Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B1: Invited Session: Logical Spin Qubits for Quantum Computation
Sponsoring Units: DCMPChair: Amir Yacoby, Harvard University
Room: Ballroom I
Monday, March 18, 2013 11:15AM - 11:51AM |
B1.00001: Entanglement of Singlet-Triplet Qubits Invited Speaker: Michael Shulman Spins in semiconductor quantum dots are promising candidates for the building blocks of a quantum information processor due to their potential for miniaturization and scalability. Singlet-triplet (S-T0) qubits, a certain type of spin qubit, store information in the joint spin state of two electrons. However, these qubits' weak interaction with the environment, which leads to their long coherence times, makes two-qubit operations challenging. We perform the first two-qubit operation between two S-T0qubits, exploiting the capacitive coupling between two adjacent qubits to generate a CPHASE gate. In order to combat low frequency noise we use a dynamically decoupled sequence that maintains the two-qubit coupling while decoupling each qubit from its fluctuating environment. Using state tomography we show that the two-qubit operation has the intended effect on the state of the qubits, and we provide definitive proof of entanglement by extracting a concurrence of 0.44 and a Bell state fidelity of 0.72. This two-qubit interaction lends itself to easily implemented improvements, which promise to generate higher fidelity entangled states that can be the basis for establishing a scalable architecture for quantum information processing. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B1.00002: The exchange-only spin qubit Invited Speaker: Charles Marcus |
Monday, March 18, 2013 12:27PM - 1:03PM |
B1.00003: Exchange-based CNOT gates for singlet-triplet qubits with spin orbit interaction. Invited Speaker: Jelena Klinovaja We propose a scheme for implementing the CNOT gate over qubits encoded in a pair of electron spins in a double quantum dot [1]. The scheme is based on exchange and spin-orbit interactions and on local gradients in Zeeman fields. We find that the optimal device geometry for this implementation involves effective magnetic fields that are parallel to the symmetry axis of the spin-orbit interaction. We show that the switching times for the CNOT gate can be as fast as a few nanoseconds for realistic parameter values in GaAs semiconductors. Guided by recent advances in surface codes, we also consider the perpendicular geometry. In this case, leakage errors due to spin-orbit interaction occur but can be suppressed in strong magnetic fields.\\[4pt] [1] J. Klinovaja, D. Stepanenko, B. I. Halperin, and D. Loss, Phys. Rev. B 86, 085423 (2012). [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:39PM |
B1.00004: Coherent Control and Manipulation of Three Spin States in a Triple Quantum Dot Invited Speaker: Andrew Sachrajda The triple quantum dot energy level spectrum is far more complex than its double quantum dot counterpart. As a result it is a challenge to cleanly manipulate only the two required qubit states without invoking more complex multi- state coherent evolution. In this talk I will describe experiments and modeling~of lateral triple quantum dot devices where by suitable device gate (i.e. energy level spectrum) tuning and pulse characteristics we were able to characterize and manipulate various three spin qubit species. In particular I will describe measurements where the Landau-Zener --St\"{u}ckelberg approach previously demonstrated in double dots is extended to three- interacting spin states permitting us to demonstrate phenomena such as pairwise exchange control. I will also~demonstrate how by tuning the experimental parameters one can~ controllably switch to coherent oscillations originating from alternative potentially~useful qubit states and how to distinguish them.\\[4pt] [1] ``Coherent Control of Three Spin States in a Triple Quantum Dot,'' L. Gaudreau et al. Nature Physics 89, 54-58 (2012)\\[0pt] [2] ``Coherent Exchange and Double Beam Splitter Oscillations in a Triple Quantum Dot,'' G.C.Aers et al. PRB 86 (2012) 045316\\[0pt] [3] ``Quantum Interference Between Three Two Spin States in a Double Quantum Dot,'' Studenikin et al. Phys. Rev. Lett. 108 (2012) 22608 [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 2:15PM |
B1.00005: Two single spin qubits with universal control and control of spin entanglement with exchange coupling Invited Speaker: Seigo Tarucha Single qubits and two-qubit gates are building blocks to prepare a universal set of logical operations. We use a micro-magnet technique to implement single spin qubits with individual quantum dots and two-qubit gates with inter-dot exchange coupling. I will talk about our recent experiments of a combined X-gate and exchange control to modulate and detect the degree of spin singlet coupling. The gate fidelity is restricted by X-gate operation time and fidelity. The X-gate is prepared by oscillating an electron inside a quantum dot with microwave (MW) in the presence of a micro-magnet induced field gradient. We have recently raised the MW power and optimized the magnet design to reduce the X-gate time $<$ 3 nsec with improved gate fidelity much shorter than the dephasing time. We have also developed a technique to use the micro-magnet induced inhomogeneous Zeeman field to make faste Z-gate and CPHASE. [Preview Abstract] |
Session B2: Invited Session: 2D Charge Ordering in Under-Doped Cuprates
Sponsoring Units: DCMPChair: Zhi-Xun Shen, Stanford University
Room: Ballroom II
Monday, March 18, 2013 11:15AM - 11:51AM |
B2.00001: Incommensurate charge density fluctuations in underdoped YBCO detected by resonant x-ray scattering Invited Speaker: Giacomo Ghiringhelli A key issue in high $T_c$ superconductivity is the short and mid range ordering of spin and charge degrees of freedom when doping disrupts the long range antiferromagnetic order of parent compounds. Cu sites are the main, although not the only, actors in the play. Inelastic and elastic scattering of x rays, when performed at the Cu $L_3$ absorption resonance, can be used to map the spin and charge excitation spectra and, simultaneously, to unveil the presence of spatial modulations in the charge or spin densities. We have used angle-resolved resonant inelastic soft x-ray scattering (RIXS) and resonant elastic soft x-ray scattering (REXS) to identify two-dimensional charge fluctuations with an incommensurate periodicity of $\sim3.2$ lattice units in the copper oxide planes of the superconductors (Y,Nd)Ba$_2$Cu$_3$O$_{6+x}$ with hole concentrations $0.09 < p < 0.13$ per planar Cu ion [G. Ghiringhelli et al, Science 337, 821 (2012)]. The intensity and correlation length of the fluctuation signal increase strongly upon cooling down to the superconducting transition temperature, $T_c$; further cooling below $T_c$ abruptly reverses the divergence of the charge correlations. In combination with prior observations of a large gap in the spin excitation spectrum, these data indicate an incipient charge-density-wave instability that competes with superconductivity. Further measurements on an Ortho III sample have confirmed that the charge fluctuations are independent of the chain ordering [A. J. Achkar et al, Phys. Rev. Lett. 109, 167001 (2012)]. Put into perspective, these results show that often elastic and inelastic x-ray scattering experiments should be ideally performed jointly, to explore with the greatest sensitivity charge and spin fluctuations [L. Braicovich et al, Phys. Rev. Lett. 104, 077002, (2010)]. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B2.00002: Charge and spin correlations in high temperature superconductors Invited Speaker: Stephen Hayden The cuprate high temperatures superconductors are characterised by numerous competing, and in some cases, co-existing broken symmetries. A important question is to what extent such additional ordered states exist for compositions with high superconducting transition temperatures. I will discuss high-energy X-ray diffraction measurements which show that a charge density wave state (CDW) develops at zero field in the normal state of superconducting YBa$_2$Cu$_3$O$_{6.67}$ ($T_c$ = 67 K). This material has a hole doping of 0.12 per copper and a well-ordered oxygen chain superstructure. Below $T_c$, the application of a magnetic field suppresses superconductivity and enhances the CDW. We find that the CDW and superconductivity are competing orders with similar energy scales, and the high-$T_c$ superconductivity forms from a pre-existing CDW environment. Our results provide a mechanism for the formation of small Fermi surface pockets which can explain the negative Hall and Seebeck effects and the $T_c$ plateau in this material. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 1:03PM |
B2.00003: Interplay between the pseudogap, mode coupling and superconductivity in Bi-based cuprates Invited Speaker: Makoto Hashimoto Complexity of the high-Tc cuprate superconductors is partly due to the coexisting energy scales that are of the order of superconducting gap ($<$50 meV). The pseudogap ($<$100 meV) and bosonic mode ($<$100 meV) could be relevant to superconductivity, but they have not been understood in a unified picture. We first show the commencement of the pseudogap state at temperature T* using three different techniques (ARPES, polar Kerr effect, and Time-resolved reflectivity) on the same optimally doped Bi2201 crystals. The result suggests that the pseudogap is a disinct phase that shows broken symmetry,\footnote{M. Hashimoto* and R.-H. He* et al., Nat. Phys. 6, 414-418, (2010).}$^,$\footnote{R.-H. He* and M. Hashimoto* et al., Science 331, 1579-1583, (2011).} which could be consistent with the two-dimentional charge ordering observed by STM and scattering measurements. Further, we discuss how this distinct pseudogap order is entangled with superconductivity below Tc. In Bi2212, by analyzing the ARPES spectral weihgt in the antinodal region, we show compelling evidence for the dynamic competition between the two order parameters for the pseudogap and superconductivity as a function of temperature.\footnote{M. Hashimoto et al., (2013)} Such competition can naturally result in the shift of the critical point for the pseudogap.\footnote{I. M. Vishik et al., PNAS 109, 18332-18337 (2012)} Moreover, by studying the detailed temperature and doping dependence of the spectral lineshape in the antinodal region, we reveal that the interplay between the pseudogap, bosonic-mode coupling and superconductivity with similar energy scales is crucial and they have to be considered in a integrated picture to understand the cuprates electronic structure.\footnote{He, Hashimoto, Science 331} \\[4pt] *These authors equally contributed to the work. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:39PM |
B2.00004: Pseudogap signatures measured in the Fermi surface of underdoped YBCO by quantum oscillations Invited Speaker: Suchitra E. Sebastian Solving the riddle of the pseudogap state in underdoped high temperature superconductors is critical to the understanding of the origin of high temperature superconductivity. Quantum oscillations performed on single crystals of the family of underdoped YBCO cuprates reveal small Fermi surface pockets in the normal state accessed at low temperatures and high magnetic fields. It has been widely thought, however, that high magnetic fields cause this state to be significantly different from the mysterious pseudogap state measured at high temperatures and low magnetic fields. In this talk I will present a quantum oscillation study of underdoped YBa$_2$Cu$_3$O$_{\rm {6+x}}$ up to magnetic fields of 100 T that reveals a dimensional collapse of the Fermi surface due to a drastic reduction in c-axis hopping, identical to the pseudogap signature measured in the low magnetic field regime. We therefore conclude that the fundamental properties of the pseudogap are encoded in the Fermi surface, an understanding of which is critical to uncovering the origin of the pseudogap in high temperature superconductors. Possible mechanisms are discussed to explain the origin of the Fermi surface in underdoped YBa$_2$Cu$_3$O$_{\rm {6+x}}$. This work was performed in collaboration with G. Lonzarich (University of Cambridge), N. Harrison, M. Altarawneh, F. Balakirev (Los Alamos National Laboratory), and R. Liang, W. Hardy, D. Bonn (University of British Columbia) [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 2:15PM |
B2.00005: Ultrasonic signatures at the superconducting and the pseudogap phase boundaries in cuprates Invited Speaker: Albert Migliori A major issue in the understanding of cuprate superconductors is the nature of the metallic state from which high temperature superconductivity emerges. Central to this issue is the pseudogap region of the doping-temperature phase diagram that extends from room temperature to the superconducting transition. To date there is no thermodynamic evidence for a phase boundary. We address this by measuring the elastic response of detwinned single crystals and observe a discontinuity in the elastic moduli across the superconducting transition with magnitude requiring that pair formation is coincident with superconducting coherence and a phase transition at the pseudogap boundary. In slightly overdoped YBCO that transition is below Tc, extending the pseudogap phase boundary inside the superconducting dome. This supports a description of the metallic state in cuprates where a pseudogap phase boundary evolves into a quantum critical point masked by the superconducting dome. [Preview Abstract] |
Session B3: Invited Session: Frustration and Quantum Criticality
Sponsoring Units: DCMPChair: Leon Balents, University of California at Santa Barbara
Room: Ballroom III
Monday, March 18, 2013 11:15AM - 11:51AM |
B3.00001: Superconductivity Near Quantum Critical Points Invited Speaker: Gilbert G. Lonzarich The study of itinerant-electron systems on the border of charge and spin density wave transitions at low temperatures is leading to an increasing number of discoveries of unusual forms of superconductivity and other types of quantum order. Examples will be reviewed of electron-electron pair instabilities in particular on the border of ferromagnetic, antiferromagnetic, ferroelectric and structural quantum phase transitions. The superconducting transition temperature in a number of nearly magnetic metals from heavy fermion compounds to the copper oxide superconductors appears to scale with the characteristic spin fluctuation temperature. These best known materials will be compared and contrasted with examples from other classes of materials in which the spin fluctuation temperature far exceeds the peak of the superconducting transition temperature in the temperature-pressure phase diagram near a magnetic quantum critical point. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B3.00002: Quantum spin liquid in organics with quasi-triangular lattices Invited Speaker: Kazushi Kanoda |
Monday, March 18, 2013 12:27PM - 1:03PM |
B3.00003: A Topological Spin Glass State of a Frustrated Magnet Invited Speaker: Seung-Hun Lee We will present a simple way of understanding the physics of the kagome-triangular-kagome trilayer antiferromagnet by mapping the magnetic interactions onto a problem of an ordered tricolor and a disordered binary sign degree of freedom. By doing so, We will show a systematic way of constructing different classical ground states, and will identify possible zero-energy excitations that involve ``partial but extended'' numbers of spins in the system. Due to the unique properties of the ground state, we argue that a topological spin glass is the ground state for the quasi-two-dimensional frustrated magnet. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:39PM |
B3.00004: On Short Ranged Resonating Valence Bond Liquids Invited Speaker: Shivaji Sondhi Over 40 years ago, P W Anderson proposed the short ranged resonating valence bond state as an alternative to Neel order in antiferromagnets with strong fluctuations---in hindsight, the first proposal for a topologically ordered $Z_2$ spin liquid. In the last year, convincing numerical evidence has accumulated for the existence of such $Z_2$ spin liquids in short ranged Hamiltonians on simple lattices in two dimensions. I will sketch the intellectually productive historical route between these two developments and survey what we now know about the physics of the short ranged RVB and allied states of matter. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 2:15PM |
B3.00005: Critical Behavior of a Strongly-Interacting 2D Electron System Invited Speaker: Myriam P. Sarachik Two-dimensional (2D) electron systems that obey Fermi liquid theory at high electron densities are expected to undergo one or more transitions to spatially and/or spin-ordered phases as the density is decreased, ultimately forming a Wigner crystal in the dilute, strongly-interacting limit. Interesting, unexpected behavior is observed with decreasing electron density as the electrons' interactions become increasingly important relative to their kinetic energy: the resistivity undergoes a transition from metallic to insulating temperature dependence; the resistance increases sharply and then saturates abruptly with increasing in-plane magnetic field; a number of experiments indicate that the electrons' effective mass exhibits a substantial increase approaching a finite ``critical'' density. There has been a great deal of debate concerning the underlying physics in these systems, and many have questioned whether the change of the resistivity from metallic to insulating signals a phase transition or a crossover. In this talk, I will report measurements [1] that show that with decreasing density $n_s$, the thermopower $S$ of a low-disorder 2D electron system in silicon exhibits a sharp increase by more than an order of magnitude, tending to a divergence at a finite, disorder-independent density $n_t$, consistent with the critical form $(-T/S) \propto (n_s - n_t)^x$ with $x = 1.0 \pm 0.1$ ($T$ is the temperature) [2]. Unlike the resistivity which may not clearly distinguish between a transition and crossover behavior, the thermopower provides clear evidence that a true phase transition occurs with decreasing density to a new low-density phase. \\[4pt] [1] Work done with S. Li and B. Wen (City College of NY), A. Mokashi and S. V. Kravchenko (Northeastern U.), A. A. Shashkin and V. T. Dolgopolov (ISSP, Chernogolovka).\\[0pt] [2] A. Mokashi, S. Li, B. Wen, S. V. Kravchenko, A. A. Shashkin and V. T. Dolgopolov, and M. P. Sarachik, Phys. Rev. Lett. {\bf 109}, 096405 (2012). [Preview Abstract] |
Session B4: Invited Session: Cold Atoms on Higher Orbital Bands
Sponsoring Units: DAMOP DCMPChair: Erhai Zhao, George Mason University
Room: Ballroom IV
Monday, March 18, 2013 11:15AM - 11:51AM |
B4.00001: Unconventional superfluidity in higher bands of an optical lattice Invited Speaker: Andreas Hemmerich Atoms trapped in optical lattices have been used successfully to study many-body phenomena. However, the shape that bosonic ground-state wavefunctions can take is limited, apparently compromising the usefulness of this approach. Such limitations, however, do not apply to excited states of bosons. The study of atomic superfluids realized in higher Bloch bands, where orbital degrees of freedom are essential, can bring the world of optical lattices closer to relevant condensed matter systems. I will discuss our observations of long coherence times, chiral superfluid order and topological features in higher bands in a square optical lattice. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B4.00002: Beyond Standard Fermi Hubbard Models Invited Speaker: Maciej Lewenstein In my talk I will focus on novel physics and novel quantum phases that are expected in a system of ultracold fermionic atoms with long range interactions, such dipolar ones. I will discuss various terms in the Hubbard model that, normally neglected, have to be included in the theory. These terms involve both lowest band physics, as well as higher bands. I will describe several exemplary effects that new terms may lead to: spontaneous breaking of symmetries, such as time-reversal, smectic-like metal phases, spontaneous formation of exotic lattices and 3D textures. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 1:03PM |
B4.00003: Higher orbital physics and artificial gauge fields with ultracold quantum gases Invited Speaker: Klaus Sengstock Recently the physics of quantum gases in higher orbitals attracted a lot of attention, theoretically and experimentally. We report on studies of a new type of superfluid described by a complex order parameter, resulting from an interaction-induced hybridization of the two lowest orbitals for a binary spin-mixture. As a main result we observe a quantum phase transition between the normal superfluid and this unconventional superfluid phase, where the local phase angle of the complex order parameter is continuously twisted between neighboring lattice sites [1]. In addition we discuss new experimental work on the creation of artificial gauge potentials for neutral atoms in 1D and 2D lattices, which do not rely on the internal structure of the atoms. Via a time-dependent driving of the optical lattice we have full control over amplitude and phase of the complex valued hopping parameters. In a 2D triangular lattice, we demonstrate the realization of gauge invariant staggered fluxes [2]. Our system consists of an array of tubes filled with bosonic atoms having a well-defined local phase. The phase distribution obtained in presence of large amplitude staggered fluxes -- where frustration plays a key role - obeys two fundamental symmetries, the discrete Ising symmetry (Z2) and a continuous global phase symmetry (U(1)). Via the full control of the staggered gauge fields [3], we are able to break the Ising symmetry on purpose which means lifting the degeneracy of the two possible Ising states, in analogy to a longitudinal homogenous magnetic field in the standard Ising-Spin model. The measurements reveal ``textbook like'' magnetization curves with the well known dependence on both, the external magnetic field and the temperature. We observe a thermally driven phase transition from an ordered Ising (ferromagnetic) to an unordered (paramagnetic) state. Future directions to combine orbital physics and gauge fields will be discussed.\\[4pt] [1] Soltan-Panahi et al., Nature Physics 8, 75 (2012)\\[0pt] [2] Struck et al., Science 333, 996 (2011)\\[0pt] [3] Struck et al., PRL 108, 225304 (2012) [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:39PM |
B4.00004: Orbital physics in one dimensional optical lattices Invited Speaker: Xiaopeng Li We explore orbital physics of fermions and bosons in one dimensional optical lattices. In a system of one dimensional $p$-orbital bosons, various phases, including anti-ferro-orbital Mott, anti-ferro-orbital superfluid and para-orbital superfluid, have been found. Signatures of phase transitions, in particular time-reversal symmetry breaking, in time-of-flight image are predicted. A fermionic ladder system composed of $s$ and $p$ orbitals is proposed, and we find a topological state featuring fractional defects. An equivalent of spin-orbit coupling naturally arises, not requiring artificial gauge field, in this quantum orbital ladder when the $s$ and $p$ orbital states are identified as a pseudo-spin 1/2. Extending this ladder system to two dimensions we find a flat-band protected by parity. The flat-band makes it plausible to study strongly correlated physics in this system. We also discuss the connection of this fermionic ladder to frustrated $\pi$ flux models and spin-orbital coupled fermions. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 2:15PM |
B4.00005: Generation and exploration of the Spin-Orbit coupled Bose gas Invited Speaker: Jian-Wei Pan To generate an artificial gauge field with ultracold quantum gas becomes a very hot topic in last few years and will continue to be attractive for ultracold atomic and condensed matter physics in the coming future. Many interesting and important topics such as Fractional Quantum Hall effect, Spin-orbit coupling and Topological insulator are connected to this topic very closely. Here we present our recent experimental progress of the synthesized gauge potential and the spin-orbit coupled Bose-Einstein condensate (BEC) in optical dipole trap. Raman coupling technique and a bias magnetic field is applied to tune the structure of the gauge potential and spin-orbit coupling. Several fundamental properties of spin-orbit coupled BEC is experimentally studied including the properties of collective dipole oscillation, the stability of excited dressed state, the critical temperature of spin-orbit coupled Bose gas and the formation of magnetic order during evaporative cooling. These studies enrich the knowledge of this field and further explorations are also in planning. [Preview Abstract] |
Session B5: Focus Session: Van der Waals Bonding in Advanced Materials - Surfaces, Growth, and Friction
Sponsoring Units: DMPChair: Jacqueline Krim, North Carolina State University
Room: 301
Monday, March 18, 2013 11:15AM - 11:51AM |
B5.00001: Exploiting London dispersion forces in nonequilibrium growth of surface-based nanostructures Invited Speaker: Zhenyu Zhang London dispersion force$^{\, }$describes the weak interaction between transient dipoles or multipoles associated with different parts of matter, and accounts for a major part of the attractive van der Waals (vdW) force. It is ubiquitous in nature, yet its importance in various physical and chemical processes just starts to be increasingly recognized. Such advances through definitive quantitative studies are largely enabled by the availability of more accurate descriptions of the weak interactions associated with long-range electron correlation effects within first-principles approaches. The present talk contains two parts, both obtained within the vdW-DF scheme on the theory side. In the first part, we critically assess the binding strengths of different classes of adatoms on ultrathin metal films of varying thicknesses. For inert gas atoms such as Xe, the London dispersion force is found to drastically enhance their adsorption, but the overall binding behavior depends only weakly on the film thickness. In contrast, for atoms with unpaired valence electrons such as H or O, the overall binding is much stronger, and also depends more sensitively on the film thickness, but with a much weaker and (in some cases) repulsive vdW contribution. These results have important implications in our developing a better understanding of atomic and molecular adsorption on different metal substrates. In the second part, we demonstrate unambiguously the decisive role of London dispersion force in non-equilibrium growth of ordered nanostructures on metal substrates using aromatic source molecules. Our multi-scale modeling integrating first-principles calculations with kinetic rate equation analysis shows that a drastic reduction in the growth temperature, from 1000$^{\circ}$C to (250-300)$^{\circ}$C, can be achieved in graphene growth on Cu(111) when the typical carbon source of methane is replaced by benzene or $p$-Terphenyl. The enhanced London dispersion forces effectively prevent easy desorption of the adsorbed molecules, facilitating their dehydrogenation, and promoting subsequent graphene growth at much lower temperatures. These strong predictions are further validated quantitatively in our experimental tests. We also demonstrate that the general trends established above are broadly applicable in graphene growth using other aromatic carbon sources. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B5.00002: Van der Waals density functional applied to adsorption systems Ikutaro Hamada The van der Waals density functional (vdW-DF) [1] is a promising density functional to describe the van der Waals forces within density functional theory. However, despite the recent efforts [2], there is still room for further improvement, especially for describing molecular adsorption on metal surfaces. I will show that by choosing appropriate exchange and nonlocal correlation functionals, it is possible to calculate geometries and electronic structures for adsorption systems accurately within the framework of vdW-DF. Applicability of the present approach will be illustrated with its applications to graphene/metal [3], fullerene/metal [4], and water/graphene interfaces [5].\\[4pt] [1] M. Dion, H. Rydberg, E. Schr{\"o}der, D. C. Langreth, B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004).\\[0pt] [2] See for e.g., K. Lee, {\'E}. D. Murray, L. Kong, B. I. Lundqvist, D. C. Langreth, Phys. Rev. B 82, 081101(R) (2010).\\[0pt] [3] I. Hamada and M. Otani, Phys. Rev. B 82, 153412 (2010).\\[0pt] [4] I. Hamada and M. Tsukada, Phys. Rev. B 83, 245437 (2011).\\[0pt] [5] I. Hamada (submitted). [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B5.00003: Nanotribological Properties of Positively and Negatively charged nanodiamonds as additives to solutions Zijian Liu, Steven Corley, Olga Shenderova, Donald Brenner, Jacqueline Krim Nano-diamond (ND) particles are known to be beneficial for wear and friction reduction when used as additives in liquids,[1] but the fundamental origins of the improvement in tribological properties has not been established. In order to explore this issue, we have investigated the nanotribological properties of ND coated with self-assembled monolayers (SAM) as additives to solutions, employing gold/chrome coated quartz crystal microbalances (QCM). Measurements were performed with the QCM initially immersed in deionized water. ND particles with positively and negatively charged SAM end groups were then added to the water, while the frequency and amplitude of the QCM were monitored. Negative shifts in both the QCM frequency and amplitude were observed when ND with positively charged SAM end groups were added, while positive shifts in both the QCM frequency and amplitude were observed when ND with negatively charged ND end groups were added . The results are consistent with a lubricating effect for the negatively charged ND, but were only observed for sufficiently small negative ND particle size. Experiments on QCM surfaces with differing textures and roughness are in progress, to determine the separate contributing effects of surface roughness charge-water interactions. 1. V. N. Mochalin, et al, Nat. Nanotech. 7, 11--23 (2012) doi:10.1038/nnano.2011.209 [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B5.00004: Adhesion in Nanodiamond Particles Vasudeva Rao Aravind, Luke Lutkus, Benjamin Legum Due to their excellent mechanical properties and biologically non-toxic nature, nanodiamonds show great promise for applications in tribology, lubrication, drug delivery, tissue scaffolds and surgical implants. In order to design effective nanocomposites and other biomedical systems exploiting these properties, it is important to understand the properties and mechanisms by which nanodiamonds adhere to other materials, and how they behave at interfaces. In this article, the adhesive force between nanodiamond particles and the silicon scanning probe microscope tip are reported. The adhesive force can be correlated to the purity and functionalization of nanodiamond surface, and the values range from 0.1nN to 2.0nN for the samples studied. It is observed that the lateral forces applied by the scanning probe tip can cause the adhesive forces to increase by an order of magnitude from 0.1 to 2.0nN at regions where the tip experiences maximum contact force. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B5.00005: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 12:39PM - 12:51PM |
B5.00006: Adsorption and intercalation of Cs atoms on epitaxial graphene on Ir(111) Predrag Lazic, Marin Petrovic, Iva Srut, Ivo Pletikosic, Milorad Milun, Petar Pervan, Sven Runte, Carsten Busse, Thomas Michely, Damir Sokcevic, Radovan Brako, Nicolae Atodiresei, Jurek Sadowski, Zhi-Hu Pan, Tonica Valla, Marko Kralj From the experimental studies of surface adsorption of Cs atoms and their intercalation under epitaxial graphene on Ir(111) it is known that both - adsorbed and intercalated phase of Cs atoms coexist. However, adsorbed phase is realized as a diluted superlattice adlayer of Cs atom while intercalated phase is a dense Cs layer. The preference for intercalated phase at large Cs layer densities can not be obtained from the DFT calculations with semilocal (GGA) functionals. Only after the van der Waals interaction is taken into account the agreement with experiment is achieved. From the results of calculations it follows that the main energy contribution responsible for the switching of preference from adsorption to intercalation is the graphene delamination energy from the Ir(111) surface which is dominantly of the van der Waals nature. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B5.00007: Structure and Morphology of Copper Phthalocyanine Films on Graphene and Graphite Terry McAfee, Tianshuai Guan, Sean Stewart, Eliot Gann, Jack Rowe, Harald Ade, Daniel Dougherty Indium tin oxide (ITO) is the most widely used anode in organic photovoltaic (OPV) applications. It has several disadvantages, including elemental scarcity and a very rough surface morphology that influences the structure of organic thin film active layers. Alternative transparent conducting anode materials must be identified for use in organic optoelectronic devices. An exciting possibility is the use of graphene since it offers high performance electrical characteristics, good transparency, and a very flat template for high quality film growth. Hard x-ray scattering reveals a ``face-on'' orientation of copper phthalocyanine (CuPc) on graphene, in contrast to its ``edge-on'' orientation on ITO. This is advantageous for solar cells due to enhanced absorption as well as improved charge transport to the anode via pi-pi stacking. Atomic force microscopy shows that CuPc forms large crystalline domains on graphene that should improve carrier mobility, as well as increase the exciton diffusion length leading to improved charge separation. These unique characteristics suggest significantly improvements in the Jsc and FF of CuPc based OPV devices using graphene as an anode material. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B5.00008: Atomic Friction Studies of Nitrogen and Oxygen Uptake on Magnetic substrates by means of the Quartz Crystal Microbalance Technique Zachary Fredricks, Keeley Stevens, Jacqueline Krim In the study of friction at the nanoscale, phononic, electrostatic, conduction electron and magnetic effects all contribute to the dissipation mechanisms [1]. Magnetic contributions have been alluded to in past studies, but remain poorly characterized. We will report here our efforts to detect magnetic friction for sliding adsorbed films on various magnetic films substrates in the presence and absence of an external field. Using a quartz crystal microbalance (QCM), we record the sliding friction of liquid monolayers of nitrogen, a diamagnetic material, as well as liquid oxygen, a paramagnetic material, on nickel alloy and graphene/nickel surfaces. In the prior literature, these systems have been reported to exhibit sensitivity to external fields.\\[4pt] [1] I. Altfeder and J. Krim, J. Appl. Phys. (2012) \\[0pt] [2] Highland et al., PRL (2006) [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B5.00009: Developing of van der Waals parameters for graphitic carbon-water interaction using ab initio methods Yanbin Wu, Narayana Aluru In this study, graphitic carbon-water van der Waals interaction parameters are developed entirely from first-principle calculation data. First, the M{\o}ller-Plesset perturbation theory of the 2nd order (MP2) method is employed to compute the polycyclic aromatic hydrocarbon-water interaction energies. The proper size of basis sets is utilized in the MP2 calculations and the energy component analysis is performed to extrapolate to infinite-sized graphene limit. Then, graphitic carbon-water interaction parameters are developed based on the MP2 results from this work and the ab initio data available in the literature from other methods such as random-phase approximation (RPA), diffusion Monte Carlo (DMC), density functional theory-symmetry-adapted perturbation theory (DFT-SAPT) and couple cluster treatment with single and double excitations and perturbative triples (CCSD(T)). We evaluate the accuracy of the interaction parameters by predicting water contact angle on graphite and compare it with experimental data. The interaction parameters based on RPA, DFT-SAPT and corrected DMC data predict contact angles which agree well with experiments, while the parameters based on MP2 and CCSD(T) data have the tendency to underestimate the contact angle. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B5.00010: Field-effect modulation of water adsorption on the TiO$_2$ (110) surface from van der Waals density functional theory Abraham Hmiel, Yongqiang Xue The interaction of water and the titanium dioxide surface has been identified as a target problem across many possible fields of application of electrochemical devices and sensors, as the surface chemistry at the interface is not well-understood. This work applies self-consistent van der Waals density functional theory and the effective screening medium theory\footnote{M. Otani, O. Sugino Phys. Rev. B \textbf{73}, 115407 (2006)} to study the surface chemistry and wetting of H$_2$O on nanostructured TiO$_2$ surfaces. Water-TiO$_2$ substrate interactions are probed from the monomer limit up to monolayer coverage under an external electric field in a charged capacitor model. We illustrate the competitive effect between the electric field and the descriptions of the hydrogen bonding induced by the application of the van der Waals functional by analyzing the energetics, charge partitioning, and bonding at the interface. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B5.00011: Thermodynamic Stability and Structure of Oxidized Cu(110) Surfaces: The Critical Role of non-Local Interactions Joseph Bamidele, Jan Brndiar, Ivan Stich, Lev Kantorovitch Thermodynamic stability of oxidized Cu(110) surface is studied using DFT techniques. At high oxygen exposures standard techniques predict more phases to be quasi-isoenergetic, whereas experiments observe only the $c$(6 $\times$ 2) phase at high oxygen exposures clearly indicating this phase to be the ground-state separated by considerable energy differences from other candidates. We show that this surface system is stabilized by a delicate coexistence and balance of chemi- and physi-sorption. Agreement with experiments is only achieved if the van der Waals interaction between the surface templates is accounted for in DFT thermodynamics. Moreover, van der Waals stabilization of the surface structure is anticipated to be a general feature present also in the cases of other related surfaces. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B5.00012: A Density Functional Theory Examination of the Local Conformational Energetics of Normal and Epigenetically Modified Duplex DNA Tahir Yusufaly, Wilma Olson We report density functional theory calculations of various local regions of duplex DNA, including hydrogen bonded base pairs, stacked nearest-neighbor bases, and sugar-phosphate backbones. Special attention is given to the methylation of 5-cytosine, an epigenetic modification believed to play a key role in eukaryotic gene regulation. Energetically stable molecular conformations are identified and their elastic properties analyzed. Our results are compared with previous ab initio studies and high-resolution crystalline structural data. [Preview Abstract] |
