DWs in wires have a well-defined structure and size, persist over a wide field range, and can be distinguished from configurations consisting of two 180$^{\circ}$ DWs pinned near each other. The formation and stability of these complex walls has implications in memory and logic devices based on field- or current-induced DW motion, where impingement of adjacent 180$^{\circ}$ DWs can produce composite DWs whose behavior and stray field distribution differ significantly from that of a 180$^{\circ} $DW, and these structures could also be used to examine intriguing resonant behavior as predicted by modeling. [Phys. Rev. B 82, 214411; Phys. Rev. B 82, 134411] [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V13.00010: Polarization Dependent Switching of Asymmetric Nanorings with a Circular field Nihar Pradhan, Mark Tuominen, Katherine Aidala We present experimental switching from the onion to vortex states in asymmetric cobalt nanorings in an applied circular field. We initialize the onion state in two polarizations, along the symmetric or asymmetric axes. We apply a circular field by passing current through a solid metal AFM tip positioned at the center of the ring [1]. The asymmetry of the ring leads to different switching fields depending on the location of the domain walls (DWs) and direction of applied field. For polarization along the asymmetric axis, the field required to move the DWs to the narrow side of the ring is smaller than moving the DWs to the larger side of the ring. The direction of the DW motion is controlled by the circular field. When polarizing the ring along the symmetric axis, establishing one DW in the narrow side and one on the wide side, the field required to switch to the vortex state is an intermediate value. We will be presenting detail of the switching field of cobalt nanoring by circular field with two different direction of polarization. \\ (1) T. Yang, N. R. Pradhan, A. Goldman, A. Licht, Y. Li, M. T. Tuominen and K. E. Aidala, \textit{Applied Physics Letter}, 98, 242505, (2011) [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V13.00011: Non-volatile electric field tuning of magnetic domains in permalloy thin films coupled to ferroelastic PZT bilayers Ichiro Takeuchi, Anbusathaiah Varatharajan, Samuel Bowden, Sean Fackler, Tieren Gao, Paris Alexander, John Cumings, Dan Pierce, John Unguris We are investigating electric field controlled magnetic domain motion in Py films on Pb(ZrxTi(1-x))O3 (PZT) bilayers. Previously, we have shown that bilayered heterostructures consisting of a tetragonal PbZr0.3Ti0.7O3 film (70 nm) above a rhombohedral PbZr0.7Ti0.3O3 film (70 nm) display large ferroelastic domains in tetragonal PZT layer. The reversible non-volatile ferroelastic domain wall motion in this layer can serve as a basis for inducing controlled strain on magnetic thin films. This results in different ferroelastic domain configurations in the tetragonal PZT layer. This in turn leads to changes in magnetic domains of Py film. We find that a Py film on the ferroelastic PZT layer exhibits sharp magnetic domain patterns usually associated with out-of-plane magnetization by MFM. SEMPA imaging reveals that the magnetic domains are indeed in-plane magnetized as expected for Pyfilms. OOMMF analysis indicates presence of unusual metastable in-plane anisotropy modulation in the Py film. [Preview Abstract] |
Session V14: Focus Session: Spins in Carbon - Spins and Magnetism in Carbon
Sponsoring Units: DMP GMAGChair: Jian Shen, Fudan University
Room: 212
Thursday, March 1, 2012 8:00AM - 8:12AM |
V14.00001: Inducing Magnetization by Flexing Graphene Nanoribbon Renat Sabirianov, Nabil Al-Aqtash Zigzag graphene nanoribbons (ZGNRs) are antiferromagnetic in the ground state with zero net magnetization due to the compensation of contributions from opposite edges. The uniform deformations (both shear and axial) do not produce magnetization due to the symmetry restrictions. However, we report the results of first-principles calculations that predict that the induction of net magnetization in the graphene nanoribbon upon non-uniform strain applied to the nanoribbon. Using density functional theory (DFT) method implemented in SIESTA code, we show that the bending or twisting of nanoribbon produces magnetization because in the presence of strain gradient the induced magnetization on opposite edges are not compensating each other. We estimate an average magnetization of $\sim $ 3.3$\mu _{B}$ that produced from the bending of nanoribbon with the sinusoidal profile $\delta $x=Asin(2$\pi $z/L) with A= 3{\AA} and L=87.4 {\AA} (z=0..L/2, i.e. the half of the period). Our study suggests that the induced magnetization can be used for the control of magnetic structure in graphene including the trap of the domain walls. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V14.00002: Numerical studies of the magnetism in graphene nanoribbons and graphene dot Tianxing Ma, Zhongbing Huang, Hai-Qing Lin Within determinant quantum Monte Carlo simulation, the magnetic properties of graphene nanoribbons and graphene dot are studied. It is predicted that the armchair graphene nanoribbons show carrier mediated ferromagnetism as electron filling is lower than 0.8. The uniform magnetic susceptibility increase as the width of nanoribbons decrease, and it increases greatly as the next-nearest-neighbor hopping energy increases. It is also shown that strain may induce magnetism in doped square graphene dot having zigzag edge. The resultant ferromagnetism in graphene-based system may facilitate the development of spintronics. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V14.00003: g-Tensor control in a bent carbon nanotube quantum dot Ruby Lai, Hugh Churchill, Charles Marcus We report low-temperature transport measurements of a carbon nanotube quantum dot containing a bend along its length. The bend occurred naturally in CVD growth, with a bend angle of 140 degrees and a radius of curvature of $\sim $1um. The device was contacted across the bend, with a global back gate and a top gate proximal to one arm of the bend. We measured the magnetic field angle dependence of conductance, tunneling rates, and bandgap in a 3-axis vector magnetic field. We characterize key signatures of carbon nanotube bends in the Kondo regime, comparing these dependencies in perpendicular and in-plane magnetic fields. We also demonstrate control of the electron spin g-tensor using gate voltages. Support from IBM, NSF-MWN, NSF-NRI through the INDEX Center, and Harvard University is acknowledged. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V14.00004: Magnetic impurities and Kondo conductance anomalies in nanotubes: the importance of being ``in'' Erio Tosatti, Pierpaolo Baruselli, Michele Fabrizio Transition metal impurities should yield zero bias anomalies in the conductance of well contacted metallic carbon nanotubes, but relevant temperatures and lineshapes cannot be anticipated without accurate ab initio calculations. Applying the density functional plus numerical renormalization group approach of Lucignano et al [1] to Co and Fe impurities in (4,4) and (8,8) nanotubes, we discover a huge difference of behaviour between outside versus inside impurity adsorption. The predicted Kondo temperatures and zero bias anomalies, tiny when the impurity is outside the nanotube [2] turn large and strongly radius dependent inside, owing to a change of symmetry of the magnetic orbital. These results foreshadow interesting field and temperature dependent nanotube electrical transport, to be addressed in future experiments.\\[4pt] [1] P. Lucignano, et al., Nature Materials 8, 563 (2009).\\[0pt] [2] P. Baruselli, et al., Physica E (2011) doi:10.1016/j.physe.2011.05.005 [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V14.00005: Spin-orbit coupling and electronic transport in carbon nanotubes in external fields Ginetom Diniz, S. E. Ulloa We have investigated theoretically the role of spin-orbit coupling (SOC) on the conductance response of carbon nanotubes (CNT) in the presence of external electric and magnetic fields. We use an equilibrium Green's function formalism to calculate the spin resolved conductance by using a four-orbital orthogonal tight-binding representation in real space, taking into account curvature effects that induce orbital hybridization and are responsible for sizable SOC enhancements [1]. Different directions of external magnetic and electric fields (transverse and longitudinal to the CNTs), as well as length and chirality of the CNT, are shown to strongly affect the transport behavior of the systems. In particular, this results in stronger SOC effects for tubes with smaller radii. The interplay of electric and magnetic fields on the possible spin polarization of conductance will be discussed, as the sizable SOC effects result in effective control of the spin selective transport in these versatile nanoeletronic devices.\\ $[1]$ J. Klinovaja et al. PRL 106, 156809 (2011). [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V14.00006: Spin-orbit induced strong coupling of a single spin to a nanomechanical resonator Philipp Struck, Andras Palyi, Mark Rudner, Karsten Flensberg, Guido Burkard We theoretically investigate the coupling of an electron spin to vibrational motion due to curvature-induced spin-orbit coupling in suspended carbon nanotube quantum dots. Our estimates indicate that, with current capabilities, a quantum dot with an odd number of electrons can serve as a realization of the Jaynes-Cummings model of quantum electrodynamics in the strong-coupling regime. A quantized flexural mode of the suspended tube plays the role of the optical mode and we identify two distinct two-level subspaces, at small and large magnetic field, which can be used as qubits in this setup. The strong intrinsic spin-mechanical coupling allows for detection, as well as manipulation of the spin qubit, and may yield enhanced performance of nanotubes in sensing applications. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V14.00007: Distinctive Magneto Conductance and Universal Scaling in One Dimensional Polymer Nanofibers Ajeong Choi, Kyung Ho Kim, Sung Ju Hong, Munju Goh, Kazuo Akagi, Richard B. Kaner, Yung Woo Park The conducting polymers are one dimensional organic hydrocarbon systems where the non-linear local excitations such as solitons, polarons and bipolarons were predicted based on the electron-phonon interactions. The local excitations have different spin-charge relations in different kinds of polymers. In this work, the magneto conductance (MC) of polymer nanofibers is investigated in high magnetic field at low temperature to understand both spin (magneto) and charge (conductance) of the charge carriers simultaneously. We discovered the distinctive zero MC in polyacetylene nanofibers while the finite MC in polyaniline and polythiophene nanofibers. On the other hand, the charge transports of polymer nanofibers as a function of temperature and bias are observed to be scaled onto the universal curve. We conclude that the universal scaling and the zero MC (the finite MC) in polyacetylene (polyaniline and polythiophene) nanofibers are from the interacting spinless charged solitons (interacting polarons which have both spin and charge). [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V14.00008: Magneto-Photoinduced Absorption in Organic Polymer Films Bhoj Gautam, Tho Nguyen, Eitan Ehrenfreund, Z.Valy Vardeny In order to elucidate the underlying mechanism of magneto-conductivity (MC) in OLEDs we studied magneto-photoinduced absorption (MPA) response in polymer films. The films were based on the MEH-PPV polymer in three different forms, namely: pristine film; film exposed to prolonged UV illumination; and MEH-PPV/PCBM blend having weight ratio 1:1. In pristine film we show that the MPA at low excitation intensity is due to sublevel spin mixing of triplet excitons; whereas at high excitation intensity the MPA is dominated by the triplet-triplet annihilation process. In UV illuminated MEH-PPV films that support photogenerated polarons we show that the MPA is due to sublevel spin-mixing of polaron-pairs via the hyperfine interaction with the closest hydrogen atoms to the chain. This mechanism also explains the MC response of OLED based on MEH-PPV, since its response is similar to that of MPA. Finally we found that the MPA in MEH-PPV/PCBM blend films is dominated by spin mixing of polaron-pair on the polymer and fullerene molecules, via the $\Delta $g mechanism. Supported by the NSF DMR-1104495, the NSF MRSEC at the UoU, and the BSF program. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V14.00009: Tunnel magnetoresistance in Self-Assembled Monolayers Based Tunnel Junctions Richard Mattana, Cl\'ement Barraud, Sergio Tatay, Marta Galbiati, Pierre Seneor, Karim Bouzehouane, Eric Jacquet, Cyrile Deranlot, Albert Fert, Fr\'ed\'eric Petroff Organic/molecular spintronics is a rising research field at the frontier between spintronics and organic chemistry. Organic molecule and semiconductors were first seen as promising for spintronics devices due to their expected long spin lifetime. But an exciting challenge has also been to find opportunities arising from chemistry to develop new spintronics functionalities. It was shown that the molecular structure and the ferromagnetic metal/molecule hybridization can strongly influence interfacial spin properties going from spin polarization enhancement to its sign control in spintronics devices. In this scenario, while scarcely studied, self-assembled monolayers (SAMs) are expected to become perfect toy barriers to further test these tailoring properties in molecular magnetic tunnel junctions (MTJs). Due to its very high spin polarization and air stability LSMO has positioned itself as the electrode of choice in most of the organic spintronics devices. We will present a missing building block for molecular spintronics tailoring: the grafting and film characterization of organic monofunctionalized long alkane chains over LSMO. We have obtained 35\% of magnetoresistance in LSMO/SAMs/Co MTJs. We will discuss the unusual behaviour of the bias voltage dependence of the TMR. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:24AM |
V14.00010: Electrical spin injection from an organic-based ferrimagnet in a hybrid organic/inorganic heterostructure Invited Speaker: Ezekiel Johnston-Halperin The development of organic-based magnets with room temperature magnetic ordering and semiconducting functionality promises to broaden the field of semiconductor spintronics, providing a route to all-organic spintronic devices and hybrid organic/inorganic structures capable of exploiting the multifunctionality and ease of production in organic systems as well as the well established spintronic functionality of inorganic materials. Our work demonstrates the successful extraction of spin polarized current from the organic-based room temperature ferrimagnetic semiconductor V[TCNE]$_{x}$ ($x\sim $2, TCNE: tetracyanoethylene; $T_{C} \quad >$ 400 K, $E_{G}$~$\sim $~0.5~eV, \textit{$\sigma $}$\sim $ 10$^{-2}$ S/cm) and its subsequent injection into a GaAs/AlGaAs light-emitting diode (LED) [1]. The spin current is detected by monitoring the polarization state of the photons emitted from the LED structure and tracks the magnetization of V[TCNE]$_{x\sim 2}$, is weakly temperature dependent, and exhibits heavy hole / light hole asymmetry. This result opens the door to a new class of active, hybrid spintronic devices with multifunctional behavior defined by the optical, electronic and chemical sensitivity of the organic layer. In addition, spin transport in these hybrid structures provides the opportunity to leverage well-characterized inorganic materials as a probe of spin physics in organic and molecular systems and to explore the impact of the hybrid interface on spin injection efficiency. Initial studies exploring the impact of surface passivation of the inorganic layer with self-assembled monolayers of various chemistries will be presented, and additional experimental probes of the interfacial exchange interaction will be discussed. \\[4pt] [1] ``Electrical Spin Injection from an Organic-Based Ferrimagnet in a Hybrid Organic-Inorganic Heterostructure,'' Lei Fang, K. Deniz Bozdag, Chia-Yi Chen, P.A. Truitt, A.J. Epstein and E. Johnston-Halperin, \textit{Phys. Rev. Lett.} \textbf{106}, 156602 (2011). [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V14.00011: The effect on the properties of V(TCNE) grown on bare substrates and self assembled monolayers Megan Harberts, Howard Yu, Mark Guseman, Yu Lu, Ezekial Johnston-Halperin, Arthur Epstein V(TCNE) is a room temperature Tc larger than 400K organic based magnetic semiconductor. It has been shown that this fully-spin polarized material can be used as a spin injector in hybrid organic inorganic spin-light emitting diode (spin-LED) as well as hybrid and all organic spin valves [1,2]. Attempts to improve the spin signal from a spin LED device are focusing on surface passivation of the III-V heterostructure surface by including a self--assembled monolayer (SAM). We present a comparison of the properties of V(TCNE)grown by chemical vapor deposition (CVD) on various SAM's and bare substrates. We find that the successful growth of uniform, thin films of CVD V(TCNE) requires careful purification of the SAM precursors. We observe bulk magnetic properties of V(TCNE) grown on SAM's to be consistent with or better than films grown on bare substrates. Therefore the presence of the SAM does not appear to adversely affect the desired properties of the V(TCNE) for use as a spin injector, dramatically expanding the range of potential hybrid device geometries. \\[4pt] [1] L. Fang, Phys Rev Lett, 106 (15), (2011). \\[0pt] [2] J. W. Yoo, Nat Mater, 9 (2010). [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V14.00012: Spin multiplicity and symmetry breaking in vanadium-benzene complexes Ivan Stich, Lucia Horvathova, Matus Dubecky, Lubos Mitas Despite use of vanadium-benzene complexes in spintronics applications [1], reliable theoretical and experimental knowledge of energetics, dissociation energy, spin multiplicity, etc. of these systems is missing. Fixed-node DMC calculations have been done with the quest to elucidate electronic and atomic structure of vanadium-benzene half-sandwiches. At variance with DFT results which favor either low- or high-spin state, depending on the functional used [2], DMC predicts degenerate energies for spin multiplicities 2, 4, and 6, irrespective of DFT functional used to fix the nodal hypersurfaces. Ultimately, we predict high-spin ground-state, based on comparison of experimental/theory ionization energy [3]. Based again on DMC, we predict vastly different gaps for spin-up/-down electrons. The DMC results indicate that both DFT [2] as well as experimental results [4] may be biased. [1] V. V. Maslyuk \textit{et al.}, Phys. Rev. Lett. \textbf{97}, 097201 (2006) [2] R. Pandey, B. K. Rao, P. Jena, M. A. Blanco, J. Am. Chem. Soc. \textbf{123}, 3799 (2001) [3] T. Kurikawa, \textit{et al.}, Organomentallics \textbf{18}, 1430 (1999). [4] R. L. Hettich, T. C. Jackson, E. M. Stanko, B. S. Freiser, J. Am. Chem. Soc. \textbf{108}, 5086 (1986). [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V14.00013: Total Scattering Study of Vanadium Tetracyanoethylene D.M. Pajerowski, J.L. Arthur, K. Page, J.S. Miller, J.W. Lynn Vanadium tetracyanoethylene powder prepared in deuterated dichloromethane (henceforth V-TCNE) was studied using neutron and X-ray diffraction. V-TCNE is a molecule-based magnet that has been shown to display magnetic order above room temperature\footnote{J.M. Manriquez, G.T. Yee, R.S. McLean, A.J. Epstein, and J.S. Miller, Science 252, 1415-1417 (1991).} as well as photocontrol of magnetism at cryogenic temperatures.\footnote{J.-W. Yoo, R.S. Edelstein, D.M. Lincoln, N.P. Raju, and A.J. Epstein, Phys. Rev. Lett. 99 157205 (2007).} To date, all reported synthesis preparations of V-TCNE yield amorphous compounds that lack Bragg peaks in diffractograms. In the absence of long range structural order, diffraction experiments may still elucidate short range structural order. The experimental results, which display short range correlations, will be presented and compared to Monte Carlo simulations. [Preview Abstract] |
Session V16: Heavy Fermions- 1-1-5 Systems and Others
Sponsoring Units: DCMPChair: Deepak Singh, NIST
Room: 251
Thursday, March 1, 2012 8:00AM - 8:12AM |
V16.00001: Fermi surface of Ce$_{2}$RhIn$_{8}$ studied by angle-resolved photoemission spectroscopy Rui Jiang, R.S. Dhaka, Chang Liu, C. Petrovic, Adam Kaminski The superconductivity in heavy fermion compounds is one of most interesting, outstanding problems in condensed matter physics. In this family of materials, Ce$_{2}$RhIn$_{8}$ and related compounds is unique because its electronic properties are believed to be quasi 2D. Here, we use angle-resolved photoemission spectroscopy to measure its electronic properties. The lack of significant $k_{z}$ dispersion confirms the quasi two dimensionality of the electronic structure. Fermi surface is quite complicated and consists of several hole and electron sheets. There are several extended segments of the Fermi surface which are likely nested. This provides clues to understanding of their unusual transport and thermodynamical properties. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V16.00002: Anomalous superfluid response in the layered heavy fermion compound Ce$_2$PdIn$_8$ near a magnetic quantum critical point Kenichiro Hashimoto, Yuta Mizukami, Ryo Katsumata, Minoru Yamashita, Hiroaki Ikeda, Takasada Shibauchi, Yuji Matsuda, Daniel Gnida, Dariusz Kaczorowski, Antony Carrington The recently discovered layered heavy fermion superconductor Ce$_2$PdIn$_8$ ($T_c$ = 0.7 K) has attracted a great interest because of the appearance of unconventional superconductivity near a quantum critical point. Here we report high-precision measurements of magnetic penetration depth $\lambda$ down to 60 mK ($\sim0.1T/T_c$) by using a tunnel diode oscillator operating at $\sim$13 MHz. A strong power-law temperature dependence of $\lambda$ demonstrates low-energy excitations of quasiparticles, which is consistent with recent thermal conductivity measurements indicative of nodal superconductivity. The observed $T^{1.5}$ behavior at low temperatures, which is commonly seen in superconductors with line nodes near a magnetic quantum critical point such as CeCoIn$_5$ or organics, can be attributed to the enhancement of effective mass towards zero temperature due to quantum magnetic fluctuations even in the superconducting state. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V16.00003: Magnetism in 2D Heavy Fermion Superconductor CePt$_{2}$In$_{7}$ Nicholas apRoberts-Warren, John Crocker, Adam Dioguardi, Peter Klavins, Ching Lin, Kent Shirer, Abigail Shockley, Nicholas Curro CePt$_{2}$In$_{7}$ was first synthesized in 2008, and it was discovered to be a superconductor under pressure shortly thereafter in 2009. It is closely related to the well-studied crystal class CeMIn$_{5}$ (M = Ir, Co, Rh), but CePt$_{2}$In$_{7}$ is considerably more anisotropic with a c/a ratio more than twice as large as the 115's. We present here nuclear quadrupole resonance results for the $^{115}$In nucleus, and follow it from the paramagnetic heavy fermion state into the anti-ferromagnetic state below T$_{N}$ = 5.2 K. The NQR data will show a rich magnetic phase diagram, with a commensurate magnetic order parameter directly below T$_{N}$, and more complex magnetic states emerging as the material is further cooled. Additionally, we will present some additional results on how hydrostatic pressure affects the magnetic order in CePt$_{2}$In$_{7}$. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V16.00004: Electric transport measurements on micro-structured CePt$_{2}$In$_{7}$ single crystals in a diamond anvil cell J. Kanter, P. Moll, F. Ronning, E. Bauer, P. Tobash, J. Thompson, B. Batlogg We report Shubnikov--de Haas and resistivity measurements of CePt$_{2}$In$_{7}$ samples under hydrostatic pressures using a diamond anvil cell. CePt$_{2}$In$_{7}$ belongs to the Ce$_{m}M_{n}$In$_{3m+2n}$ heavy fermion family. Compared to the Ce$M$In$_{5}$ members of this group, the structure of CePt$_{2}$In$_{7}$ has a more two dimensional character, but also exhibits an antiferromagnetically ordered and a superconducting phase. Upon increasing pressure the AFM order is suppressed with the N\'{e}el temperature extrapolating to a quantum critical point. The fluctuations associated with the QCP are thought to stabilize the unconventional superconducting phase. To investigate the weight of the different scattering channels the anisotropy of the resistivity above the N\'{e}el temperature was measured for various applied pressures. Shubnikov--de Haas measurements were conducted to deduce the changes in the effective electron masses in the AFM and superconducting phases under applied hydrostatic pressure. To this end we developed a method to conduct four terminal resistance measurements on micro-structured samples inside a diamond anvil cell. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V16.00005: SmPt$_4$Ge$_{12}$ -- a filled skutterudite with exotic heavy-fermion properties Andreas Leithe-Jasper, Roman Gumeniuk, Walter Schnelle, Helge Rosner, Michael Nicklas, Michael Sch\"oneich, Ulrich Schwarz, Yuri Grin Ternary samarium-filled platinum-germanium skutterudite SmPt$_4$Ge$_{12}$ was prepared at a pressure of 5.0(0.5)\,GPa and a temperature of 1070(70)\,K. The compound crystallizes in the cubic space group $Im\bar{3}$ ($a$ = 8.6069(4)\,\AA) and is isotypic with LaFe$_4$P$_{12}$. X-ray absorption spectroscopy measurements show that samarium in SmPt$_4$Ge$_{12}$ has a temperature-independent intermediate valence ($\nu = 2.90 \pm 0.03$). Magnetization data reveal van-Vleck paramagnetism above $\sim$50\,K. The low-temperature specific heat displays a broad anomaly centred at 2.9\,K and a large linear coefficient $\gamma'$ = 450 mJ\,mol$^{-1}$K$^{-2}$ suggesting heavy-fermion behaviour. This state is remarkabkly field-independent and resembles the physics of SmOs$_4$Sb$_{12}$. [1] R Gumeniuk et al. New J. Physics 12 (2010) 103035. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V16.00006: Physical properties and Ce-valence of the filled skutterudite CePt$_4$Ge$_{12}$ Walter Schnelle, Roman Gumeniuk, Michael Nicklas, Helge Rosner, Andreas Leithe-Jasper, Yuri Grin, Kristina O. Kvashnina, Yuri Skourski Filled skutterudite compounds $M$Pt$_4$Ge$_{12}$ with a Pt-Ge framework structure show intriguing physical ground states (conventional ($M$ = Sr, Ba) and unconventional superconductivity ($M$ = Pr), heavy-fermion behavior ($M$ = Sm), magnetic ordering), similar to the well-known transition-metal pnicogen skutterudites. Here, we report on the electronic, magnetic, and transport properties of CePt$_4$Ge$_{12}$ [1]. High-resolution X-ray absorption spectroscopy (XANES) measurements at the cerium L$_{\mathrm{III}}$ edge demonstrate that Ce in this compound has a temperature-independent valence close to three. However, magnetic susceptibility, thermopower, Hall effect, and electronic specific heat reveal broad maxima at $T_{\mathrm{max}}$ = 65-80\,K, suggesting the presence of valence fluctuations. The Sommerfeld coefficient $\gamma$ = 105 mJ\,mol$^{-1}$\,K$^{-2}$ indicates moderately enhanced band masses for CePt$_4$Ge$_{12}$. We discuss these findings and conclude that CePt$_4$Ge$_{12}$ represents a system at the border between intermediate valence (IV) and Kondo lattice behavior.\\[4pt] [1] R.Gumeniuk et al. J.\ Phys.: Condensed Matter 23 (2011) 456601. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V16.00007: Cu-NMR and magnetization in disordered nFL system UCu$_4$Ni O.O. Bernal, A. Valdez, M. Chiang, D.E. MacLaughlin, G.R. Stewart, J.S. Kim We present a study of the NMR spectra in a random powder of UCu$_4$Ni as a function of frequency (40-70 MHz) and temperature (5-300 K). Two types of spectral lines for each of the two isotopes of naturally abundant Cu in the material are clearly evident in the spectra. Their behavior is followed for temperature and field variations and compared/contrasted with the more studied case of UCu$_4$Pd, where only one type of Cu-NMR line has been observed clearly. Unlike in UCu$_4$Pd, the appearance of two types of signal from Cu nuclei in the Ni compound is unambiguous evidence of site disorder in UCu$_4$Ni. This alone is indication that the amount of site disorder in the Ni sample is larger than in the Pd system; however, the NMR line intensities reveal that the Ni ions do not seem to go completely randomly in the two available crystallographic sites of the underlying crystal structure as would be expected from ionic-size considerations alone. The NMR parameters for both types of spectral lines, together with complementary measurements of magnetic susceptibility performed on the same powder samples, will be discussed from the point of view of magnetic disorder and non-Fermi liquid behavior. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V16.00008: Effect of pressure on CeGe$_{2-x}$ single crystals S.L. Bud'ko, D. Braithwaite, B. Salce, E. Colombier, M.S. Torikachvili, H. Hodovanets, P.C. Canfield At ambient pressure CeGe$_{2-x}$ has an antiferromagnetic (AFM) transition at $T_N \sim 7$ K and, on further cooling, a second transition, to a weak ferromagnetic (WFM) state, at $T_C \sim 4$ K. Under pressure the $T_N$ goes through a broad maximum, and then is completely suppressed with a critical point at $\sim 85$ kbar, grossly following the Kondo-necklace picture. The WFM phase evolves into apparent AFM phase under moderate pressure of $\sim 10$ kbar, with $T_N1$ joining $T_N$ at $\sim 60$ kbar. Resulting $P - T$ phase diagram and comparison with CeRu$_2$Ge$_2$ under pressure will be presented. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V16.00009: A THz spectroscopy investigation of the heavy fermion state in CeCu$_2$Ge$_2$ Grace Bosse, L. Bilbro, R. Valdes Augilar, Y. Li, J. Eckstein, N.P. Armitage We present time-domain THz spectroscopy data of a thin film of the heavy fermion compound CeCu$_2$Ge$_2$. Measurements to obtain the frequency dependent complex conductivity were taken as a function of temperature down to temperatures well below the onset of magnetic order. At low temperatures a narrow Drude-like peak forms, which is likely associated with the heavy fermion or spin density wave state. Using this data in conjunction with DC resistivity measurements, we obtain the frequency dependence of the scattering rate and the mass renormalization through an extended Drude model analysis. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V16.00010: Influence of charge carrier doping on the $T^\ast$-scale in YbRh$_2$Si$_2$ Philipp Gegenwart, H.S. Jeevan, Y. Tokiwa, M. Schubert, M. Mchalwat, E. Blumenroether YbRh$_2$Si$_2$ is a prototype heavy-fermion metal which displays a magnetic field-induced antiferromagnetic (AF) quantum critical point (QCP). It has attracted much attention due to an additional low-energy scale $T^\star(B)$ merging at the QCP, whose origin is controversially discussed. Here, we report measurements of the electrical resistivity $\rho(T,B)$ on different single crystalline samples of charge-carrier doped Yb(Rh$_{1-x}$T$_x$)$_2$Si$_2$ (T=Fe, Ni) at temperatures down to 15~mK and in magnetic fields up to 7~T. The partial substitution of Rh by either Fe or Ni introduces holes or electrons, respectively. The evolution of the single-ion Kondo scale is similar as for isoelectronic Co substitution and in accordance with the chemical pressure effect. However, while chemical pressure has little influence on $T^\star(B)$, we observe a drastic reduction or increase of $B^\star(T=0)$ by Fe- or Ni-doping, respectively. Most interestingly, $B^\star(T=0)$ is always pinned at the field-induced AF QCP, in contrast to chemical pressure results. As AF order is completely suppressed by Fe-doping, a heavy Fermi liquid ground (without $T^\star(B)$ anomaly) is observed. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V16.00011: Quantum Criticality in the strongly correlated 3d electron system YFe$_{2}$Al$_{10}$ L.S. Wu, K. Park, Y. Janssen, M.S. Kim, C. Marques, M.C. Aronson YFe$_{2}$Al$_{10}$ has recently been identified as a strongly correlated 3d -electron system that is located very close to a quantum critical point (QCP). At low temperatures, divergences were found in the ac magnetic susceptibility ($\chi '\propto T^{-1.3})$ and magnetic specific heat ($C_M /T\propto T^{-0.47})$ based on the measurements carried out on single crystals. The magnetic Gruneisen ratio determined from these measurements ($\Gamma /H=-\frac{1}{T}\frac{\partial S/\partial B}{\partial S/\partial T}=-\frac{\partial M/\partial T}{C})$ also shows a strong divergence as $T\to 0$, which is suppressed in fields indicating a quantum critical point at B=0. When applying magnetic field, Fermi liquid like behavior with an enhanced magnetic susceptibility $\chi _{0}$ and Sommerfeld coefficient C/T emerges at T$<$T$_{FL}$, and this enabled us to establish a field temperature (B-T) phase diagram with a crossover temperature T$_{FL}$(B) coming out from the QCP at B=0. This is very similar to the quantum critical behavior observed in many f-electron based heavy fermion (HF) systems, and it makes YFe$_{2}$Al$_{10}$ an interesting 3d- electron candidate for studying quantum criticality. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V16.00012: Small-angle neutron scattering study of the vortex lattice in PrOs$_4$Sb$_{12}$ Alexandre Desilets-Benoit, Andrea Bianchi, Simon Gerber, Nikola Eigetenmeyer, Jorge Gavilano, Michel Kenzelmann, Christian Pfleiderer, Sebastian Muehlbauer, Ryan Baumbach, Brian Maple We carried out a small angle neutron scattering study of the vortex lattice in the Pr based heavy Fermion superconductor PrSb$_4$Os$_{12}$ with H applied along the c-axis. With a critical temperature T$_c$ of 1.85 K and a H$_{c2}$ of 2.5T, PrOs$_4$Sb$_{12}$ is the first Pr based heavy fermion superconductor (SC). Although PrOs$_4$Sb$_{12}$ crystallizes in a filled Skutterudite structure with a cubic lattice, this structure has a tetrahedral point group symmetry. In consequence, with the application of a magnetic field along the $c$-direction, the $a$ and $b$-axis are no longer equivalent. Measurements of the angular dependence of the thermal conductivity in PrOs$_4$Sb$_{12}$ suggest a change of the superconducting order parameter deep inside the SC phase diagram from a two fold symmetry to a fourfold symmetry. Three ways of applying field were tested and wiggle field cooling was found to produce the highest intensity. We observed a twinned distorted hexagonal VL up to 1T of applied field which is consistent with $s$-wave superconductivity. We did not observe a field dependent phase transition associated with a change in symmetry of the VL. We found a rapidly decreasing form factor |F|$^2$ of the vortex lattice (VL) in PrOs$_4$Sb$_{12}$ with increasing field. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V16.00013: In plane vortex lattice of the heavy Fermion superconductor CeCoIn5 A.D. Bianchi, P. Das, J.S. White, A.T. Holmes, E.M. Forgan, M. Kenzelmann, M. Zolliker, S. Gerber, J.L. Gavilano, E.D. Bauer, J.L. Sarrao, C. Petrovic, M.R. Eskildsen We present a small angle neutron scattering study of the vortex lattice (VL) in the unconventional superconductor CeCoIn$_5$ for magnetic fields $H$ applied in the crystallographic $ab$-plane. CeCoIn$_5$ is a $d_{x^2-y^2}$ superconductor with the lines nodes of the superconducting gap oriented along the $[110]$ direction. At low fields $H$ applied parallel to $[100]$, and $[110]$, we observed a distorted hexagonal VL with an opening angle of about 80$^{\circ}$. However, for $[110]$ we find a reorientation transition for fields above 9~T where the vortex lattice is rotated by 90$^{\circ}$. The $H$-dependence of the form factor of the vortex lattice is similar to what we observed previously along the $c$-axis: The scattering intensity increases with increasing $H$ due to Pauli paramagnetism. For fields where the transition into the SC state becomes first order, we observe a broadening of the VL. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V16.00014: Low-energy magnetic excitations in single-crystalline CeCu$_{2}$Ge$_{2}$ in fields up to 10 T Michael Loewenhaupt, Astrid Schneidewind, Enrico Faulhaber, Oliver Stockert CeCu$_{2}$Ge$_{2}$ is a magnetically ordered (T$_{N}$ = 4.1 K) Kondo lattice with a moderate enhanced Sommerfeld coefficient of 140 mJ/molK$^{2}$. Calculations of the Fermi surface showed that the observed incommensurate antiferromagnetic order can only be explained by involving an itinerant component of the Ce 4f-moments in addition to their local character. We performed low-energy inelastic neutron scattering experiments on a 2 g single crystal of CeCu$_{2}$Ge$_{2}$ using the cold triple-axis spectrometer PANDA at FRM II. Data were taken at 1.5 K and in magnetic fields up to 10 T applied perpendicular to the (110/001) scattering plane. At zero field the magnetic excitations show an energy gap of 0.5 meV at all investigated $\Gamma$ points in the (110/001) plane with similar intensities. Away from the $\Gamma$ points the magnetic excitations become dispersive merging into a band of excitations around 1 meV. For increasing magnetic fields the gap energy decreases indicating opposite action of external and internal fields. The results will be discussed in the framework of local, crystal field related, and non-local, spin-wave-like, magnetic excitations. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V16.00015: Universal Kondo effect in Ti$_{0.94}$M$_{0.06}$O$_{2}$ (M = Nb, Ta) Thin Films Kalon Gopinadhan, A. Roy Barman, A. Annadi, T.P. Sarkar, W.M. Lu, Z.Q. Liu, Amar Srivastava, - Ariando, Sankar Dhar, T. Venkatesan We show that Ti$_{0.94}$M$_{0.06}$O$_{2}$ (M = Nb, Ta) thin films are Kondo systems by angle dependent magneto-resistance measurement. Surprisingly, the data fitted by both Goldhaber-Gordon and Hamann formula yield comparable carrier dependent Kondo temperatures (10 - 60 K). The Kondo temperature dependence on carrier density was different for Ta and Nb incorporated in TiO$_{2}$ but this result could be understood on the basis of the formation of compensating defects (Ti vacancies) which also act as localized magnetic scattering centers. Using the unitarity-limit resistivity and Kondo temperature estimated from these fits the normalized resistivity versus temperature for all these films collapse into a universal curve consistent with data observed in metallic systems. However, the size of the Kondo scattering is at least an order of magnitude larger than in metallic systems. Further, from the extrapolation of the Kondo temperature we are able to predict that Ta is a better candidate for observation of ferromagnetism versus Nb. [Preview Abstract] |
Session V17: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Junctions
Sponsoring Units: DMPChair: Kevin Ingersent, University of Florida and Douglas Natelson, Rice University
Room: 252A
Thursday, March 1, 2012 8:00AM - 8:12AM |
V17.00001: Effects of electron-phonon coupling in the Kondo regime of a two-orbital single-molecule junction Edson Vernek, Gisele Iorio, Lili Deng, Kevin Ingersent, Enrique Anda Single-molecule junctions (SMJs) are electronic devices formed by a molecule bridging the gap between two metallic contacts. Despite their apparent simplicity, such systems have attracted much attention for the rich variety of experimentally accessible physics that they display. The spatial confinement of electrons in molecules can lead to collective phenomena such as Coulomb blockade and the Kondo effect, as well as to strong coupling of electrons to molecular vibrations. We explore the interesting interplay of electron-electron and electron-phonon interactions in a model of an SMJ in which the central molecule has two active orbitals. The nonperturbative numerical renormalization group method is used to treat the many-body Kondo physics and electron-phonon coupling on equal footing. Electron-phonon coupling renormalizes the energies and Coulomb interactions of the molecular orbitals. The effects are most pronounced in cases where both molecular orbitals lie close to the Fermi energy of the contacts. Here, a sufficiently strong phonon-assisted inter-orbital tunneling can suppress the Kondo effect and cause a crossover to a phonon-dominated regime having very different electrical transport properties. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V17.00002: Rectification in Symmetric Conjugated Molecules with Asymmetric Linkers Arunabh Batra, Jeffrey S. Meisner, Jonathan R. Widawsky, Eek Huisman, Colin Nuckolls, Latha Venkataraman Demonstrating single-molecule rectification is an important step towards the realization of molecule-based electronic devices. Most molecules put forward as potential rectifiers employ asymmetric molecular backbones. In contrast, we show that we can create rectifying junctions by designing asymmetry only into the linker groups used to bond the molecule to metal electrodes. Our molecules consist of a conjugated backbone terminated with methylsulfide on one end and methyl-trimethyltin on the other. These molecules couple to Au electrodes through an Au-SMe donor acceptor bond, which serves as the electronically weak link, and a Au-C covalent bond, which is created in-situ after the SnMe$_{3}$ cleaves off [1]. We create thousands of molecular junctions using a modified STM setup in a solution of molecules, measure their current-voltage (IV) characteristics and create averaged IV curves. We find that asymmetrically terminated molecules show non-linear IV curves with significant rectification, while molecules terminated symmetrically with either SMe or SnMe$_{3}$ do not show substantial rectification. We also find that the rectification direction is dependent on molecular orientation in the junction. [1] Chen, W., et al., J. Am. Chem. Soc., 2011. 133(43): p. 17160-17163 [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V17.00003: Simultaneous Determination of Conductance and Thermopower of Single Molecule Junctions Jonathan Widawsky, Pierre Darancet, Jeffrey Neaton, Latha Venkataraman We present a study of concurrent determination of conductance ($G)$ and thermopower ($S)$ of single-molecule junctions via direct measurement of electrical and thermoelectric currents. The junctions are created using the STM-based break-junction technique where a cold Au tip is repeatedly brought in and out of contact with a hot Au-on-mica substrate in an environment of the target molecule. We explore several amine-Au and pyridine-Au linked molecules that are predicted to conduct through either the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO), respectively. We find that the Seebeck coefficient is negative for pyridine-Au linked LUMO-conducting junctions and positive for amine-Au linked HOMO-conducting junctions. From histograms of thousands of junctions, we use the most probable Seebeck coefficient to determine a power factor, \textit{GS}$^{2}$, for each junction studied, and find that \textit{GS}$^{2 }$generally increases with $G$. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V17.00004: Theory of Solvent-Mediated Environmental Effects on Molecular-Scale Transport Michele Kotiuga, Pierre Darancet, Valla Fatemi, Birger Hennings, Michael Frei, Latha Venkataraman, Jeffrey Neaton Single-molecule junctions, formed with well-defined and robust metal-molecule contacts, can provide an ideal model system to study mechanisms of charge transport at the molecular scale. However, the presence of solvent is often unavoidable, and recent experiments have shown that the junction conductance can be altered by a factor of two depending on the solvent present [1]. It has been proposed that the binding of the solvent to the gold electrodes changes their local work function, which in turn alters the conductance of the junction in a predictable manner. Here, we use a first-principles scattering-state approach, based on self-energy corrected density functional theory, to explore the transmission and conductance of bipyridine- and diaminestilbene-Au molecular junctions in the presence of solvent molecules, using an analytical model to compare with experimental results. We acknowledge DOE for support, and NERSC for computational resources. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V17.00005: Tailoring IV Characteristics and Rectification in Single-Molecule Junctions Pierre Darancet, Jonathan Widawsky, Hyoung Joon Choi, Latha Venkataraman, Jeffrey Neaton Asymmetry in the current-voltage characteristics, or current rectification, of nanoscale junctions is a critical property for many optoelectronic and energy conversion applications using nanostructured materials. Here, we compute the conductance, IV characteristics, and bias-dependent rectification of a class of molecular junctions, consisting of donor-acceptor molecules in contact with Au electrodes, using quantitative first-principles calculations [1]. We relate the rectification to the identities of the donor and acceptor moieties through the junction energy level alignment and dipole moments and find, surprisingly, that a large asymmetry in the contact coupling leads to weak rectification. We explain our results with an analytic coherent tunneling model, and suggest concrete strategies for obtaining high rectification in experimentally-achievable systems. [1] Darancet et al. , submitted (2011) . [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V17.00006: Electronic transport properties of functional single molecule junctions E.S. Tam, H.D. Abruna, D.C. Ralph We report experimental studies of single molecule conductance using two techniques: statistical measurements with repeatedly-formed breakjunctions in ambient conditions and low-temperature measurements with electromigrated breakjunctions. In each case, we chose molecules with specific functions. With statistical measurements, we measured the nonconducting open and conducting closed forms of dithienylethene, a photochromic (optically switchable) molecule. These molecules were synthesized with pyridine endgroups to achieve relatively well-defined and stable contacts to Au electrodes. For the closed isomer, we find a conductance of $(3.3 \pm 0.5)$ G$_{0}$, while that of the open isomer is below the noise floor of our measurement. We can therefore set a lower limit of 30 for the on/off ratio of this molecule. We are currently investigating the use of electromigrated graphene nano-constrictions and breakjunctions for spin-polarized single molecule conductance measurements and plan to present initial results for this technique. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V17.00007: Bias-Dependent Noise Measurements in Individual Electromigrated Nanoscale Junctions Patrick Wheeler, Ruoyu Chen, Douglas Natelson Shot noise provides insight into the correlated motion of electrons in nanostructures. Previous measurements have examined shot noise in mechanically controlled break junctions (MCBJs),looking at a large ensemble of junction configurations at room temperature. Electromigrated, lithographically created Au structures at liquid nitrogen and helium temperatures allow for noise measurements of individual junction configurations. High frequency excess noise is amplified by a RF amplifier chain and measured in combination with lock-in techniques simultaneously with the dc conductance. Noise measurements across bias and temperature are compared to previous experiments preformed with MCBJs at room temperature, with an emphasis on the dependence of the noise on bias conditions. We find that measured noise in individual junctions can exhibit nonlinearities with bias as well as asymmetries as a function of the polarity of bias current. Individual junctions can also switch stochastically between different atomic configurations with nearly identical conductances but strikingly different noise properties. We discuss these observations in the context of ``traditional'' shot noise, bias-dependent noise processes, and 1/f contributions to the measured noise. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V17.00008: Shot noise measurements as a function of bias in STM-style gold junctions Ruoyu Chen, Patrick Wheeler, Douglas Natelson Shot noise measurements provide more detailed information regarding conductance channels than transport alone. Because of nearly fully transmitted modes, shot noise in nanoscale junctions is suppressed strongly near certain conductance values. In this experiment, we use a gold tip in a STM-style geometry to make nanoscale junctions, which function as the noise source when under bias. Peaks of conductance histograms and related mean square, radio frequency shot noise are successfully measured simultaneously at room temperature, at a series of voltage biases. Those peaks and related shot noise suppressions appear near integer multiples of the conductance quantum G0, especially the first three. We are able to measure shot noise at biases as low as tens of millivolts, and make use of even lower biases to estimate the systematical background existing in our measurements. Combined with a radio frequency reflection measurement, we convert the measured signal to current noise across the junction. The relevant noise processes and their evolution with bias across the junctions will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V17.00009: Testing the charge transport mechanism in molecule-nanoparticle networks by molecular exchange Venkata Kamalakar, Edwin Devid, Jean-Francois Dayen, Bernard Doudin, Sense Jan van der Molen Nanoparticles molecular networks have recently emerged as useful toolbox for molecular electronics studies. Nanoparticles bridging the size gap between molecules and macroscopic electrical interconnects make possible the realization of large self-assemblies of particles interlinked by molecules, with unique advantage of high reproducibility and robustness. This results from averaging over ensembles of molecules and make possible applications ranging from electrical, optical, mechanical, to spintronics devices. Electronic properties of well-organized two-dimensional networks, where Coulomb blockade regime is expected to be predominant, make these materials of high interest as model systems. We present temperature-dependent transport properties of networks bridging high aspect ratio trenches, making possible measurements over a wide temperature range. We study how the charge transport is modified when reversible molecular exchange is performed, tailoring the intrinsic conductance of the molecule. Quantitative agreement with experiments is obtained using a model clarifying the role played by the intrinsic conduction properties of the molecules and the geometry of the network. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V17.00010: Quantum phase transitions arising from competing electron-electron and electron-phonon interactions in a two-orbital single-molecule junction Lili Deng, Kevin Ingersent, Giselle Luiz, Edson Vernek, Enrique Anda Electron-electron and electron-phonon interactions both play important roles in determining the transport properties of nanostructures such as single-molecule junctions. We use the numerical renormalization group to study a molecule with two active electronic orbitals connecting a pair of metallic leads. We focus on quantum phase transitions (QPTs) that can be accessed by varying couplings to a local vibrational mode, particularly the strength of phonon-assisted tunneling between the two molecular orbitals. One type of QPT arises in situations where, in the absence of electron-phonon interactions, one of the molecular orbitals manifests the many-body Kondo effect with its characteristic zero-bias anomaly in the electrical conductance through the junction. At a critical coupling, the system undergoes a first-order QPT to a low-conductance phase in which the electron-phonon interaction overwhelms the strong bare electron-electron repulsion and Kondo physics is completely destroyed. A second type of first-order QPT is found in cases where there is also a Holstein coupling of local phonons to the molecular charge. We will explain the conditions that give rise to QPTs, as opposed to crossovers, between different ground states of this system. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V17.00011: Motion and Photon Emission of Single Molecules in Space-Time Joonhee Lee, Shawn Perdue, Alejandro Rodriguez Perez, V. Ara Apkarian We have visualized tunneling electron induced motion of single Zn-Etioporphyrin molecules adsorbed on the thin oxide film grown on NiAl(110) surface using scanning tunneling microscopy (STM). When tunneling electrons are injected resonantly to an unoccupied molecular orbital, nearly bistable switching in tunneling current occurs and the molecule starts exhibiting vibronic progression in its photon emission spectra. The switching behavior was spatially mapped by recording time traces on individual pixels of an STM image. We reconstruct the motion using the spatial distribution of amplitude, frequency, and on-time of the switching, and interpret it as the planar hindered rotation shuttling between two different adsorption configurations. The angle of rotation is close to 45 degrees, and the on-time reveals the nature of local potential barrier. Due to the two fold symmetry of the molecules under interrogation, the conductance switching shows different polarities. The electronic excitation of the molecule leads to vibronic transition in which the molecule emits photons and vibrates inside the local potential wells. Considering significant corrugation of oxide surfaces, rotation mediated by quantum tunneling of ethyl groups of the molecule will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V17.00012: Atomic-scale motor driven by the current-induced forces Yu-Chang Chen, Bailey C. Hsu, Allen Tseng From first-principles approaches, we investigate the current-induced forces in an asymmetric molecular junction using Hellmann-Feynman type theorem in the framework of density functional theory in scattering approaches. We observe that it is possible to construct atomic-scale systems where the current-induced forces can be used to rotate the atoms. As an example, we consider a junction formed by the benzene molecule which directly connected to the Pt electrodes, where the benzene molecule is highly tilted. The highly tilted benzene molecule causes the streamline flow of the current to curve considerably to one side of the benzene ring. This could cause a net torque due to the unbalanced current-induced forces, which tend to rotate the benzene molecule in a manner similar to a stream of water rotates a waterwheel. Thus, the highly asymmetric single molecule junctions offer the atomic-scale systems to explore the possibility of nano-motors driven by non-equilibrium electron transport. The authors thank National Science Council (Taiwan) for support under Grant NSC 100-2112-M-012-MY3 [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V17.00013: Capacitance variations of a Nanocapacitor in the Field Emission Regime Carlos Untiedt, Giovanni Saenz-Arce, Jose Ignacio Pascual The electronic transport properties and mechanical forces between two metallic electrodes separated by a nanometer-sized vacuum gap have been studied using a Scanning Tunnelling Microscope combined with a Tuning Fork Force sensor. When applying a voltage difference to the electrodes above their work function energy, the Field Emission regime can be acceded at which Field Emission Resonances take place. Under these circumstances a decrease of the capacitance has been found to occur showing a new mechanism of capacitor leaking in the quantum regime. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V17.00014: Probing Water Structures in Nanopores via Tunneling Paul Boynton, Massimiliano Di Ventra We study the effects of volumetric constraints on the structure and electronic transport properties of distilled water in a synthetic nanopore. Combining classical molecular dynamics simulations with the Landauer approach to scattering theory as originally done in the context of DNA sequencing [1], we develop a relationship between the electronic current and the structure the water assumes in the confining pore-electrode system. Prior research in the field shows a tendency for the tunneling current through water to fluctuate due to local cavities in the water's structure. We show a shift in the tunneling current's dependence on pore diameter at the transition from exclusion of water to a monolayer. Furthermore, we argue that the current with respect to pore diameter does not follow a simple exponential curve at this transition as one would expect from tunneling. This research develops our understanding of water as a complex medium and describes fundamental physics of aqueous solutions. \\[4pt] [1] J. Lagerqvist, M. Zwolak, and M. Di Ventra, \textit{Fast DNA sequencing via transverse electronic transport}, Nano Lett. \textbf{6}, 779 (2006). [Preview Abstract] |
Session V18: Focus Session: Interfaces in Complex Oxides - Interface and Surface Conductivity in SrTiO3
Sponsoring Units: DMPChair: Stefano Gariglio, Universite de Geneve
Room: 252B
Thursday, March 1, 2012 8:00AM - 8:12AM |
