Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Y1: Invited Session: New Perspectives on Kondo Systems
Sponsoring Units: DCMPChair: David Goldhaber-Gordon, Stanford University
Room: Ballroom I
Friday, March 22, 2013 8:00AM - 8:36AM |
Y1.00001: Frustration & Order in Kondo Lattice Systems Invited Speaker: Meigan Aronson |
Friday, March 22, 2013 8:36AM - 9:12AM |
Y1.00002: Visualizing heavy fermions emerging in a quantum critical Kondo lattice Invited Speaker: Pegor Aynajian In solids containing elements with $f$ orbitals, the interaction between $f$-electron spins and those of itinerant electrons leads to the development of low-energy fermionic excitations with a heavy effective mass. These excitations are fundamental to the appearance of unconventional superconductivity and non-Fermi-liquid behavior observed in actinide- and lanthanide-based compounds. We use spectroscopic mapping with the scanning tunneling microscope to detect the emergence of heavy excitations with lowering of temperature in a prototypical family of cerium-based heavy-fermion compounds. We demonstrate the sensitivity of the tunneling process to the composite nature of these heavy quasiparticles, which arises from quantum entanglement of itinerant conduction and $f$ electrons. Scattering and interference of the composite quasiparticles is used to resolve their energy--momentum structure and to extract their mass enhancement, which develops with decreasing temperature. The lifetime of the emergent heavy quasiparticles reveals signatures of enhanced scattering and their spectral lineshape shows evidence of energy--temperature scaling. These findings demonstrate that proximity to a quantum critical point results in critical damping of the emergent heavy excitation of our Kondo lattice system. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:48AM |
Y1.00003: Observation of Majorana-like Behavior at the Quantum Critical Point in a Resonant Level Coupled to a Dissipative Environment Invited Speaker: Gleb Finkelstein We investigate tunneling through a resonant level embedded in a dissipative environment, which suppresses tunneling rates at low temperatures. Specifically, the resonant level is formed in a carbon nanotube quantum dot, and the dissipative environment is realized by fabricating resistive leads. For the symmetric coupling of the resonant level to the two leads, we find that the resonant peak reaches the unitary conductance $e^2/h$ despite the presence of dissipative modes. Simultaneously, the width of the resonance tends to zero as a non-trivial power of temperature. We draw a connection between our system and a resonant tunneling in a Luttinger liquid and interpret the observed unitary resonance of vanishing width in terms of a quantum critical point (QCP). We further investigate an exotic state of electronic matter obtained by fine-tuning the system exactly to the QCP and report on several transport scaling laws both near and far from equilibrium. Particularly striking is a quasi-linear non-Fermi liquid scattering rate found at the QCP, interpreted in terms of a model with Majorana modes at the resonant level. Although unlikely to be practical for fault-tolerant quantum computing, our device constitutes a viable alternative to topological superconductors as a platform for studying strong correlation effects within Majorana physics. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:24AM |
Y1.00004: Nonequilibrium Kondo model: Real-time RG study of crossover from weak to strong coupling Invited Speaker: Mikhail Pletyukhov We analyze the nonequilibrium Kondo model at finite voltage and temperature by using a new formulation [1] of the real-time renormalization group [2] with the Laplace variable as the flow parameter. We evaluate the energy-dependent spin relaxation rate and nonlinear conductance, and derive an approximate form for the universal line shape for the latter in the whole crossover regime from weak to strong coupling (that is, from high to low energy scales). The results are shown to agree well with exact methods and the numerical renormalization group in equilibrium, Fermi liquid theory, weak-coupling expansions, and recent experiments [3].\\[4pt] References:\\[0pt] [1] M. Pletyukhov and H. Schoeller, Phys. Rev. Lett. 108, 260601 (2012).\\[0pt] [2] H. Schoeller, Eur. Phys. J. Special Topics 168, 179 (2009); H. Schoeller and F. Reininghaus, Phys. Rev. B 80, 045117 (2009).\\[0pt] [3] A. V. Kretinin, H. Shtrikman, D. Goldhaber-Gordon, M. Hanl, A. Weichselbaum, J. von Delft, T. Costi, and D. Mahalu, Phys. Rev. B 84, 245316 (2011); A. V. Kretinin, H. Shtrikman, and D. Mahalu, Phys. Rev. B 85, 201301(R) (2012). [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 11:00AM |
Y1.00005: Quantum quench of Kondo correlations in optical absorption Invited Speaker: Andreas Weichselbaum Absorption spectra of individual semiconductor quantum dots tunnel-coupled to a degenerate electron gas in the Kondo regime have recently become accessible to the experiment [1]. The absorption of a single photon leads to an abrupt change in the system Hamiltonian, which can be tailored such that it results in a quantum quench of the Kondo correlations. This is accompanied by a clear signature in the form of an Anderson orthogonality catastrophe, induced by a vanishing overlap between initial and final many-body wave functions and with power-law exponents that can be tuned by an applied magnetic field. We have modeled the experiment in terms of an Anderson impurity model undergoing an optically induced quench, and studied this \emph{Kondo exciton} in detail using both analytical methods and the Numerical Renormalization Group (NRG). Our NRG results reproduce the measured absorption line shapes very well, showing that NRG is ideally suited for the study of Kondo excitons. In summary, the experiments demonstrate that optical measurements on single artificial atoms offer new perspectives on many-body phenomena previously studied using transport spectroscopy only. \\[4pt] [1] Latta et al, Nature {\bf 474} 627 (2011). \\[0pt] [2] T{\"u}reci et al, Phys. Rev. Lett {\bf 106}, 107402 (2011). [Preview Abstract] |
Session Y2: Invited Session: Magnetism and non-Fermi Liquid in Heavy Fermion Metals
Sponsoring Units: DCMPChair: Piers Coleman, Rutgers University
Room: Ballroom II
Friday, March 22, 2013 8:00AM - 8:36AM |
Y2.00001: Dimensionality and quantum criticality in heavy fermion metals Invited Speaker: Silke Paschen Heavy fermion compounds are at the forefront of research on quantum criticality. This is due to the fact that many of these materials can be tuned to a quantum critical point (QCP) by readily accessible values of the control parameters magnetic field, pressure or substitution/doping. In recent years efforts are being made to classify the different kinds of quantum critical behavior experimentally observed, to test the extent to which heavy fermion quantum criticality is universal. We have identified a cubic heavy fermion material, Ce$_3$Pd$_{20}$Si$_6$, as exhibiting a field-induced quantum phase transition as the lower of two consecutive phase transitions is suppressed to zero. This transition is accompanied by an abrupt change of Fermi surface [1], reminiscent of what happens across the field-induced antiferromagnetic to paramagnetic transition in tetragonal YbRh$_2$Si$_2$ [2]. In Ce$_3$Pd$_{20}$Si$_6$, the QCP separates two different ordered phases. In fact, a Kondo destruction QCP [3] has been theoretically predicted to exist in the ordered portion of a global phase diagram for quantum critical heavy fermion compounds [4]. We conclude that dimensionality is an effective way to tune through such a global phase diagram and that the cubic material studied here is situated in the barely explored three-dimensional portion of this phase diagram. We believe that this finding will guide the search for further experimental anchoring points in the global phase diagram, and for a unified theoretical description.\\[0.2cm] Work done in collaboration with J. Custers, J. Larrea J., K.- A. Lorenzer, M. M\"{u}ller, A. Prokofiev, A. Sidorenko, H. Winkler, A. M. Strydom, Y. Shimura, T. Sakakibara, R. Yu and Q. Si.\\[4pt] [1] J. Custers, K.-A. Lorenzer, M. M\"{u}ller, A. Prokofiev, A. Sidorenko, H. Winkler, A. M. Strydom, Y. Shimura, T. Sakakibara, R. Yu, Q. Si, and S. Paschen, Nature Materials 11, 189 (2012).\\[0pt] [2] S. Paschen et al., Nature 432, 881 (2004). S. Friedemann et al., Proc. Natl. Acad. Sci. 107, 14547 (2010).\\[0pt] [3] Q. Si et al. Nature 413, 804 (2001). P. Coleman et al., J. Phys. Condens. Matter 13, R723 (2001). T. Senthil et al., Phys. Rev. B 69, 035111 (2004).\\[0pt] [4] Q. Si, Physica B 378-380, 23 (2006). Q. Si, Phys. Status Solidi B 247, 476 (2010). [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 9:12AM |
Y2.00002: From incommensurate correlations to mesoscopic spin resonance in YbRh2Si2 Invited Speaker: Collin Broholm Spin fluctuations are reported near the magnetic field driven quantum critical point in YbRh2Si2 [1]. On cooling, ferromagnetic fluctuations evolve into incommensurate correlations with a characteristic in-plane wave vector of $q_m = (\delta,\delta)$ with $\delta=0.14 \pm 0.04$ r.l.u. At low temperatures, an in plane magnetic field induces a sharp intra doublet resonant excitation at an energy $g\mu_B\mu_0H$ with $g=3.8\pm 0.2$. The intensity is localized at the zone center and has a width in momentum space indicating precession of spin density extending $\xi = 6 \pm 2$ \AA\ beyond the 4f site.\\[4pt] [1] C. Stock, C. Broholm, F. Demmel, J. Van Duijn, J. W. Taylor, H.J. Kang, R. Hu, and C. Petrovic, Phys. Rev. Lett. {\bf 109}, 127201 (2012). [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:48AM |
Y2.00003: Critical quasi-particle theory and scaling near a Quantum Critical Point of Heavy Fermion metals Invited Speaker: Peter W\"{o}lfle We recently developed a theory of the critical properties of a heavy fermion metal near an antiferromagnetic (AFM) quantum phase transition governed by three-dimensional spin fluctuations. The critical spin fluctuations induce critical behavior of the electron quasi-particles (qp) as seen in a diverging effective mass, leading, e.g., to a diverging specific heat coefficient. This in turn gives rise to a modification of the spin excitation spectrum [1]. We use that the concept of electron quasi-particles is well-defined as long as the qp width is less than their excitation energy, which is still the case in the so-called non-Fermi liquid regime. Impurity scattering [1,2] and/or higher order loop processes in the clean system [3] cause a redistribution of the critical scattering at the hot lines all over the Fermi surface, leading to a weakly momentum dependent critical self-energy. We derive a self-consistent equation for the qp effective mass which allows for two physical solutions: the usual weak coupling spin density wave solution and a strong coupling solution featuring a power law divergence of the effective mass as a function of energy scale. The resulting spin excitation spectrum obeys E/T scaling with dynamical exponent z$=$4 and correlation length exponent $\nu =$1/3, in excellent agreement with data for YbRh$_2$Si$_2$ [1,2]. Results of our theory applied to three-dimensional metals featuring quasi-two-dimensional spin fluctuations will be presented with the aim of explaining the observed properties of the AFM quantum critical point of CeCu$_{\mathrm{6-x}}$Au$_{\mathrm{x}}$, in particular the E/T scaling exhibited by inelastic neutron scattering data. In that case we find z$=$8/3 and $\nu =$3/7 [3]. Finally, the microscopic underpinning of our theory will be addressed, including the issues of qp renormalization, vertex corrections, interaction of bosonic fluctuations in the renormalization group sense, and higher loop corrections [3].\\[4pt] [1] P. W\"{o}lfle, and E. Abrahams, Phys. Rev. B \textbf{84}, 041101 (2011); Ann. Phys. (Berlin) \textbf{523}, 591 (2011); Phys. Rev. B \textbf{80}, 235112 (2009).\\[0pt] [2] E. Abrahams and P. W\"{o}lfle, PNAS \textbf{109}, 3228 (2012).\\[0pt] [3] E. Abrahams, J. Schmalian, and P. W\"{o}lfle, to be published. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:24AM |
Y2.00004: Ferromagnetic quantum criticality in heavy fermion systems Invited Speaker: Manuel Brando Heavy fermion (HF) systems are metals where the weak hybridisation between nearly localized $f$-electrons and the mobile conduction electrons, i.e. the Kondo effect, leads to a Fermi liquid (FL) ground state with narrow bands and quasiparticles with strongly enhanced effective electronic masses. When the magnetic RKKY interaction becomes comparable to the Kondo interaction, magnetic order can appear, mostly at very low $T$. The magnetic order can be suppressed by an external parameter, e.g. pressure or magnetic field, inducing a quantum phase transition (QPT) at $T = 0$. If this QPT is continuous, the associated quantum critical point (QCP) is surrounded by a non-FL regime of quantum critical fluctuations where unconventional superconductivity or novel phases of matter may arise [1]. The unambiguous observation of antiferromagnetic (AFM) QCPs in HF systems [2] has led to an increasing number of theoretical and experimental works in order to understand QPTs as deeply as their classical counterpart. Although it has been demonstrated that in antiferromagnets QCPs exist, in ferromagnets there is still no clear evidence. Intensive investigations have shown that metallic ferromagnets are inherently unstable [3,4] and do not exhibit a FM QCP. However, in the recently discovered HF system YbNi$_{4}$P$_{2}$, a quasi-1D ferromagnet with a remarkably-low $T_{C} = 0.15$\,K [5], the $T$-divecgence in the Gr\"uneisen ratio points to the presence of a FM QCP. I will present a general overview of the state of the art of FM quantum criticality in HF systems, discussing in particular the cases of YbNi$_{4}$P$_{2}$, CeFePO, CePd$_{1-x}$Rh$_{x}$ as well as the AFM system YbRh$_{2}$Si$_{2}$ where FM order is induced by chemical pressure.\\[4pt] [1] H. Q. Yuan \textit{et al.}, Science \textbf{302} 2104 (2003)\\[0pt] [2] J. Custers \textit{et al.}, Nature \textbf{424} 524 (2003)\\[0pt] [3] D. Belitz \textit{et al.}, Phys. Rev. Lett. \textbf{82} 4707 (1999)\\[0pt] [4] M. Uhlarz \textit{et al.}, Phys. Rev. Lett. \textbf{93} 256404 (2004)\\[0pt] [5] C. Krellner \textit{et al.}, New J. Phys. \textbf{13} 103014 (2011) [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 11:00AM |
Y2.00005: Visualizing Creation, Destruction, and Intra-Unit-Cell Symmetries of Heavy Fermion Electronic Structure Invited Speaker: J.C. Seamus Davis |
Session Y3: Invited Session: New Directions in Fractional Quantum Hall Phenomena
Sponsoring Units: DCMP DCOMPChair: Mansour Shayegan, Princeton University
Room: Ballroom III
Friday, March 22, 2013 8:00AM - 8:36AM |
Y3.00001: Local thermometry and compressibility measurements as new probes of strongly correlated states Invited Speaker: Amir Yacoby Electrons in two dimensions and strong magnetic fields can form an insulating two-dimensional system with conducting one-dimensional channels along the edge. Electron interactions in these systems can have fractionalized charge excitations and chiral edges with independent transport of charge and heat, even in opposite directions. Here, we use a quantum dot as a local thermometer to explore such heat transport along the edge at filling factor one and 2/3 in a GaAs 2DEG. Moreover, using a scanning quantum dot as a local charge sensor allows us to extract the charge of elementary excitations at filling factor 5/2 as well as to observe a delicate sequence of fractional quantum Hall states in suspended graphene. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 9:12AM |
Y3.00002: Quantum Hall Transitions and Quantum Number Fractionalization in Trapped Cold Atom Systems Invited Speaker: Kun Yang Recently there have been experimental attempts to realize quantum Hall physics in trapped cold atom systems, either through rotation or synthetic gauge fields. This can potentially open up a completely new direction in the study of quantum Hall effects. In this talk I will discuss possible quantum phase transitions between integer and fractional quantum Hall states, driven by attractive interactions between fermionic atoms. Such transitions have no counterparts in electronic quantum Hall liquids, but are related to fractionalization transitions studied in other strongly correlated systems. In one of these examples charge fractionalization is associated with the confinement-deconfinement transition of the (2$+$1D) Z2 gauge theory, which is in the Ising universality class. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:48AM |
Y3.00003: Fractional Quantum Hall in the Diluted Magnetic Semiconductor CdMnTe Invited Speaker: Dieter Weiss |
Friday, March 22, 2013 9:48AM - 10:24AM |
Y3.00004: Tunable interactions and the fractional quantum Hall effect Invited Speaker: Zlatko Papic We explore several realistic methods of tuning the interactions in two-dimensional electronic systems in high magnetic fields. We argue that these experimental probes can be useful in studying the interplay of topology, quantum geometry and symmetry breaking in the fractional quantum Hall effect (FQHE). In particular, we show that the mixing of subbands and Landau levels in GaAs wide quantum wells breaks the particle-hole symmetry between the Moore-Read Pfaffian state and its particle-hole conjugate, the anti-Pfaffian, in such a way that the latter is unambiguously favored and generically describes the ground state at 5/2 filling [1]. Furthermore, the tilting of the magnetic field, or more generally variation of the band mass tensor, probes the fluctuation of the intrinsic metric degree of freedom of the incompressible fluids, and ultimately induces the crossover to the broken-symmetry and nematic phases in higher Landau levels [2]. Some of these mechanisms also lead to an enhancement of the excitation gap of the non-Abelian states, as observed in recent experiments. Finally, we compare the tuning capabilities in conventional systems with that in multilayer graphene and related materials with Dirac-type carriers where tuning the band structure and dielectric environment provides a simple and direct method to engineer more robust FQHE states and to study quantum transitions between them [3]. \\[4pt] [1] Z. Papic, F. D. M. Haldane, and E. H. Rezayi, arXiv:1209.6606 (2012).\\[0pt] [2] Bo Yang, Z. Papic, E. H. Rezayi, R. N. Bhatt, F. D. M. Haldane, Phys. Rev. B 85, 165318 (2012).\\[0pt] [3] Z. Papic, R. Thomale, D. A. Abanin, Phys. Rev. Lett. 107, 176602 (2011); Z. Papic, D. A. Abanin, Y. Barlas, and R. N. Bhatt, Phys. Rev. B 84, 241306(R) (2011); D. A. Abanin, Z. Papic, Y. Barlas, and R. N. Bhatt, New J. Phys. 14, 025009 (2012). [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 11:00AM |
Y3.00005: Fractional quantum Hall effect in AlAs quantum wells: Role of valley degree of freedom Invited Speaker: Tayfun Gokmen When interacting two-dimensional electrons are placed in a large perpendicular magnetic field, to minimize their energy, they capture an even number of flux quanta and create new particles called composite fermions (CFs). These complex electron-flux-bound states offer an elegant explanation for the fractional quantum Hall effect. Thanks to the flux attachment, the effective field vanishes at half-filled Landau levels ($\nu =$ 1/2 and 3/2) and CFs exhibit Fermi-liquid-like properties, similar to their zero-field electron counterparts. Here, we study a two-dimensional electron system in AlAs quantum wells where the electrons occupy two conduction band valleys with anisotropic Fermi contours and strain-tunable occupation. We address a fundamental question whether the anisotropy of the electron effective mass and Fermi surface is transferred to the CFs formed around filling factors $\nu =$ 1/2 and 3/2. Similar to their electron counter parts, CFs also exhibit anisotropic transport, suggesting an anisotropy of CF effective mass and Fermi surface. We also study quantum Hall ferromagnetism for fractional quantum Hall states formed at $\nu =$ 1/3 and 5/3 as a function of valley splitting. Within the framework of the CF theory, electronic fractional filling factors $\nu =$ 1/3 and 5/3 are equivalent to the integer filling factor $p=$ 1 of CFs. Reminiscent of the quantum Hall ferromagnetism observed at $\nu =$ 1, we report persistent fractional quantum Hall states at filling factors $\nu =$ 1/3 and 5/3 when the two valleys are degenerate. However, the comparison of the energy gaps measured at $\nu =$ 1/3 and 5/3 to the available theory developed for single-valley, two-spin systems reveals that the gaps and their rates of rise with strain are much smaller than predicted.\\[4pt] [1] ``Transference of Transport Anisotropy to Composite Fermions,'' T. Gokmen, M. Padmanabhan, and M. Shayegan, \textit{Nature Physics }\textbf{6}, 621-624 (2010).\\[4pt] [2] `Ferromagnetic Fractional Quantum Hall States in a Valley-Degenerate Two-Dimensional Electron System,'' M. Padmanabhan, T. Gokmen, and M. Shayegan, \textit{Phys. Rev. Lett.} \textbf{104}, 016805 (2010). [Preview Abstract] |
Session Y5: Graphene: Transport and Optical Phenomena: Heterostructures
Sponsoring Units: DCMPChair: Sufei Shi, University of California, Berkeley
Room: 301
Friday, March 22, 2013 8:00AM - 8:12AM |
Y5.00001: Plasmons and Coulomb drag in Dirac/Schroedinger hybrid electron systems Alessandro Principi, Matteo Carrega, Reza Asgari, Vittorio Pellegrini, Marco Polini We show that the plasmon spectrum of an ordinary two-dimensional electron gas (2DEG) hosted in a GaAs heterostructure is significantly modified when a graphene sheet is placed on the surface of the semiconductor in close proximity to the 2DEG. Long-range Coulomb interactions between massive electrons and massless Dirac fermions lead to a new set of optical and acoustic intra-subband plasmons. Here we compute the dispersion of these coupled modes within the Random Phase Approximation, providing analytical expressions in the long-wavelength limit that shed light on their dependence on the Dirac velocity and Dirac-fermion density. We also evaluate the resistivity in a Coulomb-drag transport setup. These Dirac/Schroedinger hybrid electron systems are experimentally feasible and open new research opportunities for fundamental studies of electron-electron interaction effects in two spatial dimensions. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y5.00002: Enhancement of Coulomb drag in double-layer graphene structures by plasmons and dielectric background inhomogeneity Samvel M. Badalyan, Francois M. Peeters The drag of massless fermions in graphene double-layer structures has been investigated over a wide range of temperatures and interlayer separations. We have shown [1] that the inhomogeneity of the dielectric background in such graphene structures, for experimentally relevant parameters, results in a significant enhancement of the drag resistivity. At intermediate temperatures the dynamical screening via plasmon-mediated drag enhances the drag resistivity and results in an upturn in its behavior at large interlayer separations. In a range of interlayer separations, corresponding to the crossover from strong to weak coupling of graphene layers, we find that the decrease of the drag resistivity with interlayer spacing is approximately quadratic. This dependence weakens below this range of interlayer spacing while for larger separations we find a cubic (quartic) dependence at intermediate (low) temperatures. \\[4pt] [1] S. M. Badalyan and F. Peeters, Phys. Rev. B {\bf 86}, 121405(R) (2012). [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y5.00003: Energy-driven drag in Graphene Justin Song, Leonid Levitov When solid surfaces slide against each other they experience friction which can be enhanced by inserting molasses between them or reduced by using a lubricant. In the same way, two spatially isolated conducting layers that are placed in close proximity with each other feel friction because the long-ranged Coulomb interaction allows electrons in adjacent layers to ``rub shoulders at a distance.'' Recent measurements of Coulomb drag in Graphene by Gorbachev and co-workers from Manchester (doi:10.1038/nphys2441) have found that it is dramatically enhanced near the Dirac point, in stark contradiction with earlier theories predicting vanishing drag. We argue that a new kind of drag develops when heat transport in the two layers becomes strongly coupled due to efficient energy transfer between the layers. As a result, spatial charge inhomogeneity couples the motion of the electron liquid with heat transport through it, damping motion of electron flow in one layer by heat dissipation in the other. Interestingly, and somewhat paradoxically, this leads to strong drag without momentum transfer between layers. We predict distinct experimental signatures and discuss its magnetic field dependence. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y5.00004: Hydrodynamical Modes and New Transport Phenomena in Graphene: Nonlocality and Anomalous Drag Leonid Levitov The semimetal band structure of graphene givs rise to an unusually strong coupling between electrical currents and charge-neutral currents. This coupling leads to new transport phenomena mediated by neutral modes. This talk will highlight two examples connected with ongoing experiments. One is giant nonlocality observed in electric measurements.[1] This effect was explained by spin transport made possible by novel spin-Hall response near the Dirac point.[2] Another example is anomalous drag observed at charge neutrality which was attributed to the effects mediated by energy transfer in graphene heterostructures.[3,4] Drag measurements thus afford a unique probe of energy transfer at the nanoscale, a fundamental process which is not easily amenable to more conventional techniques such as calorimetry, and is key for the physics of strong interactions that occur near neutrality. \\[4pt] [1] D. A. Abanin et al, Science 332, 328-330 (2011); \\[0pt] [2] D. A. Abanin et al, Phys. Rev. Lett. 107, 096601 (2011) \\[0pt] [3] R. V. Gorbachev et al, arXiv: 1206.6626, doi:10.1038/nphys2441 \\[0pt] [4] J. W. C. Song and L. S. Levitov, arXiv:1205.5257, Phys.Rev.Lett., to be published (2012) [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y5.00005: Insulating behavior at the neutrality point in dual-gated single-layer graphene Francois Amet, James Williams, David Goldhaber-Gordon The conductivity at the neutrality point in single-layer graphene is known to saturate on the order of e$^{2}$/h due to disorder-induced density fluctuations. In this study, we report contrasting results using dual-gated graphene devices with a boron nitride back-gate dielectric and a suspended top-gate, allowing for carrier mobilities over 100 000 cm$^{2}$/Vs. As the temperature is lowered, the peak resistivity at the charge-neutrality point unexpectedly diverges with a power-law behavior and becomes as high as several megohms per square. As a transverse magnetic field is applied, our device remains insulating and directly transitions to the ?=0 quantum Hall state. We discuss possible origins for this insulating behavior. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y5.00006: Broken Symmetry Quantum Hall states in Dual Gated ABA Trilayer Graphene Yongjin Lee, Jairo Velasco Jr., David Tran, Fan Zhang, Wenzhong Bao, Lei Jing, Kevin Myhro, Dmitry Smirnov, Jeanie Lau We perform low temperature transport measurements on dual-gated suspended trilayer graphene in the quantum Hall (QH) regime. We observe QH plateaus at filling factors $\nu =$-8, -2, 2, 6, and~10, in agreement with the full-parameter tight binding calculations. In high magnetic fields, oddinteger plateaus are also resolved,~indicating almost complete lifting of the 12-fold degeneracy of~the lowest Landau levels (LL). Under an out-of-plane electric field E$\bot $. We observe degeneracy~breaking and transitions between QH plateaus. Interestingly, depending on its direction, E$\bot $selectively breaks the LL degeneracies in the electron-doped or hole-doped regimes.~ [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y5.00007: Comparison of mobility at the top and bottom surfaces of multilayer graphene placed on SiO$_2$ substrate Akinobu Kanda, Yousuke Nukui, Hikari Tomori, Hidenori Goto, Youiti Ootuka It is known that charged impurities attached to the surface of graphene films are the main source of deteriorating mobility in graphene flakes obtained by the mechanical exfoliation. There are several origins for charged impurities: charges in the substrate, to which the bottom surface of the graphene films faces, the adsorbed molecules and contaminations due to chemicals (resist residues and so on) mainly attached to the top surface of graphene. This paper aims to evaluate the influence of the charged impurities on the top and bottom surfaces separately. For this purpose, we used dual-gated multilayer graphene with a contactless top gate. We developed a method of estimating the mobility of the top and bottom surfaces of multilayer graphene (MLG), from the top- and bottom-gate voltage dependence of the conductivity. We find that in thick MLG, mobility of the top surface is more than three times larger than that of the bottom surface. This indicates that the influence of the SiO$_2$ substrate on the mobility is stronger than that of adsorbates and contaminations on the top surface of the MLG. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y5.00008: Electric charge and potential distribution in twisted multilayer graphene Natalya Zimbovskaya, Eugene Mele The specifics of charge screening and electrostatic potential spatial distribution in rotationally faulted multilayered graphene films with decoupled layers placed in between charged substrates is theoretically analyzed. The analysis is carried out using a nonlinear Thomas -Fermi approach. It is shown that by varying the areal charge densities on the substrates and/or the thickness of the graphene pack one may tune the screening length in the graphene pack. When the charge densities on the substrates are weak, the screening length is of the same order as the pack thickness, which agrees with semimetallic properties of graphene. When the amount of the donated charge is sufficiently large the screening length reduces indicating the transition to a metallic-like behavior of the graphene layers. The transition is shown to turn on rather quickly, and in occurs when the charge on the substrates/external electric field reaches a certain crossover magnitude. The possibilities for experimental observation of the predicted transition are discussed. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y5.00009: Tunable van Hove Singularities and Optical Absorption of Twisted Bilayer Graphene Yufeng Liang, Li Yang We perform the first-principles GW-Bethe-Salpeter Equation (BSE) simulation to study the optical absorption spectra of isolated twisted bilayer graphene (TBLG). The twisting generates new van Hove singularities (VHS), and these VHSs and corresponding optical absorption peaks can be tuned in a wide range by the twist angle. Enhanced electron-electron and electron-hole interactions are shown to be important to understand both optical absorption peak positions and their lineshapes. With these many-electron effects included, our calculation satisfactorily explains recent experimental measurements. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y5.00010: Gate tunable quantum transport in double layer graphene heterostructures Kostyantyn Kechedzhi, Euyheon Hwang, Sankar Das Sarma Motivated by the recently observed highly resistive state in double layer graphene heterostructures [1] we consider a system of two layers of graphene, ``studied'' and ``control,'' separated by an insulating layer. We theoretically analyze the effect of additional screening provided by Dirac electrons in the ``control'' graphene layer on the transport characteristics of the ``studied'' graphene layer. We find that in a typical device geometry fabricated on top of SiO2 substrate [1] the suppression of charge inhomogeneity is less efficient than initially expected and is limited by about a factor of 2. We also analyze the effect of additional screening on the quantum correction to the conductivity of the ``studied'' layer in this system in the metallic regime. We find that ``control'' layer screening is very efficient at suppressing electron-electron interactions in the ``studied'' layer which results in improved coherence and a novel gate tunable quantum correction to conductivity. The results of this work are summarized in [2].\\[4pt] [1] L. A. Ponomarenko et. al. Nat. Phys. 7, 958 (2011).\\[0pt] [2] K. Kechedzhi, E. H. Hwang, and S. Das Sarma Phys. Rev. B 86, 165442 (2012). [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y5.00011: Transport properties of monolayer and bilayer graphene supported by hexagonal boron nitride Jing Li, Ke Zou, Donald Seiwell, Jun Zhu We present transport studies on hexagonal boron nitride (h-BN) supported monolayer and bilayer graphene. Following the method introduced by Dean et al, we first exfoliate thin sheets of h-BN (15-20 nm) to SiO$_{\mathrm{2}}$/Si substrate then align and transfer exfoliated graphene flakes onto the h-BN sheets. E-beam lithography is used to process the samples into Hall bar devices. We find that current annealing at low temperature can increase the mobility of as-fabricated devices but often introduces large density inhomogeneity at the same time. AFM images of annealed devices reveal the limitations of this technique. In comparison, thermal annealing is much more reliable in improving the sample quality. Bilayer devices annealed in a flow of Ar/H$_{\mathrm{2}}$ at 450C for 5 hours show high mobility of 30,000 cm$^{\mathrm{2}}$/Vs at low temperature. We observe high-quality Shubnikov-de Hass (SdH) oscillations and degeneracy-lifted Landau levels in these samples. We extend existing measurements of the electron and hole effective mass in bilayer graphene[1] to lower carrier density regimes and discuss the implications of the results.[1] K. Zou, X. Hong, and J. Zhu, Phys. Rev. B 84, 085408 (2011). [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y5.00012: Ground state of double layer graphene heterostructures in the presence of charged impurities Martin Rodriguez-Vega, Jonathan Fischer, Enrico Rossi A graphene double layer heterostructure is formed by two sheets of graphene separated by a thin dielectric film. Using the Thomas-Fermi-Dirac theory we have studied the carrier density profile in the presence of charged impurities. In this talk I will present our results for the case of heterostructures formed by two sheets of single-layer-graphene (SLG) and two sheets of bilayer-graphene (BLG). As for isolated layers, we find that the presence of charged impurities induces strong carrier density inhomogeneities, especially at low dopings where the density landscape breaks up in electron-hole puddles. We find that the amplitude of the carrier density inhomogeneities in double layers can be much lower than in isolated layers due to the better screening properties of double layer systems. I will then present results for the case of ``hybrid'' structures formed by one sheet of SLG and one sheet of BLG. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y5.00013: Electronic and thermoelectric transport in graphene double layer structures with boron nitride spacers Jiuning Hu, Tailung Wu, Jifa Tian, Yong Chen Recently, much attention has been devoted to electrically isolated graphene-graphene double layers in which interaction-driven novel physics such as exciton condensation are predicted. We have used polyvinyl alcohol (PVA) based carrier films and a micro-manipulator to transfer mechanically exfoliated flakes onto desired locations with accuracy of $\sim$1 $\mu$m. We have fabricated graphene/boron nitride (BN)/graphene stacking structures on BN substrates to study their electronic and thermoelectric transport properties. We observed the low temperature mobility of graphene as high as 75000 cm$^2$/V-s. We have performed Coulomb drag measurements and observed the sign and magnitude dependence of the drag resistivity on the carrier types and densities of both graphene layers, consistent with the previous reports. We also performed thermoelectric transport measurements in such graphene double layer structures, especially in the complementary doped regime (so called excitonic regime) with one layer of electrons and the other layer of holes. Our approach may be useful to probe exciton condensation and other novel physics driven by electron-electron interactions in graphene double layers. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y5.00014: Photo doping effect in graphene/BN heterostructure Long Ju, Jairo Velasco Jr., Edwin Hwang, Jonghwan Kim, Feng Wang Boron nitride has been demonstrated as an ideal substrate to achieve high mobility in graphene. At the same time We observed strong change of graphene transport properties by shining light on graphene/BN heterostructure. This is attributed to photo doping effect induced by impurity excitation in BN. Optical spectroscopy based on this photo-doping effects enables us to probe impurities in crystalline BN. Such information will be important for potential applications based on graphene/BN heterostructures. The potential of applying similar technique to probe defects in other insulators and semiconductors will also be discussed. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y5.00015: Photon Induced Transport in Graphene-Boron Nitride-Graphene Heterostructures Nityan Nair, Nathaniel Gabor, Qiong Ma, Kenji Watanabe, Takashi Taniguchi, Wenjing Fang, Jing Kong, Pablo Jarillo-Herrero Monolayer graphene, an atomically thin sheet of hexagonally oriented carbon, is a zero band gap conductor that exhibits strong electron-electron interactions and broadband optical absorption. By combining MLG and hexagonal boron nitride into ultrathin vertical stacks, experiments have demonstrated improved mobility, Coulomb drag, and field-effect tunneling across few-layer boron nitride barriers. Here, we report on the photon-induced transport of charge carriers through a graphene-boron nitride-graphene heterostructure. The dependence of the generated photocurrent on photon energy and interlayer bias voltage is studied. The photocurrent is found to depend strongly on both these parameters, showing several interesting features. We consider several processes that may serve to explain the rich dependence of photoconductance on applied bias voltage and photon energy. [Preview Abstract] |
Session Y6: Nanotubes and Nanowires (non-carbon): Transport and Optical Phenomena
Sponsoring Units: DCMPChair: Jonathan Spanier, Drexel University
Room: 302
Friday, March 22, 2013 8:00AM - 8:12AM |
Y6.00001: Surface Passivation and Orientation Dependence in the Electronic Properties of Silicon Nanowires Keenan Zhuo, Mei-Yin Chou Different surface passivation configurations for silicon nanowires (SiNWs) have previously been studied for expanding their technological applications. Of note, methyl (CH$_3$) passivated SiNWs have enhanced ambient stability, while electronegative atoms/groups such as halogens are useful in band gap engineering and chemical post-processing. Thus far though, fundamental mechanisms for how such passivations alter the electronic properties of SiNWs have not been rigorously scrutinized. In this work, we address this issue through first-principles calculations on CH$_3$, fluorine (F) and hydrogen (H) passivated [110] and [111] SiNWs. In comparison to H passivation, we explain how CH$_3$ and F passivations cause significant band gap reductions in [110] SiNWs, through strain and quantum confinement respectively. Furthermore, we discuss how structural differences in [111] SiNWs mitigate these effects, thereby giving the electronic properties of [111] SiNWs greater stability against various surface passivations than those of [110] SiNWs. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y6.00002: Axial Si/Ge hetero-nanowires for tunneling transistors Son Le, Daniel Perea, Pooya Jannaty, Xu Luo, Shadi Dayeh, Alexander Zaslavsky, Thomas Picraux Modern vapor-liquid-solid (VLS) growth based on alloy catalysts can grow SiGe heteronanowires (hetero-NWs) with controlled axial heterojunction abruptness [1] combined with simultaneous control of material composition (Si and Ge) and doping profile. Previously, we reported on axial in-situ doped Ge NW pn junction tunneling field effect transistors (TFETs) with effective backgate control of the tunneling current [2]. In this presentation, we report on tri-gated p-Ge/i-Si/n-Si axial hetero-NWs TFET with on-state tunneling occuring in the Ge drain section and off-state leakage dominated by the Si junction in the source. The devices have high Ion of 2 uA/um, suppressed ambipolarity, and a sub-threshold slope SS of 140 mV/decade over 4 decades of current with lowest SS of 50 mV/decade. Device operation in the tunneling mode is confirmed by three-dimensional TCAD simulation. In addition, our devices work standard as NW FETs with good Ion/Ioff ratio when the source-drain junction is forward-biased [3]. [1] D. E. Perea et al., Nano Lett 11, 3117 (2011). [2] Son T. Le et al., Appl. Phys. Lett. 96, 262102 (2010). [3] Son T. Le et al., accepted to Nano Lett. (10/2012). [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y6.00003: Probing Interface Band Edge Discontinuity in Single Core-shell Nanowire by Photocurrent Spectroscopy Guannan Chen, Guan Sun, Yujie Ding, Ilio Maccoli, Nico Lovergine, Paola Prete, Jonathan Spanier Group III-V co-axial core-shell semiconducting nanowire (NW) heterostructures possess unique advantages over their planar counterparts in logic, photovoltaic and light-emitting devices. Dimensional confinement of electronic carriers and interface complexity in NWs are known to produce local electronic potential landscapes along the radial direction that deviate from those along the normal to planar heterojunction interfaces. However, understanding of electronic and optoelectronic carrier transport properties and device characteristics remains lacking without a direct measurement of band alignment in individual NWs. Photocurrent spectroscopy has proven to be effective in investigating the effects of quantum confinement and surface related properties such as bandgaps, surface adsorption/desorption, and polarization anisotropy. Here, we report on, using the GaAs/Al$_{x}$Ga$_{1-x}$As core-shell NW system ($x$ = 0.24 and 0.33), how photocurrent and photoluminescence spectroscopies can be used together to construct a band diagram of an individual heterostructure NW with high spectral resolution. This approach and results are relevant for the study of tunable hot electron transfer across NW core-shell interfaces. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y6.00004: Capacitance of Nanowire with different cross sections and different materials at different frequencies Abbas Arab, Qiliang Li During the past half century, feature-size of electronic elements has been reduced dramatically. Semiconductor industry expects this down-scaling to be continued for at least next decade. Among different approaches proposed for reducing the size of electronic elements, is nanowire (NW) based elements such as nanowire field effect transistor (NW-FET). NW approach offers a coaxial gate-dielectric-channel geometry that has advantage of electrostatic control in down-scaling the electronic elements. NWs can be grown in different cross sections depending on the material used as the core of the coaxial structure. Despite so much interest and research on this field, a complete set of study on nanowire capacitance will be very useful for nanoelectronics. In this work, we are going to study different NW structures with different materials and cross sections including: square, triangular, circular and hexagonal in different frequencies. We will study the effect of oxide thickness, oxide material and rotation of cross section, in cases that are not symmetric to rotation, on NW behavior. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y6.00005: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y6.00006: Discrete random distribution of source dopants in nanowire tunnel transistors (TFETs) Somaia Sylvia, M. Abul Khayer, Khairul Alam, Hong-Hyun Park, Gerhard Klimeck, Roger Lake InAs and InSb nanowire (NW) tunnel field effect transistors (TFETs) require highly degenerate source doping to support the high electric fields in the tunnel region. For a target on-current of 1 $\mu A$, the doping requirement may be as high as $1.5\times10^{20}~ \mathrm{cm^{-3}}$ in a NW with diameter as low as 4 nm. The small size of these devices demand that the dopants near tunneling region be treated discretely. Therefore, the effects resulting from the random distribution of dopant atoms in the source of a TFET are studied for 30 test devices. Comparing with the transfer characteristics of the same device simulated with a continuum doping model, our results show (1) a spread of $I-V$ toward the positive gate voltage axis, (2) the same average threshold voltage, (3) an average 62\% reduction in the on current, and (4) a slight degradation of the subthreshold slope. Random fluctuations in both the number and placement of dopants will be discussed. Also, as the channel length is scaled down, direct tunneling through the channel starts limiting the device performance. Therefore, a comparison of materials is also performed, showing their ability to block direct tunneling for sub-10 nm channel FETs and TFETs. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y6.00007: Observation of defect-induced Photoresponse and charge carrier transport in single GeSe2 nanobelt devices Bablu Mukherjee, Eng Soon Tok, Chorng Haur Sow Single crystal GeSe2 nanobelts were grown using chemical vapor deposition techniques. Morphology of the nanostructures was characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD) and Raman spectroscopy. Electronic transport properties, impedance spectroscopy, photoconductive characteristics and temperature-dependent electrical resistivity measurements were carried out on individual GeSe2 nanobelt devices. The photosensitivity of single GeSe2 nanobelt (NB) devices was examined with two different excitation wavelengths of laser beams with photon energies above band gap and at sub-band gap of the NB. A maximum photoconductive gain 10$^6$ \% was achieved at a wavelength of 808 nm. The magnitude of the photocurrent and response time of the individual GeSe2 NB device indicate that the photoresponse could be attributed to the presence of isolated mid band gap defect levels. Temperature dependent photocurrent measurements indicate the rough estimation of the energy levels for the defect states. Localized photostudy shows that the large photoresponse of the device primarily occurs at the metal-NB contact regions. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y6.00008: Finite Element Analysis of lateral charge distribution in ZnO nanowire Javad Usefie Mafahim, Arkadii Krokhin, Arup Neogi The coupling of piezoelectric and semiconducting properties in zinc oxide creates a strain field and charge separation across a nanowire (NW) as a result of an external or internally induced strain. The potential drop along the transverse section of a hexagonal ZnO NW is simulated by the finite element analysis method. The NW is considered to be fixed at one end and laterally deflected at the other with a uniform force on a constant area of cross-section. We numerically simulate the potential drop across a direction transverse to the growth of the NW attached to the substrate. The piezoelectric potentials difference is analyzed as a function of the lateral force, thickness, and aspect ratio of the NW. It is observed that due to a change in the component of the shear force in the transverse direction with respect to the length of the NW, a significant variation of strain in observed in the direction of the lateral force. Our analysis explains previously observed experimental results. It is also shown that the potential difference is influenced by the changing aspect ratio. The charge distribution is also analyzed in a fluid medium with a lateral flow of the liquid. Our results can be used for the design of novel biosensors. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y6.00009: Tailoring electronic properties of SnO$_2$ nanobelts via thermal annealing Timothy Keiper, Jorge Barreda, Joon-Il Kim, Jim P. Zheng, Peng Xiong Metal oxide semiconductors nanowires are a viable option for the fabrication of transistors with desirable characteristics for nanoelectronic and sensing applications. SnO$_2$ nanobelts (NBs) have been synthesized using catalyst-free chemical vapor deposition. The growth parameters have been explored, producing NBs as long as millimeters. These NBs have been demonstrated as effective channel-limited gas [1], pH [2] and protein [3] field-effect transistor (FET) sensors. Through modification of O$_2$ and vacuum thermal annealing conditions, we investigate the control and optimization of the electronic properties of the NBs to achieve desired device characteristics for biosensing applications. A pronounced increase in conductance, up to the order of microsiemens, has been observed in annealed NBs under O$_2$ environment at elevated temperatures above 600$^\circ$C. We also examine the properties of the electrical contacts with different metallization and varying NB conductivity. Optimal device characteristics for various sensing applications will be tested and discussed.\\[4pt] [1] L.L. Fields et al., Appl. Phys. Lett. 88, 263102 (2006).\\[0pt] [2] Yi Cheng et al., Nano Lett. 8, 4179–4184 (2008).\\[0pt] [3] Yi Cheng et al., Biosensors and Bioelectronics 26, 4538-4544 (2011). [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y6.00010: Gated nonlocal transport in sketched oxide nanostructures Shicheng Lu, Guanglei Cheng, Joshua P. Veazey, Patrick Irvin, Feng Bi, Mengchen Huang, Jeremy Levy, Chung-Wung Bark, Sangwoo Ryu, Kwang-Hwan Cho, Chang-Beom Eom The oxide heterostructure LaAlO$_3$/SrTiO$_3$ supports a two-dimensional electron liquid (2DEL) with a variety of competing phases including magnetism, superconductivity and weak antilocalization due to Rashba spin-orbit coupling. Further confinement of this 2DEL into quasi-one-dimensional regime can provide insight into the underlying physics of this system and reveal new behavior. Prior magnetotransport experiments on narrow LaAlO$_3$/SrTiO$_3$ structures created by a conductive atomic force microscope lithography technique have revealed large nonlocal resistances (as large as 10$^4$$\Omega$), with separations between current and voltage that are large compared to the 2D mean-free path. To help understand the origin of this remarkable behavior, we perform electric gating of nanowire structures in order to vary the carrier density and possibly other interactions such as spin-orbit coupling strength. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y6.00011: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y6.00012: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y6.00013: Atomic Hydrogen and Oxygen Adsorptions in Single-Walled Zigzag Silicon Nanotubes Haoliang Chen, Asok Ray \textit{Ab initio }calculations have been performed to study the electronic and geometric structure properties of zigzag Si nanotubes. Full geometry and spin optimizations have been performed without any symmetry constraints with an all electron 3-21G* basis set and the B3LYP functional. The largest zigzag silicon nanotube (12, 0) studied has a binding energy per atom of 3.584eV. Atomic hydrogen and oxygen adsorption on (9, 0) and (10, 0) nanotubes have been studied by optimizing the distances of the adatoms from both inside and outside the tube. The adatom can be placed initially in four adsorption sites- parallel bridge, zigzag bridge, hollow, and on-top site. The on-top site is the most preferred site for hydrogen atom adsorbed on (9, 0) with an adsorption energy of 3.0eV and an optimized distance of 1.49{\AA}. For oxygen adsorption on (9, 0), the most preferred site is the zigzag bridge site with an adsorption energy of 5.987eV. For atomic hydrogen adsorption on (10, 0), the most preferred site is also the on-top site with an adsorption energy of 2.974eV and an optimized distance of 1.49 {\AA}. For adsorption of atomic oxygen on (10, 0), the most preferred site is parallel bridge site with an adsorption energy of 6.275eV. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y6.00014: Topological Effect to Surface Plasmon Excitation in Topological Insulator Nanowires Mingda Li, Wenping Cui, Ju Li, Yimei Zhu, Lijun Wu, Qingping Meng, Weishu Liu, Zhifeng Ren, Ferhat Katmis, Peng Wei, Jagadeesh Moodera, Yong Zhang We present a theoretical investigation of the surface plasmon at the interface between topologically-non-trivial cylindrical core and topological-trivial surrounding material, from the axion electrodynamics and modified constitutive relations. We find that the topological effect lowers the SP energy in any case, while as the diameter of the core becomes smaller, the topological modification to SP energy is reduced. A qualitative picture based on perturbation theory of shifted boundary is given to explain these phenomena, from which we also infer that in order to amplify the topological effect, the difference between the inverse of dielectric constants of two materials must be increased. We also find that when the surrounding material goes magnetic, the magnetism overcomes topological effect, makes the latter seemingly suppressed. What's more, bulk plasmon energy at 17.5 $\pm$ 0.2eV for semiconducting Bi2Se3 nanoparticle is observed from high-resolution Electron Energy Loss Spectrum Image measurements. [Preview Abstract] |
Session Y7: Focus Session: Carbon Nanotubes: Sensor Applications and Gas Absoprtion
Sponsoring Units: DMPChair: Charlie Johnson, University of Pennsylvania
Room: 303
Friday, March 22, 2013 8:00AM - 8:36AM |
Y7.00001: Parts-per-quadrillion Resolution Molecular Sensor Based on Pristine Carbon Nanotubes Invited Speaker: Gugang Chen Single-walled carbon nanotube (SWNT) is probably the ultimate sensor among nanoscale semiconducting materials since a SWNT consists solely of surface so that every single carbon atom is in direct contact with the environment, allowing optimal interaction with nearby molecules. Ironically the ultrahigh sensitivity of SWNTs is easily compromised by various unintentional contaminants from the device fabrication process as well as the ambient environment. Here we show that applying continuous in situ ultraviolet (UV) light illumination during gas detection could dramatically enhance a SWNT-sensor's performance and for the first time achieve parts-per-quadrillion (PPQ) resolution with detection limit as low as 590 PPQ for nitric oxide detection at room temperature [1]. Gas detections on NO$_{\mathrm{2}}$ and NH$_{\mathrm{3}}$ further showed sensitivities 2 to 3 orders of magnitude better than what previously had reported. The much enhanced performance is apparently aroused from the UV light induced sensor surface cleaning. In addition, aiming for practical applications we illustrate how to address gas selectivity by introducing a gate bias. \\[4pt] [1] G. Chen, T. M. Paronyan, E. M. Pigos, and A. R. Harutyunyan, Scientific Reports 2, 343 (2012). [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y7.00002: Carbon nanotube based photon filter for energetic particle detection David Deglau, Stergios Papadakis, Andrew Monica, Bruce Andrews, Donald Mitchell Energetic particles (EP) ejected from a plasma carry important information about the plasma physics. To study remote plasmas in the heliosphere, space-based sensors must be used. Furthermore, only energetic \textit{neutral} atoms (ENAs) can be analyzed, since charged particle trajectories are curved by the electric and magnetic fields of the heliosphere. Because low power consumption and weight are important for spacecraft, solid-state detectors are used. The challenge with solid-state detectors is their sensitivity to light; in all observational regions of interest, photon counts are several orders of magnitude higher than ENA counts. Current state of the art solid-state detectors use ultra-thin metal or carbon films to block the photons. This sets an energy threshold for the ENAs due to the fact that the ENAs have to penetrate this film. We aim to replace the thin films with carbon nanotube (CNT) mats. The CNT mats have a much lower density while maintaining extremely high photon absorption. Thus the CNT mats will act as an excellent filter for blocking the photons while minimally affecting the ENAs of interest. We will describe the fabrication of the CNT mats and their performance characterization by optical spectroscopy and energetic particle spectroscopy using alpha particles as an ENA simulant. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y7.00003: Recyclable Buckysponges for De-emulsification and Oil-spill cleaning Mehmet Karakaya, Deepika Saini, Ramakrishna Podila, Apparao M. Rao Here we present a three dimensional, interconnected, carbon nanotube based, spongy material that is capable of efficiently separating oil from water. The buckysponge, as we term it, exhibits superhydrophobicity and oleophilicity. The adopted facile top down approach allows strong control of the porosity and is easily scalable. Due to capillary action combined with its oleophilicity, a buckysponge is capable of selectively absorbing various organic solvents up to 20 times its weight, a value comparable to existing nanosponge materials. This light weight and highly porous material is shown to work with both free and emulsified oil in water. It is not only an ideal candidate for efficient oil removal but also effective in oil recovery. The absorbed oil can be retrieved by reversibly squeezing the buckysponge, or the oil may be burnt to generate heat energy. Notably, the burnt buckysponge shows no damage to its physical structure or its absorptive properties after squeezing or having the oil burnt, and is therefore re-usable. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y7.00004: Effect of adsorbed monolayers on the conductance of single-walled carbon nanotubes Boris Dzyubenko, Hao-Chun Lee, Oscar Vilches, David Cobden We have studied the effects of adsorbing noble gases He, Ne, Ar, Kr, Xe, and diatomic gases O2, N2 and CO, on the electrical properties of individual suspended single-walled nanotubes, as a function of pressure and temperature. The quantity of gas adsorbed can be determined from the shift in the mechanical resonance frequency of the nanotube. We find that the conductance can be sensitive to small changes in density for all gases and can be measured on a timescale of milliseconds. This opens ways for studying the dynamics of adsorbed atoms/molecules on the surface of a nanotube. For some devices the conductance varies non-monotonically with coverage as a monolayer builds up. The conductance change results at least in part from a very small charge transfer between the adsorbates and nanotube. Measurements below the 2D critical point show sharp features and fluctuations in some devices but not in others. The reason for this is not currently understood. In the nonlinear regime we observe features in the I-V characteristics which occur because electrical currents cause phase transitions on the surface of a nanotube and may lead to stationary nonequilibrium states. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y7.00005: Toward Quantifying the Electrostatic Transduction Mechanism in Carbon Nanotube Biomolecular Sensors Mitchell Lerner, Nicholas Kybert, Ryan Mendoza, Jennifer Dailey, A.T. Charlie Johnson Despite the great promise of carbon nanotube field-effect transistors (CNT FETs) for applications in chemical and biochemical detection, a quantitative understanding of sensor responses is lacking. To explore the role of electrostatics in sensor transduction, experiments were conducted with a set of similar compounds designed to adsorb onto the CNT FET via a pyrene linker group and take on a set of known charge states under ambient conditions. Acidic and basic species were observed to induce threshold voltage shifts of opposite sign, consistent with gating of the CNT FET by local charges due to protonation or deprotonation of the pyrene compounds by interfacial water. The magnitude of the gate voltage shift was controlled by the distance between the charged group and the CNT. Additionally, functionalization with an uncharged pyrene compound showed a threshold shift ascribed to its molecular dipole moment. This work illustrates a method for producing CNT FETs with controlled values of the turnoff gate voltage, and more generally, these results will inform the development of quantitative models for the response of CNT FET chemical and biochemical sensors. As an example, the results of an experiment detecting biomarkers of Lyme disease will be discussed in the context of this model. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y7.00006: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y7.00007: Measurements of adsorbate binding on individual suspended carbon nanotubes Hao Chun Lee, Boris Dzyubenko, Jim Coy, David Cobden, Oscar Vilches By measuring the resonance frequency shift and the conductance change of vibrating suspended single-walled nanotubes at controlled temperature and pressure we can accurately detect the adsorption of gases including He, Ar, Kr, Xe, O2, and N2. The binding energy can then be determined from the low-coverage part of the adsorption isotherms. We find that the adsorption isotherms generally resemble those on graphite but with weaker binding energies, allowing access to behavior at lower two-dimensional (2D) chemical potential than on graphite. For He-4 the binding energy is reduced by as much as a factor of two. For Ar the binding energy on all nanotubes measured is in the range 700 - 800 K, about a third less than that on graphite. This enables us to investigate the 2D critical and triple points of Ar. Puzzlingly, we find that the devices fall into two classes: one with monolayer condensation at lower pressures and sharp 2D liquid-vapor transitions, the other with condensation at higher pressures and lacking sharp transitions even well below the 2D critical point. Possible factors that may be involved are finite-size effects, commensurability, absorption on the inside of nanotubes with holes in them, nanotube bundles containing more than one kind of nanotube and having surface grooves, and amorphous carbon or other contaminants on the surface, though no combination of these factors seems to provide a satisfactory explanation. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y7.00008: Study of Carbon Dioxide adsorption on Purified HiPco Nanotubes Shree Banjara, Vaiva Krungleviciute, Aldo Migone We have investigated the adsorption characteristics of carbon dioxide on purified HiPco single-walled carbon nanotubes. We measured four full isotherms (starting from zero CO$_{2}$ coverage an ending at the saturated vapor pressure) for temperatures between 150 K and 187 K. While a linear plot of the adsorption isotherms presents initially a relatively broad region of rapid coverage increase with pressure, logarithmic plots of the isotherms are characterized by the absence of any substeps in the data. The equilibration times for each point along the isotherms are much longer than those for other simple adsorbates (e.g., CH$_{4}$ or Ar) on the same sorbent. Results for the effective monolayer capacity as well as values for the isosteric heat of adsorption's dependence on sorbent loading will be presented. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y7.00009: Protein coronas of Graphene and Carbon Nanotubes Ramakrishna Podila, Pu Chun Ke, Jared Brown, Apparao Rao We explored the effects of protein coating on the optical and vibrational properties of single-walled carbon nanotubes (SWCNTs) and bi- and few layer graphene nanosheets using micro-Raman spectroscopy, UV-visible absorption and electron microscopy. We found that bovine serum albumin (BSA) forms a hard corona on the surfaces of both graphene and SWCNTs. Our results suggest that the BSA hard corona acted as a weak acceptor to facilitate charge transfer from the carbon nanostructures. Notably, we observed that charge transfer occurred only in the case of SWNTs possibly due to their sharp and discrete electronic density of states. On the contrary, we find that graphene did not show a similar charge transfer due to its continuous energy dispersion. Furthermore, the nanostructures induced significant changes in the secondary structure of the BSA by relaxing their external ?-helices. These results are expected to guide controlled nanostructure-biomolecule interactions and prove beneficial in developing benign nanomaterials. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y7.00010: Optimization of a carbon nanotube field emission electron gun for applications in mass spectrometry Adrian Southard, Stephanie Getty, Daniel Glavin, Gregory Hidrobo, Steven Feng, Nicholas Costen, Carl Kotecki Field emission electron guns composed of carbon nanotube (CNT) pillar arrays make a low power, robust field emission source with turn-on fields as low as 1.8 Volts/$\mu $m. Fowler-Nordheim fits to the current-voltage data exhibit field enhancement factors of greater than 1000. Scaling of a carbon nanotube field emission electron gun to an aspect ratio of 2 mm x 40 mm using MEMS fabrication techniques has increased emitted current by two orders of magnitude beyond previous designs up to a current of 0.7 mA. Enhanced sensitivity from a time-of-flight mass spectrometer compatible with such a source was also obtained. Finite difference simulations (SIMION) of emission from CNT pillar arrays indicate that the field enhancement factors measured in the experiments can't be explained by emission from smooth pillars and must be due to emission from CNTs that protrude from the top of the pillar. SIMION simulations also explain why much of the emitted current is absorbed by the extraction grid using the current geometry and provide methods for improving electron beam transmission through the addition of a second grid. Simulations of electron beam focusing also demonstrate how the addition of a second grid could enable better focusing of the electron beam. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y7.00011: Neon and Xenon adsorption on opened carbon nanohorns Carl Ziegler, Vaiva Krungleviciute, Aldo Migone, Masako Yudasaka, Sumio Iijima Adsorption isotherms were measured for neon adsorbed on opened (oxidized) carbon nanohorn aggregates. The isotherms were performed at eleven different temperatures between 19 to 40 K. Two distinct substeps are present in logarithmic plots of the adsorption data. The two substeps correspond to high and low binding energy sites present in the nanohorn aggregates. The values of the isosteric heat as a function of substrate loading was calculated; it shows features corresponding to the two adsorption isotherm substeps. The results for neon will be compared to those from ongoing measurements for xenon adsorbed on the same sample of open carbon nanohorn aggregates as well as to a previous study of neon on closed carbon nanohorns. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y7.00012: Phase transition of adsorbed noble gas on suspended graphene Zaiyao Fei, Hao-Chun Lee, Boris Dzyubenko, Sanfeng Wu, David Cobden Suspended graphene sheets are simultaneously 2D nanomechanical resonators, hosts to massless Dirac electrons, and 2D substrates for adsorption. Adsorption is expected to modulate the mechanical and elctrical properties in a number of ways. We therefore aim to investigate the effects of equilibrium adsorbates on the vibrational resonances and on the conductance. Beginning with noble gases on non-suspended graphene exfoliated on SiO2, for argon we have seen a gradual change in the conductance as a function of vapor pressure at temperatures below the 2D critical point (54 K), indicating gradual formation of a monolayer over a wide chemical potential range (although we have also seen signs of a sharp monolayer phase transition in a least one sample). The mechanism of conductance modulation is a topic of interest. The large broadening of the expected 2D vapor-liquid step is likely to be due to inhomogeneous binding caused by charge disorder, roughness, and other properties of the SiO2 substrate. We are developing pristine suspended graphene devices to eliminate these complications. [Preview Abstract] |
Session Y8: Electron-electron Interactions and Unconventional Structures
Sponsoring Units: DCMPChair: Sumit Mazumdar, University of Arizona
Room: 307
Friday, March 22, 2013 8:00AM - 8:12AM |
Y8.00001: Modeling graphene interactions beyond pairwise additivity John Dobson, Tim Gould Dispersion (van der Waals) interactions between graphenic systems are commonly modeled by summing energy contributions between pairs of atoms or `''elements''. This pairwise assumption is now known to be inaccurate for such highly polarizable, highly anisotropic systems [1-5]. Many-electron correlation theories of RPA type [6] are more accurate, but are computationally intensive. Here we present a relatively simple type of model, based on long-wavelength RPA dielectric function data for stretched bulk graphite, that captures the non-additive physics. [1] J. F. Dobson, A. White, and A. Rubio, Phys. Rev. Lett. 96, 073201 (2006) [2] H. Y. Kim, J. O. Sofo, D. Velegol, M. W. Cole, and A. A. Lucas, J. Chem. Phys. 124, 074504 (2006) [3] A. White and J. F. Dobson, Phys. Rev. B 77, 075436 (2008) [4] A. J. Misquitta, J. Spencer, A. J. Stone, and A. Alavi, Phys. Rev. B 82, 075312 (2010) [5] R.-F. Liu, J. G. Angyan and J. F. Dobson, J. Chem. Phys. 134, 114106 (2011) [6] S. Lebegue, J. Harl, T. Gould, J. G. Angyan, G. Kresse, and J. F. Dobson, Phys. Rev Lett. 105, 196401 (2010) [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y8.00002: Charge-Carrier Screening in Single-Layer Graphene David Siegel, William Regan, Alexei Fedorov, Alex Zettl, Alessandra Lanzara Unlike normal metals that have a true Fermi surface, the pointlike Fermi surface of undoped graphene allows for long-ranged coulomb interactions to be unscreened by free charges, leading to singular behaviors. Therefore, the introduction of charge to a neutral graphene sheet can have a profound effect on transport properties and device performance. In this talk I will demonstrate the effects of charge-carrier screening of the electron-electron and electron-impurity interactions on the electronic properties of graphene, as we have observed through angle-resolved photoemission spectroscopy (ARPES). These observations help us to understand the basis for the transport properties of graphene, and shed light on the fundamental physics in the vicinity of the Dirac point crossing. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y8.00003: Evidence for strong electron correlations in graphene molecular fragments: Theory and experiments on two-photon absorptions Karan Aryanpour, Adam Roberts, Arvinder Sandhu, Alok Shukla, Sumit Mazumdar Historically, the occurrence of the lowest two-photon state below the optical one-photon state in linear polyenes, polyacetylenes and polydiacetylenes provided the strongest evidence for strong electron correlations in these linear $\pi$-conjugated systems. We demonstrate similar behavior in several molecular fragments of graphene with $D_{6h}$ symmetry, theoretically and experimentally. Theoretically, we have calculated one versus two-photon absorptions in coronene, two different hexabenzocoronenes and circumcoronene, within the Pariser-Parr-Pople $\pi$-electron Hamiltonian using high order configuration interaction. Experimentally, we have performed z-scan measurements using a white light super-continuum source on coronene and hexa-peri-hexabenzocoronene to determine frequency-dependent two-photon absorption coefficients, for comparison to the ground state absorptions. Excellent agreement between experiment and theory in our work gives strong evidence for significant electron correlations between the $\pi$-electrons in the graphene molecular fragments. We particularly benchmark high order electron-hole excitations in graphene fragments as a key element behind the agreement between theory and experiment in this work. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y8.00004: Manipulating molecule-substrate exchange interactions via graphene Sumanta Bhandary, Olle Eriksson, Biplab Sanyal Organometallic molecules with a 3d metal center carrying a spin offers many interesting properties, e.g., existence of multiple spin states [1]. A recent interest has been in understanding the magnetic exchange interaction between these organometallic molecules and magnetic substrates both from experiments and theory [2]. In this work, we will show by calculations based on density functional theory how the exchange interaction is mediated via graphene in a geometry containing iron porphyrin(FeP)/graphene/Ni(111). The exchange interaction varies from a ferromagnetic to an antiferromagnetic one depending on the lattice site and type of defect in the graphene lattice along with the switching of spin state of Fe in FeP between S=1 and S=2, which should be detectable by x-ray magnetic circular dichroism experiments. This scenario of complex magnetic couplings with large magnetic moments may offer a unique spintronic logic device.\\[4pt] [1] S. Bhandary, S. Ghosh, H. Herper, H. Wende, O. Eriksson and B. Sanyal, Phys. Rev. Lett. {\bf 107}, 257202 (2011).\\[0pt] [2] H. Wende {\it et al.}, Nat. Mater. {\bf 6}, 516 (2007). [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y8.00005: Giant capacitance of a plane capacitor with a two-dimensional electron gas in a magnetic field Brian Skinner, Boris Shklovskii If a clean two-dimensional electron gas (2DEG) with small concentration comprises one (or both) electrodes of a plane capacitor, the resulting capacitance can be larger than the ``geometric capacitance" defined by the physical separation between electrodes. Such capacitance enhancement is a hallmark of the positional correlations that arise between electrons within the 2DEG at low electron density. Here we show that in the presence of a strong perpendicular magnetic field, such correlations are enhanced, leading to unusually large capacitance even for systems where the effective Bohr radius is large. The effect is perhaps most dramatic for ultrathin graphene-based capacitors, where strongly-correlated electron states appear at small filling factors, even though in the absence of magnetic field such correlated states are normally precluded by graphene's Dirac-like kinetic energy spectrum. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y8.00006: Electron-electron Interaction and Thermoelectricity in Graphene Fereshte Ghahari, Yuri Zuev, Carlos Forsythe, Kenji Watanabe, Takashi Taniguchi, Philip Kim In this presentation, we report thermoelectric power (TEP) measurements on graphene samples deposited on hexagonal boron nitride substrates where drastic suppression of disorder is achieved. Our results show that at high temperatures where the inelastic scattering rate due to electron-electron (e-e) interactions is higher than the disorder induced elastic scattering rate,~ the measured TEP deviates from the Mott relation, and can be explained by a non-relativistic hydrodynamic flow of electrons. We also investigated TEP in the quantum Hall regime at a high magnetic fields, where we observed symmetry broken integer quantum Hall due to the strong e-e interactions. The field dependence of TEP at these states reveals the important role that exchange interactions play. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y8.00007: Interface Inducing Interesting Effects on Thermal Transport in Graphene Based Systems Haiyuan Cao, Hongjun Xiang, Xingao Gong Using nonequilibrium molecular dynamics method (NEMD), we have studied how the interface affecting the thermal conductivity in multilayer graphene nanoribbons and the graphene grain boundaries. In multilayer graphene nanoribbons, the monotonous decrease of the thermal conductivity with the increase of the number of layers can be attributed to the phonon resonance effect of out-of-plane phonon modes. The reduction of thermal conductivity is proportional to the layer size, which is caused by the increase of phonon resonance. The results clearly show the dimensional evolution of thermal conductivity from quasi-one dimension to higher dimensions in graphene nanoribbons. The thermal transport across the asymmetric tilt grain boundary between armchair and zigzag graphene has also been investigated by simulations. We have observed significant temperature drop and ultra-low temperature-dependent thermal boundary resistance. More importantly, we find an unexpected thermal rectification phenomenon. The thermal conductivity and Kapitza conductance is direction-dependent. The effect of thermal rectification could be amplified by increasing the difference of temperature imposed on two sides. Our results show the interface phonon coupling could greatly change the thermal conductivity. Besides that, we have proposed a new promising kind of thermal rectifier and phonon diode based on the asymmetric interface in graphene. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y8.00008: Direct visualization of reversible dynamics in a Si$_6$ magic cluster in a graphene pore Jaekwang Lee, Wu Zhou, Stephen Pennycook, Juan-Carlos Idrobo, Sokrates Pantelides Clusters containing only a handful of atoms have been the subject of extensive theoretical and experimental studies, but direct imaging of their structure and dynamics has not been possible so far, with information provided mainly by theory. We report a direct atomically-resolved observation of a single Si$_6$ magic cluster trapped in a graphene nanopore. We report a sequence of images that show a reversible, oscillatory, conformational change: one of the Si atoms jumps back and forth between two different positions. Density functional theory shows that the cluster is exploring metastable configurations under the influence of the beam providing direct information on the atomic-scale energy landscape. The capture of a magic cluster in a graphene nanopore suggests the possibility of patterning nanopores and either capturing or assembling atomic clusters with a potential for applications. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y8.00009: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y8.00010: Brownian Dynamics Simulations of Dispersed Graphene Sheets Yueyi Xu, Micah Green Past simulations of the dynamics of dispersed graphene sheets are limited to static fluids on small timescales, with little attention devoted to flow dynamics. To address this need, we investigated how flow fields affect graphene morphology dynamics using a coarse-grained model; this relatively untouched area is critical given the importance of graphene solution-processing of multifunctional devices and materials. In particular, we developed a Brownian Dynamics (BD) algorithm to study the morphology of sheetlike macromolecules in dilute, flowing solutions. We used a bead-rod lattice to represent the mesoscopic conformation of individual two dimensional sheets. We then analyzed the morphology dynamic modes (stretching, tumbling, crumpling) of these molecules as a function of sheet size, Weissenberg number, and bending stiffness. Our results indicate the model can successfully simulate a range of dynamic modes in a given flow field and yield fundamental insight into the flow processing of graphene sheets. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y8.00011: Temperature-dependent levitation of a graphene flake due to Casimir forces Anh Phan, David Drosdoff, Lilia Woods, Igor Bondarev, Nguyen Viet We present theoretical investigations of temperature-dependent Casimir interactions of a graphene flake between substrates in a fluid. By properly choosing the materials, we propose that the graphene can be suspended in the fluid due to the balance between the Casimir, buoyancy and gravitational forces. The graphene properties, such as the Dirac-like nature of the carriers and universal optical conductivity, have a profound effect on the Casimir force making it completely thermal at room temperature. Since thermal contributions to the Casimir interaction in most materials are usually small, the graphene system offers a unique opportunity to demonstrate such effects without going to extreme temperatures. We show that the equilibrium position of the suspended flake is temperature dependent. We suggest that this maybe a promising system for observing thermal Casimir effects via levitation. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y8.00012: Properties of field-effect transistors of CVD grown MoS$_2$ single atomic layers on CVD grown hexagonal Boron Nitride Nihar Pradhan, Daniel Rhodes, Qiu Zhang, Ana Elias, N. Lopez, Zheng Liu, Sina Najmei, Jun Lou, Saikat Talapatra, Mauricio Terrones, Pulickel Ajayan, Luis Balicas Two dimensional crystalline layered materials such as MoS$_2$, WS$_2$, have recently become an intense focus of research activities due to their exceptional electronic and optical properties. A single- or a few atomic layers of these materials show quite promising charge conduction characteristics, such as large mobility or fast on/off switch ratios, which lead to a few recent examples of integrated circuits based on these materials. Here, we will present a comparison among the electronic transport properties of, either mechanically exfoliated or CVD grown MoS$_2$ under different substrates, i.e. on SiO$_2$, on exfoliated or on CVD grown h-BN, and suspended. We will also discuss results obtained from back and top gated configurations with different dielectrics. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y8.00013: Nanochannel Device with Embedded Nanopore: a New Approach for Single-Molecule DNA Analysis and Manipulation Yuning Zhang, Walter Reisner Nanopore and nanochannel based devices are robust methods for biomolecular sensing and single DNA manipulation. Nanopore-based DNA sensing has attractive features that make it a leading candidate as a single-molecule DNA sequencing technology. Nanochannel based extension of DNA, combined with enzymatic or denaturation-based barcoding schemes, is already a powerful approach for genome analysis. We believe that there is revolutionary potential in devices that combine nanochannels with embedded pore detectors. In particular, due to the fast translocation of a DNA molecule through a standard nanopore configuration, there is an unfavorable trade-off between signal and sequence resolution. With a combined nanochannel-nanopore device, based on embedding a pore inside a nanochannel, we can in principle gain independent control over both DNA translocation speed and sensing signal, solving the key draw-back of the standard nanopore configuration. We demonstrate that we can optically detect successful translocation of DNA from the nanochannel out through the nanopore, a possible method to 'select' a given barcode for further analysis. In particular, we show that in equilibrium DNA will not escape through an embedded sub-persistence length nanopore, suggesting that the pore could be used as a nanoscale window through which to interrogate a nanochannel extended DNA molecule. Furthermore, electrical measurements through the nanopore are performed, indicating that DNA sensing is feasible using the nanochannel-nanopore device. [Preview Abstract] |