Session B6: Focus Session: CVD Graphene - Growth and Characterization
Sponsoring Units: DMPChair: Luigi Colombo, Texas Instruments
Room: 302
Monday, March 18, 2013 11:15AM - 11:27AM |
B6.00001: Ultrafast dynamics of non-thermal hot electrons in chemical vapor deposited graphene Kuan-Chun Lin, Ming-Yang Li, Cheng-Chung Chi, Jeng-Chung Chen The relaxation dynamics of photoexcited carriers in a chemical vapor deposited graphene transferred on quartz substrate are investigated using ultrafast optical-pump terahertz (THz)-probe spectroscopy. Terahertz transmission through graphene sample is reduced by optical pumping. The change of transmission decays exponentially after the optical pulse. We find the decay time is insensitive to the substrate temperatures from 10~K to 300~K, but increases sublinearly with pump flunce. We model the relaxation process involving electron-phonon coupling together with a set of rate equations to describe the transient responses of quasi-particals and optical phonons. We can fit the observered transient terahertz transmission very well. The extracted carrier temperature follows the same trend of decay time as a function of fluence. High pump fluence can significantly increase the carrier temperature and broaden the carrier distributions, consequently causing the reduction of optical phonon emission efficiency and slowing down cooling rate. The differences of our results in comparison to similar measurements of expitaxial graphene on SiC will be disscussed. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B6.00002: Raman Spectroscopic Studies of Room-Temperature-Grown Graphene by Plasma-Assisted Chemical Vapor Deposition Chen-Chih Hsu, David Boyd, Wei-Hsiang Lin, Jong Yeon Lee, Nai-Chang Yeh We have synthesized graphene using plasma-assisted chemical vapor deposition (CVD) at room temperature (RT). Structural analysis through Raman spectroscopy reveals that high quality large-area graphene can be grown reproducibly. From the frequency shifts of the G-band and 2D-band, it is evident that the average strain of RT-grown graphene becomes much reduced relative to the high-temperature (1000$^{\circ}C)$ CVD-grown graphene. This finding is confirmed by the atomically resolved images taken with scanning tunneling microscopy (STM). To investigate the effect of different substrates on the resulting strain in graphene, we have grown graphene on Cu(111) and Cu(100) single crystals and polycrystalline Cu foils. Compared to high temperature CVD-grown graphene, strain is reduced no matter which substrate was used for the RT growth. However, graphene grown on Cu(111) is more inhomogeneous because anisotropic plasma etching of the substrate results in excess steps on the surface and creates stripe-like superstructures in graphene. Upon transferring the RT-grown graphene to SiO2 substrates, we find the average strain minimized. Our results suggest a promising pathway to inexpensive growth of high-quality large-area graphene. This work was supported by NSF through IQIM at Caltech. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B6.00003: Carbon atom bonding processes in CVD graphene growth on copper surface: A first principles study Takahisa Ohno, Nobuo Tajima, Tomoaki Kaneko, Jun Nara Graphene has attracted considerable research interest due to potential application to future electronic devices. Large area and high-quality graphene is needed for device applications. Chemical vapor deposition using copper surface with hydrocarbon source is one of the practical methods to produce graphene. This method is appropriate for creating large area graphene, and the graphene growth control to obtain high quality product is a challenge. The carbon atom nucleation and cluster growth processes in the CVD reactions have been studied extensively as key steps to control graphene growth. In the present study, first principles molecular dynamics calculations are performed to obtain fundamental insight into these C-C bonding process. First principles simulation code PAHSE (http://www.ciss.iis.u-tokyo.ac.jp/english/project/device/) was used in these calculations. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B6.00004: Probing dynamics in graphene with infrared spectroscopy Invited Speaker: Jie Shan Infrared and far-infrared spectroscopy provides an attractive approach for examining the properties of charge carriers in solids. For the case of graphene, while these possibilities had been recognized, experiments were hindered by the lack of samples of sufficient lateral extent to be probed by standard far-field techniques. Now, with the advent of high-quality graphene grown by chemical vapor deposition (CVD), researchers are able to overcome these limitations. Analysis of optical conductivity by infrared spectroscopy provides direct information about the carrier scattering rate, as well as the intraband and interband transition strength in graphene. Furthermore, when combined with a femtosecond excitation pulse, time-resolved terahertz (or far-infrared) spectroscopy allows us to probe the ultrafast relaxation dynamics of electrons in graphene. In this talk, I will discuss recent results on infrared spectroscopy and dynamics studies of CVD graphene, with an emphasis on identifying the role of electron-phonon and electron-electron interactions and the influence of doping on these interactions. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B6.00005: Graphene originated 3D structures grown on the assembled nickel particles Tereza Paronyan, Avetik Harutyunyan Recently, the fabrication of various morphologies of graphene originated structures became very important due to the perspective of wide range of new applications. Particularly, free standing 3D structured graphene foams could be imperative in energy related areas$.$ Here, we present the new approach of the CVD growth of 3D graphene network by using primarily sintered Ni particle's ($\sim$40$\mu $m size) assembles as a template-catalyst via decomposition of low rate of CH$_{4}$ at 1100$^{\circ}$ C based on synthesis method described earlier [1]. SEM and Raman spectra analysis revealed the formation of graphene structure containing a single up to few layers grown on the sintered metal particles served as a catalyst-template. After etching the metal frame without using any support polymer, 3D free-standing graphene microporous structure was formed demonstrating high BET surface area. Two probe measurements of frame resistance were $\sim$2-8$\Omega $. Our approach allows controllable tune the pore size and thereby the surface area of 3D graphene network through the variation of the template-catalyst particles size. \\[4pt] [1]. T. M. Paronyan et al. ACS Nano, \textbf{5}, p. 9619 (2011) [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B6.00006: Improving the quality of CVD graphene-based devices: synthesis, transfer, fabrication and measurement Junjie Wang, Bei Wang, Anna Skinner, Jun Zhu Graphene synthesized by chemical vapor deposition (CVD) is potentially useful in a wide range of electronic and optoelectronic applications. In order to obtain CVD-graphene based devices with performance comparable to their exfoliated counterparts, improvement needs to be made on the synthesis and transfer of graphene, as well as device fabrication and measurement techniques. Here we report on a low-pressure growth procedure, which successfully suppresses the growth of multilayer patches, resulting in large-scale single-layer graphene production. By following the etching of the copper substrate with a HCl/H$_{\mathrm{2}}$O$_{\mathrm{2}}$ cleaning step similar to the RCA-2 procedure used in Silicon industry, metal particle contamination is reduced. By applying the gate voltage in pulse, we eliminate the hysteresis commonly observed in the transfer curve of graphene field effect transistors. This allows us to accurately determine the charge neutrality point and carrier mobility of the device. We are able to achieve high-quality CVD-graphene devices with average carrier mobility of 7,000 cm$^{\mathrm{2}}$V$^{\mathrm{-1}}$s$^{\mathrm{-1}}$. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B6.00007: Linear magnetoresistance of graphene in contact with inhomogeneous disordered graphitic carbon Jinglei Ping, Michael Fuhrer We synthesized graphene via chemical vapor deposition(CVD) on platinum foils and transferred graphene to Si$_{\mathrm{3}}$N$_{\mathrm{4}}$ membranes for inspection by transmission electron microscope (TEM), or to SiO$_{\mathrm{2}}$/Si for fabricating field-effect transistors. Dark-field TEM shows that the graphene is decorated with disordered (nanocrystalline) graphitic carbon which is spatially inhomogeneous. The impurity layer can easily be mistaken for a second graphene layer in optical microscopy. Atomic force microscopy shows that impurities form between graphene and Pt, supporting a ``growth-from-below'' model. The impurity-decorated graphene exhibits linear magnetoresistance (LMR) which is carrier-density-dependent and nonsaturating up to 8 Tesla. No LMR is observed with graphene samples with little impurities, or in exfoliated graphene. We understand the LMR as due to an effective inhomogeneous random-resistor network arising from the spatially inhomogeneous nature of the graphene/impurity system. The results may shed light on the previously-observed LMR in graphene on Si-face SiC. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B6.00008: Triggering the Growth of Large Single Crystal Graphene by Chemical Vapor Deposition Tianru Wu, Haomin Wang, Guqiao Ding, Da Jiang, Xiaoming Xie, Mianheng Jiang Graphene, a monolayer of sp2 carbon atoms, has been attracting great interests as an ideal two dimensional crystalline material. Fabrication technique for wafer scale graphene via chemical vapor deposition (CVD) was developed several years ago [1]. However, large scale graphene films from CVD method so far are found to be polycrystalline, consisting of numerous grain boundaries, which greatly degrade the electrical and mechanical properties of graphene [2]. Recently, we obtained hexagonal-shaped single-crystal monolayer graphene domains ($\sim $1.2 mm) [3]. We adapted a strategy to synthesize larger size single crystal grains by regulating the supply of reactants and hytrogen. Nucleation density can be decreased to less than 1000 nuclei /m2. Gradually increase in the supply of reactants could break the equilibrium of growth and etching at the edge of hexagonal-shaped graphene grains. It drives the reaction toward quick growth of graphene domains during the whole CVD process. The graphene grains we obtained show high crystalline quality with high mobility of $\sim$13000 cm2V-1s-1, which is comparable to that of exfoliated graphene. The results achieved will definitely benefit for further practical application of graphene electronics. [1] Li X S, et al. Science, 2009, 324: 1312$\sim$1314. [2] Huang PY, et al. Nature 2011, 469: 389-392. [3] Wu T R, et al. Adv. Func. Mater. 2012, Doi: 10.1002/adfm.201201577. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B6.00009: Superior properties of plasma-assisted room-temperature-grown graphene from STM studies M.L. Teague, W.-H. Lin, D.A. Boyd, N.-C. Yeh, Y.-Y. Lo, C.-I. Wu, W.-Y. Chan, W.-B. Su, C.-S. Chang We report scanning tunneling microscopic and spectroscopic (STM/STS) studies of large-area monolayer graphene grown at room temperature (RT) on Cu foils, Cu (100) and Cu (111) single crystals, and compare the properties of these samples with high-temperature (1000 $^{\circ}$C) CVD-grown graphene. All RT-grown graphene exhibit highly ordered honeycomb structures over $\sim$ 1 cm$^{2}$ areas, smooth surface morphology, much reduced strain (\textless\ 0.1{\%}) and additional Moire patterns for samples grown on single crystals. The structural quality and reduced strain obtained from STM studies are consistent with finds from Raman spectra. In contrast, high-temperature CVD-grown graphene revealed strongly distorted atomic structures and large strain, giving rise to giant pseudo-magnetic fields and charging effects as manifested by the conductance peaks at quantized energies and the strongly enhanced local conductance in highly strained regions. These strain-induced effects are believed to be responsible for the reduced electrical mobility in typical CVD-grown graphene. The superior structural and electronic properties demonstrated by our RT-grown graphene are promising for a wide range of applications. This work was supported by NSF through IQIM at Caltech. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B6.00010: Structural and Electrical Properties of CVD and PECVD Grown Graphene Michelle Langhoff, W. Mitchel, E. Gallo, G. Tompa, N. Sbrockey, T. Salagaj, K. Ghosh There is a robust research effect on graphene due to its unique properties. While the ultimate goal of this research is to study the electrical properties of graphene, a multistep process of research is require to reach the point at which it is possible to make the necessary measurements. Graphene is typically grown using CVD on a copper substrate: this substrate has been found to offer the best results to date. Unfortunately, this requires the transfer to alternate, non-conducting, substrates in order to effect electrical measurements. This work seeks to determine the optimal transfer process of graphene using Raman spectroscopy and analyzing the prominence of the defect peak. Upon the success of the transfer, electrical properties are evaluated using AFM. This work will discuss the difference in growth quality between standard CVD growth and PECVD, evaluate the success of transfer to alternate substrates, and provide results from preliminary electrical measurements using AFM. We would like to acknowledge Structured Materials Industries Inc. for providing graphene samples. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B6.00011: Catalyst-free growth of nanographene and its application Dongxia Shi, Wei Yang, Donghua Liu, Rong Yang, Guangyu Zhang A new method was developed to synthesis graphene films on various substrates without catalyst at low temperature, which was performed using our home-made remote plasma enhanced chemical vapor deposition system (r-PECVD). The fabricated graphene film is composed of nanographene islands with hexagonal shape and size of several hundred nanometers. Through the adjustment of temperature, the nucleation and growth were fully controlled, in this way, nanographene films with expected crystal size and layers can be obtained. Furthermore, the fabricated nanographene films was also investigated in strain sensors, which shows ultra-sensitive properties with the highest gauge factor over 300 so far for graphene-based strain sensors. The piezoresistive characteristics of nanographene films are based on charge tunneling from neighboring nanographene islands. Besides strain sensors, this simple and scalable graphene fabrication also provides a potential way in many applications fields, such as electrode materials, transparent conductive films, thin film resistors, gas sensors and so on. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B6.00012: Engineering epitaxial graphene with oxygen Amina Kimouche, Sylvain Martin, Clemens Winkelmann, Olivier Fruchart, Herv\'e Courtois, Johann Coraux Almost free-standing graphene can be obtained on metals by decoupling graphene from its substrate, for instance by intercalation of atoms beneath graphene, as it was shown with oxygen atoms [1]. We show that the interaction of oxygen with epitaxial graphene on iridium leads to the formation of an ultrathin crystalline oxide extending between graphene and the metallic substrate via the graphene wrinkles. Graphene studied in this work was prepared under ultra-high vacuum by CVD [2,3]. The samples were studied by combining scanning probe microscopy (STM, AFM) and spatially resolved spectroscopy (Raman, STS). The ultrathin oxide forms a decoupling barrier layer between graphene and Ir, yielding truly free-standing graphene whose hybridization and charge transfers with the substrate have been quenched [4]. Our work presents novel types of graphene-based nanostructures, and opens the route to the transfer-free preparation of graphene directly onto an insulating support contacted to the metallic substrate which could serve as a gate electrode. References [1] Sutter, P. \textit{et al. }J. Am. Chem. Soc. 132, 8135 (2010). [2] Coraux, J. \textit{et al.} Nano Lett. 8, 565 (2008). [3] Vo-Van, C~; Kimouche, A \textit{et al. } Appl. Phys. Lett. 98, 181903 (2011). [4] Kimouche, A\textit{ et al. }Fully decoupling graphene from its substrate via wrinkles. \textit{Submitted} [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B6.00013: Graphene growth using Pulsed Laser Deposition Gautam Hemani, Manuel Quevedo-Lopez, Massimo V. Fischetti To obtain improved electrical performance in graphene, an unconventional growth process using pulsed laser deposition (PLD) where graphene is grown directly on a silicon substrate is proposed. Using PLD, graphene was grown directly on device quality wafer using nickel metal and then characterized with Raman spectroscopy. Also, the Electron Backscatter Diffraction technique was used to characterize the grain structure of the Nickel after deposition in order to understand how the high temperatures affected the graphene growth process. Attempts have also been made to integrate this unconventional growth with standard semiconductor device fabrication in order to explore transfer free graphene based devices. Raman spectroscopy revealed that we have well defined spectra indicating from monolayer to few layer graphene, with minimum defects. [Preview Abstract] |
Session B7: Focus Session: Graphene Devices II
Sponsoring Units: DMPChair: Arthur Hebard, University of Florida
Room: 303
Monday, March 18, 2013 11:15AM - 11:27AM |
B7.00001: Tunnel magnetoresistance of magnetic junctions based on side-wall epitaxial graphene nanoribbons Chao Huan, John Hankinson, Wenlong Yu, Rui Dong, James Palmer, Owen Vail, Ming Ruan, Claire Berger, Edward Conrad, Walter de Heer, Zhigang Jiang We report on tunnel magnetoresistance (TMR) measurements of magnetic tunnel junctions consisting of cobalt, aluminum oxide barrier, and side-wall epitaxial graphene nanoribbons (GNRs). We find that the measured resistance of tunnel junctions exhibits a spin switch behavior when the magnetic field is applied parallel to the cobalt electrode and sweeping between 1 T and -1 T. This observation indicates that the side-wall GNR is magnetic, with a spin component either parallel or antiparallel with respect to the magnetization direction of cobalt. The largest relative change of TMR observed is about 9{\%} at 6.6 K, corresponding to 14{\%} of spin polarization in GNR. In addition, we find that Rashba effect may play an important role in polarizing the electron spins in GNR; the required electric field could be due to the charge transfer between the carbon atoms on the edge of GNR and the Si atoms of the SiC substrate. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B7.00002: Superconductor-graphene based quantum entangler, a progress report Ivan Borzenets, Yuya Shimazaki, Juergen Sailer, Russell Deacon, Michihisa Yamamoto, Seigo Tarucha We report on the progress in fabricating a functioning quantum entangler. The device is based on the cooper-pair splitter ``T'' junction with either lead (Pb) or niobium (Nb) acting as the superconductor and graphene acting as the normal metal. Unlike the typically used aluminum (Al), lead and niobium have a superconducting transition at much higher temperatures (meaning a higher superconducting gap $\Delta$), thus increasing the extent of the proximity effect. Proper techniques had to be developed in order to create transparent, superconductivity inducing contacts to graphene; and graphene-based Josephson junctions were fabricated and characterized. Meanwhile, graphene features high mobility, and therefore a high coherence length. We have patterned graphene into constrictions resulting in individually gated quantum dots with consistent characteristics. This is required in order to prevent both electrons from the same cooper pair from traveling into a single normal lead. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B7.00003: C-axis magnetoresistance in epitaxially grown multilayer graphene Srikrishna Bodepudi, Abhay Singh, Sandipan Pramanik Magnetoresistance, the change in electrical resistance of a solid-state system as a function of an external magnetic field, is a key effect in condensed matter physics both for fundamental understanding of charge transport phenomena as well as immense commercial implications. Artificial layered structures, such as metallic or metal-insulator multilayers often exhibit ``giant magnetoresistance'' or ``tunnel magnetoresistance'' effects that are exploited in various state-of-the-art data storage and magnetic field sensing devices.. Graphite is a naturally occurring layered structure in which graphene layers are stacked up on each other. Magnetoresistance in graphitic systems has drawn significant attention in recent years due to the unique crystal structures of these materials, which often lead to novel physics. In this work we consider epitaxial multilayer graphene on nickel and studied c-axis charge transport when the magnetic field is applied normal to the graphene plane. We show that the electrical resistance measured across the graphene stack on nickel can be reduced by two orders of magnitude by applying a relatively small magnetic field of few kilogausses normal to the layer plane. This feature persists even at room temperature and is far stronger than any other magnetoresistance effect reported to date for comparable temperature and field conditions. Existence of such effect makes multilayer graphene an attractive platform for magnetic field sensing, data storage and exploration of fundamental insights into graphene physics. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B7.00004: Graphene barristor for high performance devices Invited Speaker: Jinseong Heo Graphene has unique properties, such as work-function tunability and high intrinsic mobility. Recently, we have introduced a new concept device, a graphene variable-barrier ``barristor'' (GB), based on those properties. In this presentation, I will describe the three-terminal active device, GB, where the key is an atomically sharp interface between graphene and hydrogenated silicon surface. Large modulation on the device current, on/off ratio of 100000, is achieved by adjusting the gate voltage to control the Schottky barrier between graphene and silicon. The barrier height was tuned to 0.2 electron volts by adjusting graphene work function which results in large shifts of diode threshold voltages. For logic application, an inverter and a half-adder were demonstrated using the complementary GB's on 150-mm wafers. In addition, recently developed vertical transistor based on asymmetric junctions will be discussed. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B7.00005: Gate Tunable Graphene-Silicon Ohmic/Schottky Contact Chun Chung Chen, Chia Chi Chang, Zhen Li, Anthony Levi, Steve Cronin We have recently demonstrated gate tunable graphene-silicon Schottky diodes, in which the low bias conductance can be varied by more than three orders of magnitude [1,2]. Here, we deposit graphene on silicon substrates and observe the rectifying $I-V$ characteristics in graphene-silicon junctions, indicating the formation of Schottky junction due to the mismatch of their work functions. By applying a polymer electrolyte gate to the graphene surface, the Fermi energy of the graphene can be shifted $\pm$ 0.85eV from its charge neutrality point ($-$4.6eV) to match the conduction ($-$4.01eV ) or valence band ($-$5.13eV) of silicon to reduce the Schottky barrier and result in Ohmic contacts with both $n$- and $p$-type silicon. The $I-V$ characteristics observed under light illumination also indicate that the short circuit current can be increased or decreased by varying graphene-silicon work function difference, further demonstrating that the graphene-silicon junction and be changed between Schottky and Ohmic contact. \\[4pt] [1] Chen, Aykol, Chang, Levi, and Cronin, ``Graphene-Silicon Schottky Diodes.'' Nano Letters, 11, 1863-1867 (2011).\\[0pt] [2] Chen, Chang, Li, Levi, Cronin, ``Gate Tunable Graphene-Silicon Ohmic/Schottky Contacts.'' Applied Physics Letters, accepted (2012). [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B7.00006: Hole Injection from Silicon to Oxide Using Graphene as Transparent Electrode Rusen Yan, Huili G. Xing, Nhan Van Nguyen We demonstrate a novel application of graphene as a transparent electrode in internal photoemission (IPE) spectroscopy. Owing to its low absorption in the IR/Visible/UV range, graphene enables the direct observation of hole injection, and thus the measurement of both conduction and valence band offsets at the semiconductor-oxide hetero-interface. The photocurrents, consisting of electron or hole transitions between Si substrate and graphene as a function of incident photon energy under various applied gate voltage are measured. The barrier height is further determined from the photoemission quantum yield, which is defined as the ratio of photocurrent and light intensity. As a result, the barrier heights, $\varphi _{e}^{0}$, from the valence band top in Si to the bottom of the conduction band in Al$_{2}$O$_{3}$, and $\varphi _{h}^{0}$, from the bottom of the conduction band in Si to the top of the valence band in Al$_{2}$O$_{3}$ are extracted to be 3.5 eV and 4.1~eV, respectively. Furthermore, the bandgap of Al$_{2}$O$_{3}$ can be simply obtained by$\mathop E\nolimits_{g}^{\mathop {Al}\nolimits_{2} \mathop O\nolimits_{3} } =\mathop \phi\nolimits_{e} +\mathop \phi\nolimits_{h} -\mathop E\nolimits_{g}^{Si} =3.5+4.1-1.1=6.5$eV, close to previously reported values. Similar phenomenon is also observed and confirmed by replacing Al$_{2}$O$_{3}$ with 10 nm SiO$_{2}$. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B7.00007: Thin film barristor: a gate tunable vertical graphene-pentacene-gold device Claudia Ojeda-Aristizabal, Wenzhong Bao, Michael S. Fuhrer Graphene, a one atom thick crystal made of carbon, shows exciting possibilities as a tunable electrode for semiconductors. Graphene's electrochemical potential can be tuned over a span of electron volts, and graphene is expected to have no interface states. Here we explore graphene as a tunable electrode contacting pentacene, a van der Waals molecular semiconductor which should also have no interface states. We fabricate a vertical thin-film barristor device consisting of highly doped silicon (gate), 300 nm SiO$_2$ (gate dielectric), monolayer graphene, pentacene, and gold top electrode. During fabrication an intermediate layer of SiO$_2$ is deposited over the graphene leaving a small hole for the pentacene contact, insuring vertical transport. We show that the current across the device is modulated by the Fermi energy level of graphene, tuned with an external gate voltage. We interpret the device current within thermionic emission theory. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B7.00008: First-Principles Study of Contact Resistance between Graphene and Metal Electrodes Tomoaki Kaneko, Takahisa Ohno Graphene attracts much interest for post-silicon electronics material due to its outstanding electronic transport properties such as considerably high mobility at room temperature. For the application of electronics devices, contacting of metal electrodes is necessary and decreasing of contact resistance between graphene and the metal electrodes is regarded as one of a key issue. In this study, we investigate the contact resistance using DFT+NEGF method. We consider the Ni and Cu electrode within LDA and TM-type norm-conserving pseudo-potential. We employed PHASE code [1] to determine the interface structures. Then, we constructed two terminal device structures in which current flows from metals to graphene. The electron transport properties were calculated using ASCOT code[2]. For Ni electrode, the dependence of the electrode size qualitatively agrees well with that obtained by the experiments. But our results suggest that contact resistance can be reduced considerably. [1] http://www.ciss.iis.u-tokyo.ac.jp/english/project/device/. [2] H. Kondo, J. Nara, H. Kino and T. Ohno, Jpn. J. Appl. Phys. 47, 4792 (2008). [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B7.00009: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 1:27PM - 1:39PM |
B7.00010: A 1D wide band gap graphene metal-semiconductor-metal junction for devices Meredith Nevius, Jeremy Hicks, Antonio Tejeda, Amina Taleb-Ibrahimi, Feng Wang, Edward Conrad Despite many advances in understanding graphene physics, progress towards a working, reproducible graphene-based switch has been nearly stagnant. Mastering obstacles like lithographic limitations, process-induced disorder, scalability, and reproducibility is absolutely crucial. We have successfully grown graphene over patterned steps on silicon carbide and, using angle resolved photoemission spectroscopy, have discovered a one-dimensional metal-semiconducting-metal junction made completely from graphene. The junction is created by inherent graphene-substrate interactions as the graphene grows over the patterned steps. The semiconducting graphene strip is connected on either side by metallic graphene sheets and has a band gap of greater than 0.5 eV.[REF] In addition, experimental results show that the average electronic band structure of thousands of ribbons varies very little even on length scales of tens of microns. We will present results on the growth of these graphene structures along with angle resolved photoemission spectroscopy measurements that reveal the band structure of both the graphene ribbons on the step facets and the 1D semiconducting strip. REF Nature to be published [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B7.00011: Probing Klein tunneling through angle dependence of resistance across graphene p-n junctions Atikur Rahman, Janice Wynn Guikema, Nina Markovic We have studied the angle-dependent resistance characteristics of ``Y''-shaped dual-gated graphene p-n junction devices. Different arms of each device share a common top gate, and the branching of current in the arms at different angles is determined by the transparency of p-n junctions formed under the top gate. For a particular back gate and top gate voltage, we first balanced the voltage drop in the straight and angled arms, and then we studied the variation of the resistance as a function of top gate keeping the back gate voltage fixed. Deviation from the balanced condition with varying top gate voltage measures the transparency of the p-n junctions in the arms. We found that this deviation is large for a p-n*-p or n-p*-n configurations, as compared to p-p*-p or n-n*-n junctions, which provides a direct evidence of the angle-selective transmission of charge carriers in graphene p-n junction. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B7.00012: Chemically functionalized graphene for bipolar electronics Bernard Matis, Jeffrey Baldwin, Brian Houston We discuss the use of chemical functionalization, in particular hydrogenation, to achieve control of the local carrier type and density in graphene, which is a prerequisite for the development of graphene-based bipolar electronics. Transport measurements are used to demonstrate independent carrier types and densities within adjacent semi-metallic graphene and semiconducting hydrogenated graphene regions. Measurements of the Hall coefficient confirm that the graphene and hydrogenated graphene charge carriers change sign about the charge neutrality point, that the graphene carrier density retains its linear dependence on a back gate voltage, and reveal that the hydrogenated graphene carrier density deviates from such a linear relationship. Measurements across the bipolar interface reveal an increasing resistance for higher hydrogen concentrations and a source of constant resistance across a range of back gate voltages for lower hydrogen concentrations. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B7.00013: Resist-free graphene/metal interaction extracted through quantum capacitance measurement R. Ifuku, K. Nagashio, T. Nishimura, A. Toriumi Understanding of the graphene/metal interaction is crucially important from both scientific and practical viewpoints. In the electric device structure, it is reported that graphene under the metal electrodes maintains the linear dispersion regardless of kinds of metals. In case of graphene grown on metals, on the other hand, the modulation of the linear dispersion strongly depends on kinds of metals, e.g. band modulation occurs on Ni and not on Au. The key issue to elucidate this discrepancy can be the resist residual in the device fabrication process. In this study, the resist-free graphene/metal interaction was studied from the density of states (DOS)--energy relation determined by the quantum capacitance measurement of metal/graphene/SiO$_{\mathrm{2}}$/n$^{\mathrm{+}}$-Si stack. Graphene in resist-free contact with Au maintains the linear DOS-energy relation, except near the Dirac point. Graphene contacting Ni shows larger DOS at the Dirac point, resulting in limited gate modulation of E$_{\mathrm{F}}$ in graphene. Resist free process reveals the intrinsic difference in the strength of the graphene/metal interaction ($\pi $-d coupling or van der Waals) for Ni and Au. [Preview Abstract] |
Session B8: Focus Session: Hexagonal BN, Graphene, and Graphene Oxide Synthesis I
Sponsoring Units: DMPChair: Mauricio Terrones, Pennsylvania State University
Room: 307
Monday, March 18, 2013 11:15AM - 11:51AM |
B8.00001: TBD Invited Speaker: Pulickel Ajayan |
Monday, March 18, 2013 11:51AM - 12:03PM |
B8.00002: Molecular beam growth of sub-monolayer and multilayer graphene on h-BN flakes Lara Fernandes dos Santos, Sheng Wang, Ulrich Wurstbauer, Jorge M. Garcia, Lei Wang, Antonio Levy, Jungsik Park, Cory Raymond Dean, Loren N. Pfeiffer, James Hone, Aron Pinczuk We report the successful growth of graphene layers on h-BN substrate flakes in a MBE environment. The growth configuration was designed to allow a gradient in the deposition rate (DR) of carbon on the substrate. The growth conditions such as the substrate temperature were highly controlled. Characterization is carried out by spatially resolved Raman spectroscopy and by AFM imaging. We investigated the graphene coverage on the h-BN flakes. The flakes could be partially covered by a sub-monolayer film, fully covered by a single layer or fully covered by a multilayer film. We find high quality graphene in sub-monolayer and single layer growths. We found a striking independence on the carbon DR, which is attributed to the high mobility of carbons atoms on the h-BN surface. This is a characteristic feature of van der Waals molecular beam growth. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B8.00003: Synthesis and Characterization of Large-Area Graphene Directly CVD-Grown on h-BN Minwoo Kim, Young Jae Song, Min Wang, Seong-Kyu Jang, Sungjoo Lee, Won-Jun Jang, Se-Jong Kahng As an ideal substrate for graphene, hexagonal boron nitride (h-BN) has been utilized and studied extensively by transfer technique, which still has a high chance to have impurities at the graphene/h-BN interface. Here we report direct CVD growth of graphene on large area h-BN film. AFM and Raman spectroscopy measurements show that there is only one monolayer of graphene, and whose unperturbed electronic structures are also confirmed by electron transport measurements and scanning tunneling spectroscopy. High resolution TEM images for cross-section taken before and after transferring graphene/h-BN on to SiO2 indicate this CVD-grown hybrid structure is robust enough. Based on this new method, high quality and large area graphene on h-BN film with a clean interface can be synthesized for the application of electronic devices, and can fill the missing steps to grow fully CVD-grown super-structure of graphene and h-BN. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B8.00004: Initial Growth of h-BN Chanyong hwang, E.K. Seo, Jun Park, Wondong Kim, Inho Lee Recently h-BN has drawn a lot of attention due to its use as an insulating layer for graphene application. Its growth on several transition metal surfaces such as Ni has been focused on their local atomic structure and superstructure formed on surfaces. However, the growth of h-BN in a large has not been studied so far. We found very interesting growth mode of h-BN on Cu surface. The shape of island is strongly dependent on the orientation of the Cu surface, which is quite different from that of the graphene on Cu. Based on the growth model, a fairly large grain size of h-BN(order of 0.1 mm) can be made. More detailed process on the growth of h-BN will be discussed. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 1:03PM |