V18.00001: Surface charges {\&} adsorbates tune the electron gas at the LaAlO$_{3}$/SrTiO$_{3 }$interface Yanwu Xie, Christopher Bell, Yasuyuki Hikita, Harold Y. Hwang The physical properties of the LaAlO$_{3}$/SrTiO$_{3}$ heterointerface are substantially determined by the density and mobility of the electron gas at the buried interface. Here, we present two ways to tune the electron gas from the LaAlO$_{3}$ surface. First, the electron gas can be locally tuned by using biased atomic force microscope probe [1]. Our studies show that this is caused by the accumulation of charge on the LaAlO$_{3}$ surface [2], and can change the density by up to 3$\times $10$^{13}$ cm$^{-2}$[3], comparable to the maximum modulation achieved in normal field effect devices. Second, we found that the electron gas can be dramatically tuned by the surface adsorption of common polar solvents such as acetone, ethanol and water [4]. This strong surface-interface coupling provides a new technique for manipulating the interface-confined electrons. Most significantly, adsorbates induce an insulator to metal transition when the thickness of the LaAlO$_{3}$ is 3 unit cells, suggesting sensor applications with extremely large sensitivity. \\[4pt] [1] C. Cen \textit{et al.}, \textit{Nature Mater. }\textbf{7}, 298 (2008). \\[0pt] [2] Y. Xie \textit{et al.}, \textit{Nano Lett.} \textbf{10}, 2588 (2010). \\[0pt] [3] Y. Xie \textit{et al.}, \textit{Adv. Mater.} \textbf{23}, 1744(2011). \\[0pt] [4] Y. Xie \textit{et al.}, \textit{Nature Commun.} \textbf{2}, 494 (2011). [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V18.00002: Unexpected Two Dimensional Electron Gas at the LaAlO$_3$/SrTiO$_3$ (110) Interface A. Annadi, X. Wang, K. Gopinadhan, W.M. Lv, Z.Q. Liu, A. Roy Barman, A. Srivastava, S. Saha, S. Dhar, H. Hilgenkamp, T. Venkatesan, - Ariando The observation of two dimensional electron gas (2DEG), superconductivity and magnetism at the interfaces of LaAlO$_3$/SrTiO$_3$ has further amplified the potential of complex oxides for novel electronics. These multifunctional properties are strongly believed to originate from the polarization discontinuity at the interface between the two oxides along (001) direction. In this scenario, the crystal orientation plays an important role and no conductivity would be expected for e.g., the interface between LaAlO$_3$ and (110)-oriented SrTiO$_3$, which should not have a polarization discontinuity. In this talk, we will show the observation of conductivity at the LaAlO$_3$/SrTiO$_3$ interface prepared on (110)-oriented SrTiO$_3$. The strong evidence for 2DEG at the interface of LaAlO$_3$/SrTiO$_3$ (110) is the observed thickness dependence of insulator-metal transition at 3-4 u.c of LaAlO$_3$. Directional dependent electrical transport is observed in resistance versus temperature measurements. We propose some models for explaining the observed phenomena. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V18.00003: Temperature Dependent Strain Relaxation in LaAlO$_{3}$ Thin Films on SrTiO$_{3}$ Substrates Guozhen Liu, Ke Chen, Qingyu Lei, Qun Li, Longqing Chen, Xiaoxing Xi LaAlO$_{3}$/SrTiO$_{3}$ interface has attracted great interest due to discoveries of rich interfacial properties. Strain and strain relaxation in LaAlO$_{3}$ films on SrTiO$_{3}$ substrates directly impact the lattice distortions and defects at the interface and therefore will influence the interfacial properties. Combining grazing incident x-ray diffraction and reciprocal space mapping, we directly measured the in-plane lattice constants of LaAlO$_{3}$ films on SrTiO$_{3}$ with thickness ranging from 4 u.c. to 250 u.c. We found a strong relationship between the strain relaxation behavior and the growth temperature of the LaAlO$_{3}$ films. Cracks were observed when the strain relaxed rapidly, consistent with the fracture theory. The processing temperature-dependent strain relaxation significantly affects the LaAlO$_{3}$/SrTiO$_{3}$ interface properties. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V18.00004: The origin of the 2DEG at oxide polar-non polar interfaces: electron reconstruction versus defect doping Claudia Cantoni, Fabio Miletto Granozio, Carmela Aruta, Jaume Gazquez, Mark Oxley, Andrew Lupini, Maria Varela, Stephen Pennycook, Umberto Scotti di Uccio, Paolo Perna, Daniele Marr\`e In spite of intense research efforts, the microscopic mechanism underlying the formation of a 2DEG at the LaAlO3(LAO)/SrTiO3(STO) interface is still controversial. In particular, there is no consensus on whether the interface charge stems from an electronic reconstruction or from mobile electrons introduced by oxygen vacancies and/or cation doping. We present direct, atomic-column-resolved scanning transmission electron microscopy and electron energy loss measurements in LAO/STO and LaGaO3(LGO)/STO interfaces. We find that both the measured injected charge, the low concentration of point defects, and the lattice polarization are consistent with a model of electronic reconstruction in LAO/STO. On the other hand, LGO/STO interfaces grown in the same conditions show negligible polar lattice distortions and an injected charge incompatible with electronic reconstruction. Conductivity here is explained by the observed cation intermixing and the presence of oxygen vacancies. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V18.00005: Computational study of oxygen vacancy defects in perovskite oxides, SrTiO$_{3}$ and LaAlO$_{3}$ Chandrima Mitra, Chungwei Lin, John Robertson, Alexander Demkov Insulating perovskite oxides, SrTiO$_{3}$ (STO) and LaAlO$_{3}$ (LAO) have attracted a lot of attention due the observed two dimensional electron gas at the interface of these two insulating oxides. Oxygen vacancies, which are easily incorporated in these oxide systems during various processes such as growth, annealing and redox reactions, form an n-type defect in these systems. Hence they are responsible for modifications of various physical properties such as conductivity and optical properties to name a few. As such, there is still a lack of comprehensive theoretical understanding of these important defects in these materials. In this work we present first principles calculations of neutral (V$^{0})$ and charged (V$^{+}$, V$^{++})$ oxygen vacancies in STO and LAO. Density functional theory within the local density approximation proves insufficient in reliably predicting the defect levels due to their well know band gap problem. We therefore employ the Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional to study these defects. Band gaps of 3.01 eV and 5.0 eV are obtained for bulk STO and LAO which are in excellent agreement to experimental values. In STO we find a localized V$^{0}$ defect state at 0.7 eV below the conduction band edge (CBE) while in LAO it is a deep lying level at 2.13 eV below the CBE. A band alignment of the defect levels in these two materials shows that upon stabilizing the singly ionized state, charge transfer could take place from the V$^{+}$ LAO state to the V$^{+}$ STO state which lies at 1.19 eV below the former. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V18.00006: Oxygen vacancy shuttle across the LaAlO3/SrTiO3 interface Evgeny Y. Tsymbal, Tula R. Paudel, Yong Wang, J.D. Burton There have been several recent indications of a switchable piezo-response in oxide materials not associated with ferroelectric polarization. We explore a LaAlO3/SrTiO3 (LAO/STO) heterostructure as a model system to understand this behavior by considering oxygen vacancies as the origin. Using first-principles calculations based on a supercell approach and an adequate electrostatic model, we show that the oxygen vacancy formation energy has two local minima: one at the surface of LAO and another in STO few unit cell below the interface. Due to inbuilt electric field in the system, electrons forming the oxygen vacancies in LAO transfer to the interface leaving behind two holes at the vacancy site irrespective of their spatial position, whereas those in the STO exhibit such a behavior only when laying within a finite distance $\sim $1nm associated with the screening length of the electric field in STO. The two local minima may accommodate oxygen vacancies reversibly when the applied electric field changes its polarity producing a switchable piezo-response behavior in the LAO/STO system. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V18.00007: Electronic phase separation at the LaAlO$_3$/SrTiO$_3$ interface Invited Speaker: - Ariando In the last few years we have seen a number of new properties to emerge at the interface between two insulating oxides. However the origin of these properties remains unclear and needs further investigation. In this talk I will discuss our recent studies on oxide interfaces in which we vary various parameters such as oxygen content and crystal orientation, and use different polar layers and heterostructures. Surprisingly, at the LaAlO$_3$/SrTiO$_3$ interface a remarkable combination of strong diamagnetism (superconductor like), paramagnetism and ferromagnetism can coexist with the quasi two dimensional electron gas (Q2DEG) when prepared under a more oxidizing condition. The ferromagnetic phase is stable even above room temperature and the diamagnetism below a relatively high temperature of 60 K. Our measurements show that the free surface of SrTiO$_3$ may be responsible for all these fascinating phenomena. The phenomena are explained due to the selective occupancy of interface/surface sub-bands of the nearly degenerate Ti orbital in the SrTiO$_3$. On changing the interface orientation, we unexpectedly can also obtain conductivity at the LaAlO$_3$/SrTiO$_3$ interface prepared on (110)-oriented SrTiO$_3$. The conductivity is found to be highly anisotropic, which is a novel feature for these oxide Q2DEGs and of potential interest for applications. I will discuss various possible models that can explain this intriguing observation and also review various interfaces prepared using different polar layers coupled to other functional materials which result in new phenomena. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V18.00008: Effect of cation-nonstoichiometry in the SrTiO$_{3}$ layer on the interfacial conductance of LaAlO$_{3}$/SrTiO$_{3}$ heterostructures Susanne Hoffmann-Eifert, Felix Gunkel, Sebastian Wicklein, Peter Brinks, Mark Huijben, Josee E. Kleibeuker, Gertjan Koster, Guus Rijnders, Regina Dittmann, Rainer Waser A study of the interface conductivity in LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) heterostructures will be presented with focus on the defect chemical state of the SrTiO$_{3}$ close to the interface. SrTiO$_{3}$ layers with a controlled deviation from cation stoichiometry (Sr/Ti=1) were grown to investigate the effect of cation-nonstoichiometry in the SrTiO$_{3}$ layer on the resulting interface conductance of the corresponding LAO/STO heterostructures. The study comprises structural information and electrical properties derived from Hall measurements and high-temperature conductance measurements performed under equilibrium oxygen atmospheres. According to the model of defect chemistry the results give further evidence for the importance of charge compensation through defects on cation lattice sites. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V18.00009: Nature of localized states induced by an oxygen vacancy in SrTiO$_3$ Chungwei Lin, Chandrima Mitra, Alexander Demkov Combining the density function theory and model analysis, we study the nature of localized states induced by an oxygen vacancy (OV) in SrTiO$_3$. The key effect caused by an OV is that it introduces a local mixing between $3z^2-r^2$ and $4s$, $4p$ orbitals of Ti next to the vacancy. According to our analysis, the OV-induced localized state is highly one-dimensional and is mainly composed of Ti $3z^2-r^2$ orbitals along the Ti-OV-Ti axis (defined as the $z$-axis) and Ti $4s$, $4p$ at the OV site. Ti $t_{2g}$ based localized states are ruled out. We suggest the $4s$, $4p$ components of the OV induced localized state should be reflected in the x-ray adsorption spectroscopy when Ti $4s$, $4p$ are served as final states. Our analysis and conclusion apply to general transition metal perovskites. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V18.00010: Oxygen deficiency on the p(1x1) structure of SrTiO$_{3}$ (001) surface Lina Chen, Zhaoliang Liao, Von Braun nascimento, Yi Li, A.C. Antony, E.W. plummer, Jiandi Zhang SrTiO$_{3}$(STO) is one of the most common substractes used for epitaxial film growth of oxide materials. However, the structure and stoichiometry of STO are still unclear even thought they may dictate the interface properties. We have studied the STO(001) surface structure by Low Energy Electron Diffraction(LEED) and angle-resolved X-ray Photoelectron Spectroscopy (AR-XPS). The unreconstructued $p$(1x1) STO(001) surface terminated with TiO$_{2}$ layer is obtained by simple chemical etching which is always accompanied by oxygen deficiency. Both LEED and AR-XPS results suggest that the surface has a considerable oxygen deficiency, but LEED I(V) analysis indicates the existence of more oxygen vacancies than that from AP-XPS results. The structureal analysis indicates a surface polarization due to a surface buckling combined with oxygen deficiency, which is comsistent with recent surface x-ray scattering results [1] with important implications on surface ferroelectric phenomena in STO. The annealing effect of the as-etched surface in oxygen atmosphere on surface oxgen deficiency as well as reconstruction will also be disussed.\\[4pt] [1] R. Herger et al., Phys. Rev. Lett. 98, 076102(2007). [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V18.00011: Strain effects on the electron effective mass in SrTiO$_3$ Anderson Janotti, Daniel Steiauf, Chris G. Van de Walle Strain can greatly affect the electronic properties of thin layers in epitaxial heterostructures, leading to improved transport properties. In SrTiO$_3$-based heterostructures, strain can lift the degeneracy and modify the order of the states near the conduction-band minimum. Using density functional theory with the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE) we study the effects of strain on the energetic ordering and effective mass of the lowest-energy conduction-band states in SrTiO$_3$3. We predict that biaxial stress in the (001) or (110) planes results in the lowest-energy conduction-band state having significantly smaller electron mass in the in-plane directions compared to unstrained SrTiO$_3$, thus suggesting that pseudomorphic growth is a promising route to increasing the electron mobility in epitaxial films. We propose possible substrates that may lead to SrTiO$_3$ films with enhanced electron mobilities, and report deformation potentials that allow accurate prediction of conduction-band splittings for arbitrary strain configurations. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V18.00012: Electrical Transport in SrTiO$_{3}$ Under Biaxial Strain Adam Kajdos, Bharat Jalan, James Allen, Susanne Stemmer Mobility engineering with strain is widely used for conventional semiconductors, but has only recently been proposed for complex oxides such as SrTiO$_{3}$. The conduction band structure of SrTiO$_{3}$ is complicated with multiple degenerate bands derived from the Ti 3d orbitals. Strain is thus expected to have a significant effect by lifting this degeneracy and altering the occupancy and curvature of the bands. Indeed, a 300{\%} increase in the electron mobility with values exceeding 128,000 cm$^{2}$/Vs at 1.8 K was demonstrated in MBE-grown SrTiO$_{3}$ films subjected to \textit{uniaxial} compressive strain [1]. For heterostructure engineering, the effect of \textit{biaxial} strain is relevant. Here, the electron mobilities in SrTiO$_{3}$ subjected to biaxial strain are investigated through growth of coherent films on lattice-mismatched substrates. Lightly-doped (high-mobility) strained SrTiO$_{3}$ films below the critical thickness are insulating because of significant surface depletion, which increases with decreasing temperature due to the high dielectric constant of SrTiO$_{3}$. We show that highly-doped, low-mobility capping layers address this problem, but require a multilayer model to analyze the Hall data in terms of the mobility in the lightly doped layer. [1] B. Jalan et al., Appl. Phys. Lett. \textbf{98}, 132102 (2011) [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V18.00013: The Influence of Oxygen in Transition Metal Oxides P.L. Bach, V. Leboran, F. Rivadulla The existence of a 2D metallic state at the interface between LaAlO$_3$ and SrTiO$_3$ (LAO/STO) has generated much excitement. Its origin has been attributed to charge redistribution to avoid a dielectric catastrophe; however, oxygen vacancies in TiO$_2$-terminated STO can play a significant role in the electronic properties of the interface. In order to determine the nature and origin of the metallic phase, we have induced vacancies in TiO$_2$-terminated STO single crystal substrates by annealing under controlled vacuum conditions. We report resistivity, Hall effect, and thermopower measurements on these materials and discuss their implications for the nature of the 2D electron gas at the STO surface. We have explored the possibility of gate-tuning these systems in order to fabricate single-oxide based devices. This work was supported by the Ministerio de Ciencia e Innovaci\'on (Spain), grant MAT2010-16157, and the European Research Council, grant ERC-2010-StG 259082 2D THERMS. [Preview Abstract] |
Session V19: Invited Session: Holography and Strongly Correlated Electron Matter
Sponsoring Units: DCMPChair: Subir Sachdev, Harvard University
Room: 253AB
Thursday, March 1, 2012 8:00AM - 8:36AM |
V19.00001: Why black holes may be useful for condensed matter physics Invited Speaker: Sean Hartnoll I will give an overview of recent applications of black hole physics to strongly correlated electron systems, via the holographic correspondence. I will briefly review the thermodynamic nature of black hole horizons and explain how black holes exhibit quantum critical dissipation. I proceed to argue that black holes are associated with fractionalized phases of matter and provide a simple setting in which transport and other properties of such phases can be nonperturbatively studied. Black holes furthermore have natural instabilities to superconducting phases. The onset of superconductivity is described in a qualitatively different way than in BCS-derived theories of pairing. The superconductivity emerges from a non-Fermi liquid state of matter, without well-defined quasiparticles, and may suggest emphasis on different classes of observables. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 9:12AM |
V19.00002: Holography and Mottness: A Discrete Marriage Invited Speaker: Philip Phillips Gauge-gravity duality has allowed us to solve the physics of certain strongly coupled quantum mechanical systems using gravity. I will show how a space-time consisting of a charged black hole and a bulk Pauli coupling corresponds to a boundary theory with a dynamically generated gap (with no obvious symmetry breaking) and a massive rearrangement of the spectral weight as in classic Mott systems such as VO$_2$. In this holographic set-up, the gap opens only when discrete scale invariance is present. This raises the possibility that the elusive symmetry that might be broken in Mott insulators, in general, might pertain to scale invariance. The relevance of this claim to recent theories of Mott systems that possess massless charged bosons is explored. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V19.00003: Charge fractionalization and gauge-gravityduality Invited Speaker: Liza Huijse We discuss zero temperature phases of compressible quantum matter, i.e. phases in which the expectation value of a globally conserved $U(1)$ density, $Q$, varies smoothly as a function of parameters. Provided the global $U(1)$ and translational symmetries are unbroken, such phases are expected to have Fermi surfaces, and the Luttinger theorem relates the volumes enclosed by these Fermi surfaces to $\langle Q \rangle $. We distinguish three compressible states: Landau's Fermi Liquid (FL), the fractionalized Fermi Liquid (FL*) and the non-Fermi Liquid (NFL). The motivation for this classification stems from the fact that compressible phases seem to be the rule rather than the exception in theories studied in the context of gauge-gravity duality. We argue that the three compressible phases we identify are indeed present in two paradigmatic supersymmetric gauge-theories underlying the duality. We then describe a gravity theory with an asymptotic electric flux dual to a zero temperature gauge theory at finite chemical potential. The flux can be sourced either by explicit charged matter in the bulk, by an extremal black hole horizon, or both. We argue that these three cases show important similarities with the three compressible states of matter. By tuning a relevant parameter we can study zero temperature phase transitions between the three phases in the dual description. The work I present was done in collaboration with S. Sachdev and S. Hartnoll. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:24AM |
V19.00004: Emergent dimensions and quantum critical points Invited Speaker: Sung-Sik Lee When quantum many-body systems are strongly correlated, the underlying particles may dynamically organize themselves to show novel collective behaviors. Under certain conditions, collective fluctuations behave as if they are living in a space which has one more dimension than the space where the original particles are defined. In this talk, I will discuss about an example where an `external' space emerges at a quantum critical point of a non-abelian gauge theory. A close analogy will be drawn between this phenomenon and fractionalization which can be viewed as an example of emergent `internal' spaces. I will also discuss about non-trivial quantum orders associated with emergent dimensions. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 11:00AM |
V19.00005: Embedding Fermi liquids in string theory Invited Speaker: Steven Gubser Evidence of Fermi surfaces in strongly coupled gauge theories comes from calculations in the gauge-string duality based on the propagation of fermion fields in charged black hole backgrounds. Most of these calculations have been done without reference to explicitly known string theory constructions. When calculations based on the most universal sector of string theory constructions were at last done, no Fermi surface was found. In addition to reviewing the work just described, I will present new work showing that Fermi surfaces do exist in the next-to-simplest string theory constructions. [Preview Abstract] |
Session V20: Invited Session: High k Dielectrics for High Carrier Mobility Channel Applications
Sponsoring Units: FIAPChair: Jueinai Kwo, National Tsing Hua University
Room: 253C
Thursday, March 1, 2012 8:00AM - 8:36AM |
V20.00001: Materials and Device Aspects of III-V 3D Transistors Invited Speaker: Peide Ye Recently, III-V MOSFETs with high drain currents (I$_{ds}>$1mA/$\mu $m) and high transconductances (g$_{m}>$1mS/$\mu $m) have been achieved at sub-micron channel lengths (L$_{ch})$, thanks to the better understanding and significant improvement in high-k/III-V interfaces. However, to realize a III-V FET at beyond 14nm technology node, one major challenge is how to effectively control the short channel effects (SCE). Due to the higher permittivity and lower bandgap of the channel materials, III-V MOSFETs are more susceptible to SCE than its Si counterpart. The scaling of planar devices stops at around 150nm L$_{ch}$. The dramatic increase in DIBL beyond 150nm indicates severe impact from 2D electrostatics. Therefore, the introduction of 3-dimensonal (3D) structures to the fabrication of sub-100nm III-V FETs is necessary. In this talk, we will review the materials and device aspects of III-V 3D transistors developed very recently [1-3]. \\[4pt] [1] Y. Q. Wu \textit{et al}. IEDM Tech. Dig. 331 (2009).\\[0pt] [2] M. Radosavljevic \textit{et al.}, IEDM Tech. Dig. 126 (2010).\\[0pt] [3] J. J. Gu \textit{et al}. IEDM Tech Dig. 2011 (in press). [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 9:12AM |
V20.00002: Pushing the material limit and physics novelty in high $\kappa$'s/high carrier mobility semiconductors for post Si CMOS Invited Speaker: Minghwei Hong The semiconductor industry is now facing unprecedented materials/physics challenges due to the scaling-limitation of Si CMOS transistor arising from non-scaling of matters, namely gate dielectrics and channel mobility. The new technology using high-$\kappa $ plus metal gate on high carrier mobility semiconductors of InGaAs and Ge will lead to faster speed at lower power. The tasks for realizing the new devices equivalent oxide thickness (EOT) $<$ 1 nm, interfacial density of state (D$_{it}) \quad \le $ 10$^{11}$ eV$^{-1}$cm$^{-2}$, self-aligned process, low parasitic, and integration with Si, have been solved or are being feverishly studied. The key of achieving the above goals is to understand/tailor interfaces of the high $\kappa $'s/InGaAs (Ge). Tremendous progress has been made using molecular beam epitaxy (MBE) and atomic layer deposition (ALD) high $\kappa $'s of Ga$_{2}$O$_{3}$(Gd$_{2}$O$_{3})$, Al$_{2}$O$_{3}$, and HfO$_{2}$, and the novel ALD/MBE dual dielectrics in attaining an EOT of 0.5 nm, D$_{it}$ of low 10$^{11}$ eV$^{-1}$cm$^{-2 }$(with a flat D$_{it}$ distribution versus energy), and thermal stability at high temperatures higher than 800\r{ }C of the MOS structures. Electronic/electrical characteristics of the hetero-structures have been studied using in-situ synchrotron radiation photo-emission, cross-sectional scanning tunneling spectroscopy, capacitance (conductance)-voltage under various temperatures, and charge pumping methods. Device performance in world-record drain currents, transconductances, sub-threshold swings, etc. in self-aligned inversion-channel high $\kappa $'s/InGaAs and /Ge MOSFET's will also be presented. This work has been supported by Nano National Program (NSC 100-2120-M-007-010) of the NSC of Taiwan, and the AOARD of the US Air Force. \\[4pt] In collaboration with J. Kwo, W. C. Lee, M. L. Huang, T. D. Lin, Y. C. Chang, Y. H. Chang, C. A. Lin, Y. M. Chang (NTHU and NTU in Taiwan), T. W. Pi, C. H. Hsu (NSRRC in Taiwan), Y. P. Chiu (NSYSU in Taiwan), C. Merckling (IMEC in Belgium), J. I. Chyi (NCU, Taiwan), and G. J. Brown (AFRL, USA). [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V20.00003: Bonding principles of the Passivation Mechanism at III-V -- oxide Interfaces Invited Speaker: John Robertson It has always been much more difficult to make FETs from GaAs than Si, because of `Fermi level pinning' and the difficulty of passivating its surfaces and interfaces. These issues have been discussed from the early days of PCSI by Spicer et al [1] with their ``unified defect model.'' Hasegawa [2] introduced the idea of ``Disorder Induced Gap states'' (DIGS). Since 1997 it has been possible to make inverted MOSFETs on GaAs using the epitaxial Gadolinium gallium oxide [3], but the main impetuous has been since 2003 to use atomic layer dpeosition to make scalable FETs, as recently acheived by Intel [4]. The obvious question is why GaAs is so much more difficult to passivate than Si. The early answer was that the native oxide was poor. But since the advent of good deposited ALD oxides on Si such as HfO2 or Al2O3, this answer is no good, as they should also work on GaAs. They do to an extent, but the interfacial density of states is still too large and the CV p~lots are distorted. The cause of the defects is cannot be due to stress. The reason must be some underlying chemical reason. I show that the reason is the polar nature of bonding in GaAa and other III-Vs, and the driving force to keep the Fermi level in a gap. The electron counting rule of Pashley [5] that describes surface reconstruction is shown to be a variant of auto-compensation, and it is proposed to work more generally, at each layer deposition or growth on GaAs [6]. This leads to a continuous generation of defects if it is not satisfied. So the answer is to deposit oxide layers that meet this rule, and also break up any surface reconstructions that would lead to As-As dimers [6]. \\[4pt] [1] W E Spicer, Phys Rev Lett \textbf{44} 420 (1980)\\[0pt] [2] H Hasegawa, J Vac Sci Technol B 5 1097 (1987)\\[0pt] [3] M Hong et al, Science \textbf{283} 1897 (1997)\\[0pt] [4] M Radosavljevic, et al, IEDM (2009) p13.1\\[0pt] [5] M D Pashley, Phys Rev B \textbf{40} 10481 (1989)\\[0pt] [6] J Robertson, L Lin, App Phys Letts (submitted), App Phys Lett \textbf{98} 082903 (2011) [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:24AM |
V20.00004: Exploring Ge and III-V devices to scale CMOS beyond the Si roadmap Invited Speaker: Marc Heyns There is lots of interest in the use of germanium and III-V compounds as channel material in future CMOS generations. Direct growth of Ge and III-V in Si trenches allows to co-integrate these materials on bulk Si substrates. The formation of antiphase domain boundaries during epitaxy of III-V materials can be avoided by creating double atomic steps at the bottom of the trench through controlled Ge surface profiling. Much effort was dedicated to the electrical passivation of the interface between the high-k dielectric and these materials. Despite its relatively poor stability GeO2-like passivation of Ge has been demonstrated for both pMOS and nMOS devices. Si capping layers of only a few monolayers were used to fabricate short channel Ge pMOS devices with high drive currents. Other successful Ge passivation methods are based on surface treatments such as in situ H2S or wet (NH4)2S. Devices in III-V materials often suffer from Fermi level pinning associated with a high density of defect states near the high-k/III-V interface. These defects can be suppressed by optimized (in-situ) surface treatments and precise control of the oxidation states at the high-k/III-V interface. The Al2O3/ InGaAs interface has been extensively investigated, often concentrating on the possibility to remove Ga and As oxides by exposure to trimethylaluminum (TMA) during atomic layer deposition (ALD), but good passivation has also been demonstrated with various other high-k materials. Since most III-V materials have a low conduction band density of states, the surface potential travels far into the conduction band before the necessary amount of mobile charge is accumulated at the interface. Therefore, the defect density at these energy levels must also be reduced, including border traps in the high-k layer that cause frequency dispersion in the capacitance-voltage behaviour. Finally, it is noteworthy to mention that the introduction of these advanced materials also allows the development of new device concepts, such as Implant-Free Quantum Well devices, heterojunction TunnelFETs and nanowire devices, that can fully exploit the properties of these new materials. [Preview Abstract] |
Session V21: Superconductivity: Mostly Cuprates
Sponsoring Units: DCMPChair: Despina Louca, University of Virginia
Room: 254A
Thursday, March 1, 2012 8:00AM - 8:12AM |
V21.00001: Evidence of strong correlations at the Van Hove singularity in the scanning-tunneling spectra of superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ single crystals Arun Bansil, Jouko Nieminen, Ilpo Suominen, Tanmoy Das, Robert Markiewicz We present realistic multiband calculations of scanning tunneling spectra in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ over a wide doping range. Our modeling incorporates effects of a competing pseudogap and pairing gap as well as effects of strong electronic correlations, which are included by introducing self-energy corrections in the one-particle propagators. The calculations provide a good description of the two-gap features seen in experiments at low energies. In particular, the Van Hove singularity (VHS) in the underlying electronic states is found to split into a prominent incoherent feature at high energies and a weaker coherent part near the Fermi level which is strongly involved in gap formation. The progressive hybridization of the localized VHS into the Fermi surface with increasing doping is suggestive of Kondo physics which has been proposed previously for cuprates and heavy fermion compounds. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V21.00002: Local correlations and deviations of energy scales from SI-STM in Bi-2212 Kyle McElroy, Jacob Alldredge, Kazuhiro Fujita, Genda Gu, Hiroshi Eisaki, S. Uchida One of the key difficulties in understanding the cuprate superconductors is reconciling the many probes that have been used to investigate them. In particular, in one of the most studied materials BSCCO-2212, this comparison is complicated by the striking atomic scale inhomogeneity that is seen in the electronic structure. We will present spectroscopic imaging scanning tunneling microscopy studies which show that the dominate energy scales are tied together not only as a function of doping but locally. This implies a single parameter that allows much of the low energy structure to be predicted. In addition, local structures which cause deviations in the ability of this single parameter to fit the data correspond to other phenomena previously described. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V21.00003: Doping dependent study of non-equilibrium quasiparticle dynamics in the superconducting and normal state of La$_{2-x}$Sr$_{x}$CuO$_{4}$ thin films Fahad Mahmood, Darius Torchinsky, A. Bollinger, I. Bozovic, Nuh Gedik We report on measurements of non-equilibrium quasiparticle dynamics in La$_{2-x}$Sr$_{x}$CuO$_{4}$ thin films via ultrafast optical pump probe spectroscopy. Quasiparticles are excited into the thin films using ultrashort laser pulses. The transient change in the quasiparticle density causes a corresponding change in the reflectivity of the sample, allowing time resolved measurements of quasiparticle recombination. We describe the doping dependence of these measurements both above and below Tc and discuss their implications on the nature of the electron-boson coupling in the cuprates. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V21.00004: Doping-dependence of Scattering Interference in Cuprates Kazuhiro Fujita, Mohammad Hamidian, Ines Firmo, Sourin Mukhopadhyay, Hiroshi Eisaki, Shin-ichi Uchida, J.C. Davis Evolution of the k-space electronic structure from the strongly underdoped to overdoped regime upon doping is studied. At the doping level coincident with the disappearance of the pseudogap phenomenology a crossover in the scattering processes is identified [1]. We discuss the significance of such transition in conjunction with other experimental facts. \\[4pt] [1] K. Fujita, M. Hamidian, I, A. Firmo, S. Mukhopadhyay, H. Eisaki, S. Uchida, and J. C. Davis \textit{Submitted} (2011). [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V21.00005: Time resolved reflectivity study of electron and hole doped cuprate superconductors James Hinton, Jake Koralek, Guichuan Yu, Mun Chan, Neven Barisic, Xudong Zhao, Martin Greven, Joseph Orenstein We study both the hole-doped cuprate HgBa$_{2}$CuO$_{4+\delta}$ (Hg1201) and the electron-doped cuprate Nd$_{2-x}$Ce$_{x}$CuO$_{4+\delta}$ (NCCO) using time-resolved optical reflectivity. In this technique, the sample is excited using an ultrashort 800 nm laser pulse, and the induced change in reflectivity is measured as a function of time after excitation. We focus on the temperature range above the superconducting transition; the fluctuating antiferromagnetic phase in NCCO and the pseudogap phase in Hg1201. In both cases we compare our results directly to neutron experiments, highlighting the relationship of the ultrafast optical response to magnetically ordered phases. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V21.00006: Magnetic, Structural and Resistivity Characterization of New R$_{3}$Gd$_{3}$Ba$_{2}$Ca$_{2}$Cu$_{7}$O$_{10}$:Li Compounds (R= rare earth) C. Aguilar, E.E. Marinero, E. Chavira, L. Bucio, H. Pfeiffer We report on the synthesis of R$_{3}$Gd$_{3}$Ba$_{2}$Ca$_{2}$Cu$_{7}$O$_{10}$:Li compounds with different rare earth elements (R=~Dy, Ho) by solid state reaction. Reaction products during the synthesis were identified by XRD and their thermal stability measured by Thermo-Gravimetric Analysis (TGA). Rietveld refinement of the XRD spectra enabled us to identify two coupled phases: a tetragonal one and an orthorhombic component. Magnetic measurements were conducted in the range of 5 to 300K to determine their Curie point and evidence for superconducting transitions. We find that in spite of the fact that both the Dy and Ho containing samples have the same crystalline structure, they exhibit different magnetic behavior over the temperature range studied. Evidence for a critical temperature at 15K for the Dy containing sample is observed exhibiting the characteristic Ms vs. T dependence ascribed to superconducting behavior. Detailed structural characterization results and measurements of R vs. T will be presented to confirm that the behavior observed at 15K is evidence for superconducting behavior. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V21.00007: External vs. ``internal'' pressure effect on the anti-ferromagnetic superexchange energy, $J$, in LnBa$_2$Cu$_3$O$_6$ (Ln=La,Nd,...,Lu) Ben Mallett, Jeffery Tallon, Grant Williams, Thomas Wolf What causes the difference between the effect of ``internal'' pressure, as caused by ionic substitution, and external pressure on $T_c^{max}$ in the cuprates [1]? Is it the density of states, the pairing boson energy scale ($\omega_B$), condensation energy (which governs fluctuations), or ...? Many models of high temperature superconductivity put the energy scale of $\omega_B$ as the anti-ferromagnetic super-exchange energy, $J$, between adjacent Cu(2) ions in the CuO$_2$ plane. We therefore investigated Raman $B_{1g}$ two-magnon scattering in high quality LnBa$_2$Cu$_3$O$_6$ (Ln123) single crystals, Ln(=La, Nd, Sm, Eu, Gd, Dy, Yb, Lu), at ambient pressure to determine the effect of internal pressure on $J$. Comparing with measurements of $J$ under external pressure reveals that internal and external pressure have \emph{quantitatively} the same effect on $J$. However, and most surprisingly, we find an anticorrelation between $J$ and $T_c^{max}$ when ion size or internal pressure is the implicit variable. Given the opposite effects of internal and external pressure on $T_c^{max}$, this result suggests that some energy scale other than short range anti-ferromagnetic interactions has a more dominant effect on $T_c^{max}$.\\[4pt] [1] e.g. M. Marezio, Physica C, 341-348, 375 (2000) [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V21.00008: Anomalous High-Energy Spin Excitations in the High-$T_c$ Superconductor Parent Antiferromagnet La$_2$CuO$_4$ Stephen Hayden, Neil Headings, Radu Coldea, Toby Perring Inelastic neutron scattering is used to investigate the collective magnetic excitations of the high-temperature superconductor parent antiferromagnet La$_2$CuO$_4$. We find that while the lower energy excitations are well described by spin-wave theory, including one- and two-magnon scattering processes, the high-energy spin waves are strongly damped near the (1/2,0) position in reciprocal space and merge into a momentum dependent continuum. This anomalous damping indicates the decay of spin waves into other excitations, possibly unbound spinon pairs. The spinon dispersion that can explain the observed spin-wave damping in our experiments has the same form as the pseudogap dispersion observed in superconducting cuprates, with a minimum at (1/4,1/4). [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V21.00009: Charge transfer at the interface between ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ and superconducting EuBa$_{2}$Cu$_{3}$O$_{7}$ probed by STM/STS Yinghao Liu, Jie Xiong, Jason Haraldsen(2), Li Yan, Alexander Balatsky(2), Quanxi Jia, Antoinette Taylor, Dmitry Yarotski La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) is a ferromagnetic half-metallic compound with nearly 100{\%} spin polarization at room temperature, making it an ideal candidate for applications in spintronic devices. However, this useful functionality disappears when the thickness of LSMO film grown on SrTiO$_{3 }$substrate is reduced to below 4 nm, limiting its application in nanoscale devices. Here, we show that metallic and ferromagnetic properties of ultrathin ($<$ 4nm) LSMO film can be restored by interfacing it with a superconductor EuBa$_{2}$Cu$_{3}$O$_{7- \delta }$ (EBCO). We use scanning tunneling microscopy and spectroscopy to probe the evolution of the electronic structure of LSMO film grown on EBCO as functions of LSMO layer thickness and aging of bilayer LSMO/EBCO. Our results reveal that the charge (hole) transfer at LSMO/EBCO interface is responsible for driving LSMO film (of only five-unit-cell thickness) to metallic state. The conductive behavior of aged LSMO/EBCO bilayers varies systematically with the thickness of LSMO layer, allowing us to estimate the charge-transfer depth to be 4$\sim $5 nm on the LSMO side. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V21.00010: Muon spin rotation investigation of the pressure effect on the magnetic penetration depth in YBa$_2$Cu$_3$O$_x$ Alexander Maisuradze, Alexander Shengelaya, Alex Amato, Ekaterina Pomjakushina, Hugo Keller The pressure dependence of the magnetic penetration depth $\lambda$ in polycrystalline samples of YBa$_2$Cu$_3$O$_x$ with different oxygen concentrations $x$ = 6.45, 6.6, 6.8, and 6.98 was studied by muon spin rotation ($\mu$SR). The pressure dependence of the superfluid density $\rho_s \propto 1/\lambda^{2}$ as a function of the superconducting transition temperature T$_{\rm c}$ is found to deviate from the usual Uemura line. The ratio $(\partial T_{\rm c}/\partial P)/(\partial\rho_s/\partial P)$ is factor of $\simeq$ 2 smaller than that of the Uemura relation. In underdoped samples, the zero temperature superconducting gap $\Delta_0$ and the BCS ratio $\Delta_0/k_{B}T_{\rm c}$ both increase with increasing external hydrostatic pressure, implying an increase of the coupling strength with pressure. The relation between the pressure effect and the oxygen isotope effect on $\lambda$ is also discussed. In order to analyze reliably the $\mu$SR spectra of samples with strong magnetic moments in a pressure cell, a special model was developed and applied. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V21.00011: Ultrafast spectroscopy of the stripe phase of underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ thin films Darius Torchinsky, Fahad Mahmood, A. Bollinger, I. Bozovic, Nuh Gedik We have performed ultrafast measurements on high-T$_c$ thin films of underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$. In these experiments, femtosecond pulses create photoexcitations derived from both the superconducting and striped phase fraction whose evolution and decay are probed as a function of time. We discuss the separate dynamics of these two electronic components as a function of temperature and excitation density. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V21.00012: Nonlinear Response of the High-Temperature Superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ to the Transmission of Intense Terahertz Pulses Iwao Kawayama, Andreas Glossner, Caihong Zhang, Shinya Kikuta, Hironaru Murakami, Paul M\"uller, Masayoshi Tonouchi High-Power Terahertz Time-Domain Spectroscopy (THz-TDS) was used to examine YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ thin films when transmitted by intense single-cycle THz pulses. This allowed for an investigation of the nonlinear, time-resolved behavior of YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ in the presence of strong THz electric fields for the first time. High field strengths of tens of kV cm$^{-1}$ were achieved by improving the efficiency of optical rectification in LiNbO$_{3}$ through the tilted-pulse-front method and by ensuring a tight focusing of the THz beam. In the case of low field strengths, the behavior of the thin films agrees with previous examinations of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ by means of conventional, low-power THz-TDS. However, for strong THz electric fields, it was found by analysis with the two-fluid model that the superfluid population decreases dramatically, possibly due to Cooper pair breakup. This was accompanied by a drop in the imaginary part of the conductivity in the investigated frequency range of 0.2 to 0.8 THz. The results further suggest a decrease of the effective mass of the carriers for strong THz fields. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V21.00013: Carrier mobility of insulating cuprates from time-resolved terahertz photoconductivity measurements J. Steven Dodge, Jesse C. Petersen, Amir D. Farahani, Ruixing Liang, Ivan Bozovic We study the ultrafast transient photoconductivity of three insulating cuprates, Sr$_2$CuO$_2$Cl$_2$, YBa$_2$Cu$_3$O$_6$, and La$_2$CuO$_4$. Terahertz spectroscopy reveals mobile dilute carriers, with a conductivity that appears promptly and decays non-exponentially in picoseconds. The peak photoconductivity is orders of magnitude larger than the static photoconductivity but smaller than that of chemically doped samples, with fast dynamics that depends weakly on material, concentration, and temperature. Decay dynamics indicate hopping transport with a low activation energy, suggesting weak to intermediate polaronic binding. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V21.00014: Study of the radiation line width and shape from the Bi2212 mesa structure Takanari Kashiwagi, Takashi Yamamoto, Kazuya Ishida, Manabu Tsujimoto, Kaveh Delfanazari, Ryo Nakayama, Takeo Kitamura, Masashi Sawamura, Hidehiro Asai, Hidetoshi Minami, Kazuo Kadowaki Continuous electromagnetic waves in terahertz (THz) range have been observed from mesa structures of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$(Bi2212) single crystals$^{1)}$ It has been established that the radiation frequency is determined by both the ac Josephson frequency and the resonance condition of the geometrical cavity$^{2)}$. In order to understand the mechanism of the radiation from the intrinsic Josephson junctions (IJJs) in Bi2212, we studied the radiation line width and shape. These might depend upon the physical parameters of the Bi2212 single crystal such as the number of IJJs the fluctuations of the quasiparticles and pairs, the non-linearity and non-equilibrium conditions, and the stability of the electrical circuit including the IJJs. However, technical difficulties have been delayed the line width measurements in the THz range. Recently, we succeeded in measuring the radiation line width using a frequency mixer. These results will be compared to those from a single junction 1) L. Ozyuzer \textit{et al.}, Science \textbf{318} (2007) 1291., 2) K. Kadowaki \textit{et al.}, J. Phys. Soc. Jpn. \textbf{79} (2010) 023703 [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V21.00015: High resolution copper M-Edge resonant inelastic X-ray scattering at MERLIN Lewis Wray, Yi-De Chuang The Advanced Light Source MERLIN RIXS spectrometer (beamline 4.0.3) is a new user endstation for resonant inelastic X-ray scattering at the Advanced Light Source. I will discuss new scientific explorations that are expected to be possible with the system, including preliminary data obtained during the commissioning period on cuprate samples. Because very few previous studies have been performed at the M-Edge, the presentation will review how M-Edge scattering data at MERLIN resemble and differ from the more broadly studied transition metal K- and L-edges, including critical parameters of the excitation process such as fitted intermediate state lifetimes, X-ray penetration depth, spin-orbit coupling and scattering intensity for different types of excitation mode. [Preview Abstract] |
Session V22: Focus Session: Fe-based Superconductors - Magnetism and Anisotropy
Sponsoring Units: DMP DCOMPChair: Rafael Fernandes, Columbia University
Room: 254B
Thursday, March 1, 2012 8:00AM - 8:12AM |
V22.00001: In Plane Resistivity Anisotropy in iron Chalcogenides Jiun-Haw Chu, Chris Kucharczyk, Ian Fisher FeTe suffers a bicolinear antiferromagnetic ordering, with a ($\frac{1}{2}$ 0) ordering wave-vector, in contrast to the ($\frac{1}{2}$ $\frac{1}{2}$) ordering wave-vector found in underdoped ``122'' and ``1111'' iron pnictides. At the optimal doping the static ($\frac{1}{2}$ 0) order disappears and a spin resonance at the ($\frac{1}{2}$ $\frac{1}{2}$) wave-vector emerges. Here we report measurements of the in-plane resistivity anisotropy of single crystals of Fe$_{1+\delta}$Te$_{1-x}$Se$_x$ for underdoped and optimally doped compositions. The underdoped compounds were partially detwinned by applying uni-axial strain along the ($\frac{1}{2}$ 0), revealing a larger resistivity along the antiferromagnetic ordering direction. However, for optimal doping uni-axial strain induces the largest resistivity anisotropy along the ($\frac{1}{2}$ $\frac{1}{2}$) direction, similar to the ``122'' family of compounds. This behaviour suggests that in addition to the presence of spin resonance, a divergent nematic susceptibility might be a key feature associated with optimal doping in iron based superconductors. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V22.00002: The temperature dependence of the conductance anisotropy of the pnictides studied via Monte Carlo simulation of the Spin-Fermion model Shuhua Liang, Cengiz Sen, Adriana Moreo, Elbio Dagotto, Gonzalo Alvarez The undoped three-orbital ($xz$, $yz$, $xy$) spin-fermion model for the pnictides is studied via Monte Carlo (MC) simulations [1], using both the standard exact diagonalization of the fermionic sector (supplemented by cooling down procedures) developed in the manganite context [2], as well as the truncated polynomial expansion techniques [3]. The magnetic order is found to be the expected $(\pi,0)$ wavevector and the angle-resolved photoemission signal is in good agreement with experiments. The low-temperature conductance reveals the experimentally observed anisotropy between the ferromagnetic and the antiferromagnetic (AFM) directions, with the largest conductance for the AFM case, similarly as observed in recent investigations using the Hartree-Fock approximation to the Hubbard model [4]. The finite temperature MC analysis also produces results in good agreement with transport experiments. [1] S. Liang, G. Alvarez, C. Sen, A. Moreo, and E. Dagotto, submitted for publication. [2] C. Sen, S. Liang, and E. Dagotto, arXiv: 1109.1797 ; and references therein. [3] C. Sen, G. Alvarez, Y. Motome, N. Furukawa, I. A. Sergienko, T. Schulthess, A. Moreo, and E. Dagotto. Phys. Rev. B 73, 224430 (2006). [4] X. Zhang and E. Dagotto, Phys. Rev. B 84 132505 (2011). [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V22.00003: Anisotropy in BaFe$_{2}$Se$_{3}$ single crystals with double chains of FeSe tetrahedra Hechang Lei, Hyejin Ryu, Anatoly Frenkel, Cedomir Petrovic Since two-dimensional (2D) FePn or FeCh (Pn = pnictogens, Ch = chalcogens) tetrahedron layers are the common structural ingredient in all iron based superconductors, they are probably related to high temperature superconductivity. In order to fully understand the nature of iron-based superconductivity, study of materials containing similar FePn or FeCh tetrahedron as building blocks is of significant interest. BaFe$_{2}$Se$_{3}$ contains one-dimensional (1D) double chains of edge shared Fe-Se tetrahedra along the b-axis, in contrast to iron chalcogenide superconductors which feature two-dimensional (2D) Fe-Se planes. We report the anisotropic physical properties and local crystal structure of Ba$_{1.00(4)}$Fe$_{1.9(1)}$Se$_{3.1(1)}$ single crystals. It shows that BaFe$_{2}$Se$_{3}$ is a semiconductor with a short-range AFM correlation at the room temperature and a long-range AFM order below 255 K. Composition analysis indicates that all crystallographic sites are fully occupied. X-ray absorption near edge structure (XANES) result shows that the valence of Fe is about 1.87+. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 9:12AM |
V22.00004: Towards an understanding of magnetic interactions and anisotropies in iron superconductors Invited Speaker: Elena Bascones The itinerant or strong coupling origin of magnetism in iron pnictides is still unsettled. The localized description generally assumes AF exchange constants satisfying $2J_2>J_1$, with$J_1$ and $J_2$ referring to first and second nearest neighbors respectively. The itinerant picture relies on the nesting of the Fermi surface. Both descriptions reproduce the columnar ordering found experimentally. The role played by the Hund's coupling $J_H$ and the orbital degree of freedom are also highly debated. Orbital ordering has been invoked to explain the anisotropic resistivity and optical conductivity. We make connection between these two pictures by studying the same five-orbital model within Heisenberg and mean field descriptions [1]. We have found that $J_2/J_1$ strongly depends on the charge and orbital filling what results in an unexpected sensitivity of the AF ordering to crystal field parameters. $J_1$ and $J_2$ can become ferromagnetic at large $J_H$. Consistent results are obtained in the mean field description. We also analyze the resistivity and optical conductivity anisotropies and show that they are a consequence of magnetism and not of orbital ordering [2].\\[4pt] [1] M.J. Calder\'on et al, arXiv:1107.2279. E. Bascones et al, PRL 104, 227201 (2010).\\[0pt] [2] B. Valenzuela et al, PRL 105, 207202 (2010). [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V22.00005: Nematic order and its implications to the iron pnictides: pseudogap behavior, orbital order, and magneto-structural phase diagram R.M. Fernandes, A.V. Chubukov, J. Knolle, I. Eremin, J. Schmalian We present an electronic model for the emergence of nematic order in the iron pnictides. In particular, we show that the degeneracy of the magnetic ground state, allied to spin fluctuations, gives rise to a state that spontaneously breaks the tetragonal symmetry of the system, but preserves its spin-rotational symmetry. The nematic state displays several anisotropic properties, inducing a weak orbital polarization as well as a small orthorhombic distortion of the lattice. Nematic order also enhances the magnetic fluctuations and facilitates the magnetic phase transition, leading to a joint magnetic and meta-nematic transition, as well as to a pseudogap behavior due to magnetic precursors. Finally, we discuss the characters of the magnetic and structural phase transitions, showing that electron doping tends to split both transitions, whereas pressure and lattice softness tend to make them simultaneous. Our model provides a simple framework to understand the interplay between the different degrees of freedom present in the iron pnictides, shedding light on the primary role played by magnetism in these materials. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V22.00006: Impurity-Induced Electronic Nematic State in Iron-Pnictide Superconductors Youichi Yamakawa, Yoshio Inoue, Hiroshi Kontani We propose that impurity-induced electronic nematic state is realized above the orthorhombic structure transition temperature $T_S$ in iron-pnictide superconductors [1]. In the presence of strong orbital fluctuations near $T_S$, it is theoretically revealed that a single impurity induces non-local orbital order with $C_2$-symmetry, consistently with recent STM/STS measurements. Each impurity-induced $C_2$ orbital order aligns along a-axis by applying tiny uniaxial pressure along b-axis. In this impurity-induced nematic phase, the resistivity shows sizable in-plane anisotropy ($\rho_b/\rho_a \sim 2$) even above $T_S$, actually observed in various ``detwinned" samples. The present study indicates the existence of strong orbital fluctuations in iron-pnictide superconductors. [1] Y. Inoue, Y. Yamakawa and H. Kontani, arXiv:1110.2401. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V22.00007: Effect of uniaxial strain on the structural and magnetic phase transitions in BaFe$_2$As$_2$ Chetan Dhital, Z. Yamani, Wei Tian, J. Zeretsky, A.S. Sefat, Ziqiang Wang, R.J. Birgeneau, Stephen Wilson We report neutron scattering experiments probing the influence of uniaxial strain on both the magnetic and structural order parameters in the parent bilayer iron pnictide compound BaFe$_2$As$_2$. Under the application of modest strain fields along the in-plane orthorhombic b-axis, we observe an upward shift in the onset of both the structural and magnetic phase transition temperatures. Our data show that modest strain fields can affect significant changes in phase behavior simultaneous to the removal of structural twinning effects. As a result, we demonstrate in BaFe$_2$As$_2$ samples detwinned via uniaxial strain that the in-plane C$_4$ symmetry is broken by both the structural lattice distortion and long-range spin ordering at temperatures far above the nominal, strain-free, phase transition temperatures. The relevance of our measurements to earlier transport measurements in detwinned crystals of BaFe$_2$As$_2$ is discussed. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V22.00008: Anisotropic transport properties of the paramagnetic state of iron-based superconductors Joerg Schmalian, Elihu Abrahams, Rafael Fernandes Recent experiments in detwinned iron-pnictide samples have revealed strong anisotropies in the in-plane transport properties of the paramagnetic state. Since these anisotropies cannot be attributed solely to the small orthorhombic distortion of the lattice, it has been proposed that an underlying anisotropic electronic order is at play. One of the candidates is the Ising-nematic order that emerges due to the degeneracy of the magnetic ground state. Here we present a microscopic model for the transport properties of this nematic phase, considering both the elastic scattering by impurities as well as the inelastic scattering by anisotropic spin fluctuations. We show that the interference between these two scattering channels give rise to anisotropic non-Fermi liquid transport properties. In particular, we explain the observed sign of the resistivity anisotropy in electron-doped samples and predict a sign-change for sufficiently hole-doped samples. We also address the suppression of the resistivity anisotropy with sample annealing, as well as its dependence on alkaline-earth substitution. Finally, we discuss the predictions of our model to the thermopower anisotropy. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V22.00009: Anisotropic magnetoelastic coupling in iron arsenide superconductors: an x-ray diffraction study in high magnetic field J.P.C. Ruff, R.K. Das, Z. Islam, J.-H. Chu, H.-H. Kuo, I.R. Fisher, H. Nojiri We report high-resolution single crystal x-ray diffraction measurements of underdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ in pulsed magnetic fields as high as 28 Tesla. Our direct measurements confirm earlier reports of strong and highly anisotropic magnetoelastic coupling in iron arsenides. We observe magnetic field induced de-twinning of orthorhombic samples, and characterize the magnitude of the effect as a function of temperature and field. We identify a range of field and temperature where samples can be 100\% de-twinned by magnetic fields less than 30 Tesla. The effect shows a notable insensitivity to SDW ordering, but varies rapidly in the vicinity of the superconducting transition. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V22.00010: In-plane structural and electronic anisotropy in de-twinned (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ Erick Blomberg, M.A. Tanatar, W.E. Straszheim, B. Shen, H.H. Wen, R. Prozorov The iron-pnictides undergo a tetragonal to orthorhombic structural transition below a doping - dependent temperature $T_s$. In the absence of external stress or strain, the orthorhombic phase is divided into four degenerate, equally populated, ``twin'' structural domains, obscuring direct measurement of in-plane anisotropy. This degeneracy may be broken through mild mechanical stress or strain leaving the sample de-twinned. The properties of detwinned (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ with x=0.1, 0.18 (hole under-doped) were discussed previously [1]. Here we report polarized-light microscopy and AC transport measurements of strain-detwinned (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ with a dopping range from x=0.15 to x=0.35. Our results provide new insight into a region of coexisting magnetic and superconducting order parameters. \\[4pt] [1] J. J. Ying, et al. Phys. Rev. Lett. \textbf{107} 067001 (2011). [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V22.00011: Nematic phase persisting above the superconducting dome of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ S. Kasahara, H.J. Shi, K. Hashimoto, T. Shibauchi, T. Terashima, Y. Matsuda, T. Fukuda, K. Sugimoto, A.H. Nevidomskyy Strongly interacting electrons can exhibit novel collective phases, among which the electronic nematic phases are perhaps the most surprising as they spontaneously break rotational symmetry of the underlying crystal lattice. Here, we provide the first thermodynamic evidence in pure crystals of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ that the nematicity develops well above the structural transition and persists to the nonmagnetic superconducting regime, resulting in a new phase diagram strikingly similar to the pseudogap phase diagram in the cuprates. Our highly sensitive magnetic anisotropy measurements using microcantilever torque-magnetometry under in-plane field rotation reveal pronounced two-fold oscillations, which break the tetragonal symmetry. Combined with complementary high-resolution synchrotron X-ray and resistivity measurements, our results consistently identify two distinct temperatures---one at $T^{\ast}$, signifying a true nematic transition, and the other at $T_s (< T^{\ast})$, which we show to be not a true phase transition, but rather what we refer to as a ``meta-nematic transition'', in analogy to the well-known metamagnetic transition in the theory of magnetism. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V22.00012: Unusual giant negative thermal expansion in La-doped CaFe$_{2}$As$_{2}$ superconducting single crystal Alwyn Rebello, John J. Neumeier, Zhaoshun Gao, Yanpeng Qi, Yanwei Ma Large negative thermal expansion (NTE), wherein a material substantially shrinks on heating, is a phenomenon that occurs only in very rare materials.\footnote{T. A. Mary et al. Science, \textbf{272}, 90 (1996)}$^,$\footnote{G. D. Barrera et al. J. Phys.: Condens. Matter, \textbf{17}, R217 (2005)} Here we present results on anisotropic and unusually large NTE in single crystalline Ca$_{0.8}$La$_{0.2}$Fe$_{2}$As$_{2}$ (CLFA), a recently discovered high temperature superconducting material. The volume thermal expansion coefficient in CLFA remains negative over the entire measured temperature range and reaches a maximum of $\Omega=-90\times10^{-6}$ K$^{-1}$ near 65 K, which is remarkably large compared to the thermal expansion (TE) of most other materials. Furthermore, we do not observe signatures of any structural transition in the linear TE in the $a$, $b$ and $c$ axes. Our results on TE and heat capacity behavior in the vicinity of superconducting transition temperature ($T_{C}=42.7$ K) indicate non-bulk superconductivity in the sample. The observed large NTE in our sample is attributed to anomalous transverse modes which may vibrate in a quartic potential as in ScF$_{3}$.\footnote{C. W. Li et al. Phys. Rev. Lett., \textbf{107}, 195504 (2011)} [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V22.00013: Lower critical field, anisotropy, and two-gap features of LiFeAs Kalyan Sasmal, B. Lv, Z. Tang, F. Wei, Y. Xue, A. Guloy, Ching-Wu Chu The magnetic properties of LiFeAs, as single crystalline and polycrystalline samples, were investigated. The lower critical field deduced from the vortex penetration of two single crystals appears to be almost isotropic with a temperature dependence closer to that of two-gap superconductors. The parameters extracted from the reversible magnetizations of sintered polycrystalline samples are in good agreement with those from the single-crystal data. [Preview Abstract] |
Session V23: Superconductivity: Mostly Transport
Sponsoring Units: DCMPChair: Wankyu Park, UI-urbana champaign
Room: 255
Thursday, March 1, 2012 8:00AM - 8:12AM |