Session Y9: Invited Session: Spin Mechanics
Sponsoring Units: GMAGChair: Sebastian Goennenwein, Bayerische Akademie der Wissenschaften
Room: 308
Friday, March 22, 2013 8:00AM - 8:36AM |
Y9.00001: Autonomous and forced dynamics in a spin-transfer nano-oscillator: Quantitative magnetic-resonance force microscopy Invited Speaker: Olivier Klein In this talk, we will discuss how magnetic-resonance force microscopy, can provide quantitative measurement of the power emitted by a spin-transfer nano-oscillator, consisting of a normally magnetized Py|Cu|Py circular nanopillar, excited both in the autonomous and forced regimes.\footnote{A. Hamadeh, et al. PHYSICAL REVIEW B 85, 140408(R) (2012)} From the power behavior in the subcritical region of the autonomous dynamics, one obtains a quantitative measurement of the threshold current and of the noise level. Their field dependence directly yields both the spin torque efficiency acting on the thin layer and the nature of the mode which first auto-oscillates: the lowest energy, spatially most uniform spin-wave mode. We will then demonstrate that the observed spin-wave spectrum in the forced regime critically depends on the method of excitation. While the spatially uniform radio-frequency (RF) magnetic field excites only the axially symmetric modes having azimuthal index $\ell=0$, the RF current flowing through the nano-pillar, creating a circular RF Oersted field, excites only the modes having azimuthal index $\ell=+1$.\footnote{V.V. Naletov et al. PHYSICAL REVIEW B 84, 224423 (2011)} It is then demonstrated that in order to phase lock this auto-oscillating mode, the external source must have the same spatial symmetry as the mode profile, i.e., a uniform microwave field must be used rather than a microwave current flowing through the nanopillar. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 9:12AM |
Y9.00002: Magneto-mechanical detection and control of the nanoscale Barkhausen effect Invited Speaker: Mark Freeman Developments in nano- and spin mechanics are driving a resurgence of interest in mechanical approaches to magnetometry. Torque methods for measurement of quasi-static magnetization or detection of spin dynamics can very fruitfully be miniaturized for application to individual magnetic nanostructures, and are complementary to magnetic force microscopy and related techniques [1]. We report a complete study of the Barkhausen effect in torsional magnetometry measurements of a micromagnetic disk. The discovery of Barkhausen noise in 1919 [2] provided the first experimental evidence of ferromagnetic domains. Within three decades elegant experiments had been performed on individual domain walls and a firm qualitative understanding had emerged [3]. Quantitative treatments of the effect have relied on statistical analysis [4], due to the collective nature of domain wall pinning by many sites. However, a vortex core effectively localizes the domain wall to the scale of an individual pining site, thereby converting the Barkhausen effect into a quantitative 2D nanoscale probe of local energetics in thin magnetic films [5]. In addition to characterization of the intrinsic disorder in a polycrystalline film, point-like tailoring of the energy landscape through low dose focussed ion beam implantation is demonstrated, and can be exploited to tune the properties of integrated magneto-mechanical devices.\\[4pt] [1] J.P. Davis et al., Appl. Phys. Lett. {\bf 96}, 072513 (2010) and New J. Phys. {\bf 12}, 093033 (2010). \newline [2] H. Barkhausen, Phys Z. {\bf 20}, 401 (1919). \newline [3] C. Kittel, Rev. Mod. Phys. {\bf 21}, 541 (1949). \newline [4] B. Alessandro et al., J. Appl. Phys. {\bf 68}, 2901 (1990). \newline [5] J.A.J. Burgess et al., arxiv.org/abs/1208.3797, and to be published. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:48AM |
Y9.00003: Coherent mechanical control of a single electronic spin Invited Speaker: Michael Grinolds Quantum control of spins via electrical, magnetic, and optical means has generated numerous applications in metrology and quantum information technology. In this talk we present an alternative control scheme that uses the mechanical motion of a resonator to coherently control spins. Specifically, by coupling the motion of a magnetically coated mechanical oscillator to a single nitrogen-vacancy (NV) defect in diamond, we demonstrate manipulations of both the amplitude and phase of the NV's electronic spin. Coherent control is achieved by synchronizing NV-addressing optical and microwave manipulations to the driven motion of the coupled mechanical oscillator, which additionally allows for a stroboscopic readout of the resonator's motion. We demonstrate applications of this mechanical spin control to sensitive nanoscale scanning magnetometry and discuss the potential for sensitive motion sensing of nanomechanical resonators. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:24AM |
Y9.00004: Laser-induced magnetization switching in ferrimagnetic alloys Invited Speaker: Andrei Kirilyuk This talk will discuss the recent studies of ultrafast switching of magnetization and the role of angular momentum in this process in ferrimagnetic rare-earth - transition metal alloys, e.g. GdFeCo, where both magnetization and angular momenta are temperature dependent. It has been experimentally demonstrated that the magnetization can be manipulated and even reversed by a single 40 fs laser pulse, without any applied magnetic field [1]. This switching is found to follow a novel reversal pathway [2], that is shown to depend crucially on the net angular momentum, reflecting the balance of the two opposite sublattices [3,4]. In particular, optical excitation of ferrimagnetic GdFeCo on a time-scale pertinent to the characteristic time of the exchange interaction between the rare earth (RE) and transition metal (TM) spins, i.e. on the time scale of tens of femtoseconds, pushes the spin dynamics into a yet unexplored regime, where the two exchange coupled magnetic sublattices demonstrate substantially different dynamics [3]. As a result, the reversal of spins appears to proceed via a novel transient state characterized by a ferromagnetic alignment of the Gd and Fe magnetic moments, despite their ground-state antiferromagnetic coupling [4]. This process is fully modeled by a system of coupled equations for the longitudinal relaxation of the sublattices [5]. The role of light helicity in this process, being a controversial issue for many years, is clarified as well [6].\\[4pt] [1] C.D. Stanciu et al., Phys. Rev. Lett. 99, 047601 (2007).\\[0pt] [2] K. Vahaplar et al., Phys. Rev. Lett. 103, 117201 (2009).\\[0pt] [3] I. Radu et al., Nature 472, 205 (2011)\\[0pt] [4] T.A. Ostler et al., Nature Comm. 3, 666 (2012).\\[0pt] [5] J.H. Mentink et al., Phys. Rev. Lett. 108, 057202 (2012).\\[0pt] [6] A.R. Khorsand et al., Phys. Rev. Lett. 108, 127205 (2012). [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 11:00AM |
Y9.00005: Spin-current generation arising from mechanical motions Invited Speaker: Mamoru Matsuo Spin current, the flow of spins, is a key concept in the field of spintronics.\footnote{S. Maekawa, S. O. Valenzuela, E. Saitoh, and T. Kimura ed. ``Spin Current,'' Oxford University Press (2012).} To create and control spin currents, magnetic dynamics, electromagnetic fields, and thermal gradient have been used. Recently, the acoustically generated spin current was observed in an insulating ferromagnet.\footnote{K. Uchida et al., Nat. Mater. 10, 737 (2011).} However, the conversion between mechanical motions and the spin current in non-magnetic materials has not been studied so far. In this talk, we will present our recent results on spin-current generation from mechanical motions, including rigid and elastic motions in non-magnetic metals and semiconductors. In a rigidly accelerating body, the spin-orbit interaction (SOI) is modulated by the mechanical motion.\footnote{M. Matsuo, J. Ieda, E. Saitoh, and S. Maekawa, Phys. Rev. Lett 106, 076601 (2011); Appl. Phys. Lett. 98, 242501 (2011); Phys. Rev. B 84, 104410 (2011).} The augmented SOI leads to the spin-current generation from both mechanical rotation and vibration. On the other hand, in the presence of the surface acoustic wave (SAW), the elastically driven rotational motion of the lattice couples to electron spins and the spin current is generated in the direction of depth. Dependence of amplitude and frequency of the SAW, the spin diffusion length, and elastic parameters on the spin current will be shown. We will also discuss the enhancement of the SOI and the spin-rotation coupling caused by an interband mixing, using an extended k.p perturbation with the gauge potential due to mechanical rotation.\footnote{M. Matsuo, J. Ieda, and S. Maekawa, arXiv:1211.0127.} [Preview Abstract] |
Session Y10: Invited Session: Advances in Actinide Measurement Techniques
Sponsoring Units: GIMSChair: Jason Cooley, Los Alamos National Laboratory
Room: 309
Friday, March 22, 2013 8:00AM - 8:36AM |
Y10.00001: Anomalous thermodynamic behavior in actinides Invited Speaker: Arkady Shekhter The thermal expansion of some of the actinides metals are strongly dependent upon doping. Extreme examples involve a change of sign of the thermal expansion coefficient upon few percent Ga doping. In contrast, resonant ultrasound spectroscopy of these doping series reveals very weak dependence of the elastic moduli on Ga content. We suggest that the anomalous thermodynamic behavior in these systems has dynamic rather than static origin. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 9:12AM |
Y10.00002: Multiconfigurational nature of 5f orbitals in uranium and plutonium and their intermetallic compounds Invited Speaker: Corwin Booth The structural, electronic, and magnetic properties of U and Pu elements and intermetallics remain poorly understood despite decades of effort, and currently represent an important scientific frontier toward understanding matter. The last decade has seen great progress both due to the discovery of superconductivity in PuCoGa$_5$ and advances in theory that finally can explain fundamental ground state properties in elemental plutonium, such as the phonon dispersion curve, the non-magnetic ground state, and the volume difference between the $\alpha$ and $\delta$ phases. A new feature of the recent calculations is the presence not only of intermediate valence of the Pu 5f electrons, but of multiconfigurational ground states, where the different properties of the $\alpha$ and $\delta$ phases are primarily governed by the different relative weights of the 5f$^4$, 5f$^5$, and 5f$^6$ electronic configurations. The usual method for measuring multiconfigurational states in the lanthanides is to measure the lanthanide $L_{III}$-edge x-ray absorption near-edge structure (XANES), a method that is severely limited for the actinides because the spectroscopic features are not well enough separated. Advances in resonant x-ray emission spectroscopy (RXES) have now allowed for spectra with sufficient resolution to resolve individual resonances associated with the various actinide valence states. Utilizing a new spectrometer at the Stanford Synchrotron Radiation Lightsource (SSRL), RXES data have been collected that show, for the first time, spectroscopic signatures of each of these configurations and their relative changes in various uranium and plutonium intermetallic compounds. In combination with conventional XANES spectra on related compounds, these data indicate such states may be ubiquitous in uranium and plutonium intermetallics, providing a new framework toward understanding properties ranging from heavy fermion behavior, superconductivity, and intermediate valence to mechanical and fundamental bonding behavior in these materials. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:48AM |
Y10.00003: Observation of $^{239}$Pu NMR in PuO$_{2-}$A new frontier for the physics and chemistry of plutonium compounds Invited Speaker: Yasuoka Hiroshi In actinide science, in general, NMR studies have been forced to limit their scope to nuclei associated with ligand atoms. The only exception of direct observation of NMR in actinide nuclei is that of $^{235}$U NMR in UO$_{2}$. There have been extensive efforts to realize NMR in actinide compounds since the electronic properties of these materials are predominantly governed by the actinide atom itself. We report the first observation of Nuclear Magnetic Resonance (NMR) on the $^{239}$Pu nucleus in any material. Our $^{239}$Pu NMR measurements were performed on plutonium dioxide, PuO$_{2}$, for a wide range of external magnetic field values (Ho$=$3$\sim $8T) at a temperature of T$=$4K. By mapping the external field dependence of the measured resonance frequency, we determined the nuclear gyromagnetic ratio to be $^{239}\gamma_{n}$(PuO$_{2})=$2.856 $\pm$ .001 MHz/T. Assuming a free ion value for the Pu$^{4+}$ hyperfine coupling constant, we estimated a bare value of $^{239}\gamma_{n}=$2.29MHz/T for the $^{239}$Pu nucleus, hence a nuclear magnetic moment of $\mu _{n}=$.15$\mu_{N}$ (where $\mu_{N}$ is the nuclear magneton). Our findings put an end to a fifty-year long search for Pu NMR and open potentially a new horizon for the solid state physics, nuclear materials science and complex chemistry in Pu compounds.\\[4pt] Work done in collaboration with G. Koutroulakis, S. Richmond, K. Veirs, E. D. Bauer, J. D. Thompson, G. Jarvinen, and D. L. Clark, Los Alamos National Laboratory, Los Alamos, NM. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:24AM |
Y10.00004: An instrument for the investigation of actinides with spin resolved photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy Invited Speaker: James Tobin A new system [1] for spin resolved photoelectron spectroscopy [2,3] and bremsstrahlung isochromat spectroscopy [4] has been built and commissioned at Lawrence Livermore National Laboratory for the investigation of the electronic structure of the actinides. Actinide materials are very toxic and radioactive and therefore cannot be brought to most general user facilities for spectroscopic studies. The technical details of the new system and preliminary data obtained therein will be presented and discussed. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy (DOE), National Nuclear Security Administration under Contract DE-AC52-07NA27344. This work was supported by the U.S. Department of Energy, Office of Basic Energy Science, Division of Materials Sciences and Engineering.\\[4pt] [1] S.-W. Yu, J. G. Tobin, and B. W. Chung, Rev. Sci. Instrum. \textbf{82}, 093903 (2011).\\[0pt] [2] S.W. Yu and J. G. Tobin, Phys. Rev. B \textbf{77}, 193409 (2008).\\[0pt] [3] J.G. Tobin, S.W. Yu, T. Komesu, B.W. Chung, S.A. Morton, and G.D. Waddill, EuroPhysics Letters \textbf{77}, 17004 (2007).\\[0pt] [4] J.G. Tobin and S.-W. Yu, Phys. Rev. Lett, \textbf{107}, 167406 (2011). [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 11:00AM |
Y10.00005: Transuranic Photoemission Using a Unique Light Source Invited Speaker: John Joyce There has been a remarkable advance in the understanding of electronic structure for complex materials in recent years. Much of this advance in understanding has been realized through advanced spectroscopy capabilities available at public synchrotron facilities. While the vast majority of materials can take advantage of facilities at public synchrotrons, transuranic materials are excluded from these facilities when multiple containment barriers are incompatible with the chosen spectroscopy. We have developed an advanced spectroscopy capability at Los Alamos for photoemission on transuranic materials including Pu. Using several different variants of photoemission we have explored a wide range of Pu materials which has lead to a significant improvement in our understanding of transuranic electronic structure. Examples of these successes will be given along with details of the unique facility. Using the unique capabilities of our transuranic photoemission system we exploit opportunities in angle-resolved photoemission (ARPES) providing insight into the details of both the energy and crystal momentum for a material. Additional information is obtained using tunable photons which may be used to isolate the 5f electron contribution to the valence electronic structure. Between ARPES and tunable photoemission, one may construct a fairly detailed picture of the bonding and hybridization for transuranic materials. By adding temperature-dependent (10 - 350K) photoemission to the suite of tools, we may cross over phase transition boundaries as well as quantify electron-phonon coupling. We also have the capability for 1.5 and 3 KeV core-level spectroscopy using a monochromatized x-ray source. By combining the above photoemission tools with a variety of surface preparation capabilities including cleaving, laser ablation, and thermal desorption, we have a flexible and capable spectroscopy facility that provides unique insight into the electronic structure of transuranic materials. [Preview Abstract] |
Session Y11: Glassy and Amorphous Systems, Including Quasicrystals followed by Epitaxial Growth and Structure of Oxides
Sponsoring Units: DCMPChair: Punit Boolchand, University of Cinncinatti
Room: 310
Friday, March 22, 2013 8:00AM - 8:12AM |
Y11.00001: Hydrogen microstructure of amorphous silicon via inversion of nuclear magnetic resonance spectra: A moment-based approach Parthapratim Biswas, Rajendra Timilsina We present an inverse approach for reconstructing hydrogen microstructure in amorphous silicon (a-Si). The approach consists of generating a prior distribution (of spins) by inverting experimental nuclear magnetic resonance (NMR) data, which is subsequently superimposed on a network of a-Si. The resulting network is then relaxed using a total-energy functional to obtain a stable, low-energy configuration such that the initial spin distribution is minimally disturbed. The efficacy of this approach is demonstrated by generating model configurations that not only have the correct NMR spectra but also satisfy simultaneously the experimental structural, electronic and vibrational properties of hydrogenated amorphous silicon. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y11.00002: Antiferromagnetic order in the Cd$_6$\textit{R} (\textit{R} $=$ rare earth) quasicrystal approximants Alan Goldman, Min Gyu Kim, Guillaume Beutier, Andreas Kreyssig, Takanobu Hiroto, Tsunetomo Yamada, Jong Woo Kim, Marc de Boissieu, Ryuji Tamura Many theoretical treatments of spins on aperiodic lattices support the notion of long-range antiferromagnetic order. However, to date, there has been no experimental confirmation of long-range magnetic order in quasicrystalline systems. The absence of long-range magnetic order extends to crystalline approximant phases of the icosahedral structures as well. Surprisingly, the 1/1 approximant to the Cd-Mg-R icosahedral phases, Cd$_{6}R$, appears to be an exception to the rule. Here, we report on the results of x-ray resonant magnetic scattering measurements on Cd$_{6}R$ approximants which show that long range antiferromagnetic order is, indeed, realized. For $R \quad =$ Tb and Ho, viewing the structure as a body-centered cubic packing of Tsai clusters, we find that the $R$ ions associated with the icosahedral cluster at the corner of the unit cell are antiferromagnetically correlated with the $R$ ions associated with the icosahedral cluster at the body-center of the unit cell. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y11.00003: Fragility, slow homogenization and Intermediate Phase in the Si$_x$Ge$_x$Te$_{100-2x}$ ternary K. Gunasekera, P. Boolchand, S. Mamedov Small sized (0.5g) melts were synthesized by reacting pure elements in 5mm ID quartz tubes at 950C, and examined after 1 week and then 2 weeks of reaction. Bulk glass formation is realized in 6{\%}\textless x\textless16{\%} range with Tg(x) increasing linearly in 6{\%}\textless x\textless12{\%} range, and decreasing thereafter (x\textgreater12{\%}). The enthalpy of relaxation at Tg shows a flat bottomed minimum in 7.5{\%}\textless x\textless9.0{\%} range with the term increasing sharply at x\textgreater9{\%} and at x\textless7.5{\%}. We identify the 7.5{\%}\textless x\textless9.0{\%} range with the Intermediate Phase. Fragility(m) of melts were established in complex Cp measurements, and show a global minimum (m\textless30) in the IP range, and a value of m=26 at x=8.5{\%}. The slow homogenization of Telluride melts results from the \textit{strong} character of IP melts. Raman scattering, excited using low power density of 785nm radiation, shows evidence of a broad mode near 160cm$^{-1}$ (characteristic of a-Te chains) and a narrower one near 127cm$^{-1}$ (group IV crosslinking units). The scattering strength of the 127cm$^{-1}$ mode increases at the expense of the 160cm$^{-1}$ mode as x increases. The nature of structure evolution with glass composition will be commented upon. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y11.00004: Slow kinetics of melt homogenization and strong nature of intermediate phase melts in chalcogenides P. Boolchand, K. Gunasekera, S. Bhosle The strong-fragile classification of melts is manifested in the T-dependence of viscosity. Strong (fragile) melts possess a T-independent (dependent) activation energy of viscosity leading to an Arrhenian (non-Arrhenian) behavior reflecting the robust (weak) nature of network structure. We have now measured [1] complex C$_p$ of binary Ge$_x$Se$_{100-x}$ glasses as a function of x, and find that in dry and homogeneous melts, fragility (m(x)) shows a global minimum (m \textless\ 20) in the Intermediate Phase (IP) compositions (19.5{\%} \textless\ x \textless\ 26{\%}) but increases rapidly outside the IP. These findings have a direct bearing on synthesis of non-stoichiometric melt compositions at elevated temperatures in which IP melt compositions serve as a bottleneck [1] to homogenize [2] batches globally. The physical properties of dry and homogeneous glasses differ significantly from their inhomogeneous counterparts, and have led, in general, to differences in results reported by various groups.\\[4pt] [1] K. Gunasekera et al, ``Fragility and kinetics of melt homogenization of network glasses''(In preparation).\\[0pt] [2] S. Bhosle et al., Solid. St. Comm. 151, 1851-1855 (2011). [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y11.00005: Fragility and slow kinetics of melt homogenization in the As-Se binary Sriram Ravindren, Kapila Gunasekera, Punit Boolchand Two gram sized As$_{x}$Se$_{100-x}$ batches at various As content x were synthesized using pure Se and As$_{2}$Se$_{3}$ as starting materials that were reacted at 700$^{o}$C. Such melts typically took 3-12 days to homogenize, as monitored in punctuated, off-line FT-Raman line profiling\footnote{S. Bhosle et al., Sol. St. Commun.151, 1851-1855 (2011)} experiments. We have now undertaken mDSC experiments as a function of modulation frequency to establish the compositional dependence of complex C$_{p}$(x), and deduce the variation of fragility m(x). We find the fragility to be rather low, m $<$ 20, across the 22\% $<$ x $<$ 38\% range, and to rapidly increase at x $<$ 22\% to acquire a value of 43 near x = 3\%. We show that the slow melt homogenization is a direct consequence of the ``strong'' character of melts that serves as a bottleneck in melt-mixing at high temperatures. Once homogenized, physical properties of glasses, such as density, glass transition temperature T$_{g}$(x), the Intermediate phase, and variation of enthalpy of relaxation at T$_{g}$(x) differ significantly from their inhomogeneous counterparts. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y11.00006: Quantitative description of orientational order in non-graphitic carbons Enshi Xu, Vincent Crespi The key factor that determines the ability to graphtize of a non-graphitic material is believed to be the level of orientational disorder which indicates how well the elemental structures are aligned. To characterize the disorder, we have developed a correlation function with multiple variable dependencies, such as radial distance and zenith angle. Through the characteristic parameter of the function, the ability to graphitize can be determined given the structure of a carbon material. The model is applied to a set of non-graphitic structures, which is generated systematically in non-conventional methods that emphasize to represent the orientational order of the carbon material rather than to match the radial distribution function. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y11.00007: Localization and percolation in random elastic networks Jacob Krich, Ariel Amir, Vincenzo Vitelli, Yuval Oreg, Yoseph Imry We consider a minimal model for a disordered phonon system that shows rich behavior in the localization properties of the phonons. We use a percolation analysis to argue for a localization/delocalization transition of the phonon modes and predict the speed of sound in the delocalized region, with comparison to numerics. We show that in contrast to the behavior in electronic systems (cf. Anderson localization), the transition exists for arbitrarily large disorder, albeit with an exponentially small critical frequency. The structure of the modes reflects a divergent percolation length that arises from the disorder in the springs without being explicitly present in the definition of our model. We calculate the critical frequency as a function of density and test the prediction numerically using a recursive Green function method. We further explore the existence of delocalized states in the two-dimensional version of this model. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y11.00008: Stoichiometric SrTiO3 Films via High Pressure Oxygen Sputter Deposition Palak Ambwani, Bharat Jalan, Chris Leighton Defect management in epilayers of semiconducting complex oxides such as SrTiO$_{3}$ is a topic of considerable contemporary interest. Recent work has shown that sufficiently precise control over stoichiometry and defects in SrTiO$_{3}$ enables facile$ n$-type doping, record high mobilities, and even simultaneous observation of quantum oscillations and superconductivity. Such progress has typically been made using techniques such as oxygen/LASER MBE or high-temperature PLD. In this work we demonstrate, via homoepitaxy on SrTiO$_{3}$(001), that RF high pressure oxygen sputtering from a ceramic target is similarly capable of growth of high-quality, stoichiometric SrTiO$_{3}$ films. We show that optimization of the deposition temperature (above 750 $^{\circ}$C) and oxygen pressure (above 2.5 mBar) leads to the deposition of films indistinguishable from the substrate via grazing incidence and wide-angle x-ray scattering. The importance of a pre-treatment of the substrates in oxygen above 900 $^{\circ}$C is emphasized. The defect density/stoichiometry was further probed via the transport properties of vacuum annealed samples with controlled O vacancy density. Finally, we also demonstrate that the stoichiometry and defect density of films deposited under non-optimal conditions can be remarkably improved via post-deposition heat treatment. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y11.00009: Time-resolved \textit{in-situ} X-ray Study of Homoepitaxial SrTiO$_3$ Growth Using Reactive Molecular-Beam Epitaxy I.C. Tung, Z.L. Luo, J.H. Lee, H. Hong, S.H. Chang, J.A. Eastman, M.J. Bedzyk, J.W. Freeland, D.D. Fong Functional materials based on complex oxides in thin film form offer new and exciting strategies for meeting many energy challenges. Unfortunately, synthesis of such oxide films can be a major challenge even when utilizing reactive molecular-beam epitaxy (MBE). To understand the fundamental physics of complex oxide thin film growth, we have developed the world's first reactive MBE system with {\it in-situ} synchrotron x-ray scattering capability at the Advanced Photon Source (APS). Here we present the results of {\it in-situ} surface x-ray scattering measurements taken during homoepitaxial growth of SrTiO$_3$ on (001) SrTiO$_3$ substrates. We compare the shuttered growth technique with codeposition to understand the nature of the distinctly different approaches. Work at the APS, Argonne is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y11.00010: STM Studies of Sub-monolayer SrO and LaAlO$_{3}$ Film Growth on SrTiO$_{3}$(001) Substrate Surfaces Takeo Ohsawa, Katsuya Iwaya, Ryota Shimizu, Susumu Shiraki, Taro Hitosugi We report atomic-scale observations of initial growth of sub-monolayer SrO and LaAlO$_{3}$ (LAO) films on the atomically-ordered ($\sqrt{13}$$\times$$\sqrt{13}$)-$\textit{R}$33.7$^{\circ}$ SrTiO$_{3}$ (STO) (001) substrate surfaces using scanning tunneling microscopy/spectroscopy (STM/STS). We found that the growth processes depend strongly on the film compositions and the investigations unveil complex chemistry of thin-film oxides. These findings will provide microscopic insights into the understanding of transport properties at the LAO/STO interface, which is known to exhibit conducting and insulating behavior depending on the termination structures of STO substrates, namely, whether ``TiO$_{2}$-" or ``SrO-terminated" surfaces, respectively. Controlling the interface structure genuinely with atomic precision will eventually lead to the creation of exotic electronic phenomena and functionalities at the complex oxide interfaces. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y11.00011: In-situ Surface X-ray Diffraction Study of Ruddlesden-Popper Series Thin Film Growth June Hyuk Lee, Seo Hyoung Chang, Zhenlin Luo, I-Cheng Tung, Milind Malshe, Julius Jellinek, Jeff Eastman, Hawoong Hong, Dillon Fong, Freeland John The layered Ruddlesden-Popper phases of A$_{n+1}$B$_{n}$O$_{3n+1}$, such as Sr$_{2}$TiO$_{4}$ and La$_{2}$NiO$_{4}$, have attracted much attention as potential materials for solid-oxide fuel cell cathodes and thermoelectrics. To understand the fundamentals of this class of layered oxide thin films, we studied the growth of (001)-oriented Sr$_{2}$TiO$_{4}$ and La$_{2}$NiO$_{4}$ on SrTiO$_{3}$ substrates by using oxide molecular beam epitaxy with in-situ surface x-ray diffraction. For Sr$_{2}$TiO$_{4}$, the synthesis of the double SrO layer followed by TiO$_{2}$ dynamically reconstructs back into the SrTiO$_{3}$ phase, which demonstrates that during thin film deposition other pathways under growth conditions can give rise to new structural arrangements. In contrast with Sr$_{2}$TiO$_{4}$, the growth of La$_{2}$NiO$_{4}$ involves the stacking of polar LaO$^{+}$ and NiO$_{2}^{-}$ layers. This raises the question of how polarity mismatch at the interface with the SrTiO$_{3}$ substrate will influence the growth process. A detail comparison of these two cases will be discussed. Work at the Advanced Photon Source, Argonne is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y11.00012: Atomic Resolution and First Principles Study of the Electronic Structure at SrTiO$_{3}$/GaAs Hetero-interfaces Qiao Qiao, Robert Klie, Serdar Ogut, Ravi Droopad, Rocio Contreras-Guerrero We examined ultra-thin SrTiO$_{3}$ films deposited on As-terminated GaAs (001) using molecular beam epitaxy under various O$_{2}$ partial pressures. Atomic-resolution Z-contrast images of different SrTiO$_{3}$ films were obtained using the aberration-corrected JEOL JEM-ARM200CF operated at 80 kV. Using atomic-column resolved EELS, our analysis of the Ti and O near-edge fine structure reveals different bonding configurations at the interface resulting from different growth methods. These results strongly suggest that a Ti pre-layer deposition alleviates the oxidation of the substrate and consequently the Fermi level pinning at the interface, as reported before. We also examined BaTiO$_{3}$ thin films grown on GaAs (001) with an ultrathin SrTiO$_{3}$ buffer layer. Interfacial charge distribution related to the polarization of BaTiO$_{3}$ thin film will be studied using atomic-resolution Z-contrast images, annular bright field images and EELS. Using first-principles DFT calculations, we analyze the formation energies of Ti-related impurity defects in different GaAs surface reconstructions to help interpret the electron microscopy experiments. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y11.00013: Temperature-driven irreversible phase transition of Sr template for epitaxial SrTiO$_{3}$ growth on vicinal Si (001) Kristy Kormondy, Agham Posadas, Alexander Demkov Strontium titanate (STO) grown epitaxially on silicon has been an area of interest both for its own properties as a high-k dielectric and its capacity to act as a substrate for other crystalline oxides. In this study, we investigate STO growth on a 4$^{\circ}$ miscut Si (001) surface with double atomic steps to enhance our understanding of submonolayer Sr deposition and STO growth. It is well-known that a half-ML of Sr on the Si surface is a necessary prerequisite for crystalline growth; however, detailed study of reflection high-energy electron diffraction (RHEED) pattern during Sr deposition at various substrate temperatures reveals two distinct surface reconstructions at half-ML coverage. At temperatures below 350$^{\circ}$C, the 2x1 pattern is nearly identical to that of clean Si, but as the temperature is increased, we see the irreversible appearance of a 2x spot parallel to the step edge while the 2x spot perpendicular to the step edge dims. We also find that crystalline STO can be grown on both of these high- and low-temperature templates, with identical RHEED and band alignment as determined by XPS, showing that this previously unexplored low-temperature template can provide an alternative route for STO growth on Si. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y11.00014: Correlation effects on the different crystal structures of AO2 (A=Na, K, and Ba) Minjae Kim, Chang-Jong Kang, B.I. Min In alkali superoxide (A=Na, and K), the structural phase transition from high symmetry to low symmetry structures occurs upon cooling. On the other hand, in BaO$_2$ peroxide, the crystal structure is always the high symmetry tetragonal structure of KO$_2$, independent of temperature. To resolve these different crystal structures of AO$_2$ (A=Na, K, and Ba), we have calculated phonon dispersions of AO$_2$, assuming the high symmetry tetragonal structure of KO$_2$ with first-principle band structure method in the generalized gradient approximation (GGA) incorporating the Coulomb interaction U (GGA+U). From softened phonon modes, we have shown that, in KO$_2$ and NaO$_2$, the degeneracy of the incomplete pi anti-bonding level is lifted with the symmetry lowering such as Jahn-Teller effect with help of Coulomb correlation U. In contrast, in BaO$_2$, the pi anti-bonding level of the peroxide is completely filled without degeneracy. Thus, U is not effective on the phonon structure so that the structural instability does not occur in BaO$_2$. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y11.00015: Fragility, Intermediate Phase and Polaronic conductivity in heavy metal oxides Shibalik Chakraborty, Kapila Gunasekera, Punit Boolchand, Mohammed Malki, Matthieu Micoulaut The (B$_{2}$O$_{3})_{5}$(TeO$_{2})_{95-x}$(V$_{2}$O$_{5})_{x}$ ternary forms bulk glasses over a wide range of compositions, 18{\%} \textless\ x \textless\ 35{\%}. Complex C$_{p}$(x) measurements as a function of modulation frequency reveal that melt fragility (m) show a global minimum (m $=$ 52(2)) in the 23{\%} \textless\ x \textless\ 26{\%} range with m \textgreater\ 65 outside that window. These results suggest more stable network structure in the window than outside it. The fragility window coincides with a global minimum of the non-reversing enthalpy of relaxation at T$_{g}$, the reversibility window (23{\%} \textless\ x \textless\ 27{\%}), a behavior also found in chalcogenide glasses. Conductivity ($\sigma )$ data show three regimes of variation; a low $\sigma $ at x \textless\ 23{\%}, a plateau in 23{\%} \textless\ x \textless\ 27{\%}, and an exponential increase at x \textgreater\ 27{\%}. The reduced activation energy for conductivity at x \textgreater\ 27{\%} is consistent with increased polaronic mobility as the network becomes flexible. These findings show glasses at x \textless\ 23{\%} are \textit{stressed-rigid}, in 23{\%} \textless\ x \textless\ 27{\%} range in the \textit{Intermediate Phase}, and at x \textgreater\ 27{\%} to be \textit{flexible}. [Preview Abstract] |
Session Y12: Focus Session: Themoelectrics Nanomaterials II
Sponsoring Units: DMP GERA FIAPChair: Austin Minnich, CalTech
Room: 314
Friday, March 22, 2013 8:00AM - 8:12AM |
Y12.00001: High Temperature Thermal Conductivity from First Principles Christian Carbogno, Rampi Ramprasad, Matthias Scheffler In spite of significant research efforts, a first principles determination of the thermal conductivity at high temperatures has remained elusive. Under such conditions, techniques that rely on the harmonic approximation are no longer valid, while standard non-equilibrium molecular dynamics methods require huge temperature gradients that lead to deviations from Fourier's law. The Green-Kubo method [1], which does not suffer from these shortcomings, involves the assessment of the thermal conductivity from the auto-correlation of the heat flux in equilibrium. In classical MD, the heat flux is computed from the energetic contributions of the individual atoms; we show that the Green-Kubo approach can be reformulated in terms of the energy and stress densities [2], which are directly accessible in DFT calculations. This approach leads to a unique definition of the heat flux that does not rely on any partitioning scheme for the total energy. We critically discuss the computational cost, the accuracy, and the applicability of this approach by investigating the thermal conductivity for oxides and semiconductors with low thermal conductivities.\\[4pt] [1] R. Kubo, M. Yokota, S. Nakajima, J. Phys. Soc. Jpn. 12, 1203 (1957).\\[0pt] [2] R. Ramprasad, J. Phys. Condens. Matter 14, 5497 (2002). [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y12.00002: Nonlinear thermoelectric transport in mesoscopic systems Jonathan Meair, Philippe Jacquod We construct a scattering theory of weakly nonlinear thermoelectric transport through mesoscopic conductors. To preserve gauge invariance interaction induced potentials within the conductor must be self-consistently determined. We describe how to do this and apply our theory to calculating the leading nonlinear contribution to both electrical and heat currents. We present sum rules for our nonlinear response coefficients that must hold for current conservation and gauge invariance to be satisfied. We illustrate the method by investigating the thermoelectric response of a quantum point contact and a resonant tunneling barrier. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y12.00003: Monte Carlo Simulations of Mode Dependent Phonon Transport in Nanostructured Thermoelectric Materials Takuma Hori, Junichiro Shiomi Nanostructuring are efficient process to lower the lattice thermal conductivity and thus enhance thermoelectric performance of semiconducting materials. Here, detailed knowledge of phonon transport properties in the nanostructures is needed for prediction of performance and/or optimization of structures. The approach to solve the linearized phonon Boltzmann transport equations stochastically by Monte Carlo method has been demonstrated to be useful to obtain phonon transport properties in mesoscale and complex structures. In this study, we have performed the Monte Carlo simulations to investigate phonon transport properties in nanostructured thermoelectric materials. With the mode-dependent bulk phonon transport properties obtained by first-principles-based calculations, the Monte Carlo simulations are performed to investigate the influence of nanostructure length-scales on the mode-dependent lattice thermal conductivity and its sensitivity to interfacial phonon transmission. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y12.00004: Ab initio thermal transport properties of nanostructures from density functional perturbation theory. Thushari Jayasekera, Arrigo Calzolari, Ki Wook Kim, Marco Buongiorno Nardelli We present a comprehensive first principles study of the thermal transport properties of low-dimensional nanostructures such as polymers and nanowires. An approach is introduced where the phonon quantum conductance is computed from the combination of accurate plane-wave density functional theory electronic structure calculations, the evaluation of interatomic force constants through density functional perturbation theory for lattice dynamics and the calculation of phonon transport properties by a real space Green's function method based on the Landauer formalism. This approach is computationally very efficient, can be straight-forwardly implemented as a post-processing step in a standard electronic-structure calculation (Quantum ESPRESSO and WanT in the present implementation), and allows us to directly link the thermal transport properties of a device to the coupling, dimensionality, and atomistic structure of the system. It provides invaluable insight into the mechanisms that govern the heat flow at the nanoscale and pave the way to the fundamental understanding of phonon engineering in nanostructures. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y12.00005: Designing Graphene-based Thermoelectric materials with Chemical Functionalization Jeong Yun Kim, Jeffrey Grossman Graphene has been explored as a thermoelectric (TE) material recently due to its superior mobility and ambipolar nature. However, the extremely high thermal conductivity ($\kappa )$ and only moderate Seebeck coefficient (S) make a graphene monolayer a highly inefficient TE material. Graphene superlattices made with chemical functionalization offer the possibility of tuning both the thermal and electronic properties via nano-patterning of the graphene surface. In this work, we investigate the effects of chemical functionalization on the thermoelectric transport properties of graphene using classical and quantum mechanical calculations. Our calculations show that chemical functionalization can control the power factor by changing the width of the pure graphene region and functionalization configuration, as well as $\kappa $ depending on the functional groups and functionalization coverage. These results suggest that chemical functionalization could be an efficient route to designing graphene-based TE materials. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y12.00006: Power-efficiency trade-off due to density of states (DOS) distortion in a molecular thermoelectric system Priyanka deSouza, Bhaskaran Muralidharan The issue of how a distortion in the electronic DOS affects nanoscale thermoelectric performance is addressed within an ``electrical engineering'' perspective. This view point is based on the direct evaluation of the overall efficiency and power from device current-voltage characteristics and gives a more complete picture of the thermoelectric performance in comparison to the traditional ``figure of merit'' based material science approach. We use representative examples from molecular conduction to study the trade-off between maximum efficiency and the maximum power generated within the set up. The trade-off is maximum for the well known example of a sharply resonant molecular level which represents the ultimate distortion in the electronic density of states. As the distortion is reduced via contact induced broadening, we obtain a smaller trade-off between maximum power and efficiency. We then present the effects of self consistent charging, contact induced asymmetry and the HOMO-LUMO gap on the thermoelectric performance. In all cases we compare our non-equilibrium calculations with zT calculations, and our results depict that zT is not the sole metric for the assessment of nanoscale thermoelectric performance. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y12.00007: Solvothermal synthesis and thermoelectric property of undoped and indium doped lead telluride nanoparticles Kamal Kadel, Wenzhi Li Undoped and indium (In) doped lead telluride (PbTe) nanostructures were synthesized via solvothermal/hydrothermal route. The crystallinity of the as-prepared un-doped and In-doped PbTe sample were examined by X-ray diffraction (XRD) which indicated the formation of face centered single phase cubic PbTe. Lattice constant calculation from XRD pattern revealed the formation of un-doped and In-doped PbTe crystals with almost similar size. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) examinations indicated that undoped and In-doped PbTe nanostructures were mostly cubically shaped and highly crystalline. The effect of the synthesis temperature on the structure and morphology of undoped PbTe was also investigated; it was found that the particle size increased with the synthesis temperature. Thermoelectric property of as-synthesized lead telluride sample was also investigated. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y12.00008: Temporal evolution of Seebeck coefficient in an ac driven strongly correlated quantum dot Ali Ihsan Goker, Elif Gedik We study the response of the thermopower of a quantum dot in the Kondo regime to sinusoidal displacement of the dot energy level via a gate voltage using time dependent non-crossing approximation and linear response Onsager relations. Instantaneous thermopower begins to exhibit complex fluctuations when the driving amplitude is increased at constant driving frequency. We also find that the time averaged thermopower decreases steadily until it saturates at constant driving amplitude as a function of inverse driving frequency. On the other hand, time averaged thermopower is found to be quite sensitive to ambient temperature at all driving frequencies for large driving amplitudes. We discuss the underlying microscopic mechanism for these peculiarities based on the behaviour of the dot density of states. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y12.00009: Thermopower Measurements of Highly Conducting Single-Molecule Devices Jonathan R. Widawsky, Wenbo Chen, Hector Vazquez, Taekyeong Kim, Mark S. Hybertsen, Ronald Breslow, Latha Venkataraman We measure the conductance ($G)$ and thermopower ($S)$ of highly conducting single-molecule junctions with Au electrodes. The junctions are formed and measured using a scanning tunneling microscope-based break-junction technique. The target molecules are synthesized with SnMe$_{\mathrm{3}}$ terminations that cleave off \textit{in situ}, allowing for the formation of direct Au-C covalent bonds to the electrodes[1,2]. We compare the conductance and thermopower for two families of molecules: pi-conjugated polyphenyls, which have a high conductance and thermopower, and sigma-bonded alkyl systems, where we observe a significant thermopower despite the low conductance. For these measurements, we use the most probable thermopower to determine a power factor, \textit{GS}$^{\mathrm{2}}$, for each molecular junction studied. Our results show that the molecular thermopower increases systematically and non-linearly with molecular length and also that the power factor is exceptionally large for the case of the biphenyl. [1] Z. L. Cheng, R. Skouta, H. Vazquez\textit{ et al.}, Nat. Nano. \textbf{6}, 353 (2011). [2] W. Chen, J. R. Widawsky, H. V\'{a}zquez\textit{ et al.}, J. Am. Chem. Soc. \textbf{133}, 17160 (2011). [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y12.00010: Operating Characteristics of a Microfabricated Phonon Spectrometer Richard Robinson, Jared Hertzberg, Obafemi Otelaja, Mahmut Aksit Phonon scattering exhibits a strong influence on the thermal properties of nanostructures. By promoting phonon scattering at surfaces and interfaces, a nanostructured thermoelectric material may achieve reduced thermal conductivity and enhanced thermoelectric efficiency. While phonons over a wide frequency range contribute to energy transport, thermal conductivity measurements capture only their combined effect. However, a window into phonon transport in nanostructures at specific frequencies could provide unique information and also serve as a crucial test platform for phonon transport theories. To this end, we have constructed a microfabricated phonon spectrometer. At a temperature of 0.3K, a superconducting tunnel junction locally generates non-thermal distributions of phonons and transmits them through adjacent silicon micro- and nanostructures.[1] We employ modulation techniques to select narrow frequency bands of phonons at frequencies up to hundreds of GHz. This prototype phonon spectrometer achieves phonon frequency resolution as low as $\sim$10 GHz, more than an order of magnitude lower than comparable thermal methods. We describe the other key parameters of this technique: spatial resolution, frequency range, dynamic range, signal-to-noise ratio and calibration methods. This work was supported in part by the National Science Foundation under Agreement No. DMR-1149036.\\[4pt][1] J. B. Hertzberg et al, Rev. Sci. Inst. 82, 104905 (2011) [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y12.00011: Nonlinear thermoelectric response of quantum dots Stefan Kirchner, Farzaneh Zamani, Enrique Munoz, Lukas Merker, Theo Costi The thermoelectric transport properties of nanostructured devices continue to attract attention from theorists and experimentalist alike as the spatial confinement allows for a controlled approach to transport properties of correlated matter. Most of the existing work, however, focuses on thermoelectric transport in the linear regime despite the fact that the nonlinear conductance of correlated quantum dots has been studied in some detail throughout the last decade. To go beyond the linear response regime, we use a recently developed scheme [1], to address the low-energy behavior near the strong-coupling fixed point at finite bias voltage and finite temperature drop at the quantum dot. We test the reliability of the method against the numerical renormalization group [2] and determine the charge, energy, and heat current through the nanostructure. This allows us to determine the nonlinear transport coefficients, the entropy production, and the fate of the Wiedemann-Franz law in the non-thermal steady-state~[3].\\[4pt] [1] E. Munoz et al, arXiv:1111.4076.\\[0pt] [2] L. Merker et al, in preparation.\\[0pt] [3] S. Kirchner, F. Zamani, and E. Munoz, in ``New Materials for Thermoelectric Applications: Theory and Experiment,'' Springer (2012). [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y12.00012: Atomistic quantum thermal conductance profile of hybrid interfaces Jeevaka Weerasinghe, Arrigo Calzolari, Marco Buongiorno Nardelli Atomistic structure at interfaces has been shown to play a critical role in quantum thermal conductance across nanoscale interfaces. In general, current models derive phonon transmission probabilities from bulk material properties. However, they do not account for the effect of atomic scale interfacial structure on thermal conductance. Here we use an ab initio approach that we have recently developed to investigate the correlation between interfacial atomic structure and quantum thermal conductance. In particular, we will discuss the electronic structure and thermal conductances in systems with hybrid metal/self-assembled monolayer (SAM) interfaces with varying chemistry in order to elucidate the role of metal-organic bonds in the thermal properties of complex assemblies. Our methodology integrates the accurate self-consistent minimization of the ground state electronic structure via first-principles density functional theory based calculations, the determination of interatomic force constants via density functional perturbation theory, and the calculation of the quantum conductance using a real space Green's function formalism based on the Landauer approach. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y12.00013: Studies of Thermal Conductivity in Hybrid Organic-Inorganic Nanocrystal Arrays Wee-Liat Ong, Sara Rupich, Dmitri Talapin, Alan McGaughey, Jonathan Malen The thermal conductivity of nanocrystal arrays (NCAs) is studied and found to be tunable through the nanocrystal diameter, and chemistry - a conclusion that is supported by our Molecular Dynamics simulation. Nanocrystal arrays self-assemble from colloidal molecule-coated nanocrystals into close-packed 3D films. It has been suggested that their electronic and thermal transport properties can be decoupled, enabling a resolution to the conflicting needs of various thermal management and solid-state energy conversion applications (e.g. high figures of merit materials for thermoelectric, high-efficiency photovoltaic materials). Although the electronic transport in NCAs has been studied extensively, little is known about their thermal transport. We herein report both experimental measurements and modeling performed to elucidate the thermal transport mechanisms in NCAs. Various factors including the geometry and chemical compositions of the NCAs will be presented. Simulation results showed good agreement with the observed experimental trends, providing a complementary computational approach for elucidating and optimizing NCA thermal properties. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y12.00014: Probing tunable thermal properties of organic hetero-junctions Shubhaditya Majumdar, Scott N. Schiffres, Jonathan A. Malen, Alan J.H. McGaughey The ability to tune physical properties of new organic-inorganic heterojunctions is essential for their popularity in the fields of molecular electronics and energy-generation devices. Intimate associations between the organic and inorganic components at the nano-scale level lead these materials to possess unique transport properties. Here, we probe the thermal conductance of self-assembled monolayer (SAM) junctions using both computational and experimental methods. SAM junctions are ordered, periodic arrays of a single layer of organic molecules chemically bonded to two inorganic substrates. Molecular dynamics simulations are performed on the SAM junctions to study the effect of physical parameters on the junction thermal conductance. These include atomic masses of leads, junction temperature, molecular chain length, and surface coverage. Another important aspect is the contribution of the stiff C-H bonds to thermal transport, an analysis of which is also presented. Lattice dynamics calculations are employed to study the effect of molecular vibrations on the thermal coupling between the leads. The SAM junctions are prepared in the laboratory through a combination of solution immersion and transfer printing techniques. Frequency domain thermo-reflectance (FDTR) -- a laser-based non-contact measurement scheme to probe the thermal properties of thin films, is employed to study the samples. A comparison between the results obtained from these studies is thus presented. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y12.00015: Seebeck Coefficient of Manganese Oxide Nanoparticles as a Function of Ohmic Resistance Nicholas Francis, Morgan Hedden, Costel Constantin Due to the ever increasing energy demand and growing global concern over the environmental impact of CO$_{2}$ emissions, there is an urging need to seek solutions to transit from fossil fuels to sustainable energy. Thermoelectric (TE) materials show great promise for converting waste heat energy into electricity. TE systems have many unique advantages such as silent operationality, time reliability, and dimensional scalability. Most recently, researchers Song et al. [1] found that MnO$_{2}$ nanoparticles show a giant Seebeck coefficient of S $=$ 20 mV/K, which is100 times higher than bismuth telluride, one of the best TE materials. Song et al.[1] concluded the paper claiming that the giant S is related to the surface density of the electronic states (DOS). However, they provided very little information about the S as a function of Ohmic resistance [R] for different nano particle sizes which can give information about the DOS. Our preliminary results show that there is a sudden increase of S from 0.33-0.63 mV/K as R increases from 80-110 Ohms. This transition has never been seen before and it can give clues as to the existence of the Giant S observed in this material.\\[4pt] [1] F. Song, L. Wu and S. Liang, Giant Seebeck coefficient thermoelectric device of MnO$_{2}$ powder, Nano. 23, 085401 (2012). [Preview Abstract] |
Session Y13: Topological Insulators: Thin Films and Interfaces
Sponsoring Units: DCMPChair: Fazel Fallah Tafti, Universite de Sherbrooke
Room: 315
Friday, March 22, 2013 8:00AM - 8:12AM |
Y13.00001: An Infrared Study of Bi2Se3 Thin Films Kirk Post, Brian Chapler, Liang He, Xufeng Kou, Alex Schafgans, Kang Wang, Dmitri Basov The experimental observation of surface states present in Bi$_2$Se$_3$ has been limited by self-doping via selenium vacancies. We have explored this issue by probing the electronic structure of Bi$_2$Se$_3$ using a combination of variable angle spectroscopic ellipsometry (VASE) and Fourier transform infrared spectroscopy (FTIR). Specifically, we have measured Bi$_2$Se$_3$ thin films grown on Si (111) substrates, ranging from 15 to 99 quintuple layers (QL) thick. These results show that both the carrier density and the energy gap are inversely related to the thickness. Surprisingly, the energy gap in all but the 15QL samples was smaller than the bulk band gap. Furthermore, the energy gap varied by over 100 meV between the 15QL and 99QL sample. The features that we observed are consistent with a modified picture of the band structure of Bi$_2$Se$_3$ that includes an impurity band below the conduction band and a Fermi level that is inversely related to the thickness. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y13.00002: Impact of growth conditions on the MBE-grown topological insulator Bi$_2$Se$_3$ thin films Y. Liu, Y.Y. Li, S. Rajput, M. Weinert, L. Li Recently, molecular beam epitaxy (MBE) has been successfully applied to prepare atomically flat topological insulator thin films that exhibit helical Dirac states. In this work, we systematically investigate the effects of substrate temperature and Bi/Se flux ratio on the morphology and properties of Bi$_{2}$Se$_{3}$ thin films grown on graphene/SiC(0001) by MBE. Under optimal growth conditions, \textit{in situ} scanning tunneling microscopy indicates spiral growth [1], characterized by atomically smooth terraces 10 to 50 nm in width, separated by steps that are one quintuple-layer in height. \textit{Ex situ} Raman spectroscopy reveals two characteristic peaks at 130 and 171 cm$^{-1}$, corresponding to the in-plane E$_{\mathrm{g}}^{2}$ and out-of-plane A$_{\mathrm{1g}}^{2}$ vibrational modes, respectively. The close resemblance of the positions and line shapes of both these peaks to those of bulk Bi$_{2}$Se$_{3}$ attest to the high quality of the film. These results and the impact of growth spirals on the properties of the topologically protected Dirac surface states of Bi$_{2}$Se$_{3}$ will be presented at the meeting. \\[4pt] [1] Y. Liu et al. PRL \textbf{108}, 115501 (2012). [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y13.00003: Epitaxial Growth of Topological Insulators on Hexagonal Boron Nitride Christopher Gutierrez, Woo Chang Chung, Chockalingam Subbaiah, Matthew Brahlek, Seongshik Oh, Abhay Pasupathy Topological insulators (TIs) have attracted much attention for exhibiting exotic, topologically-protected surface states consisting of massless Dirac fermions. Investigations on thin film TIs have primarily relied on those either grown by MBE or by mechanical exfoliation onto suitable target substrates. Taking a cue from the graphene community, hexagonal boron nitride (hBN) has proven to be an excellent insulating substrate since it is atomically flat with no surface dangling bonds. In this talk I will report on recent transport and scanning probe measurements on epitaxial thin films of bismuth selenide TI grown by MBE on hBN/SiOx. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y13.00004: Gate-tunable supercurrent in S-TI-S structures Vladimir Orlyanchik, Martin Stehno, Christopher Nugroho, Dale Van Harlingen, Matthew Brahlek, Namrata Bansal, Nikesh Koirala, Seongshik Oh Theoretical proposals for observation of the zero energy excitations (Majorana modes) involve coupling between the surface states of 3-D topological insulators (TI) and s-wave superconductors (SC). A prerequisite for such experiments is a highly tunable topological surface which is decoupled from bulk charge carriers and non-topological surface states. Here we report on measurements performed using high-quality MBE-grown thin films of Bi2Se3 patterned to create planar Josephson devices with Nb leads and a metallic top gate. We present the dependence of the conductance and proximity-induced supercurrent on the junction geometry, temperature, and the gate voltage. By analyzing the gate voltage dependence, we deduce that there are contributions to the supercurrent from two channels - topological surface states and a topologically-trivial surface accumulation layer. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y13.00005: Terahertz dynamics of gated thin films of the topological insulator Bi$_{2}$Se$_{3}$ Andreas Stier, James Neilson, Liang Wu, Namrata Bansal, Matthew Brahlek, Sean Oh, N. Peter Armitage Topological insulators are a newly discovered class of materials, which in principle exhibit bulk insulating behavior and conducting surface channels with a Dirac like dispersion relation. Real materials, however, suffer from large residual bulk conductance due to donor defect sites. This places the chemical potential in the bulk bands. Ionic liquid gating techniques are capable of moving the chemical potential into the bulk band gap, making the exotic transport characteristics predicted for the surface states accessible. Here, we present terahertz time domain spectroscopy of gated thin films of the topological insulator Bi$_{2}$Se$_{3}$ utilizing an ionic liquid gel as a top gate. The evolution of the Drude like conductivity features as a function of gate bias show a sharp decrease in the scattering rate which we interpret as the chemical potential moving from the conduction band into the surface states. We also discuss efforts to optically observe potential axionic terms in the action governing Maxwell's equations for this material class, which are reflected in a discontinuous evolution of the Faraday rotation. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y13.00006: Scanning tunneling spectroscopic (STS) studies of magnetically doped MBE-grown topological insulators (TIs) Hao Chu, Marcus Teague, Chien-Chang Chen, Nicholas Woodward, Nai-Chang Yeh, Xufeng Kou, Liang He, Murong Lang, Kang Long Wang We conduct STS studies on MBE-grown heterostructures of non-magnetic TI (Bi$_{2}$Se$_{3})$ with a range of thicknesses ($d =$ 1, 3, 5, 7 quintuple layers, QL) on top of 7-QL magnetically doped TI (Cr-doped Bi$_{2}$Se$_{3})$. For $d =$ 1 and 3-QL, a spatially homogeneous magnetism-induced surface gap (as large as about 150 meV for $d$ $=$ 1-QL) is observed at 77 K, whereas gapless Dirac spectra are found for $d =$ 5 and 7-QL, suggesting that the effective magnetic length for Cr-doped Bi$_{2}$Se$_{3}$ is approximately 4 $\sim$ 5-QL. These findings are further corroborated by ARPES and bulk electrical transport measurements. The magnetism-induced surface gap differs from those found in pure Bi$_{2}$Se$_{3}$ and (Bi$_{0.5}$Sb$_{0.5})_{2}$Te$_{3}$ films of thicknesses smaller than 6-QL, because the latter are due to overlaps of wave functions between the surface and interface layers, which lead to Rashba-like spin-orbit splitting and spin-preserving quasiparticle interference wave-vectors. In contrast, STS studies of TIs with magnetism-induced surface gap do not yield any quasiparticle interferences for energies within the bulk Bi$_{2}$Se$_{3}$ gap. Finally, comparative STS studies of pure and magnetically doped TIs in high magnetic fields will be discussed. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y13.00007: Thickness-Independent Transport Channels in Topological Insulator Bi2Se3 Thin Films Namrata Bansal, Yong-Seung Kim, Matthew Brahlek, Eliav Edrey, Nikesh Koirala, Seongshik Oh With high quality Bi2Se3 thin films grown on Al2O3(0001), we report thickness-independent transport properties over wide thickness ranges. Low temperature conductance remained nominally constant as the sample thickness changed from 256 to $\sim $8QL (where QL refers to quintuple layer, 1QL$\approx $1nm). Two surface channels with very different behaviors were identified. The sheet carrier density of one channel remained constant at $\sim $3x10$^{13}$cm$^{\mathrm{-2}}$ down to 2QL, while the other, which exhibited quantum oscillations, remained constant at $\sim $8x10$^{12}$cm$^{-2}$ only down to $\sim $8QL. The weak antilocalization effect also exhibited similar thickness independence. These two channels are most consistent with the topological surface states and the surface accumulation layers, respectively. We will also discuss surface signatures present in Bi2Se3 thin films grown on Si(111) and amorphous SiO2. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y13.00008: An abrupt change in transport dynamics across the topological phase transition in the (Bi$_{1-x}$In$_x$)$_2$Se$_3$ and ultra-thin Bi2$_2$Se$_3$ systems Liang Wu, Rolando Valdes Aguilar, Andreas V. Stier, Lucas S. Bilbro, Yuval Lubashevsky, N. Peter Armitage, Matthew Brahlek, Namrata Bansal, Sean Oh We have utilized time-domain terahertz (THz) spectroscopy to investigate the low frequency optical conductivity in (Bi$_{1-x}$In$_x$)$_2$Se$_3$ through its topological phase transition from the pure ($x=0$) compound to the topologically trivial strongly insulating material ($x>0.25$). The thickness independent Drude peak shows only minor broadening at low In substitutions. However, above $x\sim0.05$ we observe a sudden collapse in the transport lifetime. This substitution level closely coincides with a maximum in the mid-infrared (MIR) absorption coefficient which can be identified with the substitution level where the band gap closes, the band structure inverts, and hence the topological class changes. We therefore associate the collapse in the transport lifetime with the loss of topological protection of surface states as the system enters the topologically trivial phase. Topological phase transition driven by reducing film thickness is also investigated. Similar collapse in the transport lifetime is observed in the ultra-thin limit. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y13.00009: Fabrication and transport measurements of stacked double layer topological insulator devices Tai-Lung Wu, Jiuning Hu, Jifa Tian, Ireneusz Mitkowski, Yong P. Chen A double-layer structure, consisting of two separated two-dimensional electron systems close in proximity, has been an interesting system to study novel ground states and transport properties driven by electron-electron interaction, e.g. Coulomb drag, exciton condensation, and counterflow superfluidity. Recently, topological insulators (TI), such as $Bi_{2}Se_{3}$ and $Bi_{2}Te_{3}$, have attracted much attention due to their exotic topologically protected spin-helical and Dirac-particle surface states. Motivated by a recently proposed ``topological exciton condensate'' that may be formed in two interacting TI surfaces, we have fabricated stacking double-layer TI structures and studied their electrical transport properties. Using a polyvinyl alcohol (PVA) based support film and micro-manipulator, double layer TI structures ($Bi_{2}Se_{3}$/boron nitride/$Bi_{2}Se_{3}$) were fabricated with exfoliated $Bi_{2}Se_{3}$ separated by thin boron nitride flakes ($\sim$ 20 nm). We will present results from transport measurements including mutual-gated electrical field effect, Coulomb drag ,and counterflow conductivity. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y13.00010: Angle-resolved photoemission spectroscopy study of the magnetic doped topological insulator ultra-thin film Bi$_{2}$Fe$_{x}$Se$_{3}$ Yi Zhang, Bo Zhou, Yulin Chen, Sung-Kwan Mo, Zahid Hussain, Zhi-Xun Shen Topological insulator is a new type of quantum matter with gapped bulk states coexisting with a gapless surface state (SS) that is protected by time reversal symmetry and robust against non-magnetic impurities. Researches have shown that there exist two routes to open a gap in the SS: doping with magnetic impurities and the coupling of SS on opposite surfaces in ultra-thin films. In order to study the mixing of these two types of gap-opening, we prepared ultra-thin Bi$_{2}$Fe$_{x}$Se$_{3}$ films, grown by molecular beam epitaxy, with different Fe concentration and thickness. Size of the gap and its development with Fe concentration and film thickness as well as its momentum dependence have been systematically characterized by in-situ angle resolved photoemission spectroscopy using synchrotron light source. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y13.00011: Tunneling tuned spin modulations in ultrathin topological insulator films Madhab Neupane, S.-Y. Xu, N. Alidoust, I. Belopolski, Chang Liu, D.M. Zhang, A. Richardella, J. Sanchez-Barriga, D. Marchenko, A. Varykhalov, O. Rader, M. Leandersson, T. Balasubramanian, L.A. Wray, T.-R. Chang, H.-T. Jeng, H. Lin, A. Bansil, N. Samarth, M.Z. Hasan Understanding the spin behavior of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nano-devices. We report tunneling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition. We observe that for a given film thickness of the prototype topological insulator Bi$_2$Se$_3$ ultrathin films, the spin polarization is large for larger wave-vectors or for momenta far from the center of the surface Brillouin zone. In addition, the polarization is observed to decrease significantly with enhanced tunneling realized systematically in thin insulating films. We present theoretical model calculations that qualitatively capture the delicate relationship between quantum tunneling and Fermi surface spin polarization. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y13.00012: Study of proximity effect in superconductor - topological insulator heterostructures by scanning SQUID microscope Ilya Sochnikov, Andrew J. Bestwick, James R. Williams, Thomas M. Lippman, Andrew S. Bleich, James G. Analytis, Ian R. Fisher, David Goldhaber-Gordon, John R. Kirtley, Kathryn A. Moler A proximity induced superconducting state in topological insulators is potentially an enabling condition for exotic forms of superconductivity that may support Majorana fermions in some geometries. Initial studies of induced superconductivity in topological insulators have relied on transport measurements. We present a different contactless characterization approach based on a scanning SQUID microscope. We characterized Al superconducting rings with Josephson junctions made of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, long Al/Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/Al Josephson junctions, and Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/Al dots. We observe both induced proximity and inverse proximity effects in these heterostructures. Each of the structures provides unique information about the proximity effect, such as the critical current, the magnetic field penetration depth, and the critical temperatures of the induced superconducting state. These measured parameters allow the determination of limits on contributions from the surface and the bulk to the proximity effects in the topological insulator Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y13.00013: Local Magnetic Imaging of Proximity Effect-Induced Superconductivity at the Bi$_{2}$Se$_{3}$-Nb Interface Philip Kratz, John Kirtley, Ilya Sochnikov, Phillip Wu, Eric Spanton, Kristie Koski, Yi Cui, Robert Hammond, Malcolm Beasley, Kathryn Moler The interface between a topological insulator (TI) and an s-wave superconductor (SC) is predicted to host Majorana bound states analogous to vortices in a spinless p$_{\mathrm{x}}+$ip$_{\mathrm{y}}$ superconductor. For 3D TIs coupled to s-wave superconductors, the winding of the superconducting vortices can counteract the TI pi-Berry's phase, resulting in zero-energy Majorana fermion excitations at the interface. Transport measurements of Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ superconducting junctions have shown Josephson junction effects[1,3,4] and established the existence of a supercurrent that is tunable with gate voltage [2], but the relative contributions of the bulk and bound states to the supercurrent is not well-understood. We report on measurements of the local superfluid density at the interface between Bi$_{2}$Se$_{3}$ nanoplatelets and Nb using a scanning SQUID microscope and quartz tuning fork sensor for simultaneous AFM characterization. We demonstrate that the local penetration depth measurements have increased accuracy and provide an experimentally tractable method for studying proximity effect-induced superconductivity at the SC-TI interface, which is a precursor for observation of the elusive Majorana fermion in Bi$_{2}$Se$_{3}$ and other 3D TIs.\\[4pt] [1] arXiv:1209.5830 (2012). [2] Nat. Comm. \textbf{2} (2011). [3] Nat. Mat. \textbf{11}, 421 (2012). [4] Phys. Rev. Lett. \textbf{109}, 056803 (2012). [5] Phys. Rev. B \textbf{84}, 165120 (2011). [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y13.00014: Dirac cone shift and potential fluctuations in a passivated In$_2$Se$_3$/Bi$_2$Se$_3$ topological interface state Gregory S. Jenkins, A.B. Sushkov, D.C. Schmadel, M.-H. Kim, H.D. Drew, G. Koblmueller, M. Bichler, N. Bansal, M. Brahlek, S. Oh The topological interface state of Bi$_2$Se$_3$ capped with In$_2$Se$_3$ is measured by gated THz cyclotron resonance. An observed shift of 70 meV in the position of the Dirac point towards mid-gap due to the physical properties of the trivial insulator In$_2$Se$_3$ on Bi$_2$Se$_3$ opens new possibilities in tailoring Dirac cone properties in topological insulators. Modulating and sweeping a semi-transparant gate while probing at terahertz frequencies in magnetic field enables characterization of the burried In$_2$Se$_3$/Bi$_2$Se$_3$ topological interface state, even in the presence of significant bulk conductivity. Near the Dirac point, the mobility is 3500 cm$^2$/V$\cdot$s with potential fluctuations of 60 meV. The scattering rate shows a precipitous drop with Fermi energy indicating decoupling of the surface states from bulk states. At Fermi energies above the conduction band edge, a plateau is observed in the real part of the Faraday angle that is 80 times flatter than the step size expected from a single Landau Level, quantized in units of the fine structure constant. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y13.00015: Massive Dirac surface states in topological insulator/magnetic insulator heterostructures Weidong Luo, Xiao-Liang Qi We study the behavior of topological surface states in heterostructures formed by a topological insulator (TI) and a magnetic insulator (MI). Several MIs with compatible magnetic structure and relatively good lattice matching with TIs are identified, and the best candidate material is found to be MnSe, an anti-ferromagnetic insulator. We perform first-principles calculation in Bi$_2$Se$_3$/MnSe superlattices and obtain the surface state bandstructure. The magnetic exchange coupling with MnSe induces a gap of 54 meV at the surface states. In addition we tune the distance between Mn ions and TI surface to study the distance dependence of the exchange coupling. Finally, we study the band bending effect at the Bi$_2$Se$_3$/MnSe interface, and propose possible solutions to avoid band bending. [Preview Abstract] |
Session Y14: Focus Session: Thermal and Magnon Spin Currents
Sponsoring Units: DMP FIAP GMAGChair: Gabriel Chaves-O'Flynn, New Jersey Institute of Technology
Room: 316
Friday, March 22, 2013 8:00AM - 8:36AM |