B8.00005: In-situ detection of nano-crack of graphene using polarized optical microscopy Invited Speaker: Jong-Hyun Ahn Recent works for producing large-area, high quality graphene films through chemical vapor deposition (CVD) and transferring them onto various large-area substrates have offered the possibility of their use as transparent conductive films in various optoelectronic devices. However, various kinds of defects such as pinhole, nano-crack and grain boundaries incorporated for CVD growth process or transfer process of graphene to target substrates degrade the electrical and mechanical performance, which limit the quality needed for the practical use of graphene films. In particular, knowledge of the mechanism of defect generation in graphene under high strain is important to apply graphene in flexible and stretchable electronic devices. Therefore, various methods have been studied to understand the mechanism of defect generation and observe such defects directly. For example, microscopic tools such as TEM, AFM and STM have a way to observe grain boundaries and defects of graphene. However, these methods have drawbacks such as requirement of a complicated sample preparation, a time delay and limited size of observation. In this talk, we present in situ visualization method to identify the distribution of defects in graphene such as pinhole and crack created by growth and transfer process. In addition, we suggest the alignment of liquid crystal molecules on graphene shows strong correlation with domain size of graphene. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B8.00006: Enhanced catalytic reactivity of graphene and h-BN by selective substitution Junhao Lin, Bin Wang, Sokrates Pantelides Recent experiments have demonstrated that nitrogen-doped graphene is an efficient metal-free catalyst for the oxygen reduction reaction in fuel cells, but the underlying mechanism still needs to be explored. Using first-principles calculations, we find that in N-doped graphene oxygen molecules can only dissociate at carbon atoms surrounded by nitrogen. We attribute the enhanced chemical reactivity of these carbon atoms to the strong localized states near the Fermi level, which results from misalignment of pz orbitals of nitrogen and carbon atoms. We further show that the dissociation of oxygen molecules can also occur in hydrogenated graphene and h-BN based on the same mechanism. Therefore, we propose a generic way for functionalization of graphene to achieve enhanced catalytic reactivity. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B8.00007: CVD graphene growth via magnetic inductive heating of metal substrates Richard Piner, Huifeng Li, Xianghua Kong, Li Tao, Jongho Lee, Deji Akinwande, Rodney Ruoff A new route to the CVD synthesis of graphene with inductive heating of metal substrates is presented. The design and implementation of a new type of reactor that uses magnetic induction to heat metal substrates is presented. The advantages of this reactor and important parameters for the successful growth of high quality graphene or few layer graphene will be presented. Optical and SEM images, Raman spectra, and electron and hole mobility will be presented and compared to results for more traditional CVD methods [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B8.00008: Synthesis of Large-grain, Single-crystalline Monolayer and AB-stacking Bilayer Graphene Luyao Zhang, Yung-Chen Lin, Yi Zhang, Han-Wen Chang, Wen-Cheng Yeh, Chongwu Zhou We report the growth of large-grain, single-crystalline monolayer and AB-stacking bilayer graphene by the combination of ambient pressure chemical vapor deposition and low pressure chemical vapor deposition. The shape of the monolayer graphene was modified to be either hexagons or flowers under different growth conditions. The size of the bilayer graphene region was enlarged under ambient pressure growth conditions with low methane concentration. Raman spectra and selected area electron diffraction of individual graphene grain indicated that the each graphene grain is single-crystalline. With electron beam lithography patterned PMMA seeds, graphene nucleation can be controlled and graphene monolayer and bilayer arrays were synthesized on copper foil. Electron backscatter diffraction study revealed that the graphene morphology had little correlation with the crystalline orientation of underlying copper substrate. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B8.00009: CVD growth of large-grain graphene on Cu(111) thin films David L. Miller, Kyle M. Diederichsen, Mark W. Keller Chemical vapor deposition of graphene on polycrystalline Cu foils has produced high quality films with carrier mobility approaching that of exfoliated graphene. Growth on single-crystal films of Cu has received less attention, despite its potential advantages for graphene quality and its importance for eventual applications. This is likely due to the difficulty of obtaining large ($\ge$ 1 mm) grains in Cu thin films, as well as dewetting and roughening of Cu films at temperatures near the Cu melting point (1084 C). We found that 450 nm of Cu(111), epitaxially grown by sputtering onto Al$_2$O$_3$(0001), formed $> 1$ mm grains when annealed at 1065 C for 40 minutes in 40 Torr of Ar and 2.5 mTorr of H$_2$. After this annealing, adding 3 mTorr of CH$_4$ for 8 minutes produced a monolayer graphene film covering $> 99\%$ of the Cu surface. Stopping growth after 4 minutes produced dendritic graphene islands with 6-fold symmetry and diameter of 20 $\mu\textrm{m}$ to 100 $\mu\textrm{m}$. After growth, the Cu film remained smooth except for thermal grooving at grain boundaries and a few holes of diameter $\approx$ 10 $\mu\textrm{m}$ where Cu dewetted completely ($\approx10$ holes on each 5 mm x 6 mm chip). [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B8.00010: Drastic reduction in the growth temperature of graphene on Cu substrates via enhanced London dispersion force Jin-Ho Choi, Zhancheng Li, Ping Cui, Xiaodong Fan, Changgan Zeng, Zhenyu Zhang London dispersion force is ubiquitous in nature, and is increasingly recognized to be an important factor in a variety of surface processes. Here we demonstrate unambiguously the decisive role of London dispersion force in non-equilibrium growth of ordered nanostructures on metal substrates using aromatic source molecules. Our first-principles based multi-scale modeling shows that a drastic reduction in the growth temperature, from $\sim$1000 $^{\circ}$C to $\sim$300 $^{\circ}$C, can be achieved in graphene growth on Cu(111) when the typical carbon source of methane is replaced by benzene or p-Terphenyl. The London dispersion force enhances their adsorption energies by about (0.5-1.8) eV, thereby preventing their easy desorption, facilitating dehydrogenation, and promoting graphene growth at much lower temperatures. These quantitative predictions are validated in our experimental tests. The general trends established are also applicable in graphene growth using other aromatic carbon sources, and more broadly in molecular assembly and synthesis of surface-based nanostructures. [Preview Abstract] |
Session B9: Invited Session: FIP Symposium on the Science of Climate
Sponsoring Units: FIPChair: Eugene Chudnovsky, City University of New York - Lehman College
Room: 308
Monday, March 18, 2013 11:15AM - 11:51AM |
B9.00001: Climate Concerns: Asking the Right Questions Invited Speaker: Richard Lindzen |
Monday, March 18, 2013 11:51AM - 12:27PM |
B9.00002: Solar Variability and Climate Change Invited Speaker: Joanna Haigh The need to distinguish natural from anthropogenic causes of climate change makes it important to understand and quantify any impact of the Sun. In this talk I will outline what is known about variations in solar output and review the evidence for solar influences on climate over a range of timescales. When the Sun is more active our work shows the response in temperature is not a warming of the tropics but mainly of mid-latitudes, along with a weakening and poleward shift of the jet streams and storm-tracks. Using climate models we have found that an important factor driving this response is the absorption in the stratosphere of solar UV radiation and we have identified a dynamical coupling mechanism which transfers a solar signal from the stratosphere to the atmosphere below. This means that simple assessments of the solar impact based on energy balance ideas may be effective in estimating global mean temperature change but might be neglecting important effects on regional climate. During the last solar cycle minimum the Sun was in a state of very low activity and some satellite measurements have suggested that the solar spectrum has been behaving in a strange and unexpected way. The talk will finish with a discussion of recent work on the implications of these spectral variations. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 1:03PM |
B9.00003: On Winning the Race for Predicting the Indian Summer Monsoon Rainfall Invited Speaker: Bhupendra Nath Goswami Skillful prediction of Indian summer monsoon rainfall (ISMR) one season in advance remains a ``grand challenge'' for the climate science community even though such forecasts have tremendous socio-economic implications over the region. Continued poor skill of the ocean-atmosphere coupled models in predicting ISMR is an enigma in the backdrop when these models have high skill in predicting seasonal mean rainfall over the rest of the Tropics. Here, I provide an overview of the fundamental processes responsible for limited skill of climate models and outline a framework for achieving the limit on potential predictability within a reasonable time frame. I also show that monsoon intra-seasonal oscillations (MISO) act as building blocks of the Asian monsoon and provide a bridge between the two problems, the potential predictability limit and the simulation of seasonal mean climate. The correlation between observed ISMR and ensemble mean of predicted ISMR (R) can still be used as a metric for forecast verification. Estimate of potential limit of predictability of Asian monsoon indicates that the highest achievable R is about 0.75. Improvements in climate models and data assimilation over the past one decade has slowly improved R from near zero a decade ago to about 0.4 currently. The race for achieving useful prediction can be won, if we can push this skill up to about 0.7. It requires focused research in improving simulations of MISO, monsoon seasonal cycle and ENSO-monsoon relationship by the climate models. In order to achieve this goal by 2015-16 timeframe, IITM is leading a Program called Monsoon Mission supported by the Ministry of Earth Sciences, Govt. of India (MoES). As improvement in skill of forecasts can come only if R {\&} D is carried out on an operational modeling system, the Climate Forecast System of National Centre for Environmental Prediction (NCEP) of NOAA, U.S.A has been selected as our base system. The Mission envisages building partnership between operational forecasting agency and National and International R {\&} D Organizations to work on improving modeling system. MoES has provided substantial funding to the Mission to fund proposals from International R {\&} D Organizations to work with Indian Organizations in this Mission to achieve this goal. The conceptual framework and the roadmap for the Mission will be highlighted. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:39PM |
B9.00004: Stratospheric ozone: a major (long neglected) anthropogenic forcing of the climate system Invited Speaker: Darryn W. Waugh As a consequence of the Montreal Protocol, the depletion stratospheric ozone by CFCs, which occurred primarily in the last decades of the 20th Century, has noticeably slowed down in recent years. For instance, the ozone hole in 2012 has been measured to be the smallest in 20 years. In view of this, it has long been thought that the ozone hole is a ``solved problem.'' What has not been appreciated until very recently is that the large man-made perturbation of stratospheric ozone has had profound consequences on the climate system in the Southern Hemisphere. In fact, a lot of evidence is now at hand strongly suggesting that ozone depletion, not increasing greenhouse gases, have been been the major driver of observed atmospheric circulation changes in the Southern Hemisphere in the second half of the 20th Century. Furthermore, climate models robustly show that the closing of the ozone hole in the next half century will actually oppose the impact of increasing greenhouse gases, and project large cancellations between these two anthropogenic forcings resulting in greatly reduced future trends in the Southern Hemisphere. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 2:15PM |
B9.00005: Climate of Mars and Other Planets Invited Speaker: Francois Forget |
Session B10: Invited Session: Celebrating 100 Years of Physical Review at APS
Sponsoring Units: FHPChair: Don Howard, University of Notre Dame
Room: 309
Monday, March 18, 2013 11:15AM - 11:51AM |
B10.00001: In the Beginning... Invited Speaker: Martin Blume Physical Review was founded at Cornell University in 1893, by two professors, Edward L. Nichols and Ernest Merritt. Both were educated in Germany, and were familiar with the differences in publications abroad. They were enthusiastic about the idea of an American publication devoted entirely to physics. They fortunately had the full support of the then President of Cornell, J. Gould Shurman, who arranged for an initial grant of {\$}500, and eventually, for a first year total of {\$}2500. The founding editors were soon joined by Frederick Bedell, who remained an Editor into the 1920's. This talk will follow the progress (and otherwise) of the journal through the formation of the American Physical Society in 1899 and the transfer of its operation from Cornell to the APS in 1913. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B10.00002: The American Reception of the Quantum as Seen by the Physical Review, 1900-1927 Invited Speaker: Robert Crease This talk tells the story of the ``American awakening'' to quantum theory seen through the pages of the \textit{Physical Review}. It begins with the journal's first mentions of Planck and the quantum, follows the story through publication of the first papers on experiment and theory, and concludes just after Schr\"{o}dinger's 1926 \textit{Physical Review} article on wave mechanics -- which reflected the Austrian physicist's realization that at last there existed a large enough American audience interested in theoretical developments of quantum mechanics to make such an article worth writing and publishing. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 1:03PM |
B10.00003: ``Your Most Distinguished Contributor'': Einstein and the Physical Review Invited Speaker: Daniel Kennefick Einstein began to publish in the Physical Review after he began working with his first American research assistant, Nathan Rosen. They submitted three landmark papers together to the journal. These papers and their reception are discussed, along with the remarkable story of Einstein's umbrage at the referee report he received in response to his third submission. Although the referee was vindicated and Einstein eventually had to reverse his position, he never submitted a research paper to the Physical Review again. The identity of the referee, as learned from the Review's own records, will be revealed and Einstein's subsequent relationship with the journal will be discussed. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:39PM |
B10.00004: Bringing the Physical Review into the Digital Age Invited Speaker: Mark Doyle Efforts to make the entire contents of the Physical Review available digitally began early in the 1990's and closely tracked the development of the World Wide Web itself. Not satisfied with just publishing electronic versions of newly published material, the APS also embarked on one of the first systematic digitizations of the entire contents of a major series of journals in any discipline. While all APS journals were online by 1997 and the backfile archive was completed in early 2002, the journals have continued to grow and adapt as the digital environment has matured. This talk will give an account of the evolution of APS journals through the digital transition, from boxes of backup tapes to the full-fledged, invaluable online resource it is today. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 2:15PM |
B10.00005: Physical Review: a family of journals Invited Speaker: Gene Sprouse The expansion of research in physics in the last 100 years has been reflected in the expansion of the Physical Review(PR). Reviews of Modern Physics was the first ``new'' journal, starting in 1929. Physical Review Letters commenced in 1958, and was the first ``letters'' type of journal for important new results in all fields. By 1970 the Physical Review itself had grown so large that it was necessary to separate it by field into manageable volumes: PRA, PRB, PRC and PRD, and subsequently PRE, which was split off from PRA. More recently, two Special Topics journals for accelerator physics and physics education were pioneers of the open access business model, and the newest member of the family, Physical Review X, continues this trend. PRX is broad scope and very selective, setting it well above many of the new open access journals with a review standard of ``not incorrect.'' Some possible future directions for the Physical Review journals will be discussed. [Preview Abstract] |
Session B11: Invited Session: Polymer Membranes for Clean Energy and Water II
Sponsoring Units: DPOLY GERAChair: Ali Evern Ozcam, University of California, Berkeley
Room: 310
Monday, March 18, 2013 11:15AM - 11:51AM |
B11.00001: Polymer-Derived Membranes for Large Scale Energy-Efficient Separations Invited Speaker: William Koros A significant fraction of global energy is consumed to meet separation and purification needs of society, since existing processes are based primarily on energy intensive operations such as distillation. In fact, movement to alternative raw material sources tends to increase this consumption, since separation needs are more difficult to meet in such cases. Energy intensity and carbon dioxide emissions associated with many large scale separations can be reduced by a full order of magnitude by substituting membrane processes for traditional thermally-driven separation approaches. This presentation will provide a framework illustrating how such a strategy can be applied. An advanced manufacturing perspective relying upon polymer-derived materials is stressed within this framework. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B11.00002: Dramatic nano-fluidic properties of carbon nanotube membranes as a platform for protein channel mimetics Invited Speaker: Bruce Hinds Carbon nanotubes have three key attributes that make them of great interest for novel membrane applications: 1) atomically flat graphite surface allows for ideal fluid slip boundary conditions and extremely fast flow rates 2) the cutting process to open CNTs inherently places functional chemistry at CNT core entrance for chemical selectivity and 3) CNT are electrically conductive allowing for electrochemical reactions and application of electric fields gradients at CNT tips. Pressure driven flux of a variety of solvents (H2O, hexane, decane ethanol, methanol) are 4-5 orders of magnitude higher than conventional Newtonian flow [Nature 2005, 438, 44] due to atomically flat graphite planes inducing nearly ideal slip conditions. However this is eliminated with selective chemical functionalization [ACS Nano 2011 5(5) 3867-3877] needed to give chemical selectivity. These unique properties allow us to explore the hypothesis of producing ``Gatekeeper'' membranes that mimic natural protein channels to actively pump through rapid nm-scale channels. With anionic tip functionality strong electroosmotic flow is induced by unimpeded cation flow with similar 10,000 fold enhancements [Nature Nano 2012 7(2) 133-39]. With enhanced power efficiency, carbon nanotube membranes were employed as the active element of a switchable transdermal drug delivery device that can facilitate more effective treatments of drug abuse and addiction. Recently methods to deposit Pt monolayers on CNT surface have been developed making for highly efficient catalytic platforms. Discussed are other applications of CNT protein channel mimetics, for large area robust engineering platforms, including water purification, flow battery energy storage, and biochemical/biomass separations. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 1:03PM |
B11.00003: Structure Formation of Block Copolymer Membranes Invited Speaker: Volker Abetz Isoporous membranes have received increasing attention during the last couple of years. The advantage of these materials is to give access to membranes with a very high number density of pores with controlled diameters, thus leading to ultrafiltration membranes with a very high permeability, and simultaneously also with a very high selectivity in terms of size exclusion. Different approaches have been reported, which typically involve the transfer of a thin block copolymer film from a solid to a porous support, eventually followed by an edging step. An alternative strategy is to form integral asymmetric membranes, where the thin top layer is continuously changing into a spongy support layer, thus avoiding the build-up of mechanical stresses. This happens by subjecting the cast polymer solution film into a precipitant, inducing the so-called phase inversion by exchange of solvent with the non-solvent. Here it is important to have a system where solvent and nonsolvent are fully miscible. This strategy also enables the direct formation of open pores without a subsequent edging step, if the solvents and nonsolvents are appropriately chosen. Different types of amphiphilic block copolymers based on styrene, 2- or 4-vinyl pyridine, and ethylene oxide with various compositions and molecular weights will be discussed. These block copolymers were dissolved at different concentrations in various solvent mixtures, and then cast on a non-woven support, which was either pretreated with a liquid, or not. Varying the time before the cast solution was subjected to phase inversion, as well as choosing the temperature of the precipitation bath, are further parameters having strong influence on the obtained membrane film structure. Membranes with pore forming blocks showing pH or temperature sensitive behaviour can be reversibly switched from an open state to a closed state. The size of the pores can be controlled by both molecular weight and composition of the block copolymers. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:39PM |
B11.00004: Scalable Directed Self-Assembly and Anisotropic Transport Properties of Soft Mesophases for Membrane Applications Invited Speaker: Chinedum Osuji Self-assembly of block copolymers and surfactant mesophases can be can be utilized in creating composite materials with very fine periodic structures. Easy access to nm-scale features coupled with compositional variety and thus tunable physical properties makes these nanoscale heterogeneous materials excellent candidates for selective transport applications including ion-conduction, ultrafiltration and desalination. A critical limitation in their performance however arises from the tortuosity of randomly oriented self-assembled structures. We show that in appropriately engineered systems, magnetic fields provide a viable route for scalable control of morphology, producing well aligned materials over large length scales. Here we discuss this approach for the fabrication of ion conduction membranes, aligned carbon nanotube membranes and nanoporous films. We quantitatively assess the anisotropic transport properties of one such system and confront the data with models based on effective medium theory and composite conductivity calculations. The results demonstrate that directed self-assembly can provide non-trivial enhancement of the transport properties in these applications. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 2:15PM |
B11.00005: Understanding the Permeation of Solutes in Water Treatment Membranes Invited Speaker: William Phillip The responsible management of the world's water resources is essential to supporting human life on earth. The successful development of reverse osmosis seawater desalination makes it a crucial component in the portfolio of water supply options. However, other measures to alleviate the stresses on water supplies are necessary to responsibly and sustainably meet the worldwide demand for fresh water. Osmotically driven membrane processes (ODMP) are an emerging set of technologies that show promise in water conservation and reuse, as well as wastewater reclamation. The majority of research in the field has focused on predicting and enhancing water permeation through membranes, however, the effective operation of ODMP systems requires that the permeation of solutes across water treatment membranes be better understood. For example, the reverse flux of draw solute from the concentrated draw solution into the feed solution should be minimized. Additionally, due to the presence of solute-solute interactions that arise because of the unique geometry of ODMPs, the rejection of dilute solutes in these processes can be dramatically different than those observed in traditional pressure driven operations. In this talk, theoretical and experimental approaches are used to explore the permeation of solutes in osmotically driven membrane processes. Phenomenological models were developed that describe the forward and reverse permeation of the solutes across an asymmetric membrane in forward osmosis operation; and experiments were carried out to validate the model predictions. Using independently determined membrane transport coefficients, strong agreement between the model predictions and experimental results was observed. [Preview Abstract] |
Session B12: Focus Session: Complex Oxide Interfaces - Titanates
Sponsoring Units: DMPChair: Mikel Holcomb, West Virginia University
Room: 314
Monday, March 18, 2013 11:15AM - 11:51AM |
B12.00001: First-principles modeling of titanate/ruthenate superlattices Invited Speaker: Javier Junquera The possibility to create highly confined two-dimensional electron gases (2DEG) at oxide interfaces has generated much excitement during the last few years. The most widely studied system is the 2DEG formed at the LaO/TiO$_{2}$ polar interface between LaAlO$_{3}$ and SrTiO$_{3}$, where the polar catastrophe at the interface has been invoked as the driving force. More recently, partial or complete delta doping of the Sr or Ti cations at a single layer of a SrTiO$_{3}$ matrix has also been used to generate 2DEG. Following this recipe, we report first principles characterization of the structural and electronic properties of (SrTiO$_{3}$)$_{5}$/(SrRuO$_{3}$)$_{1}$ superlattices, where all the Ti of a given layer have been replaced by Ru. We show that the system exhibits a spin-polarized two-dimensional electron gas extremely confined to the 4$d$ orbitals of Ru in the SrRuO$_{3}$ layer, a fact that is independent of the level of correlation included in the simulations. For hybrid functionals or LDA+U, every interface in the superlattice behaves as minority-spin half-metal ferromagnet, with a magnetic moment of $\mu$ = 2.0 $\mu_{\rm B}$/SrRuO$_{3}$ unit. The shape of the electronic density of states, half metallicity and magnetism are explained in terms of a simplified tight-binding model, considering only the $t_{2g}$ orbitals plus (i) the bi-dimensionality of the system, and (ii) strong electron correlations. Possible applications are discussed, from their eventual role in thermoelectric applications to the possible tuning of ferromagnetic properties of the 2DEG with the polarization of the dielectric. Work done in collaboration with P. Garc\'ia, M. Verissimo-Alves, D. I. Bilc, and Ph. Ghosez. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B12.00002: Polarization controlled Ohmic to Schottky transition at a metallic oxide-doped ferroelectric interface Xiaohui Liu, Yong Wang, J.D. Burton, Evgeny Tsymbal Recently the coexistence of ferroelectricity and conductivity was observed in electron-doped BaTiO$_3$ [1], opening an exciting avenue for novel ferroelectric device applications. A basic structure which may be used for future applications is the metal/ferroelectric hetero-junction. Using first-principles methods and taking the SrRuO$_3$/BaTiO$_3$ interface as a prototypical system, we investigate the effects of polarization reversal in BaTiO$_3$ on the electronic transport across this interface. Our studies show a significant change in the resistance by switching ferroelectric polarization. This arises due to the polarization driven conversion of the interface from the Ohmic to the Schottky regime, i.e. for one polarization orientation the interface exhibits a tunneling barrier, whereas the interface is metallic for the opposite polarization orientation. Our prediction represents a new path for ferroelectric devices and may lead to exciting new applications as non-volatile memories and logic. \\[4pt] [1] T. Kolodiazhnyi et al, Phys. Rev. Lett. 104, 147602 (2010). [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B12.00003: Controlling the density of electrons in the 2DEG at complex oxide interfaces Chris Van de Walle, Lars Bjaalie, Luke Gordon, Anderson Janotti The formation of a two-dimensional electron gas (2DEG) at the interface between two insulators, SrTiO$_{\mathrm{3\thinspace }}$(STO) and LaAlO$_{\mathrm{3}}$ (LAO), has sparked huge interest in oxide electronics. In spite of almost a decade of research, the mechanisms that determine the density of this 2DEG have not yet been unravelled. The polar discontinuity at the STO/LAO interface can in principle sustain an electron density of 3.3x10$^{\mathrm{14\thinspace }}$cm$^{\mathrm{-2\thinspace }}$(0.5 electrons per unit cell). However, experimentally observed densities are more than an order of magnitude lower. Using a combination of first-principles and Schr\"{o}dinger-Poisson simulations we investigate the origin of the electrons in the 2DEG at the STO/LAO interface. We analyze the asymmetric nature of the heterostructures, i.e., the inability to form a second LAO/STO interface that is a mirror image of the first, and the effects of passivation of the LAO surface. Our results apply to oxide interfaces in general, and explain why the SrTiO$_{\mathrm{3}}$/GdTiO$_{\mathrm{3\thinspace }}$interface has been found to exhibit the full density of 0.5 electrons per unit cell. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B12.00004: Two-dimensional superconductivity induced by high-mobility carrier doping in LaTiO3/SrTiO3 hetero-structures Johan Biscaras, S. Hurand, C. Palma, J. Lesueur, N. Bergeal, D. Leboeuf, C. Proust, A. Rastogi, R.C. Budhani Transition metal oxides display a great variety of quantum electronic behaviors where correlations often play an important role. The achievement of high quality epitaxial interfaces involving such materials gives a unique opportunity to engineer artificial materials where new electronic orders take place. It has been shown recently that a two-dimensional electron gas 2DEG could form at the interface of two insulators such as LaAlO3 and SrTiO3, or LaTiO3 (a Mott insulator) and SrTiO3 [1,2]. We show that a superconducting two-dimensional electron gas is formed at the LaTiO3/SrTiO3 interface whose properties can be modulated by field effect using a metallic gate on the back of the substrate [3,4]. The gas consists of two types of carriers : a majority of low-mobility carriers always present, and a few high-mobility ones that can be injected by electrostatic doping. The calculation of the electrons spatial distribution in the confinement potential shows that the high-mobility electrons responsible for superconductivity set at the edge of the gas whose extension can be tuned by field effect [4].\\[4pt] [1] N. Reyren et al, Science 317, 1196 (2007)\\[0pt] [2] A. Ohtomo et al, Nature 419, 378 (2002)\\[0pt] [3] J. Biscaras et al, Nature Commun 1,89 (2010)\\[0pt] [4] J. Biscaras et al, PRL 108, 247004 (2012) [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B12.00005: Incipient 2D Mott insulators in extreme high electron density, ultra-thin GdTiO3/SrTiO3/GdTiO3 quantum wells S. James Allen, Daniel G. Ouellette, Pouya Moetakef, Tyler Cain, Ru Chen, Leon Balents, Susanne Stemmer By reducing the number of SrO planes in a GdTiO$_3$ /SrTiO$_3$/ GdTiO$_3$ quantum well heterostructure, an electron gas with $\sim$ fixed 2D electron density can be driven close to the Mott metal insulator transition - a quantum critical point at $\sim$1 electron per unit cell. A single interface between the Mott insulator GdTiO$_3$ and band insulator SrTiO$_3$ has been shown to introduce $\sim$ 1/2 electron per interface unit cell. Two interfaces produce a quantum well with $\sim$ 7 10$^{14}$ cm$^{-2}$ electrons: at the limit of a single SrO layer it may produce a 2D magnetic Mott insulator. We use temperature and frequency dependent (DC - 3eV) conductivity and temperature dependent magneto-transport to understand the relative importance of electron-electron interactions, electron-phonon interactions, and surface roughness scattering as the electron gas is compressed toward the quantum critical point. Terahertz time-domain and FTIR spectroscopies, measure the frequency dependent carrier mass and scattering rate, and the mid-IR polaron absorption as a function of quantum well thickness. At the extreme limit of a single SrO plane, we observe insulating behavior with an optical gap substantially less than that of the surrounding GdTiO$_3$, suggesting a novel 2D Mott insulator. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B12.00006: Possible Mott physics in SrTiO$_3$/GdTiO$_3$ superlattices Ru Chen, SungBin Lee, Leon Balents We perform generalized gradient approximation (GGA) + Hubbard U to study the thickness-dependent metal to insulator transition in SrTiO$_3$/GdTiO$_3$ superlattices. A full structural optimization procedure is applied, showing significant electronic and structural reconstruction near the interface between the band insulator SrTiO$_3$ and Mott insulator GdTiO$_3$. In addition, we find high charge density at the interface, close to half electron per interface unit cell (pseudo-cubic notation). For the insulating ultra-thin SrTiO$_3$ layer case, we are able to describe it by a low energy effective Hamiltonian. Using Hartree-Fock approximation, we find the combining effect of the hopping parameters and the correlation in the $d$ orbitals of Ti can lead to possible Mott insulating state. Finally, magnetism is also studied and compared with the GGA+U result. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:27PM |
B12.00007: Engineering new properties in PbTiO$_{3}$ based superlattices: compositionally broken inversion symmetry and polarization rotation Invited Speaker: Matthew Dawber In this talk I will present results on two superlattice systems which contain ultra fine layers of PbTiO$_{3}$ and another perovskite material. In recent years, much work has been done on the PbTiO$_{3}$/SrTiO$_{3}$ system, with a focus on improper ferroelectricity and the arrangement of ferroelectric domains. Here, we consider two different partner materials for PbTiO$_{3}$, each of which introduces markedly different behavior in the resulting superlattice. PbTiO$_{3}$/SrRuO$_{3}$ superlattices with ultra-thin SrRuO$_{3}$ layers were studied both experimentally and using density functional theory. Due to the superlattice geometry, the samples show a large anisotropy in their electrical resistivity, which can be controlled by changing the thickness of the PbTiO$_{3}$ layers. Therefore, along the ferroelectric direction, SrRuO$_{3}$ layers can act as dielectric, rather than metallic, elements. We show that, by reducing the thickness of the PbTiO$_{3}$ layers, an increasingly important effect of polarization asymmetry due to compositional inversion symmetry breaking occurs. The compositional inversion symmetry breaking is seen in this bi-color superlattice due to the combined variation of A and B site ions within the superlattice. We have also achieved an experimental enhancement of the piezoelectric response and dielectric tunability in artificially layered epitaxial PbTiO$_{3}$/CaTiO$_{3}$ superlattices through an engineered rotation of the polarization direction. As the relative layer thicknesses within the superlattice were changed from sample to sample we found evidence for polarization rotation in multiple x-ray diffraction measurements. Associated changes in functional properties were seen in electrical measurements and piezoforce microscopy. These results demonstrate a new approach to inducing polarization rotation under ambient conditions in an artificially layered thin film. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B12.00008: Observation of the cubic Rashba effect in a SrTiO$_{3}$ two-dimensional electron gas Hiroyuki Nakamura, Hisashi Inoue, Minu Kim, Chris Bell, Masayuki Hosoda, Yasuyuki Hikita, Hiroshi Kohno, Takaaki Koga, Harold Hwang, Tsuyoshi Kimura Induced spin-orbit coupling effects at oxide interfaces, where $d$-orbitals form the conduction bands, are recently attracting much interest [1-4]. Here, we report magnetotransport of normally-off SrTiO$_{3}$ field-effect transistors with parylene gate insulator at a dilution refrigerator temperatures (50 mK - 1 K). An enlarged contribution of the weak antilocalization /weak localization (WL/WAL) effect in the magnetoconductance compared to that at 2 K [4] is used to analyze the Rashba effect in detail. It will be shown that a theoretical model with effective magnetic field configuration based on the cubic Rashba term perfectly matches the observed WL/WAL data.\\[4pt] [1] H. Nakamura \textit{et al}., Phys. Rev. B, \textbf{80}, 121308(R) (2009).\\[0pt] [2] M. Ben Shalom \textit{et al}., Phys. Rev. Lett. \textbf{104}, 126802 (2010). \\[0pt][3] A. D. Caviglia \textit{et al}., Phys. Rev. Lett. \textbf{104}, 126803 (2010).\\[0pt][4] H. Nakamura \textit{et al}., Phys. Rev. Lett., \textbf{108}, 206601 (2012). [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B12.00009: Ionic liquid gating of strontium titanate nanostructures Patrick Gallagher, Sam Stanwyck, Menyoung Lee, James Williams, David Goldhaber-Gordon We present electronic transport measurements of two-dimensional electron systems (2DES) induced on strontium titanate surfaces. Using a combination of ionic liquid gates and nanopatterned metallic gates, we demonstrate the ability to isolate a nanoscale puddle of the 2DES and modulate its conductance over several orders of magnitude. Finally, we discuss the apparently gate-tunable superconducting behavior in these devices. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B12.00010: Transport Measurements of Mesoscopic Hall Bars on Strontium Titanate Sam Stanwyck, Patrick Gallagher, James Williams, David Goldhaber-Gordon We report low-temperature transport measurements of a two-dimensional electron system (2DES) at the surface of Strontium Titanate. We use electrolyte gating to create the 2DES, and then use nanopatterning techniques to define submicron constrictions with gate tunability. We observe features characteristic of superconducting transport through these small constrictions, including a critical current and critical field, but measure a nonzero resistance at zero bias. We consider possible explanations in light of these results, including large spatial inhomogeneities in the order parameter, as well as finite-size effects. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B12.00011: Importance of oxygen vacancies for the two-dimensional metallic state at the surface of SrTiO$_3$ Juan Shen, Harald O. Jeschke, Roser Valenti We analyze by means of density functional theory (DFT) the electronic structure of various oxygen-deficient SrTiO$_3$ surface slabs. We find a significant surface reconstruction after introducing oxygen vacancies and we show that the charges resulting from surface-localized oxygen vacancies --independently of the oxygen concentration-- redistribute in the surface region and deplete rapidly within a few layers from the surface suggesting the formation of a two-dimensional electron system (2DES). We also investigate possible oxygen-vacancy clustering effects and discuss our results in the context of recent angle-resolved photoemission spectroscopy observations of a highly metallic 2DES at the (001) vacuum-cleaved surface of SrTiO$_3$. [Preview Abstract] |
Session B13: Focus Session: Topological Materials - Topological Superconductors and Half Heuslers
Sponsoring Units: DMPChair: Jagadeesh Moodera, Massachusetts Institute of Technology
Room: 315
Monday, March 18, 2013 11:15AM - 11:51AM |