V23.00001: Thermal Conductivity Data Near T$_{c}$ as a Probe of Unconventional Superconductivity Brian Sales In classical phonon-mediated superconductors, the thermal conductivity, kappa, just below T$_{c}$ decreases since electrons in the superfluid state carry no heat. A decrease in kappa occurs just below T$_{c}$ even for BCS superconductors with relatively high transition temperatures such as Nb$_{3}$Sn and MgB$_{2}$. By contrast, in all of the unconventional superconductors, of which the author is aware, the thermal conductivity always increases just below T$_{c}$. It is hypothesized in these unconventional superconductors that magnetic or some other type of fluctuation that scatters phonons above T$_{c}$ are frozen out below T$_{c}$, resulting in a larger kappa. This suggests that a relatively simple thermal conductivity measurement may be a way of identifying superconductors with an unusual pairing mechanism. Although this phenomenology does not help identify new superconductors, it may help determine which materials have the potential for a much higher T$_{c}$. Several related examples of the effects of magnetic fluctuations or excitations on thermal transport also will be presented. This research was supported by the Materials Sciences and Engineering Division, Office of Science, U. S. Department of Energy. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V23.00002: Critical currents in thin-film superconductors via two-coil mutual inductance measurements John Draskovic, Jie Yong, Michael Hinton, Adam Ahmed, Song Wang, Stanley Steers, Thomas Lemberger Following Claassen et al.(RSI 1991), we determine the critical current density, $J_c(T)$, in a superconducting thin film by measuring the inductive coupling between two coils located on opposite sides of the film as a function of temperature. For several values of the AC drive magnetic field, we record the temperature at which inductive coupling between the coils jumps dramatically. The peak current density in the film at such temperature is computed by numerical simulation. For thin niobium films, we obtain $J_c(0)$ values greater than 10 percent of the Ginzburg-Landau (GL) prediction with the correct temperature dependence. This suggests the possibility of practical measurement of the GL coherence length in thin films without need for the strong magnetic fields used to measure $B_{c2}$. Application of this technique to cuprates and pnictides will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V23.00003: Anomalous dependence of interface superconductivity on carrier density in metal-insulator bilayer Jie Wu, Oshri Pelleg, Gennady Logvenov, Anthony Bollinger, Greg Boebinger, Ivan Bozovic The interface superconductivity is of particular interest for its higher superconducting temperature (Tc) and close connection to the HTS study. However, despite of the experimental and theoretical progress, the answer to the following key question still remains ambiguous: can the underlying mechanism of the interface superconductivity be interpreted solely in terms of charge transfer and Sr interdiffusion? One experimental approach to answer this question is to study the dependence of Tc on the carrier density of La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$ (LSCO/LCO) bilayer by changing the doping level in the metallic LSCO layer. The charge transfer and cation interdiffusion at the interface is proportional to the carrier density in LSCO layer; therefore, Tc of the interface is expected to show a significant and non-monotonic dependence on the carrier density. To exam this prediction, we synthesized a series of La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$ bilayers with x ranging in 0.27 to 0.47 by atomic layer-by-layer molecular beam epitaxy (ALL-MBE). The uniqueness of these samples is a 4{\%} Sr doping gradient of the central doping level across the 10mm width of the substrate. These combinatorial samples realize an extremely fine tuning of doping and enable us to construct a phase diagram that is 30 times denser than a normal method can achieve. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V23.00004: Magnetotransport of La$_{(2-x)}$Sr$_{x}$CuO$_{4}$ with Nearly Continuous Doping Zachary Stegen, Jie Wu, Jonathon Kemper, Greg Boebinger, Scott Riggs, Fedor Balakirev, Albert Migliori, Ivan Bozovic Hall resistivity and longitudinal magnetoresistance were measured in fields up to 35 T. The samples were grown using Combinatorial Molecular Beam Epitaxy (COMBE) where the strontium doping changes continuously across the sample. Thirty simultaneous transport measurements were taken on a sample from a single growth, which allows for unprecedented resolution in doping ($\Delta p \approx 0.0002 $). A series of these measurements were performed with the goal of examining the phase diagram of the hole-doped cuprates, particularly an increase in the Hall number around optimum doping. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V23.00005: Decoupling of superconducting planes of La$_{1.905}$Ba$_{0.095}$CuO$_4$ in a transverse magnetic field John Tranquada, Z. Stegen, G.S. Boebinger, Su Jung Han, Jie Wu, Zhijun Xu, Genda Gu, Qiang Li We have measured the resistivity parallel and perpendicular to the CuO$_2$ planes in single crystals of La$_{1.905}$Ba$_{0.095}$CuO$_4$ for magnetic fields up to 35~T applied along the $c$-axis. Below the zero-field superconducting transition temperature of 32 K, we observe that, above a threshold field, the $c$-axis resistivity grows with field, eventually reaching a maximum and then decreasing. At the resistivity maximum, interlayer pair tunneling becomes insignificant. Under the same field and temperature conditions, the in-plane resistivity remains quite low, reflecting robust superconductivity. We identify a regime in which the superconducting planes are effectively decoupled. At 20~K, a field much greater than 35~T would be required to destroy the in-plane pairing, despite the fact that the field also induces both charge and spin stripe order (J.S. Wen {\it et al.}, arXiv:1009.0031). [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V23.00006: Spatial Inhomogeneity in Oxygen Modulated Potassium Tungsten Oxide Thin Films: implications for superconductivity and metal-insulator transitions Ko Munakata, Katherine Luna, Akio Tsukada, Subhash Risbud, Theodore Geballe, Malcolm Beasley High quality potassium tungsten oxide (K0.33 WOy) films were synthesized by pulsed laser deposition followed by annealing in vacuum. Oxygen concentration modulated anomalous correlation of metal-insulator and superconductivity transitions were studied; a similar scenario was recently suggested in the literature [1] for polycrystalline rubidium tungsten oxide samples. Detailed studies of the transport properties below and above the superconducting transition temperature exhibit a diversity of unexpected behavior. Some of our results can be interpreted as a signature of reduced dimensionality in the ab-plane in oxygen-rich insulating samples, implying a formation of spatially inhomogeneous electronic structure. We compare such phenomenology to the behavior of other materials with strong electron-phonon interactions, and discuss its implication for the possible high temperature superconducting anomaly in sodium tungsten oxides reported in the literature [2]. [1] D. C. Ling et. al., J. Phys. Conf. Ser. 150, 052141 (2009). [2] S. Reich, and Y. Tsabba, Eur. Phys. J. B 9, 1 (1999). [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V23.00007: Thermal Hall effect in YBCO: Probing Fermi-surface reconstruction inside the superconducting state Olivier Cyr-Choini\`{e}re, Francis Lalibert\'{e}, Sophie Dufour-Beaus\'{e}jour, Ga\"{e}l Grissonnanche, Ryan T. Gordon, Nicolas Doiron-Leyraud, Louis Taillefer, Brad J. Ramshaw, Ruixing Liang, Doug A. Bonn, Walter N. Hardy, Cyril Proust The thermal Hall (Righi-Leduc) effect was measured in the cuprate superconductor YBCO at a doping $p$ = 0.11, as a function of magnetic field $H$ up to 29 T. At temperatures well below the zero-field superconducting $T_{c}$, the thermal Hall conductivity \textit{$\kappa $}$_{xy}$ is positive at low field and then turns over to become negative at fields above 15 T. The negative \textit{$\kappa $}$_{xy}$ is consistent with the negative Hall and Seebeck coefficients observed in the normal state above 25 T [1,2]. This further supports our interpretation: the Fermi surface of YBCO contains a small electron-like pocket [3] in that region of the phase diagram, the result of a Fermi-surface reconstruction attributed to stripe order [4]. In the $T $= 0 limit at $H$ = 29 T, we find reasonable agreement with the Wiedemann-Franz law, \textit{$\kappa $}$_{xy}$/$T=L_{0}$\textit{$\sigma $}$_{xy}$. The fact that \textit{$\kappa $}$_{xy}$ changes sign at $H \quad \approx $ 15 T is consistent with a scenario of phase competition whereby stripe order emerges only at finite field, in agreement with recent NMR studies that detect the onset of charge-stripe order above 15 T [5]. \\[4pt] [1] LeBoeuf \textit{et al}., \textit{PRB} \textbf{83}, 054056 (2011); [2] Lalibert\'{e} \textit{et al}., \textit{Nat. Commun.} \textbf{2}, 432 (2011); [3] LeBoeuf \textit{et al}., \textit{Nature} \textbf{450}, 533 (2007); [4] Chang \textit{et al}., \textit{PRL} \textbf{104}, 057005 (2010); [5] Wu \textit{et al., Nature }\textbf{477}$, $191 (2011). [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V23.00008: On the magnetic tricritical point in BaFe$_{2(1-x)}$Co$_{2x}$As$_{2}$ Costel Rotundu, Robert Birgeneau We present here high resolution magnetization measurements on high-quality BaFe$_{2(1-x)}$Co$_{2x}$As$_{2}$, 0$\leq$x$\leq$0.046 as-grown single crystals. The results confirm the existence of a magnetic tricritical point in the ($x$,$T$) plane at x$^{m}_{tr}$$\approx$0.022 [1,2]. We show that the extrapolated T$_{c}$ onset doping could be close to the magnetic tricritical point x$^{m}_{tr}$. It has been speculated that the magnetic critical point is relevant to the superconductivity in this series [3]. Finally, we comment on the universality of the tricritical point in the superconducting 122s.\\[4pt] [1] M. G. Kim \emph{et al.}, Phys. Rev. B {\bf 83}, 134522 (2011).\\[0pt] [2] C. R. Rotundu and R. J. Birgeneau, Phys. Rev. B {\bf 84}, 092501 (2011).\\[0pt] [3] G. Giovannetti \emph{et al.}, Nature Communications {\bf 2}, article number 398 (2011). [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V23.00009: $Rb_xFe_2Se_2$: A study of superconductivity under high pressure Melissa Gooch, Bing Lv, Liangzi Deng, Takaki Muramatsu, Jim Meen, Yuyi Xue, Bernd Lorenz, Ching-Wu Chu Superconductivity was reported in $Rb_xFe_2Se_2$ in early 2011, after the discovery of similar Fe-based chalcogenides, $K_xFe_2Se_2$ and $Cs_xFe_2Se_2$ in December 2010. These Fe-based chalcogenides have similar structures to the iron pnictides, with a superconducting transition of approximately 30 K. Here we report the results of the physical characterization and the subsequent high-pressure study on two samples with slightly different compositions of $Rb_xFe_2Se_2$. From resistivity measurements, $Rb_{0.93(2)}Fe_{1.70(2)}Se_2$ (sample A) was found to be superconducting and $Rb_{0.90(1)}Fe_{1.78(1)}Se_2$ (sample B) was found to be semiconducting. Further sample characterization was conducted through magnetic and thermoelectric power measurements, which support the initial resistivity findings. High pressure resistivity measurements were conducted with a BeCu clamp cell up to approximately 1.8 GPa. Initially, the Tc of sample A increases slightly until p reaches $\sim$ 1GPa. However, at further increasing pressure, $T_c$ starts to decrease and superconductivity is completely suppressed at about 6 GPa. Sample B was not found to be superconducting up to 1.8 GPa. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V23.00010: Unusual superconducting state in Pr-doped CaFe$_{2}$As$_{2}$ Liangzi Deng, Bing Lv, Melissa Gooch, Fengyan Wei, Yanyi Sun, James Meen, Yuyi Xue, Bernd Lorenz, Ching-Wu Chu We report the detection of unusual superconductivity up to 49 K in Pr-doped single crystalline CaFe$_{2}$As$_{2}$ as evidenced from the resistive, magnetic and thermoelectric measurements. This superconducting transition observed suggests the possible existence of two phases: a field as low as 500 Oe can totally suppress the observed diamagnetic susceptibility above 21K. The 49 K part has a low critical field $<$ 4 Oe, and the other at 21K, with a high upper critical field $>$ 5T. Our observations are in strong contrast to previous reports of doping or pressurizing layered compounds AeFe$_{2}$As$_{2}$ (Ae122), where Ae = Ca, Sr, or Ba. In Ae 122, hole-doping has been previously observed to generate superconductivity with a transition temperature (Tc) only up to 38 K and pressurization has been reported to produce superconductivity with a Tc up to 30 K. The experiment results of resistivity and inductance measurements of Pr-doped Ca122 under pressure up to 2.0 GPa will also be discussed. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V23.00011: Fermi-liquid behavior of quasiparticle scattering in the normal state of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ Ying Jia, Lei Fang, Ulrich Welp, Alexei Koshelev, George Crabtree, Wai-Kwong Kwok We present studies of the galvanomagnetic effects of compensated BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ (x=0.32$\sim $0.6) superconductors. The magnetoresistance follows the relaxed Kohler's scaling for all doping levels. Using a two-band model, we quantitatively extracted the scattering parameter m*/$\tau $ and the carrier density of the electron and hole bands. The temperature dependence of the carrier concentration reveals the semimetal properties of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$. The Fermi-liquid behavior, m*/$\tau \sim $T$^{2}$, is observed from optimal doped x=0.32 to over-doped x=0.6 crystals, suggesting that the proximity of the SDW state does not play an important role in transport. Our analysis suggests that the normal state transport properties of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ can be well understood in the framework of a compensated two-band Fermi-liquid semimetal. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V23.00012: Pressure effect on the electronic transport properties of Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ Stevan Arsenijevi\'{c}, Richard Ga\'{a}l, Henrik R{\O}nnow, Romain Viennois, Enrico Giannini, Dirk van der Marel, L\'{a}szl\'{o} Forr\'{o} We present a systematic study of electronic transport as function of pressure up to 25 kbar of Fe$_{+y}$Te$_{1-x}$Se$_{x}$ single crystalline samples (with $y=0.02$, 0.05, and $x=0$, 0.2, and 0.3). Pressure is demonstrated to be a clean control parameter to drive the system with high Fe-excess through the metal-insulator (MIT) transition, in analogy with increasing the Se-doping or reducing the Fe-excess. The scaling of resistivity $\rho (T, p)$ below 50 K identified a critical pressure of $p_{c}=8$ kbar which separates non-metallic and metallic temperature dependences. At the $p_{c}$ the low-temperature sheet resistance is in the 6.5 k$\Omega$/square range. The Seebeck coefficient ($S$) at $p_{c}$ changes sign from negative to positive indicating a change in the electronic structure and in the balance between the electron and hole carriers. The $S$ at the highest pressure exhibits low positive values similar to the metallic, superconducting cuprates. The critical MIT behavior, related to a quantum phase transition, indicates a universality of the Fe- and Cu-based high-$T_{c}$ superconductors. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V23.00013: DC transport properties of Fe(Se,Te) crystals: Effects of Se substitution into Te site Seiki Komiya, Masafumi Hanawa, Ichiro Tsukada, Atsutaka Maeda FeTe is an antiferromagnetic semimetal and superconductivity shows up when Te is substituted with Se, but effects of Se substitution are still unclear. To elucidate this issue, we study the behavior of Hall resistance in detail using single crystalline FeSe$_x$Te$_{1-x}$ with $x = 0$ to 0.4. Single crystals are grown by Bridgman method, and Hall resistance is analyzed within the standard 2-carrier model. We find that the electron density $n_e$ is larger than the hole density $n_h$ for $x \leq 0.3$ samples, but $n_h$ becomes greater than $n_e$ for $x = 0.4$ crystal. We also find that electron mobility enhances especially at low temperatures with Se substitution. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V23.00014: Ferroelectricity and magnetoelectric coupling in underdoped La$_{2}$CuO$_{4+x}$ Z. Viskadourakis, I. Radulov, A.P. Petrovic, S. Mukherjee, B. Andersen, G. Jelbert, N.S. Headings, S.M. Hayden, K. Kiefer, S. Landsgesell, D.N. Argyriou, C. Panagopoulos La$_{2}$CuO$_{4}$ is an archetypal antiferromagnetic Mott insulator. Upon oxygen doping, a wide range groundstates may be accessed, including glassy magnetic phases and high-T$_{c}$ superconductivity. However, determining the nature of the charge correlations coexistent with magnetic order has remained elusive, particularly in the highly underdoped limit. In this study, we show that the first holes added to La$_{2}$CuO$_{4}$ drive the formation of a ferroelectric phase below 4.5K, with slow charge fluctuations developing below 40K. We invoke the formation of polar nanoregions -- which are a natural consequence of non-stoichiometric oxygen doping -- to explain the emergent ferroelectricity. An anisotropic magnetoelectric coupling is observed and attributed to the Dzyaloshinskii-Moriya interaction. Although this interaction is not responsible for the electronic ordering (unlike in other multiferroic perovskites), the presence of weak magnetoelectricity allows us to confirm charge carrier doping as the cause of ferroelectricity in La$_{2}$CuO$_{4+x}$. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V23.00015: Surface pinning effects in Ag doped superconducting polycrystalline Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-x }$ Atilgan Altinkok, Murat Olutas, Kivilcim Kilic, Atilla Kilic The current-voltage measurements ($I $--$V$ curves) with different current sweep rates (d$I$/d$t)$ were carried out to investigate the effects of silver on the flux dynamics in Ag doped superconducting polycrystalline Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-x }$sample (YBCO/Ag). Standard and reverse procedures were used in the measurements of$ I $-- $V$ curves. In the standard procedure, the dc driving current is cycled up and down; whereas, in the reverse procedure, the current is first cycled down and, then, cycled up. The reverse procedure enables us to investigate the flux motion evolving from the outer surface of the sample to its interior. Upon cycling transport current, the $I $-- $V $curves of the YBCO/Ag sample exhibit hysteresis effects for both procedures and are sensitive to the variation of d$I$/d$t$. The experimental data reveal that the irreversibilities in the $I $-- $V $curves of undoped YBCO are more prominent than those of YBCO/Ag. One of the main observations in the $I $-- $V$ curves of YBCO/Ag is the peculiar voltage jumps and drops. It was found that these instabilities depend strongly on the magnitude of external parameters d$I$/d$t$ and $H$. The instabilities and short and long lived plateau regions observed in $I$ -- $V$ curves were explained in terms of plastic flow of flux lines along easy motion channels which are considered mainly as metallic silver paths in the YBCO/Ag sample. In order to understand better the flux dynamics evolving in YBCO/Ag, the results of present measurements were compared to our previous studies on polycrystalline samples of undoped YBCO and MgB$_{2}$. [Preview Abstract] |
Session V24: Low-Dimensional Semiconductors
Sponsoring Units: FIAPChair: Enrico Bellotti, Boston University
Room: 256
Thursday, March 1, 2012 8:00AM - 8:12AM |
V24.00001: 1/f Noise in Delta Doped GaAs/AlGaAs Heterostructures Yun Suk Eo, Steven Wolgast, Cagliyan Kurdak, L. N. Pfeiffer, K. W. West We studied $1/f$ noise of a two-dimensional electron gases (2DEG) in $\delta$-doped $GaAs/Al_{x}Ga_{1-x}As$ heterostructures. Three samples that we measured were identical except for the $\delta$-doping concentration: $9.1\times10^{18}(cm^{-2})$(high), $1.3\times10^{18}(cm^{-2})$(medium), $0.3\times10^{18}(cm^{-2})$(low). These $\delta$-doping layers are located in the $Al_{x}Ga_{1-x}As$ region, $800\AA$ above the $GaAs$ and $Al_{x}Ga_{1-x}As$ interface. We fabricated Corbino and Hall bar structures with different sizes. Carrier density was varied by the persistent photoconductivity effect at low temperature (4.2K). Initially, the samples did not exhibit measurable $1/f$ noise. The high $\delta$-doping concentration samples exhibited parallel conduction. As we increased the carrier concentration in the high and medium-doped samples, $1/f$ noised increased initially, but disappeared as the photo current was saturated. The low-doped samples did not exhibit $1/f$ noise as the carrier concentration was increased. We conclude that $1/f$ noise is caused by the remote ionized impurities in the $\delta$-doped region. Also, changing the DX-center configuration changes the density of the ionized impurities, which then changes the magnitude of $1/f$ noise. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V24.00002: Conductance noise in a strongly disordered 2D electron system in Si MOSFETs Ping V. Lin, Xiaoyan Shi, Jan Jaroszy\'nski, Dragana Popovi\'c Studies have shown that the metal-insulator transition (MIT) in a 2DES in Si MOSFETs is closely related to the glassy freezing of electrons as temperature $T\rightarrow0$. However, no glassy relaxations were seen after a $T$ quench. Here we first study the effect of cooling in detail: the carrier density $n_s$ is changed at a high $T\approx 20$~K, the system is then cooled to a desired $T$ with a fixed $n_s$, and fluctuations of conductance $G$ with time are measured. The analysis of the noise power spectra $S_G \propto 1/f^{\alpha}$ gives evidence for the onset of slow, glassy dynamics near the MIT as $T\rightarrow0$, supporting conclusions obtained with a different protocol. While these noise measurements were done in the Ohmic regime, we have also explored the effect of high excitation voltages, deep into the nonlinear regime, on the noise statistics. The results will be discussed in detail. Finally, we show that sweeping $n_s$ at low enough $T$ results in a reproducible fluctuation pattern of $G(n_s)$. Such a pattern, which reflects a particular realization of disorder, does not change even after warming up to 30~K. This demonstrates that the disorder does not change at low $T$, and that the observed non-Gaussianity of the noise reflects the intrinsic glassiness of the 2DES. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V24.00003: Extended States and Critical Behavior in 2D and 3D Amorphous Conductors Donald Priour With a tight binding treatment for one, two and three dimensional systems, we calculate and analyze electronic states in a conductor with topological disorder, or no correlations among the positions of the hopping sites. The Inverse Participation Ratio (IPR) is used to characterize carrier wave functions with respect to localization. We consider an exponentially decaying hopping integral with range (or ``Bohr radius'') $l$. Using two complementary finite size scaling techniques to extrapolate to the bulk limit (both methods exploit critical behavior in different ways to locate the energy marking the boundary between extended and localized wave functions) which nevertheless yield identical results, we obtain phase diagrams showing regions where states are extended and domains of localized states. We find exclusively localized wave functions for 1D geometries, irrespective of $l$. In the 2D case, states are localized below a threshold length scale $l_{c} \approx 1.0$, with a finite fraction of states extended for $l > l_{c}$. For 3D systems, the extended phase is flanked by regions of localized states and bounded by two mobility edges. The swath of extended states, broad for $l \sim 1$, becomes narrower with decreasing $l$, scaling asymptotically as $e^{-A/l}$ for $l \ll 1$. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V24.00004: The disappearance of weak localization in a strongly correlated 2D hole system Richard L.J. Qiu, Xuan P.A. Gao, Loren N. Pfeiffer, Ken W. West The origin of Metal-Insulator Transition (MIT) in strongly correlated two-dimensional (2D) electrons or holes in semiconductors has long been of great interest. We will present the low temperature (down to 0.05K) transport properties of 10nm GaAs quantum wells with metallic hole densities near the critical point of the 2D MIT (critical density p$_{c }\approx $ 0.8 $\times $ 10$^{10}$cm$^{-2})$. We found that 2D holes exhibit a negative magneto-resistance in small perpendicular magnetic fields, which has been attributed to weak localization in the literature. On the other hand, the magnitude of this negative magneto-resistance is much smaller than conventional 2D weak localization. What is more surprising is that the weak localization induced negative magneto-resistance peak around B=0 becomes weaker at lower temperature, suggesting there is a mechanism causing quasiparticles to lose coherence as temperature decreases. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V24.00005: Strong negative magnetoresistance in high-mobility 2D electron systems Michael Zudov, Anthony Hatke, John Reno, Loren Pfeiffer, Ken West This talk reports on a remarkably strong negative magnetoresistance effect in high mobility GaAs/AlGaAs heterostructures and quantum wells. The effect is the strongest at about 1 kG where a deep and strongly temperature dependent minimum is observed. At low temperature, the resistivity at this minimum is a small fraction of the zero field resistivity. The talk will discuss the effects of temperature and in-plane magnetic field on this negative magnetoresistance and compare experimental findings with existing theories. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF Cooperative Agreement No. DMR-0654118, by the State of Florida, and by the DOE and at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. The work at Minnesota was supported by the NSF Grant No. DMR-0548014 and by the DOE Grant No. DE-SC002567. The work at Princeton was partially funded by the Gordon and Betty Moore Foundation and the NSF MRSEC Program through the Princeton Center for Complex Materials (DMR-0819860) and the work at Sandia was supported by the Sandia Corporation under Contract No. DE-AC04-94AL85000. Sandia National Laboratories is a multi-program laboratory managed. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V24.00006: Nonlinear response of magnetoplasmon resonance Anthony Hatke, Michael Zudov, Michael Manfra Magnetoplasmon resonances have been observed in microwave photoresistance of Hall bar-shaped two-dimensional electron systems more than two decateds ago. This talk will report on such a resonance in a very high mobility two-dimensional electron gas where it appears as a distinct photoresistivity peak superimposed on a microwave-induced zero-resistance state. In particular, we will discuss the response of this peak to dc electric field. A portion of this work was performed at the National High Magnetic Field Laboratory which is supported by NSF Cooperative Agreement No. DMR-0654118, by the State of Florida, and the DOE. The work at Minnesota was supported by DOE Grant No. DE-SC002567 and NSF Grant No. DMR-0548014. The work at Purdue was supported by DOE grant de-sc0006671. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V24.00007: Interference effect in magneto-oscillations in two-dimensional system under bichromatic irradiation Qianhui Shi, Maxim Khodas, Michael Zudov We report on the studies of magneto-oscillations caused by bichromatic irradiation in perpendicular magnetic field. Using quantum kinetic method, we have studied the case of $\omega$, $2\omega$, and the case of $\omega$, $3\omega$ frequencies. In the latter case the interference part is shown to produce a phase-sensitive contribution. Also, the final answer depends on the polarizations of the two waves. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V24.00008: Quantum lifetime of 2D electron in magnetic field Scott Dietrich, Sergey Vitkalov, Dmitry Dmitriev, Alexey Bykov The lifetime of two dimensional electrons in GaAs quantum wells, placed in weak quantizing magnetic fields, is measured using a simple transport method in broad range of temperatures from 0.3 K to 20 K. The temperature variations of the electron lifetime are found to be in good agreement with conventional theory of electron-electron scattering in 2D systems. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V24.00009: Hall viscosity in lattice models Rudro Biswas, Taylor Hughes We present results from our investigations of the Hall viscosity in lattice models. We use the insight gained from these calculations to comment on using the Hall viscosity to characterize gapped quantum fluids. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V24.00010: Rank Saturation in finite size Entanglement Spectrum for Quantum Hall states Benoit Estienne, Bogdan A. Bernevig, Raoul Santachiara We investigate analytically in finite size the entanglement spectrum arising from real-space cuts for fractional quantum Hall states. We provide a proof that the rank of the reduced density matrix is saturated for the Laughlin state even on finite sizes. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V24.00011: Magnetic field spectra and many-body correlations of spin 3/2 holes confined to GaAs quantum well George Simion, Yuli Lyanda-Geller We consider two dimensional spin $3/2$ hole liquid in the presence of a perpendicular magnetic field. Single particle states of Luttinger Hamiltonian are calculated. For the semiclassical limit, the Hamiltonian is separated into the time-symmetric part treated exactly and time-antisymmetric part treated perturbatively. The angular momentum (spin) $3/2$ states are characterized by the Landau level index and parity with respect to reflection about the growth direction. The single-particle spectrum exhibits level-crossings as magnetic field is varied, with or without Rashba and Dresselhaus interactions. The numerical calculations were performed for infinite barrier well, finite size barrier, well doped on one side, symmetrically doped well and parabolic well. Cyclotron mass was calculated and its dependence of the type of structure, magnetic field and symmetry of states is discussed and compared with experimental values. Land\`{e} effective factor is defined and evaluated. Shubnikov-de Haas oscillations are calculated. Electron-electron interactions are accounted using (time-dependent) mean field theories. An interesting effect is a single-well non-homogeneous spin-texture state. Possible implications of shape of hole spectra for fractional quantum Hall states are explored. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V24.00012: Tunneling spectroscopy of 5/2 fractional quantum Hall excitations in etch defined quantum point contacts Madhu Thalakulam, Wei Pan, K.W. Baldwin, K.W. West, L. Pfeiffer Ever since its discovery the fractional quantum Hall (FQH) state at the even denominator filling fraction v=5/2 has generated immense interests among researchers. 5/2 FQH excitations are believed to obey non-Abelian statistics and posses topological properties making them an ideal candidate for the proposed fault tolerant topological quantum computation. Theoretical proposals to characterize the topological properties of the5/2 state are usually based on confined geometries. In this work we report the characterization of the 5/2 state using quasiparticle tunneling experiments in quantum point contacts (QPC). We have successfully fabricated QPCs on high mobility GaAs/AlGaAs heterostructures using conventional photolithography followed by etching and evaporation of Cr/Au depletion gates. Our samples show very stable FQH plateaus at v = 7/3, 5/2 and 8/3 filling fractions. Tunneling experiments are performed in the QPCs at various temperatures and also at various pinch-off voltages to characterize the effective charge and Coulomb interaction parameters of the quasiparticles. (Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000). [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V24.00013: Quasiparticles and excitons for the Pfaffian quantum Hall state Antoine Sterdyniak, Ivan Rodriguez, Maria Hermanns, Joost Slingerland, Nicolas Regnault We propose trial wave functions for quasiparticle and exciton excitations of the Moore-Read Pfaffian fractional quantum Hall states, both for bosons and for fermions, and study these numerically. Our construction of trial wave functions employs a picture of the bosonic Moore-Read state as a symmetrized double layer composite fermion state. We obtain the number of independent angular momentum multiplets of quasiparticle and exciton trial states for systems of up to $20$ electrons. We find that the counting for quasielectrons at large angular momentum on the sphere matches that expected from the CFT which describes the Moore-Read state's boundary theory. In particular, the counting for quasielectrons is the same as for quasiholes, in accordance with the idea that the CFT describing both sides of the FQH plateau should be the same. We also show that our trial wave functions have good overlaps with exact wave functions obtained using various interactions, including second Landau level Coulomb interactions and the $3$-body delta interaction for which the Pfaffian states and their quasiholes are exact ground states. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V24.00014: Exact results for a fermion chain with fractionalized excitations Masaaki Nakamura, Zheng-Yuan Wang, Emil Bergholtz We present a number of exact results for a fermion chain with center of mass conservation at $1/3$ filling. The ground state of our model, which is three-fold degenerate even in the case of site dependent interactions, show striking similarities with the superconducting BCS wave functions, the AKLT spin chain, and in particular, with the Laughlin state describing the fractional quantum Hall effect. This state supports exact zero modes with fractional charge slightly below $1/3$ filling, and it has a matrix product representation which enables us to analytically calculate correlation functions, excitation gaps, and the entanglement spectrum. Reference: M. Nakamura, Z.-Y. Wang and E. J. Bergholtz, arXiv:1110.5033 [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V24.00015: Polaronic effects in a single modulation doped GaAs quantum well Gerard Martinez, Clement Faugeras, Milan Orlita, A. Riedel, R. Rey, Klaus Friedland Absolute magneto-optical transmission measurements have been performed in the far-infra-red range under magnetic fields up to 32 T and at a temperature of 1.8 K on a single modulation doped GaAs quantum well (QW) with a width dw = 13 nm. This QW is sandwiched between two GaAs/AlAs superlattices, the whole epilayer being lift-off from the GaAs substrate and deposited on a wedged Si substrate. The carrier concentration Ns= 3.8 10$^{11}$ cm$^{-2}$ and has a mobility exceeding 10$^{6}$ cm$^{2}$/V/sec at low temperatures. Due to the absence of the GaAs substrate, the magneto-transmission of the sample, mainly governed by the cyclotron (CR) absorption line, can be followed continuously over the whole range of energies. It reveals a strong polaronic interaction with the LO GaAs-phonon: the results can be interpreted quantitatively using the FHIP model [1] and the related conductivity response [2]. \\[4pt] [1] R.P. Feynman, R.W. Hellwarth, C.K. Iddings and P.M. Platzman, Phys. Rev., 127 , 1004 (1962. \\[0pt] [2] F.M. Peeters and J.T. Devreese, Phys. Rev. B, \textbf{28}, 6051 (1983). [Preview Abstract] |
Session V25: High Pressure: Experiment
Sponsoring Units: DCOMP DCMPChair: Anatoly Belonoshko, Royal Institute of Technology
Room: 257A
Thursday, March 1, 2012 8:00AM - 8:12AM |
V25.00001: Kinetics studies across the phase transition of metals using \textit{dynamic}-DAC Jing-Yin Chen, William Evans We utilize the time-resolved synchrotron x-ray diffraction and in-situ optical spectroscopy to study the dynamic properties of several metals across the phase transition under different compression rates. The dynamic properties of metals across the pressure-induced phase transition, for example the mechanism of solidification or solid-solid transitions, are lacking. Obtaining the time-resolved structural evolutions of metals under rapid compression is critical to understanding material stability or metastability, and transition mechanism. In addition, the dynamical pressure changes can dramatically influence the microstructure and even phase boundaries, further affecting the properties of metals, such as toughness and strength. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V25.00002: Phase Transition Kinetics of GST at High Pressures Adam Cadien, Qing Yang Hu, Howard Sheng The phase change material Ge2-Sb2-Te5 (GST) undergoes an amorphous to ordered phase transition in nanoseconds. This ultra-fast phase transition kinetics in conjunction with drastic electrical property changes makes GST an ideal candidate for next generation optical and digital storage media. The origins of this fast transition have thus far been traced back to the atomic configurations of both the amorphous and ordered phases. However the precise configurations of both the ordered and amorphous phases are still unknown with advances from simulations constantly altering our understanding of these structures. To discover the structural dependence of the phase transition kinetics of GST, we have performed in-situ synchrotron X-ray diffraction experiments on GST using a new Hydrothermal Diamond Anvil Cell under a range of pressures and observed new patterns of phase transition of GST at elevated temperatures. Ab initio simulations have been performed to interpret the structural evolutions of GST phases at different pressures, with an emphasis on the role of vacancies in the phase transition. Our results provide new insight into the mechanism of the fast phase transition kinetics of GST. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V25.00003: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V25.00004: Pressure-induced phase transitions in GeS under high pressures Ranga Dias, Choong-Shik Yoo We have studied the pressure-induced structural and electronic phase transitions of layered GeS (\textit{Pnma}) to 30 GPa, using micro-Raman spectroscopy and electrical resistivity measurements in diamond anvil cells. The result shows a steady decrease in resistivity to that of metal at around 18 GPa. The visual appearance of GeS supports the insulator-metal transition: initially black GeS becomes opaque and eventually reflective with increasing pressure. The Raman result indicates that the metallization is preceded by a structural phase transition, presumably to the previously predicted \textit{Cmcm} structure. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V25.00005: High pressure behavior of Cr$_2$O$_3$ to 62~GPa Krystle Catalli, Hyunchae Cynn, William J. Evans Corundum-structured oxides are of interest for a broad range of reasons, including their mineralogical occurrences and technological uses. The high pressure behavior of Cr$_2$O$_3$ is of particular interest due to the widespread use of ruby, (Al,Cr)$_2$O$_3$, as a pressure standard in diamond anvil cells experiments. Although there have been a number of high pressure studies on Cr$_2$O$_3$, discrepancies still exist among the different data sets. Here we present synchrotron X-ray diffraction data on the structure and compressional behavior of Cr$_2$O$_3$ to 62~GPa. Although no change in crystal structure is detected within the resolution of the measurements, a change in compressional behavior occurs near 30~GPa where Cr$_2$O$_3$ changes color from red to orange. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V25.00006: Amorphous diamond -- A high-pressure superhard carbon allotrope Yu Lin, Li Zhang, Ho-kwang Mao, Paul Chow, Yuming Xiao, Maria Baldini, Jinfu Shu, Wendy Mao Compressing glassy carbon above 40 GPa, we have observed a new carbon allotrope with a fully \textit{sp}$^{3}$-bonded amorphous structure and diamond-like strength. Synchrotron x-ray Raman spectroscopy revealed a continuous pressure-induced \textit{sp}$^{2}$-to-\textit{sp}$^{3}$ bonding change, while x-ray diffraction confirmed the perseverance of non-crystallinity. The transition was reversible upon releasing pressure. Used as an indenter, the glassy carbon ball demonstrated exceptional strength by reaching 130 GPa with a confining pressure of 60 GPa. Such an extremely large stress difference of $>$70 GPa has never been observed in any material besides diamond, indicating the high hardness of this high-pressure carbon allotrope. The nanoscale transmission x-ray microscopy is being utilized for accurate pressure-volume determination of glassy carbon and its high-pressure phase. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V25.00007: Comparison of the existing internally consistent pressure scales at high pressures and high temperatures Hyunchae Cynn, B.J. Baer, S.G. MacLeod, W.J. Evans, M.J. Lipp, J.P. Klepeis, Zs. Jenei, J.Y. Chen, K. Catalli, D. Popov, C.Y. Park There have been several efforts to determine internally consistent pressure scales for static diamond anvil high pressure study. We decide to extend the choice of pressure scales to include W and Cu. A recent study of Cu claims that electronic theory can constrain cold curve and possibly room temperature isotherm (Greeff et al., 2006, JPCS). We will present our comparison of 6 different pressure scales in regards with the suggested Cu EOS. We have measured angle-dispersive x-ray diffraction of Au, Pt, W, Cu, Ne, and NaCl to directly compare with the current existing EOS. We will also discuss discrepancies in the precise determination of pressure of phase transformations. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V25.00008: Simultaneous measurement of pressure evolution of crystal structure and superconductivity in Fese$_{0.8 }$ using designer diamonds Walter Uhoya, Nathaniel Wolanyk, Georgiy Tsoi, Yogesh Vohra, Sistla M Rao, Mau-Kuen Wu, Samuel Weir Simultaneous high pressure x-ray diffraction and electrical resistance measurements have been carried out on (P4/nmm) PbO type $\alpha $-FeSe$_{0.89}$ compound to a pressure of 44 GPa and at low temperatures down to 4 K using a synchrotron source and designer diamond anvils technique. At ambient temperature, a structural phase transition from the tetragonal (I4/nmm) phase to orthorhombic (Pbnm) is observed at 11 GPa and persist up to 75 GPa. The superconducting transition temperature increases rapidly with pressure in a parabolic manner reaching a maximum of $\sim $40 K at $\sim $ 11GPa. It then decreases at higher pressures. We also performed a complimentary pressure dependence x-ray diffraction simultaneously with resistance measurement at low temperatures and observe superconductivity only in the low pressure orthorhombic phase (Cmma) of $\alpha $-FeSe$_{0.89}$ Upon increasing pressure at 10 K, structural phase change from a mixed phase of orthorhombic (Cmma) and hexagonal (P63/mmc) to a high pressure orthorhombic phase (Pbnm) is observed at around 12 GPa where Tc is maximum. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V25.00009: Melting Studies of Metals in a Paris Edinburgh Cell via Radiography Magnus Lipp, Zsolt Jenei, Dave Ruddle, Chantel Aracne-Ruddle, Hyunchae Cynn, William Evans, Yoshio Kono, Curtis Kenney-Benson, Changyong Park Equation-of-state measurements of amorphous solids and liquids suffer from the lack of distinct X-ray diffraction patterns that can be indexed and used for a precise and accurate volume determination. For relatively high Z materials, however, a possible remedy might be found in the application of X-ray radiography. The presentation will describe current efforts with regard to cerium and alloys of noble metals in the liquid state. This work was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344. The X-ray studies were performed at HPCAT (Sector 16), APS/ANL. HPCAT is supported by CIW, CDAC, UNLV and LLNL through funding from~DOE-NNSA, DOE-BES and NSF. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V25.00010: Homoepitaxial Boron Doped Diamond Anvil as Heating Element in a Diamond Anvil Cell Jeffrey Montgomery, Gopi Samudrala, Yogesh Vohra Recent advances in designer-diamond technology have allowed for the use of electrically and thermally conducting homoepitaxially-grown layers of boron-doped diamond (grown at 1200\r{ }C with a 2{\%} mixture of CH$_{4}$ in H, resulting in extremely high doping levels $\sim $ 10$^{20}$/cm$^{3})$ to be used as heating elements in a diamond anvil cell (DAC). These diamonds allow for precise control of the temperature inside of the diamond anvil itself, particularly when coupled with a cryostat. Furthermore, the unmatched thermally conducting nature of diamond ensures that no significant spatial gradient in temperature occurs across the culet area. Since a thermocouple can easily be attached anywhere on the diamond surface, we can also measure diamond temperatures directly. With two such heaters, one can raise sample temperatures uniformly, or with any desired gradient along the pressure axis while preserving optical access. In our initial experiments with these diamond anvils we report on the measurement of the thermal conductivity of copper-beryllium using a single diamond heater and two thermocouples. We augment these measurements with measurements of sample pressure via ruby fluorescence and electrical resistance of the sample and diamond heater. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V25.00011: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V25.00012: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V25.00013: High pressure study of mixed valence compound CsAuI$_{3}$ Shibing Wang, Shigeto Hirai, Alexander Kemper, Maria Baldini, Hongwei Ma, Scott Riggs, Maxwell Shapiro, Tom Devereaux, Ian Fisher, Wendy Mao, Ted Geballe CsAuI$_{3}$, chemically described as Cs$_{2}$Au$^{I}$Au$^{III}$I$_{6}$, is a mixed valence compound in the family of CsAuX$_{3}$ (X = Cl, Br, I), resembling the high Tc superconductor parent compound BaBiO$_{3}$. At ambient conditions it adopts a distorted perovskite structure with compressed Au$^{I}$X$_{6}$ octahedra and elongated Au$^{III}$X$_{6}$ octahedra along the crystallographic c-axis. The compound undergoes a pressure-induced transition into a new tetragonal phase comprising nearly equivalent AuX$_{6}$ octahedra around 5.3 GPa, which agrees with the valence transition previously reported using M\"{o}ssbauer spectroscopy between 8-12 GPa. We present a thorough high pressure studies of CsAuI3 through X-ray diffraction and Raman spectroscopy, confirming a pressure-induced band Jahn-Teller effect associated with the 5d$^{9}$ Au$^{II}$ ion. We will also report the reversible amorphization above 15 GPa. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V25.00014: Pressure tuning of the thermal conductance of weak interfaces Wen-Pin Hsieh, Austin Lyons, Eric Pop, Pawel Keblinski, David Cahill We use high pressure to reveal the dependence of interfacial heat transport on the stiffness of interfacial bonds. The combination of time-domain thermoreflectance and SiC anvil techniques is used to measure the pressure-dependent thermal conductance $G(P)$ of clean and modified Al/SiC interfaces at pressures as high as $P$=12 GPa. We create low-stiffness, van der Waals bonded interfaces by transferring a monolayer of graphene onto the SiC surface before depositing the Al film. For such weak interfaces, $G(P)$ initially increases approximately linearly with $P$, consistent with results of molecular dynamics simulations. At high pressures, $P>$8 GPa, the thermal conductance of weak interfaces approaches the high values characteristic of strongly-bonded, clean interfaces. The results provide new insight demonstrating that interface stiffness dominates thermal transport at weak interfaces, but plays a minor role for strong interfaces with stiffness similar to that in the bulk of the materials. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V25.00015: Target tracking in a dusty plasma: phase transitions and equations of state Neil Oxtoby, Jason Ralph, C\'eline Durniak, Dmitry Samsonov A dusty plasma is a low-density ionized gas containing micron-sized charged dust particles. Laboratory dusty plasmas can be used as kinematic simulators of condensed matter systems, displaying phenomena such as phase transitions. The motion of individual particles is resolvable using a high-speed video camera. We use recursive state estimation (target tracking) techniques to track the dust kinematics, from which we calculate an equation of state for the toy condensed matter system. [Preview Abstract] |
Session V26: Focus Session: Computational Frontiers in Quantum Spin Systems III
Sponsoring Units: DCOMPChair: Stefan Wessel, Aachen University
Room: 257B
Thursday, March 1, 2012 8:00AM - 8:12AM |
V26.00001: A dynamical Marshall sign convention Kevin Beach Monte Carlo sampling of quantum spin models is only practical when it is possible to gauge away simultaneously all negative signs in the coefficients of the ground state wavefunction. The existence of such a transformation is related to the possibility of establishing a bipartite pattern of magnetic order on the lattice and to the choice of a so-called Marshall sign convention. In practice, identifying the correct Marshall sign convention is the responsibility of the QMC practitioner, and the convention itself is generally hard coded. It turns out, however, that a locally optimal sign convention can be determined dynamically within the simulation---meaning that for nonfrustrated systems the simulation quickly establishes a Marshall sign convention that leads to sign-problem-free sampling and that for frustrated systems the Marshall sign convention continually evolves in Monte Carlo time so as to minimize the severity of the sign problem. For concreteness, we focus on a worm algorithm formulated in the basis of singlet product states. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V26.00002: Monte Carlo Simulations of Quantum Spin Systems in the Valence Bond Basis Fabian Zschocke We propose a quantum Monte Carlo method for frustrated spin systems that partially alleviates the sign problem --- thereby extending the range of frustrated couplings over which the system can be reliably sampled. The scheme is projective and takes advantage of the overcompleteness and nonorthogonality of the valence bond basis. It provides a framework for further semi-controlled approximations that are fully sign-problem-free in which the transition weights between bond configurations take on effective, renormalized values. We present results for the frustrated (J1-J2), spin-half Heisenberg model on the square lattice in the vicinity of its phase transition at $J2/J1 \approx 0.4$. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V26.00003: Observing spinon excitations in quantum spin models Ying Tang, Anders Sandvik We develop a technique to directly study spinons (emergent spin $S=1/2$ particles) in quantum spin models in any number of dimensions [1]. Two characteristic lengths---the size of a spinon wave packet and the size of a bound pair (a triplon)---are defined in terms of wave-function overlaps that can be evaluated by quantum Monte Carlo simulations. We find that these two lengths are well distinguishable in one-dimensional models with valence-bond-solid (VBS) ground states and explicitly dimerized models, yet hardly separable in 2-leg ladder systems. We provide some physics insights for these phenomena. We also study spinons in two-dimensional resonating-valence-bond states and models with N\'eel-VBS transitions.\\[4pt] [1] Y. Tang and A. W. Sandvik, Phys. Rev. Lett. 107, 157201 (2011). [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V26.00004: Lightcone renormalization and quantum quenches in one-dimensional Hubbard models Jesko Sirker, Tilman Enss The Lieb-Robinson bound implies that the unitary time evolution of an operator can be restricted to an effective light cone for any Hamiltonian with short-range interactions. Here we present a very efficient new renormalization group algorithm based on this light cone structure to study the time evolution of prepared initial states in the thermodynamic limit in one-dimensional quantum systems. The algorithm does not require translational invariance and allows for an easy implementation of local conservation laws. We use the algorithm to investigate the relaxation dynamics of a doublon lattice in fermionic Hubbard models as well as a possible thermalization. Furthermore, we present results for a doublon impurity in a N\'eel background. We find that the excess charge and spin spread at different velocities, providing an example of spin-charge separation in a highly excited state. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V26.00005: Time Evolution within a Comoving Window Martin Ganahl, Valentin Zauner, Tomotoshi Nishino, Hans Gerd Evertz We present a modification of Matrix Product State time evolution to simulate the propagation of a signal front on an infinite system. The time evolution is calculated within a finite window that moves along with the signal front, in such a way that boundary effects do not occur. Signal fronts can then be studied unperturbed for much longer times than on truly finite systems, where boundary perturbations and reflections would interfere. The entanglement within in the comoving window remains small. Our approach avoids the large entanglement which develops around the location of the signal source and therefore requires significantly lower computational effort. We verify our approach against exact results and show examples of propagating signals for the XXZ model and the transverse Ising model. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V26.00006: Emergence of prominent bound states in the spin-1/2 Heisenberg XXZ chain after a local quantum quench Hans Gerd Evertz, Martin Ganahl, Elias Rabel, Fabian Essler We calculate the non-equilibrium evolution in the spin-1/2 XXZ Heisenberg chain for fixed magnetization after a \emph{local quantum quench}. Initially an infinite magnetic field is applied to $n$ consecutive sites in the center of a large chain, and the ground state is determined. Then the field is switched off and the time evolution of observables such as the z-component of spin is computed using the Time Evolving Block Decimation (TEBD) algorithm. We find that the observables exhibit strong signatures of propagating spinon as well as bound state excitations. These persist even when integrability-breaking perturbations are included. Since bound states (``strings'') are notoriously difficult to observe using conventional probes such as inelastic neutron scattering we conclude that local quantum quenches are an ideal setting for studying their properties. We comment on implications of our results for cold atom experiments. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V26.00007: Non-local order parameters in 1D symmetry protected topological phases Frank Pollmann, Ari Turner, Erez Berg A topological phase is a phase of matter which cannot be characterized by a local order parameter. It has been shown that gapped phases in 1D systems can be completely characterized using tools related to projective representations of the symmetry groups. An example of a symmetry protected topological phase is the Haldane phase of S = 1 chains. Here the phase is protected by any of the following symmetries: dihedral group of $\pi$-rotations about two orthogonal axes, time-reversal symmetry, or bond centered inversion symmetry. We introduce non-local order parameters for each case which can be simply calculated using numerical methods such as Density-Matrix Renormalization Group (DMRG). These non-local order parameters provide a practical tool for numerically detecting these non-trivial phases. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V26.00008: Projected Density of Transitions for Heisenberg Models Roger Haydock, C.M.M. Nex The projected density of transitions (PDoT) is the interacting analogue of the projected or local density of states. The PDoT is proportional to the probability, averaged over all states of the system, that some disturbance (the projection) induces a transition with a specific energy. It is calculated in the same way as the density of states, but using Heisenberg's equation instead of Schr\"{o}dinger's equation. As an example we have applied it to the Heisenberg model for spin interactions of electrons on linear, square, and cubic lattices. One surprise in these calculations is what seems to be a consequence of the spin's rotational symmetry. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V26.00009: Criticality in one-dimensional supersymmetric lattice fermions Matthias Troyer, Bela Bauer, Liza Huijse, Erez Berg, Kareljan Schoutens A supersymmetric model for lattice fermions has been seen to host a plethora of interesting phenomena. On one-dimensional and quasi-one-dimensional lattices, the model naturally becomes critical and it has been conjectured that it is described by superconformal field theory. While this relation has been confirmed for the chain, establishing this link and exploring the phases adjacent to the critical point for the case of the square ladder has turned out to be a challenging problem for numerical simulations. In our work, we collect evidence in support of the conjecture and obtain insights into the adjacent phases using a variety of numerical techniques, including the density-matrix renormalization group and the multi-scale entanglement renormalization ansatz. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V26.00010: Study of Kitaev-Heisenberg model with second-neighbor Heisenberg coupling by DMRG simulations and slave-particle theories Yi-Fan Jiang, Hong-Chen Jiang, Hong Yao We study the effect of second-neighbor Heisenberg coupling $J_2$ to the first neighbor Kitaev-Heisenberg model on the honeycomb lattice by doing DMRG simulations and slave-particle theories. In the Kitaev limit, we find that the gapless spin liquid phase at $J_2=0$ survives up to a finite critical value
$J_{2c}$. In an intermediate range, namely $J_{2c} |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V26.00011: Thermodynamics and phase transitions of the pinwheel-distorted Kagome lattice Heisenberg model Ehsan Khatami, Rajiv R.P. Singh, Marcos Rigol We study the Heisenberg model on the pinwheel-distorted Kagome lattice as observed in the material $Rb_2Cu_3SnF_{12}$. Experimentally relevant thermodynamic properties at finite temperatures are computed utilizing numerical linked-cluster expansions [1]. We introduce a Lanczos-based zero-temperature numerical linked-cluster expansion and study the approach of the pinwheel distorted lattice to the uniform Kagome lattice Heisenberg model. We find strong evidence for a phase transition before the uniform limit is reached, implying that the ground state of the Kagome lattice Heisenberg model is likely not pinwheel dimerized and is stable to finite pinwheel dimerizing perturbations [2]. \\[4pt] [1] M. Rigol and R. R. P. Singh, Phys. Rev. Lett. 98, 207204 (2007); Phys. Rev. B 76, 184403 (2007). \\[0pt] [2] E. Khatami, R. R. P. Singh, M. Rigol, preprint: arXiv:1105.4147 [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V26.00012: Numerical Schwinger boson approach to the Bethe lattice antiferromagnet at percolation Shivam Ghosh, Hitesh J. Changlani, Christopher L. Henley What are the lowest energy excitations of a spin-1/2 Heisenberg antiferromagnet on a critical percolation cluster? On the square lattice, Wang et al.,\footnote{Phys. Rev. B 81, 054417 (2010)} discovered anomalously low S=1 excitations, with energy scaling as $\Delta \sim 1/N^2$. Normally the ``Anderson tower'' would be the lowest, having energy $S(S+1)/2\chi N \sim 1/N$ ($\chi$= transverse susceptibility). These anomalous excitations were attributed to ``dangling spins'' appearing in parts of the cluster where there was an imbalance of even and odd sites. Here, we look at the diluted z=3 Bethe lattice at the percolation threshold. Previous work confirmed the existence of emergent spin-1/2 degrees of freedom with an effective Heisenberg Hamiltonian but did not explain their origin. New results from DMRG (Density Matrix Renormalization Group) show strong dimerization tendency in the (singlet) ground state, yet there is long-range Neel order on the percolation cluster.\footnote{A. Sandvik, Phys. Rev. B 66,024418 (2002)} To harmonize these results, we set up a numerical Schwinger Boson mean field calculation (with site dependent parameters); we find lowering of mean field energy and spin correlations which agree well with ED and DMRG. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V26.00013: Spin Mott Glass Phase in the Disordered Spin Systems Dao-Xin Yao, Nv-Sen Ma, Anders Sandvik We use quantum Monte Carlo simulations to study a glassy ground state of S=1/2 quantum spins by using a dimerized J1-J2-J3 Heisenberg model on the square lattice. J1 corresponds to weak bonds, and J2 and J3 are stronger bonds which are randomly distributed on columnar rungs forming coupled 2-leg ladders. By tuning the average value of J2 and J3, the system undergoes Neel-glass-paramagnetic quantum phase transition. The size of the glass region is affected by the value of the disorder strength. In the glass phase, we find that the uniform susceptibility decreases with T according to exp(-b/$T^a$) with $a<1$; thus the state is incompressible at T=0 and classified as a Mott glass (MG). At the Neel-MG transition, the susceptibility behaves as $T^{2/z-1}$. The dynamical exponent z is found to be larger than 1. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V26.00014: Tuning the disorder in a bosonic superglass Derek Larson, Ying-Jer Kao We study the phase transitions associated with tuning the disorder in a Bose-Hubbard model exhibiting a superglass phase. By shifting the distribution of a nearest neighbor $\pm J$ interaction between hard-core bosons on a three-dimensional lattice, we can produce anti-ferromagnetic, glassy, and ferromagnetic (diagonal) types of ordering, each of which interacts uniquely with the superfluid (off-diagonal) ordering. Quantum Monte Carlo simulation results using the worm algorithm show that the existence of superfluidity guards against the formation of large ferromagnetic clusters, presumably due to a lowering of the cost of interfaces. This leads to a strongly-temperature-dependent phase boundary between the glassy and ferromagnetic regions. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V26.00015: Impact of spin-orbit coupling on the Holstein polaron Zhou Li, Lucian Covaci, Mona Berciu, Devin Baillie, Frank Marsiglio We utilize an exact variational numerical procedure to calculate the ground state properties of a polaron in the presence of a Rashba-like spin orbit interaction. Our results corroborate with previous work performed with the Momentum Average approximation and with weak coupling perturbation theory. We find that spin orbit coupling increases the effective mass in the regime with weak electron phonon coupling, and decreases the effective mass in the intermediate and strong electron phonon coupling regime. Analytical strong coupling perturbation theory results confirm our numerical results in the small polaron regime. A large amount of spin orbit coupling can lead to a significant lowering of the polaron effective mass [Preview Abstract] |
Session V27: Invited Session: Flexible and Rolled Up Semiconductor Nanomembranes
Sponsoring Units: DCMP FIAPChair: Pablo Bianucci, Ecole Polytechnique de Montreal
Room: 258AB
Thursday, March 1, 2012 8:00AM - 8:36AM |
V27.00001: Making Novel Materials Using Strain in Nanomembranes Invited Speaker: Max G. Lagally The controlled introduction of strain in materials offers an important degree of freedom for fundamental studies of materials as well as advanced device engineering. Strain in a crystalline solid modifies the lattice constants and reduces the crystal symmetry. Because strain energy is proportional to thickness, a free-standing crystalline thin sheet, which we call a nanomembrane (NM), can be strained to a greater degree that a bulk material with the same surface area. I show the use of nanomembrane strain engineering to make defect-free single crystals that cannot be grown any other way, and materials with strain symmetries that they do not have naturally, in both cases alloys of Si and Ge. Strain in NMs causes significant shifts in energy band edges, splitting of degenerate states, and changes in effective masses. These effects can be used to produce a desired band offset between different materials, to increase carrier mobility, and to change relative energy positions of valleys. In the latter respect, through the use of NMs it has recently become possible, using tensile strain, to make Ge direct-bandgap and light emitting at room temperature. Periodic local stress can produce strain superlattices and thus single-element heterojunctions. Work performed with the Roberto Paiella, Mark Eriksson, Feng Liu, and Irena Knezevic research groups. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 9:12AM |
V27.00002: Strain Superlattice: A Combination of Strain Induced Self-Assembly and Strain Engineered Band Structure Invited Speaker: Feng Liu Semiconductor nanomembrane affords a novel 2D platform for nanoscience and nanotechnology, especially for strain engineering of nanoelectronics. Strain is well known for band engineering to improve the performance of Si devices. The evidence that the band gap of Si changes significantly with strain suggests that by alternating regions of strained and unstrained Si one creates a single-element hetero-strain-junction electronic superlattice (SL), with the carrier confinement defined by strain rather than by the chemical differences in conventional SLs. Using first-principles calculations, we map out the electronic phase diagram of a 1D pure-silicon strain SL. It exhibits a high level of phase tunability, e.g., tuning from type I to type II. Our theory rationalizes a recent observation of a strain SL in a Si nanowire and provides general guidance for the fabrication of single-element strain SLs. The low-dimensional nanoscale strain SLs extend the concept of SL to a single element that exists in different structural states. It can be made in 1D nanowires or 2D nanomembranes by nanoscale self-assembly or by nanopatterning. It expands the application of strain engineering to new territories, by combining strain induced self-assembly with strain engineered band structure. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V27.00003: Rolled-up multifunctional materials Invited Speaker: Oliver Schmidt |
Thursday, March 1, 2012 9:48AM - 10:24AM |
V27.00004: Rolled-up Tube Based Nanophotonics Invited Speaker: Xiuling Li |
Thursday, March 1, 2012 10:24AM - 11:00AM |
V27.00005: Nanomembrane photonics for Si photonic integration and flexible optoelectronics Invited Speaker: Weidong Zhou Crystalline semiconductor nanomembranes (NMs) offer unprecedented opportunities for unique electronic and photonic devices for vertically stacked high density photonic/electronic integration, high performance flexible electronics, and adaptive flexible/conformal photonics. Research progresses have been made in the areas of optical filters/modulators, spectral selective IR photodetectors, flexible LEDs, solar cells, and novel light sources, based on quantum dots, Fano resonance photonic crystal cavities, and heterogeneous integration of III-V/Si material systems. The potentials and prospects of nanomembrane photonics will also be discussed, for a wide range of applications, in the areas of hyper-spectral imaging and gas sensing (lab-on-a-chip), high capacity data network and optical computing (WDM-on-a-chip), high performance flexible inorganic displays, solid state lighting, and photovoltaic solar cells, etc. [Preview Abstract] |
Session V28: Semiconductor Defects and Doping
Sponsoring Units: DMPChair: Walter Lambrecht, Case Western Reserve University
Room: 258C
Thursday, March 1, 2012 8:00AM - 8:12AM |
V28.00001: Transition Metal Defects in Cubic and Hexagonal Polytypes of SiC: Site Selection and Electronic Structure from ab-initio Calculations Adam Gali, Viktor Iv\'ady, Andreas G\"allstr\"om, Nguyen Son, Erik Janz\'en Relatively little is known about point defects in different polytypes of a crystal. Silicon carbide is a prototype material for polytypism. There are unidentified photoluminescence centers in SiC that are presumably originated from transition metal defects, however, the number of detected centers does not follow the number of inequivalent substitutional sites in different polytypes. In this study we applied highly convergent and sophisticated density functional theory (DFT) based methods to investigate important transition metal impurities including titanium (Ti), vanadium (V), niobium (Nb), chromium (Cr), molybdenum (Mo) and tungsten (W) in cubic 3C and hexagonal 4H and 6H polytypes of SiC. We applied DFT with PBE functional in order to calculate the ground state of the defects. We calculated the electronic structure by a screened hybrid density functional (HSE06) which was very successful in the quantitative description of native defects in SiC. We found a special asymmetric split-vacancy configuration for a class of transition metal defects. The asymmetric split-vacancy configuration exclusively prefers the hexagonal-hexagonal sites in hexagonal polytypes, thus the probability of finding these defects in 3C polytype is much smaller than in hexagonal polytypes. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V28.00002: Quantum Effects of Strain Influence on the Dopant Behavior in Semiconductors Deyan Sun, Zhentang Wang, Shiyou Chen, Xiangmei Duan, Su-Huai Wei, Xingao Gong In most fields of physics, applying external strain (pressure) provides an important technique to investigate and tune the properties of materials. Quite often the theoretical treatment is based on the continuum elastic model, in which its validity is still under debate. In this talk, by using quantum mechanical theoretical analysis, we show that if the occupation change of different orbitals caused by the strain is negligible, the continuum elastic model is valid, otherwise it will fail. Our theory is confirmed by first-principles calculation of Mn-doped GaAs system. Moreover, we show that under compressive strain the hole density, thus the Curie temperature can increase in Mn-doped spintronic materials. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V28.00003: First-principle Calculations of Donor and Acceptor Levels in PbI2 for Ultra-fast Scintillation Gaigong Zhang, Andrew Canning, Niels Gronbech-Jensen, Lin-Wang Wang, Stephen Derenzo In the past PbI2 was studied as a candidate for semi-conductor gamma ray detectors as well as more recently as an ultra-fast scintillator for time-of-flight applications. The ultra-fast scintillation properties of this materials are believed to be related to donor-acceptor recombination in both the pure and doped system. This work presents first-principles electronic structure calculations of donor and acceptor levels for intrinsic defects and doped impurities in PbI2. We performed density functional theory calculations within the generalized gradient approximation with U correction for the donor and acceptor modeling. For a more accurate description of band gaps and structures, we used more advanced methods, such as GW. Our study shows that intrinsic defects significantly affect the luminescence properties of bulk PbI2. Moreover, the relative position of intrinsic defect levels to doped impurity levels can influence the luminosity. We will compare our theoretical work to known experimental work for this material. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V28.00004: Nonradiative lifetimes in intermediate band materials -- absence of lifetime recovery Jacob J. Krich, Bertrand I. Halperin, Al\'an Aspuru-Guzik Intermediate band photovoltaics hold the promise of being highly efficient and cost effective photovoltaic cells. Intermediate states in the band gap, however, are known to facilitate nonradiative recombination. Much effort has been dedicated to producing metallic intermediate bands in hopes of producing \emph{lifetime recovery} -- an increase in carrier lifetime as doping levels increase. We show that lifetime recovery induced by the insulator-to-metal transition will not occur, because the metallic extended states will be localized by phonons during the recombination process. Only trivial forms of lifetime recovery, e.g., from an overall shift in intermediate levels, are possible. Future work in intermediate band photovoltaics must focus on optimizing subgap optical absorption and minimizing recombination, but not via lifetime recovery. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V28.00005: Imaging Defects in Semiconductors with a Snapshot Micro-Imaging Technique Kirstin Alberi, Brian Fluegel, Dan Beaton, Angelo Mascarenhas Polycrystalline or metamorphically grown single crystalline thin films are ubiquitous in energy-related technologies, such as solar cells and light emitting diodes. A detailed understanding of the defects contained within these materials (i.e. dislocations, grain boundaries, inclusions) and the ability to control them play critical roles in their development. We will present the use of a novel ``snapshot'' micro-imaging technique to evaluate the presence and behavior of defects in a variety of materials and devices. Photoluminescence from a wide area of a sample is imaged onto a Si CCD in a single exposure, enabling real-time mapping with sub-micron resolution. A tunable liquid-crystal filter selects the exact wavelength that is imaged. Combined with a tunable excitation source, this technique is ideal for selectively investigating defects in thin films as well as individual layers in a device. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V28.00006: Fluorescent nanodiamonds for biomedical thermal imaging Juan Enrique Ramirez Hernandez, Jose Garcia Sole, Martin Rafael Pedroza Montero, Marcelino Barboza Flores, Daniel Jaque, Laura Martinez Maestro, Karla Josefina Santacruz Gomez, Francisco Sanz Rodriguez, Ningning Dong, Tomas Calderon The use of nanoparticles designed for a particular purpose in biological research has increased exponentially in the last ten years. Nanoscale particles have dimensions similar to those of most biological systems and present exceptional physical, chemical and optical properties. The optical absorption and emission of nanoparticles may be tuned by varying their shape, size and composition and recent advances in their synthesis and design suggest their potential use as probes in the detection and treatment of diseases such as cancer. One of these promising materials is nanodiamond which possesses excellent surface modification capacity and high biocompatibility. In addition to being compatible with the human body, nanodiamonds can be used as radiation sensors. In this report, the ability of nanodiamonds to be used as nanothermometers was studied by the obtention of a nanothermic scale (temperature dependence of nanodiamond's emission spectrum) to accurately measure temperature in small volumes. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V28.00007: Laser induced local modification of material properties in a semiconductor alloy Yong Zhang, Naili Yue High energy (usually short pulse) lasers are routinely used for micromachining and surface modification of solid state materials. We demonstrate that a small power CW laser can be used to induce local modification of the electronic and optical properties of a semiconductor alloy at any desirable spatial location. The degree of modification can be precisely controlled by laser density and exposure time. The spatial resolution is determined currently by the beam size of a confocal optical system, but it can also be defined by the feature size of a lithographic method. The induced local changes in properties are measured by optical spectroscopy, and the results indicate the possibility of local structural modification. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V28.00008: Spatial Variation in Mobility-Lifetime Product in Bulk TlBr and CZT David Phillips, Nancy Haegel, Kevin Blaine, Hadong Kim, Guido Ciampi, Len Cirignano The energy resolution of a semiconductor radiation detector depends on the charge transport properties of the semiconductor, and the mobility-lifetime ($\mu\tau$) product is a key figure of merit for charge transport. In this work, we investigate the effects of two impurities, Na and Cu, on the $\mu\tau$ product in bulk thallium bromide (TlBr) using cathodoluminescence (CL) and transport imaging. Transport imaging uses a scanning electron microscope to generate a line of charge carriers on the surface of a bulk sample, and the intensity and spatial distribution of the recombination luminescence are recorded. A Green's function approach is used to model the generation, diffusion, and recombination of charge carriers under steady-state conditions. The luminescence distribution is fit to the model to extract the ambipolar diffusion length and the $\mu\tau$ product, providing a high-resolution correlation between the luminescence variations due to dopants/defects and the quantitative transport behavior. The $\mu\tau$ product has been mapped across a 40 $\mu$m segment of TlBr at a resolution of 2 $\mu$m. Additionally, this approach has been used to locally map variations in ambipolar diffusion length and $\mu\tau$ product due to extended defects in cadmium zinc telluride (CZT). [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V28.00009: Quantum Stress: Density Functional Theory Formulation and Physical Manifestation Hao Hu, Feng Liu The concept of ``quantum stress (QS)'' is introduced and formulated within density functional theory (DFT), to underlie extrinsic electronic effects on the stress state of solids and thin films in the absence of lattice strain. An explicit expression of QS (\textit{$\sigma $}$^{Q})$ is derived in relation to the deformation potential of electronic states ($\Xi )$ and the variation of electron density (\textit{$\Delta $n}), \textit{$\sigma $}$^{Q}=\Xi $(\textit{$\Delta $n}), as a quantum analog of classical Hook's law. Two distinct QS manifestations are demonstrated quantitatively by DFT calculations: (1) in the form of bulk stress induced by charge carriers; and (2) in the form of surface stress induced by quantum confinement. QS has broad implications in physical phenomena and technological applications that are based on coupling of electronic structure with lattice strain. [Preview Abstract] |