Y14.00001: GMAG PhD Dissertation Research Award: The Planar Nernst and Seebeck Effects in Ferromagnetic Metal Films with In-Plane Thermal Gradients Invited Speaker: Azure Avery Recently, the spin Seebeck effect (SSE) has attracted a great deal of attention as one possible source of pure spin currents. In response to a thermal gradient ($\nabla T$), the SSE is thought to produce a pure spin current detectable by measuring a transverse voltage ($V_T$) generated by the inverse spin Hall effect. However, recent work on spin-dependent transport in thin film nanostructures supported by bulk substrates suggests that early SSE experiments may have been strongly affected by unintended $\nabla T$ through the supporting substrates. They may also have been affected by thermoelectric effects generated from planar thermal gradients such as transverse thermopower, also known as the planar Nernst effect (PNE), in which a $V_T$ develops in response to a $\nabla T$ applied in the plane of a film with in-plane magnetization. In this talk, we present the first results from experiments designed to probe the SSE and related effects such as the PNE and longitudinal thermopower in 20 nm thick nickel and permalloy thin films deposited on suspended Si-N platforms. In our experiments, the background thermal conduction of the 500 nm thick platforms is at least 1000x smaller than the bulk substrates used previous experiments, thus confining $\nabla T$ to the plane of the film. The results exhibit the $\sin \theta \cos \theta$ angular dependence predicted by the PNE, where $\theta$ is the angle between film magnetization and thermal gradient, rather than the $\cos \theta$ dependence expected from SSE predictions. We demonstrate that the magnetic field dependence of the PNE, anisotropic magnetoresistance, and longitudinal thermopower ($\alpha$) is generated by spin-dependent scattering and present results confirming the Onsager reciprocity between $\alpha$ and the Peltier coefficient. Finally, we present an upper limit for the SSE coefficient in our experiment that is at least an order of magnitude smaller than previously reported by experiments conducted using bulk substrates. I would like to thank my collaborators Barry L. Zink and Matthew R. Pufall and gratefully acknowledge support from the NSF CAREER Grant No. DMR-0847796. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y14.00002: Spin-Seebeck effect in amorphous ferromagnetic alloys Hyungyu Jin, Zihao Yang, Roberto Myers, Joseph Heremans Since its first discovery in 2008 [1], continuous research on spin-Seebeck effect (SSE) has established a theory for the driving mechanisms of SSE: in the presence of a thermal gradient, the spin waves (magnons) present in ferromagnets are brought out of thermal equilibrium. It is suspected that their return to thermal equilibrium is what launches a spin flux, which then is converted into a voltage in a separate material by strong spin-orbit interactions. While it is proven that substrate phonons affect the spin-Seebeck signals [2], another possible mechanism that can drive magnons out of equilibrium can be magnon thermal conductivity. Here, to isolate the magnon and phonon contributions, we investigate the relation between SSE and magnon thermal conductivity in amorphous ferromagnetic alloys (Metglas). Because Metglas has high Curie temperature, yet mostly localized phonon modes, the magnon contribution to SSE is expected to be larger than in crystalline ferromagnets. Experimental SSE data as well as magneto-thermal conductivity data will be presented. \\[4pt] [1] K. Uchida et al., Nature 455, 778 (2008).\\[0pt] [2] C.M. Jaworski et al., PRL 106, 186601 (2011). [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y14.00003: Intrinsic Spin Seebeck Effect in Gold Danru Qu, Ssu-Yen Huang, Jun Hu, Ruqian Wu, Chia-Ling Chien In Spin Seebeck Effect (SSE), a pure spin current can be generated by a temperature gradient ($\nabla $T) and detected by the inverse spin Hall effect usually by Pt. Due to the propensity of out-of-plane $\nabla _{\mathrm{z}}$T through substrate, the SSE in the transverse configuration with an in-plane $\nabla_{\mathrm{x}}$T has been shown contaminated by the anomalous Nernst effect.\footnote{S. Y. Huang, et al. Phys. Rev. Lett. 107, 216604 (2011)} The SSE in the longitudinal configuration with $\nabla_{\mathrm{z}}$T suffers from the magnetic proximity effect (MPE) of Pt in contact with a ferromagnetic material thus also contaminated.\footnote{S. Y. Huang, et al. Phys. Rev. Lett. 109, 107204 (2012)} In this work, we demonstrate that Au does not exhibit MPE and reveals the intrinsic SSE. In contrast to Pt/YIG, Au/YIG shows no anomalous Hall signals, very weak inverse MR, and non-monotonic thickness dependence of spin thermal voltage, thus very weak if any MPE. Our results place an upper limit to the intrinsic SSE of 0.1$\mu $V/K at the Au thickness of 8nm, two orders of magnitude smaller than that in Pt/YIG. Spin-polarized density-functional calculations also show a sizable Pt but a negligible Au magnetic moment in contact with YIG, in agreement with experiments. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y14.00004: Electrical measurements of nonlinear magnetization dynamics Can-Ming Hu, Yongshen Gui, Lihui Bai, Paul Hyde A new approach to measure precisely nonlinear magnetization dynamics is demonstrated by using spin dynamos in combination with sensitive electrical probing techniques. The directly measured intrinsic foldover effect of ferromagnetic resonance in Py unravels a 50-year-old mystery of ferromagnetic metals. Pivotal importance of nonlinear ferromagnetic damping is uncovered via its distinct dependence on the frequency, amplitude, and initial conditions. The experimental results are in excellent agreement with a phenomenological model, which revises the pioneer theoretical work of Anderson and Suhl for nonlinear magnetization dynamics. New evidence for electrically detected pure spin pumping in the nonlinear dynamic regime will be briefly discussed. For more information and references, please check our group website at: http://www.physics.umanitoba.ca/\textasciitilde hu/. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:48AM |
Y14.00005: Irreversible thermodynamics of transport and relaxation of magnetic moments with applications for spin caloritronics Invited Speaker: Sylvain Brechet Spin caloritronics is mainly focused on studying the effects of a temperature gradient on the time evolution of the local spin average of a classical system. In many experimental situations, the system can be treated as a classical continuum with magnetisation on the scale of interest where the quantum fluctuations average out and the underlying microscopic structure is smoothed out. Here, we establish a clear classical formalism describing the thermodynamics of a matter continuum with magnetic moments interacting with external electromagnetic fields. Taking into account the chemical nature of the current densities -- such as the current density of magnetic moments -- and stress tensors leads to three types of dissipation terms: scalars, vectors and pseudo-vectors. The scalar terms account for the chemical reactivities, the vectorial terms account for the transport and pseudo-vectorial terms account for the relaxation. The vectorial phenomenological relations establish notably the Spin Seebeck effect first observed by Uchida and Saitoh. The pseudo-vectorial phenomenological relations establish in particular the Landau-Lifschitz relaxation of the magnetisation. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y14.00006: Spin wave mode coexistence: A consequence of the Oersted field induced asymmetric energy landscape Randy Dumas, Ezio Iacocca, Stefano Bonetti, Sohrab Sani, Majid Mohseni, Anders Eklund, Johan Persson, Olle Heinonen, Johan Akerman The emerging field of magnonics relies on the systematic generation, manipulation, and detection of spin waves (SWs). Nanocontact spin torque oscillators (NC-STOs) provide an ideal platform to study spin transfer torque induced SW emission [1,2]. In analogy to two species competing for the same food supply it has been argued that only one SW mode can survive in the steady state [3]. However, as evidenced in many experiments clear signatures of mode-hopping are often observed [1,4]. We present a third possibility, namely that under the correct experimental conditions, mode coexistence can be realized. Micromagnetic simulations reveal that the SW modes are spatially separated under the NC. Mode coexistence is facilitated by the local field asymmetries induced by the spatially inhomogeneous Oersted field in the vicinity of the NC and further promoted by SW localization. Finally, both simulation and experiment reveal a weak low frequency signal exactly at the difference of the mode frequencies, consistent with inter-modulation of two coexistent modes. [1] S. Bonetti, et al., PRL 105, 217204 (2010). [2] M. Madami, et al., Nature Nanotechnol. 6, 635 (2011). [3]$^{\, \, }$ F. M. de Aguiar, et al., PRB 75, 132404 (2007). [4]$^{\, \, }$ P. K. Muduli, et al., PRL 108, 207203 (2012). [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y14.00007: Controlling spin-wave propagation with Oersted fields K. Vogt, B. Hillebrands, H. Schultheiss, J.E. Pearson, F.Y. Fradin, S.D. Bader, A. Hoffmann The goal of magnon spintronics is to utilize the coherent propagation of spin waves for low-power data processing. Spin waves carry angular momentum and can transport spin information over distances much larger than the spin diffusion length of metals. However, in thin magnetic films the highly anisotropic dispersion relation leads to strong changes in the spin-wave energy for different angles between their propagation direction and the magnetization orientation. Consequently, spin waves only travel along a straight path if the magnetization direction is fixed by a global external magnetic field. We demonstrate that locally rotating magnetic fields generated via electric current pulses allow to vary the propagation direction of spin waves. Using spatially resolved Brillouin light scattering microscopy the propagation behavior was directly verified.\footnote{K.~Vogt, H.~Schultheiss, S.~Jain, J.E.~Pearson, A.~Hoffmann, S.D.~Bader, and B.~Hillebrands, Appl. Phys. Lett. {\bf 101}, 042410 (2012)} We have modeled the current generated magnetic fields with a finite element code and calculated the magnetic response using micro magnetic simulations. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y14.00008: Mapping microwave fields using the spin Hall effect Vincent Vlaminck, Helmut Schultheiss, John Pearson, Frank Fradin, Samuel Bader, Axel Hoffmann We present measurements of the spatial variation of the spin pumping - inverse spin Hall effect in a palladium/permalloy bilayer via a coplanar waveguide ferromagnetic resonance (CPW-FMR) broadband technique. We show that the inverse spin Hall signal is both inhomogeneous and asymmetric with respect to both the position along the CPW and the excitation port. These frequency dependent asymmetries in the measured voltage are most likely due to an impedance mismatch at the contact points and the asymmetry between the two ends of the CPW. Based on this observation we show how the inverse spin Hall effect can be used as a sensitive probe for mapping the microwave magnetic field distribution in the FMR frequency range. This work emphasizes the importance of characterizing the microwave field homogeneity in every experiment with extended samples. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y14.00009: Probing the Influence of Thermal Spin Torque on Magnetic Tunnel Junction Switching Timothy Phung, Aakash Pushp, Charles Rettner, Brian Hughes, See-Hun Yang, Stuart Parkin It has been established in the past few years that heat flow within a ferromagnet can induce a spin current and an associated voltage. This so called Spin Seebeck effect, initially reported in ferromagnetic metals, has also been observed in magnetic semiconductors, magnetic insulators as well as in strongly spin orbit coupled systems. An open question has been whether heat induced spin currents can be used in switching a magnetic tunnel junction (MTJ) via thermal spin torque (TST). In order to answer this question, we investigate the MTJ switching with TST induced by sharp temperature gradients on the order of 1-10 K/nm. We will describe our experimental setup and present data that show the various roles that temperature plays on the saturation magnetization of the material and on the induced spin currents that influence MTJ switching. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y14.00010: Temperature gradient assisted spin transport in nonlocal lateral spin valves Saidur Rahman Bakaul, Shaojie Hu, Takashi Kimura The advent of non-local spin transport devices (NSTD) provide further possibility for nano spin-electronic devices as these are capable of generating electronic charge-free and non-dissipative pure spin current. The most imperative and primary issue associated with these generic spintronic devices is finding the ways to enhance the amplitude of pure spin current and the simplest way to do that is increasing the excitation charge current density. The bottleneck for this method is the Joule heating, which reduces the pure spin current. However, recent discoveries of spintronic versions of the thermoelectric effects, such as spin-dependent Seebeck and Peltier effects$^{\, }$convincingly imply that, in a properly designed device, the thermal gradient may provide aiding impact for pure spin current. In this work we have experimentally studied the multi terminal NSTDs and observed room temperature enhancement of the spin signal at high bias current. The magnitude of the spin signal is asymmetric with respect to the DC bias polarity. We discuss about the role of different thermoelectric effects on the observed spin signal enhancement. These results are important as it may open the road to tackle the Joule heating induced degradation of spin signal in NSTDs. [Preview Abstract] |
Session Y15: Focus Session: Kagome Materials and Experiments
Sponsoring Units: GMAG DMPChair: Young Lee, Massachusetts Institute of Technology
Room: 317
Friday, March 22, 2013 8:00AM - 8:12AM |
Y15.00001: Detection of low energy spin loop excitations in rare earth kagom\'{e} systems Michael Hoch, Sanhita Ghosh, Saiti Datta, Haidong Zhou, Christopher Wiebe, Stephen Hill Collective spin excitation spectra in frustrated antiferromagnets have been detected using high field electron magnetic resonance (EMR). At low temperatures the langasite kagom\'{e} systems R$_{3}$Ga$_{5}$SiO$_{14}$(R = Pr and Nd) exhibit short range spin correlation effects. Neutron scattering has shown that these systems do not exhibit long-range magnetic order at temperatures down to 30 mK. Field-sweep EMR measurements made on single crystals of Pr$_{3}$Ga$_{5}$SiO$_{14}$ and Nd$_{3}$Ga$_{5}$SiO$_{14}$ in the temperature range 1.3 - 20 K, and in fields up to 22 T, give a series of absorption peaks which are quite different to conventional EMR spectra. The resonances are interpreted using a model which involves spin-wave excitations in short range antiferromagnetically correlated spin loops or clusters. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y15.00002: Oxygen Defect Structure in the Geometrically Frustrated Kagom\'{e} System YBaCo$_4$O$_{7+\delta}$: Impact on Structure and Magnetic Properties S. Avci, O. Chmaissem, H. Zheng, A. Huq, P. Manuel, J. F. Mitchell The $R$BaCo4O7 family ``$R$-114''(where $R =$ rare earth, Y or Ca) have been a model system due to their high oxygen affinity, significant electrochemical properties and geometric frustration, in which face-sharing tetrahedra of Co ions link to form trigonal bipyramids on a Kagom\'{e} lattice. Here we report quantitative thermogravimetric analysis (TGA), \textit{in-situ} x-ray diffraction (XRD), high resolution synchrotron x-ray and neutron diffraction data characterizing the oxygen uptake/release phenomenon and its impacts on structure and magnetic properties of YBaCo$_4$O$_{7+\delta}$. We show that YBaCo$_4$O$_{7+\delta}$ reaches an equilibrium state with $\delta \sim$0.1 when heated slightly above 350 $^{\mathrm{o}}$C. When heated slightly below 350 $^{\mathrm{o}}$C, it absorbs significantly more oxygen ($\delta = 1 \sim$1.1) and shows the orthorhombic \textit{Pbc}2$_1$ symmetry previously reported [O. Chmaissem et al. J. Solid State Chem. 181, 664 (2008)]. We also detected the existence of a miscibility gap that separates the $\delta =$ 0 and $\delta =$0.1 phases. In samples $\delta$ $\geq$ 0.1, excess oxygen suppresses the structural transition however, there are strong short range magnetic correlations below 100 K despite the preserved Kagom\'{e} structure. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y15.00003: Neutron Scattering Studies of a Flat Mode in an S=$\frac{1}{2}$ Kagome Ferromagnet Robin Chisnell, Danna Freedman, Joel Helton, Deepak Singh, Chris Stock, Franz Demmel, Robert Bewley, Daniel Nocera, Young Lee Systems with flat bands provide macroscopic degeneracy that allows for the emergence of interesting strongly correlated phenomena such as the fractional quantum Hall effect. Hopping models on geometrically frustrated lattices with spin-orbit interactions predict the existence of flat, topologically nontrivial bands. Experimental realizations of these systems have proved challenging, as the flat band is often distorted by additional interactions. Cu(1,3-bdc) is a hybrid organometallic compound featuring S=$\frac{1}{2}$ Cu$^{2+}$ ions on a kagome lattice. The magnetic moments order ferromagnetically below T=1.8K. We present neutron scattering measurements of Cu(1,3-bdc) and note the emergence of a flat magnon band in the ordered phase. The presence of a small Dzaloshinsky-Moriya interaction along with an applied magnetic field perpendicular to the kagome plane creates a gap between the flat band and lower energy dispersive band. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 9:12AM |
Y15.00004: Kapellasite: a kagome quantum spin liquid with competing interactions Invited Speaker: Edwin Kermarrec In recent years, the search for an experimental quantum spin liquid in two dimensions has attracted much interest in the community. Magnetic frustration and quantum fluctuations are believed to be key ingredients to stabilize such a spin liquid ground state in 2D. The $S=\frac{1}{2}$ kagome lattice combines these two ingredients. Among the materials available with this geometry, herbertsmithite has proven to be a very promising candidate. There, the antiferromagnetic nearest neighbor coupling $J_1$ is dominant. In this talk, I will explore the effect of frustration generated by competing interactions on the quantum kagome lattice, based on experiments performed on kapellasite Cu$_3$Zn(OH)$_6$Cl$_2$, a polymorph to herbertsmithite. The system Hamiltonian, determined from a fit of a high-temperature series expansion to magnetic susceptibility and specific heat data, points to competing interactions with a ferromagnetic nearest neighbor exchange $J_1$ and an ``across-hexagon'' antiferromagnetic one $J_d$, with a ratio $\vert J_d / J_1 \vert \simeq 0.85$. Local probes ($\mu$SR, $^{35}$Cl-NMR) and inelastic neutron scattering (INS) experiments evidence a gapless spin-liquid state down to 20 mK, showing unusual dynamic short-range correlations characteristic of a 12 spin sublattices antiferromagnetic state called Cuboc2. We further investigate the spin dynamics at different timescales by NMR, $\mu$SR and INS measurements and discuss our results within the context of theoretical calculations using the Schwinger-Boson mean field approach. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y15.00005: Signature of a Spin Liquid State in the Low-Frequency Optical Conductivity of the S $=$ 1/2 Kagome Antiferromagnet Herbertsmithite Daniel Pilon, Tianheng Han, Joshua Lui, David Shrekenhamer, Alex Frenzel, William Padilla, Young Lee, Nuh Gedik Herbertsmithite (ZnCu$_{\mathrm{3}}$(OH)$_{\mathrm{6}}$Cl$_{\mathrm{2}})$ is an antiferromagnetic Mott insulator composed of a planar kagome arrangement of S $=$ 1/2 copper atoms separated by nonmagnetic zinc atoms. It has recently emerged as one of the best candidates for exhibiting a quantum spin liquid state, showing no magnetic order down to 50 mK despite an exchange energy of 200 K. Here we report a signature of a spin liquid state in the terahertz optical conductivity of Herbertsmithite, measured via Terahertz Time-Domain Spectroscopy. A power-law dependence on frequency with exponent $\sim$ 1.4 is observed in the in-plane conductivity at low frequency, which increases in magnitude as temperature is decreased. This contribution to the conductivity is notably absent in the out-of-plane direction. Theory has predicted that the existence of a Dirac spin liquid with a gauge field serving to couple the spin and charge degrees of freedom would give rise to a power-law conductivity with exponent $\sim$ 2 inside the Mott gap. We discuss this prediction as well as other possible sources of the observed behavior. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y15.00006: Thermodynamic analysis of a kagome spin liquid candidate Tianheng Han, Craig Bonnoit, Robin Chisnell, Joel Helton, Yasu Takano, Young Lee Herbertsmithite ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$--one of the most promising quantum spin liquid candidates--presents a promising system for studies of frustrated magnetism on an S$=$1/2 kagom\'{e} lattice. Following our recent success in crystal growth, specific heat has been measured at dilution fridge temperatures up to 18 T on a single crystal sample which gives further information on the low temperature phase. Additional analysis of the thermodynamic measurements on single crystal samples lends further hints on the intrinsic spin liquid physics. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y15.00007: High field magnetic studies of S=1/2 Kagome lattice single crystalline Herbertsmithite T. Asaba, Gang Li, Ben J. Lawson, F. Yu, Z. Xiang, P. Cai, C. Tinsman, Tianheng Han, Young Lee, Lu Li Herbertsmithite ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ is a promising system to study frustrated magnetism on S=1/2 kagome lattice. A continuum of spinon excitations has been revealed by recent neutron scattering measurements on single crystals. Interesting questions arise on the fate of this spinon excitation under intense external magnetic field. We report field-driven transitions in the high field magnetization of single crystalline ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$. These transitions appear below 1 K, and the transition field values are almost independent of the magnetic field orientation. We further discuss methods to separate the magnetic contribution from the impurity to repeal the intrinsic response of the kagome lattice. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y15.00008: Fermion mediated state selection in the Kagome lattice and antiferromagnetism in FeCrAs Patrick J. O'Brien, Shivam Ghosh, Michael J. Lawler, Christopher L. Henley We study classical spins on a kagome lattice with weak Hund's coupling $J_{H}$ to hopping electrons. For each filling, the effective RKKY interactions at all distances are extracted both by fits of the total electronic energy to a database of random spin configurations, as well as second order perturbation theory in $J_{H}$. We apply this to model the Cr antiferromagnetic order found below 125K in FeCrAs [2], in which one Cr d band split by the crystal field plays the role of the itinerant fermions; the observed $\sqrt3 \times \sqrt3$ type order is indeed, close to half filling, the optimum state according to our model (out of the commonly considered alternatives) . In contrast, the limit of strong $J_{H}$ favors the cuboc1[1] state over the $\sqrt3 \times \sqrt3$ state[3], giving a bound on the possible value of the $J_{H}$ in FeCrAs. Additionally, for weak $J_{H}$, cuboc1[1] is selected instead of $\sqrt3 \times \sqrt3$ close to 5/12 filling. The complete phase diagram as a function of filling can be found using Monte Carlo (MC) minimization with the RKKY Hamiltonian. [1] Messio et al PRB 83, 184401 (2011) [2] W. Wu et al EPL 85, 17009 (2009) [3] Shivam Ghosh, Contributed talk, March Meeting 2013 [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y15.00009: Double Exchange, Berry fluxes, and fermion mediated state selection in frustrated lattices Shivam Ghosh, Christopher L. Henley We consider a Kagome or Pyrochlore magnet with local moments (treated as classical) as well as noninteracting electrons with hopping $t$ at metallic filling, in the ``Double Exchange'' (DE) limit of infinitely strong Hund's rule coupling $J_{H}$. Whereas a DE-dominated model always has a ferromagnetic ground state, we make the problem nontrivial by including a dominant separate antiferromagnetic exchange $J >> t$, so the DE is a perturbation selecting within the highly degenerate ground states of J [1]. We derive this in two stages (i) spin directions define a set of Berry fluxes for each loop in the lattice (ii) we fit an effective Hamiltonian in terms of these fluxes. The same method can be applied to the energy landscape of competing spin-liquid-like states within large-N mean field theories. Depending on filling, the stable state on the Kagome is coplanar or the non-coplanar ``cuboc1'' [2] phase.\\[4pt] [1] Motome and Furukawa, PRL 104, 106407(2010).\\[0pt] [2] Messio, Lhuillier, and Misguich, PRB 83, 184401 (2011). [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y15.00010: Neutron scattering study of the dimerized spin 1/2 AFM kagome lattice in $Rb_2Cu_3SnF_{12}$ Yang Zhao, K. Matan, Y. Nambu, T. J. Sato, Y. Fukumoto, T. Ono, H. Tanaka, C. Broholm, A. Podlesnyak, G. Ehlers The deformed AFM kagome lattice $Rb_2Cu_3SnF_{12}$ is the first realization of 'pinwheel' valence bond solid (VBS) ground state system [1]. Using inelastic neutron scattering technique, we mapped out the spin excitation spectrum up to 12 meV. The singlet to triplet transition is split by a substantial Dzyaloshinskii-Moriya (DM) interaction, with the energy gap at 2.4 meV ($S_z$= $\pm$ 1) and 6.9 meV ($S_z$ = 0), respectively. While both excitations are non-dispersive to within 1.0 meV for wave vectors, $q_z$, perpendicular to the kagome like plane, the intensity varies differently with $q_z$ for the two modes. This difference can be explained by the different polarization factor for $S_z$= $\pm$ 1 and $S_z$ = 0 excitations. Under a magnetic field along the c-axis, the low energy gap persist near 1 meV for the fields between 9 T and 15 T. Our findings emphasize the important role of DM interaction in this material.\\[4pt] [1] K. Matan, T. Ono, Y. Fukumoto, T. J. Sato, J. Yamaura, M. Yano, K. Morita, and H. Tanaka, Nature Physics 10 (2010). [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y15.00011: Electronic structure of the kagome lattice Cu$_4$(OH)$_6$FBr Kateryna Foyevtsova, Francesc Salvat-Pujol, Harald O. Jeschke, Roser Valenti, John Schlueter We investigate the electronic and magnetic properties of Cu$_4$(OH)$_6$FBr in the framework of ab initio density functional theory calculations and model considerations. This system, similarly to the well known Herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$, consists of stacked layers of Cu$^{2+}$ ions arranged in a Kagome pattern. We will discuss in terms of microscopic models the resemblances and differences between these two systems. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y15.00012: Spin configurations in the frustrated spin system YBaCo$_{4}$O$_{7}$ by $^{59}$Co NMR Shaojie Yuan, Michael Hoch, Philip Kuhns, Tiglet Besara, Jeff Whalen, Theo Siegrist, Arneil Reyes, Jim Brooks, H. Zheng, John Mitchell The frustrated spin system YBaCo$_{4}$O$_{7}$ has both kagom\'{e} and triangular planes of cobalt ions alternating with each other. The cobalt spins in the triangular layers order antiferromagnetically below the N\'{e}el temperature at 106 K. The configurations of the cobalt spins have been studied by both neutron scattering\footnote{D. D. Khalyavin, P. Manuel, J. F. Mitchell, and L. C. Chapon, Phys Rev B \textbf{82,} 094401 (2010).} and zero applied field $^{59}$Co NMR. While the triangular spin orientations are in agreement for the two approaches, this is not the case for the kagom\'{e} layers. The present in-field sample rotation NMR experiments confirm our previous finding that the triangular spins are aligned perpendicular to the [110] crystallographic direction and provide strong evidence that the spins in the kagom\'{e} layers are orthogonal those in the triangular layers in what may be described as an internal-field-induced spin-flop configuration. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y15.00013: NIR Optical Studies of the Warped-Kagome Frustrated Magnet Neodymium Langasite Christopher Ferri, Chris Weibe, Sayantani Ghosh We investigate the anti-ferromagnetic-to-spin liquid phase transition of Neodymium (Nd) Langasite, a warped Kagome lattice, using static fluorescence spectroscopy as a function of temperature. Nd3$+$ is excited at 808 nm and the fluorescence of the ground state to first excited transition is measured, the spectrum of which is a multiplet centered on 890 nm. We measure this spectrum at temperatures ranging between room temperature (295K) and 5K. The individual transitions comprising the spectrum are then fit by Lorentzians to determine the center wavelength ($\lambda _{\mathrm{c}}$) of each transition. Plots of $\lambda_{\mathrm{c}}$ versus temperature show zeros in the first derivative near 52 K, the Neel temperature, and second derivative near 33K, the anti-ferromagentic-to-spin liquid transition temperature. We attribute this to the phase transitions affecting the Zeeman energy of these levels. [Preview Abstract] |
Session Y16: Magnetic Theory II
Sponsoring Units: GMAGChair: Khorgolkhuu Odbadrakh, Oak Ridge National Laboratory
Room: 318
Friday, March 22, 2013 8:00AM - 8:12AM |
Y16.00001: Dynamics of Thermal Effects in the Spin-Wave Theory of Quantum Antiferromagnets Angel Rivas, Miguel A. Martin-Delgado The main propose of this work [1] is to study the dynamics of quantum antiferromagnets due to the interaction with a thermal environment. To this end we resort to the spin wave theory which has become by now an standard and reference tool in order to have a good approximate description of quantum antiferromagnetic systems in appropriate dimensions. We derive a master equation that allows us to study non-equilibrium dynamics due to the thermal bosons in the environment, and give closed analytic form for the magnon decay rates. Moreover, we show that these ones turn out to be closely related to form factors, which are experimentally accessible by means of neutron and Raman scattering. Furthermore, we compute the time-evolution of the staggered magnetization showing that, for moderate temperatures, the magnetic order is not spoilt even if the coupling is fully isotropic. As far as we know, this is a fundamental aspect of spin wave theory that has remained unexplored. We expect this presentation may be interesting for a broad audience as it is at the crossroads of strongly correlated systems and the physics of quantum open systems, that is so much rooted in quantum information theory.\\[4pt] [1] A. Rivas and M.A. Martin-Delgado, Ann. Phys. (N.Y.) (in press), and arXiv:1112.315. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y16.00002: Relevance of Deconfined-Criticality Action in the Light of the J-Q Spin Model Yuan Huang, Kun Chen, Youjin Deng, Anatoly Kuklov, Nikolay Prokofev, Boris Svistunov We perform large scale Monte Carlo simulations to study critical flows of 2D spin-1/2 J-Q model and 3D SU(2) symmetric discrete NCCP$^1$ model, a.k.a. deconfined-critical-point (DCP) action. The flows of the J-Q model and the DCP action collapse in a significantly large region of system sizes (up to L$\sim 60-80$), implying that the DCP theory (in general) and the discrete NCCP$^1$ model (in particular) correctly capture mesoscopic physics of the competition between the antiferromagnetic and valence-bond orders in quantum spin systems. At larger sizes we observe significant deviations between the two flows which both demonstrate strong violations of scale invariance. Furthermore, while the Neel state is perfectly space-time symmetric, the competing phase shows significant deviations from this symmetry. Possible scenarios are outlined. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y16.00003: Condensation transitions in critical spin chains Ville Lahtinen, Teresia M{\aa}nsson, Juha Suorsa, Eddy Ardonne We show that two well known one-dimensional spin chains, namely the XY spin chain and the transverse field Ising model with only next-nearest neighbor interactions, can be related at their critical points via an exact mapping. For periodic boundary conditions, the two chains only differ by a boundary term, which accounts for the differences in the critical behavior. We argue that the boundary term induces a ``condensation transition,'' which is closely related to condensation transitions between gapped two-dimensional topological phases. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y16.00004: Oxygen vacancy driven structural and orbital reconstruction on SrTiO$_{3}$ surface and subsurface Chandrima Mitra, Chungwei Lin, Alexander A. Demkov The role played by oxygen vacancies in bringing about important structural and electronic changes on oxide surfaces and interfaces have been a subject of intense scientific study. From two-dimensional electronic conductivity to the formation of magnetic states, oxygen vacancies have been suggested to be responsible for introducing a variety of interesting physical effects in bulk oxides and their surfaces. In this work, we employ Density Functional theory to perform first principles calculations of oxygen vacancy defects on SrTiO$_{3}$ surface and subsurface. In a defect free SrTiO$_{3}$ surface, the surface Ti atoms have conduction bands whose lower end comprises of split $t_{2g}$ states (lower lying degenerate $d_{xz}$ and $d_{yz}$ states and the upper lying $d_{xy}$ state). The upper conduction bands consist of split $e_{g}$ states where the $d_{z}^{2}$ orbital is shifted lower in energy with respect to the $d_{{x^2}-{y^2}}$ orbital. In the presence of an oxygen vacancy, orbitals reorder and the Ti $d_{z}^{2}$ orbitals, (which also hybridizes itself with Ti \textit{4s} state and the neighboring oxygen $p$ states) gets pushed down and occupied leading to the formation of a defect state. Formation energies of oxygen vacancies on the surface and subsurface of SrTiO$_{3}$ will be presented and the possibility of vacancy induced magnetic states on SrTiO$_{3}$ surface will be discussed. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y16.00005: A classification scheme of oxide sulfides to guide the design of new hole-conducting transparent materials Giancarlo Trimarchi, Kanber Lam, Arthur Freeman, Kenneth Poeppelmeier, Alex Zunger The addition of S to transition metal oxides has been contemplated as a way to overcome the limitations of pure oxides by producing a hybridized O-S band with lighter hole mass and narrower gap. Here, we show that O-S mixing could lead either to a continuous band broadening and an upward shift of the valence bands (``band amalgamation" scenario) or to the formation of S-localized states deep in the band gap of the host oxide above the O band (``band pinning" scenario). We survey the La-based oxide sulfides by first-principles methods and we observe the following types of VBM wavefunction in relation to the coordination of the O and S atoms: (i) O and S segregate into separate molecular units; the VBM is preferentially localized on the S units (e.g., LaOCuS). (ii) O and S segregate into separate molecular units; the VBM is delocalized on both O and S units (e.g., (LaO)$_{2}$SnS$_{3}$). (iii) O and S are spatially mixed in the lattice; the VBM is preferentially localized on S (e.g., LaGaOS$_{2}$). (iv) O and S are spatially mixed in the lattice; the VBM is delocalized on both S and O (e.g., LaCrOS$_{2}$). Thus, selecting the type of anion coordination is a posible route to tune the hole conductivity in oxide sulfides. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y16.00006: Second Order Effective Theory of Bloch Electrons in Electromagnetic Fields Yang Gao, Shengyuan Yang, Qian Niu In the first order effective theory of Bloch electrons in electromagnetic fields, the Berry curvature is introduced to yield an anomalous velocity term, which results in profound modification of the phase space density of states.~ Here we derive the second order single band effective theory, finding that the semiclassical dynamics of physical variables still follows the same structure as before, but with additional field corrections in the Berry curvature and band energy. We also discuss applications of our theory and its extension to multiple band case. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y16.00007: Efficient simulation of infinite tree tensor network states on the Bethe lattice Wei Li, Jan von Delft, Tao Xiang We show that the simple update approach proposed by Jiang et al [H.C. Jiang, Z.Y. Weng, and T. Xiang, Phys. Rev. Lett. \textbf{101}, 090603 (2008)] is an efficient and accurate method for determining the infinite tree tensor network states on the Bethe lattice. Ground state properties of the quantum transverse Ising model and the Heisenberg XXZ model on the Bethe lattice are studied. The transverse Ising model is found to undergo a second-order quantum phase transition with a diverging magnetic susceptibility but a finite correlation length which is upper-bounded by $1/\ln(q-1)$ even at the transition point ($q$ is the coordinate number of the Bethe lattice). An intuitive explanation on this peculiar ``critical'' phenomenon is given. The XXZ model on the Bethe lattice undergoes a first-order quantum phase transition at the isotropic point. Furthermore, the simple update scheme is found to be related with the Bethe approximation. Finally, by applying the simple update to various tree tensor clusters, we can obtain rather nice and scalable approximations for two-dimensional lattices. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y16.00008: A Monte Carlo Approach to Modeling Thermal Decay in Perpendicular Recording Media Tim Fal, Jason Mercer, Martin Leblanc, John Whitehead, Martin Plumer, Johannes van Ek A procedure is developed to study the evolution of high anisotropy magnetic recording media due to thermally activated grain reversal [1]. Single-domain grains evolve by passing through a sequence of relatively long-lived metastable states punctuated by abrupt reversals. Solutions to the rate equations are obtained using a stochastic integration procedure that calculates the time between successive reversals. Transition rates are formulated from the Arrhenius-Neel expression in terms of the material parameters, the temperature and the applied field. The method is applied to study the rate dependence of finite temperature MH loops and the thermal degradation of a recorded bit pattern in perpendicular recording media. A significant advantage of the method is its ability to extend simulations over time intervals many orders of magnitude greater than is feasible using standard micromagnetics with relatively modest computational effort.\\ $[1]$ T.J. Fal, J.I. Mercer, M.D. Leblanc, J.P. Whitehead, M.L. Plumer, and J. van Ek, Phys, Rev. B, submitted (2012). [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y16.00009: Numerical simulation of 2D ferromagnetic films with perpendicular magnetic anisotropy using a hexagonal lattice and a long range RKKY interaction potential Zachary Howard, Michael S. Pierce A numerical $\varphi^4$ model combined with a RKKY potential was used to simulate 2-D ferromagnetic domains. A small random field component was added to allow for a controlled amount of disorder to be introduced into the system. A hexagonal lattice allows for more realistic domains patterns than a square lattice due to the higher density of lattice sites compared to the conventional square lattice. We find that appropriate regions of parameter space produce realistic domain patterns, major hysteresis loops, and reversal curves. For parameters that produce regions of rapid nucleation and growth we observe reversal curves that can extend outside the major hysteresis loops, due to highly frustrated domain configurations as recently observed by Ref. [1]. We also observe a significant region of exponential dependence of the domain spacing upon the interaction potential. Future work will include increasing the random field contribution to determine if the dependence of the domains and hysteresis loops upon disorder matches experimental systems [2].\\[4pt] [1] J.E. Davies et al., Appl. Phys. Lett. 95, 022505 (2009).\\[0pt] [2] M.S. Pierce, et al., submission to Phys. Rev. B [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y16.00010: Inhomogeneous phases of repulsive fermions in cubic lattices Jie Xu, Simone Chiesa, Shiwei Zhang We present a fully self-consistent mean-field study of the inhomogeneous phases in the three-dimensional Hubbard model as the density deviates from half-filling. As the interaction U increases at fixed density, there is a transition from a uniform Fermi liquid to an inhomogeneous metallic phase characterized by a spin density wave along the [001] direction. Upon further increase of U the system undergoes a discontinuous transition to an insulating phase with a spin density wave along the [111] direction. We determine the evolution of the modulation wavelength of the spin density wave as a function of U and density, and discuss signature in the momentum distribution that are relevant to optical lattice experiments. Crossover from two- to three-dimensions is also studied. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y16.00011: Theoretical Scanning Probe Images of the (001) Surfaces of MnO and NiO Mihail Granovskij, Andreas Schr\"on, Friedhelm Bechstedt In the paramagnetic state the ground-state crystal structure of the 3$d$ transition metal oxides (TMOs) MnO and NiO is given by an ideal rock-salt ($rs$) structure. Below their respective N\'eel temperature, however, it is characterized by the formation of an antiferromagentic ordering AFM2 which is acompanied by a rhombohedral distortion along the [111] direction. The intersection of the thermally swichable magnetic ordering AFM2 with the crystal surfaces makes TMO surfaces ideal benchmark materials for the investigation of recent magnetic scanning probe techinques such as spin-polarized scanning tunneling microscopy (SP-STM) and magnetic exchange force microscopy (MExFM). We present a density functional theory (DFT) study of the (001) surfaces of MnO and NiO inculding an on-site interaction $U$. Different theoretical approaches for the description of magnetic scanning probe techniques are employed. the magnetic tip is modelled by a single Fe or 5-Fe-atom pyramid. For NiO, the calculated scanning probe images explain the spin contrast and the corrugation found experimentally. For MnO, the calculated images represent interesting predictions which differ from that of NiO. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y16.00012: Calculated magnetic structure of mobile defects in Fe Don Nicholson, Kh. Odbadrakh, German Samolyuk, G. Malcolm Stocks Mobile defects such as dislocations and crowdions respond to gradients of strain, temperature, concentration, and applied field, thereby, determining a material's viability in particular applications. In Fe, defects affect the magnetic state of the surrounding atoms. We discuss the defect-induced changes in magnetic moment magnitude and orientation, magnetic anisotropy and magnetic interactions. These quantities are calculated (density functional theory (DFT)) for defect models ranging in size from a few hundred to a few thousand. Comparisons are made between different DFT methods. The importance of magnetism to the response of defects to gradients is discussed. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y16.00013: Iron impurities in gold and silver: Comparison of magnetoresistance data to numerical renormalization group calculations exploiting non-Abelian symmetries Markus Hanl, Andreas Weichselbaum, Theo Costi, Christopher B\"auerle, Jan von Delft We consider iron impurities in the noble metals gold and silver and compare experimental data for the resistivity and decoherence rate to numerical renormalization group results for a fully screened $n$-channel, spin $S=n/2$ Kondo model. Our code exploits non-abelian symmetries, which increases the efficiency by orders of magnitude compared to plain abelian NRG. To be specific, the symmetries used were U(1) for charge conservation, U(1) for spin conservation in the presence of magnetic field and the SU(3) channel symmetry. Compared to previous work [1] on this subject, we show superior numerical data for both quantities at finite temperature and extend our analysis to the resistivity at finite magnetic field. We show that our results are converged and that all examined quantities can be described consistently with a single value of $T_K$. The excellent agreement between experiment and theory for $n=3$ shows that both systems are described by a spin-3/2 three-channel Kondo model. [1] T. Costi et al. Phys. Rev. Lett. \textbf{102}, 056802 (2009). [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y16.00014: Magnetic vortices induced by a moving tip Martin P. Magiera, Alfred Hucht, Dietrich E. Wolf A two-dimensional easy-plane ferromagnetic substrate interacting with a dipolar tip which is magnetized perpendicular with respect to the easy plane is studied numerically by solving the Landau-Lifshitz Gilbert equation [Europhys.\ Lett.\ \textbf{100}, 27004 (2012)]. Due to the symmetry of the dipolar field of the tip, in addition to the collinear structure a magnetic vortex structure becomes stable. It is robust against excitations caused by the motion of the tip. The moved vortex structure shows an increased energy dissipation compared to the collinear structure. We show that for high excitations the system may perform a transition between the two states. The influence of domain walls, which may also induce this transition, is examined. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y16.00015: ABSTRACT WITHDRAWN |
Session Y17: Focus Session: Magnetic Metal Insulator Transitions
Sponsoring Units: DMP GMAGChair: Eduardo Granado, Universidade Estadual de Campinas (Brazil)
Room: 319
Friday, March 22, 2013 8:00AM - 8:36AM |
Y17.00001: Magnetically driven metal-insulator transition in NaOsO3 Invited Speaker: Stuart Calder The metal-insulator transition (MIT) is one of the most dramatic manifestations of electron correlations in materials, enjoying interest both for its fundamental nature and technological application. Various mechanisms producing MITs have been extensively considered over the years, including the Mott (electron localization via Coulomb repulsion), Anderson (localization via disorder) and Peierls (localization via distortion of a periodic one-dimensional lattice). One additional route to a MIT proposed by Slater in 1951, in which long-range magnetic order in a three dimensional system drives the MIT, has received relatively little attention, particularly from an experimental viewpoint. Using neutron and x-ray scattering we have shown that the MIT in NaOsO$_3$ is coincident with the onset of long-range commensurate magnetic order at 410 K. Whilst candidate materials have been suggested, our experimental methodology allows the first definitive demonstration of the long predicted Slater MIT. We discuss our results in light of recent work on other 5d systems that contrastingly have been predicted to host a Mott spin-orbit insulating state. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y17.00002: Metal-insulator transition in pyrochlore Eu2Ir2O7 studied by infrared spectroscopy Andrei Sushkov, Dennis Drew, Jun Ishikawa, Satoru Nakatsuji, Xuan Luo, Sang-Wook Cheong The large family of pyrochlores with formula A$_2$B$_2$C$_7$ attracted a lot of early attention due to strong geometric magnetic frustration. Recent band structure calculations predict that the iridate pyrochlores A$_2$Ir$_2$O$_7$ may have nontrivial topological states. We will report the results of an infrared spectroscopic study of the metal-insulator transition in Eu$_2$Ir$_2$O$_7$ single crystal and Y$_2$Ir$_2$O$_7$ polycrystal. We will report the broad band IR reflection as a function of temperature for an overview of the M-I transition and the low frequency transmission which is more sensitive for detection of a 10 meV gap[1] and other possible excitations. We will discuss possible implications of the semimetal Weyl states. [1] J.J. Ishikawa et al., Phys. Rev. B 85, 245109 (2012). [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y17.00003: Tuning J$_{eff}$ = 1/2 Insulating State via Electron Doping and Pressure in Double-Layered Iridates L. Li, P.P. Kong, C.Q. Jin, T.F. Qi, O.B. Korneta, S.J. Yuan, G. Cao Sr$_3$Ir$_2$O$_7$ exhibits a novel J$_{eff}$=1/2 insulating state featuring a splitting between J$_{eff}$=1/2 and 3/2 bands due to spin-orbit interaction. We report that a metal-insulator transition can be induced by either dilute electron (La$^{3+}$) doping for Sr$^{2+}$ ions in Sr$_3$Ir$_2$O$_7$ or via application of high pressure. The following constitutes the central findings of our recent study of single-crystal Sr$_3$Ir$_2$O$_7$ and (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$: (1) application of high hydrostatic pressure P results in a drastic drop in the electrical resistivity by four orders of magnitude at a critical pressure, P$_C$ = 13.2 GPa, suggesting a significantly reduced splitting between J$_{eff}$=1/2 and 3/2 bands, but further increasing P up to 35 GPa produces no fully metallic state at low temperatures; (2) however, slight doping of La$^{3+}$ ions for Sr$^{2+}$ ions in Sr$_3$Ir$_2$O$_7$ readily induces a robust metallic state that follows no Fermi liquid behavior; and (3) the magnetic ordering temperature is significantly suppressed from 285 K for x=0 but remains finite for (Sr$_{0.94}$La$_{0.06}$)$_3$Ir$_2$O$_7$ where the metallic state occurs. The results will be discussed along with comparisons drawn with Sr$_2$IrO$_4$, a prototype of the J$_{eff}$ = 1/2 insulator. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y17.00004: Exploring the Unusual Physical Properties near the Metal-Insulator Transition of RNiO$_3$ Luke G. Marshall, Jinguang Cheng, Jianshi Zhou, Mar\'Ia Jes\'us Mart\'Inez-Lope, Jos\'e Antonio Alonso, John B. Goodenough Understanding the physical properties at the crossover from localized to itinerant electronic behavior in the transition-metal perovskite-related oxides remains a challenging problem of solid-state physics. This problem can manifest in mixed-valent compounds at this crossover to produce unusual properties such as high-T$_c$ superconductivity in the cuprates and colossal magnetoresistance in the manganites. RNiO$_3$ (R=lanthanide) perovskites are single-valent compounds where the $\pi$-band is filled and the $\sigma$-band is $\frac{1}{4}$ filled. The electron bandwidth can be tuned by substituting different rare earth cations at the A site, so that the system provides a unique opportunity to study this crossover more simply. While the phase diagram for this compound is well known, magnetic rare earth ions prevent the study of the evolution from Pauli to Curie-Weiss paramagnetism. To account for this, we have used high-pressure synthesis to create a series of RNiO$_3$ samples (R=La, Y, Lu) and studied their magnetic and transport properties. We have also shown that the localized to itinerant crossover can also be explored by substituting Ga$^{3+}$ for Ni$^{3+}$ in LaNi$_{1-x}$Ga$_x$O$_3$. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y17.00005: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y17.00006: Multi-orbital Mott Transition and High $T_c$ Ferromagnetism in Strongly Correlated Oxides Mohit Randeria, Onur Erten, O. Nganba Meetei, Nandini Trivedi, Patrick Woodward Amongst all perovskites with a net magnetic moment, Sr$_2$CrOsO$_6$ (SCOO) [1] has the highest $T_c = 725$K. We model this as a multi-orbital Hubbard model with different Coulomb $U$'s and Hund's coupling $J_H$'s on the Cr and Os sites along with spin-orbit coupling (SOC) $\lambda_{so}$ on Os. Using a slave-rotor approach, we find a new Mott criterion [2] $\left(\tilde{U}_{\rm Cr}\tilde{U}_{\rm Os}\right)^{1/2} > 2.5W$, where $W$ is the bandwidth and $\tilde{U}$'s are the effective charge gaps including the effects of $U$, $J_H$ and $\lambda_{so}$. Using this result, we argue that SCCO is a Mott insulator. Next, we show that the orbital moment on Os is quenched. The effective spin Hamiltonian for $S=3/2$ moments has Cr-Os and Os-Os antiferromagnetic superexchange interactions that are frustrated. Using a variational approach and Monte Carlo simulations, we show that the system has a canted ground state with a net moment at $T=0$, a non-monotonic magnetization $M(T)$ and a high $T_c$. Our results [2] are in excellent agreement with available data [1] and we predict the magnetic $S({\bf q})$ that will test our theory. [1] Y. Krockenberger {\it et al.}, Phys Rev. B {\bf 75} 020404 (2007). [2] O. N. Meetei, O. Erten, M. Randeria, N. Trivedi, and P. Woodward, arXiv:1205.1811 [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y17.00007: Resonant Ultrasound studies of spin- and orbital ordering transitions in RVO$_{3}$ M. Koehler, J.-Q. Yan, Y. Ren, B.C. Sales, D. Mandrus, V. Keppens RVO$_{3}$ perovskites (R = rare earth) have been shown to undergo multiple spin and orbital transitions due to the Jahn-Teller active V$^{3+}$ electrons. We have initiated a study of the elastic response of RVO$_{3}$, ( R = Dy, Gd, Ce) as well as Y$_{1-x}$La$_{x}$VO$_{3}$ (x = 0.05, 0.3, 1) using resonant ultrasound spectroscopy. The temperature-dependence of the elastic response is dominated by the ordering transitions, with transition temperatures that change with the size of the rare earth. For CeVO$_{3}$ and LaVO$_{3}$, two transitions are observed, separated by 17K and 2K, respectively. DyVO$_{3}$ and Y$_{0.95}$La$_{0.05}$VO$_{3}$ show three transitions below 220K while GdVO$_{3}$ only shows one. The full elastic tensor of Y${_0.7}$La$_{0.3}$VO$_{3}$ has also been determined from 300K to 50K, yielding the temperature dependence of the 9 orthorhombic elastic moduli. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y17.00008: Strong electronic correlations and spin-orbit coupling in layered ruthenates Frank Lechermann, Malte Behrmann, Christoph Piefke The combination of the local-density approximation to density functional theory with explicit many-body approaches has proven to be a powerful tool to investigate the problem of strong electronic correlations on a realistic level. Notably in quasi-twodimensional materials the interaction between the effective dimensionality and the symmetry of the underlying crystal structure with the competition between the localized and the itinerant character of electrons is indeed giving rise to highly interesting physical phenomena, especially within the family of transition-metal oxides. Here we want to focus on the intriguing interplay between rotational-invariant local Coulomb interactions and spin-orbit coupling for the case of the layered strontium ruthenates within the Sr$_{n+1}$Ru$_n$O$_{3n+1}$ Ruddlesden-Popper series. Novel results based on a generic realistic modelling of the correlated electronic structure for the n=1,2 members of this family of compounds will be discussed [1]. In this respect, also the intriguing metamagnetic behavior of Sr$_3$Ru$_2$O$_7$ will be addressed.\\[4pt] [1] M. Behrmann, C. Piefke and F. Lechermann, Phys. Rev. B 86, 045130 (2012) [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y17.00009: Pressure study of nematicity and quantum criticality in Sr$_3$Ru$_2$O$_7$ for a in-plane field Dan Sun, Wenlong Wu, Santiago A. Grigera, Robin S. Perry, Andy P. Mackenzie, Stephen R. Julian We study the relationship between the nematic phase of Sr$_3$Ru$_2$O$_7$ and quantum criticality. At ambient pressure, the nematic phase appears to be associated with a metamagnetic quantum critical end point (QCEP) when the applied magnetic field is near the c-axis. We show, however, that this metamagnetic transition does not produce the same nematic signatures when the QCEP is reached by hydrostatic pressure with the field applied in the ab-plane. Moreover, a distinct nematic phase, that is seen for field applied in the ab-plane close to, but not right at, a metamagnetic anomaly, persists with minimal change to the highest applied pressure, 16.55 kbar. Taken together our results suggest that quantum criticality may not be necessary for the formation of a nematic phase. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y17.00010: Far-infrared optical properties and the metal-insulator transition in Ti-doped Ca$_3$(Ru$_{\mathrm{1-x}}$Ti$_{\mathrm{x}})_2$O$_7$ (x$=$0.03) D. Talbayev, T. Stanislavchuk, A. Sirenko, Jin Peng, Z.Q. Mao The discovery of the intriguing phase diagram of Ca$_3$(Ru$_{\mathrm{1-x}}$Ti$_{\mathrm{x}})_2$O$_7$ is the new and exciting development in correlated electron ruthenates, as Ti doping drastically changes the material's ground state properties. The undoped Ca$_3$Ru$_2$O$_7$ is metallic at high temperature and undergoes an antiferromagnetic transition at 56 K that is followed by a metal-insulator transition at 48 K driven by the opening of a charge density wave gap. A quasi-2D metallic state develops below 30 K. At 5{\%} Ti doping, the metal-insulator transition temperature is T$_{\mathrm{MI}}=$80 K, below which the material is a Mott insulator. By contrast, a weakly localized electronic state is observed at intermediate dopings (2-4{\%} Ti) together with antiferromagnetic long range order. In the undoped Ca$_3$Ru$_2$O$_{\mathrm{7}}$, the metal-insulator transition at 48 K is accompanied by the development of a charge gap below 200 cm$^{-1}$. At low temperatures, a small Drude peak develops below 50 cm-1, resulting from small non-nested metallic pockets of the Fermi surface. We report a far-infrared spectroscopic ellipsometry study of Ca$_3$(Ru$_{\mathrm{1-x}}$Ti$_{\mathrm{x}})_2$O$_{\mathrm{7}}$ (x$=$0.03) at U4IR beamline of NSLS-BNL. Our data indicate that the low-temperature gap in optical conductivity opens at 1000 cm$^{-1}$, a dramatically different value from the one in the undoped compound. We relate our observations to the effects of Ti doping - the induced changes in carrier itinerancy and the modified double-exchange and superexchange interactions in the material. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y17.00011: Optical Polarization Microscopy of the Electron Nematic Phase in Sr$_3$Ru$_2$O$_7$ Colin Heikes, S. Ghosh, D. MacNeill, R. Perry, J.F. Mercure, E.A. Kim, A. Mackenzie, D.C. Ralph We report the implementation of a fiber-based optical microscope, capable of operating at temperatures below 100 mK and in magnetic fields in excess of 9 Tesla, with sub-micron spatial resolution. This microscope is integrated into the bore of a dilution refrigerator with an optical fiber coupling light to an external optical table. Bench-top optical elements allow for polarization analysis of the reflected light from a surface and thus the detection of magnetic or other polarization-sensitive properties of matter at low temperature and high fields. As a first application of the instrument, we are studying the proposed electron nematic phase of the n=2 Ruddlesden-Popper material Sr$_3$Ru$_2$O$_7$, which exhibits a low-temperature phase transition in the form of an in-plane conduction anisotropy. We report initial results from polarization analysis and polarization microscopy with sample temperatures below 150 mK and applied magnetic fields from 0 T to 9 T. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y17.00012: Effect of disorder on quantum phase transition in (Sr$_{1-x}$Ca$_x$)$_3$Ru$_2$O$_7$ Z. Qu, J. Peng, T.J. Liu, D. Fobes, V. Dobrosavljevic, L. Spinu, Z.Q. Mao (Sr$_{1-x}$Ca$_{x}$)$_{3}$Ru$_{2}$O$_{7}$ is characterized by complex magnetic states, spanning from anantiferromagnetic state over an unusual heavy-mass nearly ferromagnetic (NFM) state to an itinerant metamagnetic state. The NFM state, which occurs in the 0.4 \textgreater\ $x$ \textgreater\ 0.08 range, freezes into a cluster spin glass phase at low temperatures [1]. A quantum phase transition (QPT) occurs as the spin freezing temperature $T_{f}$ is suppressed to zero K near $x=$0.08. In this talk, we will report a novel quantum phase observed near the QPT [2]. The isothermal magnetization $M(H)$ and the temperature dependence of electronic specific heat \textit{Ce}($T )$ of this phaseexhibit anomalous power-law singularities and are controlled by a single exponent. Moreover, the magnetization $M(T,H)$ of this phase is found to follow a phenomenological scaling law of $M(H,T)\propto H^{\alpha}f(H$/$T^{\delta})$. These observations indicate the slow dynamics in rare regions arising from the effect of disorder on the QPT.\\[4pt] [1] Z. Qu et al., Phys. Rev. B 78, 180407(R) (2008)\\[0pt] [2] Z. Qu et al., Phys. Rev. B 86, 014434 (2012). [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y17.00013: Field-induced magnetic phase transitions in Ti-doped Ca3Ru2O7 bilayer ruthenates M. Zhu, J. Peng, Z.Q. Mao, K. Prokes, S. Matas, T. Hong, X. Ke Bilayer ruthenate Ca$_{\mathrm{3}}$Ru$_{\mathrm{2}}$O$_{\mathrm{7}}$ shows strong magnetic instability that depends sensitively on chemical doping and magnetic fields. Previously we have shown that [1] Ti doping induces Mott insulating ground state with a G-type antiferromagnetic (AFM) structure where nearest-neighbor spins align antiferromagnetically, a feature dramatically distinct from the metallic ground state with an AFM-b structure where the ferromagnetically aligned spins (pointing along the $b$-axis) within the bilayer are coupled antiferromagnetically along the $c$-axis. Here we report magnetic phases of the Ti-doped Ca$_{\mathrm{3}}$Ru$_{\mathrm{2}}$O$_{\mathrm{7}}$ in a magnetic field revealed via neutron diffraction study. In sharp contrast to pure Ca$_{\mathrm{3}}$Ru$_{\mathrm{2}}$O$_{\mathrm{7}}$ [2], below the metal-insulator transition we find a field-induced magnetic phase transition from G-type AFM to AFM-a with spins projected along the $a$-axis. Concomitantly, a sharp change in lattice parameters is observed, suggesting strong magnetoelastic coupling. The effect of such a field-induced phase transition on the magnetotransport property in the Ti-doped Ca$_{\mathrm{3}}$Ru$_{\mathrm{2}}$O$_{\mathrm{7}}$ will be discussed as well. [1] X. Ke et al., Phys. Rev. B \textbf{84}, 201102 (R) (2011). [2] W. Bao et al., Phys. Rev. Lett. \textbf{100}, 247203 (2008). [Preview Abstract] |
Session Y18: Focus Session: Spin-Dependent Phenomena in Semiconductors - Quantum Dots
Sponsoring Units: GMAG DMP FIAPChair: Avadh B. Saxena, Los Alamos National Laboratory
Room: 320
Friday, March 22, 2013 8:00AM - 8:12AM |
Y18.00001: Unconventional Nodal Wavefunctions in Quantum Dots Jeongsu Lee, Karel V\'{y}born\'{y}, Igor \v{Z}uti\'{c}, Jong Han In a single band model such as one electron in a box, it is well known that the ground state wavefunction has no node maximizing its spatial symmetry. However, the ordering of eigenstates in a multiband system e.g., p-doped semiconductor quantum dots (QDs) can be very different due to spin-orbit interaction, symmetry of the underlying lattice and geometry of the confinement. Such unconventional ordering of states has appeared in the literature {[}1, 2{]} but it is often ignored or merely considered a shortcoming of $k\cdot p$ model {[}3{]}. We investigate spatial structure of hole envelope-wavefunctions in QDs with a focus on its symmetry. Our calculation shows a counter-intuitive ordering of eigenstates where a single hole ``ground-state'' has a node at the center. For simplicity, we start with a 2D QD tight-binding model and extend the discussion to 3D QD tight-binding and $k\cdot p$ models. We also discuss experimental implications of the wavefunction ordering described above. {[}1{]} K. V\'{y}born\'{y} et al., PRB \textbf{85}, 155312 (2012) {[}2{]} A. Bagga et al., PRB \textbf{71}, 115327 (2005); P. Horodysk\'{a} et al., PRB \textbf{81}, 045301 (2010); J. Xia and J. Li, PRB \textbf{60}, 11540 (1999) {[}3{]} L. W. Wang et al., APL \textbf{76}, 339 (2000) [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y18.00002: Spin Wigner molecules in quantum dots Igor Zutic, Rafal Oszwaldowski, Peter Stano, A. G. Petukhov The interplay of confinement and Coulomb interactions in quantum dots can lead to strongly correlated phases differing qualitatively from the Fermi liquid behavior. While in three dimensions the correlation-induced Wigner crystal is elusive and expected only in the limit of an extremely low carrier density, its nanoscale analog, the Wigner molecule, has been observed in quantum dots at much higher densities [1]. We explore how the presence of magnetic impurities in quantum dots can provide additional opportunities to study correlation effects and the resulting ordering in carrier and impurity spins [2]. By employing exact diagonalization we reveal that seemingly simple two-carrier quantum dots lead to a rich phase diagram [2,3]. We propose experiments to verify our predictions; in particular, we discuss interband optical transitions as a function of temperature and magnetic field. [1] C. Ellenberger et al., Phys. Rev. Lett. {\bf 96}, 126806 (2006); A. Singha et al., Phys. Rev. Lett. {\bf 104}, 246802 (2010). [2] R. Oszwaldowski, P. Stano, A. G. Petukhov, and I. Zutic, Phys. Rev. B (Rapid Comm.), in press, arXiv:1210.6422. [3] R. Oszwaldowski, I. Zutic, and A. G. Petukhov, Phys. Rev. Lett. {\bf 106}, 177201 (2011). [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y18.00003: Spin polarized current through a quantum shuttle Jorge Villavicencio, Irene Maldonado, Ernesto Cota, Gloria Platero We study spin current through a vibrating triple quantum dot system in a linear arrangement, as a function of detuning across the device, in the presence of a magnetic field, taking into account non-spin-conserving tunneling processes induced by spin-orbit interaction (SOI). Using the density matrix master equation approach, we calculate the current and polarization for both the static and dynamic cases. In the former case the central dot is at rest, while in the latter it is oscillating (triple quantum dot shuttle, TQDS). In both cases, we find new resonances in the current with a definite spin polarization, for both symmetric and asymmetric Zeeman splitting. These resonances are shown to correspond to anticrossings in the energy spectrum reflecting coupling between states due to SOI. For the asymmetric TQDS we obtain a spin filter behavior in the weak coupling regime. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y18.00004: Nonmagnetic spin current generation as nonequilibrium Kondo effect in a spin-orbit nano interferometer Nobuhiko Taniguchi We investigate electric generation of spin-dependent transport through a single-level quantum dot embedded in a ring by help of the Rashba spin-orbit coupling\footnote{N. Taniguchi and K. Isozaki, arXiv:1210.6428 (2012).}. Although it is known for some time that applying finite bias to this type of the spin-orbit interferometer induces finite spin polarization on the dot\footnote{M. Crisan et al. Phys. Rev. B \textbf{79} 125319 (2009).}, the mechanism of driving such spin polarization to flow has not fully been understood. For instance, in spite of finite spin polarization on a noninteracting single-level dot, no spin current is found to appear. We show theoretically that it is possible to generate electrically large spin-dependent current through an interacting single-level dot, as a combined effect of the Kondo effect and finite bias as well as the Rashba spin-orbit interaction. In contrast to earlier work\footnote{H.-F. L\"{u} and Y. Guo, Phys. Rev. B \textbf{76}, 045120 (2007).}, we argue the emergent spin-dependent transport in the present model is viewed as a new type of nonequilibrium Kondo effect; it appears in the middle of the Kondo valley and is suppressed by bias voltage larger than the Kondo energy properly renormalized by the Rashba spin-orbit coupling. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y18.00005: The Optimization of Magnetic Ordering in Quantum Dots James Pientka, Rafal Oszwaldowski, Igor Zutic, Jong Han, Andre Petukhov Lately, there have been several theoretical studies that demonstrate how carrier-mediated magnetic ordering is influenced by multiple occupancies in quantum dots (QD) [1,2]. Experimentally, multiple-occupancy can be reached by high photo-excitation intensity. It was observed in type-II QDs that magnetic polaron (MP) formation persists at large temperatures [3]. We show that varying QD occupancy has important consequences, including thermally enhanced magnetic ordering in QDs [4]. We extend our method to take into account the formation of magnetic bipolarons (MBP) [1,2]. We show that a standard mean-field treatment of MBP leads to unphysical phase transitions, removed when fluctuations are taken into account. Finally, we demonstrate that for a single MP, the shrinking of the carrier wave function due to the exchange with magnetic impurities is a small effect. [1] R. Oszwaldowski, I. Zutic, and A. G. Petukhov, Phys. Rev. Lett. 106, 177201 (2011). [2] R. Oszwaldowski, P. Stano, A. G. Petukhov, and I. Zutic, accepted to Phys. Rev. B. (Rapid Communications), arxiv:1210.6422. [3] I. R. Sellers, R. Oszwaldowski, et al., Phys. Rev. B 82, 195320 (2010). [4] J. M. Pientka, R. Oszwaldowski, A. G. Petukhov, J. E. Han, and I. Zutic, Phys. Rev. B. 86, 161403(R) (2012). [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y18.00006: Single-electron transport in a magnetic quantum-dot molecule Javier Romero, Eduardo Mucciolo We study single-electron transport in a magnetic quantum-dot molecule by using a stationary rate equation approach. In the molecule, two quantum dots play the roles of magnetic ions and are connected to each other through a third quantum dot which plays the role of a nonmagnetic ion. The magnetic quantum dots are coupled to ideal metallic leads and a back gate voltage is applied to the molecule, forming a field-effect transistor setup. A hopping Hamiltonian, which includes on-site repulsion and magnetic anisotropies, is employed to describe this molecule, resulting in an energy spectrum similar to that of single molecule magnets in the giant spin approximation. An external, in-plane magnetic field is then used to drive the molecule to a diabolical point, where states with maximum total spin with opposite directions are degenerated. Both linear and nonlinear transport are evaluated near the diabolical point, showing features that can be attributed to Berry-phase interference of spin tunneling paths. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:48AM |
Y18.00007: Optical control and coherence of electron or hole spins in coupled quantum dots Invited Speaker: Samuel Carter The spin of an electron or hole in an InAs quantum dot is an attractive qubit because it combines the advantages of a semiconductor platform with the power of ultrafast optical coherent control techniques. In the last few years, basic quantum operations such as initialization, rotation, and readout have become possible using single spins, but now improvements in spin coherence and demonstrations of multi-qubit systems are needed. In this work, we combine advances in the design and growth of coupled quantum dots with optical coherent control techniques to demonstrate ultrafast manipulation and coherence improvements for one or two interacting electron [1] or hole [2] spins in a coupled pair of InAs dots. For each of these spin systems, we use a sequence of picosecond and nanosecond pulses to initialize, manipulate, and measure the coherent spin dynamics. These dynamics include precession about a magnetic field and also entangling dynamics from the exchange interaction for coupled spins. For a single electron spin, precession dephases after only a few nanoseconds due to the hyperfine interaction with nuclear spins. For hole spins, we measure a dephasing time an order of magnitude longer due to a weaker hyperfine interaction. Coupled electron and hole spins are essential for multi-qubit systems, and they can also be used to decrease sensitivity to the environment. In these systems, we typically measure the coherent dynamics of the singlet-triplet states (m$_{\mathrm{s}}=$0), which are much less sensitive to the nuclear environment. At present, dephasing is due to fluctuations in the electrical environment. With careful sample design, we can make these systems much less sensitive to electrical fluctuations, giving a powerful combination of long coherence times and ultrafast gates. Finally, we demonstrate that these spin qubits can be incorporated into a photonic crystal cavity and manipulated with optical pulses, a major step toward a quantum interface between photons and these spin qubits.\\[4pt] [1] D. Kim \textit{et al}., Nature Phys. \textbf{7}, 223 (2011).\\[0pt] [2] A. Greilich \textit{et al}., Nature Photon. \textbf{5}, 702 (2011). [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y18.00008: Dephasing and relaxation of central hole spins by nuclear spin baths in InGaAs quantum dots: role of nuclear quadrupolar coupling Yan Li, N.A. Sinitsyn, A. Saxena, D.L. Smith, D. Reuter, A.D. Wieck, D.R. Yakovlev, B. Manfred, S.A. Crooker Single electron or hole spins in III-V semiconductor quantum dots (QDs) are promising candidates for solid-state qubits. Their coherence properties are typically governed by the hyperfine coupling between these ``central'' electronic spins and the dense surrounding bath of lattice nuclear spins. Theoretically this is a challenging problem due to its many-body and strongly-correlated nature. Here we measure the spin dynamics of holes in InGaAs quantum dots by detecting their intrinsic, random spin fluctuations while in thermal equilibrium, which reveals the spin correlation time scales $\tau_h$ and the functional form of bath-induced spin relaxation. In zero magnetic field, $\tau_h$ is very long ($\sim$400 ns) and decays exponentially, in marked contrast with recent theories. $\tau_h$ increases to $\sim$5 $\mu$s in small (100 G) longitudinal fields, and the spin dynamics evolve to a very slow $\sim$1/ln(t) decay [1]. We model the influence of nuclear quadrupolar coupling on spin dynamics in these strained QDs for both electrons and holes [2], and find a good agreement with experimental data when the quadrupolar coupling exceeds the hyperfine coupling strength. [1] Yan Li, N. Sinitsyn, et al., PRL 108, 186603 (2012). [2] N. Sinitsyn, Yan Li, et al., PRL 109, 166605 (2012). [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y18.00009: Quadrupolar spectra of nuclear spins in strained InGaAs quantum dots Ceyhun Bulutay Self-assembled quantum dots (QDs) are born out of lattice mismatched ingredients where strain plays an indispensable role. Through the electric quadrupolar coupling strain affects the nuclear spins. To guide upcoming single-QD nuclear magnetic resonance (NMR) as well as dynamic nuclear spin polarization experiments, a computational atomistic insight to the strain and quadrupolar field distributions will be presented. Among our findings, a high aspect ratio of the QD geometry enhances the quadrupolar interaction; inclined interfaces introduce biaxiality and the tilting of the major quadrupolar principal axis away from the growth axis; the alloy mixing of gallium into the QD enhances both of these features while reducing the quadrupolar energy. NMR spectra in Faraday and Voigt geometries are computed, unraveling in the first place the extend of inhomogeneous broadening and the appearance of the normally-forbidden transitions. Moreover, from the main extend of the NMR spectra the alloy mole fraction of a single QD can be inferred. In the presence of an external magnetic field, the borderlines between the quadrupolar and Zeeman regimes are extracted as 1.5 T for In and 1.1 T for As nuclei. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y18.00010: Integrability-based analysis of the hyperfine interaction induced decoherence in quantum dots Alexandre Faribault, Dirk Schuricht Using the Algebraic Bethe Ansatz in conjunction with a simple Monte Carlo sampling technique, we study the problem of the decoherence of a central spin coupled to a nuclear spin bath. We describe in detail the full crossover from strong to weak external magnetic field field, a limit where a large non-decaying coherence factor is found. This feature is explained by Bose-Einstein-condensate-like physics which also allows us to argue that the corresponding zero frequency peak would not be broadened by statistical or ensemble averaging. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y18.00011: Dynamics of carrier populations and localized spins during magnetic-polaron formation in quantum dots Biplob Barman, Rafal Oszwaldowski, Lars Schweidenback, Andreas Russ, Joseph Murphy, Alexander Cartwright, Igor Zutic, Bruce McCombe, Athos Petrou, Wu-Ching Chou, Wen Chung Fan, Ian Sellers, Andre Petukhov We have extended our previous investigation of time evolution of PL from (Zn,Mn)Te/ZnSe quantum dots in a magnetic field $B$ [1]. PL studies at $T=$5 K in these type-II dots reveal formation of magnetic polarons (MP). We find their formation time $\tau_{MP}$ to be 0.5 ns, which varies little with $B$. The circular polarization $P$ of the emission shows a surprising behavior. For all fields, the characteristic time $\tau_{P}$ is longer than $\tau_{MP}$. Furthermore, $\tau_{P}$ decreases from 10 ns to 1.9 ns as $B$ increases from 1 to 4 tesla. We attribute this effect to a low-$B$ bottleneck in the $\sigma_{+}$ recombination channel, due to the almost equal populations of the spin $\pm 1/2$ electrons participating in the interband transitions. In contrast, the $\pm 3/2$ holes in the (Zn,Mn)Te QDs, are affected mostly by the effective field due to exchange interaction between hole and Mn spins around it. This effective field is much larger than $B$. \\[4pt] [1] I.R. Sellers \textit{et al}. Phys. Rev. B. 82, 195320 (2010) [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y18.00012: Room temperature spin decoherence and dephasing in CdSe nanocrystal quantum dots Ahmad Khastehdel Fumani, Reza Sharghi-Moshtaghin, Jesse Berezovsky We combine transmission electron microscopy (TEM) and Faraday-rotation-based spin measurements to reveal the connection between coherent electron spin dynamics and the shape and size distribution of an ensemble of nanocrystal quantum dots. Optically pumped spins in CdSe nanocrystal quantum dots provide a platform for studying coherent dynamics and decoherence of spins of charge carriers in a complex, room-temperature environment. In a transverse magnetic field, decay of the ensemble spin signal is often ascribed to inhomogeneous dephasing caused by the distribution of nanocrystal sizes across the ensemble. In this work, we measure the size and shape distribution of an ensemble of nanocrystals using TEM, and compare the resulting calculated spin dynamics to those measured in a time-resolved Faraday rotation experiment. We find that the size inhomogeneity alone is insufficient to explain the measured dephasing times and decay envelopes. We propose an ensemble decoherence mechanism based on the distribution of nanocrystal shapes which can account for both the magnetic field dependence of the dephasing time and the shape of the decay envelope. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y18.00013: A phonon laser using quantum dot spin states Alexander Khaetskii, Xuedong Hu, Igor Zutic Sound analog of laser (saser) has not yet been realized experimentally, though some steps in this direction have been made recently [1]. As is known, the main reason impeding coherent generation of phonons in solid state is high density of phonon states [2]. We suggest a particular realization of saser, which consists of an ensemble of quantum dots and uses the Zeeman-split spin levels of the ground orbital state in the quantum dot. We develop a complete set of saser equations taking into account the Coulomb blockade conditions for a quantum dot, and evaluate all the parameters such as the threshold, output power and efficiency of the device. Supported by NSF-ECCS and US ONR, NSF PIF,and US ARO. [1]. R.P. Beardsley et al., PRL \textbf{104,} 085501 (2010). [2]. J. Chen and J.B. Khurgin, IEEE Journal of Quantum Electronics, \textbf{39}, 600 (2003) . [Preview Abstract] |
Session Y19: Charge Density Wave Order
Sponsoring Units: DCMPChair: David Hawthorn, University of Waterloo
Room: 321
Friday, March 22, 2013 8:00AM - 8:12AM |