B13.00001: Search for Topological Superconductivity in Superconducting Doped Topological Insulators Invited Speaker: Satoshi Sasaki Recent discovery of topological insulators (TIs) which can be characterized by topologically protected gapless surface states stimulated the search for an even more exotic state of matter, a topological superconductor (TSC), which is also predicted to have a topologically protected gapless surface state consisting of massless Majorana fermions as its distinctive characteristic. Low-carrier-density semiconductors with a strong spin-orbit coupling and a Fermi surface that is centered around time-reversal-invariant momenta, such as superconducting doped TIs, are predicted to be prime candidates for TSCs [1]. Following this prediction, we studied the nature of superconductivity in doped TIs, Cu$_x$Bi$_2$Se$_3$ and Sn$_{1-x}$In$_x$Te, by employing a conductance spectroscopy [2, 3]. I will present our latest results together with recent spectroscopy data from other groups, and summarize the current understanding of topological superconductivity in superconducting doped TIs. Work in collaboration with M. Kriener, Z. Ren, A. A. Taskin, K. Segawa, Y. Ando (Osaka Univ.), K. Yada, M. Sato, Y. Tanaka (Nagoya), and L. Fu (MIT). \\[4pt] \noindent [1] L. Fu and E. Berg, Phys. Rev. Lett. {\bf 105}, 097001 (2010). \\ \noindent [2] S. Sasaki, M. Kriener, K. Segawa, K. Yada, Y. Tanaka, M. Sato, and Y. Ando Phys. Rev. Lett. {\bf 107}, 217001 (2011). \\ \noindent [3] S. Sasaki, Z. Ren, A. A. Taskin, K. Segawa, L. Fu, and Y. Ando, arXiv:1208.0059 (2012). [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B13.00002: Reversibility of Superconductivity in CuxBi2Se3 via Quenching Conditions John Schneeloch, Ruidan Zhong, Zhijun Xu, Alina Yang, Genda Gu, John Tranquada We investigated the effect of various growth and annealing conditions on Cu$_{0.3}$Bi$_{2}$Se$_{3}$, a compound proposed to host topological superconductivity. For annealing temperature $T > 580^{\circ}$C, quenching was found necessary for superconductivity, and the superconductivity loss due to not quenching after annealing was reversible by further annealing and quenching. For $T < 580^{\circ}$C, annealing was detrimental, even when followed by quenching. Floating zone growth and the annealing of thin ($< 1$ mm) crystals were found to be detrimental to superconductivity. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B13.00003: Crystal growth and physical property of Bi-Sb-Te-Se topological insulator materials, and Cu-Bi-Se and Sn-In-Te topological superconductors Genda Gu, Alina Yang, J. Schneeloch, R.D. Zhong, Z.J. Xu, J.M. Tranquada, Z.H. Pan, W.D. Si, X.Y. Shi, Q. Li, T. Valla The discovery of 3D topological insulator materials and topological superconductor opens up a new research field in the condensed matter physics. We have grown a number of Bi-Sb-Te-Se topological insulator, and Cu-Bi-Se and Sn-In-Te topological superconductor single crystals. We have measured the physical properties on these single crystals. We have studied the effect of growth condition and impurity on the bulk electrical conductivity of these single crystals. We try to answer two questions for the topological insulator materials if it is possible to grow the bulk-insulating topological insulator single crystals and Which maximum resistivity of these topological insulator single crystals we can grow. For the topological superconductor, we have got the bulk superconducting single crystals with a maximum Tc$=$4.5K. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B13.00004: Transport property of Cu-intercalated Bi$_2$Se$_3$ Atsutaka Maeda, Taiki Yoshinaka, Yoshinori Imai, Ryusuke Kondo Cu$_x$Bi$_2$Se$_3$($T_{\mathrm{c}} \sim 3.8$ K)[1] is a promising candidate material to be a topological superconductor, and it is very important to clarify the origin of its superconductivity. However, Cu$_x$Bi$_2$Se$_3$ synthesized by Hor \textit{et al.} does not show zero resistivity below $T_{\mathrm{c}}$ [1], and some concerns still remain in the quality of samples. Recently, several groups reported the successful preparation of Cu-intercalated Bi$_2$Se$_3$ with zero resistivity by an electrochemical method [2] and the Bridgman method [3]. Here, we report transport properties of single crystals of Cu$_x$Bi$_2$Se$_3$ with zero resistivity prepared by the Bridgman method. We stress that the process of the quenching from a temperature of about 1000 K into cold water is of crucial importance in the crystal growth process. The grown crystal with $x=0.10$ shows zero resistivity at about 3.2 K. We also report the results of the intercalations of different metal elements [4].\\[4pt] [1] Y.~S. Hor \textit{et al.}, PRL 104 (2010) 057001.\\[0pt] [2] M. Kriener \textit{et al.}, PRL 106 (2011) 127004.\\[0pt] [3] T. Kirzhner \textit{et al.}, arXiv:1111.5805. T. V. Bay \textit{et al.}, arXiv:1112.0102.\\[0pt] [4] Y. Imai \textit{et al.}, J. Phys. Soc. Jpn. 81 (2012) 113708. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B13.00005: Scanning Tunneling Microscopy Measurements of Superconductivity in Cu$_{\mathrm{x}}$Bi$_2$Se$_3$ Niv Levy, Tong Zhang, Jeonghoon Ha, Fred Sharifi, A. Alec Talin, Young Kuk, Joseph A. Stroscio The discovery of topological insulators has triggered the search for new topological states of matter. A Topological superconductor (TSC) is one such state, characterized by the existence of an unconventional superconducting gap in the bulk, and gapless Andreev bound states on the surface. Recently, Cu intercalated Bi$_{2}$Se$_{3}$ was found to be superconducting with T$_{\mathrm{C}}$ $\sim$ 3.8 K, and was considered a prime TSC candidate due to its band structure and strong spin-orbit coupling. A recent point contact measurement observed zero-bias conductance peaks, claiming these as evidence of surface Andreev bound states, and angle resolved photoemission spectroscopy has revealed the preservation of the topological surface states at the Fermi level. In this work we report scanning tunneling microscopy measurements of a cleaved Cu$_{0.2}$Bi$_{2}$Se$_{3}$ crystal. The measured tunneling spectrum is fully gapped and is well described by the classical s-wave BCS theory. In addition, spatially resolved measurements of the superconducting gap under an applied magnetic field found no bound states in the vortex cores. Both of these results suggest that Cu$_{0.2}$Bi$_{2}$Se$_{3}$ is a classical s-wave superconductor contrary to previous expectations and measurements. We will discuss current work examining the Cu concentration dependence. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B13.00006: Superconductivity in three dimensional topological compound via pressure C.Q. Jin, J. Zhu, J.L. Zhang, S.J. Zhang, X. Li, Q.Q. Liu, X. Dai, Z. Fang, W.G. Yang, G.Y. Shen, H.K. Mao Superconductivity in topological compounds is of great importance to the study of topological quantum phenomena. Here we report investigations of superconductivity induced via pressure in Bi$_{2}$Te$_{3}$ topological single crystals with various carrier types. We will discuss the possible relations of the superconductivity to topological scenario. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B13.00007: Effect of Indium on the Superconducting Transition Temperature of Tin Telluride Ruidan Zhong, John Schneeloch, Xiaoya Shi, Qiang Li, John Tranquada, Genda Gu Indium-doped tin telluride is one of the most appealing topological superconductors. We have grown a series of Sn$_{\mathrm{1-x}}$In$_{\mathrm{x}}$Te crystals with different indium concentrations (0.1$\le $x$\le $1.0). The results show indium doping improves the superconducting transition temperature significantly and is highly related to the indium concentration. The maximum Tc of indium-doped tin telluride polycrystalline is 4.5K for x$=$0.4. Single crystals of Sn$_{\mathrm{1-x}}$In$_{\mathrm{x}}$Te were also grown by the floating zone method, and their magnetic properties were characterized. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B13.00008: The Nature of the Superconductivity of Tl$_5$Te$_3$ Kathryn Arpino, David Wallace, Seyed Koohpayeh, Jiajia Wen, Katharine Page, Tingyong Chen, C.L. Chien, Tyrel McQueen The search for topologically non-trivial states of matter, such as topological insulators, has sparked significant interest in the impact of spin-orbit coupling on strongly correlated electronic behaviors, such as superconductivity. The known compound Tl$_5$Te$_3$ exhibits a superconducting transition at $T_c$ = 2.4 K, and contains heavy elements, making it an ideal compound in which to look for new physics at the intersection between superconductivity and strong spin-orbit coupling. In 1973, Haemmerle et al. conjectured that two-gap superconductivity might explain previous anomalous superconducting volume fractions observed in their polycrystalline samples. We have reinvestigated the superconductivity of Tl$_5$Te$_3$ using magnetic susceptibility, heat capacity, and point contact measurements on powder and single crystal samples, and resolved these previous discrepancies. Further, we report on long-range and local structure determination of superconducting and non-superconducting Tl$_5$Te$_3$ samples, as well as the relationship between structural details and the observed superconductivity. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B13.00009: Probing for Topological Superconductivity in In-doped SnTe Jeonghoon Ha, N. Levy, T. Zhang, H. Baek, D. Zhang, F. Sharifi, Y. Kuk, S. Sasaki, Z. Ren, A.A. Taskin, K. Segawa, Y. Ando, L. Fu, J.A. Stroscio Recent investigations of 3D topological insulators, which have gapless surface states protected by time reversal symmetry, have drawn attention to the search for new topological states protected by other symmetries. Theories predicted the existence of topological crystalline insulators (TCIs), which have gapless surface states protected by symmetry of the crystal lattice. In this work, we use scanning tunneling spectroscopy to investigate the superconducting properties of indium-doped tin telluride (Sn$_{\mathrm{1-x}}$In$_{\mathrm{x}}$Te), which is predicted to be a topological crystalline insulator. In a sample with T$_{\mathrm{c}}\approx $1.6 K, the tunneling spectra show a superconducting gap of 0.2 meV, which is continuous throughout the surface of the cleaved crystal. The superconducting gap is suppressed at a critical magnetic field of B$=$0.50 T and dI/dV conductance maps reveal a vortex lattice in a perpendicular applied magnetic field. Measurements will be shown as a function of In doping and discussed in relation to predictions and experiments on topological superconductivity in this material. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B13.00010: Transport and thermodynamic properties of topological semimetal candidate RPdBi (R:rare earth) Yasuyuki Nakajima, Rongwei Hu, Kevin Kirshenbaum, Alex Hughes, Paul Syers, Johnpierre Paglione The search for topologically non-trivial surface states, involving topologically protected gapless states on the boundary, is one of the central activities in the search for new quantum states of matter. Resent theoretical studies have indicated that the ternary half-Heusler system RPdBi (R:rare earth) can involve strong band inversion due to spin-orbit coupling, leading to the topologically non-trivial state. To clarify possible topological aspects of these materials, we report the characterization of single-crystal samples of RPdBi by transport and thermodynamic measurements down to very low temperatures. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B13.00011: Transport properties of the topological semi-metal LuPtBi under pressure Fazel Fallah Tafti, Takenori Fujii, Alxandre Juneau-Fecteau, Samuel Rene de Cotret, Nicolas Doiron-Leyraud, Atsushi Asamitsu, Louis Taillefer We present high-pressure magneto-transport data on single crystals of LuPtBi, a member of the ternary half-Heusler family. Recent band structure calculations show that LuPtBi is a topological semi-metal at ambient pressure due to strong spin-orbit coupling [1]. By decreasing the lattice parameter, equivalent to increasing pressure, the system should become a trivial insulator We have grown single crystals of LuPtBi and studied both the field dependence and the pressure dependence of their resistivity. The field dependence shows typical semi-metal behaviour, namely a weak temperature dependence and a large magneto-resistance. The pressure dependence shows a significant increase of resistivity and a decrease of magneto-resistance with increasing pressure. We compare our experimental results to the available theoretical work on the transport properties of topological semi-metals [2]. \\[4pt] [1] Stanislav Chadov, \textit{et al.} Nature, \textbf{9}, 541 (2010)\\[0pt] [2] W. Al-Sawai, et al. PRB, \textbf{82}, 125208 (2010) [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B13.00012: Magnetism and physical properties of topological half-Heusler compounds RPdBi Rongwei Hu, Yasuyuki Nakajima, Kevin Kirshenbaum, Alex Hughes, Paul Syers, Johnpierre Paglione, Jeffrey Lynn The non-magnetic half-Heusler compounds, YPdBi and LuPdBi, have been proposed by band structure calculations to be candidates for three-dimensional topological insulators. We present magnetic susceptibility, neutron scattering and electrical transport measurements on single-crystal samples of a series of rare earth containing half-Heulser compounds RPdBi, showing that RPdBi are semimetals with dominant p-type carriers which exhibit antiferromagnetism associated with the rare earth local moments. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B13.00013: Low temperature specific heat of YBiPt Pascoal Pagliuso, Ryan Baumbach, Priscila Rosa, Cris Adriano, Joe Thompson, Zachary Fisk We present the specific heat measured on single crystals of the putative topological superconductor YBiPt between 0.35 and 20 K. The electronic specific coefficient per mole of compound is 0.3 mJ/ K$^{2}$. A break in slope of C/T vs T at T$_{c}$ = 0.7 K is seen, but no jump in C. We speculate on possible trace second phase in the crystals. [Preview Abstract] |
Session B14: Focus Session: Magnetic Nanoparticles I
Sponsoring Units: GMAG DMPChair: Dario Arena, Brookhaven National Laboratory
Room: 316
Monday, March 18, 2013 11:15AM - 11:51AM |
B14.00001: Recent advances in magnetic nanoparticles with bulk-like properties Invited Speaker: Xavier Batlle Magnetic nanoparticles (NP) are an excellent example of nanostructured materials and exhibit fascinating properties with applications in high-density recording and biomedicine. Controlling the effects of the nanostructure and surface chemistry and magnetism at the monolayer level have become relevant issues. As the size is reduced below 100 nm, deviations from bulk behavior have been attributed to finite-size effects and changes in the magnetic ordering at the surface, thus giving rise to a significant decrease in the magnetization and increase in the magnetic anisotropy. The existence of a surface spin glass-like state due to magnetic frustration has been widely suggested in ferrimagnetic NP [1]. However, in this talk, we will show that high crystal quality magnetite Fe$_{\mathrm{3-x}}$O$_4$ NP of about a few nanometers in diameter and coated with different organic surfactants [2] display bulk-like structural, magnetic and electronic properties. Magnetic measurements, transmission electron microscopy, X-ray absorption and magnetic circular dichroism and Monte Carlo simulations, evidenced that none of the usual particle-like behavior is observed in high quality NP of a few nm [3]. Consequently, the magnetic and electronic disorder phenomena typically observed in those single-phase ferrimagnetic NP should not be considered as an intrinsic effect. We also performed a real-space characterization at the sub-nanometer scale, combining scanning transmission electron microscopy, electron energy loss spectroscopy and electron magnetic chiral dichroism. For the first time, we found that the surface magnetization is as high as about 70{\%} of that of the core [4]. The comparison to density functional theory suggested the relevance of the strong surface bond between the Fe ions and the organic surfactant. All the foregoing demonstrates the key role of both the crystal quality and surface bond on the physical properties of ferrimagnetic NP and paves the way to the fabrication of the next generation of NP with optimal magnetic properties [5]. Some bio-applications will also be discussed [6]. \\[4pt] In collaboration with A Labarta, N Perez, O Iglesias, A Fraile, C Moya(U Barcelona); A Roca, MP Morales, CJ Serna (ICMM-CSIC); F Bartolome, LM Garcia, J. Bartolome (CSIC-U Zaragoza); R Mejias, DF Barber (CNB-CSIC); M Varela, J Gazquez, J Salafranca, SJ Pennycook (ORNL), ST Pantelides (Vanderbilt U).\\[4pt] [1] X. Batlle, A. Labarta, J.Phys.D 35,R15 (2002) [2] P. Guardia, Langmuir 26,5843 (2010) [3] N. Perez, Appl.Phys.Lett. 94,093108(2009) [4] J. Salafranca, NanoLetters 12,2499 (2012) [5] X. Batlle, J.Appl.Phys. 109,07B524 (2011) [6] R. Mejias, Nanomedic. 5,397 (2010) [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B14.00002: Ligand effects on the electronic structure and magnetism of magnetite surfaces Katarzyna Brymora, Florent Calvayrac We address the effect of functionalization on the electronic and magnetic properties of magnetite surface as an indicator of the same properties in nanoparticles too big for a direct ab-initio approach. Using well-established methods and references (namely LDA+U on magnetite surfaces) we could verify the validity of our approach, and using two typical ligands, dopamine and citrate, namely $\pi$ and $\sigma$ electron donors, we could predict that those ligands would induce a different change in the electronic properties of the systems, but in both cases an enhancement of magnetization. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B14.00003: Synthesis, characterization, and fabrication of magnetic nanoparticles for low energy loss applications Hongseok Yun, Jun Chen, Vicky Doan-Nguyen, James Kikkawa, Christopher Murray It is important to increase operating frequency of power electronics for miniaturization of components. Magnetic materials are used as inductor cores to increase inductance proportional to their magnetic permeability. However, traditional magnetic materials are not used at high frequency (\textgreater 100MHz) because of large hysteresis and eddy current loss. Superparamagnetic nanoparticles are good candidates to resolve these problems because they have zero hysteresis loss. In addition, eddy currents can be reduced due to their high electric resistivity originating from the organic ligands on the surface. Magnetic nanoparticles such as NiFe$_{2}$O$_{4}$, Ni$_{1-x}$Zn$_{x}$Fe$_{2}$O$_{4}$, MnFe$_{3}$O$_{4}$ and ZnFe$_{2}$O$_{4}$ have been synthesized~via~high temperature thermal decomposition method and can be tuned to desired size, shape and chemical composition. To understand structural and magnetic properties of nanoparticles, the nanoparticles have been characterized by TEM, SQUID, PPMS, and Network Analyzer. UV-induced polymerization and pressing method have been implemented for film deposition. Finally, AC susceptibility of the nanoparticle film have been measured and discussed for low energy-loss applications. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B14.00004: Size-dependent optical properties of $\alpha$-Fe$_{2}$O$_{3}$ nanoparticles K.R. O'Neal, B.S. Holinsworth, P. Chen, J.L. Musfeldt, J.M. Patete, S.S. Wong, S.A. McGill We investigated the variable temperature optical properties of nanoscale hematite ($\alpha$-Fe$_{2}$O$_{3}$) with special attention to the parity-forbidden Fe$^{3+}$ $\textit{d-d}$ excitation that is activated by hybridization and symmetry-breaking phonons. An oscillator strength analysis of the rhombohedra, cubes, and rice reveals that the energy of the coupling phonon scales as (size)$^{-1}$. Moreover, preliminary work in high magnetic fields shows a field-induced color change. These findings are important for more deeply understanding finite length scale effects in this iconic material and other nanoscale transition metal oxides. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B14.00005: Chemical Synthesis and Physical Characterization of Hexagonal Ni Nanoparticles John Klodnicki, Brian Kelly, Karl Unruh Elemental Ni nanoparticles with a hexagonal close packed (HCP) crystal structure have been prepared by the reduction of nickel acetate in diethylene glycol (DEG) without the addition of any other reactants. No metallic Ni was formed at a reaction temperature of 195 $^{\circ}$C. At a reaction temperature of 210 $^{\circ}$C a two phase mixture of face centered cubic (FCC) and HCP Ni was obtained. With increasing temperature, the ratio of HCP to FCC Ni increased until at 245 $^{\circ}$CC (i.e. the boiling temperature of DEG) the reaction product was entirely HCP. The structural and magnetic properties of the HCP Ni were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and vibrating sample magnetometry (VSM) measurements. The SEM measurements revealed the presence of approximately spherical particles about 500 nm in diameter, as well as a number of rod-like structures. Based on a Rietveld-type analysis of the HCP Ni, best fit lattice parameters of a$=$0.26473(6) and c$=$0.43348(10) nm were obtained. Room temperature VSM measurements revealed a small magnetic moment of about 2 emu/g. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B14.00006: Synthesis and Characterization of CoNi and FeCo Nanowires With High Coercivity J. Ping Liu, Narayan Poudyal, Kinjal Gandha Ferromagnetic nanocrystals with shape anisotropy have drawn a great attention in the past decade because of their unique magnetic properties and potential applications in magnetic recording media and high performance nanocomposite magnets. CoNi and FeCo nanocrystals with different size, shape and composition were successfully synthesized via catalytic and non-catalytic chemical solution methods. It was found that the structure and morphology of the nanocrystals can be controlled by varying synthetic parameters such as solvent amount, catalyst and surfactant concentration, and heating rate. The length of the nanowires can be adjusted by changing the catalyst concentration. It has also been observed that the growth mechanisms for FeCo and CoNi nanowires are different. Magnetic properties of the FeCo and CoNi nanocrystals including coercivity and magnetization are found to be dependent on size, shape and composition of the nanowires. By optimizing the synthesis conditions, the FeCo and CoNi nanowires with enhanced magnetization and coercivity can be obtained. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B14.00007: Structural origin of low temperature glassy relaxation in magnetic nanoparticles Suvra Laha, Rajesh Regmi, Gavin Lawes Magnetic nanoparticles often exhibit glass-like relaxation features at low temperatures. Here we discuss the effects of doping boron, cobalt, gadolinium and lanthanum on the low temperature magnetic properties of Fe$_3$O$_4$ nanoparticles. We investigated the structure of the nanoparticles using both X-ray diffraction and Raman studies, and find evidence for secondary phase formation in certain samples. We acquired Transmission Electron Microscopic images to give direct information on the morphology and microstructure of these doped nanoparticles. We measured the ac out-of-phase susceptibility ($\chi^{//}$) vs temperature (T) to parameterize the low temperature glassy magnetic relaxation. All samples show low temperature magnetic relaxation, but the amplitude of the signal increases dramatically for certain dopants. We attribute these low temperature frequency-dependent magnetic relaxation features to structural defects, which are enhanced in some of the doped Fe$_3$O$_4$~nanoparticles. These studies also confirm that the low temperature relaxation in nanoparticles arises from single particle effects and are not associated with interparticle interactions. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B14.00008: Magnetic relaxation in dipolar magnetic nanoparticle clusters Ondrej Hovorka, Joe Barker, Roy Chantrell, Gary Friedman Understanding the role of dipolar interactions on thermal relaxation in magnetic nanoparticle (MNP) systems is of fundamental importance in magnetic recording, for optimizing the hysteresis heating contribution in the hyperthermia cancer treatment in biomedicine, or for biological and chemical sensing, for example. In this talk, we discuss our related efforts to quantify the influence of dipolar interactions on thermal relaxation in small clusters of MNPs. Setting up the master equation and solving the associated eigenvalue problem, we identify the observable relaxation time scale spectra for various types of MNP clusters, and demonstrate qualitatively different spectral characteristics depending on the point group of symmetries of the particle arrangement within the cluster -- being solely a dipolar interaction effect. Our findings provide insight into open questions related to magnetic relaxation in bulk MNP systems, and may prove to be also of practical relevance, e.g., for improving robustness of methodologies in biological and chemical sensing. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B14.00009: Reentrant superparamagnetism induced by spin glass behavior at the surfaces of magnetic nanoparticles Wei Qin, Xiaoguang Li, Yi Xie, Zhenyu Zhang Superparamagnetism appears when the Neel relaxation time of magnetic nanoparticles is shorter than the measurement time. Recent experimental studies of different types of magnetic nanoparticles revealed that superparamagnetic transitions could also take place below the blocking temperatures [1-3], an intriguing phenomenon tentatively termed as quantum superparamagnetism. Here we elucidate the microscopic origin of the reentrant superparamagnetism in such systems using a phenomenological model, which emphasizes the dynamical coupling between the ferromagnetic core and the spin glass surface layer of a given nanoparticle [4]. We first obtain expressions for the thermal relaxation of the total magnetization of the particle upon finite-field and zero-field cooling, then carry out numerical simulations using physically realistic materials parameters. Our findings provide a more plausible interpretation of the observed reentrant superparamagnetism beyond the previous macroscopic quantum tunneling picture.\\[4pt] [1] C. T. Hsieh, J. T. Lue, Phys. Lett. A 316 329 (2003)\\[0pt] [2] W. W. Zheng, P. Kumar, A. Washington, Z. X. Wang, et al, J. Am. Chem. Soc. 134 2172 (2012).\\[0pt] [3] C. Xiao, J. J. Zhang, Y. Xie, Sci. Rep. 2 755 (2012).\\[0pt] [4] R. H. Kodama, A. E. Berkowitz, Phys. Rev. Lett. 77 2 (1996). [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B14.00010: Voltage-controlled spin transport through a pair of Buckminster fullerene molecules encapsulating cobalt atoms Alireza Saffarzadeh, George Kirczenow Carbon-based nanostructures such as fullerenes, carbon nanotubes, and graphene, are promising candidates for spintronic applications because of their weak spin-orbit coupling and hyperfine interaction which lead to long spin coherence lengths. In particular, a fullerene C$_{60}$ molecule is an interesting carbon nanostructure which can be used as a molecular bridge in magnetic tunnel junctions due to its remarkable structural stability and electronic properties which make the molecule convenient for easier spin injection in magnetic nanojunctions. Here, we show that using cobalt atoms encapsulated in a pair of Buckminster fullerene molecules sandwiched between gold electrodes, density of states spin polarizations as large as 95\% are found by varying the gate and/or bias voltage, due to the spin-splitting of Co $3d$ orbitals. The current-voltage characteristics and strong (up to 100\%) spin polarization of the current indicate that the device can be utilized for highly efficient spin injection into nonmagnetic conductors. These results open the way to voltage-controlled spin filters and magnetic sensors using molecular magnetic junctions. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B14.00011: Magnetization Study of Sulfur-doped Graphitic Nano-platelets and Single Walled Carbon Nanotubes J. Zhu, L. Oliveira, R. Podila, S. Neeleshwar, Y.Y. Chen, J. He, M. Skove, A.M. Rao Recently we investigated the magnetic behavior of as-prepared and sulfur doped chemically exfoliated graphene nano-platelets (GNPs) and single walled carbon nanotubes (SWCNTs). The doping was achieved by annealing desired carbon nanostructures with 0, 1.0, 1.5 and 3 at{\%} sulfur in an evacuated quartz tube at 1000 $^{\circ}$C for 1 day, followed by multiple rinsing in alcohol and drying in vacuum to remove excess sulfur. The isothermal M vs. H as well as the temperature-dependent M vs. T measurements were obtained using a vibrating sample magnetometer. We found that sulfur doping drastically changes the magnetic behavior of the as-prepared samples (both SWCNTs and GNPs). The results of zero-field-cooling (ZFC) and field-cooling (FC) in M vs. T measurements indicated the existence of large amount of coupled super-paramagnetic domains, along with antiferromagnetic domains. The saturation magnetization decreased in S doped GNPs, while a contrasting trend was observed in S doped SWCNTs. The role of edge states and structural defects in carbon nanostructures in the observed magnetic properties will be discussed. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B14.00012: Enhanced Magnetic Properties in Nanoparticle-Filled CNTs K. Stojak, S. Chandra, H. Khurshid, M.H. Phan, H. Srikanth There has been much interest in magnetic polymer nanocomposites (MPNCs) recently due to potential applications for EMI shielding, tunable EM devices and flexible electronics. In past studies, using ferrite fillers, we have shown MPNCs to be magnetically tunable when passing a microwave signal through films under the influence of an external magnetic field. We extend this study to include nanoparticle-filled multi-walled carbon nanotubes (CNTs) synthesized by CVD. These high-aspect ratio magnetic nanostructures, with tunable anisotropy, are of particular interest in enhancing magnetic and microwave responses in existing MPNCs. CNTs have an average diameter and length of 300nm and 6 $\mu$m, respectively and are partially filled with CoFe$_{2}$O$_{4}$ and NiFe$_{2}$O$_{4}$ nanoparticles (NPs) ($\sim$ 7nm). When comparing NPs to NP-filled CNTs, $T_{B}$ increases by $\sim$ 40K and relaxation time, $\tau _{\mathrm{0}}$, increases several orders of magnitude, indicating that enclosing NPs in CNTs enhances interparticle interactions. Structural and magnetic characterization were completed using XRD, TEM and Quantum Design PPMS, using VSM and ACMS options. [Preview Abstract] |
Session B15: Focus Session: Spin-orbit Effects in Spin-dependent Transport and Dynamics
Sponsoring Units: GMAG DMPChair: Xin Fan, University of Delaware
Room: 317
Monday, March 18, 2013 11:15AM - 11:27AM |
B15.00001: Observation of spin Hall effective field Xin Fan, Jun Wu, Yunpeng Chen, Matthew Jerry, Huaiwu Zhang, John Xiao Recent development in spin Hall driven spin transfer torque has attracted intensive interests$^{1}$. Liu \textit{et. al.} has shown that the spin transfer torque induced by the spin Hall effect in a normal metal-ferromagnetic metal bilayer can switch the magnetization of the ferromagnetic layer, which may be a potential candidate for magnetic random access memory$^{2}$. The switching of the magnetization was primarily attributed to the Slonczewski torque$^{3}$. We show that besides the Slonczewski torque, the spin Hall effect also produces an effective field that can also facilitate the magnetization reversal. This effective field persists even with a Cu spacer layer, and reduces quickly with the increase of the ferromagnetic layer thickness. The observation of the spin Hall effective field shall have ramification on the understanding of both spin transfer torque and spin Hall effect. 1. K. Ando \textit{et. al.}, Electric manipulation of spin relaxation using the spin Hall effect, Physical Review Letters, 101, 036601 (2008). 2. L. Liu \textit{et. al}., Spin-Torque Switching with the Giant Spin Hall Effect of Tantalum. \textit{Science} 336, 555-558 (2012). 3. J. Slonczewski, Current-driven excitation of magnetic multilayers. \textit{Journal of Magnetism and Magnetic Materials}, 159, L1-L7 (1996). [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B15.00002: Analysis of the spin Hall magnetoresistance in ferromagnetic insulator/normal metal hybrids Matthias Althammer, Sibylle Meyer, Michael Schreier, Mathias Weiler, Stephan Gepr\"ags, Matthias Opel, Hans Huebl, Rudolf Gross, Timo Kuschel, Christoph Klewe, Jan-Michael Schmalhorst, G\"unter Reiss, Arunava Gupta, Yan-Ting Chen, Gerrit E.W. Bauer, Hiroyasu Nakayama, Eiji Saitoh, Sebastian T.B. Goennenwein Pure spin currents, i.e. the net flow of spin angular momentum without an accompanying charge current, represent a new paradigm for spin transport and spintronics. We have experimentally studied a new type of magnetoresistance effect, which arises from the interaction of charge and spin current flows in ferromagnetic insulator/normal metal hybrid structures. In more detail, we measured the resistance of yttrium iron garnet(YIG)/Pt, YIG/nonferromagnet/Pt, nickel ferrite/Pt, and magnetite/Pt hybrid structures as a function of the magnitude and the orientation of an external magnetic field. The resistance changes observed can be quantitatively traced back to the combined action of spin Hall and inverse spin Hall effect in the Pt metal layer, and are thus termed spin Hall magnetoresistance (SMR). We show that the SMR is qualitatively different from the conventional anisotropic magnetoresistance effect arising in magnetic metals. Furthermore, the SMR enables us to quantify the spin Hall angle in our Pt layers. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B15.00003: Transverse magnetoresistance and size effects of thin gold films: Experiment and theory. Raul C. Munoz, S. Oyarzun, R. Henriquez, M.A. Suarez, L. Moraga, G. Kremer We report new experimental data regarding the transverse magnetoresistance measured with the electric field \textbf{E} oriented perpendicular to the magnetic field \textbf{B}, both fields (\textbf{E}, \textbf{B}) contained within the plane of the film (the MacDonald configuration) performed in a family of gold films of different thickness. The signal at 4 K can be univocally attributed to electron-surface scattering. Transport measurements were performed at low temperatures T (4K $\le $ T $\le $ 50K) under magnetic field strengths B (1.5 T $\le $ B $\le $ 9 T). The magnetoresistance signal exhibits a marked thickness dependence, and its curvature as a function of magnetic field B varies with film thickness. We also present a new theoretical description based upon a solution of Boltzmann Transport Equation [MacDonald D. C. K. and Sarginson K., 1950 \textit{Proc. Roy. Soc. (London)} \textbf{A 203} 223], computed using the method of characteristics proposed by Chambers [Chambers R. G., 1950 \textit{Proc. Roy. Soc. (London)} \textbf{A 202} 378]. The theoretical description of the magnetic field dependence of the magnetoresistance requires a Hall field that varies with the thickness of the film; this Hall field is tuned to reproduce the experimental data. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B15.00004: Angular dependence of spin-orbit spin transfer torque Invited Speaker: Kyung-Jin Lee Magnetocrystalline anisotropy arises from the modification of electron states by spin-orbit coupling and is determined by integrating over all occupied electron states. On the other hand, current-induced spin transfer torques arise from the changes in torques that arise from changes in electron populations in the presence of a current. In this respect, spin transfer torques caused by spin-orbit coupling can be interpreted as current-induced corrections to the magnetic anisotropy. From this perspective, we expect a close relationship between the magnetic anisotropy and spin-orbit spin torques. We theoretically study this relationship between magnetic anisotropy and spin-orbit spin torque for a ferromagnet subject to Rashba spin-orbit coupling. For a two-dimensional free-electron model, we find that Rashba spin-orbit coupling results in perpendicular magnetic anisotropy and field-like current-induced spin transfer torques. Both quantities acquire nontrivial angular dependence as the spin-orbit coupling becomes comparable to the s-d exchange interaction. This nontrivial angular dependence can be understood from Fermi surface distortion. In the limits where either the spin-orbit coupling or the s-d exchange interaction is much greater than the other, the Fermi surface consists of two concentric circles, but when they are comparable it distorts. These free-electron calculations are in qualitative agreement with ab initio calculations for Co$|$Pt bilayers, suggesting that the spin-orbit coupling at the interface is non-negligible in comparison to the s-d exchange interaction there. The nontrivial angular dependence of spin-orbit spin torque may be used as an indicator of strong interfacial spin-orbit coupling, because the spin-orbit spin torque that is induced by the spin Hall effect, has a simple $\sin\theta$ dependence where $\theta$ is the angle between the magnetization and the spin injected into a ferromagnet. This work has been done in collaboration with M. D. Stiles and P. M. Haney. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B15.00005: Spin transfer torques in magnetic bilayers with strong spin orbit coupling M.D. Stiles, Paul M. Haney, Hyun-Woo Lee, Kyung-Jin Lee, Aurelien Manchon Current driven magnetic dynamics in ferromagnetic thin films on top of non-magnetic films with strong spin orbit coupling show strong current-induced torques. Several theoretical models have been proposed to explain these torques. In one model, the current flowing through the non-magnetic layer gives rise to a spin Hall current, leading to a spin current incident on the interface between the two layers. This spin current causes spin transfer torques similar to those that are important in magnetic multilayers with current flowing perpendicular to the plane. Another model proposes a torque due to the spin-orbit coupling at the interface where the inversion symmetry found in the bulk materials is broken. We model the spin transport with a semiclassical Boltzmann equation approach. Both torques are present in this model and for reasonable parameter sets are largely independent of each other. We compute the dependence of the torques on the thickness of the layers and find that it is difficult to reproduce the large sensitivity to the thickness of the ferromagnetic layer as found in several experiments. This disagreement indicates that structural or electronic properties are probably changing with the thickness of the films studied in experiments. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B15.00006: Spin transfer torque devices utilizing the spin Hall effect of tungsten Chi-Feng Pai, Luqiao Liu, Yun Li, Hsin-Wei Tseng, Daniel C. Ralph, Robert A. Buhrman It is recently been shown that the spin Hall effect (SHE) in $\beta $-Ta generates a transverse spin current that is sufficient for efficiently reversing the moment of adjacent thin film nanomagnets through the spin torque (ST) mechanism. Here we report the existence of an even larger SHE in $\beta $-W thin films. Using spin torque induced ferromagnetic resonance (ST-FMR) with a $\beta $-W/CoFeB bilayer microstrip we have determined the magnitude of the spin Hall angle $\theta $ to be 0.30$\pm $0.02, which is twice as large as the previously reported value for $\beta $-Ta ($\sim $0.15). From switching data obtained with 3-terminal devices consisting of a $\beta $-W channel and an adjacent CoFeB/MgO/CoFeB magnetic tunnel junction, we have independently determined \textbar $\theta $\textbar $=$ 0.33$\pm $0.06. We will also report on the variation of the spin Hall switching efficiency with W layers of different resistivities and hence of variable ($\alpha $ and $\beta )$ phase composition. Finally we have studied the SHE exhibited by several other 4d and 5d transition metals using the techniques mentioned above and we will report on those results. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B15.00007: Spin-Hall and spin-pumping effect observed in W/FeCoB thin films Yun Li, Chi-feng Pai, Hsin-wei Tseng, Luis Leao, Dan Ralph, Robert Buhrman The spin-Hall effect (SHE) and its reciprocal, the inverse spin-Hall effect (ISHE), are of great importance in spintronics since they enable, respectively, the conversion of a longitudinal charge current to a transverse spin current and the reverse process. Here we will report on a ferromagnetic resonance (FMR) study of FeCoB/W thin film bi-layer structures that incorporate different W thicknesses and hence difference phases. A very large negative spin Hall angle has been observed in the $\beta $-W samples and confirmed by spin-torque switching studies. Alternatively FMR measurements with bilayers containing $\alpha $-W suggests a strong positive SHE, but this interpretation of the experiment is not consistent with spin torque switching studies utilizing $\alpha $-W. Since the $\alpha $-W FMR results also show an enhanced magnetic damping we tentatively attribute these results to a significantly enhanced spin pumping effect in $\alpha $-W, relative to $\beta $-W. Magnetization measurements indicate that the two different types of FeCoB/W bilayers have substantially different interfacial magnetic anisotropy coefficients. We will discuss these results, together with the differing temperature dependence of the FMR signal in the two cases, which help point the way to understanding the origin of the giant SHE in $\beta $-W and the strong ISHE in $\alpha $-W. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B15.00008: Spin-orbit-induced spin-polarized surface states in one-atomic-layer Pb films on Si(111) Hyungjun Lee, Hyoung Joon Choi As a route to spintronics without magnetism, spin-orbit coupling (SOC) generates and manipulates the spin-polarized carriers, thereby providing key ingredients for spin field-effect transistors. Along this line, we investigated the spin-orbit induced effects in Pb monolayers on Si(111) substrates, modeled by $\sqrt{3}\times\sqrt{3}$ phase with Pb coverage of 4/3 ML, based on first-principles calculations with the inclusion of SOC. We focus on the electronic structures of surface states with characteristic Rashba-type spin splitting and spin texture as well as the charge flow pattern by calculating the current density distribution for the spin-polarized surface states. We also discuss our results on the difference from the spin splitting in the Shockley surface states on Au(111) surface. This work was supported by the NRF of Korea (Grant No. 2011-0018306), and computational resources have been provided by KISTI Supercomputing Center (Project No. KSC-2012-C2-14). [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B15.00009: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 1:27PM - 1:39PM |