Session V29: Focus Session: Superconducting Qubits: Epitaxial Junctions and Two-Level Systems
Sponsoring Units: GQIChair: Hanhee Paik, Yale University
Room: 259A
Thursday, March 1, 2012 8:00AM - 8:12AM |
V29.00001: Coherence in a transmon qubit with epitaxial tunnel junctions Martin Weides, Jeffrey Kline, Michael Vissers, Martin Sandberg, David Wisbey, David Pappas, Blake Johnson, Tom Ohki We developed transmon qubits based on epitaxial tunnel junctions and interdigitated capacitors. This multileveled qubit, patterned by use of all-optical lithography, is a step towards scalable qubits with a high integration density. The relaxation time $T_1$ is $.72-.86\;\rm{\mu sec}$ and the ensemble dephasing time $T_2^*$ is slightly larger than $T_1$. The dephasing time $T_2$ ($1.36\;\rm{\mu sec}$) is nearly energy-relaxation-limited. Qubit spectroscopy yields weaker level splitting than observed in qubits with amorphous barriers in equivalent-size junctions. The qubit's inferred microwave loss closely matches the weighted losses of the individual elements (junction, wiring dielectric, and interdigitated capacitor), determined by independent resonator measurements. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V29.00002: Superconducting qubits consisting of epitaxially-grown NbN/AlN/NbN Josephson junctions Yasunobu Nakamura, Hirotaka Terai, Kunihiro Inomata, Tsuyoshi Yamamoto, Wei Qiu, Zhen Wang We demonstrate superconducting qubits using epitaxially-grown Josephson junctions. A fully epitaxial NbN/AlN/NbN trilayer on MgO (100) substrate is processed by photolithography and dry-etching into transmon qubits with a large Josephson energy. The tunnel barrier made of cubic-phase AlN, rather than the ordinary hexagonal phase, is the key to avoid piezoelectric coupling to the phonon bath. The energy-relaxation time and the spin-echo decay time of $\sim 500$~ns are observed in the qubits that are coupled to a monolithically-made coplanar waveguide resonator. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V29.00003: Josephson phase qubits incorporating high-Q crystalline dielectrics U. Patel, Y. Gao, L. Maurer, S. Sendelbach, D. Hover, K.H. Cho, C.B. Eom, R. McDermott The energy relaxation times of Josephson phase qubits are currently limited by spurious coupling of the qubit to unsaturated two level system (TLS) defects of the amorphous dielectrics. It is expected that incorporation of defect-free crystalline dielectrics into qubit circuits will dramatically improve coherence. Here, we describe the growth and characterization of novel crystalline dielectrics for superconducting qubit applications, including grown Al2O3 on epitaxial Re underlayers. We discuss the incorporation of epitaxial dielectrics into phase qubit circuits, and present data on qubit coherence. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V29.00004: Growth of epitaxial Al/AlOx/Re using a sputtering PLD hybrid system with \textit{in-situ} RHEED Kwang-Hwan Cho, Jacob Podkaminer, Umeshkumar Patel, Robert McDermott, Chang-Beom Eom Our objective is the growth of epitaxial dielectrics on crystalline superconducting underlayers to improve the performance of superconducting Qubits. We have grown epitaxial Re thin films on a c-plane sapphire substrate using RF magnetron sputtering, and then transferred \textit{ex-situ} to a pulsed laser deposition (PLD) system where dielectrics thin film layer is deposited. One drawback of this fabrication approach is the necessity to expose the sample to air when the sample is transferred to different deposition chambers. In order to avoid these drawbacks, we have employed a hybrid PLD-sputtering deposition that will allow us to grow the oxide dielectric/Re heterostructures in an \textit{in-situ} environment without breaking vacuum. The system is also equipped with reflection high energy electron diffraction (RHEED) which will allow us to perform \textit{in-situ} characterization of the structure and growth dynamics. We will discuss our strategy of epitaxial growth of various single crystal dielectrics on superconducting thin films in this system and their structural and electrical properties of the heterostructures. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V29.00005: Transport Characteristics of Self-Aligned Josephson Junctions Grown using co-Deposited Molecular Beam Epitaxy Gustaf Olson, Allison Dove, Zachary Yoscovits, Chris Nugroho, Vladimir Orlyanchik, Dale Van Harlingen, James Eckstein Low noise Josephson junctions are extremely desirable in SQUIDs and qubit circuits. Sources of flux noise and critical current noise can be reduced by using both clean, single crystal junctions to lower the density of fluctuators and by decreasing the size of the junctions to lower the absolute number of fluctuators. We report transport characteristics of small, single crystal Josephson junctions grown using a co-deposited aluminum-oxide barrier molecular beam epitaxy process. We also report a novel self-aligned fabrication process that allows us to produce sub-micron junctions from these single crystal films. We show that our co-deposited junctions have more ideal transport characteristics than those junctions grown with only a diffused barrier. - [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V29.00006: Internal loss of superconducting resonators induced by interacting two level systems Lara Faoro, Lev Ioffe In a number of recent experiments with microwave superconducting resonators the anomalously slow dependence of the quality factor on the power was observed. This observation implies that the monochromatic radiation does not saturate two level systems in the surrounding oxides as predicted by the standard model of two-level systems (TLS). We argue that these observations suggest the importance of interactions between TLS in these materials. We show that interactions between TLS lead to a drift of their energies that result in much slower, logarithmic dependence of their absorption on the radiation power in a reasonable agreement with the data. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V29.00007: Two-level system dynamics in amorphous dielectrics probed with a dc electric field Moe Khalil, Sergiy Gladchenko, M.J.A. Stoutimore, F.C. Wellstood, K.D. Osborn We report loss in~a thin-film dc electric-field tunable LC resonator built with superconducting aluminum and silicon nitride dielectric. To measure the loss we continually apply microwave power on resonance and monitor the transmitted power. At milli-Kelvin temperatures, loss is limited by two-level systems in the dielectric which are saturated with high microwave excitation power. Measurements show that a sudden change of applied dc field causes the dielectric loss to increase to the intrinsic low power loss tangent of the dielectric. We study the subsequent relaxation of the loss tangent caused by two-level system saturation and interactions. We discuss how this arises from the dynamics of a distribution of two-level system defects and compare it with new theoretical work on interacting two-level systems. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V29.00008: Studying two-level systems in Josephson junctions with a Josephson junction defect spectrometer M.J.A. Stoutimore, M.S. Khalil, Sergiy Gladchenko, R.W. Simmonds, C.J. Lobb, K.D. Osborn We have fabricated and measured Josephson junction defect spectrometers (JJDSs), which are frequency-tunable, nearly-harmonic oscillators that probe two-level systems (TLSs) in the barrier of a Josephson junction (JJ). A JJDS consists of the JJ under study fabricated with a parallel capacitor and inductor such that it can accommodate a wide range of junction inductances, L$_{J0}$, while maintaining an operating frequency, f$_{01}$, in the range of 4-8 GHz. In this device, the parallel inductance helps the JJ maintain linearity over a wide range of frequencies. This architecture allows for the testing of JJs with a wide range of areas and barrier materials, and in the first devices we have tested Al/AlOx/Al JJs. By applying a magnetic flux bias to tune f$_{01}$, we detect TLSs in the JJ barrier as splittings in the device spectrum. We will present our results toward identifying and quantifying these TLSs, which are known to cause decoherence in quantum devices that rely on JJs. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V29.00009: Relaxation of a Cooper-Pair Box Coupled to Discrete Charge Fluctuators B.S. Palmer, Z. Kim, F.C. Wellstood Recently, Kim \emph{et al.} have reported that the interaction of a Cooper-pair box (CPB) with discrete charge fluctuators can decrease the relaxation time ($T_{1}$) of the first excited state of a CPB when operating the CPB near the transition frequency of a charged two-level system (TLS).\footnote{Z. Kim \emph{et al.,} Physical Review B, \textbf{78} 144506 (2008).} Using a density matrix approach and a 4-level Hilbert space, we have simulated the $T_{1}$ of a CPB coupled to a TLS and a dissipative bath. We model the TLS with asymmetry and tunneling parameters which we obtain along with the CPB parameters from fits to microwave spectroscopic measurements.\footnote{Ibid.}$^,$\footnote{F. C. Wellstood, Z. Kim, and B. S. Palmer, arXiv:0805.4429.} To model the bath, the CPB is coupled to a source of charge noise which causes relaxation and dephasing while the TLS is coupled to its own dissipative source of energy. Results of the simulation are presented and compared to the experimental measurements. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V29.00010: Microwave Frequency Loss in Aluminum Nanobridge Josephson Junctions Eli Levenson-Falk, R. Vijay, Steven Weber, Kater Murch, I. Siddiqi Dielectric loss is a major source of decoherence in many superconducting qubits. Weak link Josephson junctions have the potential advantage of eliminating any loss associated with the insulating barrier in conventional tunneling type devices. We present quality factor measurements of 6 GHz superconducting resonators realized from three dimensional aluminum nanobridges shunted by single crystal silicon overlap capacitors. We compare the measured Q values with those obtained from similar resonators with aluminum tunnel junctions, with critical currents also in the microamp range. We discuss potential relaxation mechanisms specific to weak link junctions, and describe nanobridge qubit designs. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V29.00011: $T_1$-echo sequence - Preserving the state of a qubit in the presence of coherent interaction Clemens M\"uller, Alexander Shnirman, Martin Weides We propose a sequence of pulses intended to preserve the state of a qubit in the presence of strong, coherent coupling to another quantum system. The sequence can be understood as a generalized SWAP and works in formal analogy to the well-known spin echo. Since the resulting decoherence rate of the qubits state is strongly influenced by the additional system, this sequence might serve to protect its quantum state. A possible area of application would be in superconducting circuits, where often spurious two-level system interact strongly with the qubits and might thus provide the necessary resource. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V29.00012: Circuit quantum electrodynamics with a scanning qubit Will Shanks, Devin Underwood, James Raftery, Srikanth Srinivasan, Anthony Hoffman, Andrew Houck We report measurements of the coupling between a superconducting microwave resonator and a transmon qubit fabricated on a separate chip and mounted to a three-dimensional cryogenic translation stage. The qubit-resonator system reached the strong coupling regime with a coupling strength in excess of 80 MHz. We use the translation stage to explore the position dependence of the coupling strength. With a scanning qubit stage, it is possible to measure many qubits in succession and study the statistics of the fabrication process. The system can also be used as a local probe of a large array of microwave cavities and superconducting qubits. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V29.00013: Noise and microresonance of critical current in Josephson junction induced by Kondo trap states Mohammad Ansari, Frank Wilhelm We analyze the impact of trap states in the oxide layer of a superconducting tunnel junctions, on the fluctuation of the Josephson critical current, thus on coherence in superconducting qubits. We use second order perturbation theory which allows to obtain analytical formulae for the interacting bound states and spectral weights, limited to small and intermediate repulsions. Remarkably, it still reproduces the main features of the model as identified from the Numerical Renormalization Group. We present analytical formulations for the subgap bound state energies, the singlet-doublet phase boundary, and the spectral weights. We show that interactions can reverse the supercurrent across the trap. We finally work out the spectrum of junction resonators for qubits in the presence of on-site repulsive electrons and analyze its dependence on microscopic parameters that may be controlled by fabrication. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V29.00014: Probing coupling mechanism between microscopic two-level system and superconducting qubits Yang Yu, Zhentao Zhang We propose a scheme to clarify the microscopic nature of Josephson qubits interacting with the two-level systems, coming from microscopic defects located inside insulation layer. We found that the sensitivity of the generally used spectral method in phase qubit is not sufficient to evaluate the exact form of the coupling. On the contrary, our numerical calculation shows that the coupling strength changes remarkably with flux bias for a flux qubit, providing a useful tool to investigate the coupling mechanism between the two-level systems and qubits. [Preview Abstract] |
Session V30: Focus Session: Semiconductor Qubits - Measurement
Sponsoring Units: GQIChair: Mark Gyure, HRL
Room: 259B
Thursday, March 1, 2012 8:00AM - 8:36AM |
V30.00001: Single-donor spin qubits in silicon Invited Speaker: Andrea Morello The idea of using the spin of a single donor atom in silicon to encode quantum information goes back to the Kane proposal [1] in 1998. We have now resolved the technical challenges involved in the readout and control of the electron and nuclear spin of a single atom. The key breakthrough was the development of a device structure where the donor is tunnel-coupled to the island of an electrostatically-induced single-electron transistor [2]. This device allowed the single-shot readout of the electron spin with visibility $> 90\%$ and 3 $\mu$s readout time [3]. More recently we have integrated the single-shot readout device with a broadband microwave transmission line to coherently control the electron and nuclear spins. The resonance frequency of the electron is found by monitoring the excess spin-up counts while sweeping the microwave frequency. At any time, one of two possible frequencies is found to be in resonance with the electron spin, depending on the state of the nuclear spin. Alternately probing the two frequencies yields the (quantum nondemolition) single-shot readout of the nucleus, with fidelity $> 99.99\%$. Then we demonstrate the coherent control (Rabi oscillations) of both the electron and the nucleus, both detected in single-shot mode. The $\pi$-pulse fidelity is $\sim 70\%$ for the electron and $\sim 99\%$ for the nucleus. Hahn echo and multi-pulse dynamical decoupling sequences allow us to explore the true coherence of the qubits, yielding $T_{2e} \sim 200$ $\mu$s for the electron, and $T_{2n} \sim 60$ ms for the nucleus. These results are fully consistent with the bulk values for donors in a natural Si sample. Further improvements in qubit coherence can be expected by moving to isotopically pure $^{28}$Si substrates.\\[4pt] [1] B. E. Kane, Nature \textbf{393}, 133 (1998).\\[0pt] [2] A. Morello \textit{et al.}, Phys. Rev. B \textbf{80}, 081307(R) (2009).\\[0pt] [3] A. Morello \textit{et al.}, Nature \textbf{467}, 687 (2010). [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V30.00002: Quantum read-out and fast initialization of nuclear spin qubits with electric currents Rogerio de Sousa, Noah Stemeroff Nuclear spin qubits have the longest coherence times in the solid state, but their quantum read-out and initialization is a great challenge. We present a theory for the interaction of an electric current with the nuclear spins of donor impurities in semiconductors [1]. The theory yields a sensitivity criterion for quantum detection of nuclear spin states using electrically detected magnetic resonance, as well as an all electrical method for fast nuclear spin qubit initialization.\\[4pt] [1] N. Stemeroff and R. de Sousa, Phys. Rev. Lett. {\bf 107}, 197602 (2011). [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V30.00003: Real-time readout and lifetime measurements of single-triplet states in a Si/SiGe double quantum dot Jonathan Prance, Zhan Shi, Christie Simmons, Don Savage, Max Lagally, Lars Schreiber, Lieven Vandersypen, Mark Friesen, Robert Joynt, Sue Coppersmith, Mark Eriksson The singlet and triplet states of a two-electron double quantum dot can be used as the basis for a logical qubit that combines fast gating and robust readout via Pauli spin blockade. We present measurements of the lifetimes of these states in a Si/SiGe double dot at magnetic fields between 1T and 0T [1]. The lifetimes are found by analyzing the statistics of repeated single-shot measurements of the spin state of the system. This technique allows multiple relaxation processes to be observed simultaneously. At zero magnetic field we find that all four spin states have lifetimes of approximately 10ms. With increasing magnetic field the lifetimes of the S and T0 states show no noticeable change, while the lifetime of the T- state rises, reaching 3 seconds at 1T. [1] J. R. Prance, et al., e-print: arxiv.org/abs/1110.6431 [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V30.00004: Calibrated State Tomography in Singlet-Triplet Qubits Oliver Dial, Michael Shulman, Shannon Harvey, Hendrik Bluhm, Vladimir Umansky, Amir Yacoby Quantitative and accurate state tomography is becoming increasingly necessary in spin qubits to establish gate fidelities, entanglement measures, and optimize the increasingly complex gate sequences needed to perform experiments. In spin-qubits, to perform state tomography single-qubit rotations are used to map different axes of the Bloch sphere to the singlet-triplet axis, followed by projective measurement onto the singlet-triplet axis. Two orthogonal rotations are provided by two physically distinct mechanisms: magnetic field gradients and exchange rotations. The complex interplay between these mechanisms, noise sources, and pulse distortions make it difficult to accurately predict the angle and axis of rotations from first principles, leading to a circular problem: how can one calibrate tomographic rotations without any calibrated tomographic rotations? We describe and demonstrate a method which, using minimal assumptions, makes it possible to detect and correct for both axis errors in tomography and losses during the rotations associated with state tomography. This allows state tomography with unprecedented precision in these systems. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V30.00005: Readout and Control Technology for Spin Qubits James Colless, David Reilly Scale-up of spin qubits will require the development of new technological approaches that enable readout and control in multi-qubit device architectures. We report results demonstrating a fast readout method based on quantum capacitance that is well suited to detecting spin-states in qubit geometries beyond two quantum dots. Control protocols and device architectures for the selective rotation of single spins using on-chip transmission lines and ac-magnetic field gradients will be presented. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V30.00006: Si/SiGe Quantum Dot Charge Sensing with Radio Frequency Single-Electron Transistor Mingyun Yuan, Zhen Yang, A.J. Rimberg, M.A. Eriksson, D.E. Savage We report the operation of a radio frequency superconducting single-electron transistor (rf-SSET) as a charge sensor for single and double Si/SiGe quantum dots (QDs). The charge sensitivity is on the order of $10^{-5}$ to $10^{-6}$ $e/\sqrt{Hz}$. In the reflectometry set-up, real-time electron tunneling events in a single QD are measured, which demonstrates a fast charge detection time of a few tens of microseconds. The stability diagram of a double QD is mapped out with the averaged reflected power of the rf-SSET. In addition, electron temperature is measured in a dilution refrigerator to be around 150 mK, allowing us to study spin blockade and Kondo effect. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V30.00007: Study of the QPC Back-action to Electron Spin Based Qubits Ming Xiao, Haiou Li, Gang Cao, Guoping Guo, Guangcan Guo, Hongwen Jiang The electron spin states in quantum dots (QD) are potential for implementing qubits. Quantum point contacts (QPC) are widely used to read-out these spin states. However, any read-out procedure inevitably causes back-action to the measured qubits. In this work we studied the back-action of a QPC to the electron spin states in a single GaAs QD. We found that the non-equilibrium effect in the QPC real-time charge counting statistics is a benchmark of the QPC back-action strength. The back-action driven excitations to higher energy levels contribute extra features which enabled us to study the QD's internal structures. The excitations between the two Zeeman states for odd number of electrons and between the spin singlet-triplet states for even number of electrons are respectively studied. This provides a way to quantitatively evaluate the influence of back-action on Zeeman or singlet-triplet based spin qubits. The dependence of the relaxation time of various spin excited states on the back-action strength was also studied. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V30.00008: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V30.00009: Fast charge sensing in InAs nanowire double quantum dot devices Minkyung Jung, Michael Schroer, Karl Petersson, Jiri Stehlik, Jason Petta Fast and sensitive charge and spin state readout is one of the most important requirements for quantum computing. Radio frequency (rf) reflectometry [1] provides a simple and fast charge detection scheme for charge and spin state readout in double quantum dot (DQD) devices without a separate charge detector [2]. Here, we demonstrate charge sensing measurements in InAs nanowire DQD devices using rf-reflectometry. The source electrode of the nanowire DQD is directly coupled to the tank circuit. We correlate standard dc transport measurements with the tank circuit response. We drive the resonator at its resonant frequency and detect the reflected signal via a cryogenic and room temperature amplifier. Using rf-reflectometry, we can observe charging transitions even when the device tuned to a regime where current through the device is below the noise floor of the setup. The sensor enables the occupancy of the quantum dot to be probed down to a few electron regime. We present preliminary results of spin state readout using rf-reflectometry. \noindent \\ \noindent [1] R. J. Schoelkopf {\it et al.}, Science {\bf280}, 1238 (1998)\\ \noindent [2] K. D. Petersson {\it et al.}, Nano Lett. {\bf10}, 2789 (2010)\\ [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V30.00010: Single charge sensing and transport in double quantum dots fabricated from commercially grown Si/SiGe heterostructures K. Wang, C. Payette, Y. Dovzhenko, P. Koppinen, J.R. Petta We perform quantum Hall measurements on three types of commercially available modulation doped Si/SiGe heterostructures [1] to determine their suitability for depletion gate defined quantum dot devices. By adjusting the growth parameters, we are able to achieve two dimensional electron gases with low charge densities and high mobilities. We extract an electron temperature of 100 mK in the single quantum dot regime. Double quantum dots fabricated on these heterostructures show clear evidence of single charge transitions [2] as measured in dc transport and charge sensing. \\[4pt] [1] C. B. Simmons et al, Phys. Rev. Lett. 106, 156804 (2011).\\[0pt] [2] R. Hanson et al, Rev. Mod. Phys. 79, 1217 (2007). [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V30.00011: Charge sensing in a silicon MOS double quantum dot M. Lilly, K. Nguyen, R. Young, E. Nielsen, N. Bishop, J. Wendt, R. Grubbs, T. Pluym, J. Stevens, J. Dominguez, R. Muller, M. Carroll We report charge sensing measurements on a double quantum dot fabricated in a silicon / silicon dioxide double top gated structure. Depletion gates are used to laterally define a double quantum dot, and each dot has an adjacent quantum point contact (QPC) electrometer for remote detection of the dot occupation. For charge sensing, two techniques have been employed. In one, the direct conductance of the QPC measured with a low frequency ac voltage bias, and in the other the differential change in QPC conductance is measured as an ac voltage is applied to a dot plunger gate. Simultaneous direct and differential charge sense measurements are performed using an amplitude modulation technique. We characterize the double dot using honeycomb stability diagrams and non-linear transport. Results are compared to detailed modeling of our device structure. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V30.00012: Dispersive readout of spin blockade in a carbon nanotube double quantum dot Hugh Churchill, Ruby Lai, Charles Marcus We probe the charge states of a carbon nanotube double quantum dot by coupling superconducting resonators to the leads and a gate of a device designed for operation as a spin-valley qubit. Multiplexed dispersive readout allows rapid reflectometry measurements of the device without the need for dedicated proximal charge sensors. In this way as-grown nanotubes may be used in a bottom-gated geometry to create low-disorder devices that are freely suspended and then insulated using atomic layer deposition. These techniques are demonstrated with measurements in the spin blockade regime. We acknowledge support from IBM, NSF-MWN, NSF-NRI through the INDEX Center, and Harvard University. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V30.00013: Relaxation hotspots and fast reset of a single electron spin in a double quantum dot Katja Nowack, Mohammad Shafiei, Christian Reichl, Werner Wegscheider, Lieven Vandersypen We measure the relaxation time of a single electron spin in a double quantum dot as a function of the energy detuning between the two dots for Zeeman splitting larger than the tunnel coupling. Close to the charge degeneracy point at which the electron delocalizes over both dots we observe two ``hot spots'' at which relaxation times are enhanced by almost four orders of magnitude. We identify these hot spots to occur at degeneracies of orbital and spin excitations. The spin-orbit and hyperfine interaction in the GaAs host lattice efficiently mix degenerate spin states and in combination with fast orbital relaxation this can lead to a fast pumping of the electron spin to its ground state. The enhanced spin relaxation can be exploited to achieve a fast reset of the electron spin, which might prove useful in the context of spin based quantum information processing. [Preview Abstract] |
Session V31: Focus Session: Topological Insulators: Synthesis & Characterization - ARPES
Sponsoring Units: DMPChair: Zahid Hasan, Princeton University
Room: 260
Thursday, March 1, 2012 8:00AM - 8:12AM |
V31.00001: Topological Surface States in Ternary Spin-Orbit Insulators: An ARPES Viewpoint Madhab Neupane, S.-Y. Xu, L.A. Wray, A. Petersen, R. Shankar, A. Fedorov, C. Liu, Y.S. Hor, J. Xiong, D.-X. Qu, H. Lin, N.P. Ong, A. Bansil, R.J. Cava, M.Z. Hasan Utilization of topological surface states is expected to lead to new vistas in electronics and fundamental physics. However, most of the known topological insulators either do not feature necessary band structure conditions (location of Dirac point with respect to the bulk band) or lack topological invariants essential for certain class of applications. Using angle-resolved photoemission spectroscopy (ARPES), we discuss the electronic band structure topology of a family of ternary spin-orbit insulators some of which feature functional electronic structure with in-gap Dirac point while others feature novel topological invariants (weak Z2 invariants) in crystalline form. We also present some of our recent results on ternary topological insulators. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V31.00002: Rashba Spin-Splitting Control at the Surface of the Topological Insulator Bi$_2$Se$_3$ Zhihuai Zhu, G. Levy, B. Ludbrook, C.N. Veenstra, J.A. Rosen, R. Comin, D. Wong, P. Dosanjh, A. Ubaldini, P. Syers, N.P. Butch, J. Paglione, I.S. Elfimov, A. Damascelli The electronic structure of Bi$_2$Se$_3$ is studied by angle-resolved photoemission and density functional theory. We show that the instability of the surface electronic properties, observed even in ultra-high-vacuum conditions, can be overcome via {\it in situ} potassium deposition. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with a tunable, reversible, and highly stable spin splitting. {\it Ab initio} slab calculations reveal that these Rashba states are derived from the 5-quintuple-layer quantum-well states. While the K-induced potential gradient enhances the spin splitting, this may be present on pristine surfaces due to the symmetry breaking of the vacuum-solid interface. Phys.Rev.Lett. 107 186405 (2011) [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V31.00003: Recent Results on Topological Phase Transition and Texture Inversion in Tunable Topological Insulators Su-Yang Xu, Yuqi Xia, Lewis Andrew Wray, Shuang Jia, Fabian Meier, Jan Hugo Dil, Jurg Osterwalder, Bartosz Slomski, Aron Bansil, Hsin Lin, Robert Cava, M. Zahid Hasan The recently discovered three-dimensional or bulk topological insulators are expected to exhibit exotic quantum phenomena. It is believed that a trivial insulator can be twisted into a topological state by modulating the spin-orbit interaction or the crystal lattice, driving the system through a topological quantum phase transition. By directly measuring the topological quantum numbers, we report the observation of a phase transition in a tunable spin-orbit system, BiTl(S$_{1-\delta}$Se$_{\delta}$)$_2$, in which the topological state formation is visualized (S.-Y. Xu \textit{et al}., \textit{Science} (2011)). In the topological state, vortex-like polarization states are observed to exhibit three-dimensional vectorial textures, which collectively feature a chirality transition as the spin momentum-locked electrons on the surface go through the zero carrier density point. Such phase transition and texture inversion can be the physical basis for observing fractional charge ($\pm$e/2) and other fractional topological phenomena. We also present some of our recent results that reveal further novel spin and electronic properties of the system close to the critical point of the topological phase transition. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V31.00004: Surface Termination of Cleaved Bi$_{2}$Se$_{3}$ Investigated by Low Energy Ion Scattering Xiaoxiao He, Zhiyong Wang, Jing Shi, Jory Yarmoff The 3D Topological Insulator, Bismuth Selenide (Bi$_{2}$Se$_{3})$, is investigated with low energy ion scattering (LEIS). Se vacancies are believed to be responsible for the metallic behavior in transport, and LEIS is uniquely sensitive to the outermost atomic layer composition. Bi$_{2}$Se$_{3}$ is comprised of Se-Bi-Se-Bi-Se quintuple layers (QLs). Since the van der Waals bonds between QLs is weaker than the covalent bonds within each QL, it has been assumed that it is Se-terminated when cleaved. This assumption has been used in previous surface studies, such as STM or ARPES, which do not provide the composition of the surface atoms. 3 keV Na$^{+}$ ions were scattered from single crystal Bi$_{2}$Se$_{3}$ cleaved in ultra-high vacuum. At room temperature, the spectra indicate a surface terminated with Bi, rather than Se, although some Se is still present. The samples display a sharp 1x1 LEED pattern, indicative of an ordered material. We conclude that Bi$_{2}$Se$_{3}$ cleaves between the QLs, but that the surface Se quickly desorbs, likely as Se$_{2}$ or Se$_{4}$. To test this, the Se:Bi ratio was monitored by LEIS after a sample was cleaved at liquid nitrogen temperature. It was found that the ratio starts out high, but decreases over the course of hours until it reaches the same value as that of a room temperature cleave. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V31.00005: Electronic Structure of Clean and Adsorbate-Covered Bi$_{2}$Se$_{3}$ Richard Hatch, Marco Bianchi, Tilo Planke, Jianli Mi, Bo Brummerstedt Iversen, Philip Hofmann The electronic structure of the topological insulator Bi$_{2}$Se$_{3}$ was probed using angle-resolved photoemission spectroscopy (ARPES). The electronic properties of clean and adsorbate-covered samples were studied for a combination of different bulk dopings and adsorbates. Due to contamination on the surface, the Dirac point of the topological surface states moves to higher binding energies, indicating an increasingly strong downward bending of the bands near the surface. This time-dependent band bending can be accelerated by intentionally exposing the surface to carbon monoxide, alkali atoms and other species. For a sufficiently strong band bending, new spectral features in the energy region of both the valence band and the conduction band are found. These changes are explained by a confinement of the states and the formation of quantum well states. This interpretation is supported by simple calculations of the band bending effects and by photon energy-dependent ARPES measurement that show how the band dispersion in the direction perpendicular to the surface is lost and non-dispersing, sharp states appear within the energy regions of the projected bulk bands. For a strong band bending, the conduction band quantum well states are strongly Rashba split. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V31.00006: Sharp Dirac cone at a buried superconductor/topological insulator interface Justin Waugh, Yue Cao, Qiang Wang, Alexei Fedorov, Z.J. Xu, Genda Gu, Daniel Dessau We have studied the fully buried interface of a topological insulator, Bi2Se3, with the conventional superconductor Nb. We characterized the chemical reactivity at the interface using core level photoemission spectroscopy. Using ARPES we are able to observe the sharp Dirac cone at the buried interface and characterize its details; including energy positions, band dispersion, and electronic scattering rates. All these measurements are carried out over a wide range of coverages, and set the stage for more advanced studies of this interface. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V31.00007: Mapping the Orbital Texture of the Topological Insulator Bi2Se3 Yue Cao, Justin Waugh, Seung Ryong Park, Qiang Wang, Theodore Reber, Sung-Kwan Mo, Matthew Brahlek, Namrata Bansal, Seongshik Oh, Genda Gu, Daniel Dessau The orbital texture of the topological insulator Bi2Se3 was observed with ARPES using linearly polarized light. The topological state features a superposition of all three p orbitals. We compare the measured orbitals to the existing density functional theory calculations in the literature. This illustrates some of the unusual properties of this topological state and helps clarify the origin of the currently conflicting results from the spin resolved ARPES measurements. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V31.00008: Spin resolved photoemission on surface doped topological insulator Bi$_2$Se$_3$ Zhihui Pan, Elio Vescovo, Alexei Fedorov, D. Gardner, S. Chu, Young S. Lee, Genda Gu, Tonica Valla Topological insulators (TL) have attracted much attention because of their exotic properties. Bi$_2$Se$_3$ is a model TL with a relative large bulk gap and a simple surface state structure. By depositing various impurities on the surface, we were able to fill the topological surface state and higher lying Rashba splitting surface states. The spin texture of the surface electronic structure was determined in spin resolved photoemission measurement. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V31.00009: Controlling the Bulk-surface Spectroscopic Separation in 3D Topological ``Metals'' Exploiting Hybridization Yi-Ting Hsu, Mark Fischer, Taylor Hughes, Eun-Ah Kim How to separately probe the surface state in 3D topological insulators (TI) with chemical potential crossing conduction bands, i.e. topological metals, is a key challenge in exploiting the topological nature of surface states. The advent of MBE grown thin films of TI's with varying thickness only adds to the importance of the issue. We study the effects of hybridization in the spirit of Fano model for the low-energy effective four-band model of $Bi_2Se_3$ on a slab. We find that the apparent bulk-surface spectroscopic separation in the ARPES data on 3D TI can be viewed as {\it a consequence of hybridization} rather than the evidence for the absence of hybridization. We describe how the separation depends on the film thickness and propose ways to control the separation using strain. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V31.00010: Topological insulators in the quaternary chalcogenide compounds and ternary famatinite compounds Y.J. Wang, H. Lin, Tanmoy Das, M.Z. Hasan, A. Bansil We present first-principles calculations to predict several three-dimensional (3D) topological insulators in quaternary chalcogenide compounds of compositions I$_2$-II-IV-VI$_4$ and ternary famatinite compounds of compositions I$_3$-V-VI$_4$. Among the large number of members of these two families, we give examples of naturally occurring compounds that are mainly Cu-based chalcogenides. We show that these materials are candidates for 3D topological insulators or can be tuned to obtain topologically interesting phases by manipulating the atomic number of the various cations and anions. A band inversion can occur at a single point $\Gamma$ with large inversion strength, in addition to the opening of a bulk bandgap throughout the Brillouin zone. We discuss how the two investigated families of compounds are related to each other by cross-substitution of cations in the underlying tetragonal structure. Work supported by the US DOE. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V31.00011: Quasiparticle band structures of $\beta$-HgS, HgSe, and HgTe Niels E. Christensen, Axel Svane, Manuel Cardona, Athanasios Chantis, Mark van Schilfgaarde, Takao Kotani The electronic structures of mercury chalcogenides in the zinc-blende strucrure have been calculated by the LDA, $GW$ (one-shot, $G_{0}W_{0}$) and quasi-particle self-consistent $GW$ ($QSGW$) approximations including spin-orbit coupling (SO). The slight tendency to overestimation of the band gaps by $QSGW$ is avoided by using a $hybrid$ scheme (20$\%$ LDA and 80 $\%$ $QSGW$. The results of $G_{0}W_{0}$ depend strongly starting wave functions and are thus quite different from those from $QSGW$. Within $QSGW$ HgS is found to be a semiconductor, with a $\Gamma_{6}$ s-like conduction-band minimum state above the valence-band top $\Gamma_{7}$ and $\Gamma_{8}$ (``negative'' SO splitting). HgSe and HgTe have ``negative'' gaps (inverted band structure). In HgTe the $\Gamma_{7}$ state is below $\Gamma_{6}$ due to the large Te SO splitting, in contrast HgSe where $\Gamma_{6}$ is below $\Gamma_{7}$. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V31.00012: Exceptionally Weak Electron-Phonon Coupling on the Surface of the Topological Insulator Bi$_2$Se$_3$ - A Promise for Room Temperature Applications Tonica Valla, Z.-H. Pan, A.V. Fedorov, D. Gardner, Y.S. Lee, S. Chu Gapless surface states on topological insulators are protected from elastic scattering on non-magnetic impurities which makes them promising candidates for low-power electronic applications. However, for wide-spread applications, these states should have to remain coherent at ambient temperatures. Here, we studied temperature dependence of the electronic structure and the scattering rates on the surface of a model topological insulator, Bi$_2$Se$_3$, by high resolution angle-resolved photoemission spectroscopy. We found an extremely weak broadening of the topological surface state with temperature and no anomalies in the state's dispersion, indicating exceptionally weak electron-phonon coupling. Our results demonstrate that the topological surface state is protected not only from elastic scattering on impurities, but also from scattering on low-energy phonons, suggesting that topological insulators could serve as a basis for room temperature electronic devices. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V31.00013: Persistence of Topological Order and Formation of Quantum Well States in Topological Insulators B2(Se,Te)3 under Ambient Conditions Chaoyu Chen, Xingjiang Zhou We report high resolution angle-resolved photoemission measurements on the surface state of the prototypical topological insulators, Bi2Se3, Bi2Te3 and Bi2Se0.4Te2.6, upon exposing to ambient conditions. We find that the topological order persists even when the surface is exposed to air at room temperature. However, the surface state is strongly modified after such an exposure. Particularly, we have observed the formation of two-dimensional quantum well states near the surface of the topological insulators after the exposure which depends sensitively on the original composition, x, in Bi2Se3-xTex. These rich information are crucial in utilizing the surface state and in probing its physical properties under ambient conditions. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V31.00014: Strong Warping Effects and Angular Momentum Structures of Topological Insulator Wonsig Jung, Y.K. Kim, B.Y. Kim, Y.Y. Koh, M. Matsunami, S. Kimura, M. Arita, K. Shimada, J.H. Han, B.K. Cho, C. Kim We performed angle resolved photoemission (ARPES) studies on Bi2Te3 with circularly polarized light. The the alignment of OAM is found to have a strong binding energy dependence. OAM close to Dirac point has an ideal chiral structure (sin$\theta$) without out-ofplane component. As the binding energy decreases, warping effect comes in and circular dichroism along the constant energy contour cannot be explained by a simple sin$\theta$ function but requires a sin 3$\theta$ term. When the warping effect becomes even stronger near the Fermi energy, circular dichroism has sin 6$\theta $ symmetry. Such behavior is found to be compatible with the theoretically predicted spin structure. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V31.00015: Three-Dimensional Massive Dirac Fermion in the Bulk Band Structure of Cubic Inverse Perovskite Ca$_3$PbO Toshikaze Kariyado, Masao Ogata The band structure of a cubic inverse perovskite Ca$_3$PbO, which has been proposed as a candidate for a topological insulator, is analyzed by means of the first-principles calculation. It turns out that Ca3PbO is actually not a topological insulator, but a close observation of the calculated band structure near the Fermi energy reveals that there exist Dirac fermions in the bulk, instead of the surface, band structure. The Dirac fermion in this material is found on the $\Gamma$-X line in the momentum space and remarkably exactly at the Fermi energy in the energy space. It should also be noted that the discovered Dirac fermion is three-dimensional and massive with a very small mass of about $10^{-2}$ of the bare electron mass. The origin of the Dirac fermion in Ca$_3$PbO and the band structure of the materials related to Ca$_3$PbO will also be discussed. [Preview Abstract] |
Session V32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - Interfaces, Layered Materials, and Growth
Sponsoring Units: DMP DCOMPChair: Matthew Dawber, SUNY Stony Brook
Room: 261
Thursday, March 1, 2012 8:00AM - 8:12AM |
V32.00001: Electric-field control of magnetization in Co$_{40}$Fe$_{40}$B$_{20 }$/ Pb(Mg$_{1/3}$Nb$_{2/3})_{0.7}$Ti$_{0.3}$O$_{3}$ structure at room temperature Sen Zhang, Y.G. Zhao, S. Rizwan, X.F. Han, J.X. Zhang, R. Ramesh Electric-field control of magnetization is important for new generation information storage technology with high integration density and low power consumption. A lot of work has been carried out on electric-field control of magnetization in artificial ferromagnetic-ferroelectric (FM-FE) two-phase systems via the piezo-strain effect. Beside strain, electric/elastic domains and phase structure also play important roles in the electric-field control of magnetization, especially in the case of amorphous FM film without magnetocrystalline anisotropy. We report a large magnetoelectric effect in a Co$_{40}$Fe$_{40}$B$_{20}$/Pb(Mg$_{1/3}$Nb$_{2/3})_{0.7}$Ti$_{0.3}$O$_{3}$ structure at room temperature. Investigations on the ferroelectric domains, phase structures, magnetic domains and strain with \textit{in situ} electric fields reveal a new mechanism for electric-field control of magnetization. This work provides a new way to realize large magnetoelectric coupling and is significant for applications, especially in the field of CoFeB-based spin valves to achieve electric-controlled magnetic random access memories. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V32.00002: Coherently Coupled ZnO and VO$_{2}$ Interface studied by Photoluminescence and electrical transport across a phase transition Amar Srivastava, S. Saha, A. Annadi, Y.L. Zhao, K. Gopinadhan, X. Wang, N. Naomi, Z.Q. Liu, S. Dhar, T.S. Herng, Bao Nina, - Ariando, Jun Ding, T. Venkatesan In this work we report a study of a coherently coupled interface consisting of a ZnO layer grown on top of an oriented VO$_{2}$ layer on sapphire by photoluminescence and electrical transport measurements across the VO$_{2}$ metal insulator phase transition (MIT). The photoluminescence of the ZnO layer showed a broad hysteresis induced by the phase transition of VO$_{2}$ while the width of the electrical hysteresis was narrow and unaffected by the over layer. The enhanced width of the PL hysteresis was due to the formation of defects during the MIT as evidenced by a broad hysteresis in the opposite direction to that of the band edge PL in the defect luminescense. Unlike VO$_{2}$ the defects in ZnO did not fully recover across the phase transition. From the defect luminescence data, oxygen interstitials were found to be the predominant defects in ZnO mediated by the strain from the VO$_{2}$ phase transition. Such coherently coupled interfaces could be of use in characterizing the stability of a variety of interfaces and also for novel device application. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V32.00003: Ferroelectricity and compositional inversion symmetry breaking in PbTiO$_{3}$/SrRuO$_{3}$ superlattices S.J. Callori, J. Gabel, D. Su, J. Sinsheimer, M.V. Fernandez-Serra, M. Dawber Most work to date on artificially layered ferroelectric superlattices has utilized the insulating titanium perovskite oxides (e.g. PbTiO$_{3}$, BaTiO$_{3}$, CaTiO$_{3}$ and SrTiO$_{3}$) as ``building blocks,'' from which a layered structure is assembled by sequential deposition. However, the need for new functionalities, particularly related to magnetism, demands that we expand this set. The much-studied compound SrRuO$_{3}$ provides the proof of concept that metallic magnetic oxides can transform into thin-film dielectric components in certain heterostructures, in this case PbTiO$_{3}$/SrRuO$_{3}$ superlattices. Our high quality epitaxial PbTiO$_{3}$/SrRuO$_{3}$ superlattices, grown by RF magnetron sputtering on SrTiO$_{3}$ substrates (with SrRuO$_{3}$ bottom electrodes) show both ferroelectricity, and, as they have both A and B site variation, compositional breaking of inversion symmetry. We will present experimental measurements made on this system using x-ray diffraction, transmission electron microscopy, and electrical characterization, which together with first principles density functional theory simulations, demonstrate how, as the constituent layer thicknesses are varied, the metallicity of the superlattice changes and polarization evolves. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V32.00004: Interface control of emergent ferroic order in Ruddlesden-Popper Sr$_{n+1}$Ti$_n$O$_{3n+1}$ Turan Birol, Nicole A. Benedek, Craig J. Fennie We have discovered from first-principles an unusual polar state in the low n Sr$_{n+1}$Ti$_n$O$_{3n+1}$ Ruddlesden-Popper (RP) layered perovskites in which ferroelectricity is nearly degenerate with antiferroelectricity, a relatively rare form of ferroic order. We show that epitaxial strain plays a key role in tuning the ``perpendicular coherence length'' of the ferroelectric mode, and does not induce ferroelectricity in these low dimensional RP materials as is well known to occur in SrTiO$_3$. These systems present an opportunity to manipulate the coherence length of a ferroic distortion in a controlled way, without disorder or a free surface. [T. Birol, N. A. Benedek, C. J. Fennie, Physical Review Letters, in press] [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V32.00005: Use of dimensionality to enhance tunable microwave dielectrics D.G. Schlom, Che-Hui Lee, R. Haislmaier, E. Vlahos, V. Gopalan, T. Birol, Y. Zhu, L.F. Kourkoutis, N. Benedek, Y. Kim, J.D. Brock, D.A. Muller, C.J. Fennie, N.D. Orloff, J.C. Booth, V. Goian, S. Kamba, M.D. Biegalski, M. Bernhagen, R. Uecker, X.X. Xi, I. Takeuchi The miniaturization and integration of frequency-agile microwave circuits---\textit{tunable} filters, resonators, phase shifters and more---with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at GHz frequencies can be tuned by applying a quasi-static electric field. Appropriate systems, e.g., Ba$_{x}$Sr$_{1-x}$TiO$_{3}$, have a paraelectric-to-ferroelectric transition just below ambient temperature, providing high tunability. Unfortunately such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss---Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$ phases---where in-plane crystallographic shear (SrO)$_{2}$ faults provide an alternative to point defects for accommodating non-stoichiometry. In this talk we will establish both experimentally and theoretically the emergence of a ferroelectric and highly tunable ground state in biaxially strained Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$ phases with $n\ge $3 at frequencies up to 40~GHz. With increasing $n$ the (SrO)$_{2}$ faults are separated further than the ferroelectric coherence length perpendicular to the in-plane polarization, enabling tunability with a figure of merit at room temperature that rivals all known tunable microwave dielectrics. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V32.00006: Ferroelectric-Antiferroelectric Phase Control: Interplay Between Octahedral Tilt, Polarization, and Chemistry at BFO-LSMO Interfaces Young-Min Kim, Pu Yu, Ying-Hao Chu, Stephen Pennycook, Sergei Kalinin, Albina Borisevich Atomically-defined interfaces between complex oxides offer a paradigm of novel electronic and coupled functionalities. Here, we report the stabilization of the ferroelectric and antiferroelectric phases at the BFO-LSMO interface though termination control and reveal associated atomic-scale mechanisms with the help of aberration-corrected scanning transmission electron microscopy combined with Electron Energy Loss Spectroscopy. The BFO thin film grown on MnO$_{2}$-terminated surface of LSMO exhibits stabilized ferroelectric phase at the interface. The interfacial and bulk polarization are antiparallel, giving rise to head to head ferroelectric domain wall mostly parallel to the interface. In comparison, the film grown on (Sr,La)O-terminated surface of LSMO exhibits antiferroelectric phase in the vicinity of the interface, with associated ferroelectric-antiferroelectric domain wall in the bulk. Details of tilt and polarization behavior, as well as electronic properties at the interfaces including charged domain walls and FE-AFE walls, will be presented. These results imply that the structural parameters such as octahedral tilt and ferroelectricity in BFO can be directly controlled by modifying the interface structure. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V32.00007: Interplay of couplings between antiferrodistortive, ferroelectric, and strain degrees of freedom in monodomain PbTiO$_{3}$/SrTiO$_{3}$ superlattices Pablo Aguado-Puente, Pablo Garcia-Fernandez, Javier Junquera We report first-principles calculations, within the density functional theory, on the coupling between epitaxial strain, polarization, and oxygen octahedra rotations in monodomain (PbTiO$_{3}$)$_{n}$/(SrTiO$_{3}$)$_{n}$ superlattices. We show how the interplay between (i) the epitaxial strain and (ii) the electrostatic conditions at the interfaces can be used to control the orientation of the main axis of the system -- defined by the direction of the polarization or the rotation axis of the oxygen octahedra. The electrostatic constrains at the interface facilitate the rotation of the polarization and, as a consequence, we predict large piezoelectric responses at epitaxial strains smaller than those that would be required considering only strain effects. In addition, ferroelectric (FE) and antiferrodistortive (AFD) modes are strongly coupled, with different rotation angles in the TiO$_{6}$ octahedra as a function of the polarization direction. The magnitude of the rotations cannot be explained by the usual steric arguments alone, and a covalent model is proposed to account for the large polarization-tilting coupling. The energy gain due to the FE-AFD coupling decreases with the periodicity of the superlattice, becoming negligible for $n \ge 3$. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V32.00008: Engineered polarization rotation in PbTiO$_{3}$/CaTiO$_{3}$ superlattices John Sinsheimer, Sara J. Callori, Youcef Benkara, Benjamin Bein, Jon Daley, Dong Su, Matthew Dawber Large piezoelectric responses, such as those seen in PbZr$_{x}$Ti$_{1-x}$O$_{3}$ in the vicinity of the compositional morphotropic phase boundary, can occur when the direction of the polarization in a ferroelectric material can rotate. Here we show experimentally that a similar enhancement of the piezoelectric response can be achieved in artificially layered epitaxial superlattices composed of alternating layers of PbTiO$_{3}$ and CaTiO$_{3}$ deposited on SrTiO$_{3}$ substrates by RF magnetron sputtering. The exceptional quality of our samples is demonstrated by x-ray diffraction and transmission electron microscopy. The structural and functional properties of the materials have been measured as a function of relative layer thickness. Electrically measured ferroelectric polarization and dielectric constants corroborate the enhancement of d$_{33}$ we have measured experimentally using piezoforce microscopy. The structural changes, which allow polarization rotation, have been directly measured using grazing incidence in-plane x-ray diffraction (at NSLS X21 and X22C). Finally, the as-grown domain structure has been imaged with piezoforce microscopy, further confirming polarization rotation and explaining the unusual switching dynamics observed in our electrical characterization. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V32.00009: In situ studies on ferroelectric BaTiO$_{3}$ interface Junsoo Shin, Von Braun Nascimento, Ward Plummer, Jiandi Zhang, Albina Borisevich, Vincent Meunier, Sergei Kalinin, Arthur Baddorf Ferroelectric phase stability in ferroelectric films is critically dependent on the surface and interface phenomena, especially governed by electrostatic depolarization energy. Predictions for the minimum critical film thickness for ferroelectricity have continuously decreased down to few unit cells. We have examined surface/interface atomic structures of ultrathin BaTiO$_{3}$ (BTO) films grown on conductive SrRuO$_{3}$ (SRO) and Nb-doped SrTiO$_{3}$. The surface structure of BTO/SRO was refined using in-situ Low Energy Electron Diffraction (LEED) I-V, resulting to observation of polar distortion in ultrathin ($\ge $ 4 ML) BTO films. The in-situ Scanning Tunneling Microscopy (STM) has been performed prior and after BTO deposition on SRO. However, the unusual 2x2 reconstruction is observed for 1-2 ML BTO films and bare SRO by STM. The surface reconstruction of SRO bottom electrode is shown to affect the interface of films deposited subsequently which could be reflected in ultrathin film properties. The in-situ LEED I-V structural studies on 1-2 ML BTO interface have been performed without SRO layer, which kept ultrathin BTO films from the preclusion of reconstructed SRO films. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V32.00010: Ellipsometric study of SrTiO$_{3}$ thin film grown Si(100) Yao Tian, Carolina Adamo, Kenneth Burch Recently, a new method to grow SrTiO$_{3}$ thin ferroelectric film directly on Si(100) has been demonstrated by \textit{M. P. Warusawithana, et al}. We use ellipsometry to study the film and a model based on inhomogenous gap to oxide deficiency was made to interpret the data. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V32.00011: Valence Fluctuation in UltrathinTi$_{1+x}$O$_{2}$ on Rutile TiO$_{2}$ Q.Y. Chen, P.V. Wadekar, H.J. Hunag, S.W. Yeh, N.J. Ho, H.W. Seo, W.K. Chu The physical properties of metal oxides (TMO) can change drastically depending on the non-stoichiometry of oxygen, for which rutile TiO$_{2}$ single crystals were self-implanted with Ti to acquire a layers of $\sim $6 nm populated with extra Ti found in +2, +3, and +4 charge states as revealed by X-ray photoelectron spectroscopy (XPS). The formations of the TiO and Ti$_{2}$O$_{3}$ crystalline phases were verified by high-resolution transmission electron microscopy (HRTEM). Variable-temperature electrical resistivity measurement suggests the occurrence of charge ordering at low temperatures, where weak localization of the charges take hold, commonly observed in heavily doped semiconductors. The magneto-transport behaviors follow mixed scenario of band conduction and fixed range hopping, albeit with different energy scales and weighting factors demarcated by a transition temperature of MR sign change, a signature of valence fluctuation in which the ordered low-temperature phase would melt into the high-temperature disordered phase, or by condensation in reverse. This work was supported by the National Science Council, the Ministry of Education, Taiwan, the National Science Foundation, and the State of Texas through Texas Center for Superconductivity. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V32.00012: Atomic Layer-by-Layer Growth of Homoepitaxial SrTiO$_{3}$ Films by Laser MBE Qingyu Lei, Guozhen Liu, Ke Chen, Suilin Shi, Fuqiang Huang, Xiaoxing Xi Two most effective techniques for oxides film growth are reactive MBE and laser MBE. With alternating monolayer growth, reactive MBE has shown the capacity of stoichiometry control and crystalline perfection in SrTiO$_{3}$ films on SrTiO$_{3}$, while most works of laser MBE, using compound targets, often shows non-stoichiometry and lattice defects. In order to control layer-by-layer growth to atomic level, we carried out laser MBE from separate oxide targets, for example, growing SrTiO$_{3}$ from SrO and TiO$_{2}$ targets, such that the SrO and TiO$_{2}$ layers were deposited alternatively one atomic layer at a time. X-ray diffraction spectra showed that the stoichiometric SrTiO$_{3}$ film peak overlapped with and was indistinguishable from the SrTiO$_{3 }$substrate peak, while the off stoichiometric SrTiO$_{3}$ films, either Sr rich or poor, showed lattice expansion. We conclude that laser MBE from separate oxide targets can achieve the same stoichiometry control as reactive MBE. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V32.00013: Preparation of Atomically Flat SrTiO$_3$ Surfaces Using Non-acid Etching and Thermal Annealing Process John Connell, Ambrose Seo The growth of epitaxial thin films and heterostructures requires atomically flat surfaces of substrates. So far chemical etching processes by acidic etchant have been widely used for the surface preparation of oxide substrates. Here we show that atomically flat surfaces of single crystalline SrTiO$_{3}$ substrates of both the (100) and (111) orientations can be prepared using non-acidic etching and thermal annealing techniques. Atomic force microscopy confirms the evolution of the surface of the substrates from rough to a step-terrace structure. Scanning tunneling microscopy shows that SrO segregation on the surface is removed by our etching process. This new technique replaces the use of acidic oxide etchant as the primary method for etching SrTiO$_{3}$ substrates. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V32.00014: Effect of Mg$^{2+}$ on the structure of amorphous CaCO$_3$ -- A molecular dynamics simulation Hidekazu Tomono, Hiroki Nada Molecular dynamics (MD) simulations of amorphous calcium carbonate (ACC) were carried out to investigate the effect of Mg$^{2+}$ ions on the structure of CaCO$_3$ crystal nucleus formed from ACC\@. Our systems contained 432 CaCO$_3$ with several concentrations of MgCO$_3$. In this study, our original ion model of Mg$^{2+}$ was developed and combined with Raiteri model of Ca$^{2+}$ and rigid CO$_3$$^{2-}$ [1]. The simulations indicated that the fraction of vaterite-like ion arrangement was much larger than those of calcite-like and aragonite-like ion arrangements in pure ACC\@. However, as the Mg$^{2+}$ concentration increased, the faction of vaterite-like ion arrangements decreased, which suggests that Mg$^{2+}$ ions play as inhibitors of vaterite nucleation. The result explains why calcite or aragonite is preferentially nucleated in the presence of Mg$^{2+}$, whereas vaterite is nucleated in the absence of them.[1] P. Raiteri, J.~D. Gale, D. Quigley, and P.~M. Rodger, J.~Phys.\ Chem.\ C 114, 5997 (2010). [Preview Abstract] |