Y19.00001: Specific heat studies of the chiral phase transition in charge ordered 1T-TiSe$_{2}$ Xu Luo, J.-P. Castellan, S. Rosenkranz, R. Osborn, Q. Li, G. Karapetrov, J.P.C. Ruff, U. Welp, J. Van Wezel We use high-resolution steady-state ac-micro-calorimetry to investigate the transition of 1T-TiSe$_{2}$ into the charge-ordered state. A mean-field like step of $\sim$ 0.4 J/molK in the specific heat C(T) near 193 K signals the transition into the commensurate CDW state. Upon further cooling, C(T) varies linearly in temperature until near 180 K a clear break in the slope of C(T) by 13mJ/molK$^{2}$ and possibly a small step indicate a second phase transition. Comparisons with theoretical predictions based on the Ginzburg-Landau free energy, with resistivity measurements, and with x-ray diffraction indicate that, at this transition, the commensurate CDW state changes into a helically ordered state along the crystal c-axis. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y19.00002: Electronic Properties of Cu$_x$TiSe$_2$ Single Crystals Petra Husanikova, Jan Fedor, Jan Derer, Vladimir Cambel, Goran Karapetrov We investigate the normal state and superconducting properties of 1T-TiSe$_2$ family of single crystals intercalated with different level of copper content. Magnetoresistance and Hall effect data indicate that 1T-TiSe$_2$ is a compensated narrow band-gap semiconductor or semimetal with small number of electron and hole carriers. We compare the influence of copper intercalant and titanium interstiatials on the temperature evolution of charge density waves via resistivity and Hall effect measurements. Our findings indicate that the origin of the charge density waves in 1T-TiSe$_2$ is due to the combination of exciton and Jahn-Teller mechanisms. At higher copper concentrations we investigate the superconducting properties of Cu$_x$TiSe$_2$ in overdoped regime and find that the system is a single-gap strongly type-II superconductor with in-plane Ginzburg-Landau parameter reaching 50. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y19.00003: The effect of dimensionality on the charge-density-wave phase in layered dichalcogenides Darshana Wickramaratne, Pradyumna Goli, Alexander Balandin, Roger Lake Transition-metal dichalcogenides exhibit a variety of conducting phases, which includes a charge-density wave state (CDW). Exfoliation of these layered materials allows the effect of dimensionality on the CDW state to be studied. CDW collective states are currently being considered as an alternative state variable for information processing [1]. 2H-TaSe$_{2}$ and 1T-TiSe$_{2}$ are examples of layered transition metal dichalcogenides that undergo a CDW transition. Our recent experiments demonstrated an increase in the CDW transition temperature of TiSe$_{2}$ with a decrease in film thickness [1]. This increase in temperature was attributed to the negative coefficient of the CDW transition temperature-pressure relationship. Here we present a density-functional theory investigation of the CDW instability in bulk, single and few-layer 1T-TiSe$_{2}$ and other layered dichalcogenide materials. The effect of the film thickness on the atomic structure, electronic structure, electron-phonon coupling and the CDW transition temperature will be discussed for each material. \\[4pt] [1] Goli, P., Khan, J., Wickramaratne, D., Lake, R. K., \& Balandin, A. A. (2012). Charge Density Waves in Exfoliated Films of Van der Waals Materials: Evolution of Raman Spectrum in TiSe$_{2}$. Nano Letters. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y19.00004: Charge-Density Wave and Superconducting Dome in TiSe$_2$ from Electron- Phonon Interaction Matteo Calandra, Mauri Francesco At low temperature TiSe2 undergoes a charge-density wave instability. Superconductivity is stabilized either by pressure or by Cu intercalation. We show $[1]$ that the pressure phase diagram of TiSe2 is well described byfirst-principles calculations. At pressures smaller than 4 GPa charge-density wave ordering occurs, in agreement with experiments. At larger pressures the disappearing of the charge-density wave is due to a stiffening of the short-range force constants and not to the variation of nesting with pressure. This suggests a common origin of the charge density waves instability in transition metal dichalcogenides, as also demonstrated in previous works by first principles calculations on bulk and few layers NbSe2 $[2]$. In TiSe2, the behavior of Tc as a function of pressure is entirely determined by the electron-phonon interaction without need of invoking excitonic mechanisms. Our work demonstrates that phase diagrams with competing orders and a superconducting dome are also obtained in the framework of the electron-phonon interaction.\\ $[1]$ M. Calandra and F. Mauri, PRL 106, 196406 (2011)\\ $[2]$ M. Calandra, I. I. Mazin and F. Mauri, PRB 80, 241108(R) (2009) [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y19.00005: A quantum phase transition from triangular to stripe charge order in NbSe$_2$ Eric Hudson, Anjan Soumyanarayanan, M. M. Yee, Yang He, D. J. Rahn, K. Rossnagel, Jasper van Wezel, M.R. Norman, Jennifer E. Hoffman We use scanning tunneling microscopy to reveal a previously unknown unidirectional (stripe) charge density wave (CDW) smoothly interfacing with the familiar tridirectional (triangular) CDW on the surface of the stoichiometric superconductor NbSe$_2$. Our low temperature measurements rule out thermal fluctuations, and point to local strain as the tuning parameter for this quantum phase transition. We use this discovery, in conjunction with bandstructure calculations, to resolve two longstanding debates about the anomalous spectroscopic gap and the role of Fermi surface nesting in the CDW phase of NbSe$_2$. First, the 15$\%$ wavelength difference between the two CDWs demonstrates that Fermi surface nesting plays a minor role in determining the CDW wavevectors in NbSe$_2$. Second, we disentangle a $\Delta \sim$12 meV particle-hole asymmetric CDW gap from a spectrum dominated by collective modes, resolving a longstanding debate regarding anomalous gaps previously observed by STM and ARPES. Our results highlight the importance of local strain in governing phase transitions and competing phenomena, and suggest a new direction of inquiry for resolving similarly longstanding debates in cuprate superconductors and other strongly correlated materials. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y19.00006: Role of impurities in the charge density wave state of transition metal dichalcogenides Junichi Okamoto, Andrew Millis Motivated by recent scanning tunneling microscope (STM) measurements of NbSe$_2$ which revealed the formation of charge density wave (CDW) droplets around impurities even at temperatures of the order of three times the transition temperature [1], we present a theory of impurity-induced CDW formation, and examine its consequences for the thermodynamic phase transition and low temperature ordered phases. Our fits to the STM measurements suggest that the CDW is strongly pinned by impurities, so that a standard theory predicts that even at lowest temperature the material should be in the disordered phase. We present a new theoretical picture explaining how to reconcile the experimental observation of a sharp transition with the strong pinning. [1] S. P. Chockalingam $et$ $al.$ (submitted to PNAS) [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y19.00007: Electronic Structure and Charge-Density Wave Instabilities in Monolayers of Transition Metal Dichalcogenides Pierre Darancet, Andrew J. Millis, Chris A. Marianetti Transition metal dichalcogenides (TMDC) are layered materials displaying a variety of charge-density wave (CDW) instabilities and complex phase diagrams for group IV \& V transition metals. Recent progress in mechanical exfoliation and device fabrication now allow for electrical characterization and gating of individual, 3-atom thick layers [1] of TMDCs, providing new probes of the complex many-body interactions arising in these compounds. In this talk, I will present our investigations using density functional and dynamical mean-field theory regarding the electronic structure and electronic correlations arising in distorted monolayers, bilayers, and trilayers of octahedral group V TMDCs. We will examine the importance of doping, crystal fields, and many-body interactions, and their influence on the transport and optical properties of these materials upon distortion. [1] K. S. Novoselov et al., PNAS 102, 10451 (2005) . [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y19.00008: Role of Disorder in Atomic Scale Onset of Charge Density Waves Erick Andrade, Carlos Arguello, Ethan Rosenthal, Subbaiah Chockalingam, Luiyan Zhao, Christopher Gutierrez, Woo Chung, Wencen Jin, Po-Chun Yeh, Tonica Valla, Rafael Fernandes, Shuang Jia, Richard Osgood, Andrew Millis, Robert Cava, Abhay Pasupathy How does strong disorder affect the electronic states of complex electronic materials? This question is of relevance to many quantum materials such as the cuprates and pnictides, where interesting electronic phases like superconductivity only arise in strongly disordered samples. The study of these materials is complicated by the presence of multiple electronic phases, which obscures the interpretation of local spectroscopic measurements. To gain insight into this problem, we study 2H-NbSe$_{2}$, a relatively simple material with a 2D charge density wave ground state. To tune the disorder in the sample, we use sulfur substitution to go from weak (in pristine NbSe$_{2})$ to strong disorder (in NbSe$_{2-x}$S$_{x})$. We use variable-temperature scanning tunneling microscopy and spectroscopy to visualize the electronic structure in real space. Strong changes in the local electronic spectrum are observed with the introduction of disorder, with a pseudogap appearing in the local density of states. We also observe strong changes in the quasiparticle interference from spectroscopic images. We will discuss the interpretation of quasiparticle interference in the limit of strong disorder, and its relevance to existing measurements in the cuprates and pnictides. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y19.00009: Charge Density Wave Disproportionation in Pd(III)-containing PdTeI Patrick Cottingham, John Sheckelton, David Miller, James Neilson, Tyrel McQueen Exotic electronic properties in strongly correlated materials often emerge from the interplay of structure and charge. In most Pd$^{3+}$-containing materials, Pd$^{3+}$ statically disproportionates into Pd$^{2+}$ (d$^{8}$) and Pd$^{4+}$ (d$^{6}$) with square planar and octahedral geometries, respectively. However, high-resolution diffraction data acquired for PdTeI indicate exclusively octahedral coordination of the Pd species within this compound. Temperature-dependent electrical resistivity measurements of this material performed in our lab show a hysteresis between T$_{CDW1}$ $\sim$ 120 K and T$_{CDW2}$ $\sim$ 50 K, indicative of a first-order phase transition. The most likely origin of this anomaly is the formation of a CDW involving partial, dynamic charge disproportionation of Pd$^{3+}$. In addition, low-temperature diffraction data show a broadening of Bragg peaks on cooling which is indicative of strain or of disorder concomitant with disproportion. In this presentation the temperature dependencies of the magnetic susceptibility, heat capacity, and electronic properties of PdTeI will be discussed in the context of CDW formation. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y19.00010: Short-range CDW correlations in Co$_{x}$NbSe$_{2}$ and Mn$_{x}$NbSe$_{2}$ J. Lee, R. Di Capua, G. Karapetrov, T. Nishizaki, N. Kobayashi, M. Iavarone Scanning tunneling microscopy and transport measurements were performed on NbSe$_{2}$ and Co- and Mn-intercalated NbSe$_{2}$ single crystals, to address the effect of disorder induced on the CDW structure by the effect of intercalation. We find that the CDW transition at T$_{CDW}$=33 K in the pure compound is accompanied by a small anomaly in resistivity, a strong non linearity of the Hall effect, with a sign reversal occurring at CDW transition, and high magnetoresistance in agreement with previous reports. The system remains metallic below the CDW transition. Upon increase of disorder the anomaly in resistivity moves at a lower temperature and eventually disappears for higher doping levels. By increasing the disorder also the magnetoresistance decreases and the Hall effect does not show any sign reversal. STM measurements on a pure sample reveal that CDW phase is long-range ordered below T$_{CDW}$. For doped samples short range CDW correlations dominate a large part of the phase diagram. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y19.00011: Optical Excitation Spectrum in Ni- and Cu-doped ZrTe$_3$ Chiara Mirri, Adam Dusza, Leonardo Degiorgi, Cedomir Petrovic We report on an optical study performed on Cu$_x$ZrTe$_3$ and Ni$_x$ZrTe$_3$ single crystals. ZrTe$_3$ was previously found to display a BCS-like CDW-gap opening in the optical spectra along the direction orthogonal to the Zr-chains and to undergo a filamentary superconducting transition below a T$_c$ of about 2 K. The intercalation by Cu and Ni between the ZrTe$_3$ layers partially fills the CDW gap and induces bulk superconductivity coexisting with the CDW state below T$_c$. Here we show the effect of Ni and Cu intercalation on the reflectivity and optical conductivity above and below the CDW phase-transition temperature. Furthermore, we analyze the optical spectral weight, providing equivalent information in both compounds about the partial gapping of the Fermi surface and the overall redistribution of spectral weight across the CDW phase transition. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y19.00012: Interplay between electron-electron and electron-lattice interactions in the RTe3 compounds Alexander Kemper, H.-M. Eiter, M. Lavagnini, R. Hackl, E.A. Nowadnick, T.P. Devereaux, J.-H. Chu, J.G. Analytis, I.R. Fisher, L. Degiorgi Charge and spin density waves, periodic modulations of the electron and magnetization densities, respectively, are among the most abundant and non-trivial low-temperature ordered phases in condensed matter. The ordering direction is widely believed to result from the Fermi surface topology. However, several recent studies indicate that this common view needs to be supplemented. Here, we show how an enhanced electron-lattice interaction can contribute to or even determine the selection of the ordering vector in the model charge density wave (CDW) system ErTe3. We show how the electron-phonon coupling in the vicinity of band degeneracy points is strongly enhanced, leading to a CDW direction that is different from that determined by first-principles calculations. This combination of electron-electron and electron-lattice interactions may be generally relevant for driving phase transitions in other broken-symmetry ground states. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y19.00013: Charge density wave formation in multi-band systems Rudi Hackl, Hans-Martin Eiter, Michela Lavagnini, Elizabeth A. Nowadnick, Alexander F. Kemper, Thomas P. Devereaux, Jiun-Haw Chu, James G. Analytis, Ian R. Fisher, Leonardo Degiorgi Charge and spin density waves are among the most abundant low-temperature ordered phases in condensed matter. The Fermi surface topology is widely believed to determine the ordering direction. However, several recent experimental and theoretical studies show that nesting is only one out of various other driving forces behind these instabilities. We use Raman scattering to demonstrate in which way an enhanced electron-lattice interaction can contribute to or even determine the selection of the ordering vector in the model charge density wave (CDW) system ErTe$_3$ and other rare-earth tri-tellurides. In our joint experimental and theoretical study we exploit the symmetry properties of the electron-photon and electron-phonon coupling vertex and establish a relation between the selection rules of the electronic light scattering spectra and the enhanced electron-phonon coupling in the vicinity of band degeneracy points. The proposal shown here for CDW formation, may be of more general relevance in multi-band systems for driving phase transitions into other broken-symmetry ground states. For example, the iron-based superconductors exhibit a similar phenomenology close to the intersection points of the backfolded electron bands. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y19.00014: Charge-density wave transitions of rare-earth tritellurides investigated by femtosecond electron crystallography Tzong-Ru Han, Zhensheng Tao, Subhendra D. Mahanti, Kiseok Chang, Chong-Yu Ruan, Christos D. Malliakas, Mercouri G. Kanatzidis The electron-phonon mechanism that gives rise to various charge-ordered systems depends on the topology of the Fermi surface that is subjective to the influence of hybridization, nesting, and electron correlation at low dimensions. Rare-earth tritellurides are ideal systems to investigate the two-dimensional charge-density density wave (CDW) formation as both nesting and hybridization are at play to select the unidirectional CDW at different temperatures. Using fs electron crystallography, we investigate the noncooperative suppression of the structural order parameters following ultrafast electronic quenching and correlate electronic and ionic evolutions based on a framework of three-temperature model and nonisotropic fluctuational analysis. We show that a joint consideration of the couplings between the lattice phonons, the CDW collective modes, and the corresponding electronic subsystem is required to account for the various novel structural dynamics features. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y19.00015: 1/f noise anomalies in nanoribbons of charge density wave materials Zhenzhong Shi, Adam Stabile, Peter M. Marley, Sarbajit Banerjee, Ganapathy Sambandamurthy Charge density wave (CDW) as an ordered form of matter has attracted attention for many decades. Below a critical temperature (T$_{P}$), CDW materials undergo a Peierls transition and enter the CDW ground state, where the energy is minimized by a collectively pinning mechanism. Under a moderate electric field, CDWs can be depinned and they start sliding. An onset of a large broad band noise (BBN) has been observed in bulk CDW materials as a signature of this depinning process. We report low frequency conductance fluctuation (1/f noise) measurements on single nanoribbon devices of single-crystalline NbSe$_{3}$, across both Peierls transitions. In the CDW state, a non-monotonic behavior in the noise magnitude was observed when approaching the threshold electric field for depinning: while increasing voltage from the zero-bias limit, the magnitude of BBN first decreases before increasing sharply near the threshold voltage. This unusually large BBN magnitude and the non-monotonic behavior below the depinning threshold suggest some inherent instability that could be suppressed by a small bias field, and is clearly different from results from bulk materials. Transport and noise studies from individual nanoribbons of NbSe$_{3}$, Ta-doped NbSe$_{3}$ and o-TaS$_{3}$ will be presented. [Preview Abstract] |
Session Y20: Focus Session: Electron, Ion, Exciton Transport in Nanostructures: Quantum Dots and Low-dimension Structures
Sponsoring Units: DCMPChair: Seungbum Hong, Argonne National Laboratory
Room: 322
Friday, March 22, 2013 8:00AM - 8:12AM |
Y20.00001: Flux-dependent effects in degenerate and symmetric double dot Aharonov-Bohm interferometer with and without interactions Salil Bedkihal, Malay Bandyopadhyay, Dvira Segal We study the steady-state characteristics and the transient behaviour of the non equilibrium double-dot Aharonov-Bohm interferometer using analytical tools and numerical simulations. Our simple setup includes non-interacting degenerate quantum dots that are coupled to two biased metallic leads at the same strength. A magnetic flux $\Phi$ is piercing the set-up perpendicularly. As we tune the degenerate dots energies away from the symmetric point we observe four non trivial magnetic flux control effects: (i) flux dependency of the dots occupation, (ii) magnetic flux induced occupation difference between the dots, at degeneracy, (iii) the effect of ``phase-localization" of the dots coherence holds only at symmetric point, while in general both real and imaginary parts are non-zero, and (iv) coherent evolution survives even when the dephasing strength, introduced into our model using Buttiker probe, is large and comparable to the dots energies and the bias voltage. Moreover, not only finite dephasing strength does not destroy the coherence features, it can provide a new type of coherent oscillation. These four phenomena take place when the dot energies are gated, away from the symmetric point,demonstrating the delicate controllability over the dot occupation and coherence. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y20.00002: Optical phonon lasing in transport through semiconductor double quantum dots Rin Okuyama, Mikio Eto, Tobias Brandes We theoretically propose optical phonon lasing for a double quantum dot (DQD) fabricated in a semiconductor substrate. No additional cavity or resonator is required. We show that the DQD couples to only two phonon modes that act as a natural cavity. The pumping to the upper level is realized by an electric current through the DQD under a finite bias. Using the rate equation in the Born-Markov-Secular approximation, we analyze the enhanced phonon emission when the level spacing in the DQD is tuned to the phonon energy. We find the phonon lasing when the pumping rate is much larger than the phonon decay rate, whereas anti-bunching of phonon emission is observed when the pumping rate is smaller. Both effects disappear by an effective thermalization induced by the Franck-Condon effect in a DQD fabricated in a suspended carbon nanotube with strong electron-phonon coupling. $^1$ \newline $^1$ R.\ Okuyama, M.\ Eto, and T.\ Brandes, arXiv:1205.6955 (2012). [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y20.00003: Quantum Confined Silicon Clathrate Quantum Dots Mark Lusk, Nicholas Brawand Silicon (Si) allotropes can be synthesized in such a way that tetrahedrally bonded atoms form cage-like structures with bulk mechanical and opto-electronic properties distinct from those of diamond silicon (dSi). We use DFT, supplemented with many-body Green function analysis, to explore the structural stability of clathrate Si quantum dots (QDs) and to characterize their confinement as a function of crystal symmetry and size. Our results show that that there is a simple relationship between the confinement character of the QDs and the effective mass of the associated bulk crystals. Clathrate QDs and dSiQDs of the same size can exhibit differences of gap energies by as much as 2 eV. This offers the potential of synthesizing Si dots on the order of 1 nm that have optical gaps in the visible range but that do not rely on high-pressure routes such as those explored for the metastable BC8 and R8 phases. These results prompt the question as to how minimal quantum confinement can be in dots composed of Si. More broadly, clathrate QDs can in principle be synthesized for a wide range of semiconductors, and the design space can be further enriched via doping. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y20.00004: Excess current noise in electrically conductive, crack-free, nanopatterned films of semiconductor nanocrystals Tamar Mentzel, Nirat Ray, Darcy Wanger, Moungi Bawendi, Marc Kastner We present the first electrical measurements of semiconductor nanocrystal films that have nanoscale dimensions and are crack-free. These films make it possible to study the electrical properties intrinsic to the nanocrystals unimpeded by defects such as cracking and clustering that typically exist in larger-scale films. The films' dimensions are as small as 30 nm and are positioned on a surface with 30 nm precision. The electrical conductivity of the crack-free nanoscale films is 180 times higher than that of drop-cast, microscopic films made of the same type of nanocrystal. In the nanoscopic patterns, we find excess noise in the current that is thermally activated. This noise is unusual in that it is of a comparable order of magnitude to the average current, and both the average current and the noise fluctuate by several orders of magnitude in time. The noise increases with an applied field and with a gate. The inability to explain these effects by commonly known origins of electrical noise suggests that we are observing a novel effect in the nanocrystals. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y20.00005: Energy spectrometry of electrons ejected from dynamic quantum dots driven up a potential slope by a surface acoustic wave Christopher Ford, Matthew Benesh, Seok-Kyun Son, Masaya Kataoka, Crispin Barnes, Robert McNeil, Jon Griffiths, Geb Jones, Ian Farrer, David Ritchie Surface acoustic waves (SAWs) in a GaAs/AlGaAs heterostructure generate an electrostatic wave which propagates at the sound velocity. This potential wave is capable of collecting electrons from a 2D electron gas (2DEG) and transporting them through a depleted channel. The SAW minima form a continuous series of dynamic quantum dots, each transporting a controllable number of electrons along the channel. The confinement of the electrons in each dot increases as the potential rises along the channel, ejecting electrons one-by-one back into the 2DEG above the Fermi energy. These electrons can travel several microns before thermalising. We measure their energy spectrum using a variable potential barrier upstream as the channel is squeezed by split gates, and correlate this with the SAW-driven current along the channel. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y20.00006: Measurement of Valley Kondo Effect in a Si/SiGe Quantum Dot Mingyun Yuan, Zhen Yang, Chunyang Tang, A.J. Rimberg, R. Joynt, D.E. Savage, M.G. Lagally, M.A. Eriksson The Kondo effect in Si/SiGe QDs can be enriched by the valley degree of freedom in Si. We have observed resonances showing temperature dependence characteristic of the Kondo effect in two consecutive Coulomb diamonds. These resonances exhibit unusual magnetic field dependence that we interpret as arising from Kondo screening of the valley degree of freedom. In one diamond two Kondo peaks due to screening of the valley index exist at zero magnetic field, revealing a zero-field valley splitting of $\Delta \approx$ 0.28 meV. In a non-zero magnetic field the peaks broaden and coalesce due to Zeeman splitting. In the other diamond, a single resonance at zero bias persists without Zeeman splitting for non-zero magnetic field, a phenomenon characteristic of valley non-conservation in tunneling. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y20.00007: The impact of finite-area inhomogeneities on resistive and Hall measurement Daniel Koon I derive an iterative expression for the electric potential in an otherwise homogeneous thin specimen as the result of a finite-area inhomogeneity in either the direct conductance, the Hall conductance, or both. This expression extends to the finite-area regime the calculation of the effect of such inhomogeneities on the measurement error in the sheet resistance and Hall sheet resistance. I then test these results on the exactly-solvable case of a circular inhomogeneity equally distant from the four electrodes of either a square four-point-probe array on an infinitely large conducting specimen or a circular van der Pauw specimen with symmetrically-placed electrodes. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y20.00008: Quasibound States and Evidence for a Spin 1 Kondo Effect in Asymmetric Quantum Point Contacts Hao Zhang, Phillip Wu, Albert Chang Linear conductance below $2e^2/h$ shows resonance peaks in highly asymmetric quantum point contacts (QPCs). As the channel length increases, the number of peaks also increases. At the same time, differential conductance exhibits zero bias anomalies (ZBAs) in correspondence with every other peak in the linear conductance. This even odd effect, observable in the longer channels, is consistent with the formation of quasi-localized states within the QPC. In rare cases, triple peaks are observed, indicating the formation of a spin one Kondo effect when the electron filling number is even. Changing the gate voltage tunes this spin triplet to a singlet which exhibits no ZBA. The triple-peak provides the first evidence suggestive of a spin singlet triplet transition in a QPC, and the presence of a ferromagnetic spin interaction between electrons. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y20.00009: Enhancement of the excition emission in ZnO nanowires Andrew Epps, Jamie Nowalk, Marian Tzolov The ZnO nanowires were grown by the chemical vapor transport method using a thin gold film as a catalyst. Their light emission in the visible and near UV spectral range was excited by continuous wave and pulsed UV light and by electrons within an SEM. The emission spectrum consists typically of the exciton emission band and a band in the green spectral range related to structural defects. We have followed the evolution of the ratio between the exciton and green band between our samples. The highly localized excitation by the electron beam allowed the profiling of the emission spectrum across the thickness of nanowire samples. We demonstrate that the tips of the nanowires show substantially higher exciton emission. Depth of excitation was varied independently by the electron accelerating voltage. The results have been interpreted within a model accounting for the surface effects and associated band banding at the surface. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y20.00010: BEC-BCS crossover of a dipolariton condensate in a semiconductor microcavity Jung-Jung Su, Na Young Kim, Yoshihisa Yamamoto, Allan H. MacDonald We study the electron-tunnel-coupling condensation of dipolar exciton-polariton (dipolariton) at the BEC-BCS crossover. An exciton-polariton (EP) is an extremely light bosonic quasiparticle composed of excitons and photons and can condense a temperatures as high as room temperature. Electron tunneling between nearby quantum wells can coupled spatially direct and indirect excitons and therefore also the corresponding exciton-polaritons; the indirect EPs in particular carry the interesting dipolar nature. We use a fermionic mean-field theory to examine the influence of this coupling on EP condensates from the dilute BEC to the dense BCS limits. A wide variety of distinct states are found, including mixed direct and indirect EP condensates, and metallic condensates, depending on particle-densities and on the relative positions of the quantum well states in different wells. Possible experimental manifestations of these phenomena will be discussed. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y20.00011: Strain-controlled band engineering and Self-doping in Ultrathin LaNiO3 films X. Liu, E.J. Moon, J.M. Rondinelli, N. Prasai, B.A. Gray, M. Kareev, J. Chakhalian, J.L. Cohn We discover a unique self-doping carrier transition by strain-induced in LaNiO$_3$ ultra thin film. Transport properties evolving from compressive to tensile strains are similar to those of different hole-doping superconducting cuprates. DFT calculations show the changes in low-energy electronic band structure account for the charge transfer between O p and Ni d states. The results indicate that ultrathin films can be used to change the carrier concentration transition metal oxides without resorting to chemical substitution. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y20.00012: Monte Carlo simulations of electron transport for electron beam-induced deposition of nanostructures Francesc Salvat-Pujol, Harald O. Jeschke, Roser Valenti Tungsten hexacarbonyl, W(CO)$_6$, is a particularly interesting precursor molecule for electron beam-induced deposition of nanoparticles, since it yields deposits whose electronic properties can be tuned from metallic to insulating. However, the growth of tungsten nanostructures poses experimental difficulties: the metal content of the nanostructure is variable. Furthermore, fluctuations in the tungsten content of the deposits seem to trigger the growth of the nanostructure. Monte Carlo simulations of electron transport have been carried out with the radiation-transport code Penelope in order to study the charge and energy deposition of the electron beam in the deposit and in the substrate. These simulations allow us to examine the conditions under which nanostructure growth takes place and to highlight the relevant parameters in the process. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y20.00013: Monte Carlo simulations of neon versus helium ion beam induced deposition, sputtering and etching Rajendra Timilsina, Daryl Smith, Philip Rack The ion beam induced nanoscale synthesis of PtCx (where x $\sim$ 5) using the trimethyl (methylcyclopentadienyl)platinum(IV) (MeCpPt$^{\mathrm{IV}}$Me$_3)$ precursor is investigated by performing Monte Carlo simulations of helium and neon ions. The helium beam leads to more lateral growth relative to the neon beam because of its larger interaction volume. The lateral growth of the nanopillars is dominated by molecules deposited via secondary electrons in the both simulations. Notably, the helium pillars are dominated by SE-I electrons whereas the neon pillars by SE-II electrons. Using a low precursor residence time of 70$\mu $s resulting in an equilibrium coverage of $\sim$ 4{\%}, the neon simulation has a lower deposition efficiency (3.5{\%}) compared to that of the helium simulation (6.5{\%}). At larger residence time (10ms) and consequently larger equilibrium coverage (85{\%}) the deposition efficiencies of helium and neon increased to 49{\%} and 21{\%}, respectively; which is dominated by increased lateral growth rates leading to broader pillars. The nanoscale growth is further studied by varying the ion beam diameter at 10 ms precursor residence time. The study shows that total SE yield decreases with increasing beam diameters for the both ion types. However, the helium has the larger SE yield as compared to that of neon in the both low and high precursor residence time, and thus pillars are wider in all the simulations studied. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y20.00014: Electron field emission from freestanding Diamond nanomembranes and Application to time-of-flight mass spectrometry Hyunseok Kim, Jonghoo Park, Hyuncheol Shin, Robert H. Blick We introduce a prototype of a freestanding diamond nanomembrane for large protein detection in time-of-flight mass spectrometry. Doped diamond as a material for mass spectroscopy is extremely interesting due to its mechanical and electrical properties. The freestanding diamond nanomembranes we are able to fabricate have lateral extensions of $400\,\mu m\,\times \,400\,\mu m$ with a thickness of 100nm. We employ optical lithography and a Buffered Oxide Etch (BOE) of SiO$_{\mathrm{2}}$ followed by anisotropic etching of the substrate silicon using TMAH solution and finally removing SiO$_{\mathrm{2}}$. The electron field emission from the surface of the membrane is traced in the \textit{IV} characteristics at room temperature. The membrane is then applied for detection of the large ionized proteins using time-of-flight mass spectrometry. Ion detection is demonstrated in our nanomembrane MALDI-TOF analysis of Insulin (5,735 Da). That is when the ions with a large kinetic energy bombard the nanomembrane, their energy is thermalized upon impact into phonons. The phonons give a thermal energy to the electrons with the membrane, which are then excited to higher energetic states. Given an extraction voltage this leads to electron field emission from the membrane which we labeled phonon-assisted field emission (PAFE). In other words, the MALDI mass spectra are obtained by exploiting ballistic phonon propagation and quasi-diffusive phonon propagation. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y20.00015: A Density Functional Study of the Redox Properties of H2TPP Porphyrin Dayla Morrison, Robert Thomas, Asok Ray Properties of the ground state of free base meso-tetraphenylporphyrin (H2TPP) have been calculated with various charges using the B3LYP functional and the 6-31$+$G basis set. The porphyrin skeleton was rippled and saddled and the meso-phenyl rings were twisted to yield the C$_{\mathrm{2h}}$, C$_{\mathrm{2v}}$, D$_{2}$, D$_{\mathrm{2h}}$ planar and D$_{\mathrm{2h}}$ non-planar symmetries and the structures optimized. The ground state structure was found to be of C$_{\mathrm{2v}}$ symmetry although the C$_{\mathrm{2h}}$, D$_{\mathrm{2}}$ and D$_{\mathrm{2h}}$ non-planar structures were basically degenerate, a conclusion not supported by experimental data. The C$_{\mathrm{2v}}$ structure indicated a nonzero net dipole moment for all levels of charge studied. Increase in negative charge increased the distortion of the H2TPP structure. The Raman spectra was calculated and compared with experimental data.\footnote{J.E. Parker, R. J. Thomas, D. R. Morrison, L. Brancaleon, \textit{J. Phys. Chem. B}, 2012, 116 (36), pp 11032--11040.} In addition, the results were used to select the most likely binding configuration among a set of solutions yielded by computational docking algorithms. Calculations using higher basis sets will also be presented. [Preview Abstract] |
Session Y21: Semiconductors: Thermodynamic & Transport Properties (Theory)
Sponsoring Units: FIAPChair: Chris Palmstrom, University of California, Santa Barbara
Room: 323
Friday, March 22, 2013 8:00AM - 8:12AM |
Y21.00001: Thermal transport in the two-dimensional disordered electron gas Georg Schwiete, Alexander Finkelstein We develop a theory of thermal transport in the two-dimensional disordered electron gas at low temperatures. Our approach is based on the calculation of the heat density correlation function. To this end we subject the Keldysh nonlinear sigma model in the presence of source fields to a renormalization group analysis. Special care is taken to additionally account for scattering processes with a very small frequency transfer. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y21.00002: Electronic properties of near-surface InAs heterostructures Borzoyeh Shojaei, Javad Shabani, Brian Schultz, Chris Palmstrom The interest in low-dimensional narrow gap semiconductors with large spin orbit and high electron mobility has recently surged because of novel proposals on the realization of Majorana modes in such materials. To induce the proximity effect in the semiconductor by coupling to a superconductor, and to tune parameters of the system to realize Majorana excitations the electron channel has to form at or near the interface with the superconductor. In this work we have systematically studied near surface InAs heterostructures grown by molecular beam epitaxy (MBE). We have measured magnetotransport properties in these structures and compared them to theoretical values. We also discuss the in-situ growth of s-wave superconductors on InAs heterostructures and the proximity effect. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y21.00003: Impact of Silicon Nitride Passivation Thickness on AlGaN $\backslash $GaN Transport Properties and Device Performance Helen Jackson, James Petrosky, Robert Hengehold, Zhaoqiang Fang Silicon nitride passivation (Si$_{3}$N$_{4}$) on AlGaN$\backslash $GaN heterojunction devices can improve performance by reducing electron traps at the surface. In this study, the effects of passivation layer thickness were investigated at various thicknesses (0, 20, 50 and 120 nanometers) on bare epilayer AlGaN$\backslash $GaN structures with either an AlN nucleation layer or a GaN cap. Hall system measurements were used to observe changes in carrier concentration and mobility as a function silicon nitride thickness. Mobility changes were measured and carrier scattering mechanisms are analyzed both with and without Si$_{3}$ N$_{4}$. Capacitance voltage measurements were done to give information about the surface donor states and the Si$_{3}$N$_{4}$ charge at the interface. A monatomic decrease in saturation capacitance with increasing Si$_{3}$N$_{4}$ thickness was observed. Gate current measurements were done to examine the effect of Si$_{3}$N$_{4}$ on the gate leakage current and thus device performance. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y21.00004: Anomalous Mobility Enhancement via PPC of a GaAs/AlGaAs 2DEG Yun Suk Eo, Steven Wolgast, \c{C}a\u{g}liyan Kurdak, Loren Pfeiffer, Ken West We report the unusual transport behavior of a two-dimensional electron gas (2DEG) in a $\delta$-doped $GaAs/Al_{x}Ga_{1-x}As$ heterostructure. Typically, the carrier density can be varied with a gate voltage or via the persistent photoconductivity (PPC) effect. The relationship between carrier density and mobility has often been expressed with the empirical relation $\mu\sim n^{\alpha}$, where $\alpha$ contains scattering mechanism information and typically ranges between 1 and 2. Here, we study the carrier density and mobility using gating techniques and the PPC effect with infrared and white light in small incremental exposures. At 4.2K, we find that the addition of a gate structure greatly reduces the achievable mobility. For PPC, we find that after white exposures, $\alpha$ can become unusually large. At 0.3 K, we observe an unusual decrease in carrier density, accompanied by an enhancement in mobility ($\alpha < 0$) after repeated exposures of light. When the mobility is further enhanced by PPC, the 2DEG exhibits parallel conduction in its doping layer, and the transport becomes no longer controllable. However, the drifting mobility and carrier density eventually settle to reproducible values that are independent of the light increment history or other initial conditions. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y21.00005: Parallel Magnetic Field Effect in the Insulating Phase of 2D Metal-Insulator Transition in p-GaAs with High r$_{\mathrm{s}}$ Richard L.J. Qiu, Xuan P.A. Gao, Loren N. Pfeiffer, Ken W. West We present magnetotransport measurements on the insulating side of the 2D metal-insulator transition in p-type GaAs quantum wells with 10 nm width (critical density p$_{\mathrm{c}}\sim $ 0.8*10$^{10}$/cm$^{\mathrm{2}}$, r$_{\mathrm{s}}\sim $ 36). Before entering the disorder dominated regime (p* \textless\ p \textless\ p$_{\mathrm{c}})$ (p* $\sim $ 0.5*10$^{10}$/cm$^{2})$, the conductance of the insulating phase follows a power-law like temperature dependence that is different from the well known thermally activated or variable range hopping behavior for insulators. In this unconventional insulating regime, a strong in-plane magnetic field (B$_{\mathrm{\vert \vert}}$ \textgreater\ B$_{\mathrm{c}} \sim $ 1-2 Tesla) drives the insulating phase into a ``normal'' insulating state which shows the variable range hopping behavior with Coulomb gap. Moreover, with the presence of a strong in-plane magnetic field in the hopping transport regime, large negative magnetoresistance ($\rho $ can decrease by a factor of 5) is observed when increasing the B$_{\mathrm{\bot}}$ component. The authors thank the NSF (DMR-0906415, DMR-0819860) and the Gordon and Betty Moore Foundation for funding support. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y21.00006: Temperature dependence of TCR and 1/f Noise in p-type a-Si:H Vince Lopes, Eric Hanson, Kiran Shrestha, Chris Littler, Athanasios Syllaios The amorphous semiconductor a-Si:H is used for infrared detection applications. Key figures of merit are the temperature coefficient of resistance (TCR) and 1/f noise. We report on the temperature dependence of the electrical resistivity and noise of devices fabricated on as grown boron-doped p-type a-Si:H. The 1/f noise was found to be proportional to the bias voltage and inversely proportional to the square root of the device area. As a result, it can be described by Hooge's empirical expression. However, the 1/f noise was found to be constant in the temperature range investigated, even though the resistance changes by a factor of 2.5. We conclude that the carrier concentration is nearly constant in the temperature range studied; thus, the resistance change is due to the temperature dependence of the hole mobility. This interpretation is consistent with temperature dependent hole mobility measurements of others and suggest that the TCR for p-type a-Si:H material near room temperature is determined by changes in the hole mobility. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y21.00007: Fabrication, electrical characterization and scanning gate microscopy of Schottky silicon nanowire devices Sorin Melinte, Andra Iordanescu, Constantin Dutu, Denis Flandre, Sebastien Faniel, Frederico Martins, Benoit Hackens We report the fabrication and the electrical characterization of Schottky silicon nanowire field effect transistors. Our devices are built with a top down approach on silicon-on-insulator wafers with (100) crystallographic orientation and 10 - 25 $\Omega$.cm resistivity of the silicon top layer. The transistor's channel is assured by silicon nanowires patterned by electron beam lithography and wet etching. The nanowires have nominal cross sections down to $30 \times 30 \ \rm nm^2$. For example, platinum-silicon Schottky contacts are made by physical deposition of a platinum layer followed by an annealing at 500$^{\circ}$C for 2 minutes in a $\rm N_2$ atmosphere. The devices are characterized at various temperatures by current-voltage measurements and scanning gate microscopy techniques. Varying the dimensionality and geometry of the contacts, the nature of metal-semiconductor junctions and the substrate strain, we get new insights into the influence of trapped charges at the $ \rm Si- SiO_2$ interface on transport through $\rm SiO_2$-enclosed nanowires, at the nanometer scale. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y21.00008: Investigation of two-dimensional electron systems at low density on hydrogen-terminated silicon (111) surface Binhui Hu, Tomasz M. Kott, B. E. Kane Two-dimensional electron systems (2DESs) on hydrogen-terminated Si(111) surfaces show very high quality. The peak electron mobility of 325,000 cm$^{\mathrm{2}}$/Vs can be reached at T$=$90 mK and 2D electron density $n_{2d} =4.15\times 10^{11}$ cm$^{\mathrm{-2}}$ , and the device shows the fractional quantum hall effect[1]. 2DESs on H-Si(111) at lower densities may exhibit new physics, because both valley degeneracy and effective mass lead to a large Wigner--Seitz radius $r_{s} $ at accessible densities. In these devices, phosphorus ion implantation is used to defined the contacts to the 2DESs[2]. The contacts themselves work at low temperature. However, at lower 2D electron density ($<2\times 10^{11}$ cm$^{\mathrm{-2}})$ and low temperature (\textless 1 K), the contact resistance to the 2DESs shows strong temperature dependence. This makes accurate Hall measurements difficult in this region. We have systematically investigated the contact resistance at different electron densities and temperatures. Different ion implantation annealing parameters are adjusted to mitigate the issue. Possible measurement technique is also explored to overcome the problem. [1] Tomasz M. Kott, Binhui Hu, S. H. Brown, and B. E. Kane, arXiv:1210.2386 (2012) [2] K. Eng, R. N. McFarland, and B. E. Kane, Appl. Phys. Lett. 87, 052106 (2005) [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y21.00009: Electrical Properties of p-Ge and p-GeSn materials grown on n-Si substrates Thomas R. Harris, Yung Kee Yeo, Mee-Yi Ryu, Richard Beeler, John Kouvetakis The electrical properties of {\em p\em}-Ge and {\em p\em}-Ge$_{1-y}$Sn$_{y}$ ($y$=0.06-0.1\%) grown on {\em n \em}-Si substrate were investigated through temperature-dependent Hall-effect measurements. It was found that there exists a degenerate parallel conducting layer in Ge$_{1-y}$Sn$_{y}$/Si as well as a second, deeper acceptor in addition to a shallow acceptor. Additionally, a conductivity type conversion from {\em p \em} to {\em n \em} was observed between 370 and 440 K for these samples. The parallel conducting layer dominates the electrical properties of the Ge$_{1-y}$Sn$_{y}$ layer below 50 K, and also significantly affects those properties at higher temperatures. The conductivity type conversion and causes of the degenerate conduction layer will be discussed. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y21.00010: ABSTRACT HAS BEEN MOVED TO H1.00346 |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y21.00011: Theoretical and experimental study of kinetics of photoexcited carriers in wide band gap semiconductors Sara Shishehchi, Sergey Rudin, Gregory Garrett, Michael Wraback, Enrico Bellotti We present a theoretical and experimental study of the subpicosecond kinetics of photo-excited carriers in the wide band gap semiconductors GaN and ZnO. In the theoretical model, interaction with a photo-excitation laser pulse is treated coherently and a generalized Monte Carlo simulation is used to account for scattering and dephasing. The scattering mechanisms included are carrier interactions with polar optical phonons and acoustic phonons, and carrier-carrier Coulomb interactions. For comparison, experimental time-resolved photoluminescence studies on GaN and ZnO samples are performed over a range of temperatures and excitation powers. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y21.00012: Electrical Resisitivity in Metals and Metallic Alloys from First Principles Alexander Slepko, Sadasivan Shankar, Justin Weber, Alexander Demkov We have developed a method for estimation of resistivity of metals and their alloys based on ab initio methods. The formalism is based on quantifying electron phonon interactions using Boltzmann-based electronic transport and plane wave-based density functional theory for electronic structure and phonon frequencies. We explicitly take into account long wave length scattering, energy band dispersion and interaction between impurities, often omitted in previous approaches. Given the detailed nature of our formalism, we will explain deviations from the most-used Matthiessen's Rule. We have tested our technique on Al, Cu, and Al-doping in Copper. Our resisitivity values compare very well with experimental data at room temperature; Al 2.75 $\mu\Omega$ cm (experimental, 2.83 $\mu\Omega$ cm), Cu 1.81 $\mu\Omega$ cm (experimental, 1.66 $\mu\Omega$ cm). We were also able to estimate the drops in conductivity of Cu due to alloying with Al for a wide range of composition (from dilute to concentrated alloys) which are consistent with the experiments. Given the general nature of our formalism, we believe that it is extendable to nanostructures. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y21.00013: Ab-initio calculations of the lattice thermal conductivity from an exact solution of the Boltzmann-Peierls equation Laurent Chaput In this work we present \emph{ab-initio} calculations of the lattice thermal conductivity and related quantities for several semiconductors of interest in energy transport and thermoelectricity. Excellent agreements with experiments are found. A new method is proposed to obtain a numerically exact and fast solution to the Boltzmann-Peierls equation. This is made possible using the symmetry of the systems and open the way to the theoretical design of new materials. The collision kernel of the equation is constructed using an efficient parallelization of the code over the irreducible triplets of phonon wavevectors involved in the different possible collisions. These irreducible triplets are the equivalent of the irreducible part of the Brillouin zone for single particle quantities. Therefore a formulation of the self energy and collision kernel based on their use drastically reduce the computational time. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y21.00014: Nonlinear strain generation in ultrafast laser excited semiconductors Eric Landahl, SooHeyong Lee, G. Jackson Williams, Donald Walko We have investigated the laser fluence dependence of the lattice response of Indium Antimonide and Gallium Arsenide crystals to ultrafast laser absorption using time-resolved x-ray diffraction. In both materials, slow thermal cooling follows an initial acoustic strain impulse. For Indium Antimonide, where the laser photon energy is significantly above the band gap, we find that both acoustic and thermal lattice expansions increase linearly with increasing laser fluence. The band gap and photon energy are much closer in Gallium Arsenide, where we find that while the thermal response remains linear with laser fluence, the magnitude of the acoustic impulse is highly nonlinear, exhibiting an initial saturation and recovery far below the laser damage threshold limit. Several hypotheses have been put forward of different nonlinear processes that could lead to this behavior. To place additional constraints on these models, we have recorded high-resolution diffraction lineshapes which can be directly compared to semiconductor strain models incorporating the transport of sound, heat, and charge. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y21.00015: Topological scattering of an electron gas by edge dislocations Koushik Viswanathan, Srinivasan Chandrasekar A theory of electron scattering by the strain field surrounding an edge dislocation in a linear isotropic medium is presented. When considered on a continuum scale, edge dislocations are topological defects --- the underlying elastic medium can no longer be described as a Euclidean manifold, but instead must be mapped to a Riemann--Cartan manifold with nontrivial torsion. An electron gas placed in such a background has additional covariant terms in the Hamiltonian. These act alongside the usual deformation potential arising from the shift in the conduction band minima due to the dislocation strain field. When considered as perturbations, these additional terms scatter electrons from one planewave state to another. For a group of parallel, randomly distributed edge dislocations, it is shown, through an iterative evaluation of the Boltzmann equation, that the contribution of these terms to the electrical resistivity of cold-worked Cu is larger than that of the deformation potential and the resulting specific dislocation resistivity is very close to the experimentally established value. The corresponding effect in the presence of grain boundaries (edge dislocation walls) is discussed and the application of these general results to transport in semiconductors is also presented. [Preview Abstract] |
Session Y22: Quantum Solids - He4
Sponsoring Units: DCMPChair: Matthias Graf, Los Alamos National Laboratory
Room: 324
Friday, March 22, 2013 8:00AM - 8:12AM |
Y22.00001: Dissipative superfluid mass flux through solid $^4$He Yegor Vekhov, Robert Hallock The thermo-mechanical effect in superfluid helium is used to create a chemical potential difference, $\Delta \mu$, across a liquid or solid $^4$He sample and induce a mass flux. With an improved technique, measurements of the mass flux, $F$, through a solid-filled sample cell at several fixed helium sample temperatures, $T$, have been done as a function of $\Delta \mu$. And, measurements of $F$ (in the range $100 < T < 550$~mK) have been done as a function of temperature for several fixed values of $\Delta \mu$. The temperature dependence of the flow through solid helium above $100$~mK is confirmed to show a reduction of the flux with increasing temperature, while for liquid helium there is no marked temperature dependence in the temperature range studied. The dependence of $F$ on $\Delta \mu$ documents in some detail the dissipative nature of the flow for the case of a solid helium- filled sample cell. In the case of solid helium we observe $F\sim \Delta \mu^b$ with $b \approx 0.3$, which is consistent with expectations for 1D superfluidity. The relationship between this work and the various torsional oscillator NCRI results is not clear. We may be exploring different phenomena. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y22.00002: Exact Solution for Vortex Dynamics in Temperature Quenches of Two-Dimensional Superfluids Andrew Forrester, Han-Ching Chu, Gary Williams An exact analytic solution for the dynamics of vortex pairs is obtained for rapid temperature quenches of a superfluid film starting from the line of critical points below the critical temperature $T_{KT}$. An approximate solution for quenches at and above above $T_{KT}$ provides insights into the origin of logarithmic transients in the vortex decay, and are in general agreement with recent simulations of the quenched XY model. These results confirm that there is no ``creation" of vortices whose density increases with the quench rate as predicted by the Kibble-Zurek theory, but only monotonic decay of the thermal vortices already present at the initial temperature. The problem in the Kibble-Zurek argument is the artificial restriction to measuring the vortex density only at the ``freezeout'' sampling time, which increases with the quench time. But since the the pairs continually decay, of course this will always result in lower vortex densities for a longer quench time and hence a later sampling time. But in fact the vortex densities can be measured at all times, and it then becomes quite clear that the instantaneous superfluid quench has the lowest vortex density at all times of any quench rate, since it most rapidly gets to the lowest temperature. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y22.00003: Elasticity, Plasticity and Supersolidity in Solid Helium John Beamish, Ariel Haziot, Andrew Fefferman, Xavier Rojas, Sebastien Balibar The frequency of a torsional oscillator containing solid helium depends on the helium's elastic properties, as well as its inertia. Mobile dislocations reduce the helium's shear modulus, but they are pinned at low temperatures. The resulting increase in shear stiffness raises the TO frequency and can mimic mass decoupling in a supersolid. The size of this elastic effect depends on the geometry of the oscillator and on the magnitude of the modulus changes. We recently showed that the elastic effect can be large enough to explain the apparent mass decoupling in some oscillators whose torsion rods have a central hole to admit the helium, suggesting that the apparent supersolidity is an artifact due to elastic changes. We have observed extremely large modulus changes in high quality single crystals. We were able to identify the dislocations responsible for the elastic changes and to show that they were arranged in a network with very large pinning lengths. The large modulus changes reflect the dislocations' extremely high mobility at low temperatures, which produces a ``giant plasticity'' in this quantum crystal. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y22.00004: The giant plasticity in $^4$He crystals Ariel Haziot, Andrew Fefferman, Xavier Rojas, John Beamish, Sebastien Balibar We have applied very small shear stresses (down to 1 nanobar) to oriented single $^4$He crystals, and directly measured their response as a function of temperature (from 15 mK to 1 K), orientation, crystal quality, $^3$He concentration, frequency and shear stress magnitude. For particular orientations, we have found a giant plasticity that is reversible, associated with the elastic coefficient c$_{44}$ which nearly vanishes around 200 mK. Other elastic coefficients show no measurable anomaly. The strong reduction of c$_{44}$ (80\% in high quality crystals with no impurities) shows that dislocations glide in the basal plane of the hexagonal structure with no dissipation. This plasticity disappears as soon as traces of $^3$He impurities bind to the dislocations (at low T) or if their motion is damped by collisions with thermal phonons (at higher T). It has no equivalent in classical crystals. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y22.00005: Dislocation densities and lengths in solid $^4$He from elastic measurements Andrew Fefferman, Ariel Haziot, John Beamish, Sebastien Balibar Elastic measurements on solid $^4$He show large softening of the shear modulus due to motion of dislocations, behavior which has been described as quantum plasticity. Dislocation networks may also be responsible for the unusual behavior seen at low temperatures in torsional oscillator and flow experiments. However, existing estimates of dislocation densities in helium crystals vary by many orders of magnitude. By measuring the temperature and frequency dependence of the elastic dissipation, we have determined dislocation densities and network lengths in both single crystals and polycrystals of $^4$He. The dislocation lengths are much longer than previous estimates, meaning that they are less connected than previously thought. Even in polycrystals, we find no evidence for the large densities of well-connected dislocations which would be needed to explain mass decoupling in torsional oscillators in terms of superfluidity in a dislocation network. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y22.00006: Solid $^4$He probed by both torsional oscillator and ultrasound Harry Kojima, Izumi Iwasa, John Goodkind The interpretation of observed anomalous increases in the frequencies of torsional oscillators (TO) containing solid $^4$He confined in Vycor nanopores as evidence for emergence of a supersolid phase has been met recently by conflicting experiments. Yet questions remain on the origin of the observed TO anomalies in bulk solid $^4$He samples. To search for the origin, we are carrying out simultaneous measurements of 10 MHz longitudinal ultrasound and TOs (250 $\sim$ 1100 Hz) on identical solid $^4$He samples. Temperature dependence of velocity and attenuation of ultrasound and that of amplitude and frequency of TO are measured. At the temperatures, where TO anomalies occur, anomalies in sound velocity and attenuation also appear. When solid $^4$He is doped with 20 ppm $^3$He, the tempeature of TO anomaly tracks that of ultrasound. Interpretation of these observations in terms of the motion of dislocation lines will be presented. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y22.00007: Excitations of Amorphous Solid Helium Jacques Bossy, Jacques Ollivier, Helmut Schober, Henry R. Glyde We present neutron scattering measurements of the dynamic structure factor, $S(Q,\omega)$, of amorphous solid helium confined in 47 \AA~pore diameter MCM-41 at pressure $48.6$ bar. At low temperature, $T$ = 0.05 K, we observe $S(Q,\omega)$ of the confined quantum amorphous solid plus the bulk polycrystalline solid between the MCM-41 powder grains. No liquid-like phonon-roton modes, other sharply defined modes at low energy ($\omega<$ 1.0 meV) or modes unique to a quantum amorphous solid that might suggest superflow are observed. Rather the $S(Q,\omega)$ of confined amorphous and bulk polycrystalline solid appear to be very similar. At higher temperature ($T>$ 1 K), the amorphous solid in the MCM-41 pores melts to a liquid which has a broad $S(Q,\omega)$ peaked near $\omega \simeq$ 0 characteristic of normal liquid $^4$He under pressure. Expressions for the $S(Q,\omega)$ of amorphous and polycrystalline solid helium are presented and compared. In previous measurements of liquid $^4$He confined in MCM-41 at lower pressure the intensity in the liquid roton mode decreases with increasing pressure until the roton vanishes at the solidification pressure (38 bars), consistent with no roton in the solid observed here [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y22.00008: Plasticity and dislocation-induced anomalous softening of solid helium under DC shea Irene Beyerlein, Caizhi Zhou, Jung-Jung Su, Matthias Graf, Charles Reichhardt, Alexander Balatsky The classical motion of gliding dislocation lines in slip planes of crystalline solid helium leads to plastic deformation even at temperatures far below the melting temperature and strongly affects elastic properties. In this work we propose that the gliding of dislocations and plasticity may be the origin of many observed elastic anomalies in solid He-4, which have been argued to be connected to supersolidity. We present and propose a dislocation motion model that describes the stress-strain $\tau $--$\varepsilon $ curves and work-hardening rate $\tau $/d$\varepsilon $ of a DC shear experiment to be performed at constant strain rate in solid helium. The calculated $\tau $/d$\varepsilon $ exhibits strong softening with increasing temperature owing to the motion of dislocations, which mimics anomalous softening of the elastic shear modulus $\mu $. In the same low-temperature region the classical motion of dislocations causes dissipation with a prominent peak [1] [1] C. Zhou et al., Philos. Mag. Lett. 92 (2012) 608 [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y22.00009: Simultaneous measurements of the torsional oscillator and shear modulus of solid 4He diluted with various 3He concentration Jaeho Shin, Wonsuk Choi, Jaewon Choi, Seong Jang, Keiya Shirahama, Eunseong Kim In 2004, Kim and Chan observed the non-classical rotational inertia (NCRI) of solid helium-4 by using a torsional oscillator (TO). Below 200mK, the resonance period of solid helium dropped, which was originally interpreted as the mass decoupling of the fraction of solid helium. Recently, anomalous increase in the shear modulus of solid helium was found and showed striking similarity in temperature, frequency, 3He concentration, and drive dependence to those of the NCRI [2]. To understand the connection between the NCRI and the shear modulus anomaly, we simultaneously measure the change in the resonance frequency and the stiffness of solid helium below 200mK. The torsion cell contains a pair of the concentric piezoelectric transducers (PZT) which defines an annular channel for the simultaneous measurements. We will report the interference between the motion of the TO at resonance and AC motion of the PZT in solid 4He with different 3He concentration. \\[4pt] [1] E.Kim and M.H.W Chan Nature 427, 225-227 (2004)\\[0pt] [2] J. Day and J. Beamish Nature 450, 853-856 (2007) [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y22.00010: $^4$He adsorption on $\alpha$-graphyne Yongkyung Kwon, Hoonkyung Lee, David M. Ceperley Path-integral Monte Carlo calculations have been performed to study $^4$He adsorption on a single $\alpha$-graphyne sheet that is a hexagonal network of $sp$- and $sp^2$-bonded carbon atoms. Using the $^4$He-substrate interaction described by a pairwise sum of the helium-carbon inter-atomic potentials, we have found that each hexagon of a graphyne can accomodate one $^4$He atom at its in-plane center. The first layer of $^4$He atoms adsorbed on this $^4$He-attached graphyne sheet with a composite of C$_8$He$_1$, exhibits various quantum phases depending on the helium coverage. It is found to be in a Mott insulating state at a coverage of 0.0706~\AA$^{-2}$ with three $^4$He atoms occupying each unit cell while the helium atoms form a commensurate triangular solid at 0.0941~\AA$^{-2}$. With the introduction of Ising pseudospins for two degenerate configurations of three $^4$He atoms in a hexagonal cell, the transition from the Mott insulator to the triangular solid can be interpreted as a ferromagnetic transition. In addition we find stable formation of zero-point vacancies in the commensurate triangular solid and their roles in possible realization of supersolidity are under investigation [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y22.00011: Pursuit of the Elusive Supersolid Xiao Mi, John D. Reppy The excitement following the initial report of supersolid behavior for $^{\mathrm{4}}$He embedded in porous Vycor glass has been tempered by the realization that many of the early supersolid observations were contaminated by effects arising from an anomaly in the elastic properties of solid $^{\mathrm{4}}$He. In an attempt to separate dynamic elastic effects from a true supersolid signal, we employed a torsional oscillator with two eigen frequencies to study the $^{\mathrm{4}}$He-Vycor system. We found that frequency dependent elastic signals can entirely account for the observed period shift signals. Although, we conclude that supersolid does not exist for the $^{\mathrm{4}}$He-Vycor case, the question of its presence in bulk samples remains open. In our current experiments we apply the two-frequency test to bulk samples of solid $^{\mathrm{4}}$He. Again we find a frequency dependent contribution arising from elastic effects. However, in some cases we also find a small frequency independent contribution, which may indicate the existence of a remnant supersolid phase. Given the history of this subject such results must be treated with caution. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y22.00012: An Ordered State of Dislocations in Solid Helium Hans Jochen Lauter, Eckhard Krotscheck, Efim Kats, Kenneth Herwig, Andrey Podlesnyak, Diallo Souleymane, Glyde Henry, Andreii Savici An ordered state of dislocations, see e.g. [1], is disclosed from neutron inelastic scattering data taken from solid helium at 40mK and a pressure of about 30bar. A characteristic feature is the phonon gap at the origin of about 0.15 meV, which reveals the non-equilibrium state of stressed helium created by rapid cooling with the blocked-capillary method. Energy gain scattering starts to appear at a temperature of 0.5 K that underlines the non-equilibrium state of stressed helium and the non-applicability of the detailed balance. The increasing thermal occupation of phonon-states observed as increasing intensity in energy gain scattering builds to a phase transition close to 1.4K. The creation of a helium single crystal with hcp-structure in thermal equilibrium [2] is observed at this temperature. This phase transition is in agreement with the vanishing quasi two-dimensional superfluid helium in solid helium confined in aerogel around 1.3K [3]. The event of the ``supersolid'' transition around 100mK is not observed in the two neutron scattering experiments.\\[4pt] [1] G. S\"{o}yler, et. al., Phys. Rev. Lett. {\bf 103}, 175301 (2009)\\[0pt] [2] E. Blackburn, et. al., PRAMANA {\bf 71}, 673 (2008)\\[0pt] [3] H.Lauter, et. al., Phys. Rev. Lett. {\bf 107}, 265301 (2011) [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y22.00013: Stability limit of a metastable state of hcp solid helium-4 Fabien Souris, Jules Grucker, Jacques Dupont-Roc, Philippe Jacquier Solid helium has the unique feature of having an horizontal melting curve in the P,T plane. This offers novel opportunities to study the stability limits of a metastable solid, by using the pressure as a control parameter of the metastability. The metastable state is obtained by focusing a $1$~MHz ultrasonic sound wave inside an helium-4 crystal. Around $4$~bar below the melting pressure, the metastable crystal becomes unstable. Different configurations with one or two ultrasonic emitters have been used and lead to the same stability limit. This happens at much lower depression than predicted by nucleation theory or by quantum Monte Carlo simulations. Repeated experiments show that the instability initially appears as a small defect ($\sim0.2$~mm) located at the maximum isotropic strain. Further studies are performed to understand the underlying mechanism of the instability. Possible scenarios accounting for this unexpected observation are discussed. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y22.00014: Path-Integral Monte Carlo Simulations of Ideal Strength and Peierls Stress in HCP 4He Edgar Josu\'e Landinez Borda, Maurice De Koning The ideal strength of a crystal is defined as the stress required to induce plastic deformation in a defect-free crystal. It is a theoretical upper bound to the strength of real crystals. The Peierls stress, on the other hand, is the minimum stress required to move a lattice dislocation and produce defect-mediated deformation. Here we present results for both quantities in HCP 4He as obtained from a series of Path-integral Monte Carlo simulations and discuss them in terms of its deformation behavior. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y22.00015: Superfluid transition in a correlated dislocation network Hannes Meier, Mats Wallin, Stephen Teitel The search for a supersolid state in He-4 solids has motivated theoretical investigations of 3D connected superfluid dislocation networks. It has usually been assumed that a 3DXY universality class controls the superfluid transition in such systems since the random distance between intersections of the dislocation lines carrying superfluidity appears as uncorrelated disorder which is irrelevant at the 3DXY transition. We consider the possibility that the random disorder instead has long range correlations, and investigate several different models of correlated defects. Analytic arguments and extensive Monte Carlo simulations demonstrate new disordered universality classes for the superfluid transition with a smooth temperature dependence at the transition of the superfluid density and heat capacity. [Preview Abstract] |
Session Y23: Semiconductors: Thermodynamic & Optical Properties I
Sponsoring Units: FIAPChair: Andre Sushkov, University of Maryland
Room: 325
Friday, March 22, 2013 8:00AM - 8:12AM |
Y23.00001: Temperature Dependence of Band Gaps in Semiconductors: Electron-Phonon Interaction J.S. Bhosale, A.K. Ramdas, A. Burger, A. Mu\~{n}oz, A.H. Romero, M. Cardona, R. Lauck, R.K. Kremer A theoretical investigation with \textit{ab initio} techniques of the electron-phonon interaction of semiconductors with chalcopyrite structure and its comparison with modulated reflectivity experiments yield a striking difference between those with (AgGaS$_2$) and without (ZnSnAs$_2$) $d$ electrons in their valence bands. The former exhibit a non-monotonic temperature dependence of the band gaps whose origin is not yet fully understood. The analysis of this temperature dependence with the Bose-Einstein oscillator model\footnote{G\"{o}bel \textit{et. al.} Phys. Rev. B 57, 15183 (1998).} involving two oscillator terms having weights of opposite signs, provides an excellent agreement with the experimental data and correlates well with the characteristic peaks in the phonon density of states associated with the acoustical phonon modes. This work underscores the need for theoretical understanding of the electron-phonon interaction involving $d$ electrons, particularly in \textit{ab initio} investigations. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y23.00002: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y23.00003: The Franz-Keldysh effect revisited: Electroabsorption in GaAs including interband coupling and excitonic effects Federico Duque-Gomez, J.E. Sipe We show numerical results for the linear optical absorption of bulk GaAs in the presence of a homogeneous dc electric field. Our approach, based on gauge-invariant nonequilibrium Green functions \footnote{T. Kita and H. Yamashita, J. Phys. Soc. Jpn \textbf{77}, 024711 (2008).}, is suitable for including many-body effects and using realistic band models. We calculate the time evolution of the interband polarization driven by an optical pulse and derive the absorption coefficient from it. The interband effects of the dc field are captured in a matrix transform in the band indices, which satisfies a differential equation solved efficiently in a separate numerical calculation. For the present calculation we have used a 14-band $\mathbf{k} \cdot \mathbf{p}$ model and treated excitonic effects at a Hartree-Fock level. Previous calculations in the independent particle approximation have shown interesting effects of the band structure and the importance of the interband coupling.\footnote{J. K. Wahlstrand and J. E. Sipe, Phys. Rev. B \textbf{82}, 075206 (2010).} We describe the effect of including the Coulomb interaction, which is especially relevant in low temperature and low field experiments. \footnote{A. Jaeger and G. Weiser, Phys. Rev. B \textbf{58}, 10674 (1998).} [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y23.00004: Photoreflectance and Strain Relaxation Studies of Semipolar InGaN Grace Metcalfe, Nathaniel Woodward, Hongen Shen, Michael Wraback, Po Shan Hsu, James Speck Recently, there has been a surge of interest in semipolar nitride material for quantum well devices to reduce or eliminate the quantum confined stark effect due to the strong internal polarization. Studies on the effect of the strain relaxation in semipolar nitrides are critical to the successful development and operation of long wavelength devices such as LEDs and LDs. In general, the wavefunctions associated with the A, B, and C exciton transitions in wurtzite material are mixed for crystal orientations other than c-plane. Therefore, the polarization and energy of these exciton interband transitions within wurtzite nitrides also depend on the strain and crystal orientation. In this paper, we present the effects of partial strain relaxation on the optical properties of a thickness series of semipolar (11-22) and (20-21) InGaN compressively strained to GaN using polarization-dependent photoreflectance (PR) measurements. We observe that the absolute energy of the exciton transition parallel to the c-axis is greater than that perpendicular to the c-axis, and the energy separation between them increases with strain relaxation. Our PR data compares well with strain relaxation measurements taken using X-ray diffraction, as well as with our calculations. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y23.00005: X-ray induced optical transparency and x-ray/optical photon interactions in GaAs Stephen Durbin, Tim Graber, Rob Henning An intense x-ray synchrotron pulse transforms a thin crystal of GaAs from being opaque to transparency in picoseconds for probe photon energies near the band gap energy. X-ray absorption and subsequent de-excitation processes pump a high density of electrons from the valence band into the conduction band, causing Pauli blocking of the band gap photons and hence their transmission through the bulk of the specimen. Although the GaAs photocarrier lifetime is less than 300 ps, the transmission decay time constant was as large as 2000 ps when the laser intensity was increased, an effect that can be partially understood in terms of photobleaching and the depth of x-ray absorption. Finally, the excess transmission of band gap photons due to high laser intensity could be suppressed by the onset of the x-ray pulse, evidence for x-ray quenching of laser hole burning. These effects are manifestations of x-ray/optical photon interactions mediated by their conduction band excitations in GaAs. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y23.00006: Exciton absorption of entangled photons in semiconductor quantum wells Ferney Rodriguez, David Guzman, Luis Salazar, Luis Quiroga The dependence of the excitonic two-photon absorption on the quantum correlations (entanglement) of exciting biphotons by a semiconductor quantum well is studied. We show that entangled photon absorption can display very unusual features depending on space-time-polarization biphoton parameters and absorber density of states for both bound exciton states as well as for unbound electron-hole pairs. We report on the connection between biphoton entanglement, as quantified by the Schmidt number, and absorption by a semiconductor quantum well. Comparison between frequency-anti-correlated, unentangled and frequency-correlated biphoton absorption is addressed. We found that exciton oscillator strengths are highly increased when photons arrive almost simultaneously in an entangled state. Two-photon-absorption becomes a highly sensitive probe of photon quantum correlations when narrow semiconductor quantum wells are used as two-photon absorbers. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y23.00007: Localized-delocalized transitions in GaAsN Kirstin Alberi, Brian Fluegel, Scott Crooker, Daniel Beaton, Aaron Ptak, Angelo Mascarenhas Dilute nitride semiconductors are promising materials for high efficiency multijunction solar cells and light emitting diodes, yet they exhibit an unusual evolution of their optical and electronic properties as they transition from an impurity-doped semiconductor into an alloy upon the addition of N. For example, a significant change in the photoluminescence spectrum of GaAsN is accompanied by a rapid increase in the broadening parameters of the E$_{\mathrm{0}}$ and E$_{\mathrm{1}}$ critical point transitions in electromodulated reflectance spectra as the N concentration is increased from 0.12{\%} N to 0.32{\%} N. We demonstrate that these changes result from the percolation of localized N cluster states bound below the conduction band into fully extended superclusters and the emergence of a mobility edge. Furthermore, photoluminescence studies show that we are able to reverse this localized to delocalized transition through the application of high magnetic fields to 57 tesla. These experimental results provide new insight into the percolation behavior of isoelectronic cluster states in semiconductor alloys. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y23.00008: A Tunable Terahertz Detector Based On Self Assembled Plasmonic Structure on a GaAs 2DEG Chejin Bae, Deepu George, Rohit Singh, Andrea Markelz To improve detector sensitivity, tunability and remove polarization dependence, we develop the gated grid plasmonic structure on 2DEG by using nanosphere self-assembly lithography. The measured transmission clearly is not following Drude response, but rather has three sharp resonances corresponding fundamental, 3rd, and 5th harmonics of plasmon resonance respectively. Measurements at 80K show a large transmission change of 25{\%}. We also confirmed a magneto plasmon dispersion of this device. In this paper we will discuss the radiative damping effect which affects enhanced absorption at the higher harmonics mode relative to fundamental [1] and inductive grid resonance of this self-assembled plasmonic structure by demonstrating an angular dependence of transmission due to 2D plasmon[2]. [1] V. Popov et al., J. Appl. Phys. \textbf{94}, 3556 (2003) [2] T. W. Ebbesen et al., Nature, \textbf{391}, 667 (1998) [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y23.00009: Electron-hole sound: Observation of coherent acoustic plasmons in photoexcited GaAs Prashant Padmanabhan, Steve Young, Meredith Henstridge, Sishir Bhowmick, Pallab Bhattacharya, Roberto Merlin Three-dimensional multi-component plasmas involving species with very different masses are expected to show a new branch of charge density fluctuations with a frequency dispersion that is linear with respect to the wave vector [1]. Not to be confused with similarly named modes of metallic surfaces [2], these bulk excitations are known as \textit{acoustic plasmons}. In the past, they have been identified in some gas plasmas [3] and, notably, also in electron-hole plasmas in GaAs via spontaneous Raman scattering [4]. Here, we present the first observation of \textit{coherent} acoustic plasmons in photoexcited GaAs. We utilize an ultrafast double pump-probe scheme to probe, in the time domain, the oscillations in the sample reflectivity associated with these modes. Results agree well with theoretical calculations based on the random phase approximation. The data also suggests that the coherent acoustic oscillation is driven by the interaction with modes resulting from the coupling between the longitudinal-optical-phonons and the conventional optical plasmons of the electrons. [1] J. Appel and A. W. Overhauser, Phys. Rev. B 26, 507 (1982). [2] B. Diaconescu, et al., Nature 448, 57 (2007). [3] A. Y. Wong, R. W. Motley, and N. D'Angelo, Phys. Rev. 133, A436 (1964). [4] A. Pinczuk, J. Shah, and P. A. Wolff, Phys. Rev. Lett. 47, 1487 (1981). [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y23.00010: Polarization and Interface Effects on THz Emission from c-plane InGaN/GaN Heterostructures Nathaniel Woodward, Chad Gallinat, Ryan Enck, Grace Metcalfe, Hongen Shen, Michael Wraback Nitride semiconductors have strong piezoelectric and spontaneous polarizations, which, when terminated at a heterointerface, create a large internal electric field. This field enables transport-based THz radiation with intensities comparable to that from conventional contactless semiconductor surface emitters such as InAs. We observed THz emission from 200-nm thick c-plane InGaN coherently strained to various doped GaN substrates due to photocarrier acceleration toward the surface in the field resulting from the polarization charge at the InGaN/GaN interface. We compare THz emission from the samples pumped from the substrate side as well as the epilayer side such that diffusive and polarization field-induced transport were in the same and opposite directions, respectively. When pumped from the substrate side, we observed several spectral features that did not appear when pumping the InGaN surface. These features may be attributed to effects from the InGaN/GaN heterointerface. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y23.00011: Helicity-dependent photocurrent in a (110) GaAs quantum well stack D.C. Schmadel, M.-H. Kim, A.B. Sushkov, G.S. Jenkins, J.D. Koralek, J.E. Moore, J. Orenstein, Yuzo Ohno, Hideo Ohno, H.D. Drew There have been many reports on the circular photogalvanic effect (CPGE) in GaAs quantum wells. A recent theoretical study suggests that the CPGE can be governed by a quantum confinement-induced Berry phase effect that depends only on the quantum-well width and crystal orientation (J.E. Moore, Phys. Rev. Lett. 2010). We have measured the photocurrent in a (110)-oriented GaAs quantum well stack under illumination of circularly polarized THz radiation. We will report measurements of the helicity-driven photocurrent as a function of frequency, polarization, angle of incident, and temperature, and compare with theoretical predictions of the Berry phase contribution. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y23.00012: Direct-Indirect Crossover in Ga$_{\mathrm{x}}$In$_{\mathrm{1-x}}$P Alloys Angelo Mascarenhas, Kirstin Alberi, Brian Fluegel Advances in metamorphic growth of high quality Ga$_{\mathrm{x}}$In$_{\mathrm{1-x}}$P ($x$ \textgreater\ 0.5) on GaAs substrates have improved the practicality of using these alloys in visible light emitting diodes and lasers. The wavelength range over which these materials are efficient light emitters is determined by the direct-indirect crossover energy, yet considerable discrepancies still remain in the literature regarding the precise crossover composition, $x_{C}$. We revisit this topic and present new experimental results that precisely pinpoint the crossover composition without extrapolation of the direct and indirect bandgap trends. Observation of concurrent yet distinct direct and indirect transitions in the 2 K time integrated and time resolved photoluminescence spectra of disordered Ga$_{0.719}$In$_{0.281}$P films places the crossover very near the composition $x_{C} =$ 0.71. This revised value is critical for facilitating realistic engineering of Ga$_{\mathrm{x}}$In$_{\mathrm{1-x}}$P alloys for light emitting and photovoltaic applications. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y23.00013: Temperature Dependent Dielectric Functions of MBE-grown GaMnAs Thin Films F.C. Peiris, Z.J. Weber, N. Mandel, T. Scully, X. Liu, J.K. Furdyna Spectroscopic ellipsometry was used to measure the dielectric functions of a series of Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As samples from 20 K to 300 K. Initially, by modeling the ellipsometric data in the transparent region, the film thickness and the index of refraction of Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As alloys were obtained. Extending the analysis into the absorption region, the dielectric function for the entire spectral range between 0.6 eV and 6.5 eV was determined. Monitoring the temperature dependence of the critical points, corresponding to electronic transitions in the Brillouin zone, we deduced the electron-phonon coupling parameters using Bose-Einstein occupation distributions. In comparison to GaAs, we find that the ternary alloy Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As shows a slight enhancement in its electron-phonon coupling. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y23.00014: Enhanced THz emission from stacked $+c$-plane InGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy Chad Gallinat, Nathaniel Woodward, Ryan Enck, Grace Metcalfe, Hongen Shen, Michael Wraback We have previously demonstrated THz emission in a single, fully-strained 200 nm InGaN layer grown on GaN. This emission was due to the acceleration of electrons toward the surface in the piezoelectric polarization charge-induced electric field. We observed a reduction in the THz emission from a fully relaxed InGaN layer where the piezoelectric polarization was removed. In order to increase the InGaN layer thickness to maximize the absorption of the excitation pulse, we introduced GaN spacers to limit strain relaxation. We observed an increase in THz emission strength from samples with three stacks of coherently strained 100 nm/10 nm InGaN/GaN layers over the emission from single layer structures. We explored the balance of In alloy content, InGaN layer thickness and InGaN layer strain to maximize the piezoelectric polarization for enhanced THz emission. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y23.00015: Optical spectroscopies of materials from orbital-dependent approximations Ismaila Dabo, Andrea Ferretti, Matteo Cococcioni, Nicola Marzari Electronic-structure calculations based upon density-functional theory (DFT) have been fruitful in diverse areas of materials science. Despite their exceptional success and widespread use, a range of spectroscopic properties fall beyond the scope of existing DFT approximations. Failures of DFT calculations in describing electronic and optical phenomena take root in the lack of piecewise linearity of approximate functionals. This known deficiency reverberates negatively on the spectroscopic description of systems involving fractionally occupied or spatially delocalized electronic states, such as donor-acceptor organic heterojunctions and heavy-metal organometallic complexes. In this talk, I will present a class of orbital-dependent density-functional theory (OD-DFT) methods that are derived from a multidensity formulation of the electronic-structure problem and that restore the piecewise linearity of the total energy via Koopmans' theorem. Such OD-DFT electronic-structure approximations are apt at describing full orbital spectra within a few tenths of an electron-volt relative to experimental photoemission spectroscopies and with the additional benefit of providing appreciably improved total energies for molecular systems with fractional occupations. [Preview Abstract] |