B15.00010: Spin Hall Effect induced Anisotropic Magnetoresistance Priscila Gonzalez Barba, See-Hun Yan, Luc Thomas, Kwang-Su Ryu, Stuart Parkin, Aurelien Manchon Spin-orbit-induced anisotropic transport in magnetic materials, studied for more than a century, has recently experienced a renewed interest thanks to the formulation of anisotropic spin scattering in terms of Berry's curvature. Anisotropic magnetoresistance (AMR) is related to the scattering of the transport electrons on the orbitals of localized electrons, depending on the magnetization direction. The contributions of the interfaces on AMR has been scarcely studied. We consider a trilayer composed of one ferromagnetic layer sandwiched between two normal metals. The normal metals display spin Hall effect (SHE), whereas the ferromagnetic layer polarize the flowing current. We propose that SHE present in the top and bottom layers might contribute to the AMR. The charge and spin currents are analyzed by drift-diffusion equations including the role of inverse SHE as well as anomalous Hall effect. Longitudinal and transverse spin accumulations at the interfaces are captured through spin dependent conductance and the mixing conductance. It is shown that the presence of a spin accumulation in the normal metal close to the interface is transformed into a charge current through inverse SHE hence altering the conductivity of the normal metal. The obtained total resistivity calculation indicates its own spin accumulation profile dependance. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B15.00011: Influence of spin-orbit interactions on the electronic structure and magnetic properties of IrMn alloys Hua Chen, Panteleimon Lapas, Fengcheng Wu, Allan H. MacDonald We will present a theoretical study of the electronic and magnetic properties of non-collinear antiferromagnetic metals with strong spin-orbit interactions, focusing on the particular case of the IrMn alloy. IrMn alloys are important antiferromagnetic materials often used as the pinning layer in spin-valve structures. Their electronic structure has so far not been extensively studied; in particular the influence of spin-orbit interactions which are strong in this material has not yet been addressed. We start from ab initio calculations for ordered IrMn$_{3}$ crystals, and analyze the relationships between band degeneracy, non-collinearity of the Mn spins, and the large spin-orbit coupling of Ir. We will also study the spin wave spectra in the ordered IrMn$_{3}$, and finally comment on the influence of transport currents on magnetization structure and dynamics in antiferromagnets in general, and non-collinear systems in particular. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B15.00012: Theory of Tunneling Anisotropic Magnetoresistance Using the Tight-Binding Green's Function Approach Vivek Amin, Jan Zemen, Jan Masek, Jairo Sinova, Tomas Jungwirth An increasing experimental and theoretical understanding of magnetic tunnel junctions has led to widespread application within magnetic hard drives and furthered our understanding of spin valve-like processes fundamental to Spintronics. Crucial to this understanding is the investigation of tunneling processes between single ferromagnetic layers and tunnel barriers. We present a theoretical study of the Tunneling Anisotropic Magnetoresistance (TAMR) in a Co/Pt junction with a tunnel barrier. We calculate conductance as a function of magnetization direction using the Landauer-Buttiker formula. The system Hamiltonian is obtained by means of a suitable tight-binding model fitted to ab-initio calculations, while the transmission is computed via the Green's function formalism. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B15.00013: Inelastic process at finite temperature in 2D mutil-band systems Jacob Gayles, Huawei Gao, Jairo Sinova Despite the recent progress in the understanding of the contributions to the anomalous Hall effect, however there is still a lacking in understanding the role of inelastic processes at finite temperatures and the role of strong disorder. We use numerical methods and the Kubo Formalism to explore this regime multiband systems with spin-orbit coupling. Some experiments with the use of residual conductivity have been interpreted so that extrinsic mechanisms have a strong dependence on the increase in temperature while the anomalous hall conductivity reaches a steady state value. [Preview Abstract] |
Session B16: Focus Session: Spin-Dependent Physics in Graphene
Sponsoring Units: GMAG DMPChair: Minn-Tsong Lin, National Taiwan University
Room: 318
Monday, March 18, 2013 11:15AM - 11:27AM |
B16.00001: Magnetic Moment Formation in Graphene Detected by Scattering of Pure Spin Currents Adrian Swartz, Kathy McCreary, Jen-Ru Chen, Wei Han, Jaroslav Fabian, Roland Kawakami Graphene's 2D nature and high surface sensitivity have led to fascinating predictions for induced spin-based phenomena through careful control of adsorbates, including the extrinsic spin Hall effect, band gap opening, and induced magnetism. By taking advantage of atomic scale control provided by MBE, we have investigated deposition of adsorbates and their interactions with graphene. Spin transport measurements performed in-situ during systematic introduction of atomic hydrogen demonstrated that hydrogen adsorbed on graphene forms magnetic moments that couple via exchange to the injected spin current. The observed behavior is quantitatively explained utilizing a phenomenological theory for scattering of pure spin currents by localized magnetic moments. Lattice vacancies show similar behavior, indicating that the moments originate from so called pz-orbital defects. On the other hand, experiments with charge impurity scatterers such as Mg and Au, are noticeably absent of features related to magnetic moment formation. Furthermore, we observe gate dependent effective exchange fields due to the spin-spin coupling between conduction electrons and magnetic moments, which are of interest for novel phenomena and spintronic functionality but have not been seen previously in graphene. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B16.00002: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 11:39AM - 11:51AM |
B16.00003: Spin Transport Measurements in Hydrogenated Graphene Devices Gavin Koon, Jayakumar Balakrishnan, Barbaros Oezyilmaz Graphene with all its extraordinary properties still fall short when it comes to manipulation of electron spins. Chemically modified Graphene has been explored by many to further enhance Graphene properties, tailoring it to suit desired application purposes. Here we study the effects of hydrogenation rate on graphene spin transport, spin relaxation time and length in this defected system. These findings are important for future theoretical and experimental studies on other adatoms modified Graphene.~ [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B16.00004: Magnetic Insulators-Induced Proximity Effects in Graphene Ali Hallal, Hongxin Yang, Damien Terrade, Xavier Waintal, Stephan Roche, Mairbek Chshiev Due to its very long spin diffusion lengths up to room temperature, emergence of magnetism in graphene has attracted a lot of research interest in the field of spintronics. Several methods have been proposed to magnetize graphene, from edge magnetism, to depositing magnetic atoms or molecules, and using ferromagnetic substrate. We present first-principles calculations of spin-dependent properties in graphene induced by its interaction with ferromagnetic insulator EuO, and show that this proximity effect results in spin polarization of graphene $\pi$ orbitals by up to 24\% together with large exchange splitting bandgap of about 36 meV. Moreover, the position of the Dirac cone is shown to depend strongly on the graphene-EuO interlayer distance. These findings pave the way towards the possible engineering of graphene spin gating by proximity effect especially in a view of recent experiments on successful growth of Europium oxide on top of graphene. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B16.00005: Spin pumping at Permalloy/graphene interfaces Simranjeet Singh, Brett Barin, Ajit Kumar Patra, Barbaros \"Ozyilmaz, Enrique delBarco We present evidence of large spin relaxation effects in CVD graphene observed by means of ferromagnetic resonance (FMR) measurements of Permalloy/graphene (Py/Gr) bilayers. A substantial increase of the FMR linewidth in the Py/Gr bilayer, as compared to the Py layer, is interpreted in terms of an enhancement of the Gilbert damping in the ferromagnetic layer as a consequence of spin pumping at the Py/Gr interface, which is driven by the Py magnetization dynamics (i.e. precession of the magnetization induced by the microwave stimulus at resonance). The remarkable increase in the FMR linewidth compares with observations in other bilayer systems in where thick layers (thicker than the spin diffusion length) of heavy metals with strong spin-orbit interaction are employed as the non-magnetic layer. Our results indicate that spin relaxation in CVD graphene must be greatly enhanced in order to account for the losses of angular momentum by the ferromagnet. We will also present a comparative study of the Gilbert damping in Py/NM films employing highly ordered pyrolitic graphene as the non-magnetic layer, for which a more moderate broadening of the FMR linewidth is observed. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B16.00006: Probing Spin Orbit Interaction in Single Layer Graphene via Electronic Transport Sergio Ulloa, Mahmoud Asmar An important effect on the dynamics of spins in materials is the spin orbit interaction (SOI), which may reflect or arise from intrinsic symmetries in the lattice structure, or via broken symmetries (Rashba interaction) in the system. Resonant scatterers, limiting electron mobility in graphene, are realized by impurities such as hydrogen atoms, molecules, clusters of impurities, vacancies, or metallic islands deposited on (or grown under) the surface of graphene. Resonant scatterers can also generate or enhance the Rashba SOI in graphene samples. We have developed analytical spinor solutions of the Dirac equation that include spin dependent observables, and use these to examine the role of SOI on scattering cross sections.By making use of the ratio of the total to transport cross section in the system at low energy, we predict a strong enhancement in the scattering isotropy in the presence of the intrinsic SOI. Similarly, we see fundamental changes in resonant scatterers in the presence of the Rashba SOI, which also lead to enhanced isotropy. We will discuss how these results have implications on the better characterization of impurities in graphene samples, and how typical experimental results can provide quantitative estimates of the SOI present in the system. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B16.00007: Rashba Spin Orbit Interaction and Birefringent Electron Optics in Graphene Mahmoud Asmar, Sergio Ulloa Analogies between geometrical optics and electron trajectories have resulted in a number of interesting proposals for device applications, where material interfaces play a similar role to that of transparent interfaces in physical optics. Optical birefringence in materials arising from crystal anisotropies are manifested as different group velocities for different polarizations of light. By making use of analytical solutions of the Dirac equation, and extending the partial wave component method of scattering to include spin dependent observables, we show that an equivalent phenomenon to optical birefringence in electron optics is feasible in two dimensional graphene. The electronic birefringence arises from the intrinsic graphene structure and requires the presence of Rashba spin-orbit interaction. The different group velocities depend on the chirality of the electronic states, mimicking the light polarization dependence of the group velocities in optical birefringent materials. In circular regions containing large Rashba interaction and reversed charge density (Veselago lenses), we predict the formation of sets of double caustics and cusps, where the spacing between the two different chiral cusps is proportional to the strength of the Rashba interaction in the system. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B16.00008: Colossal spin-orbit coupling in functionalized graphene Jayakumar Balakrishnan, Gavin Koon, Barbaros Oezyilmaz Graphene's low intrinsic spin orbit (SO) interaction strongly limits the realization of several functional spintronics devices. It is therefore quite desirable to develop methods to tune this SO coupling strength. Among the different approaches, the functionalization of graphene seems to be more promising from an application perspective. Recent theoretical and experimental results on functionalized graphene have shown interesting magnetic properties. Here, we will show our preliminary spin-transport results on such functionally modified graphene and discuss the various possibilities it holds for future graphene-based spintronics applications. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B16.00009: Spin waves in graphene nanoribbon devices Francisco Culchac, Andrea Latg\'e, Rodrigo Capaz, Antonio Costa We investigate spin excitations and electronic properties of graphene nanoribbon devices with zigzag edges. The magnetic region of the device is coupled to nonmagnetic metallic leads. The ground state of the magnetic region is described self-consistently within a mean-field scheme. Spin excitations are extracted from the transverse dynamic spin susceptibility. Several standing-wave modes can be identified. We study the behavior of these modes as the coupling between the magnetic region and the leads is varied. A central point found is that for a finite zigzag nanoribbon, spin excitations are damped at all finite energies. The signature of antiferromagnetic correlations is still present in the predominantly linear relationship between the standing-mode energy and the mode wave vector. The effect of an external doping is also considered and, as in the infinite case, it is found that ferromagnetic order along the ribbon's edges becomes unstable at modest doping levels. We also show the behavior of the spin excitations in the infinite zigzag nanoribbons when an electric field is applied in the transversal direction. As it is well known, this system presents a half-metallic state. A reduction of the spin-wave lifetime is found for increasing electric field intensities. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B16.00010: Spin transport studies in encapsulated CVD graphene Ahmet Avsar, Jun You Tan, Yuda Ho, Gavin Koon, Barbaros Oezyilmaz Spin transport studies in exfoliated graphene on SiO2/Si substrates have shown spin relaxation times that are orders of magnitude shorter than the theoretical predictions. Similar to the charge transport case, the underlying substrate is expected to be the limiting factor. The recent work Zomer, P. J. et al. [1] shows that spin transport over lengths up to 20um is possible in high mobility exfoliated graphene devices on boron nitride (BN) substrates. Here we discuss our initial attempts to repeat such spin transport experiments with CVD graphene on BN substrates. The effect of encapsulation of such devices with an extra BN layer will be also discussed. [1] Zomer, P. J.; Guimaraes, M. H. D.; Tombros, N.; van Wees, B. J. ArXiv:1209.1999, 2012 [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B16.00011: Suppression of spin relaxation due to weak localization in multilayer graphene spin valves Takehiro Yamaguchi, Satoru Masubuchi, Kazuyuki Iguchi, Rai Moriya, Tomoki Machida Graphene is a promising material for spintronics devices because of its long spin diffusion length. In addition, graphene is a fascinating system where quantum interference phenomena such as weak localization and Fabry-Perot interference can be observed because of its long phase coherent length at low temperature. Therefore, graphene is an ideal system for exploring the physics of spin transport and spin relaxation under the phase coherent system. In this study, we fabricated multilayer graphene spin valve devices [1] and investigated temperature dependence of spin transport and spin relaxation properties. Spin relaxation time obtained by Hanle effect with nonlocal geometry was found to start increasing below 70 K and reach 2.5 ns at 2 K. Under the same temperature range, we also found weak localization emerged. These results suggest the correlation of spin relaxation and phase coherent transport in graphene [2]. [1] T. Yamaguchi et al., J. Magn. Magn. Mater. 324, 849 (2012), [2] T. Yamaguchi et al., submitted [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B16.00012: Long Electron Spin Lifetimes in Armchair Graphene Nanoribbons Matthias Droth, Guido Burkard Armchair graphene nanoribbons (aGNR) are promising as a host material for electron spin qubits because of their potential for scalability and long coherence times [1]. The spin lifetime $T_1$ is limited by spin relaxation, where the Zeeman energy is absorbed by lattice vibrations [2], mediated by spin-orbit and electron-phonon coupling. We have calculated $T_1$ by treating all couplings analytically and find that $T_1$ can be in the range of seconds for several reasons: (i) Van Vleck cancellation; (ii) weak spin-orbit coupling; (iii) low phonon density; (iv) vanishing coupling to out-of-plane modes due to the electronic structure of the aGNR. Owing to the vanishing nuclear spin of $^{12}$C , $T_1$ is a good measure for overall coherence. These results and recent advances in the controlled production of graphene nanoribbons [3] make this system interesting for classical and quantum spintronics applications.\\[4pt] [1] B. Trauzettel, D. V. Bulaev, D. Loss, and G. Burkard, Nature Phys. 3, 192-196 (2007).\\[0pt] [2] M. Droth and G. Burkard, Phys. Rev. B 84, 155404 (2011).\\[0pt] [3] X. Zhang et al., arXiv:1205.3516 (2012). [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B16.00013: Giant magnetic anisotropy of 5d dopants in graphene and boron nitride monolayer Jun Hu, Ruqian Wu Searching for novel magnetic nanostructures is urgent due to the need for the miniaturization of spintronics devices. One of the main bottlenecks for this is the low blocking temperature (\textless 10 K) in most magnetic nanoentities studied so far. In this work, we predict that extremely high blocking temperature can be achieved in graphene or boron nitride monolayer by embedding 5d transition metal (TM) atoms, based on density functional theory calculations. For example, the size of the magnetocrystalline anisotropy energy (MAE) of Re/graphene or Re/BN can be larger than 20 meV for each Re atom, sufficient for room temperature magnetic recording and spintronics applications. We provide physical insights for the further development of nanostructures with larger MAE. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B16.00014: Magneto-Resistance in thin film boron carbides Elena Echeverria, Guangfu Luo, J. Liu, Wai-Ning Mei, F.L. Pasquale, J. Colon Santanta, P.A. Dowben, Le Zhang, J.A. Kelber Chromium doped semiconducting boron carbide devices were fabricated based on a carborane icosahedra (B$_{10}$C$_{2}$H$_{12})$ precursor via plasma enhanced chemical vapor deposition, and the transition metal atoms found to dope pairwise on adjacent icosahedra site locations. Models spin-polarized electronic structure calculations of the doped semiconducting boron carbides indicate that some transition metal (such as Cr) doped semiconducting boron carbides may act as excellent spin filters when used as the dielectric barrier in a magnetic tunnel junction structure. In the case of chromium doping, there may be considerable enhancements in the magneto-resistance of the heterostructure. To this end, current to voltage curves and magneto-transport measurements were performed in various semiconducting boron carbide both in and out plane. The I-V curves as a function of external magnetic field exhibit strong magnetoresistive effects which are enhanced at liquid Nitrogen temperatures. The mechanism for these effects will be discussed in the context of theoretical calculations. [Preview Abstract] |
Session B17: Focus Session: Iridate Mott Insulators
Sponsoring Units: DMP GMAGChair: Stephen Wilson, Boston College
Room: 319
Monday, March 18, 2013 11:15AM - 11:27AM |
B17.00001: Infrared study of the electronic structure of metallic pyrochlore iridate Bi$_2$Ir$_2$O$_7$ Yunsang Lee, S.J. Moon, Scott C. Riggs, M.C. Shapiro, I.R. Fisher, A.F. Kemper, D.N. Basov We investigated the electronic properties of a single crystal of metallic pyrochlore iridate Bi$_2$Ir$_2$O$_7$ by using the infrared spectroscopy. Our optical conductivity data show the splitting of t$_{\mathrm{2g}}$ bands into J$_{\mathrm{eff}}$ ones due to strong spin-orbit coupling. We observed a sizable mid-infrared absorption near 0.2 eV within the J$_{\mathrm{eff,1/2}}$ bands, which indicates that this material may belong to a class of correlated system. Our findings suggest that the electronic structure of Bi$_2$Ir$_2$O$_7$ is governed by the strong spin-orbit coupling and the correlation effect, which is prerequisite for theoretically proposed non-trivial topological phases in pyrochlore iridates. We also discuss possible existence of the very far-infrared region of suppression in the optical conductivity of the compound. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B17.00002: Electronic Structure of Spin-Orbital-Coupling-Driven Insulator Sr$_2$IrO$_4$ from Angle-Resolved Photoemission Spectroscopy Yan Liu, Xiaowen Jia, Daixiang Mou, Lin Zhao, Junfeng He, Guodong Liu, Shaolong He, Yingying Peng, Chaoyu Chen, Xiaoli Dong, Jun Zhang, Zuyan Xu, Chuangtian Chen, Gang Cao, X.J. Zhou Sr$_2$IrO$_4$, as a Mott Insulator, is an ideal system to study spin orbital coupling interaction in transition metal oxides. We report a comprehensive investigation on electronic structure of Sr$_2$IrO$_4$ by high resolution angle-resolved photoemission spectroscopy (ARPES). We measured the Fermi surface and band structures at different photon energies, under different photon polarizations. New features have been revealed that were not observed in previous studies. Moreover, the measurement under different polarizations helps identify different orbital characteristics of bands. The comparison between our experimental observations and theoretical calculation proves the important role of spin-orbital coupling interaction in determining its electron structure. The rich information on the electron structure of Sr$_2$IrO$_4$ will provide key insights in understanding the mechanism of various electron interactions in determining its insulator ground state. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B17.00003: Crystal field splitting and correlation effect on the electronic structure of $A_2{\rm IrO}_3$ Hlynur Gretarsson, J.P. Clancy, X. Liu, J.P. Hill, E. Bozin, Y. Singh, S. Manni, P. Gegenwart, J. Kim, A.H. Said, D. Casa, T. Gog, M.H. Upton, H.S. Kim, J. Yu, V.M. Katukuri, L. Hozoi, J.v.d. Brink, Y.J. Kim The electronic structure of the honeycomb lattice iridates Na$_2$IrO$_3$ and Li$_2$IrO$_3$ has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field split $d$--$d$ excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to non-cubic crystal fields, derived from the splitting of $j_{\rm{eff}}$=3/2 states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of $j_{\rm{eff}}$ physics in A$_2$IrO$_3$. We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both Na$_2$IrO$_3$ and Li$_2$IrO$_3$ can be described as spin-orbit Mott insulators, similar to the square lattice iridate Sr$_2$IrO$_4$. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B17.00004: Quasimolecular electronic structure of Na$_2$IrO$_3$ Igor Mazin, Harald Jeschke, Foyevtseva Kateryna, Roser Valenti, Daniel Khomskii Spin-orbit (SO) coupling can lead to many nontrivial effects such as Rashba effect, topological insulators, or topologically protected states in systems described the Heisenberg-Kitaev model, recently proposed for Na$_2$IrO$_3$. This proposal is based on the fact the SO coupling for iridium is very strong, and cannot be quenched by the small trigonal crystal field. We show, however, that Na$_2$IrO$_3$ represents a highly unusual case, in which the electronic structure is dominated by the formation of quasi-molecular composite orbitals (QMOs). The QMOs consist of six atomic orbitals on an Ir hexagon, and the orbital moment of each QMO is quenched, so that spin-orbit effects only affect the inter-QMO interaction. The concept of such composite orbitals in solids is completely new, and invokes very different physics compared to the models considered previously. For instance, one has to account for Hubbard correlations among the QMOs, and not individual atomic orbitals. Both the insulating behavior and the experimentally observed zigzag antiferromagnetism in Na$_2$IrO$_3$ naturally follow from the QMO model. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B17.00005: Magnetic properties of triangular lattice Ca$_4$IrO$_6$ and Ca$_{2.5}$Sr$_{1.5}$IrO$_6$ K.H. Butrouna, L. Li, T.F. Qi, O.B. Korneta, J. Terzic, E. Akbari, S. Parkin, S.J. Yuan, G. Cao We report a structural, thermodynamic, and transport study of single-crystal Ca$_4$IrO$_6$ and Ca$_{2.5}$Sr$_{1.5}$IrO$_6$. The isostructural Ca$_4$IrO$_6$ and Ca$_{2.5}$Sr$_{1.5}$IrO$_6$ feature a triangular lattice of spin chains running along the \emph{c} axis. The underlying properties of the two systems are characterized by a partial antiferromagnetic order occuring at 12 K and 9 K for Ca$_4$IrO$_6$ and Ca$_{2.5}$Sr$_{1.5}$IrO$_6$, respectively, a small entropy removal associated with the phase transition, and a sizable low-temperature specific heat linearly proportional to temperature. The detailed results will be discussed along with comparisons drawn with other related systems such as Ca$_5$Ir$_3$O$_{12}$. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B17.00006: Electronic and magnetic phase evolution in Sr$_3$(Ir$_{\mathrm{1-x}}$Ru$_{\mathrm{x}})_2$O$_7$ Chetan Dhital, Tom Hogan, Kevin Lukas, Steven Dissler, Cyril Opeil, Stephen Wilson A great deal of recent focus has been given to understanding how the interplay of strong spin orbit coupling effects and onsite coulomb repulsion change the conventional energy hierarchy in correlated 5d electron iridium oxides. Contrary to conventional band theory, perovskite iridate compounds Sr2IrO4 and Sr3Ir2O7 have long been known to be insulators; however many of their fundamental electronic properties and the interactions responsible for generating their antiferromagnetic insulating ground states remain under investigation. Here, we report results from our transport and magnetization study of electronic and magnetic phase of Sr3Ir1-xRuxO7. The evolution of the phase behavior as Sr3Ir2O7 is tuned from an AF insulator to a paramagnetic metal and the potential for a first order metal to insulator transition will be discussed. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B17.00007: Magnetic properties of Mn-doped Sr2IrO4 M.D. Lumsden, S. Calder, G.-X. Cao, J.W. Kim, Z. Gai, B.C. Sales, D. Mandrus, A.D. Christianson In 5d electron transition metal oxides, interplay between spin-orbit coupling and electronic interactions can lead to novel properties. One example is the Mott state in Sr$_2$IrO$_4$ which is believed to be associated with the formation of a J$_{eff}$=1/2 band due to large spin-orbit splitting of the t$_{2g}$ band. We use bulk measurements and resonant magnetic x-ray scattering to explore the effects of substituting Mn for Ir in single crystals of Sr$_2$Ir$_{0.9}$Mn$_{0.1}$O$_4$. These measurements indicate that 10\% Mn doping is sufficient to suppress the magnetic ordering temperature from 240 K to ~155 K. Resonant magnetic x-ray scattering measurements indicate a change in the long-range magnetic order when compared to that of undoped Sr$_2$IrO$_4$. Despite the large change in transition temperature and the altered magnetic structure, we observe a difference in the resonant enhancement between the L$_2$ and L$_3$ edges which is very similar to that seen in the pure material. This suggests that the magnetic structure of Sr$_2$IrO$_4$ can be altered by small perturbations whereas the J$_{eff}$=1/2 state is robust. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B17.00008: Weak magnetic transitions in pyrochlore Bi$_2$Ir$_2$O$_7$ Peter Baker, Francis Pratt, Johannes Moeller, Bil Hayes, Stephen Blundell, Tom Lancaster, Tongfei Qi, Gang Cao The pyrochlore iridate Bi$_2$Ir$_2$O$_7$ is analogous to the rare earth pyrochlores $R$Ir$_2$O$_7$ ($R=$ Y and Pr-Lu) but has no rare earth moments or $f$ electrons to interact with the Ir subsystem. This makes it an ideal system in which to study the Ir magnetism in isolation. Bulk measurements showed that it is metallic down to 2K and no indication of magnetic ordering was found down to 50mK. The magnetic field dependence of the low-temperature specific heat shows large changes in both the linear and cubic contributions and the large Wilson ratio of 53.5 suggests proximity to a quantum critical point [1]. Our muon spin relaxation measurements find a bulk magnetic transition at $1.84(3)$K and the form of the data suggests that the low-temperature state represents ordering of exceptionally small magnetic moments with persistent weak dynamics. The relaxation rate increases further below $0.23(4)$K, coincident with a growth in the specific heat, suggesting another magnetic transition. The magnetic field experienced by muons is $\sim 0.7$T at low-temperature, around two orders of magnitude smaller than that in other pyrochlore iridates, corresponding to moments $\sim 0.01~\mu_{\rm B}$/Ir.\\[4pt] [1] T. F. Qi et al., J. Phys.: Condens. Matter 24, 345601 (2012). [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B17.00009: Theory on Magnetic Excitation Spectra in Pyrochlore Iridates Eric Kin-Ho Lee, Subhro Bhattacharjee, Yong Baek Kim Metal-insulator transitions in pyrochlore iridates (A$_2$Ir$_2$O$_7$) are believed to occur due to subtle interplay of spin-orbit coupling, geometric frustration, and electron interactions. In particular, the nature of magnetic ordering of iridium ions in the insulating phase is crucial for understanding of several exotic phases recently proposed for these materials. We study the spectrum of magnetic excitations in the intermediate-coupling regime for the so-called all-in/all-out magnetic state in pyrochlore iridates with non-magnetic A-site ions (A=Eu,Y), which is found to be preferred in previous theoretical studies. We find that the effect of charge fluctuations on the spin-waves in this regime leads to strong departure from the lowest-order spin-wave calculations based on models obtained in strong-coupling calculations. We discuss the characteristic features of the magnetic excitation spectrum that can lead to conclusive identification of the magnetic order in future resonant inelastic x-ray (or neutron) scattering experiments. Knowledge of the nature of magnetic order and its low-energy features may also provide useful information on the accompanying metal-insulator transition. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:39PM |
B17.00010: Spin-Orbit Coupling in Mott Insulators: Unusual Interactions and Possible Exotic Phases Invited Speaker: George Jackeli Over the last few years, there has been an upsurge of interest in materials in which exotic states may emerge as the result of relativistic spin-orbit interactions. We will discuss insulating iridium oxides from this perspective. We show that the strong spin-orbit coupling, through the entanglement of spin and orbital spaces, leads to a variety of interesting Hamiltonians ranging from the Heisenberg model to the Kitaev or quantum compass models, for different lattice geometries [1]. Based on these effective Hamiltonians, we present a comprehensive theoretical study [1-3] of the rich phase behavior and dynamics observed in layered iridium oxides such as tetragonal Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$ and hexagonal \textit{A}$_2$IrO$_3$ (\textit{A}=Na, Li). We suggest that the hexagonal iridates might be close to the Kitaev spin-liquid state. We also discuss the layered tetragonal vanadate Sr$_2$VO$_4$ and argue that magnetically-hidden octupolar order, driven by spin-orbit coupling, is realized in this compound [4].\\[4pt] [1] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. \textbf{102}, 017205 (2009).\\[0pt] [2] J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev. Lett. \textbf{105}, 027204 (2010).\\[0pt] [3] J. Chaloupka, G. Jackeli, and G. Khaliullin, arXiv:1209.5100.\\[0pt] [4] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. \textbf{103}, 067205 (2009). [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B17.00011: ARPES Studies of Low-energy electronic structure of the strong spin-orbit semimetal SrIrO$_{3}$ Yuefeng Nie, Philip D.C. King, Haofei Wei, Masaki Uchida, John Harter, Eric Monkman, Daniel Shai, Darrell Schlom, Kyle Shen The similar energy scales of spin-orbit coupling and electron-electron correlation strength lead to exotic J$_{eff}=$ 1/2 Mott insulating ground states for layered Ruddlesden-Popper 5d iridates, Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$. A metal-insulator transition occurs upon increasing dimensionality from the two-dimensional layered Sr$_{2}$IrO$_{4}$ to the three-dimensional perovskite SrIrO$_{3}$. However, little is known about the electronic structure and nature of the metallic states in SrIrO$_{3}$. We synthesized epitaxial SrIrO$_{3}$ films on (001) LSAT substrates by molecular beam epitaxy and investigated their electronic structure using angle-resolved photoemission spectroscopy. We find an exotic semi-metallic state comprised of massive hole-like bands, whose extrema are pinned very close to the chemical potential, and rapidly dispersive electron bands which dominate the transport. Intriguingly, the bandwidths of SrIrO$_{3}$ are smaller than in its Mott insulating counterpart Sr$_{2}$IrO$_{4}$, indicating that metal-insulator transitions in Ruddlesden-Popper iridates are not simply driven by band narrowing resulting from reduced dimensionality. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B17.00012: High-magnetic-field-tuned insulating state in single-crystal BaIrO$_{3}$ O.B. Korneta, T.F. Qi, L. Li, K. Butrouna, G. Cao, E.S. Choi, Xiangang Wan BaIrO$_{3}$ is a novel magnetic insulator associated with the spin-orbit interaction. It magnetically orders at T$_{C}=182$~K, with an extremely small saturation moment M$_{S} < 0.03~\mu_{B}/Ir$. Application of high magnetic field up to 35~Tesla results in an exotic behavior characterized by: (1) a drastic rise in electrical resistivity by 250\% at low temperatures and (2) highly anisotropic magnetoresistivity with unusually strong hysteretic behavior. Our first principle calculations suggest a band structure near Fermi surface extremely sensitive to slight changes in lattice parameters, which captures underlying physical properties observed experimentally. The giant positive magnetoresistivity along with the extremely small saturation moment signals a delicate interplay between the structural and the electronic degrees of freedom in this compound. The electrical transport and magnetic properties in high magnetic field will be presented and discussed. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B17.00013: Magnetism in double-perovskite Sr$_2$GdIrO$_6$ and Sr$_2$YIrO$_6$ J. Terzic, T.F. Qi, L. Li, O.B. Korneta, S. Parkin, G. Cao {Sr$_2$GdIrO$_6$} and {Sr$_2$YIrO$_6$} with Ir$^{5+}$(5d$^4$) ions are magnetic insulators with a double-perovskite structure derived from the perovskite SrIrO$_3$, which is a paramagnetic metal. We report results of our study of structural and physical properties of single-crystal {Sr$_2$GdIrO$_6$} and {Sr$_2$YIrO$_6$}. This study reveals that while {Sr$_2$YIrO$_6$} exhibits no long-range order above 1.7 K, {Sr$_2$GdIrO$_6$} displays an anisotropic and antiferromagnetic state at low temperatures that is clearly manifested in the magnetization and specific heat. The results will be presented and discussed along with comparison drawn with other related iridates driven by the strong spin-orbit interaction. [Preview Abstract] |