Session V34: Focus Session: Impact of Ultrafast Lasers V: Applications II
Sponsoring Units: DCPChair: Amber Krummel, Colorado State University
Room: 107A
Thursday, March 1, 2012 8:00AM - 8:12AM |
V34.00001: Thermalization of photoexcited electrons in bismuth investigated by time-resolved THz spectroscopy and {\it ab initio} calculations Iurii Timrov, Tobias Kampfrath, Jerome Faure, Nathalie Vast, Christian Ast, Christian Frischkorn, Martin Wolf, Paola Gava, Luca Perfetti The charge carrier dynamics of photoexicted bismuth generates a Drude response that evolves in time. In contrast to graphite, the plasma frequency of bismuth displays an initial increase and a subsequent decay. We have performed {\it ab initio} calculations of bulk bismuth within the density functional theory, and show that the non-monotonic behaviour of the plasma frequency is due to the presence of local minima in the conduction band: most of the photoexicted electrons first accumulate in these local minima, and reach the L point only 0.6 ps after photoexcitation. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V34.00002: Spatio-temporal dynamics of plasmons on a metal cone Alejandro Rodriguez Perez, Joonhee Lee, Shawn M. Perdue, V. Ara Apkarian Localized surface plasmons (LSP) and propagating plasmon modes (PPM) field emit at the singularity of the metallic tip apex upon ultrafast optical excitation. Interferometric measurements, using frequency modulation-demodulation techniques, allow the characterization of the time profile of the optical field emission and the space-time profile of the propagating modes. A pump pulse launches a plasmon that oscillates on the cone and coherently interferes with a time-delayed probe pulse which is cross-polarized with respect to the pump. We present a simple model that accurately describes the experimental results. However, the observations of trapped propagating modes, their lifetime, and their polarization are not predicted by current theories, which describe tip-plasmons as strictly transverse magnetic modes. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V34.00003: Time-Resolved ARPES Study of Ultrafast Dynamics in Cuprate Superconductor Bi2212 Jianqiao Meng, Georgi L. Dakovski, Jian-Xin Zhu, Peter S. Riseborough, Genda Gu, Steve M. Gilbertson, George Rodriguez, Tomasz Durakiewicz The momentum-dependent ultrafast dynamics in cuprate superconductor Bi2Sr2CaCu2O8 is investigated using ultra high resolution time-resolved angle-resolved photoemission spectroscopy (tr-ARPES). In this talk, we will present our observation and analysis of dynamics of the quasiparticle states in Bi2Sr2CaCu2O8 in nodal and antinodal direction as a function of doping. Work was performed with temporal resolution of 35fs. The consequences of our findings in terms of nodal-antinodal dichotomy will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V34.00004: SFG and SPR Study of Sodium Dodecyl Sulfate Film Assembly on Positively Charged Surfaces Sanghun Song, Tobias Weidner, Matthew Wagner, David Castner This study uses sum frequency generation (SFG) vibrational spectroscopy and surface plasmon resonance (SPR) sensing to investigate the structure of sodium dodecyl sulfate (SDS) films formed on positively charged and hydrophilic surfaces. The SPR signals show a good surface coverage suggesting that full monolayer coverage is reached at 1 mM. SFG spectra of SDS adsorbed exhibits well resolved CH$_{3}$ peaks and OH peaks. At both 0.2 mM and 1 mM SDS concentration the intensity of both the CH$_{3}$ and OH peaks decreased close to background levels. We found that the loss of SFG signal at 0.2 mM occurs at this concentration independent of surface charge density. It is more likely that the loss of signal is related to structural inhomogeneity induced by a striped phase - stand-up phase transition. This is supported by a distinct change of the relative SFG phase between CH$_{3}$/OH near 0.2 mM. The second intensity minimum might be related to charge compensation effects. We observed a substrate dependence for the high concentration transition. We also observed distinct SFG signal phase changes for water molecules associated with SDS layers at different SDS solution concentrations indicating that the orientation of bound water changed with SDS surface structure. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V34.00005: Nanoscale sensing with surface-enhanced CARS spectroscopy Dmitri Voronine, Alexander Sinyukov, Xia Hua, Guowan Zhang, Wenlong Yang, Kai Wang, Pankaj Jha, George Welch, Alexei Sokolov, Marlan Scully Time-resolved coherent anti-Stokes Raman scattering (CARS) is extended to the nanoscale regime where nano-molar amounts of molecules may be investigated. We show that spectral resolution of CARS generated by ultrashort laser pulses may be improved by collecting a sequence of time-resolved spectra. This procedure provides additional linewidth information and allows detecting two species of pyridine molecules in a vicinity of aggregated gold nanoparticles. Surface-enhanced CARS (SE-CARS) signals of the adsorbed pyridine monolayer are detected in the presence of a bulk pyridine background CARS signal. Time-resolved SE-CARS signals are stronger than the conventional CARS, and more sensitive to the surface environment which makes them suitable for nano-surface characterization with high molecular specificity. This technique allows measuring the vibrational dephasing time of molecules on nano-surfaces and characterizing the effects of the surface local environment on the ultrafast molecular dynamics. This technique may be applied to a variety of artificial and biological systems and complex molecular mixtures and has a potential for nanophotonic sensing applications. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V34.00006: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V34.00007: Time-resolved photocurrent generation in graphene $p-n$ junctions; probing $e-h$ relaxation dynamics near the Fermi level Matt Graham, Su-Fei Shi, Daniel Ralph, Jiwoong Park, Paul McEuen In this contribution we probe the relevant timescales associated with photocurrent generation by optically exciting graphene at a $p-n$ junction defined by electrostatic gates. Time-resolution of the optical and electrical response are obtained by simultaneous collection of transient photocurrent and transient absorption using a two-pulse correlation approach with 160 fs resolution. The transient absorption signal decays biexponentially in the $p-n$ junction at 0.26 and 2.2 ps; timescales similar to those recently established for hot electron and hot optical phonon cooling in graphene sheets. By contrast, we find transient photocurrent decay signal is surprisingly slow, with tails extending for 100s of ps. At 10 K, the rate coefficient for transient photocurrent decay is 2.7 $\pm $ 0.6 ps$^{-1}$n$A^{-1}$, and it increases monotonically tenfold upon warming to room temperature. We attribute the stark difference in the transient photocurrent and absorption kinetics to a photocurrent response that is sensitive to recombination processes that are occurring well below optical excitation energies, near the Fermi level. Combining the measurements, transient photocurrent alongside the well-studied transient absorption allows us to construct a rough timeline of events associated with photocurrent production at a graphene $p-n$ junction. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V34.00008: Standoff detection of trace compounds enabled by continuum pulse shaping and coherent Raman scattering Marshall T. Bremer, Orin Yue, Vadim V. Lozovoy, Marcos Dantus Raman spectroscopy has long been pursued as means to detect hazards from a safe distance. This approach promises high chemical specificity, but is limited in sensitivity because of the very small Raman cross-section. We recently demonstrated detection of trace quantities using a non-linear counterpart, coherent anti-Stokes Raman scattering (CARS), which offers large signal enhancement over spontaneous Raman due to coherent signal addition. Utilizing a pulse shaper and the bandwidth inherent in a 5fs laser pulse, CARS spectra were acquired from an explosive simulant dissolved within thin polymer films. Further, the pulse shaper offers total control of the non-linear process, including selective excitation of particular vibrational modes, enabling single channel detection and associated opportunities for rapid chemical imaging. We will present standoff chemical images and associated CARS spectra acquired in a standoff configuration, demonstrating the applicability of the new spectroscopy in a realistic environment. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V34.00009: Transient Absorption Spectroscopy for the Detection of Multiexciton States in Semiconductor Quantum Dots Amanda Neukirch, Oleg Prezhdo It has been shown that excitations of quantum dots at photon energies well above the band gap can produce a superposition of single exciton (SE) and multiple exciton (ME) states. The resulting electron hole dynamics can then be monitored using transient absorption (TA) spectroscopy. We have modeled the TA signal that can be expected when Pb$_{68}$Se$_{68}$ and Si$_{29}$H$_{24}$ are excited, and subsequently probed with attosecond laser pulses. In the proposed experiment a pump pulse serves to excite the sample. The energy of the probe pulse is chosen such that any detected absorption is attributable to the presence of a ME state. Oscillations in the absorption spectrum indicate the existence of a coherent superposition. The period of the oscillation depends on the energy difference between the superimposed states. We explore how dephasing of this signal is affected by the temperature of the system, as well as the number of states involved. We predict what can and cannot be measured experimentally and propose specific attosecond experimental scenarios. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V34.00010: How much information do ultrafast spectra contain? The case for ultrafast quantum process tomography Alan Aspuru-Guzik In this talk, I will describe the concept of quantum process tomography (QPT). The QPT protocol is a novel way of thinking about ultrafast spectra that allows to design families of experiments for extracting the quantum dynamical map of any system addressable by ultrafast spectroscopy. I will explain how QPT works, and also will describe our progress towards an experimental realization in collaboration with the group of Keith Nelson (MIT). [Preview Abstract] |
Session V35: Focus Session: DFT VII: Time-Dependent Processes I: Driven Systems
Sponsoring Units: DCPChair: Adam Wasserman, Purdue University
Room: 107B
Thursday, March 1, 2012 8:00AM - 8:36AM |
V35.00001: Towards the ab-initio description of photo-induced processes Invited Speaker: E.K.U. Gross Excitons are prominent features in the optical spectra of periodic solids and molecular aggregates. Time-dependent density functional theory should, in principle, be able to describe excitonic effects. However, with standard functionals for the exchange-correlation (xc) kernel f$_{xc}$, such as adiabatic LDA and adiabatic GGA, excitonic features are completely absent. The construction of improved functionals for f$_{xc}$ yielding excitons has a long history. Here we propose a new parameter-free approximation for the xc kernel through an algorithm in which the exact Dyson equation for the response is solved self-consistently with an approximate expression for the kernel in terms of the dielectric function [PRL \textbf{107}, 186401 (2011)]. We apply this to the calculation of optical spectra for various small bandgap (Ge, Si, GaAs, AlN, TiO2, SiC), large bandgap (C, LiF, Ar, Ne) and magnetic (NiO) insulators. The calculated spectra are in very good agreement with experiment for this diverse set of materials, highlighting the universal applicability of the new kernel. These optical spectra are calculated with clamped nuclei. However, in a variety of optical phenomena, the coupling between electronic and nuclear motion plays an important role. Prominent examples are the process of vision and photo-synthesis. Standard approximations such as Ehrenfest dynamics, surface hopping, or nuclear wave-packet dynamics only partially capture the occurring non-adiabatic effects. As a first step towards a full ab-initio treatment of the coupled electron-nuclear system, we deduce an exact factorization of the complete wavefunction into a purely nuclear part and a many-electron wavefunction which parametrically depends on the nuclear configuration. We derive formally exact equations of motion for the nuclear and electronic wavefunction [PRL \textbf{105}, 123002 (2010)]. These exact equations lead to a rigorous definition of time-dependent potential energy surfaces as well as time-dependent geometric phases. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V35.00002: Exact factorization of the time-dependent electron-nuclear wavefunction: Time-dependent potential energy surface Ali Abedi, Federica Agostini, Neepa T. Maitra, E.K.U. Gross We have recently proved an exact decomposition of the electronic and nuclear degrees of freedom (Phys, Rev. Lett. 105 123002 (2010)) and introduced a set of coupled equations of motion for the electrons and nuclei that describe the evolution of the complete electron-nuclear system. The nuclear equation is particularly apealing being a Schroedinger equation with a time-dependent potential energy surface (TDPES) and a time-dependent vector potential as rigorous concepts, mediating the coupling between the nuclear and the electronic degrees of freedom in a formally exact way. By studying physically different cases, we demonstrate that the TDPES is a powerful tool to investigate molecular processes. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V35.00003: Exact factorization of the full electron-nuclear wavefunction: A quantum-classical study Federica Agostini, Ali Abedi, Eberhard Gross It was recently shown in [1] that the solution of the time-dependent Schr\"{o}dinger equation for a molecular system can be exactly factorized to an electronic and a nuclear contribution. In [1], the authors derived exact equations of motion for the coupled evolution of the electronic and nuclear wavefunctions, which are a good starting point to develop approximations, systematically. Based on this exact decomposition of the electron and nuclear motion, we present a quantum-classical scheme for the coupled electron-nuclear dynamics. Nuclear degrees of freedom evolve along a classical trajectory, affecting electronic motion and inducing quantum transitions, which in turn alter nuclear dynamics. Applications of the proposed method to model systems will be presented.\\[4pt] [1] A. Abedi, N.T. Maitra and E.K.U. Gross, \textsl{Phys. Rev. Lett.} \textbf{105} 123002 (2010). [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V35.00004: Time-dependent density functional theory for open quantum systems David Tempel, Alan Aspuru-Guzik We present the extension of time-dependent density functional theory (TDDFT) to the realm of open quantum systems (OQS). OQS-TDDFT allows a first principles description of electronic systems undergoing non-unitary dynamics due to coupling with a bath, such as that arising from molecular vibrations, solvent degrees of freedom or photon modes of the electromagnetic field. We first prove extensions of the Runge-Gross and van Leeuwen theorems to OQS-TDDFT, which rigorously establish it as a formally exact theory. We then discuss development of approximate OQS-TDDFT functionals, exact conditions on these functionals, as well as future challenges. Finally, we will discuss the application of OQS-TDDFT in obtaining broadened absorption spectra. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V35.00005: Time-Dependent Electron Localization Functions, TDELF's, for Molecular Ionization and Harmonic Generation in Intense Laser Pulses Andre D. Bandrauk, Emmanuel Fowe Penka The nonlinear nonperturbative response of N2,CO2,OCS,CS2 are studied numerically by solutions of Kohn-Sham (KS) orbital equations in the presence of few cycle intense I$>$10$^{14}$ W/cm$^2$, 800nm laser pulses. It is found generally that ionization rates depend on different functionals and ionization also occurs from inner-shell KS orbitals .This is sensitive to laser-molecule orientation as predicted earlier [1]. Ionization rate maxima correspond to the alignment of maximum KS orbital densities with the laser polarization instead of orbital ionization potentials,Ip. These results are corroborated through time analysis of TDELF's where ionization occurs from lone pair or bond regions of the corresponding molecule at various times during the pulses. Time frequency analysis of Harmonic Generation spectra allow for identification of recollision times of ionized electrons with the parent ion.\\[4pt] [1] EF Penka, AD Bandrauk, Phys Rev A81, 023411(2010); 84, 035412(2011). [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 10:00AM |
V35.00006: TDDFT for nonlinear phenomena of light-matter interactions Invited Speaker: Angel Rubio Despite the success of linear-response schemes to describe excitations of many electron systems,many physical processes stemming from the interaction of light with matter are nonlinear in nature. In this talk we will address the problems and open questions related to the description of this phenomena with the goal of providing a sound description of laser-induced-population processes within TDDFT. Through the exact solution of a few electron system interacting with a monochromatic laser we highlight some common deficiencies of all adiabatic density functionals within time-dependent density-functional theory to handle photoinduced processes leading to population changes of many-body states. One prototype case is Rabi oscillations between the ground and an excited state when a monochromatic laser with a frequency close to the resonance is applied. All adiabatic functionals are not able to discern between resonant and nonresonant (detuned) Rabi oscillations. Only the inclusion of an appropriate memory dependence can correct the fictitious time-dependence of the resonant frequency. We extend this description to dynamical induced charge transfer processes and many body tunneling. Adiabatic functionals will fail similarly in the description of all processes involving a change in the population of states. We will show our recent advances in deriving a new memory-dependent functional. The description of photo-induced processes in chemistry, physics, and biology and the newfield of attosecond electron dynamics and high-intense lasers all demand fundamental functional developments going beyond the adiabatic approximation. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V35.00007: Performance of the exact adiabatic density functional to describe Rabi physics Johanna Ildemar Fuks, Nicole Helbig, Heiko Appel, Ilya Tokatly, Angel Rubio Through the exact solution of few-electron systems interacting with a monochromatic laser we study the performance of adiabatic density functionals within time-dependent density-functional theory (TDDFT) to reproduce Rabi oscillations. The non-linear dynamics of the Kohn-Sham (KS) system shows the characteristic features of detuned Rabi oscillations even if the exact resonant frequency is used. We illustrate this effect by comparing the exact time-dependent many-body solution of a He-atom in one dimension and a few-site Hubbard model with the solution of TDDFT-KS equations for different adiabatic exchange-correlation functionals. Preventing the detuning introduces a new strong condition to be satisfied by approximate new xc-functionals. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V35.00008: Curing pathologies of TDDFT with semiclassical electron correlation Peter Elliott, Neepa Maitra Time-dependent density functional theory (TDDFT) has been identified as a prime candidate for describing the dynamics of atoms and molecules in strong laser fields. It would be especially useful in predicting the highly non-intuitive fields needed in the optimal control problem for photonic reagents. However TDDFT encounters a number of problems when simulating these systems, both in the functional approximation of the exchange and correlation potentials, but also for observables of interest, such as momentum distributions or ionization probabilities. Recently a method to overcome some of these problems was proposed[1], whereby an auxiliary semiclassical dynamics calculation produces a correlation potential, which is then used to drive a density-matrix propagation. This method incorporates memory, initial-state dependence, changing occupation numbers, all of which are known issues for TDDFT. In this talk we build on previous work[2] and examine this scheme for a number of model 1D systems including calculations for strong field dynamics, optimal control, non-sequential ionization, and double excitations [1] A.K. Rajam, I. Raczkowska,and N.T. Maitra, Phys. Rev. Lett. 105, 113002 (2010). [2] P. Elliott and N.T. Maitra, J. Chem. Phys. 135, 104110 (2011). [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V35.00009: Quantum dynamics integrators in a plane-wave implementation of time-dependent density functional theory Andr{\'e} Schleife, Alfredo Correa, Yosuke Kanai In order to develop our understanding of various dynamical phenomena in materials at the electronic-structure level, computational methods based on first-principles theory are indispensable. Such approaches can make significant contributions to improving materials for a wide range of applications reaching from photovoltaic cells to nuclear reactors. Time-dependent density functional theory is an efficient approach for describing the real-time quantum dynamics of electronic systems. However, the numerical integration of the time-dependent Kohn-Sham equations, i.e., including a density-dependent effective Hamiltonian, is highly non-trivial. We studied various integrators for propagating the single-particle wave functions explicitly in time, in order to achieve a highly-scalable implementation based on plane-waves and pseudo-potentials. We show that the fourth-order Runge-Kutta scheme is conditionally stable and accurate within this framework. Moreover, the application of our scheme to a system consisting of several hundreds of electrons unveils its excellent scalability and, hence, its applicability for large-scale simulations. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V35.00010: The effect of cusps in time-dependent quantum mechanics Zenghui Yang, Neepa Maitra, Kieron Burke Spatial cusps in initial wavefunctions can lead to non-analytic behavior in time. We suggest a method for calculating the short-time behavior in such situations. For these cases, the density does not match its Taylor-expansion in time, but the Runge-Gross proof of time-dependent density functional theory still holds, as it requires only the potential to be time-analytic. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V35.00011: Demonstration of the Gunnarsson-Lundqvist theorem and Lack of Hohenberg-Kohn Theorem for Excited States Xiao-Yin Pan, Yu-Qi Li, Biao Li, Viraht Sahni By considering two noninteracting fermions in a 1-D infinite square well we demonstrate the following: (a) The GL theorem is satisfied for the lowest excited triplet state. There exists only one potential for this density so that the HK theorem is satisfied for this lowest excited state configuration. (b) For the second excited triplet state, there exists no other potential with the original orbital configuration that reproduces the density. However, (c) for the second excited triplet state, there exist other potentials with orbital configurations different from the original that reproduce the density. Thus, there is no HK theorem for this and other excited states. The orbitals of the other set of potentials are related to the original orbitals by a rotation. The exact new eigenvalues, and solutions for the potentials and orbitals including near and at the boundaries are provided. [Preview Abstract] |
Session V36: General Chemical Physics
Sponsoring Units: DCPChair: David Leitner, University of Nevada, Reno
Room: 107C
Thursday, March 1, 2012 8:00AM - 8:12AM |
V36.00001: Measurement of kinetic and potential energy deposition in highly charged ion collisions with surfaces R.E. Lake, J.M. Pomeroy, C.E. Sosolik We measure craters in thin dielectric films formed by highly charged Xe$^{Q+}$ (26 $\leq Q \leq$ 44) projectiles [1]. Tunnel junction devices with ion-irradiated barriers were used to amplify the effect of charge-dependent cratering through the exponential dependence of tunneling conductance on barrier thickness. Electrical conductance of a crater $\sigma_{c}(Q)$ increased by 4 orders of magnitude ($7.9 \times 10^{-4}\mbox{ }\mu$S to \\6.1 $\mu$S) as $Q$ increased, corresponding to crater depths ranging from 2 to 11 \mbox{\AA}. By employing a heated spike model, we determine that the energy required to produce the craters spans from 8 to 25 keV over the investigated charge states where kinetic energies were $(8 \times Q)$ keV. We partition crater formation energy into potential and kinetic contributions to find that at least (27 $\pm$ 2) $\%$ of the available ion potential energy is required. Decreasing projectile kinetic energy at constant $Q$, provides a new test for charge-dependent kinetic energy loss theory.\\[4pt] [1] R.E. Lake, J.M. Pomeroy, H. Grube, C.E. Sosolik, Phys. Rev. Lett. \textbf{107}, 063202 (2011) [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V36.00002: Charge transport in single photochromic molecular junctions Youngsang Kim, T. Pietsch, Elke Scheer, T. Hellmuth, F. Pauly, D. Sysoiev, T. Huhn, T. Exner, U. Groth, U. Steiner, A. Erbe Recently, photoswitchable molecules, i.e. diarylethene, gained significant interest due to their applicability in data storage media, as optical switches, and in novel logic circuits [1]. Diarylethene-derivative molecules are the most promising candidates to design electronic functional elements, because of their excellent thermal stability, high fatigue resistance, and negligible change upon switching [1]. Here, we present the preferential conductance of specifically designed sulfur-free diarylethene molecules [2] bridging the mechanically controlled break-junctions at low temperatures [3]. The molecular energy levels and electrode couplings are obtained by evaluating the current-voltage characteristics using the single-level model [4]. The charge transport mechanism of different types of diarylethene molecules is investigated, and the results are discussed within the framework of novel theoretical predictions. \\[4pt] [1] M. Del Valle etal., \textit{Nat Nanotechnol} \textbf{2}, 176 (2007) S. J. van der Molen etal., \textit{Nano. Lett.} \textbf{9}, 76 (2009).\\[0pt] [2] D. Sysoiev etal., \textit{Chem. Eur. J.} \textbf{17}, 6663 (2011).\\[0pt] [3] Y. Kim etal., \textit{Phys. Rev. Lett. }\textbf{106}, 196804 (2011).\\[0pt] [4] Y. Kim etal., \textit{Nano Lett.} \textbf{11}, 3734 (2011). L. Zotti etal., \textit{Small} \textbf{6}, 1529 (2010). [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V36.00003: Length and temperature dependent crossover of charge transport across molecular junctions Ya-Lin Lo, Shih-Jye Sun, Ying-Jer Kao We study the electronic transport in a molecular junction, in which each unit is coupled to a local phonon bath, using the non-equilibrium Green's function method. We observe the conductance oscillates with the molecular chain length and the oscillation period in odd-numbered chains depends strongly on the applied bias. This oscillatory behavior is smeared out at the bias voltage near the phonon energy. For the phonon-free case, we find a crossover from tunneling to thermally activated transport as the length of the molecule increases. In the presence of electron-phonon interaction, the transport is thermally driven and a crossover from the thermally suppressed to assisted conduction is observed. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V36.00004: Electrical properties of a layered manganese vanadate Victoria Soghomonian, Qifan Yuan, Elinor Spencer, Nancy Ross We present the electrical characteristics of a layered manganese vanadate. Octahedral Mn and tetrahedral V units form layers that in turn are connected to each other via weakly bonded strontium ions. The Mn sites are also connected to each other through bridging oxygen atoms that are partially protonated, allowing for possible proton conduction in the material. The conductivity is dependent on crystal direction. Variable temperature conductivity measurements, from 160 to 830 K, show semiconducting behavior with average activation energy of 0.35 eV. Around 670 K a dip in the conductivity is observed, correlated with loss of water from the structure inferred from thermogravimetric analysis. Above 760K, an increase in conductivity is observed. Single crystal x-ray analysis is performed on samples heated above 670 K, to probe temperature induced structural changes. Preliminary results show a contraction of one of the unit cell axes corresponding to the loss of the bridging oxygen and the ensuing movement of the two Mn sites closer to each other. Single crystal x-ray investigations of the material under hydrostatic pressure in a DAC are also performed, to probe the influence of structural changes on electronic transport properties. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V36.00005: Capturing Ion-Solid Interactions with MOS structures R. Shyam, W.R. Harrell, J.E. Harriss, C.E. Sosolik We have fabricated metal-oxide-semiconductor (MOS) devices for a study of implantation rates and damage resulting from low energy ion-solid impacts. Specifically, we seek to capture ion irradiation effects on oxides by exposing as-grown SiO$_{2}$ layers (50 nm to 200 nm) to incident beams of alkali ions with energies in the range of 100 eV to 10 keV. The oxide is analyzed post exposure by encapsulating the irradiated region under a top metallic contact or within a finished MOS device. Characterization of the resulting ion-modified MOS device involves the standard techniques of room temperature and bias-dependent capacitance-voltage (C-V) measurements. The C-V results reveal alkali ion-induced changes in the flatband voltage of irradiated devices which can be used to extract both the range and implantation probabilities of the ions. Biased C-V measurements are utilized to confirm the concentration or dosage of ions in the oxide. A triangular voltage sweep (TVS) measurement at elevated temperatures also reveals the total ionic space charge in the oxide and can be used to extract a mobility for the ions as they pass through the damaged oxide. Comparisons of these measurements to standard device models as well as to ion range calculations in the oxide are presented. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V36.00006: Self-consistent full counting statistics of inelastic transport Tae-Ho Park, Michael Galperin The full counting statistics (FCS) of inelastic transport in molecular junctions is considered for the case of weak electron-vibration coupling. We introduce a self-consistent procedure for FCS within the non-equilibrium Green function (NEGF) method, and discuss its importance in two aspects. First, we show that in the case of FCS the self-consistent treatment provides a conserving approximation. Second, we discuss the importance of molecular vibration renormalization for the counting statistics of electron transport. We consider two-level bridge with diagonal and off-diagonal electron-vibration couplings. The latter model is shown to be especially sensitive to renormalization of the vibration. We show that heating the molecular vibration may lead to either an increase or a decrease in current through the junction depending on the strength of electron-vibration coupling in the bridge compared to molecule-contact coupling. We report an appearance of super-Poissonian noise induced by the non-equilibrium vibration at resonance, which is similar to the effect of the avalanche transport previously reported in the literature for a system with a strong electron-vibration interaction. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V36.00007: Measurement of the Spectral Distribution of Low Energy Electrons Emitted as a Result of M$_{3}$VV Auger Transitions in Cu(100) Suman Satyal, Prasad Joglekar, Karthik Sashtry, Alexander Weiss, Steven Hulbert Auger Photoelectron Coincidence Spectroscopy (APECS) was used to investigate the physics of the Low Energy Tail (LET) region of the Auger spectrum of a Cu(100) sample. A beam of 200eV photons was used to probe the sample. Two Cylindrical Mirror Analyzers (CMAs) were used to select the energy of electrons emitted from the sample. An APECS technique was used to obtain an Auger spectrum with one of the CMAs fixed at the core photoemission peak. The spectrum contains the extrinsic contributions from electrons excited by the M$_{3}$VV Auger transition plus a background due to true coincidence between photo-emitted valence band electrons that undergo inelastic~scattering and other valence electrons. To remove the extrinsic contribution to the LET of the Auger Spectrum, Coincidence measurements were made with the fixed analyzer set at various energies (150eV, 165eV, 180eV, 190eV and 197eV) between the core and the valence band and obtain an estimate of the background due to the inelastic scattering of the valence band electrons. The extrinsic contribution to the LET was then subtracted to get the final spectrum consisting of the secondary electrons that are intrinsic to the M$_{3}$VV Auger transition only. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V36.00008: Interaction of Water Layers on Calcite Surfaces Rashid Hamdan, Hai-Ping Cheng Calcite is a mineral of great interest because its abundance in both geological and biological systems. While the $\{10\hat{1}4\}$ surface largely dominates the calcite morphology, other surfaces consisting of $\{10\hat{1}4\}$ terraces and steps are important for the crystal dissolution or growth in aquas environment. We use ab-initio calculations to study the interaction of single water molecule and one and two water layers with the flat $\{10\hat{1}4\}$ calcite surface and two step surfaces: $\{10\hat{1}3\}$ and $\{10\hat{1}5\}$ made of $\{10\hat{1}4\}$ terraces offset by one atomic layer along the $\{10\hat{1}1\}$ and the $\{0001\}$ surface respectively. Preliminary results show that the first layer of water bond strongly to the calcite surface. However, dissociation of the water molecules is not favored on the surface. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V36.00009: Tracking Amino Acids in Chiral Quantum Corrals Esmeralda Yitamben, Rees Rankin, Erin Iski, Jeffrey Greeley, Richard Rosenberg, Nathan Guisinger Engineering molecular superstructures on metals opens great possibilities for the control and exploration of complex nanosystems for technological applications. Of particular interest is the use of chiral molecules, such as alanine, to build self-assembled nanoscale structures for the trapping of the two-dimensional free electron gas of a metal. In the present work, molecules of D- or L-alanine were deposited on Cu(111). Scanning tunneling microscopy, spectroscopy, and density functional theory (DFT) revealed the formation of a uniform network of hexagonal chiral pores of average diameter $\sim $ 1.2 nm. Each pore acts as a quantum corral by confining the two-dimensional electron gas of the Cu(111) surface state. Furthermore, each hexagonal pore acts as nanoscale tracks when excess alanine molecules were trapped at the inner perimeter of the pore, and were observed as rotating spatial states periodically moving between the six vertices of the hexagon. This study demonstrates the engineering of one of the smallest quantum confined structure, and the dynamics of molecular motion within these chiral potentials wells. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V36.00010: Influence of ligand and environment substitution on photo-triggered linkage isomerization of photochromic ruthenium sulfoxide complexes Kristin Springfeld, Volker Dieckmann, Sebastian Eicke, Mirco Imlau The group of ruthenium polypyridine sulfoxides features a pronounced photochromism in the UV/VIS spectral range based on an ultrafast photo-triggered linkage isomerization located at the SO-ligand. This isomerization exhibits a tremendous photosensitivity and a high thermal stability of the two metastable structural isomers. Here, we discuss the characteristic photochromic properties of the compounds in the frame of ligand substitution and the replacement of the dielectric environment. The complex [Ru(bpy)$_2$(ROSO)]$\cdot$PF$_6$ [1] (with OSO: 2-methylsulfinylbenzoate) has been modified with the groups R = H, Bn, BnCl and BnMe [2] and studied in different solvents as well as in polydimethylsiloxane. The analysis is performed by cw-pump-probe technique as a function of temperature and exposure. Our results reveal a selective adjustability of the thermal stability in the compounds, while the photosensitivity and the characteristic absorption spectra remain unchanged. We discuss the impact of sulfoxide compounds with the desired features in view of application in molecular photonic devices.\\[4pt] [1] V. Dieckmann et al., Opt. Express 17, 15052 (2009)\newline [2] V. Dieckmann et al., Opt. Express 18, 23495 (2010) [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V36.00011: Classical models for electron capture by highly charged ions in the thin film regime Josh Pomeroy, Russell Lake We present an extension of the classical over the barrier (COB) model[1] for highly charged ions (HCIs) that describes thin dielectric films on metal surfaces, bridging the bulk metal and bulk insulator regimes. Motivated by recent experiments [2,3], we detail the onset of charge transfer between a HCI and metal covered with a dielectric thin film. In this talk, capture distances as a function of C$_{60}$ film thickness on Au(111) will be presented. For ultrathin films, electron capture begins from filled levels in the metal and the C$_{60}$ film decreases the potential barrier for charge transfer and increases the critical distance compared to clean Au(111), increasing the time available for above-surface relaxation. ~This is consistent with the new observation of increasing HCI-induced electron emission yield as a function of film thickness [3]. As film thickness grows and reaches a critical value, the first captured electrons originate from the film at the distance expected for an insulator target.\\[4pt] [1] J. Burgd\"{o}rfer et al. Phys. Rev. A 44, 5674--5685 (1991) \\[0pt] [2] R.E. Lake et al. Phys. Rev. Lett. 107, 063202 (2011) \\[0pt] [3] E. Bodewits et al. Phys. Rev. A 84, 042901 (2011) [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V36.00012: Coupling of Cobalt-Tetraphenylporphyrin Molecules in Copper Nitride Molecular Network Vinicius Claudio Zoldan, Chunlei Gao, Ricardo Faccio, Andr\'e Avelino Pasa We have used low temperature scanning tunneling microscopy to study the interaction between individual cobalt-tetraphenylporphyrin molecules and a molecular copper nitride network. We demonstrated that the molecular Cu$_{3}$N-Cu(110) network promotes the decoupling of the porphiryn, allowing to visualize the molecular orbitals and vibronic states of the molecule, while keeping a strong coupling of the Co atom in the center of macrocycle with the substrate. The reverse behavior was observed when the molecule was sitting on the Cu(110) metallic surface. First principle calculations confirm the assembled position of the molecule on top of N atoms and the decoupling from the surface. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V36.00013: A New Series of Donor-Acceptor Substituted Small Organic Molecules With Large Third Order Molecular Polarizability Marten Beels We report on the third-order nonlinear optical properties of a series of new small organic molecules with a non-planar structure. A large third-order molecular polarizability is achieved thanks to a lower excited state energy obtained due to donor-acceptor substitution. We determined the influence of variations in the donor-acceptor substitution pattern and relate them to the nonlinear response by modeling the molecular properties computationally, and by experimentally determining the rotational average of their third-order polarizability by degenerate four-wave mixing. We found that the best molecules are extremely efficient both in relation to their size and to the fundamental quantum limit. These molecules show great potential for applications where the molecules are combined into dense supramolecular solid state assemblies in the form of high optical quality thin films obtained by molecular beam deposition. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V36.00014: Organic molecular crystals: materials with competing orders Gianluca Giovannetti, Sanjeev Kumar, Jean-Paul Pouget, Massimo Capone The search for multiferroics has become an important research topic in the last few years. Almost all newly discovered multiferroics are transition metal compounds where spin, lattice and charge degrees of freedom are strongly entangled. The possibility of finding organic multiferroics can open up a new area of research where new mechanisms, different from those active in standard transition metal oxide multiferroics, may have a role for the simultaneous occurrence of magnetic and ferroelectric order. By means of ab-initio and model calculations, we show an instability towards multiferroicity in the organic molecular crystal TMTTF$_2$-PF$_6$. Coexistence of charge ordering with a structural dimerization results in a ferroelectric phase with the tendency to the dimerization magnetically driven. The Multiferroicity is induced by competing orders: charge distribution and lattice distortions coupled with the magnetic state. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V36.00015: Energy Transport in Quantum Systems with Discrete Spectrum George Levin, Wesley Jones, Kamil Walczak, Kirk Yerkes Energy transport in quantum system driven by stochastic perturbations is examined. One of the goals of this study is to determine how the Landauer channels can be defined in a system with discrete energy spectrum. A model describes a particle trapped in a confining potential and subjected to a stochastic perturbation localized off-center of the potential well. The perturbation pumps energy into the system which results in non-zero average energy flux between different regions of the confining potential. The energy flux can be defined in terms of quantum advection modes, where each mode is associated with an off-diagonal element of the density matrix and carries a finite, quantized amount of energy per unit of the probability flux. Statistical correlations between different modes and the net energy flux will be discussed. [Preview Abstract] |
Session V37: Topological Insulators: Magnetoelectric Effect, Control, and Dynamics
Sponsoring Units: DCMPChair: Liang Fu, Harvard University
Room: 108
Thursday, March 1, 2012 8:00AM - 8:12AM |
V37.00001: A thermodynamic measure of the Magento-electric coupling in the 3D topological insulator Doron Bergman, Gil Refael We show that the magneto-electric coupling in 3D (strong) topological insulators is related to a second derivative of the bulk magnetization. The formula we derive is the non-linear response analog of the Streda formula for Hall conductivity (P. Streda, J. Phys. C: Solid State Physics, 15, 22 (1982)), which relates the Hall conductivity to the derivative of the magnetization with respect to chemical potential. Our finding allows one to extract the magneto-electric coefficient by measuring the magnetization, while varying the chemical potential and one more perturbing field. Such an experimental setup could circumvent many of the current difficulties with measuring the magneto-electric response in 3D topological insulators. The relation we find also makes transparent the effect of disorder, contained entirely in the density of states, and changing nothing as long as the system is gapped. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V37.00002: Magnetoelectric response of a topological surface under the conditions of an imperfect quantum Hall effect Allan MacDonald, Dmytro Pesin An effective magnetic monopole is induced by an external charge in a topological insulator, and in an ordinary insulator covered by a graphene sheet or another two-dimensional electron system, when it has a perfect surface quantum Hall effect [X.-L. Qi, R. Li, J. Zang, and S.-C. Zhang, Science \textbf{323}, 1184 (2009)]. We discuss the observability of this magnetoelectric response under the realistic conditions of a quantum Hall effect that is imperfect because of finite temperature, disorder, or unintended doping. By generalizing the surface electrodynamics to allow for a finite longitudinal conductivity, we analyze the transient behavior which occurs as the potential from a suddenly introduced external charge is screened. Screening severely limits the experimental time scales on which observation of magnetic-monopole-related phenomena is possible. We estimate the longitudinal conductivity values that are necessary for the monopole to survive for an extended period of time and discuss implications of our findings for other transport properties of the surface. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V37.00003: Magnetoelectric effect in topological insulator/magnetic layer nanostructure Yuriy Semenov, Ki Wook Kim The topological insulator (TI) surface band structure can be modified by contacting ferromagnetic layers due to the proximity exchange interaction. When the magnetization \textbf{M} is in the in-plane direction, the proximate exchange interaction results in a shift of the Dirac cone in the momentum space, whereas an energy gap can be generated for out-of-plane orientation culminating the maximum value at perpendicular to plane direction. Such opening of energy gap lowers the valence electron energy that can be only partially compensated by increase of the conduction electron energies depending on chemical potential $\mu $. Such correlation of electronic energy and magnetization direction open up a new way toward the electrical manipulation of \textbf{M}. To quantitative estimation of this effect, we provide the thermodynamic potential calculation of the TI surface electrons interacted with proximate ferromagnetic insulator as a function of \textbf{M} rotation by angle \textit{$\theta $} about in-plane axis \textbf{x}. The result can be described as an additive magnetic energy $E=K_{eff}$\textit{($\mu )$sin}$^{2}$\textit{$\theta $} in the form of uniaxial anisotropy, which is induced by interaction with TI surface electrons. In the case of TI Bi$_{2}$Se$_{3}$, the numerical estimations predict the $K_{eff}$\textit{($\mu )$} variation in the range of 1 meV/nm$^{2}$ if \textit{$\mu $} vary over 100 meV. The possible applications of the effect (memory and logic) are discussed. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V37.00004: Magnetoelectric and thermoelectric transport in graphene and helical metal: Effect of applied electric field Sung-Po Chao, Huazhou Wei, Vivek Aji We report on the electrical and thermoelectric transport properties of the surface state of the 3D topological insulator (TI) and graphene in a quantizing magnetic field. An unique feature of these systems is the evolution of the Landau level spectrum as a function of applied in plane electric field. We bench mark out results at small fields by computing conductivity and thermopower within linear response. We find that the universal values of thermopower in the clean limit depend on the gyromagnetic ratio in TIs, providing a clear distinction from graphene. In large electric fields we find an oscillation of conductivity as a function of applied electric field for fixed chemical potential, but not for fixed particle density. Signatures of the Landau level dependence on electric fields are also found in thermopower. These results are suggested as possible probes, in transport measurements of topological surface states. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V37.00005: Noninvasive Probe of Charge Fractionalization in Quantum Spin-Hall Insulators Ion Garate, Karyn Le Hur When an electron with well-defined momentum tunnels into a nonchiral Luttinger liquid, it breaks up into two separate wave packets that carry fractional charges and move in opposite directions. Observing this phenomenon has proven difficult, in part due to single-particle and plasmon backscattering caused by measurement probes. In this talk we propose a topological insulator RC circuit that might be ideally suited for detecting fractional charges directly and in a noninvasive fashion. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V37.00006: Topological charge pumping effect by the magnetization dynamics on the Surface of Three-Dimensional Topological Insulators Hiroaki Ueda, Akihito Takeuchi, Gen Tatara, Takehito Yokoyama We discuss a current dynamics on the surface of a 3-dimensional topological insulator induced by magnetization precession of a ferromagnet attached. It is found that the magnetization dynamics generates a direct charge current when the precession axis is within the surface plane. This rectification effect is due to a quantum anomaly (parity anomaly) and is topologically protected. The robustness of the rectification effect against first-varying exchange field is confirmed by the explicit calculation, where we adopt the dimensional regularization to remove the divergence which is inevitable in the study on the electromagnetic response of the Dirac system. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V37.00007: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V37.00008: Electric tuning of topological insulator states and their surface scattering Michael Povolotskyi, Tillmann Kubis, Gerhard Klimeck Materials that show the topological insulating (TI) properties have been extensively studied both experimentally and theoretically in recent years, but an application of those materials for electronic devices is still a challenge. In this study we explored a possibility to switch electric conductance of a surface TI states in a Bi$_{2}$Te$_{3}$ thin film. In such films there are two surfaces that have TI states located at them. We have done a theoretical study of the surface states using atomistic description within an empirical tight binding approximation. It was found that applying electric field perpendicular to the surface one can affect spin polarization of the TI states. Analysis of the scattering rates for electrons that occupy TI states close to the Fermi level shows that: a) the surface states are spin polarized under an applied electric field; b) the electron scattering between two surface states depends on whether they occupy the same surface and have parallel spins; c) by varying strength of an applied electric field it is possible to modulate the scattering rate, because at small fields the states are not spin-polarized, while at higher fields the states become spin-polarized, and at very high fields the surface states are significantly coupled to the bulk states. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V37.00009: Electrical control of the Kondo effect in a helical edge liquid Erik Eriksson, Anders Str\"{o}m, Girish Sharma, Henrik Johannesson Magnetic impurities affect the transport properties of the helical edge states of quantum spin Hall insulators by allowing single-electron backscattering. We study such a system in the presence of a Rashba spin-orbit interaction induced by an external electric field, showing that this can be used to control the Kondo temperature, as well as the correction to the electrical conductivity due to the impurity. In particular, the impurity contribution to the dc conductivity can be switched on and off by properly adjusting the strength of the Rashba coupling. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V37.00010: Nonlinear spectroscopy of non-Abelian Berry curvature Fan Yang, Ren-Bao Liu We propose a general scheme to measure the Berry curvatures of energy bands in insulators by standard nonlinear optical spectroscopy. Our method employs optical and terahertz lights to produce a signal. A general calculation shows that the third order response of the solid is directly related to the Berry curvatures of the energy bands. In particular, for a time-reversal invariant system, we get a nonzero effect compared with the linear response methods, which provides information about the underlying non-Abelian Berry curvature. For insulators with rotational symmetry, the response is proportional to the Berry curvature flux of the iso-energy surface, which enables people to determine the topological properties of the energy bands explicitly. The method is applied to the eight-band model of III-V compound semiconductors and gives a quantized susceptibility with some global coefficients. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V37.00011: The Shockley-model description of the edge states in topological insulators Victor Yakovenko, Sergey Pershoguba We show that the edge states in topological insulators can be understood based on the well-known Shockley model, a 1D tight-binding model with two atoms per elementary cell connected via alternating tunneling amplitudes. We generalize the model to a 3D Shockley-like model corresponding to the sequence of layers connected via the tunneling amplitudes dependent on the in-plane momentum $\mathbf{p = (p_x,p_y)}$. The Hamiltonian of the model is a $2\times2$ matrix with the complex off-diagonal matrix element $t(k,\mathbf{p})$ dependent on both $\mathbf{p}$ and the out-of-plane momentum $k$. The equation $t(k,\mathbf{p})=0$ defines vortex lines in the 3D momentum space. We show that the projection of the vortex lines on the 2D momentum space defines a boundary between the regions of $\mathbf{p}$ where the edge states exist or do not exist. The vorticity of the vortex lines determines the crystal sublattice on which the edge states are localized. We illustrate how our approach works for the well-established topological insulator model by Fu, Kane, and Mele. We find that different configurations of the vortex contours are responsible for the topological insulator phases with even or odd numbers of the surface Dirac cones. We discuss how real materials, such as Bi$_2$Se$_3$, can be described by this model. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V37.00012: Thermoelectric efficiency of holey topological insulators Artem Abanov, Oleg Tretiakov, Jairo Sinova We study the thermoelectric properties of three-dimensional topological insulators with many holes (or pores) in the bulk. We show that at high density of these holes the thermoelectric figure of merit, $ZT$, can be large due to the contribution of the conducting surfaces and the suppressed phonon thermal conductivity. The maximum efficiency can be tuned by an induced gap in the surface states dispersion through tunneling or external magnetic fields. The large values of $ZT$, much higher than unity for reasonable parameters, make this system a strong candidate for applications in heat management of nanodevices, especially at low temperatures. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V37.00013: Spin-charge Dynamics On Surface States of Topological Insulators and 2DEG Xin Liu, Jairo Sinova We study the spin-charge dynamics on the surface of a topological insulator and spin-orbit coupled two dimensional electron gas. A new approach is developed to study the spin-charge dynamics even valid in the very strong spin-orbit coupling regime where the spin splitting energy due to SOIs is at the same order to the Fermi energy. The effects of Coulomb interaction and external field are also considered. We predict the fast oscillation of spin polarization perpendicular and parallel to the surface. Based on our theory, we provide a scheme to measure the transport property of the surface state isolated from the bulk contribution. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V37.00014: Ultrastrong coupling cavity QED of the magnetic cyclotron transition in a 2D electron gas: massive versus Dirac fermions David Hagenm\"{u}ller, Simone De Liberato, Cristiano Ciuti We show that the cyclotron transition of a two-dimensional electron gas can be ultrastrongly coupled to a cavity photon mode. The ratio between the vacuum Rabi frequency $\Omega_0$ and the cyclotron frequency $\omega_0$ can be much larger than $1$ for large filling factor $\nu$ of the Landau levels (the normalized coupling $\Omega_0/\omega_0$ scales as $\sqrt{\alpha \, n_{QW} \nu }$, where $\alpha$ is the fine structure constant and $n_{QW}$ is the number of quantum wells). We present a comprehensive cavity QED theory both for semiconductors with massive electrons\cite{hagenmuller1,scalari} and graphene with Dirac fermions\cite{hagenmuller2}. We show the dramatic impact on the quantum ground state and excitation properties, drawing the comparison between the two different types of 2D electron gas. \\[4pt] [1] D. Hagenm\"{u}ller, S. De Liberato, and C. Ciuti, Phys. Rev. B 81, 235303 (2010) and references therein.\\[0pt] [2] Experiments demonstrating ultrastrong coupling of the cyclotron transition in a GaAs-system in the THz regime have been reported, see G. Scalari \textit{et al}., submitted.\\[0pt] [3] D. Hagenm\"{u}ller and C. Ciuti, submitted. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V37.00015: Topological Floquet Spectrum in Three Dimensions via a Two-Photon Resonance Netanel Lindner, Doron Bergman, Gil Refael, Victor Galitski A recent theoretical work [Nature Phys., 7, 490 (2011)] has demonstrated that external non-equilibrium perturbations may be used to convert a two-dimensional semiconductor, initially in a topologically trivial state, into a Floquet topological insulator. Here, we develop a non-trivial extension of these ideas to three-dimensional systems. In this case, we show that a two-photon resonance may provide the necessary twist needed to transform an initially unremarkable band structure into a topological Floquet spectrum. We provide both an intuitive, geometrical, picture of this phenomenon and also support it by an exact solution of a realistic lattice model that upon irradiation features single topological Dirac modes at the two-dimensional boundary of the system. It is shown that the surface spectrum can be controlled by choosing the polarization and frequency of the driving electromagnetic field. Specific experimental realizations of a three-dimensional Floquet topological insulator are proposed. [Preview Abstract] |
Session V40: Focus Session: Single Molecule Biological Physics - Proteins
Sponsoring Units: DBIO DPOLY DCOMPChair: Everett Lipman, University of California, Santa Barbara
Room: 156A
Thursday, March 1, 2012 8:00AM - 8:36AM |
V40.00001: From single molecule to single tubules Invited Speaker: Chin-Lin Guo Biological systems often make decisions upon conformational changes and assembly of single molecules. \textit{In vivo}, epithelial cells (such as the mammary gland cells) can respond to extracellular matrix (ECM) molecules, type I collagen (COL), and switch their morphology from a lobular lumen (100-200 micron) to a tubular lumen (1mm-1cm). However, how cells make such a morphogenetic decision through interactions with each other and with COL is unclear. Using a temporal control of cell-ECM interaction, we find that epithelial cells, in response to a fine-tuned percentage of type I collagen (COL) in ECM, develop various linear patterns. Remarkably, these patterns allow cells to self-assemble into a tubule of length $\sim $ 1cm and diameter $\sim $ 400 micron in the liquid phase (i.e., scaffold-free conditions). In contrast with conventional thought, the linear patterns arise through bi-directional transmission of traction force, but not through diffusible biochemical factors secreted by cells. In turn, the transmission of force evokes a long-range ($\sim $ 600 micron) intercellular mechanical interaction. A feedback effect is encountered when the mechanical interaction modifies cell positioning and COL alignment. Micro-patterning experiments further reveal that such a feedback is a novel cell-number-dependent, rich-get-richer process, which allows cells to integrate mechanical interactions into long-range ($>$ 1mm) linear coordination. Our results suggest a mechanism cells can use to form and coordinate long-range tubular patterns, independent of those controlled by diffusible biochemical factors, and provide a new strategy to engineer/regenerate epithelial organs using scaffold-free self-assembly methods. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V40.00002: Studying spatial gradients of signaling proteins in mitotic spindles with time-integrated multipoint moment analysis Doogie Oh, Daniel Needleman The organization of the mitotic spindle is orchestrated by the activities of multiple signaling proteins, such as the GTPase Ran.~ It has been proposed that the Ran pathway produces a cascade of events which gives rise to spatial gradients in the behavior of soluble proteins, which in turn produce spatial gradients in microtubule behaviors important for spindle assembly.~ Previous experiments have directly demonstrated the existence of gradients around the spindle in the upstream components of the Ran pathway, but it is still unclear if there are significant gradients in the downstream soluble components in this pathway.~ We recently developed a method, TIMMA, time-integrated multipoint moment analysis, a multipoint form of fluorescence fluctuation spectroscopy capable of quantitatively measuring the concentration, diffusion coefficient, and molecular brightness of soluble proteins throughout live cells.~ We are using TIMMA to characterize the behaviors of the upstream and downstream components of the Ran pathway in live mitotic cell to test the validity of the Ran gradient model. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V40.00003: New chemical kinetics for description of chemical noise in small, heterogeneous biological systems: Beyond the paradigm of the rate constant concept Jaeyoung Sung We introduce a novel chemical kinetics for quantitative description of chemical fluctuations in a small, heterogeneous biological reaction system. At first, we discuss the recently proposed renewal chemical kinetics, and its application to quantitative interpretation of the randomness in fluctuating enzymatic turnover times of a-galactosidase. From the analysis of the randomness parameter data of the single enzyme reaction, one can extract valuable quantitative information about the enzyme reaction system, beyond the reach of the conventional Michaelis-Menten analysis. Next, we discuss a new universal behavior in the time dependence of the chemical fluctuation of product density for a small, heterogeneous reaction system, which is predicted from an exact analytic study for a general reaction model and confirmed by stochastic simulation results. We also discuss the dependence of the chemical noise on substrate concentrations for a heterogeneous enzyme reaction system, which turns out qualitatively different from that for a homogeneous enzyme reaction system. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V40.00004: Generalized Michaelis-Menten equation for conformation-modulated monomeric enzyme catalysis Jianlan Wu, Jianshu Cao The conformational fluctuations induce complex enzymatic catalytic behaviors, which can be investigated by single-molecule experiments. We introduce a kinetic network model to describe conformation-modulated monomeric enzyme catalysis and formulate a generalized Michaelis-Menten (MM) rate equation for non-equilibrium steady-state turnover reactions. Using the flux balance method, we map the original kinetic network to a flux network with unbalanced population currents and derive the general substrate concentration dependence for the average turnover rate of non-equilibrium steady-state enzymatic reactions. In addition to the standard MM term, the generalized MM equation includes non-MM correction terms, which share the same functional form of substrate-dependence. Each non-MM correction term corresponds to a non-equilibrium unbalanced population current induced by conformational fluctuations. Under detailed balance conditions without population currents, non-MM terms vanish and the classical MM equation is recovered. With non-MM terms, the generalized MM equation provides a systematic approach to investigate non-MM behavior and predicts cooperativity, inhibition, and multiple-stability. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V40.00005: Conformational-Modulated Enzyme Catalysis: Generalized Michaelis-Menten Equation and Single Molecule Measurements Invited Speaker: Jianshu Cao The Michaelis-Menten (MM) equation is a basic rate equation to describe the substrate-dependence of enzymatic reactions; therefore, it is important to establish the validity of the MM-equation for complex enzymatic reactions and derive the correction terms when the MM equation fails. Indeed, single molecule experiments reveal complex catalytic behaviors induced by conformational dynamics and possible deviations from the MM rate equation. To model such complex catalytic reactions, we construct a generic kinetic network model characterized by multiple intermediates and multiple conformational sub-states and, by solving for the turnover rate of this network, we extend the MM equation into a general form. The generalized MM equation predicts that (i) the MM equation holds under detailed balance and (ii) the correction to the MM expression depends on the unbalanced conformational currents. Using these predictions, we can establish a relationship between the substrate-dependence of the turnover rate and the connectivity of the enzymatic network. To confirm these predictions, we propose several single molecule indicators to test the violations of detailed balance. However, these single molecule indicators may be difficult to resolve from noisy single molecule data. To address these issues, we propose information theory based data analysis methods to process single molecule time series, and apply the Baysian technique to analyze a single protein fluctuation experiment. \\[4pt] [1] Jianlan Wu and Jianshu Cao, ``Generalized Michaelis-Menten equation for conformation modulated monomeric enzymes,'' in Adv. Chem. Phys. (2011) \\[0pt] [2] Jianshu Cao, ``Michaelis-Menten Equation and Detailed Balance in Enzymatic Networks,'' JPC B, P5493 (2001) \\[0pt] [3] Jianshu Cao and Rob Silbey, ``Generic models of single molecule kinetics: self-consistent solutions,'' JPC B, 112, p12876 (2008) feature article \\[0pt] [4] Jim Witkoskie and Jianshu Cao, ``Analysis of the entire sequence of a single photon experiment on a Flavin Protein,'' JPC B, 112, p5988-5991 (2008) [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V40.00006: Probing short-lived protein ligand interactions with single-molecule force spectroscopy Ozgur Sahin, Mingdong Dong Hydrogen bonding plays an important role in stabilizing biomolecular complexes. Although life time of individual bonds can be extremely short, cooperativity among many interactions increase the overall life time of the complex. To probe short-lived individual interactions, we have employed a recently developed atomic force microscopy technique that can carry out single-molecule force spectroscopy experiments on the microsecond timescale. Our loading-rate dependent measurements provide experimental evidence for an additional energy barrier in the biotin-streptavidin complex. The width of this barrier, estimated from the measurements, is both close to theoretical predictions based on steered molecular dynamics simulations and to the characteristic width of individual hydrogen bonds. We will present our experimental methodology and analysis of the results on biotin-streptavidin complex. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V40.00007: Using Surface Curvature to Control the Dimerization of a Surface-Active Protein Martin Kurylowicz, Maximiliano Giuliani, John Dutcher Understanding the influence of surface geometry on adsorbed proteins promises new possibilities in biophysics, such as topographical catalysis, molecular recognition of geometric cues, and modulations of oligomerization or ligand binding. We have created nano-textured hydrophobic surfaces that are stable in buffer by spin coating polystyrene (PS) nanoparticles (NPs) to form patchy NP monolayers on a PS substrate, yielding flat and highly curved areas on the same sample. Moreover, we have separated surface chemistry from texture by floating a 10 nm thick film of monodisperse PS onto the NP-functionalized surface. Using Single Molecule Force Spectroscopy we have compared \textit{in situ} the distribution of detachment lengths for proteins on curved surfaces to that measured on flat surfaces. We have shown that $\beta $-Lactoglobulin ($\beta $-LG), a surface-active protein which helps to stabilize oil droplets in milk, forms dimers on both flat PS surfaces and surfaces with a radius of curvature of 100 nm, whereas $\beta $-LG monomers exist for more highly curved surfaces with radii of curvature of 25 and 40 nm. It is surprising that rather large radii of curvature have such a strong influence on proteins whose radius is only $\sim $2 nm. Furthermore, the transition from dimer to monomer with changes in surface curvature offers promising applications for proteins whose function can be modified by their oligomerization state. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V40.00008: Ligand Binding Stability and Site Specific Chemical Potentials in the Anisotropic Network Model Rahmi Ozisik, Osman Burak Okan, Aravind Rammohan Anisotropic network model (ANM) is a quadratic elastic model based on local force balance around each constituent material point. In ANM the potential energy functional is system specific and is built up from the connectivity and spatial distribution of elastic contacts (Atilgan et al., Biophysical J 2001, 80, 505). Because the potential energy functional is readily available in closed form, it becomes possible to derive exact expressions for energetics of the system. Recently, a simple analytical identity has been derived for free energy change associated with bond addition/removal for the Gaussian Network Model (GNM), which is one dimensional analog of ANM (Hamacher, Phys. Rev. E 2011, 84, 016703). In the current work, we generalize this formulation to ANM, and for an arbitrary potential functional which might have an acting force distribution on its constituents. Our formulation gives a complete characterization of site specific chemical potential under an arbitrary time independent force distribution. We correlate the chemical potentials with bond orientational order parameters evaluated at each site. Our results are validated on all-heavy atom networks for 15 ligand bound/unbound protein pairs, and show the decisive role of coordination geometry around each node. We show that local chemical potential is predominantly governed by changes in the bond orientational order. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 11:00AM |