Session Y24: Focus Session: Advances in Fermionic Simulatons
Sponsoring Units: DCOMPRoom: 326
Friday, March 22, 2013 8:00AM - 8:36AM |
Y24.00001: Determinantal Quantum Monte Carlo simulations of fermions in optical lattices Invited Speaker: Thereza Paiva The ability to cool fermions in optical lattices to ultra cold temperatures has led to an interdisciplinary area of research, that has attracted a lot of attention in recent years. An interesting development in this area is the possibility to realize models for strongly correlated fermions in the laboratory, such as the fermionic Hubbard Model. Determinantal Quantum Monte Carlo simulations have proven to be an important tool in the study of fermionic atoms. Nonetheless, it is important to compare the results and efficiency of different methods. Here comparisons with Numerical Linked Cluster Expansion and Dynamical Mean Field Theory data for double occupation and short range correlations, both relevant to current optical lattice experiments, will be presented and discussed. Another topic relevant in the context of optical lattice experiments is the study of metal insulator transitions. Indeed, the Mott insulating phase has been realized and observed in two-flavor mixtures of fermionic atoms loaded on optical lattices, being characterized both by the double occupation and the compressibility. An interesting point that has been addressed in the literature over the years is whether the same fermion-fermion interaction, responsible for the Mott insulating state, could drive an insulating system metallic. Here we show that, when fermions are loaded in optical lattices with spatially varying interactions a correlation induced Mott insulator to metal transition can take place. The spatial modulation of the interactions was recently demonstrated and opens the possibility for the experimental realization of such exotic phases. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y24.00002: Quantum Monte Carlo Calculations of Entanglement Norm Tubman, Jeremy McMinis Spatial entanglement properties have become increasingly important in physics which includes studies in diverse fields such as condensed matter physics, astrophysics, and quantum computation. One of the important outstanding problems in the field of entanglement is to understand the effect of many body interactions. Recent advances in quantum Monte Carlo have facilitated such studies over a range of Hamiltonians that were previously inaccessible by other techniques. We apply these techniques to interacting molecular and condensed matter systems and discuss the effect interactions have on entanglement properties. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y24.00003: Excited state calculations in solids by auxiliary-field quantum Monte Carlo Fengjie Ma, Shiwei Zhang, Henry Krakauer We present an approach for ab initio many-body calculations of excited states in solids. Using auxiliary-field quantum Monte Carlo \footnote{S.~\ Zhang and H.~\ Krakauer, Phys. Rev. Lett. {\bf 90}, 136401 (2003)}, we introduce an orthogonalization constraint with virtual orbitals to prevent collapse of the stochastic Slater determinants in the imaginary-time propagation. Trial wave functions from density-functional calculations are used for the constraints, and detailed band structures can be calculated. Results for standard semiconductors are in good agreement with GW calculations and with experiment. For the challenging ZnO, we obtain a fundamental band gap of 3.30(16) eV, consistent within the range of experimental measurements \footnote{V.~\ Srikant and D.~\ R.~\ Clarke, J. Appl. Phys. 83, 5447 (1998); S.~\ Tsoi, X.~\ Lu, A.~\ K.~\ Ramdas, H.~\ Alawadhi, M.~\ Grimsditch, M.~\ Cardona, and R.~\ Lauck, Phys. Rev. B 74, 165203 (2006); H.~\ Alawadhi, S.~\ Tsoi, X.~\ Lu, A.~\ K.~\ Ramdas, M.~\ Grimsditch, M.~\ Cardona, and R.~\ Lauck, Phys. Rev. B {\bf 75}, 205207 (2007)}. Applications to other systems are currently underway. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:36AM |
Y24.00004: Bold Diagrammatic Monte Carlo for Fermionic and Fermionized Systems Invited Speaker: Boris Svistunov In three different fermionic cases---repulsive Hubbard model, resonant fermions, and fermionized spins-1/2 (on triangular lattice)---we observe the phenomenon of sign blessing: Feynman diagrammatic series features finite convergence radius despite factorial growth of the number of diagrams with diagram order. Bold diagrammatic Monte Carlo technique allows us to sample millions of skeleton Feynman diagrams. With the universal fermionization trick we can fermionize essentially any (bosonic, spin, mixed, etc.) lattice system. The combination of fermionization and Bold diagrammatic Monte Carlo yields a universal first-principle approach to strongly correlated lattice systems, provided the sign blessing is a generic fermionic phenomenon. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y24.00005: Path Integral Quantum Monte Carlo Benchmarks for Molecules and Plasmas John Shumway Path integral quantum Monte Carlo is used to simulate hot dense plasmas and other systems where quantum and thermal fluctuations are important. The fixed node approximation---ubiquitous in ab initio ground state Quantum Monte Carlo---is more complicated at finite temperatures, with many unanswered questions. In this talk I discuss the current state of fermionic path integral quantum Monte Carlo, with an emphasis on molecular systems where good benchmark data exists. We look at two ways of formulating the fixed node constraint and strategies for constructing finite-temperature nodal surfaces. We compare different the free energies of different nodal choices by sampling an ensemble of nodal models within a Monte Carlo simulation. We also present data on imaginary-time correlation fluctuations, which can be surprisingly accurate for molecular vibrations and polarizabilty. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y24.00006: Quantum Monte Carlo simulations of complex Hamiltonians Valery Rousseau, Kalani Hettiarachchilage, Ka-Ming Tam, Juana Moreno, Mark Jarrell In the last two decades there have been tremendous advances in boson Quantum Monte Carlo methods, which allow for solving more and more complex Hamiltonians. In particular, it is now possible to simulate Hamiltonians that include terms that couple an arbitrary number of sites and/or particles, such as six-site ring-exchange terms. These ring-exchange interactions are crucial for the study of quantum fluctuations on highly frustrated systems. We illustrate how the Stochastic Green Function algorithm with Global Space-Time Update can easily simulate such complex systems, and present some results for a highly non-trivial model of bosons in a pyrochlore crystal with six-site ring-exchange terms. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y24.00007: Quasi-adiabatic Quantum Monte Carlo algorithm for non-equilibrium quantum phase transitions Cheng-Wei Liu, Anders W. Sandvik, Anatoli Polkovnikov We investigate a new quantum Monte Carlo algorithm for studying static and dynamic properties of quantum phase transitions. The method, called the quasi-adiabatic quantum Monte Carlo algorithm, is based on evolution with a changing Hamiltonian to derive information pertinent to a quantum quench according to an arbitrary protocol. We demonstrate the method with results for 1D and 2D transverse-field Ising models, showing finite-size and finite-velocity scaling according to a generalization of the Kibble-Zurek mechanism. We explore ways to extract critical points and critical exponents to high precision. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y24.00008: Ground state phases in the half-filled staggered $\pi$-flux Hubbard model on square lattices Chia-Chen Chang, Richard T. Scalettar Ground state phase diagram of the half-filled staggered $\pi$-flux Hubbard model on a square lattice are studied by means of constrained-path quantum Monte Carlo method. Charge and spin excitation gaps and magnetic order are calculated as a function of interaction strength $U/t$. Within our numerical scheme, it is found that the ground state phase is a semi-metal at $U/t < 5.6$, and a Mott insulator with long-range antiferromagnetic order at $U/t > 6.6$. In the window $5.6 < U/t < 6.6$, the system is an insulator in which both magnetic and dimer orders are absent. Spin excitation in the intermediate phase appears to be gapless, and the measured equal-time spin-spin correlation function shows a power-law dependence of relative distance. Our data suggests that the paramagnetic insulating intermediate phase might be a possible place to look for the putative algebraic spin liquid. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y24.00009: Momentum-dependent pseudogaps in the half-filled two-dimensional Hubbard model Nils Bluemer, Daniel Rost, Elena Gorelik, Fakher Assaad We compute unbiased spectral functions of the two-dimensional Hubbard model by extrapolating Green functions, obtained from determinantal quantum Monte Carlo simulations, to the thermodynamic and continuous time limits. Our results clearly resolve the pseudogap at weak to intermediate coupling, originating from a momentum selective opening of the charge gap. A characteristic pseudogap temperature $T^*$, determined consistently from the spectra and from the momentum dependence of the imaginary-time Green functions, is found to match the dynamical mean-field critical temperature, below which antiferromagnetic fluctuations become dominant. Our results identify a regime where pseudogap physics is within reach of experiments with cold fermions on optical lattices.\\[2ex] D. Rost, E. V. Gorelik, F. Assaad, N. Bl\"umer, Phys. Rev. B {\bf 86}, 155109 (2012). [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y24.00010: Series Expansion for the Green's Function of the Infinite-U Hubbard Model Ehsan Khatami, Edward Perepelitsky, B. Sriram Shastry, Marcos Rigol We implement computationally a strong-coupling expansion for the dynamical single-particle Green's function of the infinite-U Hubbard model up to the eighth order in the hopping, within the formalism introduced by Metzner [1]. We obtain analytical expressions for the finite Matsubara frequency Green's functions and the Dyson self energy in the momentum space at all densities in the thermodynamic limit. The results match those obtained up to the fourth order by means of another method devised by us. Furthermore, we employ Pade approximations and various numerical re-summation techniques to extend the region of convergence to lower temperatures.\\[4pt] Ref. [1]: W. Metzner, Phys. Rev. B 43, 8549 (1991). [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y24.00011: ABSTRACT WITHDRAWN |
Session Y25: Focus Session: Novel Theories and Methods in Computational Physics
Sponsoring Units: DCOMPChair: Rajamani Narayanan, Florida International University
Room: 327
Friday, March 22, 2013 8:00AM - 8:36AM |
Y25.00001: Beyond standard model physics using lattice techniques Invited Speaker: Ari Hietanen I will review the recent results of beyond standard model lattice calculations. The focus is on the models of dynamical electroweak symmetry breaking, Technicolor, and dark matter. A lot of effort has been devoted to finding out which models are conformal and which exhibit a chiral symmetry breaking. Lately also phenomenologically interesting observables, like mass anomalous dimension, glueball spectrum, and a contribution to scalar meson mass, have been calculated. I will briefly comment about the implications of these calculations to the phenomenology. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 9:12AM |
Y25.00002: Conformal and near-conformal field theories Invited Speaker: Anna Hasenfratz Non-Abelian gauge fermion systems could be chirally broken and confining or conformal, depending on the number of fermions and their representation. Models near the conformal boundary are important as they could be relevant in describing physics beyond the Standard Model. These models are strongly coupled and require non-perturbative investigations. Lattice techniques that were developed for QCD studies can be used to simulate these systems but there is growing evidence that new observables, new approaches are needed to study the properties of conformal or near conformal models. In this talk I will briefly summarize the most promising models and describe some standard and some promising new methods to study their properties. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y25.00003: Gradient corrections to finite-temperature exchange-correlation functionals Travis Sjostrom, James Dufty In principle, the only approximation in Kohn-Sham DFT is for the exchange-correlation (XC) energy. As such, about 40 years of development for the zero-temperature XC density functional has resulted in a ladder of functionals from simple LDA (based on essentially exact QMC results) to orbital-dependent functionals including virtuals. The non-zero temperature situation is different. To date, a handful of $T \ne 0$ K XC functionals have been introduced based on approximate electron gas calculations or interpolations. Except for a finite-T gradient expansion of X, all are local density approximations. Here we present calculations for the XC energy of the electron gas in the dielectric formalism, specifically with approximate local field corrections (LFC). Analysis of the LCF is used to evaluate the first term of the gradient expansion of the XC energy in the slowly varying limit. The resulting gradient expansion finite temperature XC functional will be presented and possible generalized gradient approximations will be considered. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y25.00004: High-Throughput Investigation of Delafossite materials Barry Haycock, M. Kylee Underwood, Jonathan Lekse, Christopher Matranga, James P. Lewis We present the application of high-throughput calculations to the intriguing problem of the forbidden optical transition in the CuGa$_{1-x}$Fe$_x$O$_2$ delafossites, which is prototypical of many delafossite systems. When 5\% or more of the Ga sites are replaced with Fe, there is a sudden shift to an optical band gap of 1.5eV from 2.5eV. Using high-throughput calculations and data mining techniques, we show the most likely positional configurations for x = 0.00 through x = 0.10 of the Fe atoms relative to one another. Implications of this result and applications of the techniques used are discussed, including the development of candidate materials via high-throughput analysis of constituent search-space. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y25.00005: Classical representation of quantum systems at equilibrium Sandipan Dutta, James Dufty A classical system has been constructed that reproduces the thermodynamics of a quantum system at equilibrium.The classical system has an effective temperature, local chemical potential, and pair interaction that are defined by requiring equivalence of the pressure, density and pair correlation functions for the classical and quantum systems. The thermodynamic parameters of the classical system are determined such that the ideal gas and weak coupling RPA limits are preserved. The pair correlations predicted from this model are in excellent agreement with Diffusion Monte Carlo results at $T=0$ and with the finite-temperature results from the Perrot-Dharmawardana model [1]. Systems in harmonic confinement have also been studied to look into the quantum effects on shell formation. [1] M. W. C. Dharma-wardana and F. Perrot, Phys. Rev. Lett. 84, 959 (2000). [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y25.00006: Phase Diagram and Isentropic Curves for Ferromagnetic and Antiferromagnetic Transverse Ising Model on a Triangular Lattice Vladimir Iglovikov, Jaan Oitmaa, Rajiv Singh, Richard Scalettar We study both the ferromagnetic and anti-ferromagnetic Ising model on a triangular lattice with a transverse magnetic field. Quantum Monte Carlo simulations and series expansions techniques are employed to determine the isentropes and phase diagrams for the system. Quantum Phase Transitions in the transverse field Ising model have recently been observed experimentally for linear chains and for small clusters with long range interactions. They are currently under investigation for triangular lattices. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y25.00007: Critical behavior of the XY model on fractal lattices Michelle Przedborski, Bozidar Mitrovic There has been considerable interest in determining whether the universality hypothesis extends to systems which are of non-integer dimension or to systems which are scale invariant (fractals). Specifically, research into these types of systems is concerned with determining the relevance of topological properties to their critical phenomena. We have performed Monte Carlo simulations for the XY model on three fractal lattices with different topological properties: the Sierpinski pyramid, Menger sponge, and Sierpinski carpet (which underwent unusual Berezinskii-Kosterlitz-Thouless transition). We will discuss the details of our results and show that while some properties, such as the order of ramification, are important in determining the critical behavior of these structures, the fractal dimension is not. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y25.00008: Strong curvature effects in wave problems Morten Willatzen, Anders Pors, Jens Gravesen Linear-in-curvature contributions to wave-problem eigenvalues in quantum mechanics and acoustics are evaluated analytically using differential geometry methods and perturbation theory. It is demonstrated that in the case of Neumann boundary conditions, relevant for electromagnetic and acoustic problems, linear-in-curvature contributions are nonvanishing if the geometry supports eigenstates that do not satisfy parity. If Dirichlet boundary conditions apply, however, linear-in-curvature vanish identically. We continue to compute analytically eigenvalue changes for a toroidal angular-sector geometry in the case of both Dirichlet and Neumann boundary conditions. Eigenstate and eigenvalue results are finally verified qualitatively and quantitatively against Comsol finite element model results. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y25.00009: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y25.00010: Computing the response functions of topological insulators with non-commutative geometry Emil Prodan For periodic systems, the correlation functions take closed-form expression involving integrations and derivations of ordinary functions defined over the Brillouin torus (Bloch-Floquet calculus). The non-commutative geometry provides an analog of the Bloch-Floquet calculus for aperiodic systems under magnetic fields, and this formalism was used in the past to derive closed-form expressions for Kubo-formula, orbital electric and magnetic polarization and much more, for strongly disordered systems under magnetic fields. In this talk I will describe how these non-commutative formulas can be evaluated on a computer, enabling us to investigate the response coefficients of strongly disordered topological with unprecedented precision and efficiency. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y25.00011: Disordered Floquet Topological Insulators Paraj Bhattacharjee, Netanel Lindner, Gil Refael We study the problem of localization in the recently proposed two-dimensional Floquet topological insulators in semiconductor quantum wells. We compute the single particle Green's function for the system using a real-time simulation. The phase diagram obtained indicates that at weak disorder the system remains delocalized. The edge-states are protected and only destroyed when the disorder closes the gap in the Floquet spectrum. The system localizes only at disorder strength which is much larger than the gap in the Floquet spectrum, long after this gap has been closed due to disorder. Analytically we compare these results with the results obtained using disorder averaged Floquet Green's functions in the Born approximation. [Preview Abstract] |
Session Y28: Rods & Buckling
Sponsoring Units: GSNPChair: Basile Audoly, University Paris 6
Room: 336
Friday, March 22, 2013 8:00AM - 8:12AM |
Y28.00001: Three-dimensional Curling of Pre-strained Elastomeric Strips: From Hemi-helix to Helix Jia Liu, Jiangshui Huang, Tianxiang Su, Katia Bertoldi, David Clarke A variety of three dimensional curls can be produced by a simple generic process consisting of pre-straining one elastomeric strip, joining it to another and then releasing the bi-strip. In thin strips we observe the formation of hemi-helices, which consists of multiple, alternating helical sections of half wavelength in opposite chiralities, separated by perversions. By contrast, helical shapes with uniform handedness are found when the cross-section is wide and flat. Finally, in the transition region between helices and hemi-helices not only the geometry effects but also boundary conditions as well as dynamic effects severely contributes. The phase separation of hemi-helical and helical structures has similarities with coiled polymer molecules and plant tendrils. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y28.00002: A rod theory for pleated elastic strips Basile Audoly, Marcelo Dias We consider the equilibrium shapes of a thin, annular strip cut out in a sheet of paper: when such a strip is folded along its circular centerline, it has been observed to buckle out-of-plane (Dias et al., PRL, 2012). We derive an equivalent Kirchhoff rod model for the folded strip. A nonlinear effective constitutive law capturing the underlying geometrical constraints is derived. In this rod model, the opening mode of the ridge appears as an internal degree of freedom. The buckling of the strip is shown to be equivalent to the buckling of a circular ring having two frozen curvatures. Another type of instability is pointed out, whereby the centerline remains planar but the ridge angle is modulated. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y28.00003: Buckling of a Flexible Strip Sliding on a Frictional Base Alexandre Huynen, Julien Marck, Vincent Denoel, Emmanuel Detournay The main motivation for this contribution is the buckling of a drillstring sliding on the bottom of the horizontal section of borehole. The open questions that remain today are related to the determination of the onset of instability, and to the conditions under which different modes of constrained buckling occur. In this presentation, we are concerned by a two-dimensional version of this problem; namely, the sliding of a flexible strip being fed inside a conduit. The ribbon, which has a flexural rigidity $EI$ and a weight per unit length $w$, is treated as an inextensible elastica of negligible thickness. The contact between the ribbon and the wall of the conduit is characterized by a friction coefficient $\mu$. First, we report the result of a stability analysis that aims at determining the critical inserted length of the ribbon $\ell_{*}(\mu)$ (scaled by the characteristic length $\lambda=(EI/w)^{1/3}$) at which there is separation between the strip and the conduit bottom, as well as the buckling mode. Next, the relationship between the feeding force $F$ and the inserted length $\ell$ after bifurcation is computed. Finally, the results of a ``kitchen table'' experiment involving a strip of silicon rubber being pushed on a plank are reported and compared with predictions. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y28.00004: Better Contact Through Twist? The skew-dependence of inter-filament adhesion Luis Cajamarca, Gregory Grason Adhesive interactions between flexible filaments maximizes their contact, though the geometry of optimal contact is far from obvious. We address a simple question: how does inter-filament twist vary the adhesive energy? We investigate two models for adhesive interactions for filaments: a Lennard-Jones potential (LJP), and a model consisting of opposite interactions, screened electrostatic repulsion and depletion attraction (SED). In both potentials the interaction energy decreases for large twist. However, for small twist the SED potential is metastable whereas the LJP is not. We understand this effect by looking at how distances between patches of area on the surface change with twist. Patches further away come into closer contact as twist increases, effectively increasing the repulsion energy. This in turn pushes the filaments away and the net result is to favor a locally parallel orientation. Finally, we predict how the geometric minima of the interaction energy varies with inter-filament spacing for the LJP, where we observe two regions dominated by geometry: threads regime, where the filaments are very thin and interactions are long-range, and contact regime, where the filaments are very thick tubes and interactions become short-range compared to tube diameter. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y28.00005: Buckling of an elastic wire inside an elastic matrix Tianxiang Su, Jia Liu, Denis Terwagne, Katia Bertoldi, Pedro Reis Using both experiment and dynamic simulation results, we will discuss in this talk how a compressed elastic wire embedded within an elastic matrix buckles into two dimensional (2D) planar shape and then three dimensional (3D) helical shape. We will show that the transitions from the initial 1D to 2D and then 3D configurations can be tuned by and are highly sensitively to the supporting matrix stiffness. This property may be useful for future photonic and piezoelectric devices. Analytic buckling and post-buckling analysis will also be presented to rationalize our results. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y28.00006: Buckling of a thin rod under cylindrical constraint Jay Miller, Tianxiang Su, Nathan Wicks, Jahir Pabon, Katia Bertoldi, Pedro Reis We investigate the buckling and post-buckling behavior of a thin elastic rod, under cylindrical constraint, with distributed loading. Our precision model experiments consist of injecting a custom-fabricated rod into a transparent glass pipe. Under imposed velocity (leading to frictional axial loading), a portion of the initially straight rod first buckles into a sinusoidal mode and eventually undergoes a secondary instability into a helical configuration. The buckling and post-buckling behavior is found to be highly dependent on the system's geometry, namely the injected rod length and the aspect ratio of the rod to pipe diameter, as well as material parameters. We quantify the critical loads for this sequence of instabilities, contrast our results with numerical experiments and rationalize the observed behavior through scaling arguments. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y28.00007: Statistical properties of an elastic rod dynamically confined in 2D Frederic Lechenault, Mokhtar Adda-Bedia We investigate the statistical properties and stationary states of an elastic rod dynamically confined in a Hele-Shaw cell. As the confined length is increased, we observe a transition from an ordered spiral-like pattern to a disordered, rearranging pack of loops. ~In the disordered phase, we decipher the trajectories of the rod from its geometric configurations, and report correlation between curvilinear and spatial energy distributions. Moreover, we establish the relationship between the number of loops and the confined length, yielding insights into the loop occupation number and the overall rigidity of the system. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y28.00008: Loops, Wrinkles and Scrolls in Twisted Ribbons Julien Chopin, Arshad Kudrolli We explore experimentally the stable and metastable configurations of an elastic ribbon under mixed twist and tension. A ribbon is a slender and thin elastic material with an extremely narrow cross section which exhibits features of rods and plates: it can coil and form loops but wrinkles and stress localization can also been seen yielding a surprisingly rich variety of shapes. Using the twist angle and the tension as control parameters, the various configurations obtained can be rationalized in a phase diagram. Using x-ray tomography, we are able to reconstruct the 3D shape of the ribbon which can then be precisely characterized by measuring locally the mean and Gaussian curvature. Guided by our experimental data, we will present a simple model for the bifurcations observed. Finally, implications for the fabrication of structured rods and yarns with novel mechanical and transport properties will be discussed. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y28.00009: Explicit solutions for the buckling of an imperfect strut on a nonlinear foundation Romain Lagrange, Daniel Averbuch We perform a theoretical and numerical study of the buckling of an imperfect finite strut on a nonlinear elastic Winkler type foundation. The imperfection is introduced by considering an initially deformed shape which is a sine function with an half wavelength. The length of the strut is chosen such that the first buckling mode is excited and the restoring force is either a bi-linear or an exponential profile. Considering these two profiles, we show (exact piecewise solution theory, explicit Galerkin method, numerical resolution) that the system is subcritical, imperfection sensitive and the deflection is an amplification of the default. For small imperfection sizes, the equilibrium paths hit a limit point which is asymptotic to the Euler load for a critical imperfection amplitude. This critical amplitude is determined analytically and does not depend on the choice of the restoring force. The decrease of the maximum value of the axial force supported by the beam as a function of the imperfection magnitude is determined. We show that the leading term of the development has a different exponent than in subcritical buckling of elastic systems, and that the exponent values depend on the regularization. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y28.00010: On the tensorial nature of chirality Efi Efrati, William Irvine Chirality occupies a central role in fields ranging from biological self assembly to the design of optical meta-materials. The definition of chirality, as given by lord Kelvin in 1893, associates handedness with the lack of mirror symmetry. However, the quantification of chirality based on this definition has proven to be an elusive task. The difficulty in quantifying chirality is contrasted by the ease with which one determines the handedness of objects with a well defined axis such as screws and helices. In this talk I will present table-top demonstrations that show that a single object can simultaneously be left handed and right handed when considered from different directions. The orientation dependence of handedness motivates a tensorial quantification of chirality relating directions to rotations. I will give an explicit example of such a tensorial measure of chirality for embedded surfaces, and show how the tensorial nature of chirality can be probed in experiments and exploited as a design principle. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y28.00011: Mechanics and Dynamics of Snapping Beams Anupam Pandey, Derek Moulton, Dominic Vella, Douglas Holmes Snap-buckling is an elastic instability that causes a rapid transition between two states separated by a finite distance. These rapid instabilities occur naturally in plants like the Bunchberry dogwood and the Venus flytrap, yet the dynamics of this phenomenon remain poorly understood. In this talk we discuss the statics and dynamics of the point load snap through of an arch. During deformation, the arch transitions from a symmetric to an asymmetric mode at a critical load and then snap-buckles at a critical indentation height. We will demonstrate that this critical force and displacement for stability loss varies nonlinearly with the amount of initial compression applied to the flat beam, and the dynamics of the snapping arch have an instability growth-rate dictated by the speed of sound within the material. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y28.00012: Effect of aspect ratio on the stress response of frictional elastic rod assemblies Vikrant Yadav, Arshad Kudrolli We discuss the effect of aspect ratio on the response of a random assembly of frictional elastic rods under repeated top loading stress-strain cycles. Random assemblies of rods of different aspect ratios were created by rain deposition of particles. Considerable hysteresis is observed over the first few cycles, but the response starts to approach a more reversible path with each cycle. The assembly was scanned after each cycle using a 3D X-ray computer aided tomography instrument to determine position, orientation, and contacts of each constituent particle. We show that rods of small aspect ratio pack tend to have small compression under the same stress as compared to rods of higher aspect ratio because they pack more densely, and thus have larger Young's modulus. By tracking motion of constituent rods over subsequent cycles we observed that larger number of rearrangements take place in the bulk away from boundaries. The mean distance over which a particle moves to rearrange also decreases with each cycle. The mean numbers of contacts were also evaluated and were found to increase rapidly with small changes in volume fraction. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y28.00013: Thin film buckling : a relation between adhesion and morphology Etienne Barthel, Jean-Yvon Faou, Sergey Grachev, Guillaume Parry When thin films with low adhesion are compressively stressed, they may buckle. These buckles exhibit interesting morphologies such as the well known telephone cord. However our understanding of this form of buckling is limited because it couples the large displacement nonlinearities of plates with the subtleties of mixed-mode adhesion. Here we investigate the morphology of the thin film buckles as a function of mode mixity by a combination of experiments and simulations. We first exhibit a linear relation between the period of the telephone cord buckles and a characteristic parameter of the mixed mode adhesion. Furthermore we evidence a rich set of new buckle morphologies through experiments, and demonstrate that these morphologies can be reproduced in the simulations. We also show that we can rationalize the transitions between morphologies through a phase diagram. This excellent agreement between experimental results and numerical predictions further validates the simulation method we have developped recently. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y28.00014: Use of magnetic micro-cantilevers to study the dynamics of 3D engineered smooth muscle constructs Alan Liu, Ruogang Zhao, Craig Copeland, Christopher Chen, Daniel Reich The normal and pathological response of arterial tissue to mechanical stimulus sheds important light on such conditions as atherosclerosis and hypertension. While most previous methods of determining the biomechanical properties of arteries have relied on excised tissue, we have devised a system that enables the growth and in situ application of forces to arrays of stable suspended microtissues consisting of arterial smooth muscle cells (SMCs). Briefly, this magnetic microtissue tester system consists of arrays of pairs of elastomeric magnetically actuated micro-cantilevers between which SMC-infused 3D collagen gels self-assemble and remodel into aligned microtissue constructs. These devices allow us to simultaneously apply force and track stress-strain relationships of multiple microtissues per substrate. We have studied the dilatory capacity and subsequent response of the tissues and find that the resulting stress-strain curves show viscoelastic behavior as well as a linear dynamic recovery. These results provide a foundation for elucidating the mechanical behavior of this novel model system as well as further experiments that simulate pathological conditions. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y28.00015: Pattern formation by deposition of a thin elastic rod on a moving substrate Mohammad Khalid Jawed, Fang Da, Eitan Grinspun, Pedro Reis We report on the formation of coiling patterns when a thin elastic rod is deposited onto a moving solid boundary. We combine precision model experiments with cutting-edge computational mechanics tools ported from the computer graphics community. In our experiments, we deposit elastomeric rods onto a conveyor belt. Our numerical tool simulates the experimental scenario by implementing a discrete notion of bending and twist of the thin rod, based on discrete differential geometry, exhibiting excellent performance and robustness. The synergy between experiments and numerics, and the excellent agreement between the two, allows us to identify the key physical ingredients of the process, explore the phase diagram of the system, quantify the influence of the control parameters and rationalize the underlying mechanical instabilities. The gained predictive understanding of this geometrically-nonlinear pattern formation process has potential applications ranging from the micron-scale (coiling of carbon nanotubes) to the kilometer-scale (laying down of transoceanic undersea cables). [Preview Abstract] |
Session Y29: Complex Networks and Their Applications I
Sponsoring Units: GSNPChair: Greg Morrison, Harvard University
Room: 337
Friday, March 22, 2013 8:00AM - 8:12AM |
Y29.00001: Phase transition and Self-Organized Criticality in the Brain Marzieh Zare, Malgorzata Turalska, Paolo Grigolini Empirical evidence for a scale free distribution of avalanche sizes in the brain as a manifestation of self-organized criticality (SOC) suggests that the brain operates near criticality. Simulations in the literature also show the optimal function of the brain at criticality. However, due to the lack of sufficient set of conditions in the SOC hypothesis for the classification of a system, there is no clear connection between the phase transition and SOC. Here we study a set of cooperative neurons in a two-dimensional regular network. Using a leaky integrate-and-fire model, we analyze the temporal complexity and find a phase transition from Poisson to periodic process for a specific value of the cooperation parameter. We also evaluate the efficiency of information transfer between two networks and find the maximum at the same critical value. To study the connection between phase transition and SOC, we measure the avalanche size distribution at the critical point.~Our results show no evidence on scaling to the popular inverse power law of 1.5 in size, while we observe this scaling in the supercritical regime. Overall, based on these results, we propose that an epileptic brain can generate power law scaling while a healthy brain works in an intermediate regime. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y29.00002: An application of a measure for organization of complex networks Georgi Georgiev, Michael Daly In order to measure self-organization in complex networks a quantitative measure for organization is necessary. This will allow us to measure their degree of organization and rate of self-organization. We apply as a measure for quantity of organization the inverse of the average sum of physical actions of all elements in a system per unit motion multiplied by the Planck's constant, using the principle of least action. The meaning of quantity of organization here is the inverse of average number of quanta of action per one node crossing of an element of the system. We apply this measure to the central processing unit (CPU) of computers. The organization for several generations of CPUs shows a double exponential rate of change of organization with time. The exact functional dependence has, S-shaped structure, suggesting some of the mechanisms of self-organization. We also study the dependence of organization on the number of transistors. This method helps us explain the mechanism of increase of organization through quantity accumulation and constraint and curvature minimization with an attractor, the least average sum of actions of all elements and for all motions. This approach can help to describe, quantify, measure, manage, design and predict future behavior of complex systems to achieve the highest rates of self-organization to improve their quality. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y29.00003: Phase Transitions in the Quadratic Contact Process on Complex Networks Chris Varghese, Rick Durrett The quadratic contact process (QCP) is a natural extension of the well studied linear contact process where a single infected (1) individual can infect a susceptible (0) neighbor and infected individuals are allowed to recover ($1 \rightarrow 0$). In the QCP, a combination of two 1's is required to effect a $0 \rightarrow 1$ change. We extend the study of the QCP, which so far has been limited to lattices, to complex networks as a model for the change in a population via sexual reproduction and death. We define two versions of the QCP -- vertex centered (VQCP) and edge centered (EQCP) with birth events $1-0-1 \rightarrow 1-1-1$ and $1-1-0 \rightarrow 1-1-1$ respectively, where `$-$' represents an edge. We investigate the effects of network topology by considering the QCP on regular, Erd\H{o}s-R\'{e}nyi and power law random graphs. We perform mean field calculations as well as simulations to find the steady state fraction of occupied vertices as a function of the birth rate. We find that on the homogeneous graphs (regular and Erd\H{o}s-R\'{e}nyi ) there is a discontinuous phase transition with a region of bistability, whereas on the heavy tailed power law graph, the transition is continuous. The critical birth rate is found to be positive in the former but zero in the latter. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y29.00004: Two-dimensional classical XY model by HOTRG Ji-Feng Yu, Zhiyuan Xie, Tao Xiang Two-dimensional (2D) classical XY model has a special phase transition, the so-called Kosterlitz-Thouless (KT) transition. Below the transtion temperature, the system has quasi long range order with all spins aligned, and the correlation function decays as power law, while the other unordered phase is exponential. Large size system study by numerical simulation is necessary, but pratically difficult.In this work, we applied a newly well-developed method: high-order tensor renormalization group (HOTRG) to investigate this model. This method is verified by 2D Ising model, and thoretially, it can deal with infinite system size. Some thermodynamic quantities such as entropy, specific heat and magnetic susceptibility etc., are computed, which may be used to find Fisher's zero of the partition function, and then to characterize the transition. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y29.00005: Optimal Phase Oscillatory Network Rosangela Follmann Important topics as preventive detection of epidemics, collective self-organization, information flow and systemic robustness in clusters are typical examples of processes that can be studied in the context of the theory of complex networks. It is an emerging theory in a field, which has recently attracted much interest, involving the synchronization of dynamical systems associated to nodes, or vertices, of the network. Studies have shown that synchronization in oscillatory networks depends not only on the individual dynamics of each element, but also on the combination of the topology of the connections as well as on the properties of the interactions of these elements. Moreover, the response of the network to small damages, caused at strategic points, can enhance the global performance of the whole network. In this presentation we explore an optimal phase oscillatory network altered by an additional term in the coupling function. The application to associative-memory network shows improvement on the correct information retrieval as well as increase of the storage capacity. The inclusion of some small deviations on the nodes, when solutions are attracted to a false state, results in additional enhancement of the performance of the associative-memory network. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y29.00006: The Dynamics of Network Coupled Phase Oscillators: An Ensemble Approach Gilad Barlev, Thomas Antonsen, Edward Ott We consider the dynamics of phase oscillators that interact through a coupling network. We further consider an ensemble of such systems where, for each ensemble member, the set of oscillator frequencies is randomly chosen according to a given distribution function. We then seek a statistical description of the dynamics of this ensemble. This approach allows us to apply the ansatz of Ott and Antonsen to the marginal distribution of the ensemble of states at each node. This results in a reduced set of ordinary differential equations determining these marginal distribution functions. The new set facilitates the analysis of network dynamics in several ways: (i) the time evolution of the reduced system of equations is smoother, and thus numerical solutions can be obtained much faster; (ii) the new set of equations can be used to obtaining analytical result; and (iii) for a certain type of network, a reduction to a low dimensional description of the entire network dynamics is possible. We illustrate our approach with numerical experiments on a network version of the classic Kuramoto problem, with both unimodal and bimodal frequency distributions. In the bimodal case, the dynamics are characterized by bifurcations and hysteresis involving a variety of steady and periodic attractors. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y29.00007: Core percolation on complex networks Yang-Yu Liu, Endre Cs\'{o}ka, Haijun Zhou, M\'{a}rton P\'{o}sfai As a fundamental structural transition in complex networks, core percolation is related to a wide range of important problems, including combinatorial optimizations and network controllability. Yet, previous theoretical studies of core percolation have been focusing on the classical Erd\H{o}s-R\'enyi random networks with Poisson degree distribution, which are quite unlike many real-world networks with scale-free or fat-tailed degree distributions. Here we show that core percolation can be analytically studied for complex networks with arbitrary degree distributions. We derive the condition for core percolation and find that purely scale-free networks have no core for any degree exponents. We show that for undirected networks if core percolation occurs then it is always continuous while for directed networks it becomes discontinuous (and hybrid) if the in- and out-degree distributions differ. We also find that core percolations on undirected and directed networks have completely different critical exponents associated with their critical singularities. Finally, we apply our theory to real-world directed networks and find, surprisingly, that they often have much larger core sizes as compared to random models. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y29.00008: Devil's Staircases, Crackling Noise and Phase Transitions in Percolation Jan Nagler We identify and study certain phenomena in percolation that can subvert predictability and controllability in networked systems. We establish devil's staircase phase transitions, non-self-averaging, and power-law fluctuations in percolation. We provide exact conditions for percolation that exhibits multiple discontinuous jumps in the order parameter where the position and magnitude of the jumps are randomly distributed - characteristic of crackling noise. The framework can be linked to magnetic effects and fragmentation processes. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y29.00009: Explosive percolation transitions in Euclidean space Young Sul Cho, Sungmin Hwang, Hans J\"urgen Herrmann, Byungnam Kahng Since the explosive percolation transition was discovered in a random graph model in the Achlioptas process, whether the explosive percolation transition is indeed discontinuous or continuous has been controversial. Even though extensive studies have been focused on the mean-field behavior of the type of the explosive percolation transition, only a few studies are carried out in Euclidean space, Here, we show that depending on a parameter we introduce, the explosive percolation transition can be either discontinuous or continuous transition in Euclidean space, and is reduced to be continuous in the mean-field limit, which can be shown using an analytic approach. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y29.00010: Effective spectral dimension in heterogeneous networks Sungmin Hwang, Deok-Sun Lee, Byungnam Kahng Random walks(RWs) approach is the simplest but the most fundamental method which encapsulates essential properties of diffusive dynamic process. Here, we study the two basic quantities, the return to origin probability and the first passage time distribution of random walks on scale-free networks. The behaviors of those quantities as a function of time typically depend on the spectral dimension $d_s$ in disordered fractal systems. However, we show that in scale-free networks, due to the heterogeneity of connectivities of each node in scale-free networks, those quantities display a crossover decay behavior from $\sim t^{-d_s^{\rm (hub)}/2}$ in early time regime to $\sim t^{-d_s/2}$ in later time regime, where $d_s^{\rm (hub)} \to 0$ as the degree exponent $\lambda$ approaches 2. This result implies that a random walker can be trapped effectively at the hub when $\lambda \to 2$. Next, we discuss the origin of the $d_s^{\rm (hub)}$ by applying the renormalization group transformation to deterministic hierarchical networks. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y29.00011: Natural emergence of clusters and bursts in network evolution James Bagrow, Dirk Brockmann Network models with \emph{preferential attachment}, where new nodes are injected into the network and form links with existing nodes proportional to their current connectivity, have been well studied for some time. Extensions have been introduced where nodes attach proportional to arbitrary fitness functions. However, in these models attaching to a node increases the ability of that node to gain more links in the future. We study network growth where nodes attach proportional to the clustering coefficients, or local densities of triangles, of existing nodes. Attaching to a node typically lowers its clustering coefficient, in contrast to preferential attachment or rich-get-richer models. This simple modification naturally leads to a variety of rich phenomena, including non-poissonian bursty dynamics, community formation, aging and renewal. This shows that complex network structure can be modeled without artificially imposing multiple dynamical mechanisms. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y29.00012: Walking and searching in time-varying networks Nicola Perra, Andrea Baronchelli, Delia Mocanu, Bruno Goncalves, Romualdo Pastor-Satorras, Alessandro Vespignani The random walk process lies underneath the description of a large number or real world phenomena. Here we provide a general framework for the study of random walk processes in time varying networks in the regime of time-scale mixing; i.e. when the network connectivity pattern and the random walk process dynamics are unfolding on the same time scale. We consider a model for time varying networks created from the activity potential of the nodes, and derive solutions of the asymptotic behavior of random walks and the mean first passage time in undirected and directed networks. Our findings show striking differences with respect to the well known results obtained in quenched and annealed networks, emphasizing the effects of dynamical connectivity patterns in the definition of proper strategies for search, retrieval and diffusion processes in time-varying networks. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y29.00013: Measuring importance in complex networks Greg Morrison, Levi Dudte, L. Mahadevan A variety of centrality measures can be defined on a network to determine the global `importance' of a node $i$. However, the inhomogeneity of complex networks implies that not all nodes $j$ will consider $i$ equally important. In this talk, we use a linearized form of the Generalized Erdos numbers [Morrison and Mahadevan EPL 93 40002 (2011)] to define a pairwise measure of the importance of a node $i$ from the perspective of node $j$ which incorporates the global network topology. This localized importance can be used to define a global measure of centrality that is consistent with other well-known centrality measures. We illustrate the use of the localized importance in both artificial and real-world networks with a complex global topology. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y29.00014: Sequential detection of temporal communities in evolving networks by estrangement confinement Sameet Sreenivasan, Vikas Kawadia Temporal communities are the result of a consistent partitioning of nodes across multiple snapshots of an evolving network, and they provide insights into how dense clusters in a network emerge, combine, split and decay over time. Reliable detection of temporal communities requires finding a good community partition in a given snapshot while simultaneously ensuring that it bears some similarity to the partition(s) found in the previous snapshot(s). This is a particularly difficult task given the extreme sensitivity of community structure yielded by current methods to changes in the network structure. Motivated by the inertia of inter-node relationships, we present a new measure of partition distance called estrangement, and show that constraining estrangement enables the detection of meaningful temporal communities at various degrees of temporal smoothness in diverse real-world datasets. Estrangement confinement consequently provides a principled approach to uncovering temporal communities in evolving networks. (V. Kawadia and S. Sreenivasan, http://arxiv.org/abs/1203.5126) [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y29.00015: Bimodality in Network Control Tao Jia, Yang-yu Liu, Marton Posfai, Jean-Jacques Slotine, Albert-Laszlo Barabasi Controlling complex systems is a fundamental challenge of network science. Recent tools enable us to identify the minimum driver nodes, from which we can control a system. They also indicate a multiplicity of minimum driver node sets (MDS's): multiple combinations of the same number of nodes can achieve control over the system. This multiplicity allows us to classify individual nodes as critical if they are involved in all control configurations, intermittent if they occasionally act as driver nodes and redundant if they do not play any role in control. We develop computational and analytical framework analyzing nodes in each category in both model and real networks. We find that networks with identical degree distribution can be in two distinct control modes, ``centralized" or ``distributed", with drastic change on the role of each node in maintaining the controllability and orders of magnitude difference in the number of MDS's. In analyzing both model and real networks, we find that the two modes can be inferred directly from the network's degree distribution. Finally we show that the two control modes can be switched by small structural perturbations, leading to potential applications of control theory in real systems. [Preview Abstract] |
Session Y30: Jamming & Shearing
Sponsoring Units: GSNPChair: Ted Brzinski, University of Pennsylvania
Room: 338
Friday, March 22, 2013 8:00AM - 8:12AM |
Y30.00001: Plastic Deformation of Semicrystalline Polyethylene under Extension, Compression, and Shear using Molecular Dynamics Simulation Jun Mo Kim, Rebecca Locker, Gregory Rutledge Molecular dynamics simulation has been performed to investigate the plastic deformation of semicrystalline polyethylene under various modes of deformation, such as extension, compression and shear. Many mechanical and structural properties of semicrystalline polyethylene are examined and compared with previous study [Lee and Rutledge, Macrmol. 44, 3096 (2011)]. Under tensile deformation, we observed crystallographic slip at low strains (e$_{3}$ \textless\ 0.08) regardless of deformation rate. However, two different yield mechanisms were monitored as a function of deformation rate at intermediate strains (e$_{3}$ \textless\ 0.25). At high strains (e$_{3}$ \textgreater\ 0.25), melting and recrystallization were observed for slow deformation (5$\times$10$^{6}$s$^{-1})$ whereas cavitations were monitored for fast deformation (5$\times$10$^{7}$s$^{-1})$. Under compressive deformation, stress-strain curve shows very similar behavior to tensile deformation at low strain, and crystallographic slip plays an important role for mechanical response of semicrystalline polyethylene. Under shear deformation, the chains tend to stretch and align into the shear direction. We also calculated stiffness constants for shear deformation and compared these to results of previous study [In't Veld et al. Macromol. 39, 439 (2006)]. Interestingly, semicyrstalline polyethylene shows typical transient behavior of Newtonian fluids under shear deformation, which we compare to various constitutive models, such as the Upper-Convected Maxwell (UCM) and Giesekus models. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y30.00002: Size Segregation in Sheared Jammed Colloids Armstrong Mbi, Daniel Blair It is well known that granular materials can spontaneously size segregate when continuously driven. However, in jammed colloidal suspensions, this phenomenon is not well understood. Colloidal dispersions provide a unique system to study the structure and dynamics of jammed matter. In this talk, we present results of size segregation of a continuously sheared binary colloidal suspension well above point J. Our colloidal system is comprised of indexed-matched bi-disperse silica particles with diameters $a = \{2.3\mu m$ and $3.2\mu m\}$ and at $\phi\approx 61\%$, well above the colloidal glass transition. We apply a highly controlled shear at a constant shear rate through the use of a rheometer. By coupling our rheometer with a high-speed laser scanning confocal microscope, we directly image the structure and flow profiles of the suspension as it un-jams. We observe migration of the small and large species; large particles move to the top while the small particles move toward the bottom conserving the total volume fraction in all regions. Moreover, we find that an associating feature of segregation is a sustained shear band. Our results are consistent with a recently proposed void filling and squeeze expulsion mechanism. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y30.00003: Reversible and irreversible deformation in hard-sphere colloidal glasses Katharine Jensen, Nobutomo Nakamura, David Weitz, Frans Spaepen Colloidal glass provides a unique experimental system with which to study the structure, defects, and dynamics of amorphous materials. We report experiments on 1.55-$\mu$m-diameter, hard-sphere silica colloidal glasses under conditions of uniform shear. We deform the samples to maximum strains ranging from 0.5\% to 10\% at various strain rates, and then reverse the deformation so that the net bulk strain is zero at the end of the experiment. We use confocal microscopy to follow the 3D, real-time trajectories of roughly 50,000 particles over the course of an experiment. In this way, we probe the elastic, anelastic, and plastic response of the system, with particular emphasis on the specific, local mechanisms of deformation. We directly observe yield as the onset of local, irreversible deformation. In both sheared and unsheared (quiescent) samples, we observe thermally-activated clusters of particles that behave as Eshelby inclusions, undergoing highly localized plastic strain that couples elastically to the surrounding material. We identify and characterize these regions as they develop in the glass, with particular focus on density-related properties including the Voronoi volume and free volume. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y30.00004: Shearbanding in Amorphous Solids and Interacting Eshelby Singularities H.G.E. Hentschel, Ratul Dasgupta, Itamar Procaccia We will describe recent work in which it was found that the fundamental shear-localizing instability of amorphous solids under external strain, which eventually results in a shear band and failure, consists of a highly correlated line of Eshelby quadrupoles all having the same orientation and some density $\rho$. We describe how the energy $E(\rho,\gamma)$ associated with such highly correlated structures as a function of the density $\rho$ and the external strain $\gamma$ can be calculated. We then show that when the strain $\gamma$ is smaller than some characteristic yield stress $\gamma_y$ the minimum energy solution is attained for $\rho=0$ (i.e. isolated localized plastic events). While for $\gamma \ge \gamma_y$ there is a bifurcation allowing a finite density of quadrupoles. We finally suggest how the universal Johnson-Samwer $T^{2/3}$ temperature reduction of the yield stress in metallic glasses can be accounted for by such ideas. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y30.00005: Compression of granular pillars with constant width at top and bottom Yuka Takehara, Jennifer Rieser, Jerry Gollub, Douglas Durian Granular media display both elastic and plastic behavior, including the formation of shear bands under extreme loading. In this study, we performed two-dimensional granular pillar compression experiments and tracked of grain- and macro- scale flows via video imaging and force measurement. Especially we focus on the condition that the top and bottom widths of the granular pillars are constrained to avoid free expansion along the contact edge. This causes more energy to be stored elastically deep inside of the pillars, which gives rise to a different kind of shear banding than for free top/bottom widths. Furthermore we tried several series of experiments with different elastic/frictional particles and also ordered/disordered systems. We demonstrate how the micro properties and packing structure contribute to the formation of shear band to discuss the mechanical failure in disordered packing. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y30.00006: The Chaotic Dynamics of Jamming David A. Egolf, Edward J. Banigan, Matthew K. Illich, Derick J. Stace-Naughton Despite the appearance of simplicity, much of the behavior of granular materials remains mysterious. One intriguing puzzle is the dynamical mechanism underlying the ``jamming'' transition, in which disordered grains become rigid at high density. By applying nonlinear dynamical techniques to simulated 2D shear cells, we reveal the mechanisms of jamming and find they conflict with the prevailing picture of growing cooperative regions. Additionally, at the density corresponding to random close packing, we find a dynamical transition from chaotic to non-chaotic states accompanied by diverging dynamical length and time scales. Furthermore, we find that the dominant cooperative dynamical modes are strongly correlated with particle rearrangements and become increasingly unstable before stress jumps, providing a way to predict the times and locations of these earthquake-like stress-release events. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y30.00007: Shear Transformation Zone theory parameters from molecular dynamics and experiment Adam R. Hinkle, Pengfei Guan, Michael L. Falk Shear Transformation Zone (STZ) theory provides a continuum framework to describe the deformation of amorphous systems. However, as a phenomenological theory it relies upon parameters which must be determined for a specific material system. We present current progress towards a set of theoretical and computational methodologies for determining the parameters of STZ theory. We investigate two distinct systems, a copper-zirconium lamellar nanocomposite, and a simple yield stress fluid (YSF), where both systems are loaded in simple shear. We show that the molecular dynamics simulations of the nanocomposite system and experimental measurements of the YSF can be used to provide the initial conditions of the dynamical fields as well as the essential STZ parameters. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y30.00008: Heterogeneous relaxation dynamics in amorphous materials under cyclic loading Nikolai Priezjev Molecular dynamics simulations are performed to investigate heterogeneous dynamics in amorphous glassy materials under oscillatory shear strain. We consider three-dimensional binary Lennard-Jones mixture well below the glass transition temperature. The structural relaxation and dynamic heterogeneity are quantified by means of the self-overlap order parameter and the four-point correlation function. We found that at small strain amplitudes, the mean square displacement develops an extended sub-diffusive plateau followed by the diffusive regime; whereas at larger amplitudes only the diffusive regime is present. At intermediate time and length scales, the dynamic susceptibility exhibits a pronounced peak, whose magnitude increases at larger strain amplitudes, indicating progressively larger size of dynamically correlated regions. The analysis of particle hopping dynamics reveals that the periodic deformation generates a heterogeneous temporal response characterized by intermittent bursts of large particle displacements. The role of dynamical facilitation in the formation of clusters of mobile particles is discussed. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y30.00009: Shear deformation of naocrystal-metallic glass composites: A computational analysis Pengfei Guan, Michael L. Falk Due to the shear strain localization, the limited ductility becomes the major drawback for the application of metallic glass materials, and the introducing of crystalline phase has been regarded as the effective method for improving the ductility of these materials. Here, we systematically investigate the nanocrystal-metallic glass composites by using Molecular Dynamic (MD) simulations. The three--dimension (3D) atomic configurations with different crystalline grain sizes and factions are constructed based on the ZrCu EAM potential. The phase diagram based on the crystalline grain size-fraction is established between single nanocrystal phase and amorphous phase. The mechanical responses of these materials are investigated by applying the shear deformation, and the relationships between the mechanical properties and atomic structure information (crystalline fraction, grain size ?) are established. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y30.00010: Criticality of non-colloidal suspensions under periodic shear Emmanouela Filippidi, David Pine Suspensions of non-colloidal particles under slow periodic strain undergo a dynamical phase transition: they can either relax to an absorbing configuration in which particles are not displaced after every cycle or can reach a stationary fluctuating state. We correlate microscopic particle motion with macroscopic rheology and explain the existence of the critical transition experimentally by comparing particles of different surface roughness and by varying the volume fraction towards jamming. Particle roughness is implicated in the transition to reversibility, as smoother particles push the critical strain to higher values. Theoretically, we attempt to construct quasi-particles that encompass the strain-induced particle interactions and discover that geometry is not sufficient to understand suspension irreversibility under strain. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y30.00011: Temperature-equivalent of strain rate for the yield stress of amorphous solids Penghui Cao, Xi Lin, Harold S. Park We couple the recently developed self-learning metabasin escape (SLME) algorithm with continuous shear deformations to probe the yield stress as a function of temperature for a binary Lennard-Jones amorphous solid. At room temperature and laboratory strain rates, the activation volume associated with yield is less than 10 atoms, while the yield stress is found to be as sensitive to a 1.5{\%}Tg increase in temperature as it is to a one order of magnitude decrease in strain rate. Our SLME results suggest a transition in yield mechanism for temperatures lower than about 0.54Tg that is not captured by extrapolating high strain rate molecular dynamics simulations to laboratory strain rates. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y30.00012: Identifying Defects in Disordered and Ordered Solids Sven Wijtmans, Lisa Manning Characterizing defects in solids is an important step to developing continuum equations for failure in materials. Defects in crystalline solids (i.e. dislocations) are easy to characterize, but in disordered solids the lack of crystalline order makes it difficult to identify where particle rearrangements are likely to occur. Here we describe simulations of quasi-statically sheared athermal jammed packings of bidisperse discs in 2D. We perform energy minimization at each step using a combination of conjugate gradient and line search algorithms. By analyzing localized excitations in low-frequency vibrational modes, one can identify flow defects in disordered solids. We have developed tools to carefully match these flow defects to corresponding plastic events, and we analyze how the properties of defects change across packings ranging from disordered to completely ordered. This will allow us to understand the fundamental connections between dislocations and flow defect dynamics in solids. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y30.00013: Atomic-scale flow defect population in Cu-Zr metallic glass Sylvain Patinet, Pengfei Guan, Adam Hinkle, Michael Falk We adapt the method developed by Manninget al.[PRL 107, 108302 (2011)] to characterize the flow defects population of a Cu-Zr metallic glass modeled using embedded atom method potentials. We investigate how the statistics of Shear Transformation Zones (STZs) change as a function of system size and quench rate during glass formation. We also consider the evolution of the STZ population during mechanical loading. On the basis of this analysis, we relate our results with predictions of the STZ theory of amorphous plasticity to consider the history dependence implicit in the strain-stress response of the metallic glass. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y30.00014: Viscous rheology of soft particles near jamming Erik Woldhuis, Brian Tighe, Martin van Hecke We investigate the effect of changing the exact nature of the viscous interaction in simulations of sheared soft, viscous, repulsive disks, which are considered to be a good model for foams and emulsions. We determine the way in which the power-law exponent of the rheological curve, in other words the shear-thinning or shear-thickening part, depends on the microscopic viscous interaction around the jamming density. We attempt to find a model that describes and predicts this dependence. [Preview Abstract] |
Session Y31: Phase Behavior of Copolymers
Sponsoring Units: DPOLYChair: Chris Ellison, University of Texas at Austin
Room: 339
Friday, March 22, 2013 8:00AM - 8:12AM |