Session B18: Focus Session: Spin-Dependent Phenomena in Semiconductors - Spin Valleytronics and Spin Orbit
Sponsoring Units: GMAG DMP FIAPChair: Aubrey Hanbicki, Naval Research Laboratory
Room: 320
Monday, March 18, 2013 11:15AM - 11:27AM |
B18.00001: Valley polarization and intervalley scattering in monolayer MoS$_{2}$ G. Kioseoglou, A.T. Hanbicki, M. Currie, A.L. Friedman, D. Gunlycke, B.T. Jonker Single layer MoS$_{2}$ is a prime candidate material for implementing valleytronics because minima in the bandstructure at inequivalent K points of the Brillouin zone can be independently populated, thus making the valley index a potential state variable for information processing. Light of a particular helicity populates only one of the two K-valleys (either K or K') resulting in a strong emission at around 1.9 eV associated with a direct transition. We use energy and helicity dependent optical pumping to analyze the coupling of the valley and spin indices to the depolarization of emitted light. The circular polarization of the photoluminescence is very high for photo-excitation near the bandgap, and has a power-law decrease as the photo-excitation energy increases. We identify phonon-assisted intervalley scattering as the primary spin relaxation mechanism and present a model of depolarization that explains the wide variation in values for the optical polarization reported in the literature. Our results elucidate the basic processes that control the unique properties of this material and should help to realize future valleytronic applications. This work was supported by core programs at NRL and the NRL Nanoscience Institute. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B18.00002: Theoretical analysis of optical excitation and luminescence in MoS$_2$ monolayers Hanan Dery, Yang Song We analyze the absorption and circularly polarized luminescence spectra of MoS$_2$ monolayers. We show that indirect optical transitions can fully explain the observed decrease in circular polarization degree when increasing the lattice temperature or the exciting photon energy. This spin-conserving optical process is assisted by electron-phonon or electron-impurity interactions giving rise to intervalley transitions to intermediate virtual states. Spin-flip mechanisms, on the other hand, are shown to be insufficient in explaining the experimental results due to their relatively long timescales compared with the radiative timescales in monolayer dichalcogenides (tens of ps). [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B18.00003: Longitudinal and spin/valley Hall optical conductivity in single layer MoS$_2$ Zhou Li, Jules Carbotte A monolayer of MoS$_2$ has a non-centrosymmetric crystal structure, with spin polarized bands. It is a two valley semiconductor with direct gap falling in the visible range of the electromagnetic spectrum. Its optical properties are of particular interest in relation to valleytronic and possible device applications. Circular polarized light associated with each of the two valleys separately is considered and results are filtered according to spin polarization. Temperature can greatly change the spin mixture seen in the frequency window where they are not closely in balance.\\[4pt] [1] Zhou Li and J. P. Carbotte, submitted to Phys. Rev. B.\\[0pt] [2] D. Xiao et.al, Phys. Rev. Lett. 108,196802 (2012). [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B18.00004: Optical and Electrical Control of Valley Pseudospin in Atomically-Thin Semiconductors Invited Speaker: Xiaodong Xu Electronic valleys are energy extrema of Bloch bands in momentum space. In analogy to electrons with spin degrees of freedom, valley indexes can be considered as pseudospins for new modes of electronic and photonic device operation. In this talk, I will discuss the experimental progress on the investigation of these pseudospins using atomically-thin semiconductors, which are either single or bilayer group VI transition metal dichalcogenides. I will show that these new 2D semiconductors not only behave as remarkable excitonic systems, but also provide an ideal system for optical and electrical control of valley degrees of freedom. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B18.00005: Theoretical design of magnetic two-dimensional transition metal dichalcogenide semiconductors Wenguang Zhu, Di Xiao We explore the possibility of making single-layer dichalcogenide semiconductors magnetic by doping transition metal ions using density functional calculations. Optimal conditions of doping are suggested based on the study of the energetics and kinetics of magnetic ions in the host materials. The magnetic ordering and magnetic coupling mechanism between the magnetic dopants will also be discussed in this talk. This work may provide a new twist to form truly two-dimensional magnetic semiconductors for spintronic applications. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B18.00006: Group theory analysis of the intrinsic momentum and spin relaxation in monolayer dichalcogenides Yang Song, Hanan Dery Using group theory, we study the intrinsic momentum and spin relaxation of electrons and holes due to scattering with phonons in monolayer dichalcogenides. Double group symmetry representations of electron and hole states at high symmetry points ($K$, $K'$, and $\Gamma$ points as well as the $T$ axis) are identified with the help of results from absorption and photoluminescence experiments. We link the leading contributions to intravalley and intervalley scattering with symmetries of the nine phonon dispersion branches. Scattering matrix elements due to short-range interaction and the corresponding Elliott-Yafet spin-flip mechanism are expressed analytically, leading to explicit wavevector-dependence and scattering integrals. Long-range interaction are similarly analyzed. Due to the absence of inversion symmetry, valley-spin coupling is revealed to be a general feature in the spin-flip scattering. Using these results we estimate the temperature-dependent relaxation times. Intervalley scattering between valleys not connected by time-reversal is shown to be compound dependent. The $K$ to $T$ transition ($K$ to $\Gamma$ transition) in the conduction (valence) band is relevant in heavier (lighter) compounds such as WSe$_2$ (MoS$_2$). [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B18.00007: Spin-dependent phonon-assisted optical transition in Si and Ge under strain Pengke Li, Dhara Trivedi, Hanan Dery In indirect bandgap semiconductors like Si and Ge, the transfer of angular momentum between free carriers and photons is intricate since they involve both radiation-matter and electron-phonon interactions. Moreover, the multi-valley conduction band of Si and Ge leads to dependence on light propagation. By breaking the degeneracies of conduction valleys and of valence bands, strain could be used as an experimental tool to regulate and validate the relation between the measured circular polarization degree of photons and the spin polarization of charge carriers. Using symmetry arguments, we present a theoretical study of the spin-dependent selection rules for various phonon-assisted optical transitions. We show how these selection rules are changed under different configurations of strain. These selection rules are verified by rigorous numerical calculation of the spin-dependent luminescence spectra in strained Si and Ge, as well as in relaxed SiGe alloys. Lastly, we also provide results of the inverse process, namely optical orientation. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B18.00008: Spin communications under optimized external electric field in strained group IV semiconductors Lan Qing, Hanan Dery We investigate factors affecting an on-chip communication paradigm that is based on modulating spin polarization of a constant current in silicon or germanium wires. Strain that quenches certain intervalley scattering can prolong the spin lifetime considerably. Necessary external electric field can accelerate the transport by increasing drift velocity, yet it also enhances the spin relaxation by heating the electrons. We predict non-monotonic behaviors of the final spin signals versus the external electric fields. Simple approximate expressions are provided for the spin lifetimes of drifting electrons in strained silicon and germanium, which enable us to choose electric fields that maximize the signals in spin transport. Even at room temperature, we can expect no significant loss of the spin signal after transport across millimeter scale. Our theoretical results are supported by recent experimental breakthroughs. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B18.00009: Enhanced intervalley splitting and reduced spin relaxation in strained thin silicon films Dmitri Osintsev, Viktor Sverdlov, Siegfried Selberherr We investigate the influence of strain and spin-orbit interaction on the valley splitting, subband structure, subband wave functions, and spin relaxation matrix elements due to surface roughness scattering in thin silicon films. A ${\mathbf{k \cdot p}}$ approach suitable to describe the electron subband structure with spin [1] is generalized to include strain. The 4$\times$4 Hamiltonian is diagonalized with respect to the spin degree by a unitary transformation. The wave functions and eigenenergies are found analytically, when the thin film is approximated by an infinite square well potential. In relaxed films the unprimed subbands are degenerate. This degeneracy produces a large mixing between the spin-up and spin-down states, resulting in spin hot spots characterized by strong spin relaxation. These hot spots are contrasted with those appearing in the bulk system [1] due to the degeneracy of the opposite valleys along certain directions close to the X-point at the edge of the Brillouin zone. Shear strain efficiently lifts the degeneracy between the unprimed subbands. This removes the origin of the spin hot spots in a confined silicon system, which substantially improves the spin lifetime in silicon films. 1.P.Li and H.Dery, {\it Phys.Rev.Lett.} {\bf 107}, 107203 (2011). [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B18.00010: Unified theory of spin-dynamics in a two dimensional electron gase with arbitrary spin-orbit coping strength at finite temperature Xin Liu, Sinova Jairo We study the spin dynamics in the presence of impurity and electron-electron (e-e) scattering in a III-V semiconductor quantum well with arbitrary spin-orbit coupling (SOC) strength and symmetry at finite temperature. In the regime where the strength of the Rashba and linear Dresselhaus SOC match, known as the SU(2) symmetry point, experiments have observed the spin-helix mode with a large spin-lifetime whose unexplained nonmonotonic temperature dependence peaks at around 75 K. As a key test of our theory, we are able to naturally explain quantitatively this nonmonotonic dependence and show that it arises as a competition between the Dyakonov-Perel mechanism, suppressed at the SU(2) point, and the Elliott-Yafet mechanism. In the strong SOC regime, we show that our theory directly reproduces the previous known analytical result at the SU(2) symmetry point in the ballistic regime. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B18.00011: A critical phase induced by interplay of spin-orbit coupling and Coulomb interaction Eun-Gook Moon, Cenke Xu, Yong Baek Kim, Leon Balents We study long range Coulomb interaction effect on the Luttinger Hamiltonian in three spatial dimensions, which describes strong spin orbit coupling intrinsically. The Hamiltonian has energy spectrum of inverted band gap semiconductors as in well-known HgTe; only one quadratic band touching point exists at the gamma point in Brillouin zone protected by the cubic and time reversal symmetries. Using controlled renormalization group techniques, we find that long-range Coulomb interaction converts the quadratic band touching state into a non-Fermi liquid (NFL) state, in some ways analogous to the Luttinger liquid state in one dimension. Consequently, all physical quantities become scale invariant and show deviations from non-interacting electrons' properties. Temperature and field dependence of various thermodynamic functions are obtained. Moreover, our ground state can be viewed as a parent state of topological insulators, magnetic metals, and Weyl semi-metals by breaking either cubic symmetry or time-reversal symmetry. The strong Coulomb interaction changes phase boundaries qualitatively and phase diagrams with the Coulomb interaction are provided. Applications to iridium-oxides materials are also discussed. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B18.00012: 3D and 4D Topological Insulators based SU(2) Landau Levels Yi Li, Shou-Cheng Zhang, Congjun Wu Current studies of 3D topological insulators (TIs) based on the Bloch-wave band inversion have made great success in lattices. Independent of current routine, we propose a novel and simple mechanism achieving exactly flat topological spectra for electrons in the continuum at 3D and 4D without magnetic fields. By introducing spin-orbit couplings, helical Dirac modes or chiral Weyl modes with opposite helicities are spatially separated along an extra spatial dimension and robust at boundaries as protected by the time-reversal symmetry. Moreover, based on elegant analytic wavefunctions of high dimensional Landau levels, we construct the Laughlin type wavefunction at the fractional filling in 4d. Further, parallel to the 2D QHE, whose quantized Hall response demonstrates spatially separated (1+1)D chiral anomaly, the 4D SU(2) Landau levels explicitly show the quantized non-linear electromagnetic response, which exhibits spatially separated (3+1)D chiral anomaly with the same quantization in the unit of fundamental physical constants. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B18.00013: Spin polarization in the Hubbard model with Rashba spin-orbit coupling on a ladder Jose Riera The competition between on-site Coulomb repulsion and Rashba spin-orbit (RSO) coupling is studied on two-leg ladders by numerical techniques. Using DMRG it is found that the contribution to the current due to the RSO coupling for a fixed value of the Hubbard repulsion $U$ reaches a maximum at intermediate values of the RSO coupling-to-hopping ratio and eventually becomes negative. This point of maximum current is correlated with the maximum value of the spin polarization between the two legs of the ladder. The most important result is that for a fixed value of the RSO coupling, the spin polarization increases with $U$ and seems to saturate as $U\rightarrow \infty$. These behaviors are studied at various fillings in the metallic regime. Further support for these conclusions is provided by the study of persistent currents in Hubbard-Rashba models on ladder rings. The implications of this enhancement of the spin Hall effect with electron correlations for spintronic devices is discussed. [Preview Abstract] |
Session B19: Metal Insulator transitions in Vanadates: exp/theory
Sponsoring Units: DCMPChair: Mumtaz Qazilbash, College of William and Mary
Room: 321
Monday, March 18, 2013 11:15AM - 11:27AM |
B19.00001: Nanoscale Thermal Mapping of VO$_2$ Adam Pivonka, Magdalena Huefner, Changhyun Ko, Alex Frenzel, Kevin O'Connor, Shriram Ramanathan, Eric Hudson, Jennifer Hoffman We present a method for nanoscale thermal imaging of insulating thin films. We image the local temperature of the metal-insulator transition in a VO$_2$ film, and investigate the role of Joule heating in two-terminal geometry. By sweeping the voltage applied to a conducting atomic force microscope tip in contact mode, we locally trigger and detect the transition to the metallic phase. By fitting the Poole-Frenkel conduction regime immediately preceding the transition, we extract the local temperature. Finally, we find grains displaying two electronic transitions, consistent with a locally stable intermediate insulating phase.\\[4pt] We acknowledge financial support from Harvard's Nanoscale Science and Engineering Center, funded by NSF grant PHY 01-17795 and the Sloan Fellowship. Adam Pivonka acknowledges the support of the New York Community Trust--George Merck Fund. Magdalena Huefner acknowledges the support of the Deutsche Forschungsgemeinschaft (HU 1960/11). [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B19.00002: Strain-dependent Metal-Insulator Tansition in VO$_{2}$ single-crystalline thin films Naga Phani Aetukuri, Alexander Gray, Matteo Cossale, Marc Drouard, Li Gao, Hermann Durr, Mahesh Samant, Stuart Parkin Vanadium dioxide (VO$_{2}$ has a near room temperature metal insulator transition (T$_{\mathrm{MIT}}$ $\sim$ 340 K) accompanied by a structural transition making the origin of this transition controversial. In this work, we have continuously changed T$_{\mathrm{MIT}}$ by as much as 60 K in VO$_{2}$ (001) single crystalline thin films by using RuO$_{2}$ buffer layers. We observe a decrease in the T$_{\mathrm{MIT}}$ as a function of decreasing c-axis length in the rutile phase which is unexpected from a one-dimensional Peierls model. By performing complementary bulk-sensitive spectroscopic measurements, namely, x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS), we identify changes in orbital occupation and electron-electron correlations as a function of strain in the metallic state that explain the observed T$_{\mathrm{MIT}}$ dependence on strain. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B19.00003: Broadband Infrared Spectroscopy of Vanadium Dioxide Films Under the Influence of Strain T.J. Huffman, Peng Xu, A.J. Hollingshad, N.E. Penthorn, D.J. Brooker, M.M. Qazilbash, Lei Wang, R.A. Lukaszew, R.D. Pike, B.-J. Kim, H.-T. Kim Vanadium dioxide (VO$_{\mathrm{2}})$ undergoes a phase transition between an insulating monoclinic phase and a conducting rutile phase. Even in this simple, stoichiometric material, a complete explanation of the phase transition has proved elusive. This transition, like phase transitions in other correlated electron systems, involves interacting electronic, lattice, and orbital degrees of freedom. This leads to physical properties that are particularly sensitive to small changes in external parameters such as strain. VO$_{\mathrm{2~}}$films grown on different substrates are subject to differing strain effects that often lead to a shift in the transition temperature. Broadband infrared (IR) and optical spectroscopy allows us to examine the electronic structure and dynamics as well as IR-active, zone-center phonons of strained films grown on sapphire and quartz. Comparing and contrasting the IR and optical properties of these films, and those of bulk crystals, will provide insight into the influence of strain on the electronic and lattice degrees of freedom. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B19.00004: In-situ studies on the Martensitic-type transition in VO$_{2}$ thin films Viswanath Balakrishnan, Shriram Ramanathan We present in-situ kinetic studies across metal-insulator transition in epitaxial and polycrystalline VO$_{2}$ thin films through electrical resistance and stress measurements along with TEM investigations. Variable temperature wafer curvature experiments enable the probing of in situ stress relaxation kinetics associated with the structural component of the metal insulator transition. Primarily, no time or drive rate dependence is observed in the stress relaxations providing insight into the athermal nature of phase transition kinetics. However, proximate to the phase transition boundary, minor fraction of isothermal component that show time dependence in both stress relaxation and electrical measurement is captured. In situ electron diffraction and micro structural observations across the metal insulator transition provide evidence for martensitic type transition in polycrystalline VO$_{2}$ thin films. The studied aspects of time independent, Martensitic type, athermal transition kinetics along with negligible fraction of isothermal kinetics have significance in understanding the dynamics of structural phase transitions that accompany electronic property changes. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B19.00005: T$_{c}$ anisotropy and phase separation in strained Vanadium Dioxide films Mengkun Liu, Martin Wagner, Elsa Abreu, Salinporn Kittiwatanakul, Alexander Mcleod, Michael Goldflam, Zhe Fei, Siyuan Dai, Michael Fogler, Jiwei Lu, Stuart Wolf, Richard Averitt, D.N. Basov We report Infrared near field study on strain induced transition temperature (T$_{c})$ anisotropy in vanadium dioxide (VO$_{2})$ films via direct visualization of a spontaneous structural and electronic phase separation. The films are epitaxially grown on [110]$_{R}$ or [100]$_{R}$ TiO$_{2}$ substrates and exhibit large uniaxial strain. By mapping the film topography with AFM and electronic percolation with Infrared scattering scanning near-field optical microscopy, a temperature dependent electron-lattice correlation can be clearly observed. Our work sheds a new light onto the nature of the Tc anomaly in metal-insulator transition and leads to the possibility of controlling the material's properties through strain induced phase separation. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B19.00006: Hydrogen doping and the metal-insulator transition in vanadium dioxide Talip Serkan Kasirga, Chunming Huang, Jae H. Park, Jim M. Coy, Zaiyao Fei, Aaron M. Jones, Xiaodong Xu, David H. Cobden Vanadium dioxide has a first-order metal-insulator transition (MIT) at 67 $^{\circ}$C. It has recently been shown [1] that hydrogen doping of VO$_{2}$ by spillover from a metal catalyst in hydrogen gas gradually reduces the gap in the insulating phase to zero, and eventually eliminates the MIT. The dependence on hydrogen concentration enables optical and electrical detection of the local hydrogen density. We exploit this to study the diffusion of hydrogen and its dependence on temperature, direction, strain, and phase in single-domain nanobeams and platelets of VO$_{2}$. For example, we find that diffusion is faster along the rutile c-axis, and can be significant even at the transition temperature. We also study the effects of hydrogen doping on the phase diagram, on the low temperature conductivity, and on the continuous-wave and ultrafast optical response.\\[4pt] [1]. Wei, J. \textit{et. al.} Nature Nano. \textbf{7}, 357 (2012) [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B19.00007: Comparative studies of electrically driven metal insulator transition in VO$_{2}$ single crystal and thin film Honglyoul Ju, Bongjin Mun, Joonseok Yoon, Sung-Kwan Mo, Kai Chen, Nobumichi Tamura, Catherine Dejoie, Martin Kunz, Zhi Liu, Yvette Lee, Kyungsun Moon, Changwoo Park Electrically driven metal-insulator transition (MIT) characteristics of VO$_{2}$ single domain crystal and thin-film were investigated by temperature and external bias voltage dependent electrical transport, optical microscopy, and synchrotron-based polychromatic x-ray micro-diffraction measurements. Our results suggest that electrically driven metallic state of VO$_{2}$ is similar to that of temperature driven metallic state. However, after the electrically driven MIT, VO$_{2}$ single crystal exhibits metallic and insulating colors on the surface of the crystals simultaneously. In addition, the origin of electrically driven MIT of crystals seems different from that of electrically driven MIT films. In this talk, we will present comparative studies of electrically driven MIT of VO$_{2}$ single crystal and thin-film, and discuss the origins of electrically driven MIT and its implications. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B19.00008: VO$_2$ and V$_2$O$_3$: different pathways for the same phase transition? E. Abreu, J. Zhang, S. Wang, K. Geng, L. Cao, S. Kittiwatanakul, J. Lu, M. Liu, J.G. Ramirez, S.A. Wolf, I.K. Schuller, R.D. Averitt Decades of investigation have led to a better understanding of the properties of vanadates but a great deal remains to be explored in these scientifically fascinating and technologically relevant systems. VO$_2$ and V$_2$O$_3$ are canonical examples of these transition metal oxides, strongly influenced by both electronic correlations and structural effects. In both materials the MIT is known to occur following a variation in temperature, the application of a dc field, optical pumping and more recently the application of transient THz pulses. The question that naturally arises is whether and how the dynamics of the MIT depend on the nature of the stimulus that induced it. We will present time-resolved optical and THz investigations, including high THz field results, of thin films of V$_2$O$_3$ and VO$_2$. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B19.00009: The Effect of Doping on the Metal-Semiconductor Transition in VO$_{2}$ Salinporn Kittiwatanakul, Stuart Wolf, Jiwei Lu Vanadium dioxide (VO$_{2})$ is a well-known correlated material that exhibits a metal-semiconductor transition at 340K, with several orders of magnitude change in the resistivity. In this study, we report the effect of Mn-doping and Al-doping, with different doping recipes; the films were deposited by Reactive Biased Target Ion Beam Deposition, and their single phase was confirmed by X-ray diffractometry. The different doping recipes had a very dramatic impact on the crystallinity of the vanadium dioxide films. It was found that using a lower frequency for the pulsed dc target bias was desirable for the improvement of the film quality. Both Al and Mn doping can enhance the transition; while the Al doped VO$_{2}$ also raises the transition temperature. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B19.00010: Benchmark study of the application of density functional theory to correlated t$_{2g}^{1}$ vanadates Danilo Puggioni, James Rondinelli SrVO$_3$ and CaVO$_3$ are strongly correlated perovskite-structured metals belonging to the class of transition-metal oxides with a 3$d^1$ electronic configuration. Both cubic SrVO$_3$ and orthorhombically distorted CaVO$_3$ are classified as Pauli paramagnets, yet their magnetic states at low temperature remain controversial. Here, we present and discuss the results of systematic density functional theory (DFT) calculations on the atomic and magnetic structures of both SrVO$_3$ and CaVO$_3$ to shed light on this issue. We use standard and ``beyond-DFT'' exchange-correlation functionals to evaluate the stable magnetic states. We conclude by discussing both the accuracy of these methods for reproducing the atomic structures of the $t_{2g}^{1}$ vanadates and their implications on artificially structured oxide superlattices. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B19.00011: Ab initio study of metal-insulator transition in VO2 Huihuo Zheng, Lucas K. Wagner The structure distortion accompanied metal-insulator transition (MIT) of vanadium dioxide (VO$_2$) at 340K has been a matter of ongoing controversy for near four decades. It is still unclear whether the nature of this transition is due to a Peierls instability, a Mott-Hubbard transition, or other physics. Most density functional theory based methods fail to describe the nature of the electronic state in this system, further complicating theoretical description of VO$_2$. We will report on progress in applying the first principles diffusion quantum Monte Carlo method to the electronic structure of VO$_2$ in the metallic and insulator phases. By examining the energetic properties, one particle reduced density matrix, as well as other static correlations in the two phases of the system, we will comment on which of the two common descriptions is a closer representation of the physical reality of VO$_2$. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B19.00012: Phonon Softenings and the Mott-spin-Peierls Transition in VO$_{2}$ Sooran Kim, Kyoo Kim, Chang-Jong Kang, B.I. Min To explore the driving mechanisms of the metal-insulator transition (MIT) and the structural transition in VO$_{2}$, we have investigated phonon dispersions of rutile VO$_{2}$ (\textit{R}-VO$_{2}$) in the DFT and the DFT+$U$ ($U$: Coulomb correlation) band calculations. We have found that the phonon softening instabilities occur in both cases, but the softened phonon mode only in the DFT+$U$ describes properly both the MIT and the structural transition from \textit{R}-VO$_{2}$ to monoclinic VO$_{2}$ (\textit{M$_{1}$}-VO$_{2}$). The present {\it ab-initio} phonon dispersion calculations clearly demonstrate that the Coulomb correlation effect plays an essential role of assisting the Peierls transition in \textit{R}-VO$_{2}$ and producing the spin-Peierls ground state in \textit{M$_{1}$}-VO$_{2}$. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B19.00013: Examining the density functional theory description of VO$_2$ above and below the metal-insulator transition Ricardo Grau-Crespo, Thomas A. Mellan, Hao Wang, Udo Schwingenschl\"ogl Vanadium oxide (VO$_2$) exhibits a metal-insulator transition at 341 K, which is accompanied by a change from a tetragonal to a mononoclinic structure. We examine the electronic and magnetic properties of VO$_2$ below and above the transition point, as calculated from density functional theory (DFT) and some extensions, including hybrid DFT / Hartree-Fock functionals and Hubbard-corrected functionals. We show that the groundstate solutions obtained with either the GGA approximation or the screened hybrid functional HSE (25\% of Hartree-Fock exchange) are at odds with experimental observations for both phases. We then discuss the effect of varying amounts of Hartree-Fock exchange and values of the Hubbard parameter U on the solutions. Although the agreement of some of the calculated properties with experiment can be tuned in this way, we conclude that no single setting can describe the properties of both VO$_2$ phases simultaneously. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B19.00014: Spatial complexity due to strong correlations in vanadium dioxide Shuo Liu, Benjamin Phillabaum, Erica Carlson, Karin Dahmen, Mumtaz Qazilbash, Dmitri Basov, Vidhyadhiraja Sudhindra Near-field scanning infrared microscopy on the Mott metal-insulator system vanadium dioxide (VO$_2$) has revealed complex nanoscale pattern formation in the form of insulating and metallic puddles near the insulator-to-metal transition [1]. We use and extend recently developed cluster techniques [2] in order to understand the fundamental physics driving this multiscale pattern formation. We map the observed metallic and insulating clusters to Ising variables by a rigorous choice of threshold amplitude, and quantify the statistics of the sizes and shapes of the geometric clusters. These in turn yield critical exponents including the cluster size distribution exponent $\tau$, and the fractal dimensions associated with the cluster formation. These quantitative measures show power-law behavior over multiple decades, revealing a delicate interplay between interactions and disorder in the material. The cluster techniques employed here can be readily applied to 2D image data in the context of other materials and measurement techniques.\newline \par \noindent [1] M. M. Qazilbash, et al., {\it Science} {\bf 318}, 1750 (2007).\newline [2] B. Phillabaum, E. W. Carlson, and K. A. Dahmen, {\it Nat. Commun.} {\bf 3}, 915 (2012). [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B19.00015: Structural and vibrational properties of VO2 from DFT and DFT+U calculations Eric J. Walter, Henry Krakauer, Tyler J. Huffman, Peng Xu, M. M. Qazilbash Vanadium dioxide (VO$_2$) undergoes a metal-insulator transition (MIT) at 340\,K from a metallic, high-temperature rutile phase to a insulating, low-temperature monoclinic phase. In thin films, the extremely fast switching times ($\simeq 100$~femtoseconds) of the MIT have led to many suggested device applications. Understanding the MIT driving mechanism and the long-debated importance of electronic correlation is important to these developments. We have computed the relaxed geometry and phonon frequencies using DFT and DFT+U for both phases of VO$_2$. The dependence of vibrational mode frequencies and oscillator strengths on the Hubbard $U$ parameter and their sensitivity to the Born effective charges in the insulating monoclinic phase will be reported. The calculated frequencies for $U=5$ eV are in good agreement with recent experimental infrared micro-spectroscopy measurements on single crystal platelets of VO$_2$ \footnote{T. J. Huffman et al., PRB, submitted.}. Our results indicate that strong electron-electron correlation must be included to describe the vibrational properties. [Preview Abstract] |
Session B20: Focus Session: Mesoscopics - Transport
Sponsoring Units: DMPChair: Yvan Bruynseraede, Catholic University-Leuven, Belgium
Room: 322
Monday, March 18, 2013 11:15AM - 11:51AM |
B20.00001: From Quanta to the Continuum: Opportunities for Mesoscale Science Invited Speaker: John Sarrao Mesoscale science embraces the regime where atomic granularity and quantization of energy yield to continuous matter and energy, collective behavior reaches its full potential, defects, fluctuations and statistical variation emerge, interacting degrees of freedom create new phenomena, and homogeneous behavior gives way to heterogeneous structure and dynamics. Mesoscale architectures form a hierarchy extending from atoms and molecules through polymers, supramolecular assemblies, periodic lattices, multilayers, nanocrystal arrays and multiphase materials. Mesoscale science builds on the foundation of nanoscale knowledge and tools that the community has developed over the last decade and continues to develop. Mesoscale phenomena offer a new scientific opportunity: designing architectures and interactions among nanoscale units to create new macroscopic behavior and functionality. Examples of mesoscale successes, challenges and opportunities will be described. A more complete discussion of mesoscale science can be found in the BESAC report, \textit{From Quanta to the Continuum: Opportunities for Mesoscale Science}, http://science.energy.gov/bes/news-and-resources/reports/basic-research-needs/ Innovative community input on opportunities for mesoscale science can be found on the \textit{Mesoscopic Materials and Chemistry} website, http://www.meso2012.com/ [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B20.00002: Magnetoresistance Jumps in Mesoscopic Hybrid Devices Ali C. Basaran, Carlos Monton, Juan Pereiro, Ivan K. Schuller We have studied the electrical transport of superconducting stripes (Nb and V) with periodically altered local magnetization. The local magnetization is controlled by the ferromagnetic states of Ni rings placed on top of the stripes. We observe a series of large resistance jumps as a function of external magnetic field (Ha). The jumps occur at temperature and current density dependent Ha values which indicate that they are probably not related to vortex pinning. Resistance jumps along with observed multiple steps in the current-voltage characteristics could be attributed to weak links induced by magnetic stray field or proximity effects originated by Ni rings in the superconducting area. The exact origin of these jumps is still under investigation. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B20.00003: Electron transport in confined oxide nanowires Guanglei Cheng, Michelle Tomczyk, Shicheng Lu, Mengchen Huang, Josh Veazey, Patrick Irvin, Chang-Beom Eom, Jeremy Levy The invention of conductive AFM lithography at the LaAlO$_3$/SrTiO$_3$ interface enables the creation of clean inter-connected oxide nanowires and artificially engineered tunnel barriers. Here we create an oxide nanowire that is confined by two tunnel barriers using this technique. Two terminal and four terminal transport studies reveal transitions among Cooper pair tunneling, Coulomb blockade and Fabry-Perot inteference that can be tuned by side gate voltages and external magnetic field. Our results indicate the presence of long-range coherence in LaAlO$_3$/SrTiO$_3$ nanowires. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B20.00004: Electric transport in Individual GaAs nanowires Zhuting Sun, Andrei Kogan, Tim Burgess, Chenupati Jagadish We report electrical transport measurements on individual GaAs nanowires approximately 50 nm in diameter contacted via lithographically patterned Al/Ti metal films. The nonlinear current-voltage characteristics show a strongly hysteretic behavior sensitive to the device temperature and the biasing history. In hysteresis-free regimes, we compare the data to a model based on two metal-semiconductor barriers in series with the wire, and find a good overall agreement. We also discuss the effects of surface treatments on the metal-wire interface resistance. The work is supported by NSF grant DMR-1206784 and DMR-0804199 and University of Cincinnati. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B20.00005: Coherent electron transport in InAs nanowires Marion J. L. Sourribes, Ivan Isakov, Marina Panfilova, Daniele Ercolani, Francesco Giazotto, Lucia Sorba, Paul A. Warburton Indium arsenide nanowires are of special interest since they exhibit high mobility, strong spin-orbit coupling and form ohmic contacts with metals which make them good candidates for the observation of Majorana fermions in semiconductor/superconductor hybrid systems. InAs nanowires have already been used as Josephson elements in superconducting devices. Here we report our low-temperature experiments on InAs nanowires grown by two methods: (i) gold-catalyzed chemical beam epitaxy on InAs (111) substrates; (ii) catalyst-free molecular beam epitaxy on Si (111) substrates. Contacts to the nanowires are defined by e-beam lithography. Before metallization of the contacts, the nanowire surface is deoxidized by an in situ sputter-cleaning process leading to a specific contact resistance of $9.8\times10^{-9}\,\Omega$.cm$^{2}$. These highly transparent contacts allowed the observation of proximity-induced superconductivity in InAs nanowires connected with Nb contacts. The critical current was tuned by changing the gate voltage. Both magnetic-field-dependent and gate-voltage-dependent measurements of universal conductance fluctuations were performed to extract information on the electron phase coherence. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B20.00006: End and Side Contacts to NiSi nanowires Abdel F. Isakovic, A. Belkadi NiSi nanowires were nanofabricated with end and side contacts. These contacts are designed to minimize spreading resistance and are tested to check whether they can aid in decreasing the energy cost of current injection and current ejection in nanotransport. It is demonstrated that the end contacts have lower power in 1/f noise spectrum. Transport data (current-voltage, differential resistance) also show quantitative differences from ``standard'' bottom or top contacts to SiNi nanowires, indicating that the presence of edge- and end-states at the termination points of the nanowires gives rise to different transport conditions. Time-dependent correlation coefficient from noise spectra is determined and it is different for different types of contacts. Structural study of nanowires contacted in this manner is also presented. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B20.00007: Strain manipulated direct-indirect band gap transition in GaAs nanowires Xihong Peng, Andrew Copple, Nathaniel Ralston One dimensional nanostructures of group III-V semiconductors have drawn broad research interests in recent years due to their potential applications in nano-electronics. In particular, GaAs has been considered as a promising channel material for the high speed NMOS beyond Si based technology. In this project, electronic structures of GaAs nanowires at both wurtzite and zinc blende phases were studied using first-principles Density Functional Theory (DFT) calculations. It was found that the band gap of GaAs nanowires experience a direct-to-indirect transition when the diameter of the nanowires is smaller than a specific value [1]. For those thin GaAs nanowires with an indirect band gap, it was found that the gap can be tuned to be direct if a moderate external strain is applied. We found many types of strains, such as tensile and compressive uniaxial strain, radial strain, strain along a specific orientation in the cross-section of the nanowires, can trigger the indirect-to-direct gap transition. The critical strains for the gap-transition are determined by the energy crossover of two states in conduction bands. \\[4pt] [1] A. Copple, N. Ralston, X.-H. Peng, Appl. Phys. Lett.100, 193108 (2012). [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B20.00008: Local transport measurement at mesoscopic lengthscales on epitaxial graphene using scanning tunneling potentiometry Weigang Wang, Ko Munakata, Michael Rozler, Malcolm R. Beasley We report direct measurement of the local transport potential at mesoscopic lengthscales in epitaxial graphene by scanning tunneling potentiometry. The measurements were made possible by using slender, sharp tips manufactured by focused ion beam that avoid the previous problem of tip jumping. The sample was measured at 17K, well below the onset of weak localization; hence locally the transport was mesoscopic. Besides local Landauer residual resistivity dipoles associated with topographical features of our sample, we observed peaks and dips in the local transport potential for which there is as yet no explanation. Work supported by AFOSR MURI Contract {\#}~FA9550-09-1-0583-P00006 [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B20.00009: ABSTRACT HAS BEEN MOVED TO V1.00314 |