V40.00009: Watching Single Enzymes and Fluorescent Proteins in Action in Solution Using a Microfluidic Trap Invited Speaker: Randall Goldsmith Observation of dynamics of single biomolecules over a prolonged time without altering the biomolecule via immobilization is achieved with a specialized microfluidic device. This device, the Anti-Brownian ELectrokinetic (ABEL) Trap, uses real-time electrokinetic feedback to cancel Brownian motion of single objects in solution. First, we use the ABEL Trap to study Allophycocyanin (APC), a photosynthetic antenna-protein and popular fluorescent probe. A complex relationship between fluorescence intensity and lifetime is observed, suggesting light-induced conformational changes and radiative and non-radiative rate fluctuations. Second, we apply the ABEL Trap to single molecules of the multi-copper enzyme blue Nitrite Reductase where a fluorescent label reports on the oxidation state of the Type I Copper. Redox cycling is observed and kinetic analysis allows extraction of the microscopic rate constants in the kinetic scheme. Evidence of a substrate-induced shift of the intramolecular electron transfer rate is seen. Taken together, these observations provide windows of unprecedented detail into the dynamics of solution-phase biomolecules. [Preview Abstract] |
Session V41: Focus Session: Physics of Proteins I: Dynamics and Function
Sponsoring Units: DBIO DPOLYChair: Aihua Xie, Oklahoma State University
Room: 156B
Thursday, March 1, 2012 8:00AM - 8:36AM |
V41.00001: Two tyrosyl radicals stabilize high oxidation states in cytochrome c oxidase for efficient energy conservation and proton translocation Invited Speaker: Denis Rousseau The reaction of hydrogen peroxide (H$_{2}$O$_{2})$ with oxidized bovine cytochrome $c$ oxidase (bC$c$O) was studied by electron paramagnetic resonance (EPR) to determine the properties of radical intermediates. Two distinct radicals with widths of 12 and 46 G are directly observed by X-band CW-EPR in the reaction of bC$c$O with H$_{2}$O$_{2}$ at pH 6 and pH 8. High-frequency EPR (D-band) provides assignments to tyrosine for both radicals based on well-resolved $g$-tensors. The 46 G wide radical has extensive hyperfine structure and can be fit with parameters consistent with Y129. However, the 12 G wide radical has minimal hyperfine structure and can be fit using parameters unique to the post-translationally modified Y244 in C$c$O. The results are supported by mixed quantum mechanics and molecular mechanics calculations. This study reports spectroscopic evidence of a radical formed on the modified tyrosine in C$c$O and resolves the much debated controversy of whether the wide radical seen at low pH in the bovine system is a tyrosine or tryptophan. A model is presented showing how radical formation and migration may play an essential role in proton translocation. This work was done in collaboration with Michelle A. Yu, Tsuyoshi Egawa, Syun-Ru Yeh and Gary J. Gerfen from Albert Einstein College of Medicine; Kyoko Shinzawa-Itoh and Shinya Yoshikawa from the University of Hyogo; and Victor Guallar from the Barcelona Supercomputing Center. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V41.00002: Structural and electronic properties of copper-containing nitrite reductase (CuNiR): elucidating the mechanism of nitrite reduction at the T$_2$Cu center Yan Li, Miroslav Hodak, Jerry Bernholc Copper-containing nitrite reductases (CuNiRs) play an important role in catalyzing the reduction of NO$_2^{-}$ to NO during the bacterial denitrification process. Experimental studies have provided the structures of various states of CuNiR in the catalytic reaction, but many important aspects of the initial and intermediate attachments as well as the mechanism of the enzyme function remain unclear. We present a density-functional-theory-based study of the structural and electronic properties of different coordination forms at the T$_2$Cu center. The nudged elastic band (NEB) method is used to examine the activation energy barriers and to determine the minimum energy pathways (MEP) of the reaction processes. Our results reveal the role of the Asp$^{98}$ residue in the enzymatic function of CuNiR and also address the transformation from the initial O-coordinated binding of NO$_2^-$ to the N-coordinated attachment of the NO during the enzymatic reaction. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V41.00003: Nanometer Scale Distance Measurements for Biological Systems using Gd$^{3+}$-based Spin Probes at High Magnetic Fields Devin Edwards, Daniella Goldfarb, Songi Han, Mark Sherwin Determination of nanometer-scale distances is critical for understanding structure and dynamics of proteins. Electron Paramagnetic Resonance (EPR), primarily below 1 T, is used to complement other structural techniques by quantifying sparse distances up to 8 nm in biomolecules labeled with nitroxide-based radicals. EPR becomes more powerful with increasing magnetic fields and frequencies. At 95 GHz (3.5 T), Gd$^{3+}$ ions have shown clear advantages over nitroxide probes (Potapov, JACS 2010). We show that these advantages are even more dramatic at 240 GHz (8.5 T). The width of Gd$^{3+}$'s central EPR transition narrows with increasing average distance between Gd$^{3+}$ ions out to distances as long as 5 nm. This doubles the distances accessible with nitroxides in continuous wave measurements, which can be carried out above the 200K protein-glass transition and with broad distance distributions. Temperature-dependent measurements of the phase memory times at 8.5 T and low temperatures show distance dependence out to 10 nm. Measurements of Gd$^{3+}$ labeled Proteorhodopsin confirm that phase memory times remain long enough to observe distance dependence in a spin-labeled protein. This work is supported by the National Science Foundation and the Binational Science Foundation. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V41.00004: Metal-like transport in proteins: A new paradigm for biological electron transfer Nikhil Malvankar, Madeline Vargas, Mark Tuominen, Derek Lovley Electron flow in biologically proteins generally occurs via tunneling or hopping and the possibility of electron delocalization has long been discounted. Here we report metal-like transport in protein nanofilaments, pili, of bacteria \textit{Geobacter sulfurreducens }that challenges this long-standing belief [1]. Pili exhibit conductivities comparable to synthetic organic metallic nanostructures. The temperature, magnetic field and gate-voltage dependence of pili conductivity is akin to that of quasi-1D disordered metals, suggesting a metal-insulator transition. Magnetoresistance (MR) data provide evidence for quantum interference and weak localization at room temperature, as well as a temperature and field-induced crossover from negative to positive MR. Furthermore, pili can be doped with protons. Structural studies suggest the possibility of molecular pi stacking in pili, causing electron delocalization. Reducing the disorder increases the metallic nature of pili. These electronically functional proteins are a new class of electrically conductive biological proteins that can be used to generate future generation of inexpensive and environmentally-sustainable nanomaterials and nanolectronic devices such as transistors and supercapacitors. [1] Malvankar et al. Nature Nanotechnology, 6, 573-579 (2011) [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V41.00005: Behind the Scene Role of Conserved Threonine in Intein Splicing Albert Dearden, Brian Callahan, Marlene Belfort, Saroj Nayak Protein splicing is an autocatalytic process where an ``intein'' self-cleaves from a precursor protein and catalyzes ligation of the flanking fragments. Inteins occur in all domains of life and have myriad uses in biotechnology. While reaction steps of intein splicing are known, mechanistic details remain incomplete. Here, we investigate the possible role of a highly conserved active-site Threonine residue in bringing about the initial step of splicing: peptide bond rearrangement at a conserved Glycine-Cysteine motif. We report that although not part of the active transition state in this reaction, Threonine plays an important role in reducing the energy barrier through charge screening of active residues in the transition state. Interestingly, Threonine-Glycine hydrogen bonding makes sulfur of the attacking Cysteine less nucleophilic, thereby minimizing Coulomb repulsion in the transition state. These non-intuitive results are obtained through a combination of crystal structure, quantum mechanical simulations, and mutagenesis data. Our results further predict that the sluggish reaction rates observed with intein mutants harboring Threonine-Alanine substitutions can be accelerated in the presence of non-aqueous solvents. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V41.00006: Coherent Oscillations in Hemoglobin and Myoglobin Ligand Complexes and their Dynamical Connection to Global Protein Motion and Gas Discrimination Jacob Jantzi, Kristina Woods THz spectroscopy and Molecular Dynamics (MD) simulations are used to investigate the coherent oscillations in the heme group of two heme proteins (hemoglobin and myoglobin) and the affect of the heme dynamics on the collective fluctuations taking place in the proteins. Preliminary experiments have confirmed that the deoxy state (no gas in the active site) of both proteins do not possess a 40 cm$^{-1}$ mode in their THz spectra. The 40 cm$^{-1 }$mode has been observed in both experimental and theoretical investigations of myoglobin dynamics. The low-frequency mode at 40 cm$^{-1}$ has been hypothesized to be connected with energy transport between the active site and the protein-solvent interface. Once a gas is introduced into the ligand, both proteins contain the 40 cm$^{-1}$ mode in their experimental spectra. But both the shape and intensity of the myoglobin peak differs from that of hemoglobin. Additionally, we observe a number of collective protein fluctuations ($\le $ 100 cm$^{-1})$ that are altered in the myoglobin spectrum but remain unchanged in the hemoglobin spectrum when a gas is introduced into the protein active site. We will present experimental data of both proteins that have been exposed to a number of different gases. The reaction of the protein collective motions in the gas is linked with the difference in the coupling of the coherent oscillations of the heme group with the protein global modes but also with the mechanism of protein relaxation that controls ligand migration. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V41.00007: Vibrational Dynamics of Ferric MbCN-A Revisit by Resonance Raman and Vibrational Coherence Spectroscopy Weiqiao Zeng, Yuhan Sun, Abdelkrim Benabbas, Paul M. Champion Ultrafast pump-probe spectroscopy has indicated that there exists a photoproduct state following the excitation of ferric MbCN$^{[1][2]}$. This excited state decays with a time constant of 3.6 ps$^{[1]}$. Previous studies on this system have suggested that in this photoproduct state, the heme is either (i) still six-coordinated but vibrationally hot in the electronic ground state$^{[1]}$ or (ii) the proximal histidine residue (His93) is transiently dissociated, while CN$^{-}$ is still bound$^{[2]}$. Recent resonance Raman measurements on ferric MbCN in static solution yield spectra that are very similar to ferric myoglobin, which has His93 and a water molecule as axial ligands. This indicates that a water molecule replaces CN$^{-}$ in ferric MbCN under continuous laser excitation. Photolysis of CN$^{-}$ from the heme iron is necessary to make this happen, which is not consistent with the above two suggestions. In this presentation we will revisit the dynamics of ferric MbCN with resonance Raman and vibrational coherence spectroscopy and try to explain how a water molecule competes with CN$^{-}$ in binding to the heme under photo excitation$^{[3]}$. References: [1]Helbing J. et al., Biophys J, vol 87, 1881(2004) [2]Gruia F. et al., Biophys J, vol 94, 2252(2008) [3]Cao W. et al., Biochemistry, vol 40, 5728(2001) [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V41.00008: Effect of DNA binding on geminate CO recombination kinetics in CooA Abdelkrim Benabbas, Venugopal Karunakaran, Hwan Youn, Thomas Poulos, Paul Champion CooA proteins are heme-based CO-sensing transcription factors. Here we study the ultrafast dynamics of geminate CO rebinding to RrCooA. The effects of DNA binding and the truncation of the DNA binding domain on the CO geminate recombination kinetics were investigated. The CO rebinding kinetics in these CooA complexes takes place on ultrafast timescales but remains non-exponential over many decades in time. We show that this non-exponential kinetic response is due to a quenched enthalpic barrier distribution resulting from a distribution of heme geometries that is frozen or slowly evolving on the timescale of CO rebinding. We also show that, upon CO binding, the distal pocket of the heme in RrCooA relaxes to form a very efficient hydrophobic trap for CO. DNA binding further tightens the narrow distal pocket and slightly weakens the iron-proximal histidine bond. Analysis of our data reveals that the uncomplexed and inherently flexible DNA binding domain adds additional structural heterogeneity to the heme doming coordinate. When CooA forms a complex with DNA, the flexibility of the DNA-binding domain decreases and the distribution of the conformations available in the heme domain becomes restricted. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V41.00009: Physical control of carrier-mediated ion-transporters by entrainment of their turnover rate Wei Chen, Clausell Mathis, Zhihui Fang, Jason Mast, Karim Hamidi, Patrick Kelly, Max Eve In the past, tremendous efforts have been made to physically activate carrier-mediated ion-transporters, such as Na/K pumps. However, the outcome is not significant. Recently, we developed a new technique which can effectively and efficiently control the pumping rate by introducing a concept of an electronic synchrotron accelerator to the biological system. The approach consists of two steps. First, a specially designed oscillating electric field is used to force or synchronize individual pump molecules to run at the same turnover rate and phase as the field oscillation frequency. Then, by gradually changing the field frequency and carefully keeping the pump synchronization we can entrain the pump molecules so that their pumping rate can be progressively modulated, either decelerated or accelerated, following the field frequency to a defined value. Based on theoretical analysis of the underlying mechanisms involved in the technique, computer simulation of the entrainment process, and intensive experimental studies we have realized significant activation of the Na/K pumping rate up to ten-folds quickly in less than ten seconds. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V41.00010: Molecular dynamics simulation studies of dielectric response and vibrational energy relaxation in photoactive yellow protein and green fluorescent protein Yao Xu, Ramachandran Gnanasekaran, David Leitner The first step in the photocycle of many proteins involves conformational change of a chromophore or a charge transfer reaction following photoexcitation. To explore the response of the protein and solvent environment to photoexcitation of the chromophore in photoactive yellow protein (PYP) and green fluorescent protein (GFP) we carried out molecular dynamics simulations of the dielectric response and vibrational energy relaxation (VER) from the chromophore to the protein and solvent. In PYP the time scale of the protein response, mainly contributed by Tyr42 and Glu46, to photoexcitation appears prominently between 0.1 and 0.3 picoseconds. The frequency-dependent VER rate also reveals dynamic coupling between the chromophore and residues that hydrogen bond to it. Resonances in the VER rate appear at frequencies comparable to the oscillations observed in recent fluorescence decay studies. In GFP, which undergoes excited state proton transfer about 10 ps following photoexcitation that may be assisted by specific chromophore vibrations, both the protein and water molecules inside the $\beta $-barrel surrounding the chromophore mediate the dielectric response. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V41.00011: Employing Solution X-Ray Scattering to Investigate Conformational Changes of Proteins Mustafa Tekpinar, Wenjun Zheng Many proteins undergo global conformational changes to perform their functions. Small/Wide Angle Solution X-Ray Scattering (S/WAXS) is an increasingly used experimental technique to probe conformational changes of proteins in aqueous environment. We have developed a new modeling method to reconstruct coarse-grained and atomistic 3D models from S/WAXS data. The method has been applied to a variety of proteins including the R-T transition of hemoglobin. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V41.00012: Near-Field Orientation Sensitive Terahertz Micro-Spectroscopy of Single Crystals Gheorghe Acbas, Rohit Singh, Edward Snell, Andrea Markelz We present spectroscopic imaging studies of molecular crystals. These measurements examine the anisotropy of the intra and inter-molecular vibrational modes of single crystals at terahertz frequencies. The method is based on the technique developed in [1-2] for sub-wavelength resolution time domain terahertz spectroscopy (THz TDS), with added polarization orientation dependent measurements and hydration control. This method allows us to study the spectroscopic properties of small single crystals with sizes down to 20 micrometers. In addition, mapping the spectroscopic information at such small spatial scales allows us to reduce the water absorption and interference artifacts that usually affect protein THz TDS measurements. We show the polarization sensitive terahertz absorption spectra in the (0.3-3THz) range of sucrose, oxalic acid and lysozyme protein crystals. \begin{enumerate} \item M. A. Seo, et. al., Opt. Express, 15(19):11781--11789, 09 (2007) \item J. R Knab, et. al., App. Phys. Lett.,97, 031115 (2010) \end{enumerate} [Preview Abstract] |
Session V42: Focus Session: Systems Biology - Stochastic Gene Expression
Sponsoring Units: DBIOChair: Stephen Hagen, University of Florida, Physics Dept
Room: 156C
Thursday, March 1, 2012 8:00AM - 8:36AM |
V42.00001: Mapping the environmental fitness landscape: Lessons from a noisy synthetic gene circuit Invited Speaker: Gabor Balazsi Gene expression actualizes the organismal phenotypes encoded within the genome in an environment-dependent manner. Among all encoded phenotypes, cell population growth rate (cell population fitness) is perhaps the most important, since it determines how well-adapted a genotype is in various environments. Currently it remains unclear how a cell population's growth rate and its subpopulation fractions in specific environments emerge from the stochastic molecular-level kinetics of gene networks and the division rates of single cells. To address this question we developed and quantitatively characterized synthetic a gene circuit controlling the expression of a bifunctional antibiotic resistance gene in \textit{Saccharomyces cerevisiae}. We found that knowing the cell division rates and nongenetic (cellular) memory of gene expression states were necessary for predicting the overall fitness of cell populations in specific antibiotic- and inducer-containing environments. We validated these predictions experimentally and identified environmental conditions that determined a ``sweet spot'' of drug resistance. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V42.00002: Stochastic description of birth and death processes governed by a mixture of exponential and non-exponential waiting times Stephan Eule The dynamics of complex systems is significantly influenced by fluctuations originating from intrinsic as well as extrinsic sources. In general, the discrete nature of individual events, such as the birth and death of an individual in a population or the production and degradation of a molecule in a chemical reaction, is the main source of intrinsic noise. The occurrence of such events is usually modeled by Poissonian statistics, implying that the probability per unit time for an event to happen is assumed to be constant. Many complex systems however exhibit deviations from elementary Poissonian statistics. Such deviations can arise for example in coarse-grained stochastic models of gene expression, where the waiting time distribution can be more general than the simple exponential distribution. In this contribution we consider birth and death processes which are governed by both, exponential as well as non-exponential waiting times. We derive the corresponding master equation and present methods to approach this equation analytically. As an example we consider a reaction where the production of molecules is governed by a non-exponential waiting time distribution and the degradation follows regular Poissonian statistics. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V42.00003: Mechanistic basis for transcriptional bursting of ribosomal genes in E. coli Sandeep Choubey, Alvaro Sanchez, Jane Kondev Upon adding more ribosomal genes to the E. coli cell, it adjusts the overall transcription of these genes by reducing the average transcription rate per gene, so as to keep constant the level of ribosomal RNA in the cell. It was observed that this reduction in the average transcription level per gene is accompanied by the generation of transcriptional bursts. The biophysical mechanism responsible for this type of transcriptional control is not yet known. We consider three possible mechanisms suggested in the literature: proximal pausing by RNA polymerase, cooperative recruitment of RNA polymerase by DNA supercoiling, and competition between RNA polymerase and a transcription factor for binding to regulatory DNA. We compute the expected statistical properties of transcription initiation for each one of these models,and compare our predictions with published distributions of distances between the polymerases transcribing the ribosomal genes, obtained from electron micrographs.We use this data to estimate the rates of transcription initiation, which are found to be in good agreement with independent measurements. We also show that the three mechanisms considered here can be discriminated by comparing their predictions for the mean and the variance of interpolymerase distances. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V42.00004: Decreasing the stochasticity of mammalian gene expression by a synthetic gene circuit Dmitry Nevozhay, Tomasz Zal, Gabor Balazsi Gene therapy and functional genetic studies usually require precisely controlled and uniform gene expression in a population of cells for reliable level of protein production. Due to this requirement, stochastic gene expression is perceived as undesirable in these fields and ideally has to be minimized. The number of approaches for decreasing gene expression stochasticity in mammalian cells is limited. This creates an unmet need to develop new gene expression systems for this purpose. Based on earlier synthetic constructs in yeast, we developed and assessed a negative feedback-based mammalian gene circuit, with uniform and low level of stochasticity in gene expression at different levels of induction. In addition, this new synthetic construct enables highly precise gene expression control in mammalian cells, due to the linear dependence of gene expression on the inducer concentration applied to the system. This mammalian gene expression circuit has potential applicability for the development of new treatment modalities in gene therapy and research tools in functional genetics. In addition, this work creates a roadmap for moving synthetic gene circuits from microbes into mammalian cells. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V42.00005: Applications of queueing theory to stochastic models of gene expression Invited Speaker: Rahul Kulkarni The intrinsic stochasticity of cellular processes implies that analysis of fluctuations (`noise') is often essential for quantitative modeling of gene expression. Recent single-cell experiments have carried out such analysis to characterize moments and entire probability distributions for quantities of interest, e.g.\ mRNA and protein levels across a population of cells. Correspondingly, there is a need to develop general analytical tools for modeling and interpretation of data obtained from such single-cell experiments. One such approach involves the mapping between models of stochastic gene expression and systems analyzed in queueing theory. The talk will provide an overview of this approach and discuss how theorems from queueing theory (e.g. Little's Law) can be used to derive exact results for general stochastic models of gene expression. In the limit that gene expression occurs in bursts, analytical results can be obtained which provide insight into the effects of different regulatory mechanisms on the noise in protein steady-state distributions. In particular, the approach can be used to analyze the effect of post-transcriptional regulation by non-coding RNAs leading to new insights and experimentally testable predictions. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V42.00006: Cross-talk and interference can enhance information capacity of a signaling pathway Sahand Hormoz A recurring theme in gene regulatory networks is transcription factors (TFs) that regulate each other, and then bind to overlapping sites on DNA, where they interact and synergistically control transcription of a target gene. TF binding is inherently a noisy process due to thermal fluctuations and the small number of molecules involved. A consequence of multiple TFs interacting at the binding site through competition or cooperativity is that their binding noise becomes correlated. Using concepts from information theory, we show that a correlated-noise channel can enhance its capacity if the TFs are no longer independent but regulating each other. Essentially, the frequency of observing each TF at a given concentration is no longer separable, but ``entangled.'' The form of this entanglement elucidates the upstream TF cross-regulation (cross-talk). We demonstrate these ideas using a cartoon model of two TFs competing for the same binding site. Surprisingly, competition can enhance the information transmission rate. We suggest that this mechanism explains the motif of a coherent feed-forward loop terminating in overlapping binding sites commonly found in developmental networks, and discuss specific examples. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V42.00007: Exact results for integral thresholds in models of stochastic oscillatory gene expression Srividya Iyer Biswas, Norbert Scherer, Aaron Dinner Oscillatory stochastic gene expression is often combined with threshold regulation to ensure periodic occurrence of some cellular activity, such as cell division. In this work we first demonstrate the virtue of implementing such regulation using an integral threshold, rather than a step threshold, in the fluctuating numbers of the regulator. We then develop a general theoretical framework using which we derive a model independent result that relates the stochastic distribution of the time oscillating regulator numbers to the distribution of event (cell division) times, regardless of the underlying mechanism that generates a specific form of oscillations in the regulator copy numbers. We then use this result in conjunction with a simple model of stochastically oscillating gene expression to show how the shape of the division time distribution can be used to make deductions about the underlying stochastic dynamics of the oscillating regulator. Specifically, we show that bimodal division time distributions can occur, even in the absence of any bistability in the underlying model, and connect that observation to general features of the underlying stochastic model. Finally, we discuss connections to ongoing single cell experimental studies of Caulobacter cell-cycle division times. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V42.00008: Fitness Effects of Network Non-Linearity Induced by Gene Expression Noise Christian Ray, Tim Cooper, Gabor Balazsi In the non-equilibrium dynamics of growing microbial cells, metabolic enzymes can create non-linearities in metabolite concentration because of non-linear degradation (utilization): an enzyme can saturate in the process of metabolite utilization. Increasing metabolite production past the saturation point then results in an ultrasensitive metabolite response. If the production rate of a metabolite depends on a second enzyme or other protein-mediated process, uncorrelated gene expression noise can thus cause transient metabolite concentration bursts. Such bursts are physiologically unnecessary and may represent a source of selection against the ultrasensitive switch, especially if the fluctuating metabolic intermediate is toxic. Selection may therefore favor correlated gene expression fluctuations for enzymes in the same pathway, such as by same-operon membership in bacteria. Using a modified experimental \textit{lac} operon system, we are undertaking a combined theoretical-experimental approach to demonstrate that ($i)$ the \textit{lac} operon has an implicit ultrasensitive switch that we predict is avoided by gene expression correlations induced by same-operon membership; (\textit{ii}) bacterial growth rates are sensitive to crossing the ultrasensitive threshold. Our results suggest that correlations in intrinsic gene expression noise are exploited by evolution to ameliorate the detrimental effects of nonlinearities in metabolite concentrations. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V42.00009: Stochastic Gene Expression in Networks of Post-transcriptional Regulators Charles Baker, Tao Jia, Hodjat Pendar, Rahul Kulkarni Post-transcriptional regulators, such as small RNAs and microRNAs, are critical elements of diverse cellular pathways. It has been postulated that, in several important cases, the role of these regulators is to to modulate the noise in gene expression for the regulated target. Correspondingly, general stochastic models have been developed, and results obtained, for the case in which a single sRNA regulates a single mRNA target. We generalize these results to networks containing a single mRNA regulated by multiple sRNAs and to networks containing multiple mRNAs regulated by a single sRNA. For these systems, we obtain exact expressions relating the mean levels of the sRNAs to the mean levels of the mRNAs. Additionally, we consider the convergence of the original model to an approximate model which considers sRNA concentrations to be high; for the latter model we derive an analytic form for the generating function of the protein distribution. Finally, we discuss potential experimental protocols which, in combination with the derived results, can be used to infer the underlying gene expression parameters. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V42.00010: Accurate analytical distributions for stochastic gene expression Hodjat Pendar, Rahul Kulkarni Gene expression is significantly stochastic process that can give rise to phenotypic heterogeneity across a population of genetically identical cells. Gene expression variability is generally characterized by the mean and variance of associated distributions, however the entire distributions are often not adequately characterized by the first two moments. For stochastic models of gene expression, exact analytic results for protein steady-state distributions have been obtained only for the simplest case. In this talk, we show how to obtain approximate but accurate representations of protein steady-state distributions for a broad class of models of stochastic gene expression. We first present a procedure to obtain analytical solutions in two limiting cases as the ratio of mRNA to protein lifetimes is varied. We then propose a general strategy for constructing an analytical distribution that interpolates these limits while reproducing the exact mean and variance. ~The corresponding analytical distributions show excellent agreement with results from stochastic simulations throughout parameter space. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V42.00011: Stochastic Hopf bifurcation in transcription networks with delayed feedback John Wentworth, Mathieu Gaudreault, Jorge Vinals We study the oscillatory instabilities of two model systems that rely on delayed negative feedback to induce oscillation: a single gene auto repressor system, and a dimer negative autoregulation system. We focus on fluctuations of intrinsic origin in the range of low copy number. The bifurcation diagram is obtained for these stochastic models, and shown to differ significantly from that of a macroscopic description that neglects fluctuations. Bifurcation lines remain sharp under fluctuations, but their location is a function of the relative size of the fluctuations. Shifts in the stability threshold of the oscillators can be traced back to the interplay between statistical correlations and delayed feedback. We finally show that there results cannot be captured by weak noise approximations (the diffusion limit), but instead result from strong fluctuations associated with low copy numbers. [Preview Abstract] |
Session V43: Invited Session: Entrepreneurship - The Quest for Start-Up Success Based on Research Advances
Sponsoring Units: FEd FGSAChair: John M. Newsam, Windhover Ventures LLC
Room: 157AB
Thursday, March 1, 2012 8:00AM - 8:36AM |
V43.00001: Start-ups for Dummies Invited Speaker: Lawrence Bock I will present the best practices I have learned from founding, co-founding or seeding the early stage growth of 50 high technology and life sciences companies. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 9:12AM |
V43.00002: Pushing a physics discovery towards commercial impact Invited Speaker: Eric Mazur In 1997 my research group discovered that shining intense, ultrashort laser pulses on the surface of a crystalline silicon wafer drastically changes the optical, material and electronic properties of the wafer. The resulting textured surface is highly absorbing and looks black to the eye, making this 'black silicon' useful for a wide range of commercial devices, from highly-sensitive detectors to improved photovoltaics. Over the following ten years we investigated this material and developed a prototype detector. The prototype gave us the confidence to commercialize black silicon. Togethe r with a graduate student, I founded SiOnyx. The company, based in Beverly, MA, is in the process of manufacturing the first commercial products based on black silicon. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V43.00003: How a Venture Capitalist Approaches an Investment Decision Invited Speaker: Daniel Colbert |
Thursday, March 1, 2012 9:48AM - 10:24AM |
V43.00004: Having your cake and eating it too; effective engagement in start-ups from an academic seat Invited Speaker: Chad Mirkin In order for scientific advances to have a positive impact on society, they must be successfully transitioned from conceptually fundamental endeavors in academic research laboratories to valuable enabling technologies at start-up companies. Nanosphere, NanoInk, and AuraSense are three start-up companies that have been spun out of Northwestern based on research initiated in my laboratory. These companies are focused on commercializing nanotechnology-based applications in the life science and semiconductor industries and have turned discoveries from my lab into viable commercial products. For example, several of the systems developed at these start-ups are in the clinical trial phase, with one already approved by the FDA, and they are poised to have a positive world-wide impact. Herein, I discuss the challenges associated with identifying commercial value in academic research projects, securing intellectual property, forming a company as a legal entity, and locating sources of start-up funds. Further, I will discuss the rewards of venturing into such enterprises and the ways of ensuring a start-up company's long-term success, while juggling the numerous responsibilities of an academic seat. I argue that these two activities are done not in competition, but rather are integral for driving the type of high-level, synergistic scientific research that is being done today. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 11:00AM |
V43.00005: An entrepreneurial physics method and its experimental test Invited Speaker: Robert Brown As faculty in a master's program for entrepreneurial physics and in an applied physics PhD program, I have advised upwards of 40 master and doctoral theses in industrial physics. I have been closely involved with four robust start-up manufacturing companies focused on physics high-technology and I have spent 30 years collaborating with industrial physicists on research and development. Thus I am in a position to reflect on many articles and advice columns centered on entrepreneurship. What about the goals, strategies, resources, skills, and the 10,000 hours needed to be an entrepreneur? What about business plans, partners, financing, patents, networking, salesmanship and regulatory affairs? What about learning new technology, how to solve problems and, in fact, learning innovation itself? At this point, I have my own method to propose to physicists in academia for incorporating entrepreneurship into their research lives. With this method, we do not start with a major invention or discovery, or even with a search for one. The method is based on the training we have, and the teaching we do (even quantum electrodynamics!), as physicists. It is based on the networking we build by 1) providing courses of continuing education for people working in industry and 2) through our undergraduate as well as graduate students who have gone on to work in industry. In fact, if we were to be limited to two words to describe the method, they are ``former students.'' Data from local and international medical imaging manufacturing industry are presented. [Preview Abstract] |
Session V44: Focus Session: Interparticle Interactions in Polymer Nanocomposites - Surface Interactions
Sponsoring Units: DPOLYChair: Dilip Gersappe, SUNY Stony Brook
Room: 157C
Thursday, March 1, 2012 8:00AM - 8:12AM |
V44.00001: The synthesis of metal nanoparticulate catalysts within functional microgel particles Maria Kaliva, Eleni Pavlopoulou, Konstantinos Christodoulakis, Maria Vamvakaki, Spiros H. Anastasiadis Electrostatically and sterically stabilized polymer microgel particles have been prepared containing either amino (poly(2-(diethylamino)ethyl methacrylate), PDEA) or carboxylic acid (poly(acrylic acid), PAA; poly(methacrylic acid), PMMA) functional groups. The PDEA, PAA and PMAA particles can be used for the incorporation of a large variety of metal nanoparticulate catalysts due to their functional amine and carboxylic acid groups; Pd, Ru and Ni nanoparticles have been synthesized. The more polar PAA microgels were designed as the nanocatalyst carrier system in aqueous reaction media while the less polar PMAA particles were prepared as the metal nanoparticle template for use in catalytic reactions that take place in organic solvents. The sterically and electrostatically stabilized microgel particles possess surface functional groups that can potentially interact with the microchannel walls of microfluidic catalytic reactors. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V44.00002: The role of nanoparticle synergies in modifying the thermal properties of biodegradable polymer blends Kai Yang, Shan He, Rachel Davis, Miriam Rafailovich, Takashi Kashiwagi Most of thermoplastic polymers are brittle, when sufficient amounts are added to get flame retardant properties.Furthermore, melt-blending starch with other biodegradable polymers is difficult since very few polymers are compatible with starches.We have developed a new nanoparticles where resorcinol diphenyl phosphates (RDP) is used to modify the surface energy, allowing the particles to be dispersed within polymer.When multiple types of particles share the same coating,they can be melt blended simultaneously and synergies can be achieved, imparting properties to the nanocomposite, which cannot be achieved by any single additive. Here we show that RDP modified starch, can be extruded together with the biodegradable polymers,Ecoflex and polylactic acid,to produce flame retardant nanocomposites which can pass the UL-94-V0 test.TEM images of the blend show that the RDP-coated starch particles were well dispersed within the polymer matrix providing the flame retardant properties,while the RDP clays are reducing the interfacial tension and contributing to compatibilization. Nanomechanical measurements of the chars remaining after cone calorimetric measurements indicate that maintaining flexibility of the chars may be an additional factor in achieving good flame retardant properties. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V44.00003: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V44.00004: Supramolecular Assembly in a Polymer Nanocomposite Rick Beyer, Aaron Jackson, Samuel Price, Christopher Gold, Andrew Duncan Overcoming the inherent tendency of nanoparticles to aggregate is critical and has typically been the main focus of research on the fabrication of polymer nanocomposites. Developing specific or organized arrangements of nanoparticles is even more technically challenging, but also more appealing due to the possibility of creating nanocomposites with desired optical, electrical, or magnetic properties. Successful approaches in this area often rely on the polymer matrix to organize the nanoparticle additive, but are limited when one considers the high-throughput processing procedures used industrially. In this presentation, the preliminary findings of a research effort to use the strong, reversible, interparticle or intermolecular interactions found in supramolecular assembly to promote synergistic organization of the polymer matrix and dispersion of nanoparticles in a polymer nanocomposite will be described. Here, the effect of incorporating functionalized metal nanoparticles into a supramolecularly assembling metallopolymer based on reversible bonds formed between a metal atom (such as Zn$^{2+})$ and the ``mebip'' ligand (2,6-bis(19-methylbenzimidazolyl)pyridine) will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V44.00005: Stabilizing nanotube films with thin polymer layers: Mitigating van der Waals forces through excluded-volume interactions Matthew R. Semler, John M. Harris, Erik K. Hobbie Thin membranes of single-wall carbon nanotubes (SWCNTs) on elastic polymer substrates show considerable promise for flexible electronics applications, but the modulus and conductivity of these films decrease dramatically in response to applied strains. This softening arises from the strong van der Waals interactions between contacted nanotubes, which favor the parallel coarsening of SWCNT bundles in response to even very small external forces. By capping the SWCNT membranes with a thin layer of glassy polymer, we demonstrate a dramatic improvement in the mechanical response of the strained films. We link this behavior to the stabilizing influence of excluded-volume interactions mediated by the glassy polymer layer. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V44.00006: SANS Studies of DNA-Templated Silver Nanoclusters Hongyu Guo, Sunil Sinha, Jaswinder Sharma, Jennifer Martinez, Andrew Shreve DNA-templated silver nanoclusters have received significant attention due to their useful properties, including high molar absorptivities, good quantum yields and photostability, and small size. Their potential use may range from biology to nanoscience. For example, they are promising biological fluorescence probes due to their fluorescence properties dependence to DNA template sequence. However, some basic features, like the structure of the DNA/Ag complex, are still unclear. We have conducted Small Angle Neutron Scattering (SANS) experiments to investigate the formation of the Nanoclusters. By comparing the SANS data from conjugated samples, pure DNA and DNA/Ag complex, we can characterize the size and position of the Ag clusters along the DNA strand. The time evolution of the DNA/Ag complex can also be studied since such aging process is kind slow. We found that the formation and aging of the Ag Nanoclusters are also strongly dependent on the DNA template sequence. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V44.00007: Theory of the Structure and Miscibility of Soft Filler Polymer Nanocomposites Jian Yang, Kenneth Schweizer A new hybrid theory for soft filler polymer nanocomposites is constructed based on combining integral equation methods and small scale Monte Carlo simulation. The consequences of nanoparticle softness (surface fluctuations) and corrugation (discrete roughness) on the equilibrium behavior is investigated in the dilute filler limit. Under athermal (entropic) conditions, the monomer-particle pair correlations exhibit qualitatively different features relative to hard smooth spheres. Polymer-mediated depletion attractions in the particle potential-of-mean-force (PMF) are qualitatively modified by surface corrugation and/or fluctuations. With increasing particle softness, monomer-scale PMF oscillations are destroyed, and the interparticle separation and effective maximum attraction strength depend sensitively on surface fluctuation amplitude and monomer-nanoparticle size ratio. Second virial coefficient calculations are performed to estimate how particle softness/roughness modifies miscibility, and a mechanism is identified for entropically stabilizing rough nanofillers in the absence of a cohesive interface. Surface corrugation and softness is also found to significantly modify the bridging and sterically stabilized states associated with adsorbing polymers. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V44.00008: Self-assembly of Nano-rods in Photosensitive Phase Separation Ya Liu, Olga Kuksenok, Egor Maresov, Anna Balazs Computer simulations reveal how photo-induced chemical reactions in polymeric mixtures can be exploited to create long-range order in materials whose features range from the sub-micron to the nanoscale. The process is initiated by shining a spatially uniform light on a photosensitive AB binary blend, which thereby undergoes both a reversible chemical reaction and phase separation. When a well-collimated, higher intensity light is rastered over the sample, the system forms defect-free, spatially periodic structures. We now build on this approach by introducing nanorods that have a preferential affinity for one the phases in a binary mixture. By rastering over the sample with the higher intensity light, we can create ordered arrays of rods within periodically ordered materials in essentially one processing step. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V44.00009: Dynamic mechanical properties of hydroxyapatite/polyethylene oxide nanocomposites: characterizing isotropic and post-processing microstructures Meisha Shofner, Ji Hoon Lee Compatible component interfaces in polymer nanocomposites can be used to facilitate a dispersed morphology and improved physical properties as has been shown extensively in experimental results concerning amorphous matrix nanocomposites. In this research, a block copolymer compatibilized interface is employed in a semi-crystalline matrix to prevent large scale nanoparticle clustering and enable microstructure construction with post-processing drawing. The specific materials used are hydroxyapatite nanoparticles coated with a polyethylene oxide-b-polymethacrylic acid block copolymer and a polyethylene oxide matrix. Two particle shapes are used: spherical and needle-shaped. Characterization of the dynamic mechanical properties indicated that the two nanoparticle systems provided similar levels of reinforcement to the matrix. For the needle-shaped nanoparticles, the post-processing step increased matrix crystallinity and changed the thermomechanical reinforcement trends. These results will be used to further refine the post-processing parameters to achieve a nanocomposite microstructure with triangulated arrays of nanoparticles. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V44.00010: Thermodynamic Interactions in Polymer Nanocomposites towards Controlled Nanoparticle Dispersion Alisyn Nedoma, Rajeev Dattani, Joao Cabral Thermodynamic interactions in polymer/nanoparticle blends can be used to control the dispersion of nanoparticles. Here we introduce the use of polymer blends comprising immiscible A and B homopolymers, an \mbox{A-B} diblock copolymer ``surfactant,'' and nanoparticles. Upon thermal annealing, the composites self-assemble into equilibrium morphologies with well-dispersed nanoparticles.Polystyrene (PS), polymethyl methacrylate (PMMA), and a PS-PMMA diblock were chosen as the model polymers; C$_{60}$ fullerene was the model nanoparticle. Blends were prepared for C$_{60}$ loadings from 0.1 to 2 mass\% for blends with symmetric homopolymers and a symmetric diblock copolymer. The molecular weight of the matched homopolymers was varied as 2, 20, and 40 kDa whilst the same 60-60 kDA diblock copolymer was used. Samples were studied using small angle neutron scattering, and the resulting morphologies were found to be lamellar for all C$_{60}$ loadings. Ongoing work is exploring the effects of polymer asymmetry on the nanostructure of the composites. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V44.00011: Effect of competing monomer-monomer and monomer-particle interactions on the assembly of copolymer grafted nanoparticles Tyler Martin, Arthi Jayaraman Functionalizing nanoparticles with copolymer ligands is an attractive method to tailor the assembly of the nanoparticles. In this talk we present simulation results that show the effect of competing monomer-monomer and monomer-particle interactions on assembly of nanoparticles grafted with AB copolymers with diblock or alternating sequence. We vary strengths of like-monomer (A-A and/or B-B) attractive interactions in the presence of strong or negligible monomer A- particle (A-P) attraction or monomer B- particle (B-P) interaction. At a constant particle size and graft length, the competing interactions and copolymer sequence dictate the amount of inter-grafted particle monomer aggregation, inter- and intra-graft monomer aggregation within the same grafted particle. The resulting monomer aggregation on/near the particle surface imparts an effective patchiness to the particle. For e.g., in case of particles grafted with AB diblock copolymer (with A block closer to the surface) the presence of strong B-P and B-B attractions leads to smaller attractive patches that extend from the surface, and in turn lead to anistropic assembly, while presence of strong A-P and A-A attractions lead to larger attractive patches closer to the particle surface and in turn isotropic assembly. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V44.00012: Dielectric Breakdown in Filled Silicone Elastomers at the Particle/Matrix Interface Roger Diebold, Michael Gordon, David Clarke Silicone elastomers, widely used as electrical insulators and potting agents, are typically filled with large amounts of fumed silica nanoparticles to enhance their mechanical strength and toughness. It is well known that as the filler particle diameter decreases to the nanometer length scale, the interfacial surface area to volume ratio increases dramatically, consequently determining many of the physical properties of the macroscopic composite. Thus, to understand electrical breakdown in these materials, it is imperative to investigate the interface between the filler particle and the bulk elastomer. In order to eliminate some of the complexities of studying dielectric breakdown at the filler/matrix interface, it is necessary to remove particle-particle interactions. In this paper, the authors will present `creep' (surface) and `strike' (through-thickness) dielectric breakdown results at idealized oxide/polymer interfaces which closely emulate those found in filled elastomer systems. Weibull and I-V curve analysis will be used to describe the effects of silane coupling agent functionalization on dielectric breakdown, and in particular, how different chemical moieties play a role in high-field interfacial electronic transport. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 11:00AM |
V44.00013: The Development of Filler Structure in Colloidal Silica -- Polymer Nanocomposites Invited Speaker: Jeffrey Meth The realization of the full potential for polymeric nanocomposites to manifest their entitled property improvements relies, for some properties, on the ability to achieve maximum particle-matrix interfacial area. Well-dispersed nanocomposites incorporating colloidal silica as the filler can be realized in both polystyrene and poly(methylmethacrylate) matrices by exploiting the charge stabilized nature of silica in nonaqueous solvents which act as Bronsted bases. We demonstrate that dispersions of colloidal silica in dimethylformamide are charge stabilized, regardless of organosilyl surface functionalization. When formulated with polymer solutions, the charge stabilized structure is maintained during drying until the charged double layer collapses. Although particles are free to diffuse and cluster after this neutralization, increased matrix viscosity retards the kinetics. We demonstrate how high molecular weight polymers assist in immobilizing the structure of the silica to produce well-dispersed composites. The glass transition temperatures of these composites do not vary, even at loadings up to 50 v{\%}. [Preview Abstract] |
Session V45: Soft Matter Physics of Heterogeneous Membranes - Experiments
Sponsoring Units: DPOLY DBIOChair: Gary Grest, Sandia National Laboratories
Room: 159
Thursday, March 1, 2012 8:00AM - 8:12AM |