Y31.00001: Phase Behavior of All-Hydrocarbon ``Diblock-Random'' Copolymers Bryan Beckingham, Richard Register ``Block-random'' copolymers (A$_{x}$B$_{1-x})$-(A$_{y}$B$_{1-y})$, where each of the two blocks is a random copolymer of monomers A and B, present a convenient and useful variation on the typical block copolymer architecture, as the interblock interactions and physical properties can be tuned continuously through the random block's composition. The ability to tune the effective interaction parameter between the blocks continuously, allows for the order-disorder transition temperature (T$_{ODT})$ to be tuned independently of molecular weight using only two monomers. This flexibility makes block-random copolymers a versatile platform for the exploration of polymer phase behavior and structure-property relationships. Here, we present the phase behavior of hydrogenated derivatives of various lamellae-forming diblock-random copolymers where one block is a styrene/isoprene (S$r$I) random copolymer. Using small-angle x-ray scattering, we investigate a series of isoprene hydrogenated hI-S$r$hI with varying styrene content, determine order-disorder transition temperatures and compare the observed phase behavior to that of more typical S-hI block copolymers via mean-field theory. Additionally, diblock-random copolymers, 50 wt. {\%} styrene in the S$r$I block, are synthesized with polyisoprene, polybutadiene or polystyrene blocks and we examine the phase behavior of both their hydrogenated derivatives, prepared with catalysts which either leave the S units intact or saturate them to vinylcyclohexane. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y31.00002: Self-consistent field theory for directed self-assembly in non-cylindrical confinement Tatsuhiro Iwama, Nabil Laachi, Bongkeun Kim, Kris Delaney, Glenn Fredrickson We use self-consistent field theory to study the directed self-assembly (DSA) of diblock copolymers under non-cylindrical pore confinement such as oval, rectangular or the like. Our goal is to understand whether block copolymers can rectify non-cylindrical holes with reduced critical dimension in both minor direction and major direction of the non-cylindrical prepatterns. We explore a wide range of prepattern shapes, polymer characteristics to optimize DSA non-cylindrical holes. We also discover defects of DSA morphologies in the non-cylindrical prepattterns. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y31.00003: Phase behavior of binary blends of asymmetric diblock copolymers: Bulk and thin films Adetunji Onikoyi, Edward Kramer Experimental and theoretical investigations of the phase behavior of binary blends of block copolymers in bulk state have been published, yet there is little work on the effects of confinement of such blends to thin films. We investigate the phase behavior of blends of two poly(styrene-b-2vinylpyridine) diblock copolymers; one sphere forming (BCP1, with f$_{\mathrm{P2VP}} =$ 0.12, N$=$538) and the other cylinder forming (BCP2, with f$_{\mathrm{P2VP}}=$0.25, N$=$355). SAXS, TEM and SFM are used to characterize the microstructures formed as a function of temperature, blend compositions and film thickness. Results show that increased surface-induced free energy penalties in thin films lead to a significant change in phase behavior when compared to similar samples in bulk. Order disorder temperatures (ODT), mixing regimes and overall microstructure are strongly affected by the dominant contributions of the surface to the overall free energy of the system. Furthermore, for a regime (with BCP1 blend composition $\sim$ 0.4-0.6) of expected mixed phases of 2D hexagonal spheres and parallel cylinders in thin films, we are able to selectively stabilize 2D hexagonal sphere phases rather than the cylindrical phase by appropriate choice of confinement size and geometry. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y31.00004: Identifying the ODT in simulations of diblock copolymers using thermodynamic integration with a flexible simulation cell Pavani Medapuram, Jens Glaser, David Morse The order-disorder transition (ODT) has been precisely identified in several simulation models by using a thermodynamic integration procedure introduced by Mueller and Daoulas (\textit{J. Chem. Phys.}, 128, 024903, 2008). We have applied the method to constant pressure simulations with a flexible tetragonal simulation unit cell to avoid incommensurability effects. The transition is found to be surprisingly weakly first order, even for very short chains, in agreement with recent experiment results on short, strongly-incompatible diblocks. Precise values for the value of $\chi$ N at the transition are obtained by combining this free energy method with a fit of the disordered state scattering data to the renormalized one-loop theory, which is found to give an excellent fit for several different models over a wide range of molecular weights. Results from different chain lengths and models are compared to test the degree of universality of the ODT, and to test the accuracy of the Fredrickson-Helfand theory predictions for the ODT. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y31.00005: Identifying the ODT in simulations of diblock copolymers using metadynamics Jens Glaser, David Morse We propose a novel approach based on the structure factor as an order parameter and metadynamics as a free-energy technique to precisely locate the order-disorder transition in melts of symmetric diblock copolymers, which is flucutation-induced first-order. We are able to directly measure the height of the free energy barrier separating the disordered and the ordered phase. We quantify finite size effects on the free energy minima and barrier. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y31.00006: Rod-Coil Block Copolymer Simulation With SCFT Lee Trask, Eric Cochran Theoretical and experimental investigations of rod-coil block copolymer systems have made leaps forward recently. Fully 3D computer simulations of rod-coil diblock copolymer systems using self-consistent field theory (SCFT) have become feasible due to advances in theory and computer resources, while a number of experimental papers have illustrated a wide array of phases. These simulations include the use of all spatial and orientational degrees of freedom along with a Maier-Saupe interaction to describe the rod-rod alignment interactions. However, these 3D simulations have not been compared to experimental data. Simulations of moderately segregated poly(alkoxyphenylenevinylene-\textsl{b}-isoprene) (PPV-\textsl{b}-PI) are performed for a range of characteristic parameters linked to these systems. For different Flory-Huggins parameters, compositions, Maier-Saupe parameters, and geometric asymmetries, phase diagrams are constructed and compared with the phase diagrams previously reported in the literature. Along with phase space information, the domain space scaling relationship of the lamellar phase as a function of molecular weight is compared. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y31.00007: Self-consistent Field Theory Simulations of the Phase Behavior of Tapered Diblock Copolymers Jonathan Brown, Lisa Hall Phase diagrams of tapered and inverse-tapered diblock copolymers were computed by self-consistent field theory. These copolymers consist of three ``blocks'': a pure A block, a linear gradient ``block'' that is either A to B (tapered) or B to A (inverse-tapered), and a pure B block. This composition was approximated using a multi-block model in which the tapered region consisted of alternating A and B blocks of appropriate size to approximate the gradient. Phase diagrams were produced for varying sizes of the tapered region, showing a shift of the ordered phases to higher $\chi N$ for larger tapered regions (and higher still for inverse-tapered systems), while preserving non-lamellar phases in some cases. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y31.00008: Theory of Chiral Block Copolymer Melts: Mesoscopic Helicity from Inter-Segment Twist Gregory Grason, Wei Zhao, Thomas Russell We study the effects of chirality at the segment scale on the thermodynamics of block copolymer melts using self consistent field theory. In linear diblock melts where segments of one block prefer a twisted, or cholesteric, texture, we show that melt assembly is critically sensitive to the ratio of random coil size to the preferred pitch of cholesteric twist. For weakly-chiral melts (large pitch), mesophases remain achiral, while below a critical value of pitch, two mesocopically chiral phases are stable: an undulated lamellar phase; and a phase of hexagonally-ordered helices. We show that the non-linear sensitivity of meso-scale chiral order to preferred pitch derives specifically from the geometric and thermodynamic coupling of the helical mesodomain shape to the twisted packing of chiral segments within the core, giving rise to a second-order cylinder-to-helix transition. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y31.00009: Self-assembly of peptoid block copolymers with tunable conformational asymmetry Adrianne Rosales, Ronald Zuckermann, Rachel Segalman Functional polymers such as conjugated or biological molecules have been shown to have a variety of chain conformations that affect their self-assembly. Polypeptoids are sequence-specific biomimetic polymers for which the statistical segment length can be tuned by the introduction of monomers with bulky, chiral side chains, allowing one to change the polymer conformation independent of chemical structure or molecular weight. Furthermore, sequence specificity enables the precise placement of those chiral monomers along the polymer chain. This work presents a systematic study of block copolymer self-assembly using chiral polypeptoids or their racemic analogs and poly(n-butyl acrylate). For the chiral block copolymers, SAXS measurements reveal that the change in conformational asymmetry increases the morphological domain spacing and decreases the corresponding interfacial area per chain, indicating that the chiral peptoid chains can pack more closely within the domain compared to the racemic peptoid chains. The effect on domain spacing is also probed by changing the position of the chiral monomers with respect to the block copolymer junction. These results lend insight to the design of block copolymers with secondary structure. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y31.00010: Phase coexistence calculations via a unit-cell Gibbs ensemble formalism for melts of reversibly bonded block copolymers Zoltan Mester, Nathaniel Lynd, Glenn Fredrickson Melts of block copolymer blends can exhibit coexistence between compositionally and morphologically distinct phases. We derived a unit-cell approach for a field theoretic Gibbs ensemble formalism to rapidly map out such coexistence regions. We also developed a canonical ensemble model for the reversible reaction of supramolecular polymers and integrated it into the Gibbs ensemble scheme. This creates a faster method for generating phase diagrams in complex supramolecular systems than the usual grand canonical ensemble method and allows us to specify the system in experimentally accessible volume fractions rather than chemical potentials. The integrated approach is used to calculate phase diagrams for AB diblock copolymers reversibly reacting with B homopolymers to form a new diblocks we term ``ABB.'' For our case, we use a diblock that is sixty percent A monomer and a homopolymer that is the same length as the diblock. In the limits of infinite reaction favorability (large equilibrium constant), the system approaches cases of an ABB diblock-B homopolymer blend when the AB diblock is the limiting reactant and AB diblock-ABB diblock blend when the homopolymer is the limiting reactant. As reaction favorability is decreased, the phase boundaries shift towards higher homopolymer compositions so that sufficient reaction can take place to produce the ABB diblock that has a deciding role stabilizing the observed phases. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y31.00011: Phase behavior of multi-arm star-shaped polystyrene-\textit{block}-poly(methyl methacrylate) copolymer Sangshin Jang, Hong Chul Moon, Dusik Bae, Jonghen Kwak, Jin Kon Kim We synthesized star-shaped polystyrene-\textit{block}-poly(methyl methacrylate) copolymer (PS-$b$-PMMA) by utilizing $\alpha $-cyclodextrin ($\alpha $-CD) as a core of the star-shaped block copolymer. Eighteen hydroxyl groups on $\alpha $-CD were transformed to bromine by the reaction with $\alpha $-bromoisobutyryl bromide. We found that the number of bromine substituted arms per one $\alpha $-CD was higher than 16, which was determined by nuclear magnetic resonance and Matrix-assisted laser desorption/ionization. We could control molecular weight of block copolymers by changing polymerization times. The block copolymers were characterized by gel permeation chromatography and nuclear magnetic resonance. Phase behaviors of these star-shaped block copolymers were investigated by small angle X-ray scattering and transmission electron microscopy. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y31.00012: Pressure Effect of Various Inert Gases on the Phase Behavior of Polystyrene-\textit{block}-Poly(n-pentyl methacrylate) Copolymer Hong Chul Moon, Hye Jeong Kim, Junhan Cho, Jin Kon Kim We investigated the pressure effect of three inert gases (nitrogen, helium and argon) on the phase behavior of polystyrene-\textit{block}-poly(n-pentylmethacrylate) copolymer (PS-$b$-PnPMA) showing closed-loop phase behavior and baroplasticity. Helium gas pressure enhanced the miscibility between PS and PnPMA blocks similar to the hydrostatic pressure. Very interestingly, however, with increasing nitrogen and argon gas pressure, the miscibility between the two blocks decreased even though these two are also considered as inert gases. To explain these unexpected results, we measured the amount of gas absorption into each block. The experimentally measured gas absorption results are consistent with the theoretical ones based on the Sanchez-Lacombe theory. The results in this study imply that well-known and widely employed inert gases such as nitrogen and argon could significantly affect the phase behavior of a weakly interacting block copolymer at high pressures. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y31.00013: Micellization behavior of A-$b$-(B-\textit{alt}-C)$_{\mathrm{n}}$ multiblock terpolymers in a selective solvent for one terminal A-block Yu-Chieh Hsu, Ching-I Huang, Weihua Li, Feng Qiu, An-Chang Shi We used self-consistent field theory to investigate the micellization behavior of A-$b$-(B-\textit{alt}-C)$_{\mathrm{n}}$ multiblock terpolymers in the presence of a solvent that is selective to the terminal A-block. In particular, we focused on the effects of $\chi_{\mathrm{BC}}$, and $f_{\mathrm{A}}$, on the formation of micelles from ABC triblock and A(BC)$_3$ multiblock terpolymers, respectively. We observed a general trend that a segmented packing of B- and C-layers along the axial direction of the micelles is favored than the coaxial packing with the increasing of $\chi _{\mathrm{BC}}$ or decreasing of $f_{\mathrm{A}}$. The separation of B and C blocks within a micelle leads to the formation of a variety of multicompartment micelle morphologies, such as core-shell-corona spherical micelles, hamburgers, and bump-surface micelles, in the ABC triblock copolymers. In the A(BC)$_3$ multiblock terpolymers, we discovered more fascinating micelles by implementing the SCFT simulation than by the DPD simulation. Besides the BC-segmented worm-like micelles, which have been found in the DPD simulation work, concentric multilayer spheres and vesicles can be formed by the solvent-induced effect when the solvophilic A-block is a majority component. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y31.00014: Micellar Packing in Aqueous Solutions of As-Received and Pure Pluronic Block Copolymers Chang Ryu, Han Jin Park Pluronic block copolymers (Pluronics) are produced on a commercial scale to enable wide range of novel applications from emulsification and colloidal stabilization as nonionic surfactants. While the Pluronic block copolymers offer the advantages of being readily available for such applications, it contains non-micellizable low molecular weight (MW) impurities that would interfere with the self-assembly and micellar packing of PEO-PPO-PEO triblock copolymers in aqueous solutions. The impacts of the low MW impurities will be discussed on the micellar packing of Pluronics F108 and F127 solutions, which form BCC and FCC. While as-received Pluronic samples typically contain about 20 wt.{\%} low MW impurities, we were able to reduce the impurity level to less than 2 wt.{\%} using our large scale purification technique. Comparative studies on small angle x-ray scattering (SAXS) experiments on as-received and purified Pluronics solutions revealed that the contents of triblock copolymers in solutions essentially governs the inter-micellar distance of Pluronic cubic structures. A universal relationship between triblock copolymer concentration and SAXS-based domain spacing has been finally discussed. [Preview Abstract] |
Session Y32: Polymer Nanocomposites III
Sponsoring Units: DPOLYChair: Jeff Meth, DuPont Chemicals
Room: 340
Friday, March 22, 2013 8:00AM - 8:12AM |
Y32.00001: Layered polymer nanocomposite films of type-specific single wall carbon nanotubes Matthew R. Semler, John M. Harris, Jeffrey A. Fagan, Erik K. Hobbie Thin networks of single-wall carbon nanotubes (SWCNTs) on elastic polymer substrates show significant promise for applications in flexible electronics, but the modulus and conductivity of such films can degrade significantly under an applied strain. This softening occurs because strong van der Waals interactions between adjoining nanotubes promote coarsening into a preferred parallel alignment under even modest compression. We demonstrate that by capping the nanotube layer with a thin glassy polymer film, the mechanical properties of networks can be substantially improved, which we attribute to the stabilizing influence of excluded-volume interactions. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y32.00002: Influence of Thermal History on Microphase Separation and Morphology of Elastomeric Polyureas James Runt, Alicia Castagna, Autchara Pangon Polyureas are versatile elastomers consisting of alternating soft and hard segments. These polymers tend to form a nanophase-segregated morphology consisting of high aspect ratio hard domain ribbons in a low Tg matrix, the details of which are key in tailoring the unique characteristics of this family of materials. In the present work, bulk-polymerized polyureas were synthesized from a modified diphenylmethane diisocyanate and a polytetramethyleneoxide based diamine (1000 g/mol) and annealed at selected elevated temperatures. Various experimental probes (e.g. atomic force microscopy and small-angle X-ray scattering) reveal significant changes in hard domain ordering as a function of thermal history. Time-resolved synchrotron X-ray scattering was also conducted as a function of temperature to augment these findings. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y32.00003: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y32.00004: Optical characterization of isotactic polypropylene and carbon nanotube composites using spectroscopic ellipsometry Sabyasachi Sarkar, Parvathalu Kalakonda, Georgi Georgiev, Germano Iannacchione We report the dielectric properties of optically characterized isotactic polypropylene (iPP) and its composites with carbon nanotubes (CNTs) using spectroscopic ellipsometry. Characterization was performed at angles ranging from 50 to 70 degrees and for the spectral range between 300-1000 nm. CNT concentrations varied from 0 to 5 wt\% in the iPP/CNT composites investigated. Ellipsometry is a non-invasive and non-destructive technique that enabled us to determine the dielectric properties of the materials investigated. A concentration dependency on CNT wt\% was found to exist for both the refractive index and the extinction coefficient for the iPP/CNT composites. At higher concentrations however, this distinction was not very clear, suggesting that saturation levels were reached in the material. We will also discuss our efforts to separate the optical properties of bound CNT from the analyzed nanocomposites. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y32.00005: Temperature dependent photoluminescence from polymer nanocomposites of size-puified silicon quantum dots Austin R. Vansickle, Joseph B. Miller, Rebecca J. Anthony, Uwe R. Kortshagen, Erik K. Hobbie The photoluminescence (PL) of polydimethylsiloxane (PDMS) nanocomposites of size-purified silicon nanocrystals is measured as a function of temperature and nanoparticle size. The overall behavior is in agreement with the trends imposed by quantum confinement, where the temperature dependence of the nanocrystal bandgap is governed primarily by intrinsic electron-phonon coupling. The response of the PDMS nanocomposites provides a consistent measure of local temperature through intensity and lifetime in a polymer-dispersed morphology suitable for biomedical applications, and we exploit this to fabricate a small-footprint fiber-optic cryothermometer. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y32.00006: Non-Bleaching Photoluminescent Magnetic Nanoparticles Lu Zou, Chanjoong Kim, Emad Girgis, Wagdy K. B. Khalil We report a new type of photoluminescent magnetic nanoparticles produced by a very simple process. The nanoparticle consists of an ordinary magnetic nanoparticle as core and a non-toxic polymer shell. The biocompatibility is evaluated using in-vivo tests on mice. They are non-bleaching photoluminescent without any addition of fluorophores, such as quantum dots or fluorescent dyes that can be toxic and easily photobleached, respectively. This work provides a low-cost, bio-safe, non-bleaching alternative of conventional fluoroscent magnetic nanoparticles which covers a wide range of applications, from bio-imaging to biomedical diagnostics and therapeutics, such as hyperthermia. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y32.00007: Electrically Percolating Clusters in Sheared Carbon Nanotube Composites Kalman Migler, Doyoung Moon, Jan Obrzut, Jack Douglas, Thomas Lam, Renu Sharma, Alex James Liddle The electrical conductivity of polymer nanotube composites can be dramatically modified by processing flows and subsequent annealing. The mechanism is widely believed to be nanotube structural rearrangements that occur during flow and alter the percolating pathways. We seek to directly visualize these flow-induced three-dimensional percolating clusters through three-dimensional confocal microscopy and image analysis. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y32.00008: Electrical properties of isotactic polypropylene loaded with carbon nanofibers Mircea Chipara, Magdalena L. Ciurea, Karen Lozano, Gheorghe V. Aldica, Dorina M. Chipara, Stelian Popa, Ionel Stavarache Nanocomposites have been obtained by dispersing vapor grown carbon nanofibers (VGCNF) within isotactic polypropylene (iPP) via melt mixing. VGCNFs were purified and disentangled before blending with iPP. The mixing was performed by using HAAKE Rheomix, at 180 $^{o}$C and 65 rpm for 9 minutes followed by an additional mixing at 90 rpm for 5 minutes (same temperature). The electrical properties of nanocomposites loaded with various amounts of VGCNFs (0{\%}, 1{\%}, 2.5{\%}, 5{\%}, 7.5{\%}, 10{\%}, 15{\%}, and 20{\%} wt.) have been investigated. DC electrical measurements revealed a percolation threshold at about 12 {\%} wt. VGCNFs. The DC electrical characteristics of the nanocomposites located above the percolation threshold were investigated in detail, in a wide temperature range starting from 20 K up to about 750 K. The investigations revealed small changes of the DC conductivity within the glass and melting transition range of the polymeric matrix. The dominant charge transport mechanism below the glass transition temperature as well as between the glass and melting transition temperature is the variable range hopping. Above the melting temperature an Arrhenius like dependence of the DC conductivity was noticed. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y32.00009: Polyaniline-SnO$_2$ Nanocomposites for Better Sensitivity of NO$_{\mathrm{X}}$ gases at Lower Temperatures Navendu Goswami, Anjali Sharma, Monika Tomar, Vinay Gupta We demonstrate that the sensor based on Polyaniline (PAni) nanofibers, simply prepared by the interfacial polymerization, has advantages of sensitivity, spatial resolution and rapid time response for NO$_2$ gas at room temperature. Although PAni is one of the most studied conducting polymers due of its good electrical conductivity, environmental stability and relative easier synthesis, yet due to poor solubility of PAni, it is difficult to form the film adopting conventional methods. Nonetheless, nanomaterials of conjugated polymers are found to exhibit superior performance as compared to conventional materials due to their larger exposed surface area. The objective of this work is to study the PAni doped SnO$_2$ nanocomposite as novel sensing system and to probe the NOx sensing characteristics of this sensor at room temperature. Here we focus on the effect of doping ratio of sensor material, gas flow time and response time. PAni with different amounts has been stirred with SnO$_2$ solution to obtain SnO$_2$/PAni mixture. In present work, sensors with different PAni doping ratio were prepared and characterized so as to ascertain the favorable conditions for higher sensitivity, selectivity and better gas sensing characteristics. The as-grown films characterized employing various techniques and revealed that PAni/SnO$_2$ nanocomposite show good gas sensitivity at 30-100 $^{\circ}$C. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y32.00010: Self-healing of polymeric materials: The effect of the amount of DCPD confined within microcapsules Dorina M. Chipara, Alma Perez, Karen Lozano, Ibrahim Elamin, Jahaziel Villarreal, Alfonso Salinas, Mircea Chipara The self-healing SH) of polymers is based on the dispersion of a catalyst and of microcapsules filled with monomer within the polymeric matrix. Sufficiently large external stresses will rupture the microcapsule, releasing the monomer which will diffuse through the polymer and eventually will reach a catalyst particle igniting a polymerization reaction. The classical SH system includes first generation Grubbs catalyst and poly-urea formaldehyde microcapsules filled with DCPD. The polymerization reaction is a ring-opening metathesis. The size and the mechanical features of microcapsules are critical in controlling the SH process. Research was focused on the effect of DCPD on the size and thickness of microcapsules. Microscopy was used to determine the size of microcapsules (typically in the range of 10$^{-4}$ m) and the thickness of the microcapsules (ranging between 10$^{-6}$ to 10$^{-8}$ m). Research revealed a thick disordered layer over a thin and more compact wall. Raman spectroscopy confirmed the confinement of DCPD, TGA measurements aimed to a better understanding of the degradation processes in inert atmosphere, and mechanical tests supported the ignition of self-healing properties. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y32.00011: Engineering Flame Retardant Biodegradable Nanocomposites Shan He, Kai Yang, Yichen Guo, Linxi Zhang, Seongchan Pack, Rachel Davis, Menahem Lewin, Harald Ade, Chad Korach, Takashi Kashiwagi, Miriam Rafailovich Cellulose-based PLA/PBAT polymer blends can potentially be a promising class of biodegradable nanocomposites. Adding cellulose fiber reinforcement can improve mechanical properties of biodegradable plastics, but homogeneously dispersing hydrophilic cellulose in the hydrophobic polymer matrix poses a significant challenge. We here show that resorcinol diphenyl phosphates (RDP) can be used to modify the surface energy, not only reducing phase separation between two polymer kinds but also allowing the cellulose particles and the Halloysite clay to be easily dispersed within polymer matrices to achieve synergy effect using melt blending. Here in this study we describe the use of cellulose fiber and Halloysite clay, coated with RDP surfactant, in producing the flame retardant polymer blends of PBAT(Ecoflex) and PLA which can pass the stringent UL-94 V0 test. We also utilized FTIR, SEM and AFM nanoindentation to elucidate the role RDP plays in improving the compatibility of biodegradable polymers, and to determine structure property of chars that resulted in composites that could have optimized mechanical and thermal properties. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y32.00012: Designing high hard block Content TPU resins for composite application Alberto Saiani, Chinemelum Nedolisa, Christopher I. Lindsay Thermoplastic Polyurethanes (TPU) are linear block copolymers typically constructed of statistically alternating soft (SS) and hard (HS) segments. Due to their numerous industrial applications these materials have received considerable attention. We have recently investigated the phase behavior and morphology of a set of high hard block content polyurethanes. Using mainly calorimetry, scattering and microscopy techniques we were able to elucidate the origins of all the thermal events observed through differential scanning calorimetry and propose a new morphological model of the structure and the phase behavior of these high hard block content polyurethanes [A. Saiani et al. Macromolecules, 34, 9059-9068 (2001); 37, 1411-1421 (2004); 40, 7252-7262 (2007)]. We have now shown that these new materials can potentially be used as resins for designing fiber based composites and investigated the effect of processing on conditions the final properties of the composites [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y32.00013: Structure and Dynamics Characterization of HMDI- and MDI-based Poly(urethane urea) Elastomers via Solid- State NMR Weiguo Hu, Alex Hsieh, B. Christopher Rinderspacher, Tanya Chantawansri High performance elastomers have recently gained considerable interest throughout DoD, particularly for their potential in ballistic impact protection and blast mitigation capabilities. Recent simulation results based on coarse-grained modeling have revealed the role of the intermolecular interaction and the flexibility of interface between hard and soft segments on the morphology and mechanical deformation behavior of poly(urethane urea), PUU, elastomers. In this work, we exploit solid-state nuclear magnetic resonance (NMR) techniques to investigate the influence of hard domain size on molecular dynamics by comparing the diisocyanate chemistry (aliphatic 4,4'-dicyclohexylmethane diisocyanate (HMDI) vs. aromatic 4,4'-diphenylmethane diisocyanate (MDI)) in PUU elastomers. Despite identical stoichiometry and soft segment chemical structure, large difference in the molecular dynamics, indicated by the $^{\mathrm{1}}$H dipolar dephasing time (T$_{\mathrm{d}})$, is observed. The T$_{\mathrm{d}}$ of HMDI-PUU is shorter and it exhibits higher activation energy, suggesting finer phase mixing. Results from $^{\mathrm{1}}$H spin echo measurements are also included for comparison. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y32.00014: Thermal Boundary Resistance Across Solid-Fluid Interface Sanghamitra Neogi, Davide Donadio The recent advances in the field of nanotechnology, specially the advent of nanostractures and nanocomposite materials, have prompted an increased interest in the study of thermal transport across interfaces. When heat flows across an interface, the local temperature presents a discontinuity which is related to the thermal boundary resistance (TBR), also known as the Kapitza resistance. The investigation of Kapitza resistance has important technological applications in the improvement of the thermal performances of composite materials. The current theoretical understanding of TBR is primarily based on the ``acoustic mismatch theory'' or the ``diffusive mismatch model.'' Both these models consider only the bulk properties of the two materials, with no account being taken of the details of the material properties near the interface. Here, we investigate the thermal transport across a model solid-fluid interface using the technique of reverse non-equilibrium molecular dynamics simulations. The interaction potentials between the particles in our system are governed by the Lennard-Jones potential. We study the influence of pressure on the thermal boundary resistance for a range of mismatched interfaces and compare our results to the existing analytical models. [Preview Abstract] |
Session Y33: Focus Session: Organic Electronics and Photonics - Morphology and Structure I
Sponsoring Units: DMPChair: Penpeng Zhang, Michigan State University
Room: 341
Friday, March 22, 2013 8:00AM - 8:12AM |
Y33.00001: Molecular simulation studies of morphology in blends of conjugated polymers and fullerene derivatives for organic photovoltaic applications Eric Jankowski, Hilary Marsh, Arthi Jayaraman The device efficiency of organic solar cells is dependent on the microstructure of the active layer, which is typically a mixture of conjugated polymer electron donor molecules and fullerene based acceptor molecules. Active layer morphology can be tuned by choosing these acceptor and donor components that self-assemble into thermodynamically stable structures and by choosing processing conditions that facilitate the formation of equilibrium structures or that ``trap'' the active layer in an optimal metastable configuration. We present the results of molecular dynamics studies of model conjugated polymers and fullerene derivatives performed on GPUs. We show that the ordered structures that are self-assembled from initially disordered configurations depend strongly upon the strength of the attractions between acceptor and donor molecules, the relative amounts of each component, and the architecture of the donor molecules. Further we quantify the relaxation times and suggest processing strategies for obtaining optimal morphologies for charge transport. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y33.00002: Correlation of Fullerene Structure to its Miscibility in P3HT and OPV Function Mark Dadmun, Huipeng Chen, Jeff Peet The miscibility of four fullerenes, bis-PCBM, ICBA, Thio-PCBM and PC$_{\mathrm{70}}$BM in poly(3-hexylthiophene) (P3HT) is determined by neutron reflectivity by monitoring the intermixing of P3HT and fullerene bilayers with thermal annealing. The miscibility limit of these fullerenes in P3HT ranges from 11{\%} to 26{\%}, where the bis-adduct fullerenes exhibit a lower miscibility in P3HT than singly functionalized fullerenes. The correlation of miscibility to device performance indicates that sufficient polymer/fullerene miscibility is crucial to rationally optimize organic photovoltaic active layers. Low miscibility of conjugated polymer and fullerene in the amorphous phase decreases the probability of exciton dissociation and enhances the recombination of free charge-carriers. Moreover, the results indicate that the average surface-to-surface distance between fullerenes must be less than approximately of 5-7 {\AA} to minimize charge traps and allow sufficient charge transport in the mixed phase to improve photovoltaic performance. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y33.00003: Mixing-Induced Anisotropic Correlations in Molecular Crystalline Systems: Rationalizing the Behavior of Organic Semiconductor Blends Katharina Broch, Antje Aufderheide, Jiri Novak, Alexander Hinderhofer, Alexander Gerlach, Rupak Banerjee, Frank Schreiber Binary mixtures of organic semiconductors (OSCs) have recently become an important field of research, as they find applications in opto-electronic devices [1]. In these systems, the mixing (intermixing vs. phase separation) and ordering behavior is crucial, since it affects the optical and electronic properties. We present a comprehensive study of binary mixtures of the three prototypical OSCs pentacene (PEN), perfluoropentacene (PFP) and diindenoperlyene (DIP) in all possible combinations [1,2]. Using X-ray reflectivity and grazing incidence X-ray diffraction we investigate the stuctural properties of the mixed films as well as their impact on the optical spectra obtained by spectroscopic ellipsometry. For PEN:DIP we find an anisotropic ordering behavior, comparable to that observed in some liquid crystals, which is fundamentally new for OSCs [2]. The influence of sterical compatibility and the strength of the intermolecular interactions on the mixing and ordering behavior in the different blends will be discussed by extending a conventional mean-field model [1]. Finally, we discuss general rules for the targeted preparation of blends of OSCs. [1] A. Hinderhofer and F. Schreiber, Chem. Phys. Chem., 13, 628 (2012); [2] A. Aufderheide et al., Phys. Rev. Lett., 109, 156102 (2012) [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y33.00004: Domain compositions in the active layer of low band gap polymer/fullerene solar cells strongly affect device performance Sameer Vajjala Kesava, Zhuping Fei, Martin Heeney, Cheng Wang, Alexander Hexemer, Enrique Gomez We have characterized the morphology of mixtures of a germole-containing polymer, poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]germole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PGeBTBT), and PCBM using Resonance Soft X-ray Scattering (RSOXS) and Energy-Filtered Transmission Electron Microscopy (EFTEM). PGeBTBT belongs to cyclopentadithiophene-based polymer family with a band gap of 1.5 eV. Analyses of RSOXS data and EFTEM images have shown that the volume fraction of polymer in the fullerene matrix enveloping PGeBTBT fibers ($\sim$10 nm diameter) decreases with increasing overall composition of PCBM. Furthermore, PGeBTBT/PCBM devices demonstrate a correlation between the short circuit current and the purity of the PCBM-rich phase. We hypothesize that the relationship between PCBM domain composition and device performance is related to charge recombination, where increasing the polymer content suppresses charge transport thereby increasing the transit time. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y33.00005: The importance of domain purity for performance in P(NDI2OD-T2)-based all-polymer solar cells revealed by resonant x-ray scattering Harald Ade, Brian Collins, Marcel Schubert, Steffen Roland, Robert Steyrluethner, Zhihua Chen, Antonio Facchetti, Dieter Neher The nanostructure of bulk heterojunction organic solar cells has long been recognized as critical to their performance. To date, the primary morphological characteristics under investigation have been the level and nature of crystallinity of the materials. Yet the recent and wide-spread measurement of molecular mixing and diffusion of the electron donor and acceptor materials in amorphous regions has focused attention on the non-crystalline portions in these films as well. Here we investigate both aspects using x-ray diffraction and resonant scattering techniques to measure crystallinity and the domain sizes and purities, respectively, of devices based on P3HT:P(NDI2OD-T2) blends. The repercussions of the nanostructure is revealed in measurements of exciton bandwidth and photoluminescence quenching. We find that through variation of solvent blends and film drying conditions can significantly alter domain size and purity. This results in significant increases in device performance that correlate with increased domain purity and exciton bandwith of the P3HT crystals. This strongly indicates that molecular mixing of these materials is detrimental to performance in harvesting solar energy. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y33.00006: Probing the morphology of novel non-fullerene based bulk heterojunction solar cells Gregory Su, Toan Pho, Fred Wudl, Edward Kramer, Michael Chabinyc Organic semiconductors are promising for low-cost, large-area electronics such as organic photovoltaics (OPVs). OPVs require an active layer that is an intimate mixture of an electron donor, usually a conjugated polymer, and an electron acceptor, typically a fullerene. While fullerene-based OPVs show high efficiencies, the inability to tune its electronic levels limits the open circuit voltage, so alternative acceptors are desirable. Here, we report on blend films consisting of a polymer donor, poly(3-hexylthiophene) (P3HT), and a novel acceptor, decacyclene triimide (DTI), that display good solar power conversion efficiencies (PCE) as-cast. The PCE of these blends decreases significantly with thermal annealing, unlike P3HT:fullerene blends. NEXAFS spectroscopy and grazing incidence wide angle X-ray scattering suggest the PCE decrease is due to the formation of hexagonally packed DTI columns with an in-plane $\pi$-$\pi$ stacking direction, resulting in poor electron transport out-of-plane. Polarized resonant soft X-ray scattering suggests that the $\pi$-$\pi$ stacking directions are orthogonal at interfaces between DTI and P3HT domains. These results demonstrate the importance of blend morphology in OPV efficiency and key differences between DTI-based and fullerene-based blends. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y33.00007: Tuning polymer/inorganic blend morphology using pyridine terminated poly(3-hexylthiophene)s: Novel ligands for potential OPV applications W. Michael Kochemba, S. Michael Kilbey II, Deanna L. Pickel, Bobby G. Sumpter End-functional pi-conjugated polymers are promising materials for the improvement of organic electronic devices due to their high hole mobility and ease of processability. Here we describe a ``materials by design'' approach to create 2- and 3-pyridyl, end-functionalized poly(3-hexylthiophene)s (P3HTs) that possess the capacity to ligate semiconductor quantum dots (SQDs). The replacement of native ligands on the SQD surface by pyridyl-terminated P3HTs provides the opportunity to manipulate the morphology of polymer/inorganic blends created by dispersing the P3HT-ligated SQDs in a P3HT matrix. TEM imaging and small angle x-ray scattering were used to assess the morphological traits of the blends as a function of ligand type, processing condition, and matrix molecular weight, which in general show that the P3HT ligands improve dispersion of the nanoparticles upon thermal annealing. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y33.00008: Determination of the Crystallinity of Semicrystalline Poly(3-hexyl thiophene) by Means of Wide Angle X-Ray Scattering Jens Balko, Ruth Lohwasser, Mukundan Thelakkat, Michael Sommer, Ovidiu Pascui, Kay Saalwaechter, Thomas Thurn-Albrecht Poly(3-hexyl thiophene) (P3HT) is a common polymer semiconductor, often used as material or component in organic field effect transistors or solar cells. The crystallinity of this semicrystalline material is among other parameters governing the electronic mobility. However, at present there is no routine method available to determine an absolute value for the crystallinity, and the values given in the literature e.g. for the enthalpy of melting vary by a factor of three. Wide Angle X-Ray Scattering (WAXS) probes the crystals as well the amorphous parts of the sample. We present an approach for the determination of the crystallinity based on the evaluation of WAXS intensities at low scattering vectors emanating from the amorphous regions. The result is used for a calibration of the melting enthalpy (34 J/g) that can serve as a reference value for more convenient calorimetric techniques and compared to the results of recent NMR investigations. We discuss the crystallinity for a number of chemically well-defined samples, with different molecular weight and a typical commercial sample with broad molecular weight distribution. Despite the high crystallinities of 60 to 80{\%} the crystallites exhibit a large amount of disorder. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y33.00009: Structural and Morphological Analysis of Poly(3-hexylthiophene ) at Surfaces and Interfaces Yeneneh Yimer, Mesfin Tsige The structure and morphology of semiconducting polymers such as Poly(3-hexylthiophene) (P3HT) at surfaces and interfaces have significant influence over the performance of organic solar cell devices. Because charge-carrier generation, transport to and collection at the electrodes depend on the material properties of P3HT - themselves controlled by factors including packing, orientation and environment - analysis of the mechanisms that contribute to efficient charge generation and minimization of recombination is necessary. Using molecular dynamics simulation, we have investigated the structural properties and morphological evolution of P3HT at different surfaces and interfaces. We have also investigated the dependence of those properties on temperature, chain length, and interfacial energies. The morphology of P3HT is correlated to efficient charge transport. Using our analyses, we have attempted to elucidate these correlations, which should help lead to optimization of the morphology of P3HT in devices in the pursuit of increasing the efficiency of polymeric devices. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y33.00010: Conjugated backbone orientation variation in high mobility regioregular PT based copolymers Louis Perez, Lei Ying, Guillermo Bazan, Edward Kramer The synthesis of novel solution processable conjugated polymers is an active field of study due to the potential to fabricate low cost, high though-put electronic devices such as organic field effect transistors (OFET). A regioregular copolymer based on cyclopenta[2,1-$b$:3,4-$b'$]dithiophene (CDT) and pyridal[2,1,3]thiadiazole (PT) structural units has been prepared by using polymerization reactions involving reactants specifically designed to avoid random orientation of the asymmetric PT heterocycle. Compared to it's regiorandom counterpart, the regioregular polymer exhibits a two orders of magnitude increase in hole mobility from 0.005 to 0.6 cm$^{\mathrm{2\thinspace }}$V$^{\mathrm{-1}}$ s$^{\mathrm{-1}}$. A combination of X-ray scattering techniques were employed to quantitatively access the degree of orientation and crystallinity in thin films (15-20 nm) that matched device architecture. We examined the backbone orientation dependence as a function of depth via grazing incidence wide angle X-ray scattering (GIWAXS) and found significant differences in the backbone stacking orientation between the regiorandom and regioregular copolymers. These experiments suggest the backbone regularity leads to significant differences in the structural arrangement and it is another important design criteria to consider in the design of new conjugated copolymers with asymmetric structural units. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y33.00011: Poly(3-hexylthiophene) Brush-Modified Interfaces for Control of Active Layer Morphology and Properties S. Michael Kilbey, W. Michael Kochemba, Deanna Pickel, Jose Alonzo Tailoring the morphology of donor-acceptor blends based on conjugated polymers and fullerenes is an essential part of optimizing the power conversion efficiency of organic photovoltaic (OPV) devices. While a variety of studies have demonstrated the importance of the nanoscale morphology of donor-acceptor blends on efficiency, a clear understanding of the links between morphology, processing, interfacial structure and device-level properties is yet to emerge. Here we turn to well-defined layers of end-tethered poly(3-hexylthiophene) (P3HT) chains as modifiers, or buffer layers, that straddle the inorganic/organic interface and exert control over the morphology of donor-acceptor blends. In addition to improving device performance characteristics, ostensibly due the presence of surface dipoles brought about by confinement, P3HT brushes affect the penetration of the fullerene derivative, 6,6-phenyl-C61-butyric acid methyl ester, PCBM, into the brush as well as the morphology of bilayers and blends of P3HT and PCBM coated atop the brushes. The role of molecular weight, chain grafting density, and thermal aging and light cycling on these behaviors will be highlighted. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y33.00012: Rod-Coil Copolymer as Efficient Compatibilizer for Thermally-Stable Polymer Solar Cell H.J. Kim, K. Paek, H. Yang, B.J. Kim Improving the thermal stability of polymer solar cells (PSC) is important for the future application of these devices since any heat generated by solar irradiation could be detrimental to the performance as a result of the relatively low Tg of polymers and the strong immiscibility of components in the active layer. Herein we have developed new type of compatibilizers having two different blocks of conjugated polymer and poly(2-vinyl pyridine)(P2VP). The P2VP and fullerene are mixed together by supramolecular interaction resulting conjugated polymer-P2VP copolymers act as a compatibilizer reducing the interfacial tension between the two dissimilar components of the PSC. Our compatibilizer successfully suppresses the macrophase separation of donor and acceptor blended films made of either singly functionalized PCBM or bisadduct fullerene derivatives. P3HT/o-xylene-C60-bissadduct fullerne(OXCBA) blended device containing 10 vol percent of compatibilizer shows an average efficiency higher than 4.3 percent after 60 h annealing at an elevated temperature of 150'C. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y33.00013: Percolating bulk-heterostructures from neutron reflectometry and small angle scattering data Daniel Olds, Phillip Duxbury We present a novel algorithm for efficiently calculating the simulated small angle scattering data of any discretized morphological model of arbitrary scale and resolution, referred to as the distribution function method (DFM). Unlike standard SAS fitting methods, the DFM algorithm allows for the calculation of form factors and structure factors from complex nanoscale morphologies commonly encountered in many modern polymeric and nanoparticle based systems, which have no exact analytical corollary. The computational efficiency of the DFM algorithm suggests it's use in morphological model refinement. We will present a number of simple examples to demonstrate the accuracy and limits of the algorithm, followed by an example of incorporation of the DFM algorithm into reverse Monte Carlo structural refinement of bulk-heterojunction two-phase morphologies, such as those commonly found in organic photovoltaic devices. We will show that morphological features introduced via direct incorporation of experimental neutron reflectometry and SANS data to the models has a direct effect on the results of device simulations. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y33.00014: In-situ Neutron Scattering Determination of 3D Phase-Morphology Correlations in Fullerene -Polymer Organic Photovoltaic Thin Films Alamgir Karim, David Bucknall, Dharmaraj Raghavan, Bobby Sumpter, Scott Sides The tunability of the morphology and structure of conjugated polymer-fullerene bulk heterojunctions (BHJs) is being investigated through synthesis of new materials, novel processing strategies and advanced characterization (experimental and computational). We are using this integrated approach to test currently poorly understood fundamental issues in organic photovoltaic (OPV) performance relating to structure-property and very importantly processing relationships. Using model conjugated polymer-fullerene systems, we are investigating how the phase morphology of the BHJs correlate with OPV efficiency. A range of fullerenes is being investigated that include a number of new derivatives that we have synthesized. We are currently investigating the use of surface energy confinement and block copolymer templating to control both phase domain segregation and orientation relative to the film normal to allow us to test morphology-device efficiency hypotheses in OPVs. Using both neutron scattering and computational modeling we have developed important correlations that establish relationships between the polymer-fullerene miscibility, phase domain orientation and interfacial behavior with the corresponding photoelectronic properties. [Preview Abstract] |
Session Y34: Focus Session: Microfluidics, Nanofluidics Applications
Sponsoring Units: DPOLYChair: Alberto Fernandez-Nieves, Georgia Institute of Technology
Room: 342
Friday, March 22, 2013 8:00AM - 8:36AM |
Y34.00001: Acoustic Microfluidics for Bioanalytical Application Invited Speaker: Gabriel Lopez This talk will present new methods the use of ultrasonic standing waves in microfluidic systems to manipulate microparticles for the purpose of bioassays and bioseparations. We have recently developed multi-node acoustic focusing flow cells that can position particles into many parallel flow streams and have demonstrated the potential of such flow cells in the development of high throughput, parallel flow cytometers. These experiments show the potential for the creation of high throughput flow cytometers in applications requiring high flow rates and rapid detection of rare cells. This talk will also present the development of elastomeric capture microparticles and their use in acoustophoretic separations. We have developed simple methods to form elastomeric particles that are surface functionalized with biomolecular recognition reagents. These compressible particles exhibit negative acoustic contrast in ultrasound when suspended in aqueous media, blood serum or diluted blood. These particles can be continuously separated from cells by flowing them through a microfluidic device that uses an ultrasonic standing wave to align the blood cells, which exhibit positive acoustic contrast, at a node in the acoustic pressure distribution while aligning the negative acoustic contrast elastomeric particles at the antinodes. Laminar flow of the separated particles to downstream collection ports allows for collection of the separated negative contrast particles and cells. Separated elastomeric particles were analyzed via flow cytometry to demonstrate nanomolar detection for prostate specific antigen in aqueous buffer and picomolar detection for IgG in plasma and diluted blood samples. This approach has potential applications in the development of rapid assays that detect the presence of low concentrations of biomarkers (including biomolecules and cells) in a number of biological sample types. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y34.00002: Electrokinetic device for three-dimensional trapping of single fluorescent emitters Jason K. King, Brian K. Canfield, Lloyd M. Davis Trapping by use of actively controlled electric fields is a valuable tool for studies of single biological molecules and nanoparticles. Devices have been developed to trap in one and two dimensions, but these rely on physically constraining the molecule along one or more directions. However, behavior of trapped molecules may be perturbed due to high collision rates with walls. Here we report on the development of a three-dimensional (3D) electrokinetic trap to counteract Brownian motion. Two pairs of electrodes arranged in a crossed configuration on separate planes allow generation of an electric field of variable orientation and magnitude. A custom forward-illuminated microscope with astigmatism introduced to the tube lens is used to determine the nanoparticle's 3D position in real time. This device has demonstrated the capability to manipulate and confine single 40 nm fluorescent latex beads in glycerol-water solution. The use of an electron-multiplying CCD camera allows for faster detection rates (\textgreater 100 Hz) and single-photon sensitivity. Characterization of particle motion and performance analysis of trapping methods is investigated. The use of alternative 3D detection methods is discussed, as well as applications to studies of single biomolecules and nanoparticles. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y34.00003: It may be possible to construct a Chemical Synthesizing Computer based on Capillary Action Richard Kriske This author had previously proposed that Capillary Action has a Quantum Mechanical Model. This model can be easily constructed by noting that when a photon of the heat wavelength evaporates one molecule of water at the top of a capillary column, a ``hole'' is transmitted from the top of the column to the roots and into the water reservoir sustaining the capillary tube. This ``hole'' is a true hole (a true particle) in that it is transmitted as a quantized unit through the capillary tube. The mathematics of this process are the same as used in Quantum Field Theory, with the capillary acting as a perfect spring (like the spring used on a ``stack'' of dishes). When the external field using a force to pull the water molecule off the stack, an equal and opposite spring force (which is quantized), is transmitted down the column to the reservoir. When the water is not pure, this author proposes that each of the elements in the unpure water act linearly, each with its own quantized spring constant that does not interact with the other quantized spring constants, so it is possible to pull a single electron off the top of the water stack, yet the water in the stack is undisturbed (the reservoir is disturbed). Likewise it is possible to pull a sugar molecule off and balance chemical equations. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y34.00004: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y34.00005: Integrated optics for Lab-On-Chip Yu Gu, Andrea Crespi, Lisa Mariani, Gianna Valentino, Giulio Cerullo, Roberto Osellame The miniaturization of traditional chemical and biochemical functionalities called Lab-On-Chip has many advantageous over existing methods, such as portability, small sample size, multiplexing and simpler automation and standardization. In recent years, the integration of microfluidic and microoptical elements together onto monolithic platforms has led to the new term optofluidics. We present novel optofluidic devices based on integrated waveguides, microfluidic channels and high-index fluids. Such devices have a variety of applications including label-free biochemical sensing and telecommunications. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y34.00006: A microfluidic separation platform using an array of slanted ramps Sumedh Risbud, Jorge Bernate, German Drazer The separation of the different components of a sample is a crucial step in many micro- and nano-fluidic applications, including the detection of infections, the capture of circulating tumor cells, the isolation of proteins, RNA and DNA, to mention but a few. Vector chromatography, in which different species migrate in different directions in a planar microfluidic device thus achieving spatial as well as temporal resolution, offers the promise of high selectivity along with high throughput. In this work, we present a microfluidic vector chromatography platform consisting of slanted ramps in a microfluidic channel for the separation of suspended particles. We construct these ramps using inclined UV lithography, such that the inclined portion of the ramps is upstream. We show that particles of different size displace laterally to a different extent when driven by a flow field over a slanted ramp. The flow close to the ramp reorients along the ramp, causing the size-dependent deflection of the particles. The cumulative effect of an array of these ramps would cause particles of different size to migrate in different directions, thus allowing their passive and continuous separation. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y34.00007: Size based separation of micro-particles using adhesive ciliated surfaces: Mimicing the behaviour of suspension feeders Anurag Tripathi, Amitabh Bhattacharya, Anna Balazs Separation of different size micro-particles in microfluidic devices is important for many biomedical applications. Inspired by the selective intake of small food particles by marine suspension feeders, we propose a novel separation mechanism of micro-particles using active cilia arrays with adhesive tips. By means of Lattice Boltzmann simulations, we show that mixture of two different size particles with size ratio greater than or equal to two can be nearly completely separated by tuning adhesion strength and cilia stiffness. The proposed technique can be used even at low Reynolds number (Re $\ll$ 1) where separation mechanisms based on inertial effects will be of little use. For a given cilia-particle interaction, the balance of hydrodynamic and adhesive forces favors capture of particles below a critical size, which can be predicted by a simple analytical model. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y34.00008: Stiffness Dependent Separation of Cells in a Microfluidic Device Todd Sulchek, Gonghao Wang, Wenbin Mao, Alexander Alexeev Abnormal cell mechanical stiffness can point to the development of various diseases including cancers and infections. We report a high-throughput technique for continuous cell separation utilizing variation in cell stiffness. We use a microfluidic channel that is decorated by periodic diagonal ridges to force cells of different stiffness values to follow different trajectories. The ridges within the microfluidic flow channel compress and deform the cells in rapid succession to translate each cell perpendicular to the channel axis in proportion to its stiffness. We report the experimental demonstration of separation as well as computational validation of the mechanism of separation. Atomic force microscopy (AFM) was used to independently measure cell stiffness. By flowing cells through the microfluidic device, we can quickly and efficiently separate mixtures into subpopulations of stiff cells and soft cells. We then summarize how we expect this technology may produce new biomedical diagnostic capabilities. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y34.00009: Process development for perfectly concentric droplets-within-droplets and uniform-walled shells Greg Randall, Brent Blue Compound droplets, or droplets-within-droplets, are currently precursors for shell targets used in intertial fusion experiments. To implode properly, each shell requires a uniform wall thickness, which in turn requires a centered core droplet in the compound droplet precursor. Previously, Bei et al. (2009, 2010) have shown that stationary compound droplets could be centered in a static fluid using an electric field of 0.7 kV/cm at 20 MHz. We present our recent results in developing a continuous microfluidic process to mass fabricate these uniform-walled shells. This includes: using electric fields to center the core of moving compound droplets, inhibiting droplet stretching by using protein emulsifiers, and maintaining a centered core during polymerization. We apply a physical scaling analysis from a fluid mechanics perspective to aid process design. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y34.00010: Optical Nanodozers: A New Tool for Probing Single-Molecule Conformation and Confinement Free Energy in Cavities of Adjustable Nanoscale Dimension Ahmed Khorshid, Walter Reisner Experiments probing single-molecule DNA statics and dynamics in nanoconfined systems are typically performed via fluoresence microscopy, yielding access to information regarding molecule conformation but no direct information regarding nanoscale forces. In our experiment we combine two single-molecule manipulation tools, optical trapping and nanoconfinement, to develop a novel assay that can yield information regarding both molecule conformation and forces experienced in confinement. Polystyrene beads are trapped inside 300x300nm silica nanochannels. These beads are then used as ``nano-pistons'' or ``nanodozers,'' to apply compressive forces to single-molecules confined inside the nanochannels. In particular, a single nanodozer is used to push a DNA molecule against a nanoslit barrier, enabling measurements of force versus molecule compression. By carefully calibrating our trap via assessing Brownian motion of the nanochannel confined bead we are able to obtain a force-compression curve that we are comparing to polymer physics models for a cavity confined chain. In addition, we can determine the force required to drive the polymer across the entropic barrier as the critical force applied when the polymer jumps out of the cavity and over the slit. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y34.00011: DNA in Nanochannels: A Multistage Free Energy Perturbation Approach Yanwei Wang, Douglas R. Tree, Kevin D. Dorfman Nanochannels are ideal platforms for studying the basic physics of confined polymers, using DNA as the model polymer. While the scaling laws for strong (Odijk) and weak (de Gennes) confinement were established decades ago, recent experiments and computer simulations have illuminated the complex physics arising between these limiting cases. To understand fully the transition region between the classical regimes of de Gennes and Odijk, it is necessary to examine the underlying free energy behavior of DNA in nanochannels. This presentation reports our studies on the confinement free energy and other properties of nanochannel-confined DNA by the multistage free energy perturbation (MFEP) technique. Emphases are focused on the methodology, the role of the aspect ratio of the channel on the confinement free energy and the force-extension relation of DNA confined in nanochannels. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y34.00012: Molecular Dynamics Study of Polymer Separation Using a Nanofluidic Staircase Frederick Phelan Jr., Christopher Forrey The diffusive behavior of isolated polymer chains in a nanofluidic staircase has recently been studied experimentally [Strychalski et al., Macromolecules, 45(3), 1602, (2012); Stavis et al., Lab Chip, 12(19), 1174, (2012)] and by simulation [Phelan et al., in preparation, (2012)]. Chains are observed to exhibit spontaneous 1-D biased diffusion from regions of high to low confinement, without the use of external forces, under conditions where the local confinement lies in either the Odijk or de Gennes regimes. The transport mechanism is that of a Brownian motor, where the polymer free energy is used to generate directed transport using thermal fluctuations and the biased structural features of the device. The nanostaircase has potential for a number of applications in polymer measurement science and transport, an important one of which could be separations. To study this, we examine polymer separation in the nanofluidic staircase using the molecular dynamics simulation software LAMMPS. Length based separations of linear polymers as applicable to DNA separations are the main topic of the study, but the effect of more complex architectures such as branching are also examined. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y34.00013: Measuring the confinement free energy of DNA in nanofluidic cavities Alexander Klotz, Walter Reisner It is possible to dictate the equilibrium conformation of single DNA molecules in nanofluidic systems by creating topographies where confinement varies over scales of nanometers to microns. Much work has been done to elucidate the polymer physics of systems with simple 1D or 2D confinement, but there is little quantitative understanding of behavior in more complex systems.~ Using single-molecule fluorescence microscopy, we study the equilibrium conformation of single DNA molecules partitioning into a single nanoscale pit etched in a nanoscale slit. In this system the polymer exists in a conformation which is partially occupying the nanopit and partially outside in the slit: the fraction of contour filling the pit is determined by a balance of confinement free energy and self-avoidance.~ We measure statistical distributions of this filling fraction resulting from fluctuations of contour in and out of the slit. These distributions are measured as a function of slit height and pit width and interpreted in terms of free energy models based on the balance of confinement free energy and self-avoidance.~ These measurements serve as a unique experimental probe of cavity-like polymer confinement, a system with rich phase behavior that has not been probed experimentally.~ Together with previous work on the statistics of molecules spanning multiple pits, we can use this system to make measurements of the free energy of confinement and self-avoidance effects in confined systems, essential quantities in the design of nanofluidic devices for DNA manipulation. [Preview Abstract] |
Session Y35: Low TC: 2-D Superconductor-insulator Transition
Sponsoring Units: DMPChair: Greg Boyd, Georgetown University
Room: 343
Friday, March 22, 2013 8:00AM - 8:12AM |
Y35.00001: Strongly disordered s-wave superconductors probed by microwave electrodynamics E.F.C. Driessen, P.C.J.J. Coumou, R.R. Tromp, P.J. de Visser, T.M. Klapwijk In contrast to Anderson's theorem, recently evidence has emerged that superconductivity is susceptible to strong disorder and that there is a disorder-induced superconductor-to-insulator transition (SIT). We probe the effects of strong disorder ($8.6> k_Fl > 2.4$, approaching the SIT) in thin films of niobium titanium nitride and titanium nitride by measuring the microwave electrodynamics in coplanar waveguide resonators. The electromagnetic response gradually evolves with disorder, deviating from conventional Mattis-Bardeen theory, for both materials. The result is understood as due to changes in the quasiparticle density of states, as a consequence of the short elastic scattering length. Our observations are consistent with a model that uses an effective pair breaker, which is inversely proportional to the value of $k_Fl$. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y35.00002: Berezinsky– Kosterlitz– Thouless transition in ultrathin NbN films near superconductor-insulator transition Jie Yong, K. Il'in, M. Siegel, Thomas Lemberger We report temperature dependent superfluid densities $\lambda$ $^{-2}$(T) in ultrathin NbN films near thickness-tuned superconductor-insulator transition (SIT). Superfluid densities in these films are measured by two-coil mutual inductance apparatus. For thick films, dirty limit BCS theory fits experimental data well and this verifies the correctness of this technique. As films get thinner and closer to SIT, sharp downturns near transition temperatures (T$_c$), signature of Berezinsky-Kosterlitz-Thouless transition, are observed. This downturn occurs much earlier than what 2-D XY theory predicts. This might due to smaller vortex core energy than expected in 2-D XY model. The superconducting gap, deduced from fitting low temperature $\lambda$ $^{-2}$(T), is linear with T$_c$ for most films but remain finite across SIT. This is consistent with the scenario that superconductivity is destroyed by phase fluctuations. Zero temperature sheet superfluid density also shows correlation with T$_c$, further proving the importance of fluctuations near SIT. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y35.00003: Universal Scaling of the order parameter distribution in strongly disordered superconductors A. Kamlapure, S.C. Ganguli, G. Lemari\'e, D. Bucheli, L. Benfatto, J. Lorenzana, C. Castellani, G. Seibold, P. Raychaudhuri We present scanning tunneling spectroscopy measurements on strongly disordered s-wave superconductor, NbN, close to Anderson metal insulator transition. At low temperatures all our samples show superconducting spectra with dip close to zero bias and two coherence peaks after correcting with large V shaped background. Although spectra do not show significant variation in the superconducting energy gap but we see large distribution in the coherence peak heights characteristic to the strength of disorder. We take average value of the coherence peak heights on positive and negative bias as a measure of local order parameter $S$ [1]. We observe that maxima of order parameter distribution (OPD) steadily decrease with increasing disorder. On rescaling with the new scaling variable $R_{S}$ as logarithm of order parameter normalized to its variance, OPD for all the samples collapse into single curve showing universality of the OPD. In addition OPD is in good agreement with the universal Tracy-Widom distribution in finite dimension. We also identify similar scaling relation of the OPD within two prototype fermionic and bosonic models for disordered superconductors showing an excellent agreement between experiment and theory in the current field.\\[4pt][1]arXiv:1208.3336 [cond-mat.supr-con] [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y35.00004: The effects of disorder and temperature on the glassy dynamics of the first-order spin-paramagnetic transition in ultrathin granular Al films Joseph Prestigiacomo, Philip Adams We report an ongoing experimental study of the effects of disorder and temperature on the glassy dynamics of the first-order spin-paramagnetic transition in ultrathin granular Al films. The disorder of the films is gauged primarily by their proximity to the quantum sheet resistance $R_Q \approx$ 6.45 K$\Omega/$sq at temperatures slightly above $T_c$. In general, thicker films with $R \ll R_Q$ achieve equilibrium almost entirely through avalanches in resistance while thinner films with $R \sim R_Q$ exhibit slow stretched-exponential relaxation with very few detectable avalanches. Preliminary observations indicate that increasing measurement temperatures to near the tricritical point has the effect of speeding up the relaxations, thereby reducing the time constants involved. Similarities between this system and other glassy systems will be discussed. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y35.00005: Transport Behavior of Ultrathin Films with NanoThickness Undulations in the Strongly Localized Regime J.C. Joy, S.M. Hollen, C. Zhao, G. Fernandes, J.M. Xu, J.M. Valles, Jr. Recent work on thin films of superconducting material grown on anodized aluminum oxide (AAO) has revealed the existence of a Cooper Pair Insulator (CPI), a state in which superconducting pair correlations survive, but with activated transport dominated by electron pairs. AFM data has revealed that the AAO substrates have a regular undulating structure, which causes films to grow with predictable variations in thickness. These thickness undulations, which have a spatial period greater than the superconducting coherence length, work to localize Cooper pairs in the insulating state. To gain insight into the properties of the normal state of the CPI, we are investigating the transport properties of Copper films grown on AAO substrates. Early data indicate activated transport with activation energies of approximately 20 K in the most insulating films. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y35.00006: Superconductor-Metal-Insulator transition in two dimensional Ta thin Films Sun-gyu Park, Eunseong Kim Superconductor-insulator transition has been induced by tuning film thickness or magnetic field. Recent electrical transport measurements of MoGe, Bi, Ta thin films revealed an interesting intermediate metallic phase which intervened superconducting and insulating phases at certain range of magnetic field. Especially, Ta thin films show the characteristic IV behavior at each phase and the disorder tuned intermediate metallic phase [Y. Li, C. L. Vicente, and J. Yoon, Physical Review B 81, 020505 (2010)]. This unexpected metallic phase can be interpreted as a consequence of vortex motion or contribution of fermionic quasiparticles. In this presentation, we report the scaling behavior during the transitions in Ta thin film as well as the transport measurements in various phases. Critical exponents v and z are obtained in samples with wide ranges of disorder. These results reveal new universality class appears when disorder exceeds a critical value. Dynamical exponent z of Superconducting sample is found to be 1, which is consistent with theoretical prediction of unity. z in a metallic sample is suddenly increased to be approximately 2.5. This critical exponent is much larger than the value found in other system and theoretical prediction. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y35.00007: Magneto-transport Measurements of Electrostatically Tuned Disordered In2O3 Films near the Superconductor-Insulator Transition Yeonbae Lee, Aviad Frydman, Allen Goldman We have used an electric double layer transistor configuration employing an ionic liquid to modify the carrier density and resultant properties of disordered In$_2$O$_3$ films near the superconductor-insulator (SI) transition. By carrier density modulation up to 7 X 10$^{14}$ carriers-cm$^{-2}$, we have been able to traverse the SI transition as well as significantly alter the strength and location of the large magnetoresistance peak found in the insulating regime. We have also been able to correlate the magnetic length associated with the largest magnetoresistance peak with a length scale for granularity of the film obtained from a spectral analysis of surface profile data obtained using atomic force microscopy. The latter suggests that film morphology may play an important role in the peak. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y35.00008: AC evidence of a field tuned 2D superconductor-metal transition in a low-disorder InO$_x$ film Wei Liu, LiDong Pan, Jiajia Wen, Minsoo Kim, Sambandamurthy Ganapathy, Peter Armitage Employing microwave spectroscopy, we investigated the field tuned quantum phase transition between the superconducting and the resistive states in a low-disorder amorphous InO$_x$ film in the frequency range of 0.05 to 16 GHz. Our AC measurements are explicitly sensitive to the critical slowing down of the characteristic frequency scales approaching a transition. The relevant frequency scale of superconducting fluctuations approaches zero at a field $B_{sm}$ far below the field $B_{cross}$ where different isotherms of resistance as a function of magnetic field cross each other. The phase stiffness at the lowest frequency vanishes from the superconducting side at B $\approx B_{sm}$, while the high frequency limit extrapolates to zero near $B_{cross}$. Our data are consistent with a scenario where $B_{sm}$ is the true quantum critical point for a transition from a superconductor to an anomalous metal, while $B_{cross}$ only signifies a crossover to a regime where superconducting correlations make a vanishing contribution to both AC and DC transport measurements in the low-disorder limit. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y35.00009: Observation of the Collapse of the Cooper Pair Phase Coherence Length at a Superconductor to Insulator Transition James Valles, Shawna Hollen, Gustavo Fernandes, Jimmy Xu Experiments on ultrathin amorphous Bi films provided one of the best known examples of a Superconductor to Insulator quantum phase transition (SIT). Nevertheless, controversy persists over whether this thickness tuned SIT is ``fermionic'' or ``bosonic''. Early data suggested fermionic with the suppression of the amplitude of the superconductor order parameter creating a weakly-localized, phase incoherent, single electron insulator. However, recent work on other uniformly disordered materials suggests that bosonic physics universally dominates at the SIT to produce insulators of locally phase coherent Cooper pair islands. To address this issue, we used a technique that previously revealed local Cooper pair phase coherence in insulating non-uniformly thick films. We measured the strength of flux periodic magneto-resistance oscillations of ultrathin a-Bi films patterned with a nano-array of holes. The data indicate that the Cooper pair phase coherence length collapses at this SIT. This collapse is inconsistent with the continuous decrease of the phase coherence length expected for a bosonic SIT. It is consistent with the order parameter amplitude disappearing at a fermionic SIT. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y35.00010: Transport of thin superconducting films and multilayer heterostructure made by Atomic layer deposition Thomas Proslier, Jeffrey Klug, Nickolas Groll, Nicholas Becker, Andreas Glatz, Valerii Vinokur, Michael Pellin, Tatyana Baturina, Jeffrey Elam, John Zasadzsinki We report the use of atomic layer deposition (ALD) to synthesize thin superconducting films and multilayer superconductor-insulator (S-I) heterostructures. The ALD technique applied to superconducting films opens the way for a variety of applications, including improving the performance and decreasing the cost of high energy particle accelerators, superconducting wires for energy storage, and bolometers for radiation detection. Furthermore, the atomic-scale thickness control afforded by ALD enables the study of superconductivity and associated phenomena in homogeneous layers in the ultra-thin film limit. In this respect, we will present results of ALD-grown transition metal-based superconductors, including nitrides, carbides, and silicides of niobium, nitrides of molybdenum and titanium, and Nb$_{\mathrm{1-x}}$Ti$_{\mathrm{x}}$N/AlN-based S-I heterostructures. Transport measurement for various composition and film thicknesses will be presented. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y35.00011: Dynamical conductivity across the superconductor-insulator transition Mason Swanson, Yen Lee Loh, Mohit Randeria, Nandini Trivedi Thin superconducting films can exhibit a quantum phase transition from a superconductor to an insulator with increasing disorder. While the exact mechanism of the transition is not completely understood, there is strong evidence that it is bosonic in nature in some models and materials, with disorder acting to localize the superconducting pairs [1]. Previous studies of bosonic models of the superconductor-insulator transition (SIT) have focused almost entirely on criticality and dc properties at the transition. We go beyond these studies by calculating the dynamical conductivity of a disordered (2$+$1)D XY model using quantum Monte Carlo simulations that capture the phase fluctuations driving the SIT. Our results obey standard sum rule constraints for the longitudinal and transverse current correlation functions and show a build-up of integrated spectral weight near the transition. We will discuss the low frequency spectral weight in terms of a possible intermediate bose-metal phase between the superconductor and insulator. [1] K. Bouadim, Y. L. Loh, M. Randeria, and N. Trivedi, \textit{Nat. Phys. }\textbf{7}, 884 -- 889 (2011). We acknowledge support from the NSF Graduate Research Fellowship Program (MS), NSF DMR-1006532 (MR), and DOE DE-FG02-07ER46423 (NT). [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y35.00012: Off-diagonal disorder in two-dimentional attractive Hubbard model: A Bogoliubov-deGennes study Sanjeev Kumar, Prabuddha Chakraborty We present a detailed computational study of the two-dimensional attractive Hubbard model on a square lattice in the presence of off-diagonal disorder. The focus is on the superconductor to insulator transition, and on the comparison between the effects of diagonal disorder and those of off-diagonal disorder. We decouple the attractive Hubbard model in the pairing channel and make use of the Bogoliubov deGennes methodology to study the resulting model numerically on finite lattices. The work is motivated by recent observation of Anderson localization in optical lattices, and the possibility of tuning the sign of interactions between the atoms [Rev. Mod. Phys. {\bf 80}, 885 (2008)]. We find very interesting qualitative differences between the models of diagonal and off-diagonal disorder. The average amplitude of superconducting order parameter and spectral gap are strongly suppressed with off-diagonal disorder. This is in contrast to the case of diagonal disorder, where the spectral gap has a minumum and the amplitude of superconducting order parameter remains finite. We also present systematic lattice-size dependence of results. In addition, we compare the results of binary (discrete), and box (continuous) distributions, for both diagonal and off-diagonal disorder models. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y35.00013: Random-Field Model of a Cooper Pair Insulator Thomas Proctor, Eugene Chudnovsky, Dmitry Garanin The model of a disordered superconducting film with quantum phase fluctuations is mapped on a random-field XY spin model in 2+1 dimensions. Analytical studies within continuum field theory, supported by our recent numerical calculations on discrete lattices, show the onset of the low-temperature Cooper pair insulator phase. The constant external field in the random-field spin model maps on the Josephson coupling between the disordered film and a bulk superconductor. Such a coupling, if sufficiently strong, restores superconductivity in the film. This provides an experimental test for the quantum fluctuation model of a superinsulator. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y35.00014: Collective effects in the two-dimensional Josephson junction array Valerii Vinokour, Ivan Sadovskyy, Alexey Galda We study collective quantum effects in the two-dimensional Josephson junction arrays (JJA) in the vicinity of the superconductor-insulator transition (SIT). We find the contribution of the quantum coherent phase slips (QCPS) into the formation of thermodynamic properties of the JJA, including critical current, as a function of the magnetic field. We investigate the response of the 2D JJA to the external bias and the contribution from QCPS to this response. [Preview Abstract] |