Monday, March 18, 2013 1:27PM - 1:39PM |
B20.00010: Probing spin-charge relation by magnetoconductance in one-dimensional polymer nanofibers Yung Woo Park Polymer nanofibers are one dimensional (1-D) organic hydrocarbon systems containing conducting polymers where the non-linear local excitations such as solitons, polarons and bipolarons formed by the electron-phonon interaction were predicted. Magnetoconductance (MC) can simultaneously probe both the spin and charge of these mobile species and identify the effects of electron-electron interactions on these nonlinear excitations. Here we report our observations of a qualitatively different MC in polyacetylene (PA) and in polyaniline (PANI) and polythiophene (PT) nanofibers. In PA the MC is essentially zero, but it is present in PANI and PT. The universal scaling behavior and the zero (finite) MC in PA (PANI and PT) nanofibers provide evidence of Coulomb interactions between spinless charged solitons (interacting polarons which carry both spin and charge). [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B20.00011: On the Lifetimes of Nonaxisymmetirc Metallic Nanowires Lan Gong, Jerome Buerki, Charles Stafford, Daniel Stein We present a theoretical approach for understanding the stability of simple metallic nanowires, in particular monovalent metals such as the alkalis and noble metals. Their cross sections are of order one nanometer so that small perturbations from external (usually thermal) noise can cause large geometrical deformations. The nanowire lifetime is defined as the time required for making a transition into a state with different cross-sectional geometry. This can be a simple overall change in radius, or a quadrupolar deformation, or both. We develop a stochastic field theoretical model to describe this noise-induced transition process, in which the initial and final states correspond to locally stable states on a potential surface derived numerically from a nearly free electron model. The numerical ``string method" is implemented to determine the optimal transition path governing the lifetime. Using these results, we tabulate the lifetimes of sodium and gold nanowires of several different initial geometries. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B20.00012: Making quantum devices with electrical properties that are robust to thermal cycling using AlGaAs/GaAs HIGFET structures Adam Micolich, Andrew See, Oleh Klochan, Adam Burke, Alex Hamilton, Ian Pilgrim, Billy Scannell, Rick Montgomery, Richard Taylor, Martin Aagesen, Poul Lindelof, Ian Farrer, David Ritchie The transport properties of quantum devices on modulation-doped AlGaAs/GaAs heterostructures change after thermal cycling above $\sim$130 K due to charge redistribution in the modulation doping layer. This is particularly evident in a quantum dot's magnetoconductance fluctuations (MCF) which provide a sensitive fingerprint of electron trajectories through the dot. We show that the MCF become reproducible with high-fidelity after thermal cycling to 300 K in quantum dots made using AlGaAs/GaAs heterostructures without modulation doping. This is achieved by populating the dot electrostatically using a Heterostructure Insulated Gate Field Effect Transistor (HIGFET) architecture. Our result demonstrates ionized impurity scattering has a measurable effect on transport in quantum dots, even in the ballistic transport regime. It highlights the potential for HIGFET-based architectures to provide devices with significantly reduced small-angle scattering at equivalent transport mobility, and more thermally robust electrical properties. More broadly, we suggest a quantum dot's MCF may be a useful tool for studying the temporal/thermal stability of disorder in other semiconductor materials. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B20.00013: Scattering phase of quantum dots: Emergence of universal behavior Philippe Jacquod, Rodolfo Jalabert, Rafael Molina, Dietmar Weinmann We investigate scattering through chaotic ballistic quantum dots in the Coulomb-blockade regime. Focusing on the scattering phase, we show that long universal sequences emerge in the short wavelength limit of many electrons on the dot, where phase lapses of $\pi$ systematically occur in between two consecutive resonances. We further argue that such universal sequences become shorter and shorter as the wavelength becomes larger/the number of electrons on the dot is reduced. Our results are corroborated by numerics and are in qualitative and quantitative agreement with experimental results. We finally present numerical data on models of interacting electrons to show that strong correlations do not alter our conclusions. [Preview Abstract] |
Session B21: Focus Session: Hexagonal Ferrites and Manganites
Sponsoring Units: DMPChair: Nicole Benedek, University of Texas at Austin
Room: 323
Monday, March 18, 2013 11:15AM - 11:27AM |
B21.00001: Multiferroic vortices Sang-Wook Cheong, Seung Chul Chae, Xueyun Wang, Fei-Ting Huang, Yoichi Horibe Hexagonal REMnO$_3$ (RE=Ho, Er, Tm, Yb, Lu) is an improper ferroelectric where the size mismatch between RE layers and Mn-O layers induces a simultaneous ferroelectric-trimerization structural phase transition [1]. The six types of ferroelectric-trimerization domains merge one point, and form a vortex or antivortices, depending on vorticity. A zoo of vortices and antivortices form into a topologically-nontrivial network, and intriguing collective magnetism develops at the domain walls of the vortex-antivortex network. In addition, we found that bound states of vortices and antivortices can also develop. We will discuss the formation of the vortex-antivortex network and the vortex-antivortex bound states in terms of the interaction between two neighboring domain walls and that between a vortex and an antivortex.\\[4pt] [1] T. Choi et al., Nature Mater. \textbf{9}, 253 (2010).\\[0pt] [2] S. C. Chae et al., PRL \textbf{108}, 167603 (2012). [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B21.00002: Unfolding Vortices to Topological stripes in a multiferroic Xueyun Wang, Myung-Geun Han, Yoichi Horibe, Toshihiro Aoki, Yimei Zhu, Sang-Wook Cheong Hexagonal REMnO$_{\mathrm{3}}$ (RE$=$Ho, Er, Tm, Yb, Lu) is an improper ferroelectric where the size mismatch between RE layers and Mn-O layers induces a simultaneous ferroelectric-trimerization structural phase transition [1]. Two distinct domain configurations have been observed in REMnO$_{\mathrm{3}}$ (RE$=$rare earths): vortex domains vs. stripe domains [2]. However, the rapport between those topologically distinct domain patterns has never been studied. We have investigated the transformation process between vortex domains and stripe domains with the variation of temperature and the application of various strains on thin-plate-like crystals. [1] T. Choi \textit{et al.}, Nature Mater. 9, 253 (2010). [2] S. C. Chae \textit{et al}., PRL 108, 167603 (2012). [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B21.00003: Ferroelectric domain formation and reversal by cation substitution in magnetoelectric gallium ferrite epitaxial thin films R. H. Shin, S. H. Oh, W. Jo, C. Lefevre, F. Roulland, A. Thomasson, C. Meny, N. Viart Linear magnetoelectric Ga$_{\mathrm{2-x}}$Fe$_{\mathrm{x}}$O$_{\mathrm{3}}$ (GFO) is ferrimagnet at room temperature (RT) (T$_{\mathrm{C}}=$370 K at x$=$1.4) from Fe spin of d orbitals in octahedral Fe1(opposite direction), Fe2, and Ga2 sites along c-axis. According space group as Pc2$_{\mathrm{1}}$n, ferroelectric ordering should be here along b-axis but have not observed, experimentally. Several scenarios of ferroelectricity in GFO have been suggested such as displacement of Fe ions, structural change, and so on. In the scenarios, it is very difficult to obtain their polarization because of tiny quantity and high domain wall (DW) formation energy. In this talk, we suggest the cation substituted GFO can be promising RT multiferroic showing ferroelectric ordering. We tried two kinds of direction: 1. Obtain polarization reversal under high magnetic and electric field to overcome high DW formation energy. 2. Apply chemical strain to make DW formation energy low. Though, when we applied chemical strain by substituting divalent cations, leakage current that overshadow polarization reversal was strongly reduced, ferromagnetic ordering was lost at RT owing to magnetic dilution by nonmagnetic cation like Mg$^{\mathrm{2+}}$. Therefore, we discuss how to obtain ferroelectric polarization in the GFO thin films conserving RT ferrimagnet. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B21.00004: Strong coupling of ferroelectricity and magnetism in the hexagonal ferrites Invited Speaker: Hena Das During the last decade one of the most extensively studied class of multiferroics has been the hexagonal rare-earth manganites RMnO$_3$ where R=Dy-Lu, Y, Sc. These compounds exhibit antiferromagnetic (AFM) order with a N\'eel temperature $\rm T_N\approx 100 K$. In addition, they are improper ferroelectrics ($\rm T_C > 1200K$) driven a by zone-tripling structural distortion associated with a buckling of the R-planes and a rotation of the oxygen trigonal bipyramids. The improper nature of the transition is responsible for the fascinating, topologically protected trimer-domains. Even though magnetism and ferroelectricity in these materials are not intrinsically coupled, there is a non-trivial interaction between the structural and magnetic domain walls. In contrast to the manganites, the ground state structure of the rare-earth ferrites RFeO$_3$ is the orthorhombic perovskite. Recently, however, thin films of RFeO$_3$ have been epitaxially stabilized in the hexagonal rare-earth manganite structure. This development has triggered several new studies of these hexagonal ferrite systems. Similar to manganites, ferrites exhibit ferroelectricity above room temperature and crystallize in P6$_3$cm polar structure but conflicting results have been reported as to the origin of ferroelectricity in these materials. Unlike the manganites, recent neutron diffraction measurements suggest a considerably high AFM ordering temperature, $T_N=$440 K. Additionally there is an indication of a second temperature, $ T_{\rm wFM}\sim100K$, at which weak ferromagnetism has been observed. In this work my collaborators (Alex Wysocki and Craig J. Fennie) and I address the nature of ferroelectricity and magnetic order in the RFeO$_3$ systems from first-principles. We elucidate the origin of ferroelectricity in the rare-earth ferrites and provide many useful insights into their magnetic behavior, which we will show is fundamentally different than that observed in the manganites. Combining first-principles calculations with a detailed modeling of the magnetic structure we will also show how this difference leads to an interplay between ferroelectricity and magnetism in the ferrites. This strong coupling, absent in the hexagonal manganites, manifests itself in a nontrivial way that may be useful for voltage controlled magnetic functionalities. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B21.00005: Is hexagonal InMnO$_3$ ferroelectric? Fei-Ting Huang, Yoichi Horibe, Xueyun Wang, Sang-Wook Cheong, Shigeo Mori Hexagonal manganite (h-REMnO$_3$; RE=rare earths) shows a unique improper ferroelectricity, accompanying a structural trimerization. RE can be replaced by In, which is much smaller than any RE ions. Recently, Oak {\it{et al.}} [1] suggested InMnO$_3$ is ferroelectric from the results of first-principles calculations, while Kumagai {\it{et al.}} [2] proposed a non-ferroelectric ground state. In this talk, we will report the results of our investigation on the structural domains and local structural distortions of InMnO$_3$ using dark-field transmission electron microscopy. We demonstrate that InMnO$_3$ shows a distinct $\sqrt{3}\times\sqrt{3}$-type superstructure from the high-temperature paraelectric phase (P6$_3$/mmc), and the domain structure can be delicately controlled by varying the synthesis and annealing conditions. The correlation between physical properties and local structural distortions in the InMnO$_3$ will be discussed in detail. [1] M.-A. Oak, J.-H. Lee, H. M. Jang, J. S. Goh, H. J. Choi and J. F. Scott, PRL \textbf{106}, 047601 (2011). [2] Y. Kumagai, A. A. Belik, M. Lilienblum, N. Leo, M. Fiebig, and N. A. Spaldin, PRB \textbf{85}, 174422 (2012). [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B21.00006: Far- and mid-infrared emission and reflection of magnetoelectric RMnO$_3$ and RCrO$_3$ (R$=$Rare Earth) Nestor E. Massa, Leire del Campo, Domingos De Sousa Meneses, Patrick Echegut, Maria Jesus Martinez-Lope, Jose Antonio Alonso Far- and mid-infrared emission and reflection spectra of ferrielectric hexagonal TmMnO$_{3}$ show that small polarons, a paramagnetic collective electronic mode, and lower than T$_{\mathrm{N}}$ soft hybrid modes are in concomitant relation. CO$_{\mathrm{2}}$ laser heating in dry air triggers oxidation and Mn$^{3+}$- Mn$^{4+}$ double exchange hopping conductivity. A collective excitation in the paramagnetic phase is assigned to e$_{\mathrm{g}}$ electrons in THz low energy d-orbital fluctuations. It locks-in at the E-type antiferromagnetic onset (T$_{\mathrm{N}}$ $\sim$ 80K) into soft bands that harden simultaneously down to 4 K with temperature dependence given by the magnetic long range order coupling of the collective electric dipole. They have T$_{\mathrm{N}}$ as critical temperature and critical exponents suggesting a second order phase transition. They also match zone center spin wave modes measured in isomorphous LuMnO$_{3}$ (Lewtas et al, Phys. Rev. B~\textbf{82}, 184420 (2010)). Both excitations, magnons y electric dipoles, are generated by electrons e$_{\mathrm{g}}$ in deformed d-orbitals. Sharing this behavior with orthorhombic NdMnO$_{3}$ there is no evidence of new phonons in a structural deformation down to 4K Preliminary results in ErCrO$_{3}$ (T$_{\mathrm{N}}$ $\sim$ 130 K) show the emerging soft bands in an order-disorder scenario. Overall, we conclude that magnetoelastic deformations in an orbital fluctuating environment are close related to magnetoelectric couplings. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B21.00007: Synthesis, Structural Characterization and Magnetic Properties of YbFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$O$_3$ (0.0 $\le$ x $\le$ 1.0) Perovskites C. Hernandez, E. Chavira, I. Rosales, A. Tejada, L. Huerta, E.E. Marinero We report on the synthesis, structural characterization and magnetic properties of YbFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$O$_3$ (0.0$ \le $ x $\le$ 1.0) perovskites. Compounds with x$=$\textbf{ 0, 0.2, 0.4, 0.6, 0.8 and 1.0} were synthesized by solid state reaction. We find that the perovskites with x$=$0, 0.2 exhibit an orthorombic crystalline structure, whereas those with x$=$0.6, and 1.0 are hexagonal. A mixture of both hexagonal and orthorhombic phases are observed for x$=$ 0.4 and 0.8. The magnetic properties of these materials have been studied as a function of temperature (2K -- 300K). In the low temperature regime (2K -- 30K), we observe previously unreported magnetic transitions whose transition temperature depends on the amount of Mn incorporated in the perovskite. We will describe the possible origin of these magnetic transitions in terms of the structural properties of the materials studied. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B21.00008: Crystal Structure, Electric Polarization and Heat Capacity Measurements on Small R-Ion Multiferroic Hexagonal RMnO$_{3}$ Tian Yu, Peng Gao, Tao Wu, Trevor Tyson, Roger Lalancette Crystal structure, electric polarization and heat capacity measurements on the hexagonal multiferroic RMnO$_{3}$ reveal that small R ion (Lu and lower cation size) systems are ferroelectric and possess the same space-group as YMnO$_{3}$. Combined local and long range structural measurements were conducted by XAFS, PDF and single crystal and powder XRD methods. The influence of the Mn-O and R-O distribution on the electric polarization is discussed. Point charge estimates of the electrical polarization are given for comparison with the YMnO$_{3}$ system. This work is supported by DOE Grant DE-FG02-07ER46402. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B21.00009: Temperature Dependent Properties of Perovskite Small R-Ion RMnO$_{3}$ Systems Haiyan Chen, Tian Yu, Peng Gao, Trevor Tyson, Keun Hyuk Ahn Perovskite small ion E-type systems with radii smaller than that of Lu have been synthesized. The structure, heat capacity and magnetic measurements have been used to compare them with standard E-type systems such as LuMnO$_{3}$ and HoMnO$_{3}$. Analysis of the structure is combined with theoretical estimates to relate the electronically driven polarization with the atomic structure. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B21.00010: Critical Structural Parameters Influencing Magnetic Transition Temperatures in Multiferroic Hexagonal RMnO$_{3}$ Trevor Tyson, Tian Yu, Tao Wu, Catherine Dubourdieu, Sang-wook Cheong Multiferroic hexagonal RMnO$_{3}$ systems with a broad range of transition temperatures and including some with spin rotation transitions have been studied. Detailed temperature depended structural measurements have been conducted to extract the static and dynamic changes. The structural measurements are combined with qualitative theoretical arguments to determine the critical parameters which influence the magnitude of the magnetic ordering temperature. Suggestion are made on ways to optimize it to enable higher temperature multiferroic behavior. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B21.00011: Insights on Electric Polarization in $E$-type $R$MnO$_{3}$ Tao Wu, Trevor A. Tyson, Haiyan Chen, Zhiqiang Chen, Ryan Tappero, Keun H. Ahn, Sungbaek Kim, Sang-Wook Cheong Orthorhombic perovskite $E$-type $R$MnO$_{3}$ multiferroic systems were prepared by high pressure synthesis and solid state reaction. High pressure synchrotron x-ray diffraction and x-ray absorption spectroscopy measurements were performed to explore the structural changes. The influence of the pressure on the electrical polarization is discussed. Theoretical analysis is used to predict pressure dependence of the polarization from the structural data derived from the refinements. This work is supported by DOE Grant DE-FG02-07ER46402. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B21.00012: Ferroelectric and ferromagnetic properties of Ga$_{\mathrm{x}}$ CoFe$_{\mathrm{2-x}}$O$_{4}$ /BaTiO$_{3}$ Yan Ni, Cajetan Nlebedim, David Jiles Single phase magnetoelectric materials are limited in application. Consequently, practical application of magnetoelectric materials requires the development of composite materials in which piezoelectric and magnetostrictive phases are coupled via interfacial strain. In addition to strong coupling, it is desirable that both the magnetostrictive and piezoelectric phases possess high sensitivity, d$\lambda $/dH and dP/d$\sigma $ respectively. Of all the substituted cobalt ferrite studies, CoGaxFe2-xO4 has been shown to have the highest strain sensitivity. In the present study, CoGaxFe2-xO4 (x$=$0.1, 0.2, 0.3) has been combined with BaTiO3 to fabricate a y(CoGaxFe2-xO4)-(1-y)BaTiO3 (y $=$ 0.4, 0.5 and 0.6) magnetoelectric composite samples. Crystal structure, microstructure and compositions of the samples were verified by XRD, SEM and EDX. The effect of the BaTiO3 phase on the magnetostrictive properties of CoGaxFe2-xO4 and the effect of the CoGaxFe2-xO4 phase on the piezoelectric properties of BaTiO3 will be presented with respect to the magnetoelectric properties of the composites. [Preview Abstract] |
Session B22: Nano Particles, Wires, and Cavities
Sponsoring Units: DCMPChair: Sergio Ulloa, Ohio University
Room: 324
Monday, March 18, 2013 11:15AM - 11:27AM |
B22.00001: Optical properties and circular dichroism of chiral metal nanoparticles Zhiyuan Fan, Alexander Govorov In nature, biological systems are built up by homochiral building blocks, such as a sugar and protein. Circular dichroism (CD) is an effective tool of resolving molecular conformations. It utilizes circularly polarized light to detect differential absorption of chiral materials. In medicine, it will help us to develop new drugs and therapies, if we understand the connection between the physical or chemical properties of drug molecules and their conformations. With the rapid development of nanotechnologies, chiral nanomaterials attract lots of attention nowadays. CD signals of chiral molecules can be enhanced or shifted to the visible band in the presence of plasmonic nanocrystals. Here we present a plasmonic CD mechanism from a single chiral metal nanocrystal[1]. The mechanism is essentially different from the dipolar plasmon-plasmon interaction in a chiral NP assembly[2], which mimics the CD mechanism of chiral molecules. Chiral metal nanocrystals are expected to have promising applications in biosensing. Recently a few experimental papers reported successful realizations of chiral nanocrystals in a macroscopic ensemble in solution. Particularly the paper[3] described silver nanoparticles grown on chiral template molecules and demonstrating characteristic CD signals at a plasmonic wavelength. The plasmonic CD signals in Ref.[3] can come from a dipolar plasmon-molecule interaction or from a chiral shape of nanocrystals. [1] Z.Fan, et al. Nano Lett.,12, 3283 (2012). [2] A. Kuzyk, et al., Nature 483, 311 (2012). [3] B.Maoz, et al. J. Am. Chem. Soc.134, 17807 (2012). [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B22.00002: Transport measurements across single nanoparticles Qian Yu, Limin Cui, Christian Ulysse, Alireza Mottaghizadeh, Alexandre Zimmers, Herv\'e Aubin During this last decade, numerous progresses have been obtained in the chemical synthesis of nanoparticle. Various materials (oxides, chalcogenides) known for their peculiar electronic or magnetic properties -- superconductivity, Mott localization, topological protection -- can now be obtained as nanoparticles through chemical synthesis. These new nano-materials are offering a unique opportunity to study the effect of quantum confinement on unconventional electronic orders. To improve the preparation of samples with single nanoparticles trapped within a nanogap, we developed a new method where nanoparticles are projected in-vacuum on chip circuits covered by nanogap spaced electrodes. Continuous current measurements during the projection allow identifying the trapping of a single nanoparticle within the nanogap. We apply the method for trapping single gold nanoparticles, which led to the observation of Coulomb blockade. We also applied the method to magnetite (Fe3O4) nanoparticles, which allows to study the electric field induced insulator to metal transition in only a few nanoparticles. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B22.00003: Characterization of TbAs nanoparticles embedded in GaAs using pump-probe measurements of carrier relaxation dynamics Laura R. Vanderhoef, Abul K. Azad, Dibakar R. Chowdhury, Cory Bomberger, Joshua M. O. Zide, Matthew F. Doty Rare-earth-monopnictide nanoparticles epitaxially deposited within III-V semiconductors have been shown to improve the performance of devices for applications ranging from thermoelectrics to THz pulse generation. However, the electronic structure of small (approximately 1.5 nm diameter) TbAs nanoparticles remains poorly understood. We use ultrafast pump-probe spectroscopy to investigate the electronic structure of the TbAs nanoparticles. The samples studied were grown by co-deposition of Tb, Ga, and As on a GaAs substrate, resulting in TbAs nanoparticles embedded within a GaAs host. We study the dynamics of carrier relaxation into the TbAs states, which essentially act as traps, using both optical-pump terahertz-probe and optical-pump optical-probe techniques. By analyzing how the carrier relaxation rates depend on both pump fluence and sample temperature we conclude that the TbAs states are saturable, which suggests the existence of a bandgap for TbAs nanoparticles. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B22.00004: Optical and electronic properties of self-assembled nanoparticle-ligand metasurfaces Jake Fontana, John Livenere, Joshua Caldwell, Christopher Spillmann, Jawad Naciri, Ronald Rendell, Banahalli Ratna The optical and electronic properties of inorganic nanoparticles organized into two-dimensional lattices sensitively depend on the properties of the organic ligand shell coating the nanoparticles. We study the optical and electronic properties of these two-dimensional metasurfaces consisting of gold nanoparticles functionalized with ligands and self-assembled into macroscopic monolayers on non-templated substrates. Using these metasurfaces we demonstrate an average surface-enhanced Raman scattering (SERS) enhancement factor on the order of 10$^{\mathrm{8}}$ for benzenethiol ligands and study the mechanisms that influence the enhancement. These metasurfaces may provide a platform for the development of low-power, low-cost next-generation chem/bio-sensors and new insights into the organic-inorganic interface at the nanoscale. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B22.00005: Two photon excitation fluorescence from Ag nanotriangles and nanohexagons Chi-yu Jao, Brenden Magill, Hans Robinson We report on measurements of two photon excitation fluorescence (TPEF) from arrays of silver nanotriangles and nanohexagons fabricated by nanosphere lithography. The silver nanoparticles exhibit localized surface plasmon resonances (LSPRs) that depend on the size, shape and aspect ratio of the particles. When the particles are excited by femtosecond pulsed laser light resonant with the LSPRs, they emit TPEF with significantly higher intensity than when excited off resonance. Moreover, if the light intensity is turned up sufficiently to cause some of the particles to melt into spherical particles, we observed an increase in the TPEF from the spheres by as much as an order of magnitude, even though their LSPRs are no longer resonant with the laser. Finally, we note that the silver particles also generate light at the second harmonic of the laser frequency, although the efficiency of this process depends strongly on the dielectric environment of the silver particles, which is not the case for the TPEF. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B22.00006: Investigation of the electronic transport in polarization-induced nanowires using conductive atomic force microscopy (AFM) Camelia Selcu, Santino C. Carnevale, Thomas F. Kent, Fatih Akyol, Patrick J. Phillips, Michael J. Mills, Siddharth Rajan, Jonathan P. Pelz, Roberto C. Myers In the search to improve short wavelength light emitting diodes (LED's), where the dislocations limit their performance and hole doping (Mg) is a fundamental challenge, the III-Nitride polarization-induced nanowire LED provides a promising system to address these problems. The new type of pn diode, polarization-induced nanowire LED (PINLED), was developed by linearly grading AlGaN composition of the nanowires (from GaN to AlN and back to GaN) from 0{\%} to 100{\%} and back to 0{\%} Al (Carnevale et al, \textit{Nano Lett.}, \textbf{12}, 915 (2012)). In III-Nitrides (Ga,Al/N), the effects of polarization are commonly observed at the surfaces and interfaces. Thus, in the case of the polarization-induced nanowire LEDs, taking advantage of the bound polarization charge, due to the grading of the AlGaN, the pn diodes are formed. The polarity of the nanowires determines the carrier type in each graded region, and therefore the diode orientation (n/p vs p/n). We used conductive AFM to investigate polarity of the PINLED's as well as hole conductivity in PINLED's made of AlGaN with and without acceptor doping. The results reveal that most of the wires are n-top/p-bottom (N-face), but some are p-top/n-bottom (Ga-face). Also, we found that the current density is 3 orders of magnitude larger in the case of the doped nanowires than the nanowires with no impurity doping. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B22.00007: Critical Role of Modal Spatial Overlap in Nanoscale Nonlinear Optics Jimin Zhao, Rui Wang, Ben-Li Wang, R.J. Liu, X.H. Lu, Zhi-Yuan Li We unambiguously demonstrate the critical role of modal spatial overlap in \textit{nonlinear} optics for nanoscale structures. Our experimental and theoretical investigations show that, within a sub-wavelength metallic hole, spatial overlap between the linear and nonlinear modes strongly correlates to the conversion efficiency. Our results provide an accurate explanation for the long-emphasized but elusive shape effect. Moreover, our investigation stimulates new angles for and deeper insights into general nonlinear optics at nanoscale. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B22.00008: Nanocluster effects on magneto-resistance and optical second-harmonic generation in Au-Co composite films Kaida Yang, Tatiana Murzina, Ale Lukaszew Magnetic nanomaterials typically exhibit significant differences in their magnetic and magnetic-optical properties compared to bulk. A viable nanoscale platform to investigate the magnetic and magneto-optical properties of magnetic nanomaterials is in composite thin films to have magnetic clusters embedded on a different matrix material which size can be tailored. The Au-Co binary system is a typical phase-separation system in bulk phase diagram . The nanocomposite geometry allows tailoring the actual composition and microstructure of the composite by exploiting different temperature during deposition. In our previous studies, Au/Co/Au trilayers as well as Au-Co nanocomposite thin films exhibit strong enhancement of the magneto-optical activities due to surface plasmon polariton excitation in the noble metal. In this study, we investigate other non-linear optical properties such as second harmonic generation (SHG) in Au-Co nanocomposite thin films and understand its correlation with the magneto-transport properties of the composite. Optical SHG is a sensitive probe of surface and buried interfaces due to inversion symmetry breaking at the interfaces of centrosymmetric materials which allows probing of the structural and morphological properties near interfaces. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B22.00009: Far-infrared transmission through periodic arrays of cross-shaped holes Luyi Yan, Chang Long, David Tanner, N. Bradman, N. McFarland, J.B. Marbruger The far-infrared transmission of light incident on a free-standing metal film perforated with periodic cross-shaped holes is investigated. These metal-mesh filters show enhanced ``extraordinary'' infrared transmission at particular wavelengths. A number of filter samples having different periodicities and geometries have been measured over frequencies from 20-650 cm$^{-1}$/0.6-19.5 THz. The results will be compared with calculations from surface plasmon polariton (SPP) theory. It is shown that for certain periodicity and geometry, the SPP mode and the localized surface plasmon (LSP) mode may have their resonance peaks nearly superimposed on each other. The bandwidth of this transmission peak is related to the ratio of the width and length of the cross-shaped holes. The correlation between transmission properties and the incident angle of the far-infrared light has also been measured for both polarization conditions. As the incident angle is increased, the transmission peak shows a blue shift when illuminated by s-polarized light, while for p-polarized light it splits into two parts which shift in opposite directions. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B22.00010: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 1:15PM - 1:27PM |
B22.00011: Time-resolved nonlinear dynamics of quantum dots coupled to a photonic crystal cavity in the Purcell regime Jieun Lee, Timothy Saucer, Andrew Martin, Joanna Millunchick, Vanessa Sih Recently, there has been great interest in studying the optical nonlinearities of light confined in a solid-state nano-cavity interacting with a quantum emitter for on-chip applications. The nonlinearity in the strong coupling regime has enabled ultrafast all-optical switching at low incident power using exciton-photon coupled systems. In this report, we show that nonlinear optical properties can also be observed in the Purcell regime using a cavity with a moderate quality factor (Q), which arises from the saturation of a single quantum dot and describes the time-resolved dynamics of two transitions (exciton and biexciton) exhibiting different nonlinearities. In order to conduct these investigations, we used the luminescence intensity autocorrelation method and measured the variation of nonlinear emission dynamics while varying the incident power over nearly three orders of magnitude and found excellent agreement with a numerical simulation. We expect the method and the theoretical model will be applicable for understanding other nonlinear effects such as lasing and cavity-QED. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B22.00012: Entangled photons from the polariton vacuum in a switchable optical cavity Adrian Auer, Guido Burkard We study theoretically the entanglement of two-photon states in the ground state of the intersubband (ISB) cavity system, called polariton vacuum. The system is formed by a sequence of quantum wells (QWs) located inside a microcavity and the interaction of cavity photons with ISB excitations inside the QWs leads to the formation of polariton states. In the ultrastrong coupling regime, the polariton vacuum already contains a finite number of photons, of which pairs with opposite in-plane wave vectors are correlated. In an explicit solution for the polariton vacuum, we only consider certain two-photon states by post-selection and analyze them for mode entanglement, i.e.~in the momentum degree of freedom. We find an analytical expression for the entanglement using the concurrence [1], which depends on the absolute values of the in-plane wave vectors of the photons. In the limit of large cavities and for photon energies around the ISB resonance in the mid infrared regime, the photons are almost maximally entangled, which is fundamentally important for their possible use in quantum information processing. Furthermore, there exists a continuous set of mode pairs, for which the photons are maximally entangled.\\[4pt] [1] A. Auer and G. Burkard, Phys. Rev. B \textbf{85}, 235140 (2012). [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B22.00013: Coherent flow and Bose-Einstein Condensation of Long Lifetime Polaritons Gangqiang Liu, Bryan Nelson, Mark Steger, Ryan Balili, David Snoke, Ken West, Loren Pfeiffer Exciton-polaritons with lifetimes of the order of 100ps are created in semiconductor microcavity of extremely high quality factor (? 106). Due to this long lifetime and very few defects in the sample, the polaritons can travel ballistically over macroscopic distances up to millimeter. The properties of the system changes dramatically with the particle density. At moderate density, the polaritons behave like a superfluid, maintaining phase coherence after propagating over hundreds of microns. This indicates the existence of long range spatial coherence in the system. As the density increases above a threshold value, the polaritons condense into the lowest-energy state of the effective trap produced by the repulsive interaction between the polaritons and excitons within the excitation region and the cavity gradient across the sample. The coherence time of this polariton BEC is measured to be at least 280ps. By creating a exciton barrier at the center of a stress trap, we are able to obtain a ring shape polariton BEC which provides the opportunity for studying the constant flow of a superfluid in the polariton system. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B22.00014: ABSTRACT WITHDRAWN |