V45.00001: Structure and mechanics of biological fiber-laden membranes Yogesh Kumar Murugesan, Alejandro Rey, Damiano Pasini We present a mechanical model of the plant cell wall viewed as anisotropic 2D soft matter, where a dilute dispersion of cellulose fibrils of variable orientations is on a curved deformable viscoelastic membrane. The model integrates the elastic energy of the curved membrane, the nematic fiber orientation energy, and competing curvophilic and curvophobic interactions mediated by the membrane geometry and the fibrils' orientation. The selected membrane geometry is a straight cylinder, whose cross-sectional shape varies from a circle to a super-ellipse as in many plant species, and its size increases as in plant cell wall during growth. Model predictions indicate that due to curvature--orientation couplings, the fiber orientation displays three modes: axial (at large curvature), helical and transverse (at small curvature). The high curvature also promotes the order of the fibrils. The predicted fiber structure is validated with that in the cell wall of tracheids. To gain insight into the role of fiber structure on the elasticity of fiber-laden membranes, the effect of fiber orientation and order on the effective bending modulus of the fiber-laden membrane cross-section is investigated. The structure--property relations for super-elliptical membranes are established. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V45.00002: From a single molecule to a membrane of structured ionic polymers: A molecular dynamic simulation study Dipak Aryal, Dvora Perahia, Gary S. Grest The association of an A-B-C-B-A co-polymer with an ionizable center and a bulky end block has been investigated using molecular dynamic simulations. The center block consists of a randomly sulfonated polystyrene connected to a flexible poly (ethylene-r-propylene) bridge and end caped with poly (t-butyl styrene). Tailoring the nature of individual segments within a block co-polymer is a potential design tool to form membranes with desired properties. The association mode and the dynamics of the segments control the overall characteristics. The membranes with three sulfonation level for the center block were made by evaporating a common solvent for all blocks. The local structure including size and distribution of the ionic blocks and the continuity of the styrene phase as well as long range correlations were identified at 300 and 500K. The initial membrane structure is affected by the structure in solution. Studies on changes that take place above the glass transition temperature for each of the blocks will also be presented. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V45.00003: Investigation of the elastic properties of a lipid bilayer by fluorescence interferometry Dong Gui, Hsiang-ku Lin, Ehsan Noruzifar, Leonid Pryadko, Roya Zandi, Umar Mohideen Freestanding curved lipid bilayers were formed on micron diameter wells fabricated on a silicon chip. The height profile of the lipid bilayers was measured using fluorescence interference contrast microscopy. Dark and bright rings resulted from the interference of emission from the fluorophores in the lipid bilayers with the same light reflected from the bottom surface of the well. By changing the osmotic pressure difference across the bilayers, the relationship between the pressure and the membrane curvature was studied. By using Helfrich theory, the surface tension of the bilayer was extracted. The influence of detergents and antibiotics on the elastic property of lipid bilayers was also investigated. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V45.00004: Regulating the Size and Stabilization of Lipid Raft-Like Domains and Using Calcium Ions as Their Probe Uri Raviv, Or Szekely In this paper, we apply means to probe, stabilize and control the size of lipid raft-like domains in vitro. In biomembranes the size of lipid rafts is ca. 10 - 30 nm. In vitro, mixing saturated and unsaturated lipids results in micro-domains, which are unstable and coalesce. Using solution X-ray scattering, we studied the structure of binary and ternary lipid mixtures in the presence of calcium ions. Three lipids were used: saturated, unsaturated and a hybrid (1-saturated-2-unsaturated) lipid that is predominant in the phospholipids of cellular membranes. Only membranes composed of the saturated lipid can adsorb calcium ions, become charged and therefore considerably swell. The selective calcium affinity was used to show that binary mixtures, containing the saturated lipid, phase separated into large-scale domains. Our data suggests that by introducing the hybrid lipid to a mixture of the saturated and unsaturated lipids, the size of the domains decreased with the concentration of the hybrid lipid, until the three lipids could completely mix. We attribute this behavior to the tendency of the hybrid lipid to act as a line-active co-surfactant that can easily reside at the interface between the saturated and the unsaturated lipids and reduce the line-tension between them. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V45.00005: Floret-shaped solid domains on giant fluid lipid vesicles induced by pH Stavroula Sofou, Amey Bandekar Lateral lipid phase separation and domain formation induced by changes in pH is significant in liposome-based drug delivery: environmentally responsive lipid heterogeneities can be tuned to alter collective membrane properties such as drug release and drug carrier reactivity impacting, therefore, the therapeutic outcomes. At the micron-meter scale, fluorescence microscopy on Giant Unilamellar fluid Vesicles (GUVs) shows that lowering pH (from 7.0 to 5.0) promotes the condensation of titratable PS or PA lipids into beautiful floret-shaped solid domains in which lipids are tightly packed via H-bonding and VdWs interactions. Solid domains phenomenologically comprise a circular ``core'' cap beyond which interfacial instabilities emerge resembling leaf-like stripes of almost vanishing Gaussian curvature independent of GUVs' preparation path and in agreement with a general condensation mechanism. Increasing \textit{incompressibility} of domains is strongly correlated with larger \textit{number of thinner stripes }per domain, and increasing \textit{relative} \textit{rigidity} of domains with smaller \textit{core cap areas}. Line tension drives domain ripening, however the final domain shape is a result of enhanced incompressibility and rigidity maximized by domain coupling across the bilayer. Introduction of a transmembrane osmotic gradient (hyperosmotic on the outer lipid leaflet) allows the domain condensation process to reach its maximum extent which, however, is limited by the minimal expansivity of the continuous fluid membrane. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V45.00006: The influence of membrane stress on phase separation and domain shape in phospholipid vesicles Dong Chen, Maria Santore Phase separation of mixed phospholipid bilayers is of interest due to the potential role of phospholipid rafts in cell adhesion and signaling. Studies of membrane dynamics and the phase diagram itself typically neglect the role of tension, though it is expected that imposition of moderate membrane tensions might mildly shift the phase separation temperature, as anticipated by Clausius Clapeyron. We show here, using a simple binary system (DOPC/DPPC), a more dramatic effect: The tension imposed on giant unilamellar phospholipid vesicles can alter the phase and the domain shape, completely changing the composition of the liquid and solid phases, their proportions, and the transition temperature. The example in this talk demonstrates how striped or patchy hexagonal phases can develop, depending on thermal history and tension. Different incorporation of tracers into the ordered phases suggests fundamental differences in their structure at the molecular level. Rapid quenching and low tensions favor hexagonal patches while increased tension and slower quenching favors a striped phase. For this reason it is believed that the patches contain corrugations such that the structure of the ordered phase is metastable. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V45.00007: Membrane fluctuations alter the fluidity of clathrin protein lattices Andrew Spakowitz, Nicholas Cordella, Shafigh Mehraeen Clathrin is a protein that plays a major role in the creation of membrane-bound transport vesicles in cells. The pinwheel subunits of clathrin assemble into closed, nanoscale assemblies with various shapes and sizes. We develop a model for clathrin, facilitating the study of membrane, surface, and bulk assembly. The clathrin are modeled as pinwheels that form leg-leg associations and resist bending and stretching deformations. Invoking theories of dislocation-mediated melting in two dimensions, we discuss the phase behavior for clathrin. We demonstrate that the generation of defects resembles creation of two dislocations, and we use orientational- and translational-order correlation functions to predict the crystalline-hexatic and hexatic-liquid phase transitions. Accounting for membrane fluctuations, we address the phase behavior of clathrin on a membrane surface. Membrane fluctuations act to soften the elastic coupling between defects in the clathrin lattice, altering the conditions for the crystalline-hexatic phase transition. This effect offers a mechanism for altering the fluidity of protein or polymer films. Furthermore, these results illustrate the pivotal role that molecular elasticity plays in the physical behavior of self-assembling and self-healing materials. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V45.00008: The structure of unsupported, self-assembled phospholipid bilayers on an artificially nano-patterned surface Gregory Smith, Seung-Yong Jung, James Browning, Jong Keum, Nickolay Lavrik, Pat Collier We present neutron reflectivity measurements of the in-situ microscopic architecture of phospholipid molecules at the interface between a regularly nano-patterned surface and an aqueous sub-phase using neutron reflectometry. 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) single bilayers were deposited on a patterned silicon substrate. The substrate was patterned with a rectangular array of nano-scaled holes using e-beam nano-lithographic techniques. The goal of these experiments is to produce a set of small freely-suspended bilayers spanning the nanostructured surface. We compare results for films deposited by vesicle adsorption or by the Langmuir--Shafer (L-S) technique. Initial data analysis shows that there are well formed bilayers on the surface. Detailed analysis of the reflectivity curves will be presented to confirm details of the architecture of these bilayer films. Bilayers prepared in this way may serve as model single bilayer systems with freely suspended areas for the study of membrane functionality in biological and biomimetic materials and systems. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V45.00009: Peptide-induced Asymmetric Distribution of Charged Lipids in a Vesicle Bilayer Revealed by Small-Angle Neutron Scattering William Heller, Shuo Qian Cellular membranes are complex mixtures of lipids, proteins and other small molecules that provide functional, dynamic barriers between the cell and its environment, as well as between environments within the cell. The lipid composition of the membrane is highly specific and controlled in terms of both content and lipid localization. Here, small-angle neutron scattering and selective deuterium labeling were used to probe the impact of the membrane-active peptides melittin and alamethicin on the structure of lipid bilayers composed of a mixture of the lipids dimyristoyl phosphatidylglycerol (DMPG) and chain-perdeuterated dimyristoyl phosphatidylcholine (DMPC). We found that both peptides enriched the outer leaflet of the bilayer with the negatively charged DMPG, creating an asymmetric distribution of lipids. The level of enrichment is peptide concentration-dependent and is stronger for melittin than alamethicin. The enrichment between the inner and outer bilayer leaflets occurs at very low peptide concentrations, and increases with peptide concentration, including when the peptide adopts a membrane-spanning, pore-forming state. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V45.00010: Cooperative assembly in targeted drug delivery Debra Auguste Described as cell analogues, liposomes are self-assembled lipid bilayer spheres that encapsulate aqueous volumes. Liposomes offer several drug delivery advantages due to their structural versatility related to size, composition, bilayer fluidity, and ability to encapsulate a large variety of compounds non-covalently. However, liposomes lack the structural information embedded within cell membranes. Partitioning of unsaturated and saturated lipids into liquid crystalline (L$\alpha )$ and gel phase (L$\beta )$ domains, respectively, affects local molecular diffusion and elasticity. Liposome microdomains may be used to pattern molecules, such as antibodies, on the liposome surface to create concentrated, segregated binding regions. We have synthesized, characterized, and evaluated a series of homogeneous and heterogeneous liposomal vehicles that target inflamed endothelium. These drug delivery vehicles are designed to complement the heterogeneous presentation of lipids and receptors on endothelial cells (ECs). EC surfaces are dynamic; they segregate receptors within saturated lipid microdomains on the cell surface to regulate binding and signaling events. We have demonstrated that cooperative binding of two antibodies enhances targeting by multiple fold. Further, we have shown that organization of these antibodies on the surface can further enhance cell uptake. The data suggest that EC targeting may be enhanced by designing liposomes that mirror the segregated structure of lipid and receptor molecules involved in neutrophil-EC adhesion. This strategy is employed in an atherosclerotic mouse model in vivo. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V45.00011: Polymer-Protein interaction at air/liquid interfaces: X-ray reflectivity and surface spectroscopy studies Wenjie Wang, Nathaniel Anderson, Sanjeeva Murthy, David Vaknin Adsorption of proteins onto a substrate is the first and a critical step that determines the cellular response of substrates. To understand the adsorption and distribution of proteins on surfaces, we employ surface sensitive X-ray scattering and spectroscopic techniques to monitor the adsorption of plasma proteins (fibrinogen) onto surfaces of polymers, poly(DTE carbonate), on aqueous surfaces. Our X-ray measurements provide the density profiles of the polymers-proteins systems on aqueous surfaces, with details on the interactions between the polymers and the protein, and distribution of the protein within and on the polymer surface. The hydrophobic and hydrophilic behaviors of these polymers are modified by incorporating poly(ethylene glycol) (PEG) and by iodinating the tyrosine rings. Our results confirm the inhibition of the adsorption of fibrinogen onto polymer surfaces by PEG, and the counteraction of this influence when the polymers are iodinated. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V45.00012: Microporous device for local electric recordings on lipid bilayers Theresa Kaufeld, Christopher Battle, Conrad Weichbrodt, Claudia Steinem, Christoph Schmidt Many methods for artificial membrane formation are available. We focus on the reconstitution of lipid bilayers on porous substrates combining the stability of solid supports and the accessibility of both sides of the bilayer of the classical BLM which is necessary for low noise electric experiments. Most commercially available porous substrates however are not suitable for electric experiments or a combination of several measuring techniques. Therefore, we designed a microporous substrate, which meets several demands: We wanted to have the possibility to perform multiple experiments in one, so we chose to divide the device into several individually addressable arrays of pores with separate electrolyte compartments and integrated electronic connections. Also, to perform electrical and fluorescence experiments at the same time, we designed a PDMS sample chamber so that the substrate is accessible to a microscope objective. By having separated electrolyte compartments, we are also able to exchange solutions or introduce chemicals throughout the experiment. Bilayer formation can be probed by impedance spectroscopy and fluorescence microscopy. The function of inserted ion channels can be measured by current recordings. [Preview Abstract] |
Session V46: Invited Session: Active Matter and Dynamical Systems
Sponsoring Units: DPOLY DBIOChair: Cristina Marchetti, Syracuse University
Room: 160AB
Thursday, March 1, 2012 8:00AM - 8:36AM |
V46.00001: From filamentous bundles to active random flows Invited Speaker: Zvonimir Dogic The cytoskeleton has a number of highly unusual material properties which are essential for the reproduction and survival of the cell. However, the extraordinary structural complexity of the cytoskeleton, which contains hundreds of different proteins, presents a particular challenge to any study of its material properties. I will outline experiments whose goal is to reconstitute certain material properties of the cytoskeleton using a few well-defined biochemical components. The long term goal is to understand the behavior of the resulting materials at all levels of hierarchy. For this reason, I will first describe experiments that probe the behavior of equilibrium filamentous bundles and their dependence on the properties of the constituent filaments and their interactions. Subsequently, I will describe the emergent dynamical patterns that form when molecular motors drive an assembly of filamentous bundles to highly out of equilibrium steady states. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 9:12AM |
V46.00002: Spindle Assembly and Architecture: From Laser Ablation to Microtubule Invited Speaker: Daniel Needleman The spindle is a dynamic steady-state structure composed of microtubules and a wide range of factors which control microtubule nucleation, growth, and motion. While many of the individual components of the spindle have been studied in detail, it is still unclear how these molecular constituents self-organize into this structure. Crucially, the extent to which microtubule behaviors are spatially regulated is not known. Here I describe how we are using laser ablation experiments to obtain detailed structural information in spindles: the location of microtubule plus ends, microtubule minus ends, and the length distribution of microtubules in different regions. All of our data can be explained by a very simple model which provides surprising insight into how the regulation of microtubule nucleation and stability gives rise to spindle architecture. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V46.00003: Synchronization of eukaryotic flagella {\it in vivo}: from two to thousands Invited Speaker: Raymond E. Goldstein From unicellular organisms as small as a few microns to the largest vertebrates on Earth, we find groups of beating flagella or cilia that exhibit striking spatiotemporal organization. This may take the form of precise frequency and phase locking, as frequently found in the swimming of green algae, or beating with long-wavelength phase modulations known as metachronal waves, seen in ciliates such as {\it Paramecium} and in our own respiratory systems. The remarkable similarity in the underlying molecular structure of flagella across the whole eukaryotic world leads naturally to the hypothesis that a similarly universal mechanism might be responsible for synchronization. Although this mechanism is poorly understood, one appealing hypothesis is that it results from hydrodynamic interactions between flagella. This talk will summarize recent work using the unicellular alga {\it Chlamydomonas reinhardtii} and its multicellular cousin {\it Volvox carteri} to study in detail the nature of flagellar synchronization and its possible hydrodynamic origins. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:24AM |
V46.00004: Dynamic Patterns in Active Fluids Invited Speaker: Frank J\"ulicher Biological matter is inherently dynamic and exhibits active properties. A key example is the force generation by molecular motors in the cell cytoskeleton. Such active processes give rise to the generation of active mechanical stresses and spontaneous flows in gel-like cytoskeletal networks. Active material behaviors play a key role for the dynamics of cellular processes such as cell locomotion and cell division. We will discuss intracellular flow patterns that are created by active processes in the cell cortex. By combining theory with quantitative experiments we show that observed flow patterns result from profiles of active stress generation in the system. We will also consider the situation where active stress is regulated by a diffusing molecular species. In this case, spatial concentration patterns are generated by the interplay of stress regulation and self-generated flow fields. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 11:00AM |
V46.00005: Order and instabilities in dense bacterial colonies Invited Speaker: Lev Tsimring The structure of cell colonies is governed by the interplay of many physical and biological factors, ranging from properties of surrounding media to cell-cell communication and gene expression in individual cells. The biomechanical interactions arising from the growth and division of individual cells in confined environments are ubiquitous, yet little work has focused on this fundamental aspect of colony formation. By combining experimental observations of growing monolayers of non-motile strain of bacteria Escherichia coli in a shallow microfluidic chemostat with discrete-element simulations and continuous theory, we demonstrate that expansion of a dense colony leads to rapid orientational alignment of rod-like cells. However, in larger colonies, anisotropic compression may lead to buckling instability which breaks perfect nematic order. Furthermore, we found that in shallow cavities feedback between cell growth and mobility in a confined environment leads to a novel cell streaming instability. Joint work with W. Mather, D. Volfson, O. Mondrag\'{o}n-Palomino, T. Danino, S. Cookson, and J. Hasty (UCSD) and D. Boyer, S. Orozco-Fuentes (UNAM, Mexico). [Preview Abstract] |
Session V47: Focus Session: Heterogeneous Colloids II
Sponsoring Units: DPOLYChair: David Pine, New York University
Room: 160C
Thursday, March 1, 2012 8:00AM - 8:12AM |
V47.00001: Self Assembly of Soft Matter Quasicrystals and Their Approximants Christopher Iacovella, Aaron Keys, Sharon Glotzer The discovery of soft-matter quasicrystals (QCs) and their approximants [1-4] hints at a unique thermodynamic mechanism underlying their stability. In the past, specific interaction potentials have been contrived to stabilize QCs and their approximants in computer simulations, but such interactions are difficult to achieve in colloidal systems. Here, we use molecular simulation to demonstrate an alternative approach for assembling dodecagonal QCs and their approximants based solely on particle functionalization and shape [5]. Our approach replaces complex energetic interactions with simpler-to-achieve bonded and excluded-volume interactions, encouraging the formation of structures with low surface contact area, including non-close-packed and polytetrahedral structures. We argue that this mechanism can be widely exploited to assemble QCs and approximants in colloidal systems, and may further elucidate the formation of soft matter QCs in experiment [1-4]. \\[4pt] [1] G. Ungar, et al., Science 299 (2003) \\[0pt] [2] X. Zeng, et al., Nature 428, (2004) \\[0pt] [3] S. Lee, M.J. Bluemle, F.S. Bates, Science, 330 (2010) \\[0pt] [4] S. Fischer, et al. Proc. Natl. Acad. Sci., 108, (2011) \\[0pt] [5] C.R. Iacovella, A.S. Keys, S.C. Glotzer, Proc. Natl. Acad. Sci., in press (2011) arXiv:1102.5589 [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V47.00002: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V47.00003: Self-assembly of anisotropic colloidal particles under confinement Carlos Avendano Jimenez, Fernando Escobedo We perform molecular simulations of two novel anisotropic colloidal particles under confinement. The first is an ensemble of hard-hemispheres (mimicking mushroom cap colloids [1]) confined between two parallel walls separated by a distance H. We simulated: a) Low values of H that restrict the rotation of the particles (in a monolayer) where three main dense crystal structures are found: buckled phase, square, and triangular structures. (b) Large values of H where the particles are able to fully rotate; here parallel (spheres-like) or anti-parallel (column-like) dimers are observed which form ordered structures with triangular and rectangular symmetries at high densities. The second system is a model of hard-square particles with rounded corners. Recently, Zhao et. al. have reported the phase behaviour of monolayers of polymeric squares platelets with rounded corners, assembled at the bottom of the container [2]. This system exhibits the formation of a hexagonal rotator phase and the rhombic crystal phase that were not observed in earlier simulations of squares [3] which found instead a tetratic and square crystal phases. By interpolating between hard discs and hard squares, we map out the phase diagram as a function of the roundness of the particles and resolve the discrepancies of the earlier studies. [1] Riley and Liddell, Langmuir 26, 11648 (2010). [2] Zhao, Bruinsma, and Mason, PNAS 108, 2684 (2011). [3] Wojciechowski and Frenkel, Comp. Met. Sci. Technol. 10, 235 (2004). [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 9:12AM |
V47.00004: Towards Structural Complexity with Colloids Invited Speaker: Michael Engel Colloids rather easily assemble into simple crystal structures like the face-centered cubic lattice or the body-centered cubic lattice. More complex phases are harder to achieve, but have recently been reported using a number of approaches. Yet, assembling complex structures often results from trial-and-error and is not well understood. In this presentation, we show how novel crystals, quasicrystals, and liquid crystals can be achieved with colloidal building blocks by varying the interactions and the shapes of the building blocks. Using computer simulations, we demonstrate the formation of unusually ordered phases both with isotropic pair potentials, as well as with facetted shapes like polyhedra. We describe new tools we have developed to perform complex structural analysis on simulated systems and show how they may be used to analyze real space images from colloid experiments. We also compare the assembled structures with densest packings of the building blocks and show that good packings can often be distinct from what is observed to assemble from the disordered state. This suggests that dense packings may not be illustrative of what is achievable in colloid experiments. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V47.00005: Novel Non-Close-Packed Structures Assembled by Spherical Colloids with Anisotropic Interactions under Electric Field Fuduo Ma, Ning Wu Spherical colloids with isotropic properties have been used as building blocks to assemble a variety of 2D and 3D structures in past, such as FCC, HCP, and BCT crystals. We recently, however, have observed new type of two-dimensional structures under the influence of electric field at the liquid-solid interface. This is primarily due to anisotropic interactions arising from electric field on both particles and aqueous solution. At low concentrations and low frequencies of the electric field, the isotropic spheres can form a series of colloidal clusters, ranging from 3 to 10. The analysis of cluster distributions shows non-trivial peaks for trimer, tetramer, hexamer, and nanomer. Those clusters can change bond angles freely while maintaining the overall structures intact. At high concentrations, those colloidal clusters with flexible bond angles can further assemble and connect themselves into a good variety of two-dimensional non-close-packed networks that have not been observed before. By precisely controlling the electric field strength, frequency, volume fraction of colloids, and ionic strength, we have made diversified non-close-packed structures that have potential applications in photonic crystal, catalysis, or filtration. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V47.00006: Self-assembly of polydisperse nanoparticles into monodisperse supraparticles: a computer simulation study Trung D. Nguyen, Yunsheng Xia, Byeongdu Lee, Ming Yang, Aaron Santos, Paul Podsiadlo, Zhiyong Tang, Sharon C. Glotzer, Nicholas A. Kotov Experiments have shown that polydisperse inorganic nanoparticles such as CdSe, CdS and PbS self-assemble into highly uniform supraparticles with a core-shell morphology[1]. The self-assembly process is believed to be self-limiting as indicated by the time evolution of the measured surface potential and supraparticle size distribution. We performed molecular dynamics simulations to demonstrate that the balance between van der Waals attraction and Coulombic repulsion leads to the self-limiting growth of the supraparticles[2-4]. That the uniform supraparticles are stable over a wide range of density indicates that they are thermodynamically stable, rather than consequences of limited diffusion. Our simulation results further reveal that the broad nanoparticle polydispersity leads to the core-shell morphology of the supraparticles. The generic nature of the governing interactions suggests great versatility in the composition, size and shape of the constituent building blocks, and allows for a large family of hierarchical self-assembled structures, including colloidal crystals. References 1. Y. Xia et al, Nat. Nano. 6, 580, 2011. 2. Gonzalez-Mozuelos et al, J. Chem. Phys. 103, 3145, 1995. 3. Van Hyning et al, Langmuir 17, 3120, 2001. 4. J. Ramsden, Proc. R. Soc. Lond. A 413, 407, 1987 [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V47.00007: Particle dynamics in colloidal glasses with short-range attraction Piotr Habdas, Ke Chen, Martin Iwanicki, Daniel Flynn, John Michael Devany, Lisa Mariani, Arjun G. Yodh We study colloidal particle dynamics of a model glass system using confocal microscopy as the sample evolves from a repulsive glass towards an attractive glass. Short-range depletion forces induce the transition from a repulsive glass to the attractive glass. We identify particles which exhibit substantial motional events and characterize the transition using the properties of these motional events. It appears that number of particles that exhibit motional events doesn't change as the system is brought from a repulsive glass towards the attractive glass. Also, we investigate vibrational properties of these dense colloidal suspensions. Our preliminary results show that the boson peak for an attractive glass is lower than that for a repulsive system and it is shifted towards higher frequencies. To our knowledge, this is the first experimental investigation of the evolution of vibrational modes in colloidal glasses when particle interaction potential changes from repulsive to attractive. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:24AM |
V47.00008: Self-assembly of colloidal surfactants Invited Speaker: Willem Kegel We developed colloidal dumbbells with a rough and a smooth part, based on a method reported in Ref. [1]. Specific attraction between the smooth parts occurs upon addition of non-adsorbing polymers of appropriate size. We present the first results in terms of the assemblies that emerge in these systems. \\[4pt] [1] D.J. Kraft, W.S. Vlug, C.M. van Kats, A. van Blaaderen, A. Imhof and W.K. Kegel, \textit{Self-assembly of colloids with liquid protrusions}, J. Am. Chem. Soc. \textbf{131}, 1182, (2009) [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V47.00009: The Structure of Dipolar Colloidal Gels in the Dilute Regime Veronica I. Marconi, Marcelo A. Carignano Dipolar colloidal systems are one of the simpler material presenting anisotropic interactions and are highly relevant for designing new soft materials easy to control. Using stochastic dynamics simulations we investigate the gelation process in a high dilute system of dipolar colloidal particles immerse in an implicit dielectric solvent. This system self assembles in a rich variety of structures, from open percolated networks or gels to short chains of particles that self cross. We perform simulations using a ``continues model'' for the dipolar particles interactions and we present a phase diagram, density vs temperature. Each phase is characterized in detail as a function of the spatial correlation (clusters) and response. Interestingly at this low density regime, it is possible to find, characterize and follow the dynamics of a clear and huge variety of short strings configurations (cross loops, triple pointed chains, bundles), mainly at very low temperature. In addition, studying the dynamics of gelation in detail we observe an increasing gelation time while density is decreased in good agreement with previous simulations and experiments on colloidal systems with directional interactions. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V47.00010: Two-Dimensional Crystals of Icosahedral Viruses at Liquid interfaces Masafumi Fukuto, Lin Yang, Antonio Checco, Ivan Kuzmenko, Quyen Nguyen, Nick Mank, Qian Wang Two-dimensional (2D) assembly of turnip yellow mosaic virus (TYMV) on cationic lipid monolayers is investigated at the air-water interface. TYMV, an icosahedral virus with a diameter of 28 nm, exhibits well-defined roughness, charge distribution, and hydrophilic/hydrophobic patches on its surface. The electrostatic attraction to the lipid-coated aqueous interface provides means to impose a specific virus orientation and hence reduce the number of possible inter-particle interactions. The 2D geometry is particularly advantageous in dissecting the role of anisotropy in aqueous-media assembly, which involves various types of similarly weak interactions. We show that the assembly approach used not only facilitates crystallization but also provides insights on how complex anisotropic interactions can be exploited to generate long-range order. Specifically, we report an \emph{in situ} x-ray scattering observation of novel 2D crystal forms of TYMV that reflect the virus' icosahedral symmetry. The symmetry, shape, and surface heterogeneities of TYMV suggest a mechanism by which these crystals are stabilized by a combination of hydrophobic, electrostatic, and steric interactions. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V47.00011: Active colloids at liquid-liquid interfaces: dynamic self-assembly and functionality Alexey Snezhko, Igor Aranson Self-assembled materials must actively consume energy and remain out of equilibrium in order to support structural complexity and functional diversity. Colloids of interacting particles suspended at liquid-liquid interfaces and maintained out of equilibrium by external alternating electromagnetic fields develop nontrivial collective dynamics and self-assembly. We use ferromagnetic colloidal micro-particles (so the magnetic moment is fixed in each particle and interactions between colloids is highly anisotropic and directional) suspended over an interface of two immiscible liquids and energized by vertical alternating magnetic fields to demonstrate novel dynamic and active self-assembled structures (``asters'') which are not accessible through thermodynamic assembly. Structures are attributed to the interplay between surface waves, generated at the liquid/liquid interface by the collective response of magnetic microparticles to the alternating magnetic field, and hydrodynamic fields induced in the boundary layers of $\it{both}$ liquids forming the interface. Two types of magnetic order are reported. We demonstrate that asters develop self-propulsion in the presence of a small in-plane dc magnetic field. We show that asters can capture, transport, and position target microparticles. [Preview Abstract] |
Session V48: Focus Session: Advanced Optical Probes of Soft Matter - Forces, Imaging, Excitons
Sponsoring Units: DPOLY DBIOChair: Maria Kilfoil, University of Massachusetts
Room: 161
Thursday, March 1, 2012 8:00AM - 8:36AM |
V48.00001: TBD |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V48.00002: Brownian vortex circulation due to spin orbit conversion in a circularly polarized optical tweezer David Ruffner, David G. Grier Strong focusing of circularly polarized beams converts spin angular momentum into orbital angular momentum. We describe this process in terms of a generalized vector potential, involving the amplitude, phase, and polarization of the light. This gives a more general understanding of this force in terms of experimentally accessible parameters. In addition, this formalism provides a framework for understanding other polarization induced forces, which arise from the curl of the spin angular momentum density. Experimentally we demonstrate deterministic polarization-induced circulation with trapped clusters of 1$\mu$m polystyrene spheres, and Brownian vortex circulation for a single sphere trapped in elliptically polarized optical tweezers. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V48.00003: Pulling Particles Backward Using a Forward Propagating Beam Jack Ng, Jun Chen, Zhifang Lin, Che Ting Chan Can the scattering force of a forward propagating beam pull a particle backward? A photon carries a momentum of $\hbar k$, so one may expect light will push against any object standing in its path. However, light can indeed ``attract'' in some cases. For example, if light is focused to a spot, small particles will be attracted towards it due to the gradient force. But it is probably more appropriate to say that the gradient force ``grabs'' rather than ``pulls'', as the particle will remain stable in the trap after being drawn to the focus. Here, we discuss another possibility---a backward scattering force which is always opposite to the propagation direction of the beam so that the beam keeps on pulling an object towards the source without an equilibrium point. In the absence of intensity gradient, using a light beam to pull a particle backwards is counter intuitive. The underlining physics is the maximization of forward scattering via interference of the radiation multipoles. We show explicitly that the necessary condition to realize a pulling force is the simultaneous excitation of multipoles in the particle and if the projection of the total photon momentum along the propagation direction is small, attractive optical force is possible. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V48.00004: Lateral optical binding forces between two colloidal Mie particles Ming-Tzo Wei, Jack Ng, C.T. Chan, H. Daniel Ou-Yang Micro particles in an intense optical field can self-organize into an array with well defined structure. This phenomenon, first reported by Burns et al., as optical binding, was believed to be caused by the optical gradient force. In spite of many attempts to calculate the binding forces and the colloidal structures, there has been a lack of experiments directly measuring the forces between the particles. Positioning two micron-sized polystyrene particles, each held by a tightly focused laser beam from a single coherence laser source, we found the lateral optical binding force oscillates with the inter particle separation, as well as the relative phase between the beams due to retardation. By independently changing the polarization directions at each optical trap, we examined the periodicity and magnitude of the forces. Our research indicates the forces under such conditions require a model beyond dipole approximation. In addition, an accurate calculation based on Mie theory with consideration of high focusing compare well with our experimental findings. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V48.00005: Super-Resolution Imaging Using Randomly Diffusing Probes Anna Labno, Hu Cang, Christopher Gladden, Xiang Zhang Recent advances in super-resolution microscopy allow imaging of biological tissues labeled with fluorescent dyes with unprecedented resolution. These techniques often rely on the fact that single emitters can be localized with nm accuracy. When multiple emitters reside within a diffraction-limited spot they are serially photo-switched to ensure that they emit one at the time. This approach has not been applied to other imaging modalities, for example imaging local electromagnetic field enhancement, mainly because photo-switching would be infeasible. Here we present a super-resolution imaging technique which circumvents the requirement for serial photoswitching by using the random motion of single dye molecules to scan the surface in a stochastic manner. This technique allowed us to image electromagnetic field enhancement of a single spot formed on thin metallic film with 1.2nmn accuracy and gain insight into the mechanism for generating field enhancement. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V48.00006: Three-dimensional thermal noise images of single biopolymer filaments allows to determine their orientation and quantify their mechanical properties Martin Kochanczyk, Tobias Bartsch, Pinyu Thrasher, Ernst-Ludwig Florin Intracellular biopolymer networks perform many essential functions for living cells. Most of these networks show a highly nonlinear mechanical response that is well-studied on the macroscopic scale. While much work has been done to connect the macroscopic responses of networks to specific network properties, such as filament persistence length, cross-linking geometry and pore size, there is a lack of experimental techniques that can simultaneously determine the structure and the mechanical properties of a network in situ on the single filament level. Thermal Noise Imaging is a scanning probe technique that utilizes the confined thermal motion of an optically trapped particle as a three-dimensional, noninvasive scanner for soft, biological material. It achieves nanometer precision in probe position detection at MHz bandwidth. Thermal noise imaging visualizes single biopolymer filaments as nanoscale tunnels and allows for the quantification of their mechanical properties from their transversal fluctuations. The experiments presented here pave the way for investigating force distributions inside biopolymer networks on the single filament level, as well as establish thermal noise imaging as a quantitative tool for studying biological material on the nanometer scale. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V48.00007: Microscopy method for the characterization of structural color on a single wing scale of Morpho butterfly Beom-Jin Yoon, Matija Crne, Jung Ok Park, Mohan Srinivasarao The structural color and the iridescence of Morpho rhetenor were investigated using an optical microscope and a digital camera. Incoherent white light source was used for both spatial and spectral analysis. The scattering pattern from the micrometer sized single scale in the back focal plane of the objective lens was observed with Bertrand lens equipped in the optical microscope. We precisely controlled incident angle of the light using common components typically embedded in most optical microscope; aligning aperture stop at the center or off-center. Wide range of the angular scattering pattern from a single scale was measured and the iridescence of Morpho rhetenor was measured quantitatively. The single scale of Morpho rhetenor diffusively reflected the normally illuminated light, while blue band was more effectively reflected than green and red band. We retrieved the raw intensity data generated at the imaging sensor of the digital camera and quantitatively analyzed the spatial distribution of the scattered light. The reflectivity measured by the digital camera was comparable to the result from microspectrometer reported earlier. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V48.00008: High Fidelity Detection of Defects in Polymer Films using Surface-Modified Nanoparticles Chaitanya Pratiwada, Sindhuja Chari, Jolanta Marszalek, Matthew Becker, Alamgir Karim Defects are ubiquitous to materials and material surfaces. As we push the thresholds of length scale producing defect free materials, surfaces and interfaces, it becomes increasingly difficult to detect their presence over multiple length scales. The ideal example is the semiconductor industry where the driving force is higher performance and lower cost devices with material interfaces. In this regard, Chemical mechanical planarization, CMP, emerged as the premier method for achieving ultra flat surfaces below the 0.35 micron technology node, enabling many of the advanced electronic devices currently in production. The interactions between numerous process settings and output metrics are difficult to predict and often lead to defects. Failure analysis (FA) is critical and the current methods involve advanced imaging methods such as SEM. We design a cost effective method to detect physico-chemical defects at multiple length scales through Polymer-Nanoparticles (NPs) interactions in relatively shorter period of time (time of imaging is reduced 50 fold). This method can be used in conjunction with the traditional imaging methods to pin point location of these defects, which can be further analyzed. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V48.00009: A Study of the Polarizability of Single-Walled Carbon Nanotubes in an Optical Field Jingyu Wang, H. Daniel Ou-Yang Whereas the behavior of single-walled carbon nanotubes (SWCNT) in an electric field has been extensively studied, the polarizability of SWCNTs at optical frequencies remains unclear due to the difficulty in direct detection. It was demonstrated by utilizing Raman spectroscopy as a characterization means, optical tweezers could selectively aggregate SWCNTs. While it was commonly believed that the trapping effect due to the large optical field gradient caused the strong response of tubes to the laser beam, we expect the aligning effect due to the optical polarization also has a considerable contribution. To quantify these two possible effects experienced by an ensemble of individual DNA-SWCNTs of different chiralities, and address the issue of tube-tube interaction, We design an experiment by applying optical tweezers with variable polarization states and insepcting resonance Raman excitation for sensitive detection. Specifically, we measure the radial breathing mode signal of SWCNTs as a function of laser power and the direction of polarization for different tube types and concentrations. The research may lead to a more complete understanding of sorting phenomenon of individual SWCNTs in an optical field at microscopic level. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V48.00010: Dynamics of Aerial Tower Formation in Bacillus subtilis Biofilms Naveen Sinha, Agnese Seminara, James Wilking, Michael Brenner, Dave Weitz Biofilms are highly-organized colonies of bacteria that form on surfaces. These colonies form sophisticated structures which make them robust and difficult to remove from environments such as catheters, where they pose serious infection problems. Previous work has shown that sub-mm sized aerial towers form on the surface of Bacillus subtilis colony biofilms. Spore-formation is located preferentially at the tops of these towers, known as fruiting bodies, which aid in the dispersal and propagation of the colony to new sites. The formation of towers is strongly affected by the quorum-sensing molecule surfactin and the cannibalism pathway of the bacteria. In the present work, we use confocal fluorescence microscopy to study the development of individual fruiting bodies, allowing us to visualize the time-dependent spatial distribution of matrix-forming and sporulating bacteria within the towers. With this information, we investigate the physical mechanisms, such as surface tension and polymer concentration gradients, that drive the formation of these structures. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V48.00011: The Orientation of Luminescent Excitons in Layered Organic Nanomaterials Jon Schuller, Sinan Karavelli, Keliang He, Shyuan Yang, Jie Shan, John Kymissis, Rashid Zia A fundamental understanding of optoelectronics in organic semiconductors is complicated by the diversity of excitons which can exist within a single material system. Measurements that distinguish between different exciton types are crucial for a complete understanding of organic materials. By fitting experimental curves of angle-, polarization-, and energy-dependent PL to analytical Purcell calculations we quantify the relative dipole moments for in-plane and out-of-plane oriented excitons in organic and inorganic layered nanomaterials. In mono- and bi-layers of Molybdenum Disulfide (MoS2) and Graphene Oxide the luminescence arises only from in-plane oriented excitons. In the perylene derivative PTCDA, however, we show that PL arises from both in-plane and out-of-plane excitons. We observe a difference in emission frequency between the dipole orientations which indicates the existence of two distinct exciton species: an in-plane oriented Frenkel exciton and an out-of-plane oriented Charge Transfer exciton. Based on these results we devise and implement a method for isolating luminescence from either exciton species. We observe different temporal dynamics for the two distinct excitons, highlighting the power of this technique for fundamental studies of organic materials. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V48.00012: Accessing exciton transport in light-harvesting structures with plasmonic nanotip Semion K. Saikin, Johannes Feist, M.T. Homer Reid, Mikhail D. Lukin, Alan Aspuru-Guzik Natural light-harvesting complexes, such as that of plant cells or photosynthetic bacteria, are considered as possible prototypes for artificially designed solar cell materials. In these structures the energy of light absorbed by a peripheral antenna is transmitted very efficiently in a form of excitons to a reaction center. Usually, information about the exciton transport is obtained from time-resolved nonlinear optical experiments where the frequencies of a pump and a probe fields select particular electronic transitions in the light-harvesting complex. We explore a complimentary setup utilizing a plasmonic nanotip as a local sub-wavelength probe of excitation dynamics. As specific examples we consider an LHII complex involved in the light-harvesting process of purple bacteria and a Fenna-Matthews-Olson pigment-protein complex of green-sulphur bacteria. [Preview Abstract] |
Session V49: Focus Session: Organic Electronics and Photonics - Solar Cells and Light Emitting Devices
Sponsoring Units: DMP DPOLYChair: Garry Rumbles, National Renewable Energy Laboratory
Room: 162A
Thursday, March 1, 2012 8:00AM - 8:12AM |
V49.00001: Frequency dependence of magnetoresistance in MEH-PPV Thaddee Kamdem Djidjou, Tho Nguyen, Z. Valy Vardeny, Andrey Rogachev The organic magnetoresistance (OMAR) in organic light emitting diodes (OLED) made of MEH-PPV was investigated by means of DC transport and the admittance spectroscopy in the range of 1 Hz to 10 MHz at room temperature. The measurements were carried out on unipolar and bipolar OLEDs made of pristine MEH-PPV as well as MEH-PPV with traps introduced by the UV light irradiation. We found that in bipolar, UV-exposed OLEDs, the magnitude of magnetoresistance effect in real part of admittance increases with DC bias, reaches very high value of 35 {\%} (in the field 30mT) at bias 4.8 V and decreases at higher bias voltages. Also, we observed that the cutoff frequency of OMAR effect monotonically increases with DC bias voltage. The cutoff has extrinsic origin and is likely caused by a dissipative process related to the reorientation of permanent dipoles. At the highest tested bias voltage 6.7 V, we were able to detect the OMAR at the highest frequency of our system, 10 MHz. We have found that imaginary part of the admittance is also affected by magnetic field. The effect of magnetic field on dynamical capacitance of the device at low frequencies is very strong and opens up a possibility of using these devices as magnetic field sensors. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V49.00002: Graphene-Based Polymer Bulk Heterojunction Solar Cells Fei Yu, Vikram Kuppa The requirement of exciton dissociation in organic photovoltaics necessitates the presence of a large-area interface accessible to the interior of the active layer. Traditionally, such bulk heterojunctions (BHJ) have been spin-coated from a blend of conjugated polymer and functionalized fullerene molecules. We propose and demonstrate BHJs that utilize chemically modified graphene nanoparticles in order to facilitate charge transfer in polymeric solar cells. Devices based on P3HT:PCBM:graphene were fabricated on patterned ITO glass, and the effect of graphene on performance was investigated. Various device parameters including short-circuit current density, open-circuit voltage, fill factor, power conversion efficiency, and external quantum efficiency are compared with traditional BHJs. Results are discussed in the context of the morphology of the active layer, and the distribution and orientation of graphene platelets, as characterized by SEM, AFM and TEM. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V49.00003: Planar Organic Photovoltaics for more Opportunities of Efficiency Enhancement and Parameters Controlling Feras G. Alzubi, Saiful I. Khondaker Organic photovoltaics have been intensively studied as a cheap, easy processed and reliable source of energy that will eventually substitute the inorganic photovoltaics. Commonly, PV devices are made in vertical geometry where the BHJ active material is sandwiched between two electrodes one of them must be transparent to shin the light through. This vertical geometry created some challenges such as requirement of transparent ITO electrode as well as tying up the active material film thickness and electrodes separation. As an approach to overcome these challenges, we utilize the planar geometry to fabricate PV device where the poly (3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methylester (P3HT/PCBM) blend is deposited between two asymmetric metallic electrodes. We investigated the PV behavior for different metal electrodes which is an advantage provided by planar structure. Also, we discuss the behavior of the power conversion efficiency (PCE) with independently varying the active material film thickness and electrodes separation. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V49.00004: Low band gap polymer bulk heterojunction solar cell on a co-planar digitated electrodes structure Abrar Quadery, Feras Alzubi, Simon Tang, Andre J. Gesquiere, Saiful Khondaker Usage of additive in a bulk heterojunction organic solar (BHJ) cell to enhance absorption of solar spectrum in the NIR range is attracting significant research interest. At present, the BHJ solar cells are fabricated in a vertical geometry where a transparent electrode is necessary limiting the choice to mostly ITO. This may create a significant challenge as the performance of organic solar cell is dependent on the work function matching of active materials and the electrode. In order to address this challenge, we fabricated solar cell by dropcasting a ternary blend of P3HT:PC$_{60}$BM: PCPDTBT on a simple structure comprising of co-planar digitated electrodes with different work functions. We will discuss the performance of our device and its implication. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V49.00005: Charge Accumulation and Internal Photovoltaic Processes in Organic Solar Cells Huidong Zang, Yu-Che Hsiao, Qing Liu, Ilia Ivanov, Bin Hu The accumulation of dissociated charge carriers plays an important role in reducing the loss occurring in open-circuit voltage (Voc), short-circuit photocurrent (Isc), fill factor (FF) in organic solar cells. We found from light-assisted capacitance measurements that the charge accumulation inevitably occurs at device interfaces in bulk-heterojunction ITO/PEDOT/P3HT:PCBM/Ca/Al solar cells. Our experimental studies have indicated that the charge accumulation can reduce the Voc through charge injection, Isc through charge collection, and FF through charge transport. Furthermore, our light-assisted capacitance measurements reveal that using a dielectric thin film of TiOx can decrease charge accumulation in the ITO/PEDOT/P3HT:PCBM/TiOx/Ca/Al solar cell. In particular, we find that decreasing the charge accumulation can reduce the loss occurring in Voc, Isc, and FF. Clearly, controlling charge accumulation presents a new mechanism to improve photovoltaic performance in organic solar cells. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V49.00006: Photocurrent noise in organic bulk heterojunction solar cells N.S. Vidhyadhiraja, Monojit Bag, K.S. Narayan We report the first electrical noise measurements from illuminated bulk heterojunction polymer based solar cells. The dependence of photocurrent fluctuations on temperature, light intensity and device conditions was analyzed. We find flicker noise of the form 1/f$^\alpha$ at low frequencies ($<$1 kHz). An unusual log-normal feature in the noise power spectrum is observed in the frequency regime $>$ 5 kHz. We develop a theoretical description employing kinetic Monte-Carlo simulations that points to the importance of a mobility edge in the understanding of fluctuations in the low frequency regime, while also capturing the temperature dependence of the noise amplitude. We find that a Gaussian disorder model with uncorrelated traps is not sufficient to describe the log-normal feature, thus highlighting the significance of spatial and/or energetic correlations among the trap states. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:48AM |
V49.00007: Relating charge transport and performance in single-layer graded-composition organic light-emitting devices Invited Speaker: Russell Holmes Organic light-emitting devices (OLEDs) continue to receive interest for application in displays and as solid-state lighting sources. While OLEDs can exhibit high external quantum and luminous power efficiencies, high performance often requires the use of complex, multilayer architectures. Consequently, there has been interest in the development of OLED architectures containing fewer device active layers or perhaps, a single active layer. Here, we describe an approach to realize efficient electroluminescence from a single active layer through the use of engineered composition gradients. Graded-emissive layer (G-EML) devices contain a single layer consisting of nearly 100{\%} hole-transporting material (HTM) at the anode and nearly 100{\%} electron-transport material (ETM) at the cathode, and having a continuously varying HTM:ETM composition across the active layer. Electroluminescence originates from a phosphorescent guest that is uniformly doped throughout the G-EML. For red-, green-, and blue-emitting phosphors, efficiencies are realized that rival those of more complex, multilayer structures. The G-EML balances electron and hole injection and transport leading to effective charge carrier confinement and exciton formation. This talk will examine how charge confinement in the G-EML is realized through a spatial variation in the carrier mobility across the active layer. In addition, separate measurements of the G-EML exciton recombination zone show that it is substantially broader than that of conventional, abrupt heterojunction OLEDs, a feature which may help to reduce bimolecular exciton quenching in these structures. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V49.00008: Sources of high photo-current in inverted organic solar cells Abay Gadisa, Yingchi Liu, Rene Lopez, Edward Samulski Inverted organic solar cells have been proved to render exceptional environmental stability compared to the conventional solar cell architecture. On the other hand, polymer/fullerene based inverted solar cells produce more photo-current compared to conventional cells comprising the same active layer thickness. The origin of this current has never been clearly stated so far. We have investigated the photovoltaic properties of inverted solar cells comprising a bulk heterojunction layer of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). The blend layer was formed by spin casting the blend solution on ITO substrate, covered with an Al doped zinc-oxide layer (ZnO-Al) deposited through pulsed laser deposition technique. The inverted solar cells show over 15{\%} increase in photo-current yield compared to conventional solar cells. We have discovered that the inverted solar cells produce additional photo-current through dissociation of excited electron-hole pairs near the polymer/ZnO-Al interfaces. Since ZnO-Al is a good electron conductor, the electrons generated at the polymer/ZnO-Al interface are more efficiently collected compared to photo-current produced in the bulk of the active film. External quantum efficiency exceeding 70{\%} was recorded in the ZnO-Al based inverted solar cells. In general, ZnO-Al is not only characterized by its high electron conductivity, and transparency but also serves as electron acceptor. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V49.00009: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V49.00010: Near-Infrared Photodetector Consisting of J-Aggregating Cyanine Dye and Metal Oxide Thin Films Timothy Osedach, Antonio Iacchetti, Richard Lunt, Trisha Andrew, Patrick Brown, Gleb Akselrod, Vladimir Bulovic We demonstrate a photodetector structure that employs metal-oxide charge transport layers and that is sensitized at near-infrared wavelengths by a thin film of a J-aggregating cyanine dye. The high absorption coefficient of the J-aggregate film, combined with the use of a reflective anode and optical spacer layer, enables an external quantum efficiency (EQE) of 16.1 $\pm $ 0.1{\%} ($\lambda $ = 756 nm) to be achieved at zero-bias in a device consisting of an 8.1 $\pm $ 0.3 nm-thick dye film. The specific detectivity (D*) and response speed (f$_{3dB})$ of the fully-optimized device are measured to be (4.3 $\pm $ 0.1)$\times $10$^{11}$ cm Hz$^{1/2}$ W$^{-1 }$and 91.5 kHz, respectively. Modeling of our structure reveals that the photocurrent is limited by the diffusion of photo-generated excitons to the metal oxide/J-aggregate hetero-interface and we determine the exciton diffusion length in the J-aggregate film to be L$_{D}$ = 2.0 $\pm $ 0.4 nm. This work provides insights relevant to the use of J-aggregating cyanine dyes in photodetector and photovoltaic applications and highlights the importance of engineering the optical field profile within such structures in order to maximize performance. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V49.00011: Electrical Stability Tests of Polymer Light Emitting Devices Jacob Cox, Zac Barcikowski, Marian Tzolov The degradation of polymer light emitting devices (PLEDs) is a main concern influencing the commercial production of functioning devices. There are various sources of degradation related to the polymer film, the interfaces, and the cathodes. The fundamental understanding of these processes helps to develop strategies for fabrication of devices with longer lifetimes. We are reporting on devices in which delamination of the metal cathode is the dominating degradation mechanism. We have performed stability tests at constant current and constant voltage accompanied by current-voltage characteristics. The results indicate initial improvement of functionality followed by degradation. The change in the current-voltage characteristics indicates modifications of the electron and hole transport through the polymer layer in addition to the delamination. The delamination appears only if the current is above a certain threshold value. We have studied the kinetics of the delamination which gradually increased with time. Several types of semitransparent anodes were used to clarify the origin of the observed delamination, e.g. gold, platinum, and ZnO:Al. The devices were also exposed to thermal stress tests to verify if the evolution of volatile molecules is involved in the observed degradation. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V49.00012: Rational design of charge transport molecules for blue organic light emitting devices Asanga Padmaperuma, Lelia Cosimbescu, Phillip Koech, Evgueni Polikarpov, James Swensen, Daniel Gaspar The efficiency and stability of blue OLEDs continue to be the primary roadblock to developing organic solid-state white lighting as well as power efficient displays. It is generally accepted that such high quantum efficiency can be achieved with the use of organometallic phosphor doped OLEDs. The transport layers can be designed to increase the carrier density as a way to reduce the drive voltage. We have developed a comprehensive library of charge transporting molecules using combination of theoretical modeling and experimental evidence. Our work focuses on using chemical structure design and computational methods to develop host, transport, emitter, and blocking materials for high efficiency blue OLEDs, along with device architectures to take advantage of these new materials. Through chemical modification of materials we are able to influence both the charge balance and emission efficiency of OLEDs, and understand the influence of the location of photon emission in OLEDs as a function of minor chemical modifications of host and electron transport materials. Design rules, structure-property relationships and results from state of the art OLEDs will be presented. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V49.00013: Transient Capacitance of Light-Emitting Electrochemical Cells Nancy Haegel, Yevtte Davis, Peter Crooker, J. Devin MacKenzie, Yuka Yoshioka Although the steady-state behavior of light-emitting electrochemical cells (LECs) has been addressed theoretically, the transient properties of LECs have yet to be studied in detail. We present time- and frequency-dependent measurements of the capacitance, current, and optical emission of LECs as a constant voltage bias is applied and removed. We find that the capacitance increases more rapidly than the light or current and, unlike the light and current behavior, can be oscillatory and even negative at lower frequencies. Variable temperature experiments were performed to enable observation of a range of transient phenomena that cannot be fully explored at room temperature. The transient behavior suggests that the capacitance is determined by a combination of ion distribution, free carrier screening, and junction width. We interpret our data by qualitatively extending the ideas of existing steady-state theory. [Preview Abstract] |
Session V50: Structure and Properties of Copolymers
Sponsoring Units: DPOLYChair: Bradley Olsen, Massachusetts Institute of Technology
Room: 162B
Thursday, March 1, 2012 8:00AM - 8:12AM |