Session Y36: Novel Superconductors III
Sponsoring Units: DCMPChair: N. Peter Armitage, Johns Hopkins University
Room: 344
Friday, March 22, 2013 8:00AM - 8:12AM |
Y36.00001: Spin incommensurability varies linearly with hole content in single-layer Bi2201 cuprate John Tranquada, M. Enoki, M. Fujita, T. Nishizaki, K. Yamada, S. Iikubo, D.K. Singh, S. Chang We have performed inelastic neutron scattering measurements on the single-layer cuprate Bi$_{2+x}$Sr$_{2-x}$CuO$_{6+y}$ (Bi2201) with $x=0.2$, 0.3, 0.4 and 0.5, a doping range that spans the spin-glass (SG) to superconducting (SC) phase boundary [1]. The doping evolution of low energy spin fluctuations ($\la 11$~meV) was found to be characterized by a change of incommensurate modulation wave vector from the tetragonal [110] to [100]/[010] directions, while maintaining a linear relation between the incommensurability and the hole concentration, $\delta\approx p$. In the SC regime, the spectral weight is strongly suppressed below $\sim4$~meV. Similarities and differences in the spin correlations between Bi2201 and the prototypical single-layer system La$_{2-x}$Sr$_x$CuO$_4$ will be discussed.\\[4pt] [1] M. Enoki {\it et al.}, arXiv:1205.3301. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y36.00002: Strong coupling behavior of the neutron resonance mode in unconventional superconductors Patrik Hlobil, Boris Narozhny, Joerg Schmalian A number of unconventional superconductors are characterized by a resonance mode in the spin excitation spectrum, measured via inelastic neutron scattering, which emerges below the superconducting transition temperature and is sharp as function of momentum and energy. A promising theory for the resonance is based on the analysis of the particle-hole spectrum in the superconducting state and in the presence of antiferromagnetic fluctuations. In this theory, a resonance occurs in case of a sign change of the superconducting gap function for momenta on the Fermi surface that are coupled by the antiferromagnetic ordering vector. So far, the theory was analyzed without including higher order vertex corrections of the particle-hole spectrum. In this work we analyze such vertex correction and show that: i) the qualitative difference in the behavior between a gap that changes sign and that doesn't change sign remains if one includes higher order vertex corrections, ii) vertex corrections are of order unity and cannot be ignored. Thus, while the resonance mode does seem to be a reliable fingerprint for an unconventional, sign-changing order parameter, it is a strong coupling phenomenon and no reliable approach to determine its detailed behavior seems to exist. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y36.00003: Distinct Fe-induced magnetic states in the underdoped and overdoped regimes of La$_{2-x}$Sr$_x$Cu$_{1-y}$Fe$_y$O$_4$ revealed by muon spin relaxation Kensuke Suzuki, Tadashi Adachi, Youichi Tanabe, Hidetaka Sato, Risdy Risdiana, Yasuyuki Ishii, Takao Suzuki, Isao Watanabe, Yoji Koike Zero-field muon-spin-relaxation measurements have been performed in partially Fe-substituted La$_{2-x}$Sr$_x$Cu$_{1-y}$Fe$_y$O$_4$ in a wide range of hole concentration, to investigate the magnetic state induced by the Fe substitution recently suggested from the neutron-scattering measurements [1]. It has been found that a static magnetic order is formed in 1\% Fe-substituted La$_{2-x}$Sr$_x$Cu$_{1-y}$Fe$_y$O$_4$ in a wide range of hole concentration where superconductivity appears in Fe-free La$_{2-x}$Sr$_x$CuO$_4$. In the underdoped regime, the Fe-induced magnetic order can be understood in terms of the concept of stripe pinning by Fe. In the overdoped regime, on the other hand, the Fe-induced magnetic order is short-ranged, which is distinct from the stripes. It is plausible that a spin-glass state of Fe spins derived from the RKKY interaction is realized in the overdoped regime. These results suggest a change of the electronic state from the strongly correlated electron state to the Fermi-liquid-like state with hole doping in La-214 high-$T_{\rm{c}}$ cuprates [2,3].\\[4pt] [1] R.-H. He et al., Phys. Rev. Lett. 107, 127002 (2011).\\[0pt] [2] K. Suzuki et al., Phys. Procedia 30, 275 (2012).\\[0pt] [3] K. M. Suzuki et al., Phys. Rev. B 86, 014522 (2012). [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y36.00004: Test of Variational Methods for Electronic Structures of Solid State and Molecular Systems by Application to Atomic Systems R.H. Pink, S.R. Badu, R.H. Scheicher, T.P. Das The Linked Cluster Many-Body Perturbation Theory [1,2] uses energies and wave-functions obtained from the one-electron Hartree-Fock equations for the ground state to determine the occupied states' contribution to properties such as magnetic hyperfine interaction. Both the occupied and unoccupied bound and continuum state energies and wave-functions are then used to include many-body effects through perturbation theory. This method has been found to provide excellent agreement between theoretical and experimental values for hyperfine constants for atomic systems [3,4]. Due to their multi-center nature, one cannot solve the Hartree-Fock differential equations by numerical integration methods for solid state and molecular systems, and must instead use variational methods [5,6,7]. We shall present our assessment of the accuracy of the variational procedure by determining the hyperfine constants for the Phosphorous [3] and Lithium [2] atoms. [1] Hugh P. Kelly, Phys. Rev. 144, 39 (1966) [2] E.S. Chang, R.T. Pu and T.P. Das, Phys. Rev. 174, 1 (1968) [3] N.C. Dutta, C. Matsubara, R. T. Pu, and T.P. Das, Phys. Rev. Lett. 21, 1139 (1968) [4] J. Andriessen, K. Raghunathan, S.N. Ray and T.P. Das, Phys Rev. B15, 2533 (1977) [5] C.C.J. Roothaan, Rev. Mod. Phys. 23, 69 (1951) [6] J.E. Rodgers and T.P. Das, Phys. Rev. A8, 2195 (1973) [7] W.J. Hehre, R.F. Stewart, and J.A. Pople, J. Chem. Phys. 51, 2657 (1969) [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y36.00005: Understanding of Nuclear Quadrupole Interaction of $^{19}$F* and Binding Mechanism in Solid Fluorine at First-Principles Level D.R. Mishra, M.M. Aryal, N.P. Adhikari, R.H. Pink, T.P. Das We have carried out a theoretical study of the nuclear quadrupole interaction (NQI) parameters of $^{19}$F* excited nuclear state in solid fluorine as well as the intermolecular binding of fluorine molecules in the solid. This is in continuation of our investigation [1] of the properties of solid halogens using the first-principles Hartree-Fock (HF) cluster procedure combined with many-body perturbation theory (MBPT), implemented by the Gaussian 03 set of programs. For the NQI parameters, the value of ($e^{2}$\textit{qQ/h}) obtained from our investigation for the $^{19}$F* excited nuclear state in solid fluorine is 120.9 MHz, which agrees with the experimental value 127.2 MHz, quoted in [2], within 5{\%} and the asymmetry parameter, $\eta $ is essentially zero. For obtaining ($e^{2}$\textit{qQ/h}) the value of the quadrupole moment, $Q$ for $^{19}$F* is taken from [3] as 0.072 x 10$^{28}$ m$^{2}$. As regards the binding of fluorine molecules in solid fluorine, our quantitative binding energy results show that the binding arises mainly from the van der Waals interaction obtained from intermolecular many-body effects with the one electron HF contribution being weak and repulsive in nature.\\[4pt] [1] M.M. Aryal et al., Hyperfine Interact, 176, 51 (2007). \\[0pt] [2] K.C.Mishra et al.,Phys. Rev.B25, 3389(1982).\\[0pt] [3] H. Barfuss et al., Phys. Lett. 90A, 33(1982). [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y36.00006: Enhanced charge stripe order in superconducting La$_{\mathrm{2-x}}$Ba$_{\mathrm{x}}$CuO$_{4}$ in high magnetic fields M. Huecker, M. v. Zimmermann, Z.J. Xu, J.S. Wen, G.D. Gu, J.M. Tranquada There is mounting evidence for a proximity of the superconducting ground state in the cuprates to competing states with static spin and/or charge density modulations. One such competing state is the spin and charge stripe phase in La$_{\mathrm{2-x}}$Ba$_{\mathrm{x}}$CuO$_4$. By means of high energy (100 keV) x-ray diffraction we have studied the effect of a high magnetic field (H\textbar \textbar c) on the charge stripe order in a broad range of doping (0.095 $\le $ x $\le $ 0.155). We find that the field can significantly enhance the charge stripe order, but only at temperatures and dopings where it coexists with bulk superconductivity at zero field. The field also increases stripe correlations between the planes, which can result in an enhanced frustration of the interlayer Josephson coupling. Close to the famous x$=$1/8 compound, where zero field stripe order is pronounced and bulk superconductivity is suppressed, charge stripe order is independent of the field. The results imply that static stripe order and three-dimensionally coherent superconductivity are competing ground states. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y36.00007: Lifetime of Skyrmions in Cuprates and Other Layered Materials Liufei Cai, Eugene Chudnovsky, Dmitry Garanin Collapse of a skyrmion due to the discreteness of a crystal lattice in isotropic two-dimensional ferro- and antiferromagnets has been studied analytically and by numerical solution of equations of motion for up to 2000$\times$2000 classical spins on a square lattice coupled via Heisenberg exchange interaction. Excellent agreement between analytical and numerical results has been achieved. The lifetime of the skyrmion scales with its initial size, $\lambda_0$, as $(\lambda_{0}/a)^{5}$ in ferromagnets and as $(\lambda_{0}/a)^{2.15}$ in antiferromagnets, with $a$ being the lattice parameter. This makes antiferromagnetic skyrmions significantly shorter lived than ferromagnetic skyrmions. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y36.00008: Fractional Flux Quantization in Loops of Unconventional Superconductors Florian Loder, Arno Kampf, Thilo Kopp The magnetic flux threading a conventional superconducting ring is typically quantized in units of $\Phi_0=hc/2e$. The factor 2 in the denominator of $\Phi_0$ originates from the existence of two different types of pairing states with minima of the free energy at even and odd multiples of $\Phi_0$. Here we show that spatially modulated pairing states exist with energy minima at fractional flux values, in particular at multiples of $\Phi_0/2$. In such states condensates with different center-of-mass momenta of the Cooper pairs coexist. The proposed mechanism for fractional flux quantization is discussed in the context of cuprate superconductors, where $hc/4e$ flux periodicities as well as uniaxially modulated superconducting states were observed. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y36.00009: Magnetic structures in YBCO single crystals under tilted magnetic fields Vitalii Vlasko-Vlasov, Ulrich Welp, Alexei Koshelev, Wai Kwok We study magnetic flux distributions in YBCO single crystals remagnetized by magnetic fields of different orientations using the magneto-optic indicator technique. Application of the perpendicular field to the crystals cooled in the in-plane magnetic field, application of the in-plane field to the crystals cooled in the normal magnetic field, and remagnetization by magnetic field tilted to the sample surface result in unusual quasiperiodic vortex structures. These strongly inhomogeneous vortex patterns can be associated with the flux cutting and strong anisotropy of the vortex kink motion depending on the trapped flux and external field orientations. We discuss the effect of resulting inhomogeneous current distributions on the current carrying ability of the YBCO coated conductors. Work supported by the US DoE-BES funded Energy Frontier Research Center and by Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y36.00010: Vortex lock-in transition coinciding with the 3D to 2D crossover in YBa$_2$Cu$_3$O$_7$ Saskia Bosma, Stephen Weyeneth, Roman Puzniak, Andreas Erb, Hugo Keller Dimensionality is essential to understand the behavior of vortices in layered cuprate superconductors. A 3D (three-dimensional) to 2D (two-dimensional) crossover takes place when the out-of-plane coherence length becomes smaller than the interplane distance. We directly detected a vortex lock-in transition by torque magnetometry in an overdoped YBa$_2$Cu$_3$O$_{7-\delta}$ single crystal of low anisotropy. The locked-in state was observed below the 3D to 2D crossover temperature, independently of extrinsic pinning effects thanks to a high quality clean crystal and the use of a vortex shaking technique. The lock-in is enhanced by decreasing temperature and increasing magnetic field. The shape of the torque signal is in very good agreement with the model developped by Feinberg and Ettouhami [Int. J. Mod. Phys. B {\bf 7}, 2085 (1993)] for quasi-2D superconductors, despite the low anisotropy of the material. Additionally, we present a new torque magnetometer design featuring vortex shaking, and compatible with the {\it Quantum Design} PPMS system. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y36.00011: Investigating the low-field vortex lattice phase diagram in CeCoIn$_5$ with $H \parallel c$ P. Das, M. R. Eskildsen, E. M. Forgan, H. Kawano-Furukawa, C. Petrovic Here we present small angle neutron scattering studies of the vortex lattice (VL) phase diagram in CeCoIn$_5$ in the low-field high-temperature regime with $H \parallel c$ which remained unexplored. While previous studies [A. D. Bianchi $et. al.$ Science {\bf{319}}, 177 (2008)] reported the phase boundary between the high-field square $\rightarrow$ rhombic $\rightarrow$ hexagonal VLs, the lower boundary between hexagonal $\rightarrow$ rhombic $\rightarrow$ square remained unexplored at higher temperatures where only estimates were provided. We have investigated this regime and mapped out these VL transitions. Interestingly, at the base temperature, no rhombic phase is observed but a direct transition from hexagonal to square phase. A possible explanation for this deviation from earlier reports may be that the current measurements were done following a field-ramp at base temperature rather than field-cool used in previous measurements. This indicates a slight hysteresis associated with this transition. While the measured hexagonal to rhombic phase transition agrees with the earlier estimates, the square VL phase occupies a larger region at higher temperatures than previously estimated. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y36.00012: Vortices in superconducting MoGe pentagon Takekazu Ishida, Ho Thanh Huy, Masaru Kato, Masahiko Hayashhi Vortices in bulk prefer to form a triangular lattice while a mesoscopic superconductor with a size comparable to coherence length $\xi $ or the magnetic penetration depth $\lambda $ is quite different so as to create particular configuration of vortices. The behavior of such structures in an external magnetic field is strongly influenced by the boundary conditions. Vortex states in superconducting disk, triangle and square pattern have been extensively studied both theoretically and experimentally [B. J. Baelus et al., Phys. Rev. B 69, 064506 (2004)]. We present vortex structures in MoGe pentagon disks imaged by means of a scanning quantum interference device (SQUID) microscopy [Ho Thanh Huy et al., Physica C, in press; DOI 10.1016/j.physc.2012.03.037.] Systematic measurements allow us to reveal how vortex arrangement evolves with the applied magnetic field. Moreover, we found that shell filling rule is subjected to change when a pinning center is introduced. Numerical calculations of vortex structure in pentagon disks on the basis of the nonlinear Ginzburg-Landau theory reveal that there are good agreement between experimental data and theoretical calculations. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y36.00013: Unconventional Vortex States in Nanoscale Superconductors Due to Shape-Induced Resonances in the Inhomogeneous Cooper-pair Condensate Ling-Feng Zhang, Lucian Covaci, Milorad Milosevic, Golibjon Berdiyorov, Francois Peeters Vortex matter in mesoscopic superconductors is known to be strongly affected by the geometry of the sample. Here we show that in nanoscale superconductors with coherence length comparable to the Fermi wavelength the shape resonances of the order parameter results in an additional contribution to the quantum topological confinement leading to unconventional vortex configurations. Our Bogoliubov de Gennes calculations in a square geometry reveal a plethora of asymmetric, giant multivortex, and vortex antivortex structures, stable over a wide range of parameters and which are very different from those predicted by the Ginzburg Landau theory. By modifying the size of the system and the Fermi energy we show that ground states with different symmetries can be obtained. By increasing the temperature we observe first-order transitions from multivortex to giant vortex states. These unconventional states are relevant for high Tc nanograins, confined Bose Einstein condensates, and graphene flakes with proximity induced superconductivity. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y36.00014: Competition between covalent bonding and charge transfer tendencies at complex-oxides interfaces J. Salafranca, J. Tornos, J. Garc\'Ia-Barriocanal, C. Le\'on, J. Santamaria, J. Rinc\'on, G. \'Alvarez, S.J. Pennycook, E. Dagotto, M. Varela Interfaces alter the subtle balance among different degrees of freedom responsible for exotic phenomena in complex oxides, such as cuprate-manganite interfaces. We study these interfaces by means of scanning transmission electron microscopy and theoretical calculations. Microscopy and EEL spectroscopy indicate that the interfaces are sharp, and the chemical profile is symmetric with two equivalent interfaces. Spectroscopy also allows us to establish an oxidation state profile with sub-nanometer resolution. We find an anomalous charge redistribution: a non-monotonic behavior of the occupancy of d orbitals in the manganite layers as a function of distance to the interface. Relying on model calculations, we establish that this profile is a result of the competition between standard charge transfer tendencies involving materials with different chemical potentials and strong bonding effects across the interface. The competition can be tuned by different factors (temperature, doping, magnetic fields...). As examples, we report different charge distributions as a function of doping of the manganite layers. ACKNOWLEDGEMENTS ORNL:U.S. DOE-BES, Material Sciences and Engineering Division \& ORNL's ShaRE. UCM:Juan de la Cierva, Ramon y Cajal, \& ERC Starting Investigator Award programs. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y36.00015: ABSTRACT WITHDRAWN |
Session Y37: Focus Session: Fe-based Superconductors: Vortices and Critical Fields
Sponsoring Units: DMP DCOMPChair: Morten Eskildsen, University of Notre Dame
Room: 345/346
Friday, March 22, 2013 8:00AM - 8:36AM |
Y37.00001: Slow Abrikosov- to fast moving Josephson-vortex transition in iron-pnictide superconductors Invited Speaker: Philip Moll We have observed a novel type of transition of vortex matter from well-pinned Abrikosov to highly mobile Josephson vortices in the iron pnictide high-$T_c$ superconductor SmFeAs(O,F) ($T_c \sim 50K$). This A-to-J transition between the two regimes upon cooling through the temperature $T^*$ is hallmarked by an extraordinary jump of vortex mobility and a pronounced peak in the critical current density. The dissipation below $T^*$ reaches significant fractions of the normal state resistance at all temperatures and fields, far below $H_{c2}\|ab$, estimated well above $100T$ at low temperatures. We show the temperature $T^*$ to coincide with the temperature at which the interlayer coherence length $\xi_c(T)$ equals the SmO layer thickness, hence leading to Josephson-like vortices below and Abrikosov-like vortices above $T^*$. This transition is surprising, as the material is an only moderately anisotropic superconductor ($\gamma \sim 5 - 7$), unlike strongly anisotropic, clearly two-dimensional cuprates. The observation of this A-to-J transition highlights the significance of structural layeredness and gives microscopic information about the order parameter in SmFeAs(O,F). This profound change in the nature of the vortex matter in these compounds has eluded discovery until now, as its detection poses two main experimental challenges: The Josephson nature of the vortex matter may only be observed (1) for fields precisely aligned with the FeAs layers($<0.1$deg). Even slightest field misalignments away from the FeAs planes ($>0.1$deg) restore dissipation free current transport and very high critical current densities ($\sim 10^6 A/cm^2$) at low temperatures. Secondly (2), currents flowing perpendicular to the layers are essential for the observation, forcing the vortices to slide between the layers. To this end, thin ($<10\mu m$) high quality single crystals of SmFeAs(O,F) were microstructured and contacted using a Focused Ion Beam. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y37.00002: Superconductivity amid phase inhomogeneity: the case of K$_{x}$Fe$_{2-y}$Se$_{2}$ Despina Louca The recently discovered Fe-based superconductors, K$_{x}$Fe$_{2-y}$Se$_{2}$, is studied using neutron diffraction and the pair density function analysis to investigate the nature of the atomic disorder induced by the K and Fe site vacancies. In this system, both superconductivity and magnetic ordering can coexist, while superconductivity is observed in a narrow range of potassium concentration, between 0.6 $<$ x $<$ 0.8. While no crystal transition occurs across with x, the Fe site vacancies are ordered in the $\sqrt{5} \times \sqrt{5}$ structure. At high temperatures, the Fe vacancies are not ordered. Why does superconductivity appear in the vicinity of the 0.8 composition? To provide a clue towards the answer, instead of probing the periodic structure, we probed the local atomic structure that provides information regarding the short-range correlations in real space. The results suggest a strong dependence of the Fe-Fe bond lengths to the K concentration. What is unique to this system is that a double-well bond distribution of short and long Fe - Fe bonds exists, originating from the fully occupied Fe site. As the K concentration increases to x=1, the distribution shifts weight from the short to the long while in the superconducting case, it is equal between the two. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y37.00003: Angle and frequency dependent low field microwave absorption in electronically doped Ca 122 pnictides: Comparison of high Tc = 42 K phase in Pr, Nd, Ce and La doping Austin Howard, Jonathan Yuen, Myron Salamon, Anvar Zakhidov, Bing Lv, Paul C. W. Chu, Daniel Sells The motivation of this study is to investigate the properties of a unique interfacial superconducting phase in electron-doped Ca 122 pnictides by the Low Field Microwave Absorption (LFMA) technique. Samples are exposed to microwave radiation with frequency $\nu_{MW}$ between 1 and 24 GHz, and also to a low strength magnetic field which modulates at $\nu_{mag} = 100$ kHz. Due to their single crystalline nature, the pnictides can be oriented relative to the MW polarization and magnetic field direction. Studying this orientation dependence reveals filament-like micro-interfaces between highly doped regions and poorly doped regions. These interfaces may be responsible for a high $T_c \sim 42$ K superconducting phase. We demonstrate that this higher $T_c$ can be clearly distinguished from the low temperature bulk SC phase by the angle-dependent LFMA method. Additionally, variation of the MW frequency yields changes in the spectra which are in agreement with theoretical predictions. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y37.00004: High, magnetic field independent critical currents in Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ with composite defects K. Kihlstrom, L. Fang, Y. Jia, C. Chaparro, G. Sheet, H. Claus, A. Koshelev, U. Welp, G. Crabtree, W. Kwok, S. Zhu, A. Kayani, H.F. Hu, J.M. Zuo, H.H. Wen, B. Shen We investigate the enhancement of vortex pinning by compound defects that are composed of correlated and point defects in Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ crystals with T$_{c}$ 37.5. Initial irradiation by high-energy heavy ions to a dose matching field of $B=$\textit{21}T increases vortex pinning via columnar defects with no degradation of the superconducting transition temperature. Subsequent proton irradiations further enhance the critical current \textit{Jc(H) }by suppressing the motion of vortex kinks between the columnar defects. At a temperature of 5K, we find a critical current density of 5.8 MA/cm$^{2}$ that is essentially magnetic field independent in fields up to 7 T. This work supported by the Center for Emergent~Superconductivity, an Energy Frontier Research Center funded by the U.S. D.O.E., Office of Science, Office of Basic Energy Sciences and by the D.O.E, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The operation of the ATLAS facility was supported by the U.S. D.O.E., Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357. The work in China was supported by the NSF of China, the MOST of China (2011CBA00102 and 2012CB821403) and PAPD. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y37.00005: Angle - dependent upper critical field of overdoped Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ Jason Murphy, M.A. Tanatar, N. Ni, S.L. Bud'ko, P.C. Canfield, R. Prozorov, D. Graf In-plane resistivity measurements were used to study the upper critical field, $H_{c2}$, of single crystals of iron-based superconductor Ba(Fe$_{1-x}$Ni$_{x}$)$_{2}$As$_{2}$ ($x=0.054$ and $x=0.072$). An applied magnetic field (up to 35 T) was precisely aligned (with the accuracy better than 0.1$^{o}$) parallel to the Fe-As layers and the measurements were taken for $H\parallel ab-$ plane and $H\parallel c-$axis as function of temperature. The determined $H_{c2}(T)$ clearly differs for the two principal directions. The dependence of the upper critical field on the angle $\Theta$ between the field direction and the $ab-$plane was measured in isothermal conditions at temperatures close to $T_{c0}$ and at low temperatures $T\ll T_{c}(H=0)$. In both temperature regimes $H_{c2}(\Theta)$ clearly deviates from sinusoidal function, expected for orbital $H_{c2}$ [1]. We discuss the origin of this behavior as possible reflection of the angular modulation of the superconducting gap magnitude and the complex warping of the Fermi surface along the $c$-axis. Work in Ames was supported by the Department of Energy Office of Science, Basic Energy Sciences under Contract No. DE-AC02-O7CH11358.\\[4pt] [1] V. G. Kogan and R. Prozorov, Rep. Prog. Phys. \textbf{75}, 114502 (2012). [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y37.00006: Multiband, paramagnetic effects and vortices in KFe$_2$As$_2$ Frederic Hardy, Dai Aoki, Robert Eder, Ilya Vekhter, Philipp Burger, Anna Boehmer, Robert Fisher, Thomas Wolf, Christoph Meingast We study the normal- and superconducting-state properties of the iron pnictide superconductor KFe$_{2}$As$_{2}$ using heat-capacity, thermal-expansion and magnetization measurements. In the normal state, our data show strong evidence of the existence of strong local fluctuations and of the coherence-incoherence crossover predicted by theory. In zero field, for T \textless\ T$_{\mathrm{c}}$, the temperature dependence of the heat capacity provides evidence for the existence of extremely small energy gaps. The (H,T) phase diagram is also determined down to 80 mK using calorimetric measurements. We reveal the existence of strong paramagnetic effects for field parallel to the Fe$_{\mathrm{2}}$As$_{2}$ planes. We discuss the symmetry of the order parameter and the interplay between multiband, paramagnetic and orbital effects. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y37.00007: Field dependence of the thermal conductivity in the iron-based superconductor KFe$_2$As$_2$ A. Juneau-Fecteau, F.F. Tafti, S. Ren\'e de Cotret, N. Doiron-Leyraud, L. Taillefer, A.F. Wang, X.G. Luo, X.H. Chen The behavior of the thermal conductivity in the iron-arsenide KFe$_2$As$_2$ at low temperature provides compelling evidence of d-wave superconductivity [1]. Here we report a detailed study of the thermal conductivity in KFe$_2$As$_2$ as a function of magnetic field, for two field orientations: perpendicular and parallel to the FeAs planes. The data are in excellent quantitative agreement with theoretical calculations for a d-wave superconductor [2]. Our study also highlights the power of thermal conductivity as a technique to directly measure the upper critical field H$_{c2}$ in a clean type-II superconductor. \\[4pt] [1] J.-Ph. Reid et al., Phys. Rev. Lett. 109, 087001 (2012).\\[0pt] [2] A. B. Vorontsov and I. Vekhter, Phys. Rev. B 75, 224502 (2007). [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y37.00008: Microwave Surface Impedance Measurements on Fe(Se,Te) Single Crystals under Finite Magnetic Fields Hideyuki Takahashi, Tatsunori Okada, Fuyuki Nabeshima, Shinji Koshida, Yoshinori Imai, Atsutaka Maeda We measured the microwave surface impedances of Fe(Se,Te) single crystals under magnetic fields up to 8 Tesla and extracted the flux flow resistivity, $\rho_f$, to investigate the quasiparticle dynamics inside the vortex core. Previously performed $\rho_f$ measurements on several iron-based superconductors have revealed that the quasiparticle dynamics inside the vortex core can be described as that in the so-called ``moderately clean'' regime, in which the mean free path is comparable to the coherence length [1,2]. The mean free path in Fe(Se,Te) in the normal state is smaller than those in other superconductors. In addition, London penetration depth shows quadratic temperature dependence because of the strong pair-breaking [3]. Therefore, it is interesting to investigate the $\rho_f$ to clarify whether the strong quasiparticle scattering affects the quasiparticle dynamics inside the vortex core. We also discuss the surface impedances of Fe(Se,Te) thin films which have a higher $T_c$ than the bulk crystals. \\[4pt] [1] T. Okada $et$ $al.$, Phys. Rev. B {\bfseries 86} (2012) 064516.\\[0pt] [2] H. Takahashi $et$ $al.$, Phys. Rev. B {\bfseries 86} (2012) 144525.\\[0pt] [3] H. Takahashi $et$ $al.$, Phys. Rev. B {\bfseries 84} (2011) 132503.\\ [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y37.00009: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 10:12AM - 10:48AM |
Y37.00010: Flux flow of iron based superconductors Invited Speaker: Atsutaka Maeda Flux flow measured by the microwave technique is the only one possible technique to pick up the information on the quasiparticles in the vortex core. Theoretically, novel features have been suggested to show up in the flux flow of Fe-based superconductors (SCs) as multiple gapped SCs with possible sign changes. We investigated the flux flow resistivity of various different types of Fe based SCs, such as 111, 122, and 11 systems [1]. It is found that (1) the sign change is not important for the flux flow, (2) we can discuss the gap structure based on the flux flow data, even without performing angle dependent measurement, (3) vortices of Fe-based SC dissipate more energy than expected from the properties in the Meissner state. Together with the flux flow result in many other superconductors such as cuprates, Y$_{2}$C$_{3}$, and boron carbides, our result suggests the existence of a universal mechanism of dissipation for quasiparticles in the vortex core, probably related with the Andreev reflection at the core boundary. We also discuss the flux flow of cuprate superconductors, in terms of superconductivity fluctuation investigated by ac conductivity and diamagnetisms.\\[4pt] [1] K. Okada \textit{et al}.: Phys. Rev. B86 (2012) 064516, H. Takahashi \textit{et al}.: Phys. Rev. B86 (2012) 144525. [Preview Abstract] |
Session Y38: Focus Session: Scalable Technologies for Photovoltaics II
Sponsoring Units: GERA FIAPChair: Janelle Leger, Western Washington University
Room: 347
Friday, March 22, 2013 8:00AM - 8:36AM |
Y38.00001: High Efficiency Photovoltaics -- The Key to Grid Parity Invited Speaker: David L. Young For three decades the photovoltaic (PV) industry has enjoyed roughly a 22\% price reduction for each doubling of cumulative production volume. Recently, the PV market has exceeded this trend with module prices dropping to all-time lows. This trend has come mainly from economies-of-scale, incremental efficiency increases, and over supply. However, this PV learning curve is likely to flatten (or even rise) as unsustainable profit margins weed competition and devices near minimal material usage and practical efficiencies. The current market climate, and the strong weighting factor of balance-of-system costs, favor higher efficiency devices. Technologies that cannot reach a minimum module efficiency of about 18\% will likely not be competitive. This paper will discuss several evolutionary and revolutionary scalable wafer and thin-film photovoltaic technologies that are likely to remain competitive, and will identify several areas within these technologies in need of scientific breakthroughs. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y38.00002: Local excitation and local collection of photocurrent in thin-film polycrystalline photovoltaic devices Nikolai Zhitenev, Heayoung Yoon, Marina Leite, Youngmin Lee, Sarah Ko, Yue Zhao, Anthony Gianfrancesco, Paul Haney, Alec Talin The power conversion efficiency of commercial solar modules based on thin-film chalcogenide materials is well below the theoretical limits. To understand the underlying physical mechanisms limiting the efficiency, we investigate local photovoltaic properties isolating the difference between the grain bulk (0.5-2 mkm in size) and the grain boundary in CdTe absorber. Local current-voltage measurements are performed using nano-contacts in conjunction with local electron-hole pairs generation comparing multiple injection techniques. First, the carriers are excited using variable energy electron beam enabling measurements with a spatial resolution down to 20 nm. Second, we have developed a novel approach for high-resolution and high-throughput photocurrent imaging downconverting electron beam into a near-field optical source using a thin film (\textless 50 nm) of phosphors. The electron beam is fully absorbed in the phosphors layer, and the cathodoluminescence is used as a local photon source. Third, we generate carriers using a near-filed optical microscope varying the excitation wavelength. The results show that, in a well-optimized material, a large fraction of grain boundaries displays higher photocurrent as compared to grain bulk effectively serving as a three-dimensional distributed photocurrent collector. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y38.00003: Development of InP Based Quantum Well Tunnel Junctions Michael Yakes, Matthew Lumb, Maria Gonzalez, Christopher Bailey, Igor Vurgaftman, Robert Walters In this presentation we demonstrate lattice-matched InAlGaAs quantum well tunnel junctions for an InP-based multi-junction cell. By including two 0.74 eV bandgap InGaAs quantum wells in InP-lattice matched InAlGaAs tunnel junctions with a 1.18eV bandgap, a peak tunnel current density of 113 A/cm$^{\mathrm{2}}$ was observed, 45 times greater than a baseline bulk InAlGaAs tunnel junction. The differential resistance of the quantum well device is 7.52 x 10$^{\mathrm{4}} \quad \Omega $cm$^{\mathrm{2}}$, a 15-fold improvement over the baseline device. The upper bound of the transmission loss to the bottom cell is estimated to be approximately 1.7{\%}. Strain balanced quantum wells will be discussed which have the same benefits of the latticed matched tunnel junctions, but can be made accessible to both InP and GaAs based multi-junction architectures. We will also show the results of a study where a bulk, double heterostructure design is used to mitigate the effects of dopant diffusion and maximize the peak tunnel current, achieving a 15 times improvement in peak tunnel current over the baseline device. We propose that quantum well tunnel junctions with bulk heterostructure diffusion barriers could play a key role in improving performance both at one sun and high sun concentrations. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y38.00004: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y38.00005: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y38.00006: Energy Alignment at Organic/Oxide and Organic/Metal Interfaces: The Effects of Molecular Overlayer Thickness on the HOMO/LUMO Gap and Interfacial Dipole Charles Ruggieri, Sylvie Rangan, Senia Coh, Robert Bartynski Dye-sensitized solar cells offer the potential for low-cost production with comparable efficiencies to traditional Si-based solar cells. Energy alignment of the dye orbitals with respect to the band edges of the oxide semiconductor substrate is a key parameter in device performance. Using direct and inverse photoemission, XPS, and STM we have investigated the electronic structure and bonding geometry of zinc tetraphenylporphyrin (ZnTPP) molecules adsorbed on a set of four oxide semiconductor and metallic substrates [TiO$_{2}$(110), ZnO(11-20), Ag(100) and Au(111)] at monolayer and multilayer coverages. The vacuum levels of the organic/oxide and organic/metal systems were also measured and the interface dipoles determined. The energy level shifts and the width of adsorbate spectral features are qualitatively different for molecules adsorbed on the oxide versus the metal substrates. The HOMO-LUMO energy separation decreases with decreasing molecular overlayer thickness, which is thought to be due to substrate screening properties, but these shifts occur in different ways for the two classes of substrates. Possible origins of this distinct behavior will be discussed. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y38.00007: Simulation and Testing of Type-II Strained-Layer Superlattices for Long Wavelength Thermophotovoltaics Abigail Licht, Dante DeMeo, Thomas Vandervelde In this presentation we detail our research on long wavelength thermophotovoltaic (TPV) cells, with cutoff wavelength in the 7-9 micron range, which hold the potential for a wide array of applications due to their ability to work with lower temperature sources. We will discuss simulation results on the optimization of structures utilizing type II strained-layer superlattice (SLS) cells and unipolar barriers. The performance of these simulated cells is compared with fabricated cells which were characterizing using calibrated blackbody sources. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y38.00008: Optimization of gain and energy conversion efficiency using front-facing photovoltaic cell luminescent solar concentrator design Melissa Osborn, Carley Corrado, Shin Woei Leow, Emory Chan, Ben Balaban, Sue Carter Luminescent solar concentrator (LSC) windows with front-facing photovoltaic (PV) cells were built and their gain and power efficiency were investigated. Conventional LSCs employ a photovoltaic (PV) cell that is placed on the edge of the LSC, facing inward. This paper describes a new design with the PV cells on the front-face allowing them to receive both direct solar irradiation and wave-guided photons emitted from a dye embedded in an acrylic sheet, which is optically coupled to the PV cells. Parameters investigated include the thickness of the waveguide, edge treatment of the window, cell width, and cell placement. The data allowed us to make projections that aided in designing windows for maximized overall efficiency. A gain in power of 2.2x over the PV cells alone was obtained with PV cell coverage of 5{\%}, and a power conversion efficiency as high as 6.8{\%} was obtained with a PV cell coverage of 31{\%}. Balancing the trade-offs between gain and efficiency, the design with the lowest cost per watt attained a power efficiency of 3.8{\%} and a gain of 1.6x. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y38.00009: Theoretical and Practical Limits for Transparent Photovoltaics Richard Lunt Transparent photovoltaics (TPVs) offer a new paradigm for solar energy harvesting, integration, and deployment. These devices have recently been shown to be enabled by exploiting the excitonic nature of molecular and organic semiconductors.\footnote{R. R. Lunt, and V. Bulovi\'{c}. Appl. Phys. Lett. 98, 113305, 2011.} Here, we present the theoretical and practical efficiency limits of these novel electronic architectures as a function of bandgap, transparency and aesthetic quality for both single and multi-junction cells. For example, power-production from ultraviolet and near-infrared photons alone leads to a theoretical single-junction efficiency of 21{\%} in completely transparent structures, compared to 33{\%} for opaque-junctions. This approach for transparent photovoltaics will be contrasted with other semi-transparent approaches, where TPVs with selective absorption offer the highest possible potential for combined transparency and efficiency. The impact of transparent PVs will be discussed for a range of applications from electronic displays to window integration. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y38.00010: Luminescent Solar Concentrators in the Algal Industry Katie Hellier, Carley Corrado, Sue Carter, Angela Detweiler, Leslie Bebout Today's industry for renewable energy sources and highly efficient energy management systems is rapidly increasing. Development of increased efficiency Luminescent Solar Concentrators (LSCs) has brought about new applications for commercial interests, including greenhouses for agricultural crops. This project is taking first steps to explore the potential of LSCs to enhance production and reduce costs for algae and cyanobacteria used in biofuels and nutraceuticals. This pilot phase uses LSC filtered light for algal growth trials in greenhouses and laboratory experiments, creating specific wavelength combinations to determine effects of discrete solar light regimes on algal growth and the reduction of heating and water loss in the system. Enhancing the optimal spectra for specific algae will not only increase production, but has the potential to lessen contamination of large scale production due to competition from other algae and bacteria. Providing LSC filtered light will reduce evaporation and heating in regions with limited water supply, while the increased energy output from photovoltaic cells will reduce costs of heating and mixing cultures, thus creating a more efficient and cost effective production system. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y38.00011: Monte Carlo Simulations of Luminescent Solar Concentrators with Front-Facing Photovoltaic Cells for Building Integrated Photovoltaics Shin Woei Leow, Carley Corrado, Melissa Osborn, Sue Carter Luminescent solar concentrators (LSCs) have the ability to receive light from a wide range of angles and concentrate the captured light on to small photo active areas. This enables LSCs to be integrated more extensively into buildings as windows and wall claddings on top of roof installations. LSCs with front facing PV cells collect both direct and concentrated light ensuring a gain factor greater than one. It also allows for flexibility in determining the placement and percentage coverage of PV cells when designing panels to balance reabsorption losses, power output and the level of concentration desired. A Monte-Carlo ray tracing program was developed to study the transport of photons and loss mechanisms in LSC panels and aid in design optimization. The program imports measured absorption/emission spectra and transmission coefficients as simulation parameters. Interactions of photons with the LSC panel are determined by comparing calculated probabilities with random number generators. Simulation results reveal optimal panel dimensions and PV cell layouts to achieve maximum power output. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y38.00012: Porous Silicon as Antireflecting Layer Gulsen Kosoglu, Mehmet Yumak, Selim Okmen, Ozhan Ozatay, Yani Skarlatos, Carlos Garcia The main aim in photovoltaic industry is to produce efficient and energy competitive solar cell modules at low cost. Efficient AntiReflection Coatings (ARC) improve light collection and thereby increase the current output of solar cells. Broadband ARCs are desirable for efficient application over the entire solar spectrum and porous silicon layers as antireflective coating layers provide successful light collection. In the study the most critical physical parameters of porous silicon are examined, homogeneous and uniform porous layers are produced. The photoluminescence spectrum and optical parameters of porous layers have been investigated, and we are now in the process of improving the efficiency of the device by modulating the structure of the porous silicon layers and studying its photovoltaic characteristics. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y38.00013: Transparent Luminescent Solar Concentrators for Large-area Solar Windows Yimu Zhao, Richard Lunt Luminescent solar concentrators (LSCs) have recently regained attention as a route for integration into the building envelope. To date, however, these systems have been limited to absorption and emission (glow) in the visible part of spectrum. We have designed and fabricated novel transparent luminescent solar concentrators devices composed of synthesized metal halide nanocrystal phosphorescent luminophores that allow for efficient and selective harvesting of ultraviolet (UV) photons with a near perfect absorption cutoff at the edge of the visible spectrum (430nm) while efficiently down-converting emitted light with a massive stoke shift to the near-infrared (800nm). We have demonstrated transparent LSCs with power efficiency of 0.8{\%} $\pm$ 0.5{\%}, system external quantum efficiency exceeding 35{\%}, and an average transmittance of 82{\%} $\pm$ 1{\%}. We show through experiments and modeling that this architecture has the potential to exhibit up to 1-2{\%} power conversion over module areas \textgreater\ 1 m$^{2}$. These concentrators present new opportunities for non-tinted and highly-adoptable solar- windows that can translate into improved building efficiency, enhanced UV-barrier layers, and lower cost solar harvesting systems. [Preview Abstract] |
Session Y39: Swimming, Motility and Locomotion
Sponsoring Units: DFDChair: Alexander Alexeev, Georgia Institute of Technology
Room: 348
Friday, March 22, 2013 8:00AM - 8:12AM |
Y39.00001: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y39.00002: Navigation and chemotaxis of nematodes in bulk and confined fluids Alejandro Bilbao, Venkat Padmanabhan, Kendra Rumbaugh, Siva Vanapalli, Jerzy Blawzdziewicz Small nematodes, such as the model organism \textit{C.\ elegans}, propel themselves by producing sinuous undulations along the body and perform turns by varying the undulation amplitude. We have recently demonstrated [PLoS ONE 7(7) e40121 (2012)] that such motions can be accurately represented in terms of a piecewise-harmonic body curvature. We combine our harmonic-curvature description with highly accurate hydrodynamic bead-chain models to investigate the swimming efficiency and turning capabilities of the worm in bulk and confined fluids. Our results indicate that for the same change of the curvature-wave amplitude, a swimming nematode turns by a smaller angle compared to a crawling worm. The difference is due to rotational slip with respect to the surrounding medium, but the angles are sufficiently large to allow for efficient turning maneuvers. We use our description of nematode maneuverability to study chemotaxis in both confined and unconfined fluids. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y39.00003: Nematode Chemotaxis: Gradual Turns, Sharp Turns, and Modulated Turn Angles Amar Patel, Venkat Padmanabhan, Kendra Rumbaugh, Siva Vanapalli, Jerzy Blawzdziewicz We examine strategies used by the soil-dwelling nematode \textit{Caenorhabditis Elegans} for chemotaxis in complex environments. The proposed description is based on our recently developed piecewise-harmonic-curvature model of nematode locomotion [PLoS ONE, 7(7) e40121 (2012)], where random harmonic-curvature modes represent elementary locomotory movements. We show that the previously described gradual-turn and sharp-turn chemotaxis strategies can be unified in our model. The gradual-turn mechanism relies on crawling amplitude changes commensurate with the undulation frequency. The sharp-turn mechanism consists in modulation of the frequency of jumps to large-amplitude modes. We hypothesize that there exists a third strategy, where the nematode adjusts the variance of the amplitude distribution. Such adjustments result in a modulation of the magnitude of random turns, with smaller turns performed when the nematode moves toward the increasing chemoatractant concentration. Experiments are proposed to determine if the third strategy is present in the nematode behavior. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y39.00004: Simulation of model swimmers near ciliated surfaces Henry Shum, Anurag Tripathi, Julia Yeomans, Anna Balazs Biofouling by micro-organisms is problematic on scales from microfluidic devices to the largest ships in the ocean. One solution found in nature for clearing undesired material from surfaces is to employ active cilia, for example, in the respiratory tract. It is feasible to fabricate surfaces covered with artificial cilia actuated by an externally imposed field. Using numerical simulation, we investigate the interactions between these artificial cilia and self-propelled model swimmers. One of the key aims is to explore the possibility of steering swimmers to influence their trajectories through the flow field produced by the cilia. In our simulations, the fluid dynamics is solved using the lattice Boltzmann method while the cilia and model swimmers are governed by elastic internal mechanics. We implement an immersed boundary approach to couple the solid and fluid dynamics. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y39.00005: Underwater propulsion of an internally actuated elastic plate Peter Yeh, Lejun Cen, Alper Erturk, Alexander Alexeev Combining experiments and numerical simulations we examine underwater locomotion of an active (internally powered) flexible bimorph composite. We use Macro-Fiber Composite (MFC) piezoelectric laminates that are actuated by a sinusoidally varying voltage generating thrust similar to that of a flapping fin in carangiform motion. In our fully-coupled three dimensional simulations, we model this MFC bimorph fin as a thin, elastic plate that is actuated by a time-varying internal moment producing periodic fin bending and oscillations. The steady state swim velocity and thrust are experimentally measured and compared to the theoretical predictions. Our simulations provide detailed information about the flow structures around the swimming fin and show how they affect the forward motion. The results are useful for designing self-propelling fish-like robots driven by internally powered fins. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y39.00006: Flow generated by an oscillated elastic filament in viscous fluids Moumita Dasgupta, Arshad Kudrolli We discuss with experiments the interplay of periodic driving, elasticity, and damping of a cilium in a viscous fluid and the resulting fluid flow. In particular, we oscillate an elastic filament made of PDMS in a viscous Newtonian fluid and observe the generated flow using PIV techniques. The competition between viscous drag and elasticity of the filament is observed to lead to symmetry breaking, resulting in a net flow. The length of the filament is varied to find an optimum length at which maximum net flow is obtained for a given elastic constant of the material and oscillating frequency. We discuss the related coupled oscillator system, and the rich dynamics observed in the context of fluid flow generated by elastic flagella and cilia. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y39.00007: Transmutation of rotational motion into translational diffusion in 3D rotary powered random walkers Amir Nourhani, Paul Lammert, Ali Borhan, Vincent Crespi Experimenters have for several years been studying motors with sizes in the $10^{-1}$--$10^{0}$ micron range which execute circular motion on scales as small as the motor dimensions in an aqueous environment. Previously, we have studied the normal situation wherein the motor is confined to a plane. Here we consider the case where such confinement is absent. The orbital motion of a particle undergoing regular circular motion in 3D has three rotational degrees of freedom. The introduction of stochasticity into them gives rise to 3D translational motion. A special, and apparently experimentally relevant, case is that of an orbiter in the plane which can flip over, reversing its chirality. We present analytical and simulation results on these transmutations of rotational motion into translational motion [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y39.00008: ABSTRACT WITHDRAWN |
Session Y40: Surfaces, Interfaces, and Thin Films: Oxides
Sponsoring Units: DCMPChair: Michael Pierce, Rochester Institute of Technology
Room: 349
Friday, March 22, 2013 8:00AM - 8:12AM |
Y40.00001: Metal Oxide Growth and Characterization on CVD Graphene Akitomo Matsubayashi, Joseph Abel, Dhiraj Prasad Sinha, Ji Ung Lee, VIncent LaBella Thin metal oxide layers deposited on graphene can be utilized as dielectric barriers between metals and graphene to help isolate a metal contact from the graphene channel. This is important for graphene based spintronic devices as dielectric layers between the ferromagnetic electrode and graphene have been shown to increase the spin relaxation time measured utilizing non-local detection and spin precession measurements. However, simply depositing metal oxide layers such as aluminum oxide on graphene results in non-uniform film lowering the quality of the interface barrier. We will present a systematic study of aluminum oxide layers grown on CVD (chemical vapor deposition) graphene under ultra-high vacuum conditions with and without titanium seed layers. The aluminum oxide layers with the titanium seed layers showed reduced surface roughness. The chemical and structural composition determined by XPS (X-ray photoelectron spectroscopy) will be also presented that shows full oxidation of the aluminum and partial oxidation of the titanium. Our previous work which demonstrated that introducing HfO2 barrier layer in the epitaxial graphene devices on SiC wafer improves the measured lifetime and spin injection efficiency will be briefly presented as well. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y40.00002: Atomically Resolved Surface of Laser-MBE Grown SrRuO$_ 3$ Thin Films A. Tselev, P. Ganesh, A.P. Baddorf, S.V. Kalinin Surface of SrRuO$_3$ (SRO) thin films is of high interest since SRO layers are used as bottom electrodes in oxide heterostructures demanding sharp interfaces. Here we studied SRO films in-situ using STM with atomic resolution. Films were grown on undoped, SrTiO$_3$ substrates by laser-MBE. Depending on preparation conditions, the film surfaces exhibited varying reconstructions. Films deposited at 650$^{\circ}$C and annealed at deposition conditions for 15 min. revealed surfaces with double-row 1D-structures along $<110>_{pc}$ of SRO. Atoms in the 1D-structures are packed in square or zigzag arrangements. The surface in-between the structures appeared poorly ordered. Similar patterns were observed on surfaces of films deposited at 700$^{\circ}$C without anneal. In turn, deposition at 700$^{\circ}$C with post-anneal resulted in well-ordered surfaces covered by double-rowed structures with square atomic arrangement. Ab initio DFT calculations show a high local DOS from oxygen adatoms with zigzag and square patterns contributing to STM images. Oxygen atoms have high adsorption energies and will be present at our growth conditions. Surface O-adatoms show AFM coupling to the film, with possible ramifications to understand interfacial bonding/magnetism between SRO and oxide-insulators. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y40.00003: Transport phenomena in SrVO$_3$ thin films Man Gu, Stuart Wolf, Jiwei Lu Bulk SrVO$_{3}$ (SVO) with a 3$d^{1}$ electronic configuration has been found to exhibit metallic and Pauli paramagnetic behavior. We have obtained epitaxial SVO films grown on various substrates (STO, SLAO, LSAT and LAO) using a pulsed electron-beam deposition (PED) technique. The film transport properties were found to be strongly dependent on the substrate. A 40 nm SVO film deposited on an STO substrate exhibited metallic behavior with the electrical resistivity following a T$^{2}$ law that corresponds to a Fermi liquid system, the resistance ratio R(300K)/R(2K) was $\sim$ 1.66. Hall measurements showed that the mobility increased slightly as the temperature was decreased. A small positive out-of-plane magnetoresistance was observed, it was only 0.045{\%} at 5 K and 7 Tesla. SVO films with the same thickness grown on SLAO, LSAT and LAO showed semiconducting behavior, the different transport properties in the SVO films could be attributed to the compressive film strain or the different film-substrate interfaces. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y40.00004: High quality, hybrid-MBE growth of SrVO$_{3}$ thin films Jarrett Moyer, Craig Eaton, Roman Engel-Herbert Vanadium-based transition metal oxides are an intriguing class of materials to study due to the metal-to-insulator (MIT) transitions that arise in many of the binary oxides (i.e. VO$_{2}$, V$_{2}$O$_{3}$, V$_{2}$O$_{5})$. The perovskite SrVO$_{3}$ is metallic in bulk; however, it is possible to induce an MIT by modulating the bandwidth through strain or dimensional confinement. A mandatory requirement for controlling the electronic phase transition properties in material systems with strong correlation is the growth of high quality, stoichiometric thin films. This is demonstrated here with the growth of SrVO$_{3}$ on LSAT (001) substrates using a hybrid-MBE technique, where the Sr is evaporated from an effusion cell and the V is provided through the metal-organic precursor vanadium oxo-tri-isopropoxide (VTIP). The structural properties of films with varying VTIP:Sr ratios are characterized by RHEED, XRD, AFM and TEM. These measurements demonstrate that SrVO$_{3}$ can be grown with excellent structural quality, atomically flat surfaces and rocking curves of the same width as the substrate, accomplishing a necessary first step in controlling the MIT in SrVO$_{3}$. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y40.00005: Optimizing Pt/TiO$_{2}$ templates for textured PZT growth and MEMS devices Daniel Potrepka, Glenn Fox, Luz Sanchez, Ronald Polcawich Crystallographic texture of lead zirconate titanate (PZT) thin films strongly influences piezoelectric properties used in MEMS applications. Textured growth can be achieved by relying on crystal growth habit and can also be initiated by the use of a seed-layer heteroepitaxial template. Template choice and the process used to form it determine structural quality, ultimately influencing performance and reliability of MEMS PZT devices such as switches, filters, and actuators. This study focuses on how 111-textured PZT is generated by a combination of crystal habit and templating mechanisms that occur in the PZT/bottom-electrode stack. The sequence begins with 0001-textured Ti deposited on thermally grown SiO$_{2}$ on a Si wafer. The Ti is converted to 100-textured TiO$_{2}$ (rutile) through thermal oxidation. Then 111-textured Pt can be grown to act as a template for 111-textured PZT. Ti and Pt are deposited by DC magnetron sputtering. TiO$_{2}$ and Pt film textures and structure were optimized by variation of sputtering deposition times, temperatures and power levels, and post-deposition anneal conditions. The relationship between Ti, TiO$_{2}$, and Pt texture and their impact on PZT growth will be presented. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y40.00006: First-principles calculations of water-based surfactant-assisted growth of polar CaO(111) oxide film Xin Tan, Peter Zapol Despite many attempts to grow rocksalt (111) oxide surfaces, the growth of an atomically flat polar oxide film with an arbitrary thickness still remains challenging because of surface roughening during the growth process, such as faceting into neutral \textbraceleft 100\textbraceright\ surface planes. This seemingly unavoidable behavior leads to a grainy morphology and diminished functionality. Here, we present a first-principles investigation of the surfactant-assisted growth of polar CaO(111) film in the presence of a water-based surfactant, both from thermodynamic and kinetic points of view. We show that water molecules not only supply a surfactant by depositing hydrogen on the surface throughout the growth process, but also supply oxygen atoms as an elemental constituent in the film growth, i.e. water oxygen atoms are easily inserted into the top surface layer of the growing film. We suggest that adding water surfactants to conventional synthesis techniques leads to the continuous presence of hydrogen atoms in the surface region during the growth process, which efficiently quenches polarity and dynamically stabilizes the growth of the polar surface, and thus facilitates the growth of defect-free CaO(111) films with arbitrary thickness. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y40.00007: The Electronic Structure of Nonpolar Surfaces in Insulating Metal Oxides Danylo Zherebetskyy, Lin-Wang Wang Understanding the electronic and geometric structures of metal oxide surfaces has a key interest in many technological areas. A randomly chosen crystal surface has a high probability of being polar, unstable and containing in-gap states due to surface dangling bonds. As a result, the surface should be stabilized by passivation or reconstruction. However, do the nonpolar surfaces of ionic crystals of insulating metal oxides need the passivation or reconstruction similar to covalent crystals? We address this question by analyzing the nonpolar surfaces and their electronic structure for the common crystal structures of metal oxides. The study using periodic DFT calculations is performed for following representatives: Cu2O, ZnO, Al2O3, TiO2, V2O5, WO3, CaTiO3, Mg2SiO4. It has been shown that the nonpolar surface can be constructed out of dipole-free, charge-neutral and stoichiometric unit cells for each crystal. We demonstrate that all constructed and relaxed nonpolar surfaces of the metal oxides show a clear band gap. It should be emphasized that the constructed surfaces are neither reconstructed nor passivated. Additionally, we show a correlation between the electronic structure of the relaxed surfaces and Ewald energies calculated for the surface ions. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y40.00008: In-situ study of Nb oxide and hydride for SRF cavity applications using aberration-corrected STEM and electron energy loss spectroscopy Runzhe Tao, Robert Klie, YoonJun Kim, David Seidman, Lance Cooley, Alexander Romanenko We present an atomic-resolution study of the effects that a 48 hour bake at 120 $^{\circ}$C in vacuum has on the high-field properties of Nb-based SRF cavities. This bake results a significant increase in the high-field quality factor Q, reversely, 800 $^{\circ}$C bake for 2 hour reduces the H$_{\mathrm{c3}}$/H$_{\mathrm{c2}}$-ratio of cavities. Several mechanisms have been proposed, including an increased NbO$_{\mathrm{x}}$ surface layer thickness and the precipitation of NbH$_{\mathrm{y}}$. Using a combination of atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy with in-situ heating and cooling experiments, we examine the atomic and electronic structures of Nb and related oxides/hydrides near the cavity surface. We quantify the oxygen diffusion on surface during bake by measuring the local Nb valence using EEL spectra. Also, we demonstrate that hydrogen atoms incorporated into the Nb crystal, forming $\beta $-NbH precipitates, can be directly visualized using annular bright field imaging in our aberration-corrected JEOL ARM-200CF. Finally, the effects of the 800 $^{\circ}$C baking process on the local hydrogen and other impurity will be examined by in-situ heating and cooling experiments. Our results will be combined with atom-probe tomography to develop a 3-D impurity and phase profile of Nb near the SRF cavity surface. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y40.00009: Incorporation of Non-metal Impurities at the Anatase TiO$_2$(001)-(1 $\times$ 4) Surface Jun Hee Lee, Daniel Fernandez Hevia, Annabella Selloni Surface doping of TiO$_2$ is of special interest because the distribution of impurities at or near the surface can have a significant influence on the photocatalytic properties of TiO$_2$. We have used first-principles density functional theory (DFT) calculations to determine the incorporation mechanisms of nitrogen (N) and carbon (C), two widely used $p$-type dopants, at the reconstructed (001) surface of anatase, the TiO$_2$ polymorph most relevant for photocatalysis. Starting from adsorbed impurities, we identify various incorporation pathways and show that the non-exposed oxygen sites just below the surface play a crucial role in accomodating non-metal impurities at the TiO$_2$(001) surface. Based on the obtained results, we propose strategies which could help to increase the doping concentration and the photocatalytic activity at the TiO$_2$ surface by exploiting the morphology of the reconstructed surface [1].\\[4pt] [1] Incorporation of Non-metal Impurities at the Anatase TiO$_2$(001)-(1 $\times$ 4) Surface, \underline {arXiv:1209.1602}. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y40.00010: TiO$_{2}$ Surface Defects with the Tetrahedral Cationic Coordination Ken Park, Vincent Meunier, Nan Hsin Yu, Ward Plummer Titanium dioxide is one of the most extensively investigated transition metal oxide. It has well-known applications in catalytically converting toxic organic and inorganic materials to benign products, as well as turning solar energy into a chemical. In these processes, it is believed that surface defects with lower coordination and/or stoichiometry play crucial roles. Our study of a (2$\surd$2 $\times \surd $2) R45 reconstructed TiO$_{2}$(001) using scanning tunneling microscopy and density functional theory reveals that the basic building blocks of the reconstruction can be modeled as fully stoichiometric nanocluster defects. As in the bulk-terminated (001) surface, the atoms in the nanocluster are under-coordinated, for example, 4-coordinated Ti, 1-coordinated, and 2-coordinated O atoms. However, the absence of neighboring atoms drives the nanocluster to relax into a structure, which possesses tetrahedrally coordinated Ti atoms. This result will be compared and discussed with the reported nanocluster defects on TiO$_{2}$(110). [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y40.00011: Increase of Structural Phase Transition Temperature with Cr doping in Cr:VO$_{2}$ Thin Films B.L. Brown, Mark Lee, P. Clem, C.D. Nordquist, T.S. Jordan, S.L. Wolfley, D. Leonhardt, J.A. Custer Bulk crystal VO$_{2}$ has a well-known structural phase transition near T$_{c} =$ 68 $^{\circ}$C that separates a low-temperature insulating phase from a high-temperature metallic phase with several orders-of-magnitude resistance contrast between the two phases. We report electrical and optical studies of the effect of Cr doping on the T$_{c}$ in Cr:VO$_{2}$ thin films. Resistivity, Hall effect, and infrared reflectivity all show that Cr doping systematically increases T$_{c}$ from 50 $^{\circ}$C up to $\sim$ 75 $^{\circ}$C at 11{\%} Cr with similar transition width and hysteresis from DC to infrared, but the effect appears to saturate. At the same time, there is a modest decrease in resistance contrast. We will discuss possible effects of both carrier density and scattering changes across T$_{c}$ on the resistance. 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] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y40.00012: Non-Destructive Element Specific Density Depth Profiling by Resonant Soft X-ray Reflectometry Sebastian Macke, Adriano Verna, Maurits Haverkort, Abdullah Radi, Ronny Sutarto, Georg Christiani, Gennady Logvenov, Bernhard Keimer, George Sawatzky, Vladimir Hinkov X-ray resonant reflectometry (XRR) is the ideal tool to study the depth resolved and element-specific electronic structure of multilayer films. By changing angle, energy and polarization of the incoming beam complete reflectivity maps can be measured leading in principle to an accurate picture of the depth resolved electronic states of thin films. The standard model used in reflectometry is based on compound layers with a defined thickness, roughness and dielectric tensor. But such a simple model is usually not capable to reproduce a full measured reflectivity map. The main reasons are especially contaminations, additional oxide layers and interdiffusion between layers. However, introducing a layer system based on the element specific atomic density and scattering factors instead of dielectrics tensors allows more degrees of freedom for the system and allows to reproduce the reflectivity maps. Thereby the advanced model is capable to retrieve the element specific density profiles of thin films. The method is introduced by analyzing a simple film of PrNiO3 grown on an [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y40.00013: Investigation of electronic and magnetic properties of wurtzite NiO thin films Brian Busemeyer, Michael Shaughnessy, Liam Damewood, C.Y. Fong We study the electronic and magnetic properties of wurtzite (B4) NiO thin films grown epitaxially on wide gap semiconductors to understand factors affecting their half-metallic properties, in particular, the effect of film thickness, interface geometry, and dangling bonds. One, two, and four consecutive layers of NiO are considered, both buried within bulk ZnO, and as thin surface films on bulk ZnO. We perform GGA+U calculations, with the U value determined via a self consistent linear response approach. We find that the interface generates small s-p hybridized states at the Fermi energy, which can possibly destroy the half metallicity; these states are likely due to effects from strain at the interface. We also find that the interface can influence the Ni d states in markedly different ways, depending on the geometry and the presence of dangling bonds. These factors can determine whether the interface Ni d states resemble those from Ni in bulk wurtzite NiO, or demonstrate semiconductivity, more akin to d states of Zn atoms within bulk ZnO. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y40.00014: Structure of Oxide Surfaces Rong Yu Surfaces of metal oxides are of crucial importance for a variety of technological applications such as heterogeneous catalysis, thin film growth, gas sensing, and corrosion prevention. Due to the complexities of oxides in crystal structure and electronic structure, however, the surface science of oxides lags far behind that of metals or semiconductors. Conventional surface-science techniques are usually limited to surfaces of single crystals of conductors. Metal oxides are usually good insulators, making them difficult for conventional surface science techniques. On the other hand, the complex atomic structures of oxides results in too many structural parameters to be determined by spectroscopy or diffraction methods. We will show that the surface structure of oxides can be directly imaged and measured at the sub-angstrom scale using aberration-corrected transmission electron microscopy. The atomic positions of oxide surfaces can be measured to an accuracy of picometers, comparable to that obtained by conventional surface science techniques on single crystals.\\[4pt] [1] R. Yu, L.H. Hu, Z.Y. Cheng, Y.D. Li, H.Q. Ye, J. Zhu, Phys. Rev. Lett., 105, 226101 (2010).\\[0pt] [2] M.R. He, R. Yu, J. Zhu, Angew. Chem. Int. Ed., 124, 7864 (2012). [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y40.00015: UV-induced stable photoconductivity in Indium Oxide films Ehab Abdelhamid, Rupam Mukherjee, Debabrata Mishra, Ambesh Dixit, Boris Nadgorny, Gavin Lawes We have investigated the effects of UV radiation on the electrical conductivity of sputter deposited Indium Oxide films for samples annealed under different conditions. The films were annealed in air, hydrogen, argon, nitrogen, and vacuum to modify the microstructure and distribution of point defects. X-ray diffraction shows the formation of polycrystalline single phase films, with the average crystallite size changing under different annealing conditions. We find that the resistance sharply decreases to between 0.1{\%} and 50{\%} of its initial value on exposure to UV irradiation. The magnitude of the decrease depending on the annealing conditions, with the largest relative change occurring in the as-prepared sample (high initial resistance), and the smallest decrease observed in the Hydrogen-annealed film (low initial resistance). This low resistant state is surprisingly stable, having a time constant of several hours or longer to relax to the initial value after the UV illumination is removed. [Preview Abstract] |
Session Y42: Focus Session: Single Molecule Studies of Nucleotides and Nanomachines
Sponsoring Units: DBIOChair: Keir Neuman, NIH
Room: Hilton Baltimore Holiday Ballroom 3
Friday, March 22, 2013 8:00AM - 8:12AM |
Y42.00001: Length selective accumulation of oligonucleotides in thermal gradients Moritz Kreysing, Simon Lanzmich, Dieter Braun Central to most Origin-of-Life scenarios is the possibility for pre-biotic organic molecules to interact in order to form increasingly complex, catalytic molecular machinery ultimately capable of autonomous replication. While strong evidence for the spontaneous synthesis of single nucleotides [1] recently arose, concentrations required to allow these building blocks to polymerize [2] and gain functionality, still seem improbable for early earth conditions. Here, we demonstrate experimentally that temperature gradients across pores, as found in rocks near hydrothermal vents [3], are sufficient to accumulate nucleotides efficiently from dilute solutions. In particular we show that depending on the pores' dimensions, it can act as a length-selective molecular filter. We suggest that equivalent systems could have served as meeting points for long and complex molecules, too rare to find each other in a dilute primordial ocean. Furthermore, we discuss under which conditions this selection could have triggered the evolutionary adaptation of molecular replicators, and how polymerase chain reaction assays could nowadays benefit from the presented concept. References: 1. M. Powner et al., Nature 459 (2009), 2. G. Costanzo et al., ChemBioChem 13 (2012), 3. P. Baaske et al., PNAS (2007) [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y42.00002: Reconstructing kinetic pathways from single-molecule FRET experiments using Bayesian inference Jan-Willem van de Meent, Ruben L. Gonzalez, Jr., Chris H. Wiggins Single-molecule FRET studies have enabled observation of conformational transitions in individual molecules, allowing targeted investigations into the mechanistic function of molecular machines. Like in many single-molecule platforms, sm-FRET studies yield observations of hundreds of noisy time series, which report on the same underlying conformational steps, but exhibit significant variations in photophysical properties and kinetic rates. Reconstruction of a consensus kinetic pathway from such noisy measurements is statistically challenging. Hidden Markov Models are widely used to identify states and estimate the associated kinetic rates. Existing techniques perform inference on one time series at a time, yielding variable parameter estimates that must now be `averaged' using ad-hoc experiment specific post-processing steps. Here, we propose a technique known as Empirical Bayes estimation, which performs simultaneous analysis on a collection of trajectories in an experiment. This results in a single estimate for a consensus kinetic model, as well as a significantly reduced estimation error. By comparing models with different constraints, we show how these methods may be used to test detailed mechanistic hypotheses in a statistically principled, adaptable manner. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y42.00003: Single Molecule Measurements Using Correlation Force Spectroscopy Milad Radiom, Brian Robbins, John Walz, Mark Paul, William Ducker Thermal noise represents a fundamental limit in force measurements. We describe single molecule measurements using two AFM cantilevers that have lower thermal noise than single-cantilever measurements. We achieve this by measuring the correlated thermal motions of two closely spaced cantilevers. Because only correlated thermal noise is measured, there is lower noise. In addition, the use of two cantilevers produces both decreased hydrodynamic fluid damping and decreased van der Waals forces acting on an AFM probe, both of which are interferences in single molecule measurements. Analysis of the correlated motions reveals molecular damping, a parameter that is not sensed with conventional (pulling) AFM single molecule force spectroscopy. When a molecule is straddled between the two cantilevers, the correlation arises from the solvent coupling as well as stiffness and damping of the molecule. We will describe the technique of correlation force spectroscopy and measurements of the mechanical properties of single polymer chains such as dextran. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 9:12AM |
Y42.00004: A kinetic clutch governs uncoiling by type IB topoisomerases Invited Speaker: Keir Neuman Type IB topoisomerases (Top1B) are essential enzymes that relax excessive DNA supercoiling associated with replication and transcription and are important drug targets for cancer chemotherapy. The natural compound camptothecin (CPT) and the cancer chemotherapeutics derived from it, irinotecan and topotecan, are highly specific inhibitors of human nuclear Type IB topoisomerase (nTop1). We employed a magnetic-tweezers based single-molecule DNA supercoil relaxation assay to measure the torque dependence of human nuclear Top1 relaxation (nTop1) and inhibition by CPT. For comparison, we examined the human mitochondrial (Top1mt) topoisomerase and an N-terminal deletion mutant of nTop1 (Top68). Despite substantial sequence homology in their core domains, nTop1 and Top1mt exhibit dramatic differences in sensitivity to torque and CPT, with Top68 betraying intermediate characteristics. In particular, nTop1 displays nearly torque-independent religation probability, distinguishing it from other Top1B enzymes studied to date. Kinetic modeling reveals a hitherto unobserved torque-independent transition linking the DNA rotation and religation phases of the enzymatic cycle. The parameters of this transition determine the torque sensitivity of religation, and the efficiency of CPT binding. This ``kinetic clutch'' mechanism explains the molecular basis of CPT sensitivity and more generally provides a framework with which to interpret Top1B activity and inhibition. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y42.00005: Stretch Moduli of Ribonucleotide Embedded Short DNAs Hsiang-Chih Chiu, Kyung Duk Koh, Elisa Riedo, Francesca Storici Understanding the mechanical properties of DNA is essential to comprehending the dynamics of many cellular functions. DNA deformations are involved in many mechanisms when genetic information needs to be stored and used. In addition, recent studies have found that Ribonucleotides (rNMPs) are among the most common non-standard nucleotides present in DNA. The presences of rNMPs in DNA might cause mutation, fragility or genotoxicity of chromosome but how they influence the structure and mechanical properties of DNA remains unclear. By means of Atomic Force Microscopy (AFM) based single molecule spectroscopy, we measure the stretch moduli of double stranded DNAs (dsDNA) with 30 base pairs and 5 equally embedded rNMPs. The dsDNAs are anchored on gold substrate via thiol chemistry, while the AFM tip is used to pick up and stretch the dsDNA from its free end through biotin-streptavidin bonding. Our preliminary results indicate that the inclusion of rNMPs in dsDNA might significantly change its stretch modulus, which might be important in some biological processes. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y42.00006: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y42.00007: The interplay between single-stranded binding proteins on RNA secondary structure Yi-Hsuan Lin, Ralf Bundschuh RNA-protein interactions are critical for Biology because of their regulatory effects on mRNA and protein levels. There are typically several specific protein binding sites on an RNA molecule. A protein can bind one of these sites only if the RNA folds into a structure that leaves the entire binding site free of base pairs. Therefore, a protein binding to an RNA excludes some of the originally permitted RNA structures, causing a change in the structural ensemble. Thus, the probability of another protein to bind the same RNA at a different site will change upon binding of the first protein. To discover such effects, we combine methods of RNA secondary structure prediction with models of protein-RNA interaction. We focus on an RNA molecule with two protein binding sites. The ensemble of secondary structures of random RNA sequences is considered, and numerical calculations show the existence of a semi-long-range interaction between the protein binding sites mediated by the thermodynamics of the RNA structures. A brief analytic argument for this correlation is given, and a phase transition to a high-temperature phase, possibly related to the molten-glass phase transition of secondary RNA structures, is discussed. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:24AM |
Y42.00008: Mechanostability of Proteins and Virus Capsids Invited Speaker: Marek Cieplak Molecular dynamics of proteins within coarse grained models have become a useful tool in studies of large scale systems. The talk will discuss two applications of such modeling. The first is a theoretical survey of proteins' resistance to constant speed stretching as performed for a set of 17134 simple and 318 multidomain proteins. The survey has uncovered new potent force clamps. They involve formation of cysteine slipknots or dragging of a cystine plug through the cystine ring and lead to characteristic forces that are significantly larger than the common shear-based clamp such as observed in titin. The second application involves studies of nanoindentation processes in virus capsids and elucidates their molecular aspects by showing deviations in behavior compared to the continuum shell model. Across the 35 capsids studied, both the collapse force and the elastic stiffness are observed to vary by a factor of 20. The changes in mechanical properties do not correlate simply with virus size or symmetry. There is a strong connection to the mean coordination number $< z >$, defined as the mean number of interactions to neighboring amino acids. The Young's modulus for thin shell capsids rises roughly quadratically with $< z >$ - 6, where 6 is the minimum coordination for elastic stability in three dimensions. \\[4pt] [1] M. Sikora, J. I. Sulkowska, and M. Cieplak, Mechanical strength of 17134 model proteins and cysteine slipknots. PLoS Computational Biology, 5:e1000547 (2009).\\[0pt] [2] M. Sikora nd M. Cieplak, Mechanical stability of multidomain proteins and novel mechanical clamps. Proteins. Struct. Fun. Bioinf. 79:1786-1799 (2011).\\[0pt] [3] M. Sikora and M. Cieplak, Formation of cystine slipknots in dimeric proteins. Phys. Rev. Lett. 109 208101 (2012).\\[0pt] [4] M. Cieplak and M. O. Robbins, Nanoindentation of virus capsids in a molecular model. J. Chem. Phys. 132:015101 (2010).\\[0pt] [5] M. Cieplak and M. O. Robbins, Nanoindentation of 35 virus capsids in a molecular model: Relating mechanical properties to structure (submitted). [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y42.00009: Nanomechanical Response of \textit{Pseudomonas aeruginosa} PAO1 Bacterial Cells to Cationic Antimicrobial Peptides Shun Lu, Grant Walters, John Dutcher We have used an atomic force microscopy (AFM)-based creep deformation technique to study changes to the viscoelastic properties of individual Gram-negative \textit{Pseudomonas aeruginosa} PAO1 cells as a function of time of exposure to two cationic peptides: polymyxin B (PMB), a cyclic antimicrobial peptide, and the structurally-related compound, polymyxin B nonapeptide (PMBN). The measurements provide a direct measure of the mechanical integrity of the bacterial cell envelope, and the results can be understood in terms of simple viscoelastic models of arrangements of springs and dashpots, which can be ascribed to different components within the bacterial cell. Time-resolved creep deformation experiments reveal abrupt changes to the viscoelastic properties of \textit{P. aeruginosa} bacterial cells after exposure to both PMB and PMBN, with quantitatively different changes for the two cationic peptides. These measurements provide new insights into the kinetics and mechanism of action of antimicrobial peptides on bacterial cells. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y42.00010: Ion Discrimination by Nanoscale Design Susan Rempe, David Rogers Proteins that form membrane-spanning channels excel at discriminating between molecules on the basis of subtle structural and chemical differences. For example, some channels distinguish between water and ions; others between Na+ (sodium) and K+ (potassium) despite identical charges and only sub-Angstrom differences in size. If we could understand these structure/function relationships, we could potentially harness biological design principles in robust nanoscale devices that mimic biological function for efficient separations. Using ab initio molecular simulations, we have interrogated the link between channel structure and selective transport, both in cellular channels and polymer membranes. Our results emphasize the surprisingly important role of the environment that surrounds ion-binding sites, as well as the coordination chemistry of the binding site for raising or lowering the free energy barrier to transport in both systems. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y42.00011: Extracting Models in Single Molecule Experiments Steve Presse Single molecule experiments can now monitor the journey of a protein from its assembly near a ribosome to its proteolytic demise. Ideally all single molecule data should be self-explanatory. However data originating from single molecule experiments is particularly challenging to interpret on account of fluctuations and noise at such small scales. Realistically, basic understanding comes from models carefully extracted from the noisy data. Statistical mechanics, and maximum entropy in particular, provide a powerful framework for accomplishing this task in a principled fashion. Here I will discuss our work in extracting conformational memory from single molecule force spectroscopy experiments on large biomolecules. One clear advantage of this method is that we let the data tend towards the correct model, we do not fit the data. I will show that the dynamical model of the single molecule dynamics which emerges from this analysis is often more textured and complex than could otherwise come from fitting the data to a pre-conceived model. [Preview Abstract] |
Session Y43: Spectroscopy, Photochemistry, and Electrochemistry
Sponsoring Units: DCPChair: Jeff Owrutsky, Naval Research Laboratory
Room: Hilton Baltimore Holiday Ballroom 2
Friday, March 22, 2013 8:00AM - 8:12AM |
Y43.00001: Quasiparticle Representation of Coherent Nonlinear Optical Signals of Multiexcitons Benjamin Fingerhut, Kochise Bennet, Oleksiy Roslyak, Shaul Mukamel Elementary excitations of many-Fermion systems can be described within the quasiparticle approach which is widely used in the calculation of transport and optical properties of metals, semiconductors, molecular aggregates and strongly correlated quantum materials. The excitations are then viewed as independent harmonic oscillators where the many-body interactions between the oscillators are mapped into anharmonicities. We present a Green's function approach based on coboson algebra for calculating nonlinear optical signals and apply it onwards the study of two and three exciton states. The method only requires the diagonalization of the single exciton manifold and avoids equations of motion of multi-exciton manifolds. Using coboson algebra many body effects are recast in terms of tetradic exciton-exciton interactions: Coulomb scattering and Pauli exchange. The physical space of Fermions is recovered by singular-value decomposition of the over-complete coboson basis set. The approach is used to calculate third and fifth order quantum coherence optical signals that directly probe correlations in two- and three exciton states and their projections on the two and single exciton manifold. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y43.00002: Surface induced selective deposition of Dysprosium Polyoxometalate on HOPG surface studied by STM and STS David Costa Milan, Elena Pinilla Cienfuegos, Salvador Cardona Serra, Eugenio Coronado Miralles, Carlos Untiedt Lecuona Scanning Tunneling Microscope (STM) and scanning Tunnelling spectroscopy (STS) techniques have been used to study the Preyssler type Polyoxometalate K$_{12}$[DyP$_{5}$W$_{30}$O$_{110}$] molecules deposited on Highly Oriented Pyrolytic Graphite surface (HOPG). Chainlike arrangements of clusters containing two or three molecules, as well as different cluster sizes are observed. As many structural artifacts are present on the graphite surface, like Moir\'{e} patterns, that could look like the molecular deposits, we have studied their STS and size to ensure the presence of the POM molecules on the surface. This article shows the possibility of addressing POMs on a flat surface to obtain their electronic properties through STS. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y43.00003: Detecting excited-state vibrational dynamics by broadband infrared or Raman probes; A unified picture based on loop diagrams Konstantin E. Dorfman, Benjamin Fingerhut, Shaul Mukamel Vibrational motions in electronically excited states can be probed either by time and frequency resolved infrared or by off resonant stimulated Raman techniques. Using loop diagrams, which represent forward and backward propagation of the wavefunction we derive similar multipoint correlation function expressions for both signals which are suitable for quantum microscopic simulations. The effective temporal ($\Delta $t) and spectral ($\Delta \omega )$ resolution of the techniques is not solely controlled by experimental knobs since it also depends on the system dynamics being probed. The Fourier uncertainty $\Delta \omega \Delta $t \textgreater\ 1 is never violated. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y43.00004: Interfacial Matrix Stabilization Spectroscopy (IMSS) studies of CO and O2 interactions with thin films of oxide-supported Au nanoparticles Nina K. Jarrah, David T. Moore Interfacial Matrix Stabilization Spectroscopy (IMSS) employs energy-dissipating cryogenic matrix isolation techniques combined with FTIR to enable stabilization and detection of pre-reactive complexes of CO and O2 formed on oxide-supported gold nanoparticles (AuNPs). Following deposition of CO and O2 in an argon matrix at 10-20K, annealing to warmer temperatures (28-32K) promotes diffusion of isolated dopant molecules through the matrix to binding sites on a thin film of catalyst. Matrix-solvated pre-reactive complexes form at the surface and are characterized spectroscopically. Comparison of observed complexes in IMSS experiments with results from direct adsorption studies, in absence of a matrix, can provide a measure for the stabilizing effects of matrix solvation. Subsequent surface warming following stabilization of the pre-reactive complexes reveals qualitative information about relative binding energies of formed intermediates of CO, O2, and the supported AuNPs. A series of FTIR spectra mapping the evolution of vibrational bands during the annealing process and tracking the various surface-bound species will be presented and comparisons to direct adsorption experiments will be discussed. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y43.00005: Rotational Spectroscopy at Sub-Angstrom Level: Rotationaland Vibrational Excitations of Molecular Hydrogen measured by the Scanning Tunneling Microscope Shaowei Li, Arthur Yu, Hui Wang, Freddy Toledo, Zhumim Han, Ruqian Wu, Wilson Ho The power of rotational spectroscopy has long been demonstrated in the frequency domain by microwave spectroscopy, but its application in real space has been limited. Using a scanning tunneling microscope (STM) and inelastic electron tunneling spectroscopy (IETS), we are able to conduct real-space measurements of rotational transitions of gaseous hydrogen molecules physisorbed on Au(110) surface at 10 K. The j=0 to j=2 rotational transition for para-H$_{2}$ and HD as well as the v=0 to v=1 vibrational transitions for H$_{2}$, D$_{2}$ and HD were observed by STM-IETS. By varying the tip-substrate distance, we could precisely investigate how the environmental coupling modifies the structure, including the bond length, of a single molecule with sub-Angstrom resolution. Rotational spectroscopy at the single molecule level provides a powerful tool for chemical identification as well as bond length measurement in both the frequency and space domains. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y43.00006: Two-Photon Ionization of a Low IP Molecule (TDAE) Byron Smith, Robert Compton Very low ionization potential molecules (\textless\ 6 eV) have been studied thoroughly for their use in low temperature plasmas, charge-transfer salts, and as an alternative to liquid scintillation in photomultiplier tubes. One such molecule is tetrakis(dimethylamino)ethylene (TDAE) with a previously measured IP of 5.2 $\pm$ 0.05 eV using electron and photon impact time of flight mass spectrometry.\footnote{N. Mirsaleh-Kohan, et al., \textit{Int. J. Mass Spectrom.} \textbf{304}, 56-65(2011).} Two-photon ionization photoelectron spectroscopy of TDAE at 441 nm and 355 nm results in an IP of 5.22 $\pm$ 0.14 eV. In addition to the photoelectron peak associated with direct ionization, a peak was observed corresponding to thermal energy electrons ($\sim$ 0 eV). This has been previously assigned to an intense short-lived auto-ionizing state\footnote{B. Soep, et al., \textit{J. Eur. Phys. D} \textbf{14}, 203 (2001).} which quickly cools to a zwitterionic intermediate. We assign this state to a collective excitation as an alternative explanation of the source of the slow electrons. The collective state involves electron correlation within the parent molecule as well as the degeneracy of the auto-ionizing state. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y43.00007: Modeling the color of natural dyes Xiaochuan Ge, Arrigo Calzolari, Simon Binnie, Stefano Baroni We report on a theoretical study, based on time-dependent density-functional theory, of various factors affecting the optical properties of a few representative anthocyanins, a class of molecules responsible for the color of many fruits, flowers, and leaves, which have also aroused some interest for photovoltaic applications. We first address the influence of substituting different side groups in the phenyl ring of flavylium dyes. We find that these dyes can be classified into three broad classes, according to the number of peaks (1, 2, or 3) featured in the visible range, and give a rationale to this finding. We then examine the effects of solvent-induced thermal fluctuations and dielectric screening, by calculating the spectrum of a representative molecule in solution, for each one these classes. This is achieved by first running an ab initio molecular dynamics simulation of an explicit model for the water-solvated molecule, and then accumulating time averages of the optical spectra calculated on the fly. The effects of thermal fluctuations are shown to overshadow those of dielectric screening, and more dramatic the larger the number of peaks in the gas phase. The effects of different functionals (GGA vs. hybrids) on the calculated spectra are also addressed. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y43.00008: Terahertz Time-Domain Spectroscopy of Nitrogen Ice Bagvanth R. Sangala, Perry A. Gerakines, David J. Hilton We have used terahertz time-domain spectroscopy from 0.1-1.6 THz to study thin films of solid N$_2$ from 10-25 K. A temperature dependent absorption line shift was observed near 1.46 THz as the temperature increased from 10 to 25 K, where the center frequency of the absorption line decreased with temperature. We can fit these data to a model assuming a standard Lennard-Jones potential with the addition of a quadrupole-quadrupole interaction. We modeled the shift in the resonant absorption with a lattice expansion that includes previously published thermal expansion coefficients in N$_2$ ice, the gas-phase Lennard-Jones parameters, and the gas-phase quadrupole moments. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y43.00009: Temperature-dependent and time-resolved emission studies of \textit{cis}-Ru(bpy)$_2$(N$_3$)$_2$ Helen K. Gerardi, Douglas J. Brown, Ryan Compton, Walter J. Dressick, Edwin J. Heilweil, Jeffrey C. Owrutsky The electronic properties of a Ru$^{II}$ cyclometalated dye complex, \textit{cis}-Ru(bpy)$_{2}$(N$_{3})_{2}$, were examined with time-resolved and temperature-dependent visible emission measurements. Compared to two related solar cell dye-sensitizer species, \textit{cis}-Ru(bpy)$_{2}$(NCS)$_{2}$ and \textit{cis}-Ru(bpy)$_{2}$(CN)$_{2}$, the azide (N$_{3})$ pseudohalide ligand dramatically changes the electronic properties of the dye. The uv-vis absorption spectra of \textit{cis}-Ru(bpy)$_{2}$(N$_{3})_{2}$ in various solvents reveal that its metal-to-ligand charge transfer band (MLCT) is located more than 50 nm to the red of the MLCT bands found for the other two complexes. Furthermore, while room temperature emission is readily observed for \textit{cis}-Ru(bpy)$_{2}$(NCS)$_{2}$ and \textit{cis}-Ru(bpy)$_{2}$(CN)$_{2,}$ the emission is much weaker for \textit{cis}-Ru(bpy)$_{2}$(N$_{3})_{2}$. We report the first observation of luminescence from \textit{cis}-Ru(bpy)$_{2}$(N$_{3})_{2}$ by measuring it in 4:1 EtOH:MeOH matrices at temperatures below 140~K. Emission bands are observed at 665 nm and 620 nm (514 nm excitation). The quantum yield of this species was estimated by comparing the integrated emission signal of \textit{cis}-Ru(bpy)$_{2}$(N$_{3})_{2}$ to that of \textit{cis}-Ru(bpy)$_{2}$(CN)$_{2}$ at 77~K and was determined to be exceptionally low (6~x~10$^{-4}$~). The luminescence lifetime of \textit{cis}-Ru(bpy)$_{2}$(N$_{3})_{2}$ at 77 K was measured to be approx. 800~ns, implying an extremely slow radiative rate of 780 s$^{-1}$. The long radiative rate and low quantum yield led us to further investigate the photolability and electrochemical behavior of the azide complex. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y43.00010: How Fissors works: tracking vibrationally adiabatic conformational change with femtosecond stimulated Raman spectroscopy Jeffrey Cina, Phil Kovac With the help of a two-dimensional model system comprising a slow conformational degree of freedom and a higher-frequency vibration, we investigate the optical generation and dynamical information content of femtosecond stimulated Raman spectroscopy (FSRS or fissors). Our treatment makes use of an assumption that the motion of a wave packet describing the relatively slow---but still ultrafast---conformational change is vibrationally adiabatic. We present calculated fissors signals for regimes in which the conformational change is or is not sufficiently slow to result in an evolving fissors lineshape whose center frequency tracks the ``instantaneous'' vibrational frequency. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y43.00011: A photoelectrochemical (PEC) study on graphene oxide based hematite thin films heterojunction (R-GO/Fe$_2$O$_3)$ Poonam Sharma, Michael Zachariah, Sheryl Ehrman, Rohit Shrivastava, Sahab Dass, Vibha R. Satsangi Graphene has an excellent electronic conductivity, a high theoretical surface area of 2630 m$^2$/g and excellent mechanical properties and, thus, is a promising component for high-performance electrode materials. Following this, GO has been used to modify the PEC response of photoactive material hematite thin films in PEC cell. A reduced graphene oxide/iron oxide (R-GO/Fe$_2$O$_3)$ thin film structure has been successfully prepared on ITO by directly growing iron oxide particles on the thermally reduced graphene oxide sheets prepared from suspension of exfoliated graphene oxide. R-GO/Fe$_2$O$_3$ thin films were tested in PEC cell and offered ten times higher photocurrent density than pristine Fe$_2$O$_3$ thin film sample. XRD, SEM, EDS, UV-Vis, Mott-Schottky and Raman studies were carried out to study spectro-electrochemical properties. Enhanced PEC performance of these photoelectrodes was attributed to its porous morphology, improved conductivity upon favorable carrier transfer across the oxides interface. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y43.00012: Site-Specific Photosimulated Reactions of O$_{2}$ on TiO$_{2}$(110) Zhi-Tao Wang, N. Aaron Deskins, Igor Lyubinetsky We report the direct observation at an atomic level with high-resolution scanning tunneling microscopy of photostimulated reactions of single O$_{2}$ molecules on reduced TiO$_{2}$(110) surfaces at 50 K. Two distinct reactions of O$_{2}$ desorption and dissociation occur at different active sites of terminal Ti atoms and bridging O vacancies, respectively demonstrating the critical relation between photoreactivity and adsorption sites on TiO$_{2}$. These two reaction channels follow very different kinetics. Hole-mediated O$_{2}$ desorption is promptly and fully completed, while electron-mediated O$_{2}$ dissociation is much slower and is quenched above some critical O$_{2}$ coverage. Density functional theory calculations indicate that both coordination and charge state of an O$_{2}$ molecule chemisorbed at specific site largely determine a particular reaction pathway. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y43.00013: Cobalt (hydro)oxide electrodes under electrochemical conditions: a first principle study Jia Chen, Annabella Selloni There is currently much interest in photoelectrochemical water splitting as a promising pathway towards sustainable energy production. A major issue of such photoelectrochemical devices is the limited efficiency of the anode, where the oxygen evolution reaction (OER) takes place. Cobalt (hydro)oxides, particularly Co3O4 and Co(OH)2, have emerged as promising candidates for use as OER anode materials. Interestingly, recent in-situ Raman spectroscopy studies have shown that Co3O4 electrodes undergo progressive oxidation and transform into oxyhydroxide, CoO(OH), under electrochemical working conditions. (Journal of the American Chemical Society \textbf{133}, 5587 (2011))Using first principle electronic structure calculations, we provide insight into these findings by presenting results on the structural, thermodynamic, and electronic properties of cobalt oxide, hydroxide and oxydroxide CoO(OH), and on their relative stabilities when in contact with water under external voltage. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y43.00014: Near-field Nanoscopy and Spectroscopy of Phase Coexistence in LiFePO$_4$ Electrode Microcrystals Ivan T. Lucas, Alexander McLeod, Jaroslaw S. Syzdek, Derek S. Middlemiss, Robert Kostecki, D.N. Basov Due to instrumental limitations, the microscopic description of lithiation and delithiation processes in low-cost LiFePO$_4$ electrodes has remained uncertain and subject to controversy. Using infrared near-field imaging, we present evidence for a novel coexistence of phases within single LiFePO$_4$ microcrystals. First-principles calculations of the phonon response of lithiated and delithiated end-phases are compared with broadband nano-FTIR (Fourier transform infrared) spectroscopy data to reveal the mid-infrared vibrational signature of lithiation. By resolving this signature at the nano-scale, we observe a propagation of phase boundaries within these crystals over the course of chemical delithiation. In addition, by comparing theoretical modeling with spatially resolved nano-FTIR spectra measured across a single crystal at partial delithiation, we assemble a tomographic view of phases distributed hundreds nanometers beneath the crystal surface. These experiments set the stage for quantitative nano-spectroscopy of new composite electrode materials, assisting in the rational design of next-generation electrical energy storage systems. [Preview Abstract] |
Session Y44: Focus Session: Novel Experimental Techniques for Probing Cellular Mechanics
Sponsoring Units: DBIOChair: Cristian Staii, Tufts
Room: Hilton Baltimore Holiday Ballroom 1
Friday, March 22, 2013 8:00AM - 8:36AM |
Y44.00001: Mechanosensitivity in axon growth and guidance Invited Speaker: Jeff Urbach In the developing nervous system, axons respond to a diverse array of cues to generate the intricate connection network required for proper function. The growth cone, a highly motile structure at the tip of a growing axon, integrates information about the local environment and modulates outgrowth and guidance, but little is known about effects of external mechanical cues and internal mechanical forces on growth cone behavior. We have investigated axon outgrowth and force generation on soft elastic substrates for dorsal root ganglion (DRG) neurons (from the peripheral nervous system) and hippocampal neurons (from the central) to see how the mechanics of the microenvironment affect different populations. We find that force generation and stiffness-dependent outgrowth are strongly dependent on cell type. We also observe very different internal dynamics and substrate coupling in the two populations, suggesting that the difference in force generation is due to stronger adhesions and therefore stronger substrate engagement in the peripheral nervous system neurons. We will discuss the biological origins of these differences, and recent analyses of the dynamic aspects of growth cone force generation and the implications for the role of mechanosensitivity in axon guidance. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y44.00002: Electrophysiology of Axonal Constrictions Christopher Johnson, Peter Jung, Anthony Brown Axons of myelinated neurons are constricted at the nodes of Ranvier, where they are directly exposed to the extracellular space and where the vast majority of the ion channels are located. These constrictions are generated by local regulation of the kinetics of neurofilaments the most important cytoskeletal elements of the axon. In this paper we discuss how this shape affects the electrophysiological function of the neuron. Specifically, although the nodes are short (about $1\mu m$) in comparison to the distance between nodes (hundreds of $\mu m$) they have a substantial influence on the conduction velocity of neurons. We show through computational modeling that nodal constrictions (all other features such as numbers of ion channels left constant) reduce the required fiber diameter for a given target conduction velocity by up to 50\% in comparison to an unconstricted axon. We further show that the predicted optimal fiber morphologies closely match reported fiber morphologies. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y44.00003: Role of biomechanical cues on neuronal growth on asymmetric textured surfaces Cristian Staii, Elise Spedden, Timothy Atherton, Koray Sekeroglu, Melik Demirel Axonal growth and the formation of synaptic connections are key steps in the development of the nervous system. Here we present experimental and theoretical results on axonal growth on unidirectional nanotextured surface, and demonstrate that these surface can bias axonal growth. We also perform a systematic investigation of neuronal processes on these surfaces and quantify the role that biomechanical surface cues play in neuronal growth. We show that these surfaces provide a model growth substrates, which allow us to perform systematic studies of the interplay between mechanical, biochemical and topographical cues that contribute to neuronal growth. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y44.00004: The formation of axonal caliber and nodes of Ranvier Yinyun Li, Peter Jung, Anthony Brown A remarkable feature of myelinated neurons is that their axons are constricted at the nodes of Ranvier. These are the locations where axons are directly exposed to the extracellular space and where the vast majority of the ion channels are located. These constrictions emerge during development and have been observed to reduce axonal cross sectional area by factors of more than 10. Combining fluorescent imaging methods with computational modeling, we describe how the nervous system regulates the local caliber of its axons through the regulation of the transport kinetics of its most important cytoskeletal elements, the neurofilaments, matching axon caliber and shape to its physiologic function. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y44.00005: Studying neuronal biomechanics and its role in CNS development Kristian Franze, Hanno Svoboda, Luciano da F. Costa, Jochen Guck, Christine Holt During the development of the nervous system, neurons migrate and grow over great distances. Currently, our understanding of nervous tissue development is, in large part, based on studies of biochemical signaling. Despite the fact that forces are involved in any kind of cell motion, mechanical aspects have so far rarely been considered. Here we used deformable cell culture substrates, traction force microscopy and calcium imaging to investigate how neurons probe and respond to their mechanical environment. While the growth rate of retinal ganglion cell axons was increased on stiffer substrates, their tendency to grow in bundles, which they show \textit{in vivo}, was significantly enhanced on more compliant substrates. Moreover, if grown on substrates incorporating linear stiffness gradients, neuronal axons were repelled by stiff substrates. Mechanosensing involved the application of forces driven by the interaction of actin and myosin II, and the activation of stretch-activated ion channels leading to calcium influxes into the cells. Applying a modified atomic force microscopy technique\textit{ in vivo}, we found mechanical gradients in developing brain tissue along which neurons grow. The application of chondroitin sulfate, which is a major extracellular matrix component in the developing brain, changed tissue mechanics and disrupted axonal pathfinding. Hence, our data suggest that neuronal growth is not only guided by chemical signals -- as it is currently assumed -- but also by the nervous tissue's mechanical properties. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y44.00006: Contact nanomechanical measurements with the AFM Nicholas Geisse The atomic force microscope (AFM) has found broad use in the biological sciences largely due to its ability to make measurements on unfixed and unstained samples under liquid. In addition to imaging at multiple spatial scales ranging from micro- to nanometer, AFMs are commonly used as nanomechanical probes. This is pertinent for cell biology, as it has been demonstrated that the geometrical and mechanical properties of the extracellular microenvironment are important in such processes as cancer, cardiovascular disease, muscular dystrophy, and even the control of cell life and death. Indeed, the ability to control and quantify these external geometrical and mechanical parameters arises as a key issue in the field. Because AFM can quantitatively measure the mechanical properties of various biological samples, novel insights to cell function and to cell-substrate interactions are now possible. As the application of AFM to these types of problems is widened, it is important to understand the performance envelope of the technique and its associated data analyses. This talk will discuss the important issues that must be considered when mechanical models are applied to real-world data. Examples of the effect of different model assumptions on our understanding of the measured material properties will be shown. Furthermore, specific examples of the importance of mechanical stimuli and the micromechanical environment to the structure and function of biological materials will be presented. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 10:12AM |
Y44.00007: Quantitative nano-mechanics of biological cells with AFM Invited Speaker: Igor Sokolov The importance of study of living cells is hard to overestimate. Cell mechanics is a relatively young, yet not a well-developed area. Besides just a fundamental interest, large practical need has emerged to measure cell mechanics quantitatively. Recent studies revealed a significant correlation between stiffness of biological cells and various human diseases, such as cancer, malaria, arthritis, and even aging. However, really quantitative studies of mechanics of biological cells are virtually absent. It is not even clear if the cell, being a complex and heterogeneous object, can be described by the elastic modulus at all. Atomic force microscopy (AFM) is a natural instrument to study properties of cells in their native environments. Here we will demonstrate that quantitative measurements of elastic modulus of cells with AFM are possible. Specifically, we will show that the ``cell body'' (cell without ``brush'' surface layer, a non-elastic layer surrounding cells) typically demonstrates the response of a homogeneous elastic medium up to the deformation of 10-20{\%}, but if and only if a) the cellular brush layer is taken into account, b) rather dull AFM probes are used. This will be justified with the help of the strong condition of elastic behavior of material: the elastic modulus is shown to be independent on the indentation depth. We will also demonstrate that an attempt either to ignore the brush layer or to use sharp AFM probes will result in the violation of the strong condition, which implies impossibility to use the concept of the elastic modulus to describe cell mechanics in such experiments. Examples of quantitative measurements of the Young's modulus of the cell body and the cell brush parameters will be given for various cells. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y44.00008: Tracking Cytoskeletal Dynamics in Living Neurons via Combined Atomic Force and Fluorescence Microscopy Elise Spedden, David Kaplan, Cristian Staii Living cells are active mechanical structures which evolve within and in response to their local microenvironments. Various cell types possess different mechanical properties and respond uniquely to growth, environmental changes, and the application of chemical stimuli. Here we present a powerful approach which combines high resolution Atomic Force Microscopy with Fluorescence Microscopy to systematically obtain real-time micrometer and sub-micrometer resolution elasticity maps for live neuronal cells cultured on glass substrates. Through this approach we measure the topography, the elastic properties, and the dynamics of neuronal cells, and identify changes in cytoskeletal components during axonal growth, chemical modification, and changes in ambient temperature. We will also show high resolution elasticity measurements of the cell body and of axons/dendrites during growth, as well as identification of cytoskeletal components during cell growth and environmental changes. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y44.00009: Atomic Force Microscopy Based Cell Shape Index Usienemfon Adia-Nimuwa, Volkan Mujdat Tiryaki, Steven Hartz, Kan Xie, Virginia Ayres Stellation is a measure of cell physiology and pathology for several cell groups including neural, liver and pancreatic cells. In the present work, we compare the results of a conventional two-dimensional shape index study of both atomic force microscopy (AFM) and fluorescent microscopy images with the results obtained using a new three-dimensional AFM-based shape index similar to sphericity index [1]. The stellation of astrocytes is investigated on nanofibrillar scaffolds composed of electrospun polyamide nanofibers that has demonstrated promise for central nervous system (CNS) repair. Recent work by our group has given us the ability to clearly segment the cells from nanofibrillar scaffolds in AFM images [2]. The clear-featured AFM images indicated that the astrocyte processes were longer than previously identified at 24h. It was furthermore shown that cell spreading could vary significantly as a function of environmental parameters, and that AFM images could record these variations [3]. The new three-dimensional AFM-based shape index incorporates the new information: longer stellate processes and cell spreading. [1] AWl. Jay, Biophys. J.:15, 205 (1975) [2] VM Tiryaki, et al, Scanning:34, 316 (2012) [3] VM Tiryaki, et al, Int. J. Nanomed.:07, 3891 (2012) [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y44.00010: Response of Quiescent Cerebral Cortical Astrocytes to Nanofibrillar Scaffold Properties Virginia Ayres, Volkan Mujdat Tiryaki, Kan Xie, Ijaz Ahmed, David I. Shreiber We present results of an investigation to examine the hypothesis that the extracellular environment can trigger specific signaling cascades with morphological consequences [1]. Differences in the morphological responses of quiescent cerebral cortical astrocytes cultured on the nanofibrillar matrices versus poly-L-lysine functionalized glass and Aclar, and unfunctionalized Aclar surfaces were demonstrated using atomic force microscopy (AFM) and phalloidin staining of F-actin. The differences and similarities of the morphological responses were consistent with differences and similarities of the surface polarity and surface roughness of the four surfaces investigated in this work, characterized using contact angle and AFM measurements. The three-dimensional capability of AFM was also used to identify differences in cell spreading. An initial quantitative immunolabeling study further identified significant differences in the activation of the Rho GTPases: Cdc42, Rac1, and RhoA, which are upstream regulators of the observed morphological responses: filopodia, lamellipodia, and stress fiber formation. The results support the hypothesis that the extracellular environment can trigger preferential activation of members of the Rho GTPase family with demonstrable morphological consequences for cerebral cortical astrocytes. [1] VM Tiryaki et al, Int. J. Nanomed.: 07, 3891 (2012) [Preview Abstract] |
Session Y45: Focus Session: Physics of Cancer II
Sponsoring Units: DBIOChair: Larry Nagahara, National Institutes of Health
Room: Hilton Baltimore Holiday Ballroom 4
Friday, March 22, 2013 8:00AM - 8:12AM |
Y45.00001: Collective Behavior of Brain Tumor Cells: the Role of Hypoxia Evgeniy Khain, Mark Katakowski, Scott Hopkins, Alexandra Szalad, Xuguang Zheng, Feng Jiang, Michael Chopp We consider emergent collective behavior of a multicellular biological system. Specifically we investigate the role of hypoxia (lack of oxygen) in migration of brain tumor cells [1]. We performed two series of cell migration experiments. The first set of experiments was performed in a typical wound healing geometry: cells were placed on a substrate, and a scratch was done. In the second set of experiments, cell migration away from a tumor spheroid was investigated. Experiments show a controversy: cells under normal and hypoxic conditions have migrated the same distance in the ``spheroid'' experiment, while in the ``scratch'' experiment cells under normal conditions migrated much faster than under hypoxic conditions. To explain this paradox, we formulate a discrete stochastic model for cell dynamics [1,2]. The theoretical model explains our experimental observations and suggests that hypoxia decreases both the motility of cells and the strength of cell-cell adhesion. The theoretical predictions were further verified in independent experiments [1]. \\[4pt] [1]. E. Khain, M. Katakowski, S. Hopkins, A. Szalad, X.G. Zheng, F. Jiang, M. Chopp, Physical Review E 83, 031920 (2011). \\[0pt] [2]. E. Khain, C. M. Schneider-Mizell, M. O. Nowicki, E. A. Chiocca, S. E. Lawler and L. M. Sander, EPL (Europhysics Letters) 88, 28006 (2009). [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y45.00002: Rapid evolution of drug resistance of multiple myeloma in the microenvironment with drug gradients Amy Wu, Qiucen Zhang, Guillaume Lambert, Zayar Khin, Ariosto Silva, Robert Gatenby, John Kim, Nader Pourmand, Robert Austin, James Sturm Drug resistance in cancer is usually caused by the spatial drug gradients in tumor environment. Here, we culture multiple myeloma in a gradient from 0 to 20 nM of doxorubicin (genotoxic drug) across 2 mm wide region for 12 days. The myeloma cells grew rapidly and formed 3D colonies in the regions with less drug concentration. However, we have seen emergent colonies forming in regions with drug concentration above the minimal inhibitory concentration in less than one week. Once the cells have occupied the regions with less drug concentration, they tend to migrate toward the regions with higher drug concentration in a collective behavior. To characterize their resistance, we collect them from this microfluidic system, for further analysis of the dose response. We find that the IC50 (drug concentration that inhibits 50\% of controlled population) of the cells, undergone a drug gradient, increase 16-fold of the wildtype cells. We further discover that these resistant cells express more Multidrug Resistance (mdr) protein, which pumps out the drugs and causes drug resistance, than the wildtype. Our current works on RNA-sequencing analysis may discover other biomolecular mechanisms that may confer the drug resistance. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y45.00003: Hierarchy of Gene Expression as a Biomarker for Breast Cancer Prognosis Man Chen Cancer is a dedifferentiation of healthy cellular and genetic processes. At the same time, specific oncological pathways are activated in the cancer state [1]. Cancer metastasis exposes cancer cells to a variety of microenvironments, in which physics of evolution suggests modularity is a relevant order parameter [2]. We were thus motivated to examine the structure in gene and tissue networks of breast cancer patients. We studied the relation between metastasis and breast cancer network structure. We found that hierarchy of cancer networks distinguishes non-metastatic from metastatic patient populations. We also found that for cancer-associated genes, likelihood of metastasis is correlated with increased network hierarchy. Conversely for tissue networks using all gene data, reduced network structure is correlated with likelihood of metastasis. We suggest hierarchy of gene expression may be useful as a biomarker for breast cancer breast cancer metastasis and recurrence. For those patients with reduced structure, which is at least 5\% of the patient population, this biomarker provides a strong signal for likelihood of cancer metastasis.\\[4pt] [1] Paul Davis, Physical Biology vol (2011) page\\[0pt] [2] Jun Sun Phys. Rev. Lett vol (2007) page [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y45.00004: Ultrasensitive Mirco-Hall Detector for Enumeration and Molecular Profiling of Rare Cells Changwook Min, David Issadore, Jaehoon Chung, Huilin Shao, Monty Liong, Arezou A. Ghazani, Cesar M. Castro, Ralph Weissleder, Hakho Lee We have recently developed a miniaturized microfluidic chip-based technology, the micro-Hall detector (uHD), that can perform rapid, highly sensitive, and quantitative measurement of individual cells in unprocessed biological samples. The uHD detects the Hall voltage induced by magnetic moments of cells in-flow that have been immunomagnetically tagged with magnetic nanoparticles (MNPs) and bio-orthogonal chemistry. The entire assay is performed on a single microfluidic chip with minimal sample preparation to avoid sample loss and to simplify assay procedure, eliminating the need for any washing and purification steps, and thereby allows cellular diagnostics to be conducted in point-of-care clinical settings. We also demonstrated simultaneous detection of heterogeneous biomarkers on individual cells by targeting different cellular markers with a panel of MNPs. The quantity of each MNP type, and hence the expression level of a target biomarker in a single cell, could be obtained using the particles' distinctive magnetization properties. The clinical use of the uHD was explored by the detection of circulating tumor cells (CTCs) in whole blood of 20 ovarian cancer patients, and drug treatment efficacy was monitored in a mouse tumor model. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y45.00005: Miniaturized holographic imaging system for real-time cellular detection Jun Song, Hyungsoon Im, Monty Liong, Lioubov Fexon, Misha Pivovarov, Ralph Weissleder, Hakho Lee We herein present a miniaturized holographic imaging system for high throughput cellular detection. The system consists of an imager chip with a microfluidic channel built on top. Clinical samples (e.g., blood) are introduced into the fluidic channel, and holographic images of cells are recorded by the imager chip. We then perform computational reconstruction of original cell images, retrieving both the intensity and phase information. For fast image reconstruction, we have implemented parallel computing software and utilized multicore GPU (graphics processing unit) chips. The resulting imaging system enabled high throughput cellular detection; up to 1000 cells/$\mu$L could be imaged over a wide detection area (20 mm$^{2}$), and cellular images could be reconstructed in real time (20 frames/sec). Furthermore, assays can be performed without extra dilution and washing steps, which significantly simplifies the diagnosis process. This cost-effective, real-time holographic imaging system can be used for target cell detection in point-of-care applications. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y45.00006: Rapid detection and profiling of rare cancer cells with a portable holographic imaging system Hyungsoon Im, Jun Song, Monty Liong, Lioubov Fexon, Misha Pivovarov, Ralph Weissleder, Hakho Lee We herein present the detection and molecular profiling of rare cancer cells, using a chip-based holographic imaging system. In this approach, target cancer cells are labeled with molecular-specific microbeads. Such labeling enables 1) a reliable differentiation between cancer cells and host cells (e.g., leukocytes); and 2) quantitative profiling of target marker expression through bead-counting. A new algorithm for digital image reconstruction and bead counting was developed as well to facilitate the assay. The developed system were able to accurately count more than thousands of beads and cells in a single image. Importantly, the assay could be performed without any dilution or washing steps, minimizing cell loss and simplifying the assay procedure. By counting the number of beads attached on cells, we could also measure the expression levels of different cancer markers, which showed good agreement with profiling results by flow cytometry and fluorescence microscopy. This cost-effective, portable, flow-based holographic imaging system is applicable to detecting rare cancer cells in a large volume of blood samples for point-of-care applications. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:48AM |
Y45.00007: A single-molecule view of gene regulation in cancer Invited Speaker: Daniel Larson Single-cell analysis has revealed that transcription is dynamic and stochastic, but tools are lacking that can determine the mechanism operating at a single gene. Here we utilize single-molecule observations of RNA in fixed and living cells to develop a single-cell model of steroid-receptor mediated gene activation. Steroid receptors coordinate a diverse range of responses in higher eukaryotes and are involved in a wide range of human diseases, including cancer. Steroid receptor response elements are present throughout the human genome and modulate chromatin remodeling and transcription in both a local and long-range fashion. As such, steroid receptor-mediated transcription is a paradigm of genetic control in the metazoan nucleus. Moreover, the ligand-dependent nature of these transcription factors makes them appealing targets for therapeutic intervention, necessitating a quantitative understanding of how receptors control output from target genes. We determine that steroids drive mRNA synthesis by frequency modulation of transcription. This digital behavior in single cells gives rise to the well-known analog dose response across the population. To test this model, we developed a light-activation technology to turn on a single gene and follow dynamic synthesis of RNA from the activated locus. The response delay is a measure of time required for chromatin remodeling at a single gene. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y45.00008: Non-covalent interactions of the carcinogen (+)-anti-BPDE with exon 1 of the human K-ras proto-oncogene Jorge H. Rodriguez, Christos Deligkaris Investigating the complementary, but different, effects of physical (non-covalent) and chemical (covalent) mutagen-DNA and carcinogen-DNA interactions is important for understanding possible mechanisms of development and prevention of mutagenesis and carcinogenesis. A highly mutagenic and carcinogenic metabolite of the polycyclic aromatic hydrocarbon benzo[$\alpha$]pyrene, namely (+)-anti-BPDE, is known to undergo both physical and chemical complexation with DNA. The major covalent adduct, a promutagenic, is known to be an external (+)-trans-anti-BPDE-N$^2$-dGuanosine configuration whose origins are not fully understood. Thus, it is desirable to study the mechanisms of external non-covalent BPDE-DNA binding and their possible relationships to external covalent trans adduct formation. We present a detailed codon-by-codon computational study of the non-covalent interactions of (+)-anti-BPDE with DNA which explains and correctly predicts preferential (+)-anti-BPDE binding at minor groove guanosines. Due to its relevance to carcinogenesis, the interaction of (+)-anti-BPDE with exon 1 of the human K-ras gene has been studied in detail. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y45.00009: The Causality of Evolution on Different Fitness Landscapes Saurabh Vyawahare, Robert Austin, Qiucen Zhang, Hyunsung Kim, John Bestoso Evolution of antibiotic resistance is a growing problem. One major reason why most antibiotics fail is because of mutations on drug targets (e.g. essential enzymes). Sequencing of clinically resistant isolates have shown that multiple mutational-hotspots exist in coding regions, which could potentially prohibit the binding of drugs. However, it is not clear whether the appearance of each mutation is random or influenced by other factors. In this paper, we compare evolution of resistance to ciprofloxacin from two distinct but well characterized genetic backgrounds. By combining our recently developed evolution reactor and deep whole-genome sequencing, we show different alleles of $\sigma_s$ factor lead to fixation of different mutations in {\em gyrA} gene that confer ciprofloxacin resistance to bacteria {\em Escherichia coli}. Such causality of evolution in different genes provides an opportunity to control the evolution of antibiotic resistance. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y45.00010: Evolution of radiation resistance in a complex microenvironment So Hyun Kim, Robert Austin, Monal Mehta, Atif Kahn Radiation treatment responses in brain cancers are typically associated with short progression-free intervals in highly lethal malignancies such as glioblastomas. Even as patients routinely progress through second and third line salvage therapies, which are usually empirically selected, surprisingly little information exists on how cancer cells evolve resistance. We will present experimental results showing how in the presence of complex radiation gradients evolution of resistance to radiation occurs. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y45.00011: Modeling growth and dissemination of lymphoma in a co-evolving lymph node: a diffuse-domain approach Yao-Li Chuang, Vittorio Cristini, Ying Chen, Xiangrong Li, Hermann Frieboes, John Lowengrub While partial differential equation models of tumor growth have successfully described various spatiotemporal phenomena observed for in-vitro tumor spheroid experiments, one challenge towards taking these models to further study in-vivo tumors is that instead of relatively static tissue culture with regular boundary conditions, in-vivo tumors are often confined in organ tissues that co-evolve with the tumor growth. Here we adopt a recently developed diffuse-domain method to account for the co-evolving domain boundaries, adapting our previous in-vitro tumor model for the development of lymphoma encapsulated in a lymph node, which may swell or shrink due to proliferation and dissemination of lymphoma cells and treatment by chemotherapy. We use the model to study the induced spatial heterogeneity, which may arise as an emerging phenomenon in experimental observations and model analysis. Spatial heterogeneity is believed to lead to tumor infiltration patterns and reduce the efficacy of chemotherapy, leaving residuals that cause cancer relapse after the treatment. Understanding the spatiotemporal evolution of in-vivo tumors can be an essential step towards more effective strategies of curing cancer. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y45.00012: Computational studies on DNA recognition of novel organic and copper anti-tumor compounds Rafael R. Nascimento, Marcos B. Gon\c{c}alves, Helena M. Petrilli, Ana M. D.C. Ferreira, Emiliano Ippoliti, Jens Dreyer, Paolo Carloni The ability of many organic and coordination compounds to bind to DNA and/or damage cellular structures has been largely exploited in anticancer research. Identifying DNA recognition mechanisms have thus important impact on the chemical biology of gene expression and the development of new drugs and therapies. Previous studies on copper(II) complexes with oxindole-Schiff base ligands have shown their potential anti-tumor activity towards different cells, inducing apoptosis through a preferential attack to DNA and/or mitochondria [SIL11]. The binding mechanism of the organic and copper(II) complexes [Cu(isaepy)2]2$+$ (1) and [Cu(isaenim)]2$+$ (2) and their modulation at DNA is investigated through theoretical studies. Here we adopted a multi-scale procedure to simulate this large system using molecular docking and classical molecular dynamics. Hybrid Car-Parrinello/Molecular Mechanics calculations were applied to parameterize the copper(II) complexes by using the force matching approach. Free energies of binding are investigated by metadynamics enhanced sampling methods[VAR08]. [SIL11] V. C. da Silveira et. al. JIB 105 (2011) 1692.[VAR08] A. V. Vargiu et. al. Nucl. Acids Res. 36 (2008) 5910. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y45.00013: The positioning logic and copy number control of genes in bacteria under stress Qiucen Zhang, Robert Austin, Saurabh Vyawahare, Alexandra Lau {\em Escherichia coli} ({\em E. coli}) cells when challenged with sublethal concentrations of the genotoxic antibiotic ciprofloxacin cease to divide and form long filaments which contain multiple bacterial chromosomes. These filaments are individual mesoscopic environmental niches which provide protection for a community of chromosomes (as opposed to cells) under mutagenic stress and can provide an evolutionary fitness advantage within the niche. We use comparative genomic hybridization to show that the mesoscopic niche evolves within 20 minutes of ciprofloxacin exposure via replication of multiple copies of genes expressing ATP dependent transporters. We show that this rapid genomic amplification is done in a time efficient manner via placement of the genes encoding the pumps near the origin of replication on the bacterial chromosome. The de-amplification of multiple copies back to the wild type number is a function of the duration is a function of the ciprofloxacin exposure duration: the longer the exposure, the slower the removal of the multiple copies. [Preview Abstract] |
Session Y46: Focus Session: Physics of Proteins III
Sponsoring Units: DBIO DPOLYChair: Corey O'Hern, Yale University
Room: Hilton Baltimore Holiday Ballroom 5
Friday, March 22, 2013 8:00AM - 8:12AM |
Y46.00001: Copper Chelation in Alzheimer's Disease Protein Frisco Rose, Miroslav Hodak, Jerry Bernholc Alzheimer's disease (AD) is a neurodegenerative disorder affecting millions of people in the U.S. AD is primarily characterized at the cellular level by densely tangled fibrils of amyloid-$\beta$ protein. These protein clusters have been found in association with elevated levels of multiple transition metals, with copper being the most egregious. Interestingly, metal chelation has shown promise in attenuating the symptoms of AD in recent clinical studies. We investigate this process by constructing an atomistic model of the amyloid-$\beta$-copper complex and profile the energetic viability in each of its subsequent disassociation stages. Our results indicate that five energetic barriers must be overcome for full metal chelation. The energy barriers are biologically viable in the presence water mediated bond and proton transfer between the metal and the protein. We model the chelation reaction using a consecutive path nudged elastic band method implemented in our {\it ab initio} real-space multi-grid code to obtain a viable sequence. This reaction model details a physically consistent explanation of the chelation process that could lead to the discovery of more effective chelation agents in the treatment of AD. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y46.00002: Low resolution structures of cold, warm, and chemically denatured cytochrome-c via SAXS Christopher Asta, Anthony Banks, Margaret Elmer, Trevor GrandPre, Eric Landahl The results of a small-angle x-ray scattering (SAXS) study of equine cytochrome-c protein under different unfolding conditions are discussed. Although the measured radius of gyration of this protein over a wide range of temperatures and GuHCl concentrations conform to a two-state model, we find different levels of residual structure present depending on whether the protein is cold- or warm- denatured. We present DAMMIF reconstructions of these different unfolded states using 1532 dummy atoms with a 1.5 Angstrom radius, and suggest ways that these different states may be described by the same folding free energy. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 9:00AM |
Y46.00003: Structural dynamics of membrane proteins - time-resolved and surface-enhanced IR spectroscopy Invited Speaker: Joachim Heberle Membrane proteins are the target of more than 50{\%} of all drugs and are encoded by about 30{\%} of the human genome. Electrophysiological techniques, like patch-clamp, unravelled many functional aspects of membrane proteins but suffer from structural sensitivity. We have developed Surface Enhanced Infrared Difference Absorption Spectroscopy (SEIDAS) to probe potential-induced structural changes of a protein on the level of a monolayer. A novel concept is introduced to incorporate membrane proteins into solid supported lipid bilayers in an orientated manner via the affinity of the His-tag to the Ni-NTA terminated gold surface. General applicability of the methodological approach is shown by tethering photosystem II to the gold surface. In conjunction with hydrogenase, the basis is set towards a biomimetic system for hydrogen production. Recently, we succeeded to record IR difference spectra of a monolayer of sensory rhodopsin II under voltage-clamp conditions. This approach opens an avenue towards mechanistic studies of voltage-gated ion channels with unprecedented structural and temporal sensitivity. Initial vibrational studies on the novel light-gated channelrhodopsin-2 (ChR2) will be presented. ChR2 represents a versatile tool in the new field of optogenetics where physiological reactions are controlled by light. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y46.00004: Multistage Enzymatic Pathways of the Copper-containing Nitrite Reductase (CuNiR) Yan Li, Miroslav Hodak, Jerry Bernholc Copper-containing nitrite reductase (CuNiR) catalyzes the reduction of NO$_2^-$ to NO in the global nitrogen cycle. Experimental X-ray data have provided good insight into the overall function of CuNiR. However, many important questions, such as the relevance of the conformational change of Asp$^{98}$ as well as the transformation from the O-coordination of the substrate to the N-coordination of the product remain unanswered. We present a computational study of the enzymatic mechanism of CuNiR based on density functional theory. The climbing-image nudged elastic band (CI-NEB) method is used to find the minimum energy pathways and the activation energy barriers of the reaction. Furthermore, the effects of hybrid functionals and solvent on the activation barriers are investigated. A critical residue Asp$^{98}$ is found to control the access to the binding site and to stabilize a previously reported ``side-on'' coordination of the nitrosyl intermediate, although this geometry does not occur during the reaction. We also find that the transformation of the O- to N-attachment is achieved by an electron transfer from Type I copper. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y46.00005: Single molecule processivity and dynamics of cAMP-dependent protein kinase (PKA) Patrick C. Sims, Yongki Choi, Chengjun Dong, Issa S. Moody, Mariam Iftikhar, O. Tolga Gul, Gregory A. Weiss, Philip G. Collins Using single-walled carbon nanotube (SWNT) transistors, we monitored the processivity and dynamics of single molecules of cAMP-dependent protein kinase (PKA). As PKA enzymatically phosphorylates its peptide substrate, it generates an electronic signal in the transistor that can be monitored continuously and with 20 $\mu $s resolution. The electronic recording directly resolves substrate binding, ATP binding, and cooperative formation of PKA's catalytically functional, ternary complex. Statistical analysis of many events determines on- and off-rates for each of these events, as well as the full transistion probability matrix between them. Long duration monitoring further revealed minute-to-minute rate variability for a single molecule, and different mechanistic statistics for ATP binding than for substrate. The results depict a highly dynamic enzyme offering dramatic possibilities for regulated activity, an attribute that is useful for an enzyme that plays crucial roles in cell signaling. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y46.00006: The ribosome as an optimal decoder: a lesson in molecular recognition Tsvi Tlusty, Yonatan Savir The ribosome is a complex molecular machine that, in order to synthesize proteins, has to decode mRNAs by pairing their codons with matching tRNAs. Decoding is a major determinant of fitness and requires accurate and fast selection of correct tRNAs among many similar competitors. However, it is unclear whether the present ribosome, and in particular its large deformations during decoding, are the outcome of adaptation to its task as a decoder or the result of other constraints. Here, we derive the energy landscape that provides optimal discrimination between competing substrates, and thereby optimal tRNA decoding. We show that the measured landscape of the prokaryotic ribosome is indeed sculpted in this way. This suggests that conformational changes of the ribosome and tRNA during decoding are means to obtain an optimal decoder. Our analysis puts forward a generic mechanism that may be utilized by other ribosomes and other molecular recognition systems. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y46.00007: A Bayesian Statistical Approach for Improving Scoring Functions for Protein-Ligand Interactions Sam Z Grinter, Xiaoqin Zou Even with large training sets, knowledge-based scoring functions face the inevitable problem of sparse data. In this work, we present a novel approach for handing the sparse data problem, which is based on estimating the inaccuracy caused by sparse count data in a potential of mean force (PMF). Our new scoring function, STScore, uses a consensus approach to combine a PMF with a simple force-field-based potential (FFP), where the relative weight given to the PMF and FFP is a function of their estimated inaccuracies. This weighting scheme implies that less weight will be given to the PMF for any pairs or distances that occur rarely in the training data, thus providing a natural way to deal with the sparse data problem. Simultaneously, by providing the FFP as a substitute, the method provides an improved approximation of the interactions between rare chemical groups, which tend to be excluded or reduced in influence by purely PMF-based approaches. Using several common test sets for protein-ligand interaction studies, we demonstrate that this sparse data method effectively combines the PMF and FFP, exceeding the performance of either potential alone, and is competitive with other commonly-used sparse data methods. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y46.00008: Refinement and Selection of Near-native Protein Structures Jiong Zhang, Jingfen Zhang, Yi Shang, Dong Xu, Ioan Kosztin In recent years \textit{in silico} protein structure prediction reached a level where a variety of servers can generate large pools of near-native structures. However, the identification and further refinement of the best structures from the pool of decoys continue to be problematic. To address these issues, we have developed a selective refinement protocol (based on the Rosetta software package), and a molecular dynamics (MD) simulation based ranking method (MDR). The refinement of the selected structures is done by employing Rosetta's relax mode, subject to certain constraints. The selection of the final best models is done with MDR by testing their relative stability against gradual heating during all atom MD simulations. We have implemented the selective refinement protocol and the MDR method in our fully automated server Mufold-MD. Assessments of the performance of the Mufold-MD server in the CASP10 competition and other tests will be presented. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y46.00009: ABSTRACT HAS BEEN MOVED TO H1.00345 |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y46.00010: Diffusion and internal dynamics of proteins in crowded solutions Felix Roosen-Runge, Marcus Hennig, Tilo Seydel, Fajun Zhang, Frank Schreiber Protein function is determined through the interplay of structure, dynamics and the aqueous, but crowded cellular environment. We present a comprehensive study accessing the full hierarchy of protein dynamics in solutions, e.g. vibrations, interdomain motions and diffusion of the entire protein. Quasi-elastic neutron and dynamic light scattering experiments are performed and compared to theoretical predictions. In crowded solutions, both self diffusion $D_s$ and collective diffusion $D_c$ of protein solutions are well described by colloidal concepts, with $D_s$ reduced to $20 \%$ at $\approx 20 \%$ volume fraction [1,2]. Separating the motion of the entire protein molecule, the internal motions are accessed under native conditions [3]. We studied the dynamics before, during and after thermal denaturation, supporting the notion of protein unfolding with subsequent chain entanglement. While long-range motions are {\it reduced} in the denatured state, the local flexibility of side chains is found to be {\it enhanced}. The frameworks enable further experimental access to the relation of protein function and dynamics at fast time scales. [1] F. Roosen-Runge et al., PNAS 108 (2011) 11815; [2] M. Heinen et al., Soft Matter 8 (2012) 1404; [3] M. Hennig et al., Soft Matter 8 (2012) 1628 [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y46.00011: Diffusion of molecular oxygen in the red fluorescent protein mCherry Chola Regmi, Yuba Bhandari, Bernard Gerstman, Prem Chapagain The monomeric variants of red fluorescent proteins (RFPs), known as mFruits, have been especially valuable for tagging and tracking cellular processes \textit{in vivo. } Determining the oxygen diffusion pathways in FPs can be important for improving photostability and for understanding maturation of the chromophore. We use molecular dynamics (MD) calculations to investigate the diffusion of molecular oxygen in one of the most useful monomeric RFPs, mCherry. We investigate a pathway that allows oxygen molecules to enter from the solvent and travel through the protein barrel to the chromophore. The pathway contains several oxygen hosting pockets, which are identified by the amino acid residues that form the pocket. The results provide a better understanding of the mechanism of molecular oxygen access into the fully folded mCherry protein barrel and provide insight into the one of the photobleaching processes in this protein. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y46.00012: New insights into the picosecond dynamics solvated proteins Nguyen Vinh, Jim Allen, Kevin Plaxco According to computer simulations, the slowest, largest-scale harmonic motions of solvated biomolecules and the relaxation times of water occur on the picosecond regime. Experimental methods for the characterization of these collective vibrational modes, however, have been severely lacking. In response, we have developed a unique precision and sensitivity dielectric spectrometer. Operating over the frequency range from 0.5 GHz up to 1.1 THz, this spectrometer provides an unparalleled ability to probe the dynamics of water and aqueous proteins over the 100 fs to 100 ps timescale. Using this spectrometer to characterize the collective dynamics of solvated lysozyme we find that the collective vibrational modes of this protein are characterized by a hitherto unrecognized cutoff at 250 GHz (corresponding to 0.6 ps) arising due to the finite size of the molecule. Employing an effective medium approximation to describe the complex dielectric response of the protein in solution we find that each molecule is surrounded by a tightly held layer of 164 $\pm$ 5 water molecules that behave as if they are an integral part of the protein. The observation sheds new light on the femtosecond to picosecond collective dynamics of water and solvated biomolecules. [Preview Abstract] |
Session Y47: Invited Session: Controlling and Exploiting Topological Defects in Liquid Crystals
Sponsoring Units: DFD GSNPChair: Kathleen Stebe, University of Pennsylvania
Room: Hilton Baltimore Holiday Ballroom 6
Friday, March 22, 2013 8:00AM - 8:36AM |
Y47.00001: Colloid-in-liquid crystal gels Invited Speaker: Nicholas Abbott This presentation will describe investigations of the collective properties of colloidal particles that are dispersed in liquid crystalline solvents. A focus will be directed to recent observations of the gelation of particles dispersed in thermotropic liquid crystals. While a series of studies over the past decade have revealed two distinct mechanisms leading to gelation of particles in liquid crystalline solvents, our recent observations are inconsistent with both and hint at a third mechanism of gelation. These observations will be described along with examples of how the unique mechanical and optical properties of colloid-in-liquid crystal gels enable the design of biotic-abiotic interfaces. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 9:12AM |
Y47.00002: TopologicallyRequired Defects in Nematic Liquid Films over Microposts or in contact with Anisotropic Particles Invited Speaker: Mohamed Amine Gharbi In this work we present an experimental investigation of topological defects in nematic liquid crystals formed over micropost array with a LC-air interface pinning to the pillar edges or containing washer-shaped microparticles in suspension. For nematic-LC covered microposts with homeotropic anchoring conditions on all boundaries, including the LC-air and LC-substrate interfaces, disclination lines form that bear the signature of the micropost and satisfy global topological constraints of the system. When washer particles with different anchoring conditions are dispersed in homeotropic liquid crystal cells, new topological configurations are observed. In each case, defects are described from both a geometric and topological perspective. Finally, we demonstrate that topological defects created by microposts and washers can generate elastic interactions with dispersed microparticles in nematic liquid crystals. We believe this is a promising route to controlling colloidal self-assembly in complex media. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:48AM |
Y47.00003: Control of periodic, quasicrystalline, and arbitrary arrays of liquid crystal defects stabilized by topological colloids and chirality Invited Speaker: Ivan Smalyukh Condensed matter systems with ground-state arrays of defects range from the Abrikosov phases in superconductors, to various blue phases and twist grain boundary phases in liquid crystals, and to skyrmion lattices in chiral ferromagnets. In nematic and chiral nematic liquid crystals, which are true fluids with long-range orientational ordering of constituent anisotropic molecules, point and line defects spontaneously occur as a result of symmetry-breaking phase transitions or due to flow, but they typically annihilate with time and cannot be controlled. This lecture will discuss physical underpinnings of optically patterned and self-assembled two-dimensional arrays of long-term stable point defects and disclination loops bound together by elastic energy-minimizing twisted director structures and/or stabilized by colloids. The topological charge conservation and the interplay of topologies of genus g\textgreater\ 1 particles, fields, and defects provide robust means for controlling three-dimensional textures with arrays of optically- and electrically-reconfigurable defects. In the periodic lattices of defects, we introduce various dislocations (i.e., defects in positional ordering of defects) and use them to generate optical vortices in diffracted laser beams. The lecture will conclude with a discussion of how these findings bridge the studies of defects in condensed matter physics and optics and may enable applications in data storage, singular optics, displays, electro-optic devices, and diffraction gratings. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:24AM |
Y47.00004: Nanoparticles at fluid interfaces: how capping ligands control adsorption, stability and dynamics Invited Speaker: Valeria Garbin The spontaneous assembly of nanoparticles at fluid-fluid interfaces is exploited in microfluidic encapsulation, fabrication of nanomaterials, oil recovery, and catalysis. Control over the microstructure formed by interfacial nanoparticles is an important goal in these contexts: the ability to \emph{reversibly} tune the packing fraction enables for nanomaterials with tunable properties, while control over nanoparticle removal and recycling is desirable for green processes. I will discuss how capping ligands can promote interfacial self-assembly by tuning the interfacial energies of the nanoparticles with the fluids. Ligand-mediated particle interactions at the interface then affect the formation of equilibrium and non-equilibrium two-dimensional phases. Important differences with colloidal interactions in a bulk suspension arise due to the discontinuity in solvent properties at the interface, which cause the ligand brushes to rearrange in asymmetric configurations. I will present experimental results for gold nanoparticles capped with short amphiphilic ligands, which spontaneously adsorb at an oil-water interface. Using pendant drop tensiometry, we measured the surface pressure of the nanoparticle monolayer during adsorption and subsequent compression. In contrast to the commonly observed buckling of solid-like films of interfacial particles, upon compression these nanoparticles are mechanically forced out of the interface and into suspension. Area density measurements by a newly developed optical method reveal that ligand-mediated short-range interparticle repulsion enables desorption upon compression. Brownian dynamics simulations corroborate this picture. Therefore, ligand-mediated interactions also determine the fate of nanoparticle monolayers upon out-of-plane deformation. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 11:00AM |
Y47.00005: Resolving Defect Formation and Dynamics of the Smectic-A Mesophase Invited Speaker: Nasser Mohieddin Abukhdeir The formation and interaction of defects in liquid crystalline (LC) phases are fascinating both from a fundamental and applied perspective. Smectic LC phases, which have both orientational and translational order, exhibit relatively complex defect structures [1] and dynamics compared to lower order nematics (possessing only orientational order). A simple example of this complexity is that smectic disclination dynamics differ from those of nematics due to additional topological constraints imposed by the presence of translational order. A far less simple example is the presence of focal conic defect domains [1] that arise due to smectic elasticity favouring layer curvature over compression/dilation. Direct experimental observation of defect formation and dynamics of the smectic-A mesophase is challenging due to them occurring on the nano-scale. Theoretical approaches have had substantial success, particularly extensions of the tensorial Landau-de Gennes free energy for nematics [2] to smectic order [3]. Modelling dynamics via the time-dependent Landau-Ginzburg equation [4] has been shown to resolve topologically consistent smectic dynamics which agree with experimentally determined phase transition kinetics [5]. This talk will present an overview of recent research in this area, including the effects of an external field. The results of this research support the use of a relatively complex model of smectic dynamics. Specifically, it is shown that couplings between both short- and long-range orientational/translational order play an important role in smectic defect formation and interaction.\\[4pt] [1] Kleman, M. (1982) ``Points Lines and Walls''\\[0pt] [2] de Gennes, P. \& Prost, J. (1995) ``The Physics of Liquid Crystals''\\[0pt] [3] Mukherjee, P. K.; Pleiner, H. \& Brand, H. R. (2001) \textit{Eur. Phys. J. E}\\[0pt] [4] Desai, R. C. \& Kapral, R. (2009) ``Dynamics of Self-Organized and Self-Assembled Structures''\\[0pt] [5] Abukhdeir, N. M. \& Rey, A. D. (2008) \textit{New Journal of Physics} [Preview Abstract] |
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