V50.00001: Designing 100 K Glass Transition Breadths in Bulk Polymer Systems: Effects of Architecture in Homopolymers, Copolymers, and Copolymer Blends Stephen Marrou, Sean Wundrow, John Torkelson Gradient copolymers have attracted interest as vibration or acoustic damping materials due to their extremely broad, tunable glass transition temperature (Tg) responses, up to 100 K in breadth. This behavior is caused by large compositional heterogeneity resulting from sinusoidal composition profiles in nanophase-separated systems. We have also found that some homopolymers and random copolymers exhibit large Tg breadths caused by incompatible main- and side-chain interactions. For example, the Tg response broadens with increasing side-chain length in the poly(n-alkyl methacrylate) series by more than a factor of 2 in going from poly(methyl methacrylate) to poly(n-hexyl methacrylate). We have also blended weakly-segregating styrene/n-butyl acrylate random copolymers of different compositions to allow for tunable Tg breadths over a 100 K temperature range. Finally, we have shown that blending a selective plasticizer into a styrene/4-vinylpyridine gradient copolymer results in a dramatic shift in the Tg response of a single nanophase region, increasing the Tg breadth above 100 K. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V50.00002: Structure and Mechanical Behavior of Elastomeric Multiblock Terpolymers Containing Glassy, Rubbery, and Semicrystalline Blocks Feng Zuo, C. Guillermo Alfonzo, Frank Bates Multiblock terpolymers containing poly(cyclohexylethylene) (C), poly(ethylene-\textit{alt}-propylene) (P), and poly(ethylene) (E) were synthesized. The CECPCEC (denoted XPX) and CECP (XP) each contain 50 v{\%} P and equal amounts of C and E. These materials have been studied by DSC, DMS, TEM, SAXS, WAXS, and tensile deformation to characterize the morphology, phase behavior, and mechanical properties. Microphase separation is induced by crystallization of E and/or chemical incompatibility between the three blocks, leading to a morphology which contains continuous region of P and continuous region of microphase separated X, resulting in mechanically resilient materials. High $M_{w}$ block copolymers microphase separate with two different length scales associated with segregation between C and E, and X and P. These structural features produce a non-classical scaling relationship for the C-E domain spacing, d $\sim $ N$^{0.31}$. The role of semicrystalline E domains during uniaxial deformation has been exposed with WAXS experiments, which support a two-step mechanism involving recoverable and non-recoverable deformation to different extents. Strain hardening is observed in double-anchored XPX, but not in single-anchored XP, at large tensile strains. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V50.00003: Energy Storage and Dissipation in Random Copolymers during Biaxial Loading Hansohl Cho, Mary Boyce Random copolymers composed of hard and soft segments in a glassy and rubbery state at the ambient conditions exhibit phase-separated morphologies which can be tailored to provide hybrid mechanical behaviors of the constituents. Here, phase-separated copolymers with hard and soft contents which form co-continuous structures are explored through experiments and modeling. The mechanics of the highly dissipative yet resilient behavior of an exemplar polyurea are studied under biaxial loading. The hard phase governs the initially stiff response followed by a highly dissipative viscoplasticity where dissipation arises from viscous relaxation as well as structural breakdown in the network structure that still provides energy storage resulting in the shape recovery. The soft phase provides additional energy storage that drives the resilience in high strain rate events. Biaxial experiments reveal the anisotropy and loading history dependence of energy storage and dissipation, validating the three-dimensional predictive capabilities of the microstructurally-based constitutive model. The combination of a highly dissipative and resilient behavior provides a versatile material for a myriad of applications ranging from self-healing microcapsules to ballistic protective coatings. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V50.00004: X-ray scattering studies of ordered block copolymer melts during uniaxial extensional flow Wesley Burghardt, Ruinan Mao, Erica McCready We present the design and implementation of a new apparatus for in situ x-ray scattering studies of polymer melts during homogenous uniaxial extensional flow. The instrument is based on the commercial SER extensional flow fixture, which employs counter-rotating drums to deform a strip of polymer melt, which is incorporated into a custom-built convection oven designed to facilitate x-ray access to the sample and operation in a synchrotron environment. Here we report measurements of extensional flow-induced structural changes in a cylindrically ordered styrene-ethylene butylene-styrene triblock copolymer melt. At early stages, SAXS data reveal that the ordered microstructure deforms affinely until Hencky strains of $\sim$ 0.2. A global re-orientation process leads to alignment of microdomains predominantly along the stretching direction after Hencky strains of $\sim$ 1. Further stretching does not lead to further qualitative changes in 2-D SAXS patterns. Relaxation of both microdomain orientation and d-spacing is observed following cessation of extensional flow, albeit with different characteristic time scales. In situ x-ray scattering data are compared with off-line measurements of transient extensional viscosity, performed using the SER fixture in a rotational rheometer. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V50.00005: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V50.00006: Understanding the Dynamics of Magnetic Field Alignment for Rod-Coil Block Copolymers Bryan McCulloch, Giuseppe Portale, Wim Bras, Alexander Hexemer, Rachel A. Segalman Alignment of semiconducting block copolymer nanostructures is crucial to optimize charge transport in these materials. Magnetic fields can act on the liquid crystalline conjugated polymers, inducing alignment in rod-coil block copolymers. By using a combination of small angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) we have studied the magnetic field alignment of poly(alkoxy phenylene vinylene-b-isoprene) (PPV-PI) rod-coil block copolymers. In situ measurements have also shown the magnetic field leads to a stabilization of the ordered phase. Furthermore, there appear to be two distinct timescales for alignment: at short times the alignment of these materials is fast likely caused by preferential growth of aligned domains, and at long times alignment increases by the very slow process of defect annihilation. Further, there is an optimum temperature where the kinetics and thermodynamic driving forces for alignment are balanced, producing very highly aligned samples. Understanding the mechanisms by which alignment occurs has lead to knowledge helping to rationally optimize the magnetic alignment of rod-coil block copolymers. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V50.00007: In-situ SAXS observation of magnetic field effects on block copolymer ordering and alignment Chinedum Osuji, Manesh Gopinadhan, Pawel Majewksi The use of external fields to direct block copolymer self-assembly is well documented. Magnetic fields offer particular promise due to their space-pervasive nature and the ability to produce arbitrary alignments over truly macroscopic length scales in appropriate systems. We present here the results of in-situ SAXS studies of side-chain liquid crystalline diblock copolymers ordering under high magnetic fields and ex-situ GISAXS data on thin films. Despite the coincidence of the block copolymer order-disorder transition (ODT) and the LC clearing temperature in these weakly segregated materials, there is no measurable effect of the field on the ODT of the system, up to 6 T. This is in line with rough estimates based simply on the magnitudes of the relevant energy scales - the free energy of field interaction and the enthalpy of the isotropic-LC transition. We show that the alignment of the system is critically limited by the viscosity of the mesophase such that alignment can only be advanced by residence in a small temperature window near $T_{ODT}$. This residence produces a weakly aligned system which thereafter transitions to a strongly aligned state on cooling even in the absence of the field. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V50.00008: Characterizing Mesoporous Block Copolymers by Resonant Soft X-ray Scattering David Wong, Keith Beers, Cheng Wang, Jeffrey Kortright, Nitash Balsara Mesoporous block copolymers membranes can be used as water filtration membranes and battery separators. In these applications, it is often advantageous to generate a structure where one block serves as a structural component, and the second block lines the pores. The membrane thus has 3 components: the two blocks and vacuum (or air). Small Angle X-ray Scattering (SAXS), which relies on electron density for contrast, only distinguishes between vacuum and polymer. This is because the scattering contrast between the two blocks is much less than that between the polymer and vacuum. Resonant Soft X-ray Scattering (RSoXS) experiments can be used to adjust the contrast between block copolymer phases by tuning the energy of the incident x-ray beam. We have studied mesoporous poly(styrene-block-ethylene-block-polystyrene) (SES) films, where the semicrystalline polyethylene serves as a structural phase, and the polystyrene lines the pores. At a particular energy, the scattering contrast between PS and vacuum becomes negligible, while contrast between PS and PE is enhanced. We present RSoXS data at different x-ray energies to demonstrate this contrast enhancement. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V50.00009: Self-assembly Morphology and Crystallinity Control of Di-block Copolymer Inspired by Spider Silk Wenwen Huang, Sreevidhya Krishnaji, David Kaplan, Peggy Cebe To obtain a fuller understanding of the origin of self-assembly behavior, and thus be able to control the morphology of biomaterials with well defined amino acid sequences for tissue regeneration and drug delivery, we created a family of synthetic silk-based block copolymers inspired by the genetic sequences found in spider dragline, HABn and HBAn (n=1,2,3,6), where B = hydrophilic block, A = hydrophobic block, and H is a histidine tag. We assessed the secondary structure of water cast films by Fourier transform infrared spectroscopy (FTIR). The crystallinity was determined by Fourier self-deconvolution of amide I spectra and confirmed by wide angle X-ray diffraction (WAXD). Results indicate that we can control the self-assembled morphology and the crystallinity by varying the block length, and a minimum of 3 A-blocks are required to form beta sheet crystalline regions in water-cast spider silk block copolymers. The morphology and crystallinity can also be tuned by annealing. Thermal properties of water cast films and films annealed at 120 C were determined by differential scanning calorimetry and thermogravimetry. The sample films were also treated with 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) to obtain wholly amorphous samples, and crystallized by exposure to methanol. Using scanning and transmission electron microscopies, we observe that fibrillar networks and hollow micelles are formed in water cast and methanol cast samples, but not in samples cast from HFIP. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V50.00010: The Effects of Blockiness on the Chemical Composition Distribution of Partially Functionalized Polystyrene Wayne Powers, Jan Genzer, Chang Y. Ryu Monodisperse polystyrene has been functionalized chemically to make random copolymers with controlled sequence distribution of the unmodified and modified styrene segments. The sequence blockiness of the resulting random copolymers can be controlled via the temperature of reaction, with a high temperature reaction resulting in a ``truly random'' copolymer, and a low temperature resulting in a ``random blocky'' copolymer. Interaction chromatography has been employed to estimate the chemical composition distribution of these partially functionalized polystyrenes. Two different chemical systems will be discussed; i.e., the brominated and borylated polystyrene systems. The results of our analysis reveal that the chemical composition distribution of ``random blocky'' copolymers is narrower than that of the corresponding ``truly random'' copolymers. The chemical composition of the two systems will be compared directly, and the influence of ``chain conformation inversion'' will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V50.00011: Large Scale Purification and Characterization of Model Poly(isopropyl methacrylate)-block-Poly(styrene) Diblock Copolymers Asem Abdulahad, Du Yeol Ryu, Chang Yeol Ryu The development of purification techniques of block copolymers is vital for overcoming the synthetic difficulty of preparing well-defined block copolymers using various living polymerization techniques. A large scale separation technique would lead us to obtaining sufficient amounts of homopolymer-free block copolymers for subsequent physical characterization. This can potentially aid in the elucidation of the role of chemical heterogeneity on the thermodynamic transitions and viscoelastic properties of block copolymer materials. Employing an acute understanding of polymer adsorption/desorption onto nanoporous silica during solvent gradient interaction chromatography, we demonstrate the large scale purification of anionic polymerized poly(isopropyl methacrylate)-\textit{block}-poly(styrene) diblock copolymers with narrow molecular weight distribution. Additionally, we address the impact of removing early-terminated poly(styrene) homopolymers on the viscoelasticity of these model diblock copolymers, as well as their impact on block copolymer assembly as analyzed by small-angle X-ray scattering. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V50.00012: Quantifying Fluctuation Effects on the Order-Disorder Transition of Symmetric Diblock Copolymers Jing Zong, Qiang (David) Wang How fluctuations change the order-disorder transition (ODT) of symmetric diblock copolymers is a classic yet unsolved problem in polymer physics.\footnote{\textit{L. Leibler}, \textbf{Macromolecules, 13}, 1602 (1980); \textit{G. H. Fredrickson and E. Helfand}, \textbf{J. Chem. Phys., 87}, 697 (1987).} Here we unambiguously quantify the fluctuation effects by direct comparisons between fast off-lattice Monte Carlo (FOMC) simulations\footnote{\textit{Q. Wang and Y. Yin}, \textbf{J. Chem. Phys., 130}, 104903 (2009).} and mean-field theory using exactly the same model system (Hamiltonian), thus without any parameter-fitting. The symmetric diblock copolymers are modeled as discrete Gaussian chains with soft, finite-range repulsions as commonly used in dissipative-particle dynamics simulations. The effects of chain discretization and finite-range interactions on ODT are properly accounted for in our mean-field theory.\footnote{\textit{Q. Wang}, \textbf{J. Chem. Phys.}, \textbf{129}, 054904 (2008); \textbf{131}, 234903 (2009).} Our FOMC simulations are performed in a canonical ensemble with variable box lengths to eliminate the adverse effects of fixed box sizes on ODT.\footnote{\textit{Q. Wang et al.}, \textbf{J. Chem. Phys.}, \textbf{112}, 450 (2000).} Furthermore, with a new order parameter for the lamellar phase, we use replica exchange and multiple histogram reweighting to accurately locate ODT in our simulations. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V50.00013: Simulation of micelle lattice ordering via nucleation from disorder in a diblock copolymer melt Russell Spencer, Robert Wickham We examine the dynamics of the order-disorder transition (ODT) in diblock copolymer melts by simulating the nucleation of the BCC phase of spherical micelles out of the disordered phase, using the time-dependent Landau-Brazovskii model. Questions about the description of the disordered phase as a phase of disordered micelles, and the role of intermediate close-packed structures suggest a rich dynamics for this ordering transition. For a copolymer composition $f = 0.39$, we find that above a critical size, which diverges at the ODT, a spherical nucleus of the BCC micelle phase grows, forming the BCC phase directly. The growth rate varies linearly with undercooling. We also examine the growth of nuclei of the close-packed phases. For more asymmetric copolymers, $f = 0.35$ and $f = 0.3$, a region of disordered micelles surrounds a core of the BCC phase in the growing nucleus, suggesting that, for asymmetric copolymers, the ODT proceeds via a two-step mechanism. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V50.00014: Order-to-order Transition in Closed-loop type Block Copolymer Hyungju Ahn, Hoyeon Lee, Sungmin Park, Du Yeol Ryu Recently, the complex morphologies such as gyroid (GYR), hexagonally perforated layer (HPL) and non-cubic network phase (Fddd) observed in block copolymer (BCP) melts. In this study, we investigated the unique and unusual OOT behavior of asymmetric deuterated polystyrene-block-poly(n-pentyl methacrylate) copolymers, denoted as dPS-b-PnPMA at narrow range of volume fraction. Scattering results and transmission electron microscopy show that dPS-b-PnPMAs exhibit an unusual OOT phase behavior of GYR?HPL and GYR?HPL?GYR, whereas a general morphological transition occur from HPL to GYR with increasing temperature. The OOT process found in this work provides a physical insight into a new type of morphological transition in diblock copolymer system. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V50.00015: Self-assembly Behavior of Poly(3-alkylthiophene)-\textit{block}-poly(methyl methacrylate) Block Copolymers Prepared by Anionic Coupling Reaction Jin Kon Kim, Hong Chul Moon We synthesized rod-coil block copolymers composed of regioregular poly(3-alkylthiopene) (P3AT) and poly(methyl methacrylate) (PMMA) via anionic coupling reaction. For poly(3-hexylthiopene)-b-PMMA, the morphology was mainly determined by self-crystallization of P3HT moieties due to strong rod-rod interaction. On the other hand, poly(3-dedecyl thiophene) (P3DDT)-b-PMMA, the self-crystallization was effectively suppressed. Detail phase behaviors were investigated at temperatures higher melting point (T$_{m})$ of P3DDT using small-angle x-ray scattering (SAXS), wide-angle x-ray scattering (WAXS) and transmission electron microscopy (TEM). [Preview Abstract] |
Session V52: Focus Session: Extreme Mechanics - Biological Systems and Structures
Sponsoring Units: GSNP DFDChair: Jacy Bird, Massachusetts Institute of Technology
Room: 153C
Thursday, March 1, 2012 8:00AM - 8:36AM |
V52.00001: Micro-actuation through swelling and tissue engineering Invited Speaker: Nicholas Fang |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V52.00002: Plant tendrils: Nature's hygroscopic springs Sharon Gerbode, Joshua Puzey, Andrew McCormick, L. Mahadevan Plant tendrils are specialized climbing organs that have fascinated biologists and physicists alike for centuries. Initially straight tendrils attach at the tip to an elevated rigid support and then winch the plant upward by coiling into a helical morphology characterized by two helices of opposite handedness connected by a helical perversion. In his renowned treatise on twining and tendril-bearing plants, Charles Darwin surmised that coiled tendrils serve as soft, springy attachments for the climbing plant. Yet, the true effect of the perverted helical shape of a coiled plant tendril has not been fully revealed. Using a combination of experiments on Cucurbitaceae tendrils, physical models constructed from strained rubber sheets, and numerical models of helical perversions, we have uncovered that tendril coiling occurs via anisotropic shrinkage of a strip of specialized cells in the interior of the tendril. Furthermore, variations in the mechanical behavior of tendrils as they become drier and ``woodier'' adds a new twist to the story of tendril coiling. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V52.00003: Radial force development during root growth measured by photoelasticity Evelyne Kolb, Christian Hartmann, Patricia Genet The mechanical and topological properties of a soil like the global porosity and the distribution of void sizes greatly affect the development of a plant root, which in turn affects the shoot development. In particular, plant roots growing in heterogeneous medium like sandy soils or cracked substrates have to adapt their morphology and exert radial forces depending on the pore size in which they penetrate. We propose a model experiment in which a pivot root (chick-pea seeds) of millimetric diameter has to grow in a size-controlled gap $\delta $ ($\delta $ ranging 0.5-2.3 mm) between two photoelastic grains. By time-lapse imaging, we continuously monitored the root growth and the development of optical fringes in the photoelastic neighbouring grains when the root enters the gap. Thus we measured simultaneously and in situ the root morphological changes (length and diameter growth rates, circumnutation) as well as the radial forces the root exerts. Radial forces were increasing in relation with gap constriction and experiment duration but a levelling of the force was not observed, even after 5 days and for narrow gaps. The inferred mechanical stress was consistent with the turgor pressure of compressed cells. Therefore our set-up could be a basis for testing mechanical models of cellular growth. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V52.00004: Digging Like Plants: Flexible Intruders in Granular Materials Dawn Wendell, Katharine Luginbuhl, Diego Solano, Peko Hosoi Inspired by plant root growth in granular media, we report on the effects of flexibility on the mechanical work required to dig through granular systems. In the case where the digger is significantly thinner than the grain diameter, increased flexibility in one-dimension leads to savings of nearly 50\%. A simple numerical model based solely on the variability of forces in the granular substrate and the flexibility of the digger gives similar results to those observed in experiments. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V52.00005: Helical Buckling of Plant Roots: Mechanics and Morphology Jesse Silverberg, Roslyn Noar, Michael Packer, Maria Harrison, Chris Henley, Itai Cohen, Sharon Gerbode How do plant roots respond to heterogeneities in their environment as they grow? Using a simple model system consisting of a layered hydrogel, we present a controlled mechanical barrier to the roots allowing us to perturb their growth. Interestingly, we find a localized helical root morphology which forms prior to the root passing through the gel layer interface. We interpret this geometry as a combination of a purely mechanical buckling caused by continued root elongation modified by the growth medium and a simultaneous twisting near the root tip. We study the morphology of the helical deformation as the modulus of the gel is varied using 3D time-lapse imaging and demonstrate that its shape scales with gel stiffness as expected by a simple model based on the theory of buckled rods. Our results demonstrate that mechanics is sufficient to account for the shape and its variations. In addition, we hypothesize that the twisted growth near the root tip arises from a touch-activated growth response that we call thigmotorsion. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V52.00006: Dislocations and Grain Boundaries in Optimally-Packed, Twisted Filament Bundles Amir Azadi, Gregory Grason From the collagen fiber to the parallel-actin bundle, twisted and rope-like assemblies of filamentous molecules are common and vital structural elements in cells and tissue of living organisms. We study the intrinsic frustration occurring in these materials between the two-dimensional organization of filaments in cross-section and out-of-plane interfilament twist in bundles based on the non-linear continuum elasticity theory of columnar materials. We find that interfilament twist generates in-plane stresses that couple favorably to the presence of topological defects, edge dislocations, in the cross-sectional packing, thereby restructuring the ground state filament packing of twisted bundles. The stability of dislocations increases with increases in both the degree of twist and lateral bundle size. We show that in ground states of large bundles, multiple dislocations pile up into linear arrays, radial grain boundaries, whose number and length grows with bundle twist. Remarkably, the ``polycrystalline'' texture of these optimal packings of twisted bundles show a striking similarity to models of the ``almost crystalline'' cross-section of collagen fibers. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V52.00007: Mechanical Behavior of Bio-inspired Model Suture Joints Yaning Li, Erica Lin, Christine Ortiz, Mary Boyce Suture joints of varying degrees of geometric complexity are prevalent throughout nature as a means of joining structural elements while providing locally tailored mechanical performance. Here, micromechanical models of general trapezoidal waveforms of varying hierarchy are formulated to reveal the role of geometric complexity in governing stiffness, strength, toughness and corresponding deformation and failure mechanisms. Physical constructs of model composite suture systems are fabricated via multi-material 3D printing (Object Connex500). Tensile tests are conducted on samples covering a range in geometry, thus providing quantitative measures of stiffness, strength, and failure. The experiments include direct visualization of the deformation and failure mechanisms and their progression, as well as their dependence on suture geometry, showing the interplay between shear and tension/compression of the interfacial layers and tension of the skeletal teeth and the transition in failure modes with geometry. The results provide quantitative guidelines for the design and tailoring of suture geometry to achieve the desired mechanical properties and also facilitate understanding of suture growth and fusion, and evolutionary phenotype. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V52.00008: Morphogenesis of protrusions from confined lipid bilayers mediated by mechanics Marino Arroyo, Margarita Staykova, Mohammad Rahimi, Howard A Stone Biological membranes adopt a wide range of shapes that structure and give functionality to cells, compartmentalizing the cytosol, forming organelles, or regulating their area. The formation, stabilization, and remodeling of these structures is generally attributed to localized forces or to biochemical processes (insertion of proteins, active compositional regulation). Noting that in the crowded intra and extra-cellular environments membranes are highly constrained, we explore to what extent can mechanics explain the shape of protrusions out of confined membranes. For this purpose, we developed an in-vitro system coupling a lipid bilayer to the strain-controlled deformation of an elastic sheet (Staykova et al, PNAS 108, 2011). We show that upon contracting the elastic support, tubular or spherical protrusions grow out of the adhered membrane, which can be reversibly controlled with strain and osmolarity without resorting to localized forces or chemical alterations of the bilayer. The morphologies produced by our minimal system are ubiquitous in cells, suggesting mechanics may be a simple and generic organizing principle. We can understand most of our observations in terms of a phase diagram accounting for elasticity, adhesion, and the limited amount of area and volume available. [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V52.00009: Thin-shell model for faceting of multicomponent elastic vesicles Rastko Sknepnek, Monica Olvera de la Cruz We use a discretized version of a thin elastic shell model to show that a two-component elastic vesicle can lower its energy by faceting into a wide variety of polyhedral shapes. The elastic shell model allows us to completely remove effects of the topological defects necessarily present in spherical topology. Therefore, we show that the faceting mechanism of multicomponent elastic vesicles is fundamentally different than the familiar defect-driven buckling into icosahedra. We present a detailed gallery of faceted shapes and discuss how the interplay between bending and stretching energies leads to faceting. Present work extends our recent study of the faceting of a two-component shell in the presence of topological defects [1]. [1] G. Vernizzi, R. Sknepnek, M. Olvera de la Cruz, Proc. Natl. Acad. Sci. USA 108, 4292 (2011). [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V52.00010: Charge Effects on Mechanical Properties of Elastomeric Proteins Ravi Kappiyoor, Ganesh Balasubramanian, Daniel Dudek, Ishwar Puri Several biological molecules of nanoscale dimensions, such as elastin and resilin, are capable of performing diverse tasks with minimal energy loss. These molecules are efficient in that the ratio of energy output to energy consumed is very close to unity. This is in stark contrast to some of the best synthetic materials that have been created. For example, it is known that resilin found in dragonflies has a hysteresis loss of only 0.8{\%} of the energy input while the best synthetic rubber made to date, polybutadiene, has a loss of roughly 20{\%}.We simulate tensile tests of naturally occurring motifs found in resilin (a highly hydrophilic protein), as well as similar simulations found in reduced-polarity counterparts (i.e. the same motif with the charge on each individual atom set to half the natural value, the same motif with the charge on each individual atom set to zero, and a motif in which all the polar amino acids have been replaced with nonpolar amino acids). The results show a strong correlation between charge and extensibility. In order to further understand the effect of properties such as charge on the system, we will run simulations of elastomeric proteins such as resilin in different solvents. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V52.00011: Denaturation of Circular DNA: Supercoils, overtwist and condensation Alkan Kabakcioglu, Amir Bar, David Mukamel The statistical mechanics of DNA denaturation under fixed linking number is qualitatively different from that of the unconstrained DNA. Past work suggests that the nature of this constrained melting transition is sensitive to the mechanism that relaxes the torsional stress induced on the bound portions by the loops. Quantitatively different melting scenarios are reached from two alternative assumptions, namely, that the denatured loops are formed in expense of 1) overtwist, 2) supercoils. Recent work has shown that the supercoiling mechanism results in a BEC-like picture where a macroscopic loop appears at $T_c$ and grows steadily with temperature while no such phenomenon has been reported for the overtwisting case. By extending an earlier result, we show here that a macroscopic loop appears in the overtwisting scenario as well. We calculate its size as a function of temperature and show that the fraction of the total sum of microscopic loops decreases above $T_c$, with a cusp at the critical point. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V52.00012: Mechanics of short rod-like molecules in tension Prashant Purohit Rod like macromolecules such as actin, DNA etc., are most commonly stretched using optical tweezers or fluid flow. In this presentation we will describe the mechanics of short rod like molecules in tension. The mechanics is dominated by the competition between tensile forces (exerted by fluid flow, or by a device, such as, optical tweezers) and the thermal fluctuations of the molecule. For molecules whose contour length is comparable to the persistence length we show that the boundary conditions play major role in determining the mechanical behavior. We use the equipartition theorem of statistical mechanics to obtain expressions for the amplitude of the transverse fluctuations of the molecule and its force-extension relation for various boundary conditions. We then apply our theory to an experiment on short fluctuating actin filaments trapped by various means. We estimate the tension in these filaments by fitting our theory to the measured values of transverse fluctuations as a function of the position along the filament. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V52.00013: Turning by buckling: a cheap evolutionary strategy for turning among marine bacteria Kwangmin Son, Jeffrey Guasto, Roman Stocker Marine bacteria have long been known to swim forward and backward (`run and reverse') by controlling the rotational direction of a 20 nm helical flagellum. Recent detailed observations have shown that these bacteria can also make sharp, $\sim90^{\circ}$ turns, an astounding feature for a micron-scale organism with just one degree of freedom under its control. We demonstrate that a buckling instability originating from the flexible linkage (`hook') between the body and the flagellum is responsible for the reorientation. Using high-magnification (40$\sim$100X) observations based on high-speed video microscopy (420$\sim$1000 fps), we captured the extreme deformation of the flagellum and the hook involved in this process. The mechanical properties of the hook are finely tuned to the hydrodynamic loads experienced by the cell: the hook becomes unstable only when the compressive load during the onset of forward swimming exceeds the threshold for Euler buckling. Combining the data with a model of buckling of thin structures, we show that bacteria take advantage of the flexibility of the flagellum and the hook to generate a turn, which may represent the evolutionarily cheapest bacterial strategy to actively change direction. [Preview Abstract] |
Session V53: Wet and Cohesive Granular Materials
Sponsoring Units: GSNPChair: Arshad Kudrolli, Clark University
Room: 153B
Thursday, March 1, 2012 8:00AM - 8:12AM |
V53.00001: An analytical framework for aeolian saltation Meredith Reitz, Douglas Jerolmack We construct an analytical framework for steady state aeolian saltation, using experimentally-derived splash function relationships and the additional constraint of a threshold height, above which the wind velocity is strong enough to carry a grain from reptation into saltation. This threshold height rises as the wind profile magnitude is lowered by the increasing number of saltating grains being accelerated by the wind, until the number of grains being demoted below this threshold due to loss of energy to collisions with the bed equals the number being promoted. The balance of these populations at steady state determines both the flux of grains in saltation and the saturated wind velocity profile, while the approach to this balance describes the transient evolution to this state. We also formulate the difference between the critical impact Shields stress, defined as the stress below which transport ceases, and the higher critical fluid Shields stress, at which transport is initiated. Finally, we test the dependence of grain flux and trajectory lengths and speeds on the erodibility of the bed, and compare these results with observed differences in scaling. We also compare our results to findings for saltation under water. [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V53.00002: X-Ray Fluoroscopy of Sedimentation in Elastic Fluids T.A. Brzinski, R. Karunamuni, A.D.A. Maidment, P.E. Arratia, D.J. Durian Dispersions of spheres in a Newtonian fluid will sediment until all grains form a packing. While the system approaches its final configuration, the dispersion is roughly homogenous in space and time except at two well-defined interfaces: a dispersion-supernatant interface, and an interface below the dispersion at which grains stack to form a packing. In order to better understand this packing process we perturb the dynamics at the lower interface by adding a flexible, high molecular weight polymer that enhances fluid elasticity. The fluid strain-rate between spheres has an extensional component that is inversely proportional to grain separation, so elasticity dominates the fluid forces as the grain separation becomes small, thus frustrating the packing process. In order to observe the effect of this perturbation, we utilize x-ray fluoroscopy. In the case of a system without polymer we observe settling rates in accordance with a typical Stokes' model until all grains have settled into a random packing. In the polymeric case we see that, alongside the Newtonian-like settling, there's a time- and depth-dependent compression of the disperse phase, resulting in a smooth transition between dispersion and packing rather than the sharp interface observed in the Newtonian case. [Preview Abstract] |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V53.00003: Three dimensional imaging of soft sphere packings under shear Joshua Dijksman, Hu Zheng, Robert Behringer The (microscopic) flow of three dimensional disordered athermal granular packings remains poorly understood. However, experimentally studying flow and deformations in a three dimensional packing of grains is challenging due to the opacity of such packings. We use refractive index matched scanning with hydrogel spheres to image such flows. Hydrogel is soft and has low friction, which allows for the study of contact forces via contact deformations. We look at how force networks develop in sheared packings close to the onset of mechanical rigidity. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V53.00004: Fluidization of wet granulates under hydrodynamic shear - experiments Christoph G\"ogelein, Ilenia Battiato, Matthias Schr\"oter, Stephan Herminghaus, J\"urgen Vollmer Very recently, the fluidization threshold of a wet granular bed under hydrodynamic shear forces were predicted theoretically [1]. This theory described the flow through a wet granular bed by a continuum model and provides analytical expressions for the averaged drag foce on a single particle. Moreover, the theory predicts the stability of the granular bed in dependence of the strength of the capillary and buoyancy forces. These theoretical predictions are tested in the present study by a newly designed flow channel. We will present our first experimental results for the fluidization onset of granular beds. \\[4pt] [1] I. Battiato, and J. Vollmer,``Fluidization of wet granulates under hydrpdynamic shear,'' submitted for pubblication. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V53.00005: Building designed granular towers one drop at a time Julien Chopin, Arshad Kudrolli The impact of a drop on a surface leads to beautiful dynamical shapes that result from a subtle interplay between inertial effects, fluid properties and substrate characteristics. In this talk, we will present an experiment where the successive impacts of drops lead to surprisingly slender mechanically stable structures that we called granular towers. They are created by dripping a dense granular suspension on a liquid absorbing surface such as a blotter paper or a dry granular bed. These towers formed by rapid solidification of the drop upon impact are analogous to many natural structures found in nature including frozen lava flows, icicles and stalagmites. We find that the height can be determined by balancing the excess liquid flux and the drainage through the granular tower. The velocity impact, the free fall time and the density of the suspension are found to control the tower width and its detailed morphology. We show that these facts can be manipulated to obtain various symmetric, smooth, corrugated, zigzag, and chiral structures. Further, the shape of the tower can be used as a quick diagnostic tool to characterize the rheology of a granular suspension. [J. Chopin and A. Kudrolli, Phys. Rev. Lett. 107, 208304 (2011)] [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V53.00006: Dynamics of failure in cohesive granular packings Jennifer Rieser, Wenbin Li, Ju Li, Jerry Gollub, Douglas Durian We explore the grain-scale interactions that precede large-scale deformations and mark the onset of mechanical failure in two-dimensional disordered granular packings. The two-dimensionality of the system allows for direct observation of all particle dynamics during the compression of a pillar. The grains are cohesive, with an attraction governed by tunable capillary forces induced through an interstitial fluid. For our analysis, we focus on the evolution of local rearrangements into shear bands within the pillar, quantifying the structural differences within the packing due to the presence of cohesion. We observe that cohesion results in greater spatial heterogeneity within the packing during compression. We also compare the compression of ordered and disordered packings. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V53.00007: Cohesive granular aggregates under punching: theory and experiments Alexei Perelet, Tapan Sabuwala, Gustavo Gioia When poured into a container, cohesive granular materials form low-density, open granular aggregates. If put under compression, these aggregates densify by particle rearrangement. We seek experimental evidence that particle rearrangement occurs in the form of a phase transition between two configurational phases of the aggregate (G.\ Gioia, A.\ M.\ Cuiti\~no, S.\ Zheng, and T.\ Uribe, PRL {\bf 88}, 204302, 2002). We use a simple model to show that when an open granular aggregate with two configurational phases is penetrated by a punch that lacks a characteristic length scale, the functional relation between the punching force and the penetration of the punch depends solely on the dimensionality of the punch: for a two-dimensional, wedge-shaped punch the force--penetration curve is linear whereas for a three-dimensional, conical punch the force--penetration curve is quadratic. To test these predictions we carry out experiments with open granular aggregates of a fine powder. The experimental measurements are in accord with the theoretical predictions. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V53.00008: Discrete Particle Dynamics Simulations of Adhesive Systems with Thermostatting Flint Pierce, Jeremy Lechman, John Hewson Aggregation/coagulation/flocculation processes are ubiquitous in modern industry from fields as diverse as waste water treatment, the food industry, algae biofuel production, and materials processing where control of the size and morphology of aggregates is paramount to the application of interest. Population balance models have historically been used with success in predicting aggregation kinetics and size distributions for these processes. However, even the most robust population balance schemes can lack an exact description of the underlying physical processes governing attractive or adhesive particulate matter suspended in a background medium, including finite aggregate strength and yield stress, restructuring length and time scales, and response to hydrodynamic forces. In order to elucidate these phenomena, We develop and use a JKR type model for simulating adhesive particulate matter in a background medium varying from dilute gas to liquid. We evaluate the time and length scales for restructuring/fragmentation that result from this model as a function of aggregate size and fractal dimension. We additionally introduce a method for pairwise thermostatting of the adhesive potential and discuss the applicability of this model to various adhesive systems. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V53.00009: Normal Mode Spectrum of Finite Sized Granular Systems: The Effects of Fluid Viscosity at the Grain Contacts John Valenza, David Johnson We investigate the effects of adsorbed films on the attenuative properties of loose granular media occupying a finite sized rigid container, which is open on the top. We measure the effective mass, $\tilde{M}(\omega)$, of loose tungsten particles prepared under two different sets of conditions: 1) We lightly coat tungsten grains with a fixed volume fraction of silicone oil (PDMS), where the liquid viscosity is varied for individual realizations. 2) In the other set of experiments we vary the humidity. On a theoretical level we are able to decompose the effective mass into a sum over the contributions from each of the normal modes of the granular medium. Our results indicate that increasing either the PDMS viscosity or the humidity, as the case may be, does markedly increase the damping rate of each normal mode relevant to our measurements. However, there is appreciable damping even in the absence of any macroscopic film. With a notable exception in the case of the highest humidity in the humidity controlled experiments, all the relevant modes are weakly damped in the sense of a microscopic theory based on damped contact forces between rigid particles. [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V53.00010: Influence of liquid bridges on the macroscopic properties of granular assemblies Geoffroy Lumay, Jorge Fiscina, Francois Ludewig, Nicolas Vandewalle We present the results of two experimental studies concerning the compaction dynamics of cohesive granular materials. In the first study, the cohesion between neighboring grains is induced by capillary bridges in a wet granular material. The cohesiveness is tuned using different liquids having specific surface tension values. The second study concerns initially dry granular materials surrounded by a well controlled air humidity. Then, the cohesion inside the packing is controlled through the relative humidity which influence both triboelectric and capilary effects. For both cases, the evolution of the parameters extracted from the compaction curves (the compaction characteristic time $\tau$, the initial and final packing fractions) have been analyzed as a function of the cohesiveness. A model, based on free volume kinetic equations and the presence of a capillary energy barrier, is able to reproduce quantitatively the experimental results (Phys. Rev. Lett. 105, 048001 (2010)). [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V53.00011: Wetting and packing effects on evaporation out of a porous medium Cesare Mikhail Cejas, Bertrand Selva, Raphael Beaufret, Larry Hough, Christian Fretigny, Remi Dreyfus Evaporation through granular media involves complex fluid transport and exhibits two regimes: (1) a capillary-supported regime maintaining hydraulic continuity to the surface and vapor exchange with the atmosphere followed by (2) a diffusion-limited regime through the medium. A well-defined intermediate partially saturated zone (PSZ) has already been observed in the past. It is evidently seen from our experimental investigations using a 2D model soil of glass beads. The PSZ is the region identified above the interface formed by the drying front, which separates the PSZ from the fully-saturated wet region. This intermediate zone is filled with a dynamic mixture of vapor and liquid. The existence of this zone is of significant importance as it sets the kinetics of the evaporation process. Here we will present how wetting and packing effects influence the size of this partially saturated zone and we will show how a simple model based on geometrical considerations can explain our observations. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V53.00012: A coupled deformation-diffusion theory for fluid-saturated porous solids David Henann, Ken Kamrin, Lallit Anand Fluid-saturated porous materials are important in several familiar applications, such as the response of soils in geomechanics, food processing, pharmaceuticals, and the biomechanics of living bone tissue. An appropriate constitutive theory describing the coupling of the mechanical behavior of the porous solid with the transport of the fluid is a crucial ingredient towards understanding the material behavior in these varied applications. In this work, we formulate and numerically implement in a finite-element framework a large-deformation theory for coupled deformation-diffusion in isotropic, fluid-saturated porous solids. The theory synthesizes the classical Biot theory of linear poroelasticity and the more-recent Coussy theory of poroplasticity in a large deformation framework. In this talk, we highlight several salient features of our theory and discuss representative examples of the application of our numerical simulation capability to problems of consolidation as well as deformation localization in granular materials. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V53.00013: Water Retention of Sandy Soil with Hydrogel Particle Additives under Steady Rain Yuli Wei, Douglas Durian We probe the water retention behavior of a dry model sandy soil with hydrogel particle additives under a steady rain using a self-built raindrop impingement set-up. The 0.4mm dry hydrogel particles are sent into a dry model sandy soil, 1mm glass beads, in different methods and a steady rain is created to irrigate the soil packing. The mass of the retained water in the packing is measured as a function of rain time. The influences of packing height, gel concentration, and gel location are examined respectively. For the model sandy soil alone, the packing height has little effect on the results. Rain water wets a shallow top region and flows out through a narrow path in the packing. With hydrogel particles uniformly mixed into the top region of a model sandy soil packing, the retained water increases as the gel number ratio increases or when the hydrogel particles are concentrated into the wet top region. A better way is to place hydrogel particles in a layer at certain depths under soil surface. The wet gel layers formed during the rain not only lock water inside but also clog the water path and force rain water to wet other dry soil regions. The clogging efficiency is determined by the wet gel layer number and the soil confinement. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V53.00014: Period tripling causes rotating spirals in agitated wet granular matter Kai Huang, Ingo Rehberg Pattern formation of a thin layer of vertically agitated wet granular matter is investigated experimentally. Due to the strong cohesion arising from the capillary bridges formed between adjacent particles, agitated wet granular matter exhibits a different scenario as its dry counter-part. Rotating spirals with three arms, which correspond to the kinks between regions with different colliding phases, are the dominating pattern. This preferred number of arms corresponds to period tripling of the agitated granular layer, unlike predominantly subharmonic Faraday crispations in dry granular matter. The chirality of the spatiotemporal pattern corresponds to the rotation direction of the spirals. Understanding this well traceable instability could pave a way for testifying elaborate theories on dense flow of wet granular matter. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V53.00015: The melting of an ice blok assembly Stephane Dorbolo, Nicolas Vandewalle, Claude Laroche The melting of an assembly of ice blocks under an unidirectional controlled load was investigated. The volume occupied by the ice blocks and the volume of ice were simultaneously measured. While the ice volume continuously decreases, sudden breakdown of the total volume was observed suggesting large reorganization of the whole assembly. The waiting-times between two successive collapses and the magnitudes of the collapse have been correlated. The pile structure was studied using a x-ray tomography before and after a collapse. The arch network re-organization is responsible for the melting dynamics as the pile becomes more and more ordered during the melting. [Preview Abstract] |
Session V54: Superconductivity: Magnetic Field Effects Including Vortex Related Phenomena (Theory)
Sponsoring Units: DCMPChair: David Singh, Oak Ridge National Laboratory
Room: 152
Thursday, March 1, 2012 8:00AM - 8:12AM |
V54.00001: Vortex Dynamics in Ferromagnetic Superconductors: Excitations of Domain Walls and Enhanced Viscosity Shizeng Lin, Lev Bulaevskii, Cristian Batista As more and more superconductors with coexisting magnetic order are found in the last decades, understanding of the vortex dynamics in these superconductors becomes a relevant issue [1, 2]. We investigate vortex dynamics in ferromagnetic superconductors both numerically and analytically. Driven by the Lorentz force, the vortices perturb the magnetic moments and excite magnons. At some velocities, the motion of vortices is resonant with magnetic moments, and the amplitude of magnon excitation is enhanced. When the relaxation rate of magnon is smaller than the pumping rate, the magnon becomes unstable and domain walls are created. The domain walls interact strongly with vortices motion and greatly enhance the viscosity of vortices. Depending on the density of vortices, the vortex configuration may be modulated by the magnetic system. The underlying dynamics of the vortices and magnetic moments can be probed by transport measurement. \\[4pt] [1] L. N. Bulaevskii, A. I. Buzdin, M. L. Kulic, and S. V. Panjukov, Adv. Phys. 34, 175 (1985).\\[0pt] [2] A. Shekhter, L. N. Bulaevskii, and C. D. Batista, Phys. Rev. Lett. 106, 037001 (2011). [Preview Abstract] |
Thursday, March 1, 2012 8:12AM - 8:24AM |
V54.00002: The effect of impurities on the Superheating field of Type II superconductors Fareh Pei-Jen Lin, Alexander Gurevich We calculate the superheating field $H_s(T)$, the maximum field at which the Meissner state exists, for a type-II, single band s-wave superconductor with nonmagnetic and magnetic impurities. $H_s(T)$ was calculated for the entire temperature region $0 |
Thursday, March 1, 2012 8:24AM - 8:36AM |
V54.00003: Gap structure probed by field-angle resolved thermal oscillations in CeCoIn5 superconductor Matthias J. Graf, Tanmoy Das, Anton B. Vorontsov, Ilya Vekhter We calculate the angle-resolved oscillations of the specific heat and thermal conductivity in a rotating in-plane magnetic field in the multiband superconductor CeCoIn$_5$ using realistic tight-binding Fermi surfaces. We find that an electron pocket at the $M$ point and a hole pocket at the $\Gamma$ point of the Brillouin zone yield sufficiently large Fermi surface anisotropies to produce fourfold oscillations not only for $d$-wave pairing, but also for $s$-wave pairing in the regime where our approximations are valid for both nodal and isotropic gap, namely near the upper critical field $H_{c2}$ and down to fields of order $H_{c2}/2$. More importantly, in this region we find a sign reversal in the oscillations as a function of temperature and fixed field for all gap symmetries investigated. We compare our results with available data on CeCoIn$_5$ and CeIrIn$_5$ and discuss how Fermi surface anisotropies affect the identification of gap structures and symmetries. [Preview Abstract] |
Thursday, March 1, 2012 8:36AM - 8:48AM |
V54.00004: Pnictide Half-Dirac Nodal Quasiparticle Scaling Properties in Vortex State Imam Makhfudz We investigate the scaling properties of quasiparticles of Pnictide with ``half-Dirac'' node under magnetic field in vortex state. By computing the density of states, we aim to find in vortex state the form of non-Simon-Lee scaling predicted for such system by several recent works in non-vortex state. We find by exact diagonalization of the BdG Hamiltonian and finite size scaling a $N(E)\sim \sqrt{E}$ power law in the case without magnetic field which agrees with analytical prediction. We consider the vortex state by first studying the hypothetical situation of uniform magnetic field without vortices and then we properly treat the magnetic field-induced vortex lattice by expressing the BdG Hamiltonian in terms of superfluid velocity and Berry's gauge field. The two calculations are shown to agree with each other. We then analyze quantitatively, the effects of anisotropic dispersion to the quasiparticles scaling properties in vortices. A very crucial prediction is also made on an upper bound to the value of ``anomalous dimension'' $\delta$ of density of states scaling with magnetic field, a quantity that can be measured experimentally. [Preview Abstract] |
Thursday, March 1, 2012 8:48AM - 9:00AM |
V54.00005: Vortex-assisted photon counts and their magnetic field dependence in superconducting nano-wire single-photon detectors Lev Bulaevskii, Matthias Graf, Shizeng Lin, Vladimir Kogan We argue that photon counts in a superconducting nano-wire single-photon detector (SNSPD) are caused by the transition from a current-biased metastable superconducting state into the normal state. Such a transition is triggered by photons with the frequency $\omega$ above the critical frequency $\omega_c$ or by photons with the frequency $\omega<\omega_c$ and subsequent vortex crossing from one edge of the superconducting strip to the other. The vortex is pushed across the strip due to the Lorentz force in the presence of the bias current $I$. We calculate the efficiency of photon counts as a function of $\omega$, $I$, bath temperature, and the strip geometry. We derive the dependence of the rate of vortex-assisted photon counts on the bias current at given $\omega$ and strip geometry. The resulting photon count rate has a plateau at high currents, close to the critical current, and drops as a power-law with high exponent at lower currents. While a magnetic field applied perpendicular to the superconducting strip does not affect the formation of hot spots by photons, it increases the rate of vortex crossings (with and without photons). We show that by applying a magnetic field one may identify the origin of dark and photon-assisted counts. [Preview Abstract] |
Thursday, March 1, 2012 9:00AM - 9:12AM |
V54.00006: Type-1.5 superconductivity: effects of interband interactions and Semi-Meissner State Egor Babaev, Johan Carlstrom, Julien Garaud A conventional superconductor is described by a single complex order parameter field which has two fundamental length scales, the magnetic field penetration depth $\lambda$ and the coherence length $\xi$. Their ratio $\kappa$ determines the response of a superconductor to an external field, sorting them into two categories as follows; type-I when $\kappa <1/\sqrt{2}$ and type-II when $\kappa >1/\sqrt{2}$. Multicomponent systems can possess three or more fundamental length scales and allow a separate ``type-1.5'' superconducting state when $\xi_1<\sqrt{2}\lambda<\xi_2$. In that state, as a consequence of the extra fundamental length scale vortices attract one another at long range but repel at shorter ranges. As a consequence the system should form an additional Semi-Meissner state. In that state vortices form clusters in low magnetic fields. In the vortex clusters the component with larger coherence length is depleted and consequently its current is mostly concentrated on the boundaries of the vortex cluster, thus resembling the screening currents in type-I superconductors. We discuss how the vortex clusters and type-1.5 regimes are affected by various interband couplings in multiband superconductors. [Preview Abstract] |
Thursday, March 1, 2012 9:12AM - 9:24AM |
V54.00007: Length scales, collective modes, and type-1.5 regimes in three-band superconductors Johan Carlstrom, Julien Garaud, Egor Babaev The recent discovery of iron pnictide superconductors has resulted in a rapidly growing interest in multiband models with more than two bands. We derive normal modes and characteristic length scales in the conventional $U(1)$ three-band Ginzburg-Landau model as well as in its time-reversal symmetry-broken counterpart with $U(1)\times Z_2$ symmetry. We show that, in the latter case, the collective modes are associated with the mixed phase-density modes and thus are different from the Leggetts modes in two band superconductors. Next we show that gradients of densities and phase differences can be inextricably intertwined in vortex excitations in three-band models. This can lead to very long-range attractive intervortex interactions and the appearance of type-1.5 regimes even when the intercomponent Josephson coupling is large. We next show that field-induced vortices can lead to a change of broken symmetry from $U(1)$ to $U(1)\times Z_2$ in the system. In the type-1.5 regime, it results in a semi-Meissner state where the system has a macroscopic phase separation in domains with broken $U(1)$ and $U(1) \times Z_2$ symmetries. [Preview Abstract] |
Thursday, March 1, 2012 9:24AM - 9:36AM |
V54.00008: Molecular Dynamics of Type-1.5 Superconductors Qingze Wang, Christopher Varney, Hans Fangohr, Egor Babaev In multi-component superconductors and structures of interlaced layered type-I type-II layers there exists a range of parameters where inter-vortex interaction is repulsive at short ranges and attractive at long range. Such regimes were termed ``type-1.5 superconductivity'' recently. This vortex interaction leads to a microscopic phase separation in Meissner and vortex droplets. In this talk, we utilize molecular dynamics to investigate the magnetic response of such superconductors in the type-1.5 regimes. [Preview Abstract] |
Thursday, March 1, 2012 9:36AM - 9:48AM |
V54.00009: Stabilization of a vortex-antivortex lattice created by a magnetic-field pulse V.N. Gladilin, J. Tempere, J.T. Devreese, V.V. Moshchalkov Using the time-dependent Ginzburg-Landau approach, we theoretically investigate the formation and evolution of vortex-antivortex patterns, which are created in a thin superconducting film by a single pulse of an inhomogeneous magnetic field. The field pulse is induced by a periodic square array of current loops, where the current direction is either the same for all the loops or changes from one loop to another in the checkerboard order. In an ideally homogeneous superconductor film, the vortices and antivortices, generated by an applied magnetic field pulse, fully recombine within a relatively short time interval after switching off the current in the loops. We demonstrate that in the presence of- even relatively weak- pinning centers the vortex-antivortex distributions, induced by a short magnetic-field pulse, can eventually evolve into vortex-antivortex lattices, which remain stable for an arbitrarily long time. This work was supported by the Methusalem Funding of the Flemish Government, the NES-ESF program, the Belgian IAP, the Fund for Scientific Research-Flanders (FWO- Vlaanderen). [Preview Abstract] |
Thursday, March 1, 2012 9:48AM - 10:00AM |
V54.00010: Topological solitons in three-band superconductors with broken time reversal symmetry Julien Garaud, Johan Carlstrom, Egor Babaev We report that three-band superconductors with Broken Time Reversal Symmetry allow magnetic field-induced topological solitons. When time reversal symmetry is broken, ground state exhibits $\mbox{U(1)}\times Z_2$ symmetry. Domain-wall, are natural solutions when theories exhibit such a discrete symmetry. Closed domain-walls are unstable to collapse because of their line tension. We show that closed domain-walls can be stabilized by confining vortices. The resulting topological solitons are stable and can be induced by fluctuations or quenching the system through a phase transition. This new kind of solitons can provide an experimental signature of the Time Reversal Symmetry Breakdown. Based on : J. Garaud, J. Carlstr\"om, and E. Babaev, Phys. Rev. Lett. 107, 197001 (Nov 2011). [Preview Abstract] |
Thursday, March 1, 2012 10:00AM - 10:12AM |
V54.00011: Vortex dynamics in Superconducting Corbino Disks: Molucular Dynamics and Heat Transport Simulations Masaru Kato, David Fujibayashi Understanding vortex dynamics is important for application of superconductivity, because vortex motion causes resistive state of superconductors and controlling vortex motion is useful for superconducting devices. Also vortex dynamics shows much variety of phenomena. In a corbino disk geometry, where electric current is injected at the center of the disk and flows toward the perimeter of the disk, vortex moves circular by the Lorentz force from this current. But the Lorentz force depend on the distance from the center as $1/r$, and therefore the vortex velocity faster in the center region than those in the perimeter region. Then vortex motion generates heat and causes non-uniform temperature distribution. Non-uniform temperature distribution causes further vortex motion. Therefore in the superconducting corbino disk, vortex dynamics is not a simple problem. In order to investigate this vortex dynamics, we combine the molecular dynamics and the heat transport simulations. Our simulation results show that dynamical structure of vortices depends on the heat resistance between the superconductor and a substrate as well as the heat capacity and heat conductance of superconductors. Especially there is a transition from laminar flow to the spiral or non-uniform flow. [Preview Abstract] |
Thursday, March 1, 2012 10:12AM - 10:24AM |
V54.00012: $\Phi_{0}$ as a smallest unit of the intermediate state of a type-I superconductor: Revelation through nonlinear dynamics Golibjon Berdiyorov, Alexander Hernandez-Nieves, Francois Peeters, Daniel Dominguez, Milorad Milosevic We study by time-dependent Ginzburg-Landau simulations the nonlinear dynamics of the intermediate state in a current-carrying type-I superconductor. The stray magnetic field of the current induces the intermediate state, where nucleation of flux domains is \textit{discretized to a single fluxoid at a time}, while their final shape (tubular or laminar), size and nucleation rate depend on applied current and edge conditions. The current induces opposite flux domains on opposite sides of the sample, and subsequently drives them to annihilation -- which is \textit{also discretized}, as a sequence of vortex-antivortex pairs. Discretization of both nucleation and annihilation leave measurable traces in the voltage signal across the sample. These dynamic phenomena provide an unambiguous proof of a flux quantum being the smallest building block of the intermediate state in type-I superconductors. [Preview Abstract] |
Thursday, March 1, 2012 10:24AM - 10:36AM |
V54.00013: Nucleation and development of dendritic flux avalanches in superconducting films J{\O}rn Inge Vestg{\aa}rden, Daniil Shantsev, Yuri Galperin, Tom Henning Johansen The stability of superconducting films is threatened by thermomagnetic runaways commonly observed as abrupt dendritic flux avalanches. We report numerical simulations of the electrodynamics and thermal behavior of superconducting films, where the gradual flux penetration is interrupted by such avalanches. The simulation formalism is based on an efficient method for treating the nonlinear and nonlocal electrodynamics, and it handles both the slow flux creep dynamics prior to the avalanches and the transition to the many orders of magnitude faster instability regime. Then the temperature rises quickly above the critical temperature, and the avalanche develops fully in less than 100 nanoseconds, with an initial velocity of approximately 100 km/s. Both the morphology and speed of the avalanches are in excellent agreement with results from magneto-optical imaging experiments. The sample is seeded with randomly distributed disorder, which results in a significantly reduced threshold for onset of avalanches. Interaction with the material disorder also contributes to branching and irreprodusibility of the flux structures. However, disorder is not the main mechanism behind branching and dendritic structures are also found to develop in completely uniform samples. [Preview Abstract] |
Thursday, March 1, 2012 10:36AM - 10:48AM |
V54.00014: Josephson and pancake vortices in Bi-2212 with anti-dots array Kazuto Hirata, Shuuichi Ooi, Takashi Mochiku Josephson and pancake vortices in Bi-2212 with anti-dots (holes) array have been studied with measuring the flow-resistance and the c-axis resistance. The samples for the measurements were prepared by a focused ion-bean milling with a diameter of 200 nm and 1000 nm pitch hole-array and in the in-line structure. Angular dependence of the flow-resistance does not show lock-in phenomenon as usually observed in the behaviors of Josephson vortices in Bi-2212. Instead, there are several peaks observed in the flow-resistance. A couple of peak can be explained with the matching field effect of pancake vortices to the array, but the others not. The c-axis transport measurements show characteristic transitions near the matching fields and between them in the c-axis resistance with the perpendicular field to the superconducting layers. This is related to the accommodation rate of pancake vortices into the holes and to the interaction between/among the vortices outside the holes and the trapped vortices. [Preview Abstract] |
Thursday, March 1, 2012 10:48AM - 11:00AM |
V54.00015: Effect of pinning force on critical current using molecular dynamic simulation Abdalla Obeidat, Hadeel Abu Lahim Molecular dynamics have been used to study the effect of pinning force on the critical current at different temperatures. Our simulation is built on assuming a two dimensional periodic arrays of vortices and pins at different sites. The critical current density has been studied at different temperatures by solving the over damped equation of vortex motion taking into account the vortex-vortex interaction, the thermal force, the vortex-pinning interaction as well as the driving Lorentz force. The results show a second phase transition at specific low temperature at all pinning forces with a definite pinning size. [Preview Abstract] |
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