Session B23: Focus Session: Dopants and Defects in Semiconductors II
Sponsoring Units: FIAPChair: Gerd J. Duscher, University of Tennessee at Knoxville
Room: 325
Monday, March 18, 2013 11:15AM - 11:27AM |
B23.00001: A modification of Eu incorporation sites by the dissociation of hydrogen defect complexes in Mg co-doped Eu doped gallium nitride Brandon Mitchell, Jonathan Poplawsky, Volkmar Dierolf Europium doped gallium nitride (Eu:GaN) is a promising candidate as a material for red LEDs that can monolithically be integrated with existing nitride based lighting technology. Photoluminescence (PL) and cathodoluminescence (CL) studies have revealed, however, that the majority incorporation environment (site) for the Eu is not efficiently excited by electron hole pairs. To improve this efficiency, Mg was co-doped into Eu:GaN during metal organic chemical vapor deposition and multiple new incorporation environments were discovered. These new sites show a high efficiency at room temperature and have been attributed to the coupling of a Mg-H complex to the majority Eu site. However, we also observe that sustained electron beam irradiation produced a semi-permanent change in the CL spectra of the sample. It was demonstrated that this change occurs in two distinct steps which exhibit a pronounced temperature dependence. Our observations point toward a dynamic system in which the Mg-H bond is broken and the hydrogen moves within the epi-layer. Details of this behavior will be discussed. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B23.00002: The origin of the high hole density in In$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N:Mg William Willoughby, Mary Ellen Zvanut InGaN is the nitride of choice for applications requiring high hole density and emission tunability. The increased hole density with In incorporation may be explained by several different mechanisms; however, our electron paramagnetic resonance (EPR) studies reveal a surprising feature: the number of Mg-related acceptors decreases with increasing hole density. In$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N films, with x between 0.02 and 0.11 and thickness between 0.25 and 0.44 $\mu$m, were grown p-type by doping with Mg to a concentration of 2-3 $\times$ 10$^{19}$ cm$^{-3}$. Hall measurements reveal the expected hole density increase from 5-30x10$^{17}$ cm$^{-3}$ with increasing In mole fraction. However, unlike GaN:Mg where the EPR Mg signal tracks the hole density, the EPR intensity of the Mg-related signal in InGaN is found to decrease as the hole density increases. Together, compensating defects and a lowering of the acceptor level may explain the decrease in EPR intensity and the increase in hole density observed as the In mole fraction is increased. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B23.00003: Stability and electronic structure of Mg dopants in InGaN alloys Ji-Sang Park, K.J. Chang Nitride semiconductors have attracted much attention due to their applications for light emitting and laser diodes. High conductivity p-type nitride layers are demanding for various optoelectronic devices, however, hole concentrations are generally low because of the deep acceptor level of Mg and the compensation of hole carriers by donor defects. In this work, we investigate the stability and electronic properties of Mg dopants in InGaN alloys through first-principles density functional calculations. We generate the alloy structure with the In content of 10\% by using the special quasi-random structure approach. Considering various Mg sites surrounded with different numbers of the In atoms in the second nearest neighborhood, we find that Mg dopants prefer to be located near the Ga atoms rather than the In atoms due to the local bonding effect. Incorporation of the In atoms not only reduces the band gap but also decreases the ionization energy of Mg in Ga-rich regions. However, the ionization energy tends to increase as the number of the In atoms in the second nearest neighborhood increases, although this configuration is energetically unfavorable. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B23.00004: How localized acceptors limit $p$-type conductivity in GaN Invited Speaker: John L. Lyons Despite the impressive development of GaN as an optoelectronic material, $p$-type conductivity is still limited. Only a single acceptor impurity, magnesium, is known to lead to $p$-type GaN. But Mg is far from a well-behaved acceptor. Hydrogen is known to passivate Mg, necessitating a post-growth anneal for acceptor activation. In addition, the ionization energy is quite large ($\sim$ 200 meV in GaN), meaning only a few percent of Mg acceptors are ionized at room temperature. Thus, hole conductivity is limited, and high concentrations of Mg are required to achieve moderately $p$-type GaN. Other acceptor impurities have not proven to be effective $p$-type dopants, for reasons that are still unresolved. Using advanced first-principles calculations based on a hybrid functional, we investigate the electrical and optical properties of the isolated Mg acceptor and its complexes with hydrogen in GaN, InN, and AlN.\footnote{J. L. Lyons, A. Janotti, and C. G. Van de Walle, Phys. Rev. Lett. \textbf{108}, 156403 (2012).} We employ a technique that overcomes the band-gap-problem of traditional density functional theory, and allows for quantitative predictions of acceptor ionization energies and optical transition energies. Our results allow us to explain the deep or shallow nature of the Mg acceptor and its relation to the optical signals observed in Mg-doped GaN. We also revisit the properties of other group-II acceptors in GaN. We find that all cation-site acceptors show behavior similar to Mg$_{\mathrm{Ga}}$, and lead to highly localized holes. The Zn$_{\mathrm{Ga}}$ and Be$_{\mathrm{Ga}}$ acceptors have ionization energies that are even larger than that of Mg, making them ineffective dopants. All acceptors cause large lattice distortions in their neutral charge state, in turn leading to deep, broad luminescence signals that can serve as a means of experimentally verifying the deep nature of these acceptors. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B23.00005: Role of self-trapping in luminescence and $p$-type conductivity of wide-band-gap oxides Joel Varley, Anderson Janotti, Cesare Franchini, Chris Van de Walle Using hybrid functional calculations, we investigate the behavior of holes in the valence band of a range of wide-band-gap oxides including ZnO, MgO, In$_2$O$_3$, Ga$_2$O$_3$, Al$_2$O$_3$, SnO$_2$, SiO$_2$, and TiO$_2$. We find that, due to the orbital composition of the valence band, holes tend to form localized small polarons with characteristic lattice distortions, even in the absence of defects or impurities. These self-trapped holes (STHs) are energetically more favorable than delocalized, free holes in the valence band in all materials but ZnO and SiO$_2$. Based on calculated optical absorption and emission energies we show that STHs provide an explanation for the luminescence peaks that have been observed in many of these oxides. Additionally, we demonstrate that polaron formation prohibits $p$-type conductivity in this class of materials. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B23.00006: Carbon Defect Complex as a Source of Yellow Luminescence in GaN Denis Demchenko, Mikhail Reshchikov Using hybrid functional theory compared with experimental measurements, we demonstrate that yellow luminescence often observed in both carbon-doped and pristine GaN is the result of electronic transitions via C$_N$-O$_N$ complex. In contrast to the common isolated defects, C$_N$-O$_N$ complex is energetically favorable, and its calculated optical properties as well as the thermodynamic transition level show excellent agreement with the measured luminescence data. Calculated transitions via the localized defect states of this complex are (experimental values are given in brackets): thermodynamic transition level of 0.75 eV (0.85 eV), absorption energy 3.30 eV (3.32 eV), emission energy 2.25 eV (2.20 eV), and zero phonon transition 2.70 eV (2.60 eV). This complex has not been proposed as a source of the yellow band in GaN, while all other defects previously suggested to be sources of this band exhibit high formation energies and would produce red or infrared photoluminescence. Thus, combining hybrid density functional theory and experimental measurements we propose a solution to a long-standing problem of the GaN yellow luminescence. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B23.00007: Identification of the defect responsible for current collapse in GaN/AlGaN HEMTs Yevgeniy Puzyrev, Xiao Shen, Sokrates Pantelides Recent experiments show that GaN/AlGaN high-electron-mobility transistors (HEMTs) suffer significant current collapse during stress conditions characterized by the presence of charge trap level $\sim$ 0.50 eV below conduction band. This phenomenon has been attributed to thermally activated defect diffusion without specifying responsible defects. Here we report first-principles density-functional calculations of the hydrogenated substitutional oxygen complexes and show that the electric-field-enhanced formation of this defect complex provides an explanation for observed phenomenon. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B23.00008: Fe charge state kinetics in semi-insulating Fe-doped GaN Ustun Sunay, Jamiyanaa Dashdorj, Mary Ellen, Kevin Udwary, Jacob Leach GaN is a wide bandgap semiconductor with applications in LEDs and high-power devices. One of the problems plaguing this material is a high concentration of residual donors. This issue can be resolved by doping GaN with deep acceptors such as Fe, which compensates donors and creates semi-insulating material. Recently, a photo-induced electron paramagnetic resonance (EPR) spectroscopy study of Fe-doped GaN showed significantly long relaxation times [1]. The study proposed a charge transfer mechanism between Fe$^{3+}$ and Fe$^{4+}$ as an explanation for the phenomenon. However, absorption data from the same samples showed the existence of both Fe$^{2+}$ and Fe$^{3+}$ which suggests that the proposed model involving Fe$^{4+}$ is incorrect and a theory involving an intermediate center is more likely. 3.5 K 10 GHz EPR was performed on HVPE grown free-standing Fe/Si co-doped GaN. Data show an unexpected situation where both donor and Fe$^{3+}$ acceptor signals exist simultaneously. Together with the photo-EPR results, these data reinforce the necessity of invoking a multi-step mechanism for compensation. A model for compensation based on charge transfer between Fe3$+$ and a donor will be described based on EPR and additional material characterization measurements. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B23.00009: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 1:27PM - 1:39PM |
B23.00010: Nonradiative carrier capture rates at defects from first-principles calculations Qimin Yan, Audrius Alkauskas, Chris G. Van de Walle We develop a computational methodology to determine nonradiative carrier capture rates at defects in wide-band-gap semiconductors. In our theoretical framework, we consider carrier capture via multiphonon emission as the dominant nonradiative mechanism for deep defects in wide-band-gap materials at low and moderate carrier densities. Our methodology is based on the static approximation for the electron-phonon coupling. We employ a state-of-the-art hybrid density functional approach to describe the electronic structure. For charged defect systems, the screening effect by excess carriers is taken into account. As test cases, we investigate deep centers including C$_{\rm N}$ and $V_{\rm Ga}$ in GaN and Li$_{\rm Zn}$ in ZnO. Calculated carrier capture rates are in good agreement with available experimental data. This work was supported by DOE, NSF, Swiss NSF, and by the UCSB SSLEC. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B23.00011: Optical properties of Ga$_{1-x}$Mn$_x$As from large scale ab initio calculations J. Jackson, R. Cardenas, G. Bester The properties of Mn impurities in GaAs are revisited employing a new methodology based on atomic effective potentials (AEPs [1]) which yields LDA accuracy at considerably reduced computational expense. We consider the case of very low Mn concentrations that cannot be considered using conventional ab initio methods and discuss the metal/insulator transition in terms of the Mn-d band localization and its interpretation as a shallow acceptor. We discuss practical methods to improve upon the LDA bandgap in GaAs together with the excessive delocalization of the Mn states. Using a configuration-interaction technique we calculate the optical spectra of Ga$_{1-x}$Mn$_x$As including the fine-structure (FSS) splitting which is of importance to the development of quantum computing devices based upon magnetic impurities in semiconductors [2].\\[4pt] [1] J. R. C\'{a}rdenas and G. Bester, Phys. Rev. B {\bf 86}, 115332 (2012)\\[0pt] [2] D. E. Reiter, T. Kuhn, V.M. Axt, Phys. Rev. B {\bf 83}, 155322 (2011) [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B23.00012: Impact of gamma-irradiation on the properties of n-type AlGaN/GaN heterostructures Elena Flitsiyan, Leonid Chernyak Gamma-photon irradiation of AlGaN/GaN HEMTs with the modest dose of 700 Gy$^{\ast}$ resulted in significant deterioration of their DC characteristics. To understand the nature of the observed effect, we carried out a series of variable temperature EBIC measurements in the vicinity of HEMT's gate in-situ in Scanning Electron Microscope. The measurements were performed on 3 different devices, which were exposed to various gamma-irradiation doses. Temperature dependent EBIC measurements allowed obtaining activation energies for levels in the material's forbidden gap, which are responsible for carrier recombination. While the diffusion length decreases significantly with increasing irradiation dose, the activation energy, associated with carrier recombination, gets larger. This fact indicates generation of new deep levels caused by gamma-photon irradiation. These levels act as traps for electrons in AlGaN/GaN HEMT channel, thus reducing the drain current and leading to degradation of other device characteristics. The investigated effects of gamma irradiation are likely related to, epitaxial layer quality and composition. Therefore, the study of materials with variations in these properties is necessary to fully understand the irradiation-induced mechanisms. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B23.00013: Electronic band structure, doping, and defects in the semiconducting Half Heusler compound CoTiSb Jason Kawasaki, Linda Johansson, Martin Hjort, Rainer Timm, Brian Schultz, Thiagarajan Balasubramanian, Anders Mikkelsen, Chris Palmstrom We report transport and electronic band structure measurements on epitaxial films of the Half Heusler compound CoTiSb. CoTiSb belongs to the family of Half Heuslers with 18 valence electrons per formula unit that are predicted to be semiconducting despite being composed of all metallic components. Here the CoTiSb films were grown by molecular beam epitaxy on a lattice matched InAlAs buffer. The films are epitaxial and single crystalline, as measured by reflection high-energy electron diffraction and X-ray diffraction. Scanning tunnelling spectroscopy and temperature-dependent transport measurements reveal that the films are semiconducting, with unintentionally doped carrier concentrations comparable to that of highly doped conventional compound semiconductors. These carrier concentrations can be modulated by doping with Sn. The band structure of the films was measured by angle resolved photoemission spectroscopy at the MAX-Lab Synchrotron facility. The bulk bands are in general agreement with density functional theory calculations, with a valence band maximum at $\Gamma $ and surface states within the bulk band gap. The effects of defects are explored in order to explain the ARPES results. [Preview Abstract] |
Session B24: Focus Session: Materials in Extremes: High-Strain-Rate Phenomena
Sponsoring Units: GSCCM DCOMP DMPChair: Igor Schweigert, Naval Research Laboratory
Room: 326
Monday, March 18, 2013 11:15AM - 11:51AM |
B24.00001: Iron and Aluminum at Ultrahigh Strain Rates Invited Speaker: Jonathan Crowhurst In recent years, techniques based on table-top laser systems have shown promise for investigating dynamic material behavior at high rates of both compressive and tensile strain. Common to these techniques is a laser pulse (the ``pump'') that is used in some manner to rapidly deliver energy to the sample; while the energy itself is often comparatively very small, the intensity can be made high by tightly focusing the pump light. In this way pressures or stresses can be obtained that are sufficiently large to have relevance to a wide range of basic and applied fields. Inherent to these techniques too, is relatively low cost and high throughput. Also, by using additional laser pulses (the ``probe'') to measure the response of the sample, very high time resolution can be achieved. The latter in particular is desirable when studying, for example shock waves, in which the time for the material to pass from undisturbed to fully compressed (the ``rise time'') can be extremely short (order 10 ps or less) even at fairly small peaks stresses. Since much of the most interesting physics comes into play during this process it is important to be able to adequately resolve the shock rise. Furthermore, the associated time scale is comparable to that typically considered in state-of-the-art molecular dynamics simulations which are emerging as the theoretical tool of choice for investigating shock waves in condensed matter. It should be pointed out however, that a general drawback to these techniques is that, depending on the aim of the experiment, a small pump energy imposes limits on the nature of the sample; if for example the aim is to study steady shock waves, the compressed region has to be thin, and its internal structure cannot vary on a scale that is not much smaller than the compressed dimensions. We consider and illustrate these concepts in the context of various metals, primarily aluminum and iron, and show how current methods are capable of making meaningful and useful measurements of material behavior at ultrahigh strain rates up to or exceeding 10$^{\mathrm{10}}$ s$^{\mathrm{-1}}$, corresponding to more than 40 GPa in aluminum. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344 with Laboratory directed Research and Development funding (12ERD042), as well as being based on work supported as part of the EFree, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DESC0001057. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B24.00002: Orientation-dependent structure of elastic and plastic shock waves in Nickel single crystals Brian Demaske, Vasily Zhakhovsky, Nail Inogamov, Ivan Oleynik The response of Ni single crystals to shock loading has been investigated using molecular dynamics (MD) simulations. It was found that within the elastic-plastic split-shock-wave regime, the amplitude of the elastic precursor in the [111] direction depends strongly on the pressure of the plastic wave; whereas in the [110] direction the pressure of the elastic precursor is pinned. Coupling of the elastic and plastic waves in the [111] direction and lack thereof in the [110] direction is attributed to different activation mechanisms for homogeneous dislocation nucleation (HDN), the major relaxation process observed in our MD simulations. In the [111] direction, thermodynamic fluctuations activate HDN randomly within a metastable elastic zone separating the elastic and plastic fronts, while in the [110] direction HDN is induced by the high levels of shear stresses produced at the plastic front. We will discuss how thermally-activated HDN gives rise to a new pulsating regime of single two-zone elastic-plastic shock waves, where the elastic zone width undergoes significant oscillations in time. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B24.00003: Atomistic simulations of high strain rate loading of nanocrystals E.M. Bringa, D. Tramontina, C.J. Ruestes, Y. Tang, M.A. Meyers, N. Gunkelmann, H.M. Urbassek Materials loaded at high strain rates can reach extreme temperature and pressure conditions. Most experiments on loading of simple materials use poly crystals, while most atomistic simulations of shock wave loading deal with single crystals, due to the higher computational cost of running polycrystal samples. Of course, atomistic simulations of polycrystals with micron-sized grains are beyond the capabilities of current supercomputers. On the other hand, nanocrystals (nc) with grain sizes below 50 nm can be obtained experimentally and modeled reasonably well at high strain rates, opening the possibility of nearly direct comparison between atomistic molecular dynamics (MD) simulations and experiments using high power lasers. We will discuss MD simulations and links to experiments for nc Cu and Ni, as model f.c.c. solids, and nc Ta and Fe, as model b.c.c. solids. In all cases, the microstructure resulting from loading depends strongly on grain size, strain rate and peak applied pressure. We will also discuss effects related to target porosity in nc's. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B24.00004: Rarefaction shock waves in shock-compressed diamond \textless 110\textgreater\ crystal Romain Perriot, You Lin, Vasily Zhakhovsky, Carter White, Ivan Oleynik Piston-driven shock compression of diamond \textless 110\textgreater\ crystal was simulated by molecular dynamics using the REBO potential. At piston velocities between 2 and 5 km/s and corresponding pressures 117 GPA \textless\ P \textless\ 278 GPa, diamond sample undergoes a polymorphic phase transition, characterized by the coexistence of two elastically compressed phases, low-pressure phase A and high-pressure phase B. This phase transition results in the splitting of the shock wave into two elastic shock waves, composed of pure phase A and a mixture of phases A and B. Upon removal of the piston, a release wave is observed at the rear of the sample, turning into a rarefaction shock wave where the material undergoes the reverse phase transition from coexisting phases to the original low-pressure phase. For strong plastic waves induced by larger piston velocities the release wave propagates as a rarefaction wave without any phase transition corresponding to the adiabatic expansion along the plastic branch of the Hugoniot. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 1:03PM |
B24.00005: Efficient semiclassical quantum nuclear effects for shock compression studies Invited Speaker: Evan Reed A fast methodology is described for atomistic simulations of shock-compressed materials that incorporates quantum nuclear effects in a self-consistent fashion. We introduce a modification of the multiscale shock technique (MSST) that couples to a quantum thermal bath described by a colored noise Langevin thermostat. The new approach, which we call QB-MSST, is of comparable computational cost to MSST and self-consistently incorporates quantum heat capacities and Bose-Einstein harmonic vibrational distributions. As a first test, we study shock-compressed methane using the ReaxFF potential. The Hugoniot curves predicted from the new approach are found comparable with existing experimental data. We find that the self-consistent nature of the method results in the onset of chemistry at 40\% lower pressure on the shock Hugoniot than observed with classical molecular dynamics. The temperature shift associated with quantum heat capacity is determined to be the primary factor in this shift.\\[4pt] In collaboration with Tingting Qi, Department of Materials Science and Engineering, Stanford University. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B24.00006: Study of Plastic flow at high pressures and strain rates via the Rayleigh-Taylor instability Hye-Sook Park, J. Belof, K. Blobaum, R. Cavallo, B. Maddox, C. Plechaty, S. Prisbrey, B. Remington, R. Rudd, C. Wehrenberg, M. Wilson We present the results from study of tantalum material strength at high pressures and high strain rates using the Omega laser system. The Ta sample is maintained in the solid state via a quasi-isentropic ramped drive using a reservoir-gap-sample configuration at high pressures (\textgreater\ 1 Mbar) and high strain rates (10$^{6}$ - 10$^{8}$ sec$^{-1})$. The strength is inferred by measurement of Rayleigh-Taylor induced growth in pre-imposed sinusoidal ripples on a Ta sample [1]. Our study of the samples with single crystal, 0.25, 15 and 90 micron average grain sizes shows that there is no obvious Hall-Petch effect under such extreme conditions. We also show that RT growth is linear as long as the RT growth is below 0.15 of the original sample thickness. We show a comparison of experimental results with the recently developed Livermore Multiscale model that integrates the atomistic scale physics to macro hydro flow simulations. The NIF experimental design will also be presented \\[4pt] [1] H. S. Park et al., PRL. 104, 135504 (2010). [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B24.00007: Dynamic diffraction measurements of Ta lattice response under Mbar shock loading conditions Bruce Remington We will report on experiments done on the Omega laser to determine the strength of shock-loaded single-crystal [100] tantalum using in-situ broadband x-ray Laue diffraction. The inferred strength reaches 350 kbar at a shock pressure of 1.8 Mbar and is in excellent agreement with a multiscale strength model, which employs a hierarchy of simulation methods over a range of length and time scales. Laser driven shock experiments using in situ Bragg diffraction were also performed at the Omega-EP laser on single crystal tantalum to study the dynamic yield strength and lattice dynamics. Both techniques will be described, comparisons to the strength models made, and interpretations of the results given. ~Recent results from recovery experiments in shocked single crystal Ta will also be given, showing features such as the residual dislocation density and slip-twinning threshold. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B24.00008: Mesoscale Modeling of Shock Wave Propagation and Dynamic Failure in Metallic Systems Avinash Dongare The response of materials under conditions of thermomechanical extremes is very complex and involves damage creation and propagation, phase transformation, heat generation and transfer, etc. A principal challenge in predictive modeling of failure behavior is presented by the gap between the atomistic description of micromechanisms of the relevant processes and the macoscale response in continuum simulations/experiments. This difficulty can be approached through the development of a robust mesoscopic computational model that retains the relevant physics and is capable of representing the material behavior at time- and length-scales intermediate between the atomistic or continuum levels. \quad Mesoscale models typically reduce a group of atoms by a mesoparticle system with much smaller number of collective degrees of freedom, and hence are often difficult to apply for problems such as heat transfer, phase transformation, and dissipation of mechanical energy during wave propagation. To achieve this goal, a novel mesoscopic model is being developed based on the idea of coarse-graining with the energetics defined for the particles based on interatomic potentials used in molecular dynamics (MD) simulations. The coarse-grained molecular dynamics simulations (CGMD) allows larger size systems and improved time-steps for simulations and thus able to extend the capabilities of MD simulations to model materials behavior at mesoscales. The successful application of the CGMD method is demonstrated by prediction of the phase-transformation, heat generation and wave-propagation behavior under the conditions of shock loading, as would be predicted using MD simulations. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B24.00009: Microstructure in the Extreme Environment: Understanding and Predicting Dynamic Damage Processes Darcie Dennis-Koller, Ellen Cerreta, Curt Bronkhorst, Pablo Escobedo-Diaz, Ricardo Lebensohn The future of materials science: strategic application for functionally controlled materials properties is emphasized by the need to control material performance in extreme environments. This study examines the separate effects of kinetics (in the form of dynamic loading rate and shock wave shape) from that of length-scale effects (in the form of microstructural defect distributions). Recently available mesoscale modeling techniques are being used to capture a physical link between kinetic and length-scale influences on dynamic loading. This work contributes innovative new tools in the form of shock-wave shaping techniques in dynamic experimentation, materials characterization, lending insight into 3D damage field analysis at micron resolution, and the physics necessary to provide predictive capabilities for dynamic damage evolution. Experimental are obtained to provide the basis for the development of process-aware material performance models. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B24.00010: Kinetics of a Fast Moving Partial Dislocation Nitin Daphalapurkar, K.T. Ramesh Plastic deformation in materials under extreme stresses requires a kinetic description of moving dislocations. The velocities with which the partial dislocations can propagate under an applied stress has implications for plasticity at high strain rates, specifically, the rate of plastic deformation and the rate-sensitivity. In this work, we focus our attention on motion of a twinning partial dislocation in a face-centered cubic (FCC) material, Ni. We use molecular dynamics simulations to simulate the velocity of a propagating twinning partial dislocation and investigate the effect of applied shear stress. Results suggest a limiting value for the speeds of a propagating partial dislocation. The material speeds based on the nonlinear part (under high stresses) of the stress-strain curve are shown to have an influence on the velocity with which a partial dislocation can propagate. Predicted velocities from simulations will be related to observations from high rate impact experiments. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B24.00011: Characterization of several martensitic phase transitions under extreme conditions Manling Sui, Shujuan Wang, Wei Zhang, Pengfei Yan In shock-compressed $\alpha $-iron, transmission electron microscopy (TEM) investigations revealed a refined microstructure with tale-telling features that are indicative of $\alpha \to \varepsilon \to \alpha $ sequential martensitic transformations, even though no $\varepsilon $ phase was retained. The unique microstructural fingerprints enable a quantitative assessment of the volume fraction transformed during explosive loading. In a Ti--6Al--4V alloy, an unusual martensitic transformation from $\alpha $-Ti to $\beta $-Ti occurred by a high-density current pulse, instead of the conventional martensitic transformation from $\beta $-Ti to $\alpha $-Ti. A large amount of the high-temperature phase remained. By pulsed laser irradiation, a solid-state phase transition from the $\alpha $ to the $\gamma $ phase of aluminum oxide was observed for the first time. High resolution TEM reveals that the transformation is achieved via the glide of quarter partial dislocations on every other basal plane of $\alpha $-Al$_{2}$O$_{3}$. This martensitic transformation is associated with a positive volume change and substantial shear strain. [Preview Abstract] |
Session B25: Superconducting Qubits: Loss Mechanisms (TLS) and Novel Materials
Sponsoring Units: GQIChair: David Pappas, National Institute of Standards and Technology
Room: 327
Monday, March 18, 2013 11:15AM - 11:27AM |
B25.00001: Delocalised oxygen as the origin of two-level defects in Josephson junctions Jared Cole, Timothy DuBois, Manolo Per, Salvy Russo One of the key problems facing superconducting qubits and other Josephson junction devices is the decohering effects of bi-stable material defects. Although a variety of phenomenological models exist, the true microscopic origin of these defects remains elusive. We show that these defects can arise from delocalisation of the atomic position of the oxygen in the oxide forming the Josephson junction barrier. Using a microscopic model, we compute experimentally observable parameters for phase qubits. Such defects are charge neutral but have non-zero response to both applied electric field and strain. This explains the observed long coherence time of two-level defects in the presence of charge noise, while still coupling to the junction electric field and substrate phonons. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B25.00002: Noise from Two-Level Systems in Superconducting Resonators C. Neill, R. Barends, Y. Chen, B. Chiaro, E. Jeffrey, J. Kelly, M. Mariantoni, A. Megrant, J. Mutus, S. Ohya, D. Sank, A. Vainsencher, J. Wenner, T. White, A. N. Cleland, J. M. Martinis Two-level systems (TLSs) present in amorphous dielectrics and surface interfaces are a significant source of decoherence in superconducting qubits. Linear microwave resonators offer a valuable instrument for characterizing the strongly power-dependent response of these TLSs. Using quarter-wavelength coplanar waveguide resonators, we monitored the microwave response of the resonator at a single near-resonant frequency versus time at varying microwave drive powers. We observe a time dependent variation of the resonator's internal dissipation and resonance frequency. The amplitude of these variations saturates with power in a manner similar to loss from TLSs. These results provide a means for quantifying the number and distribution of TLSs. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B25.00003: Universal dielectric loss in amorphous solids in Josephson qubits from simultaneous bias and microwave fields Alexander Burin, Kevin Osborn, Khalil Moe We calculate the microwave dielectric loss of an ensemble of two-level systems in amorphous solids within superconducting qubits during the application of a time-varying electric bias field. We find that this loss becomes universal in a wide range of temperatures and frequencies of the AC drive field, corresponding to the bare linear dielectric permittivity in the low-temperature limit. This non-equilibrium theory allows the separate extraction of the TLS density and their dipole size in experiments and can be used to reduce the destructive effect of decoherence. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B25.00004: Polaronic model of Two Level Systems in amorphous solids Kartiek Agarwal, Ivar Martin, Eugene Demler, Mikhail Lukin Motivated by recent experiments studying effects of elastic strain on two level systems (TLSs) in Josephson Junctions, we consider interaction of the electronic TLS with phonons. We demonstrate that including strong polaronic effects is crucial for analyzing these systems. Our model not only gives a quantitative understanding of the TLS relaxation and dephasing as probed in Josephson junction qubits, but also provides a microscopic justification for phenomenological models used to describe experiments with bulk amorphous solids. Our model explains such surprising observations of recent experiments as the existence of minima in the energy of some TLSs as a function of strain and maximum of the relaxation time in such minima. We argue that better understanding of the microscopic nature of TLSs can be used to improve properties of quantum devices, from dramatic enhancement of TLS relaxation time by putting them inside phononic crystals to creating new types of strongly interacting optomechanical systems. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B25.00005: Superconducting Titanium Nitride Coplanar Resonators: Relationships between performance and deposition parameters B. Chiaro, S. Ohya, A. Megrant, C. Neill, R. Barends, B. Campbell, Y. Chen, J. Kelly, M. Mariantoni, J. Mutus, P. O'Malley, P. Roushan, D. Sank, A. Vainsencher, J. Wenner, T. White, C.J. Palmstrom, B.A. Mazin, A.N. Cleland, J.M. Martinis Superconducting coplanar waveguide (CPW) resonators are widely used structures in the fields of photon detection and quantum information processing. Recently, there has been a growing interest in titanium nitride (TiN) thin films due to their widely tunable critical temperature, large surface inductance, and ability to produce high intrinsic quality factor (Q$_{i}$) resonators. We have deposited nearly stoichiometric TiN films on Si substrates by reactive magnetron sputtering. By increasing the deposition pressure and adjusting the N2 flow rate to maintain stoichiometry, the film stress was changed from $\sim 100$ MPa to $>3000$ MPa and the Q$_{i}$ of CPW resonators made from these films increased from $\sim 10^{4}$ to $\sim 10^{6}$ for single photon excitations measured at $\sim 100$ mK. In this talk, we discuss relationships between deposition parameters, film properties, and microwave electrodynamic responses in these resonators. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B25.00006: Characterization of quantum-regime dielectric loss of aluminum oxide using superconducting LC resonators Chunqing Deng, Martin Otto, Adrian Lupascu We report low-temperature measurements of dielectric loss of thin layers of aluminum oxide. The experiments are performed by measuring the microwave transmission of coplanar waveguides coupled to LC resonators where the capacitor contains the dielectric to be characterized. We develop a method, based on systematic approximations of transfer functions, to analyze the measured transmission curves. The fit of the resonance curves yields not only the loss tangent of the dielectric, but also the relation between the voltage on the capacitor and the excitation voltage. The latter is a nonlinear relation which has to be properly taken into account when analyzing the power dependence of dielectric loss. We find that the loss tangent of the aluminum oxide increases with decreasing capacitor voltage and temperature and reaches a constant value around $2\times 10^{-3}$ at sub-single photon levels. Our results are qualitatively in agreement with the two-level system defect model. Despite large loss, compact resonators based on these dielectrics have potential applications in microwave amplifiers. These results are relevant to understanding decoherence in superconducting quantum devices. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B25.00007: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 12:39PM - 12:51PM |
B25.00008: Observation of Cavity QED in thick dielectric films Bahman Sarabi, A.N. Ramanayaka, S. Gladchenko, M.J.A. Stoutimore, M.S. Khalil, K.D. Osborn Cavity QED in amorphous dielectrics is investigated by measuring five linear superconducting resonators with thick dielectric films and capacitor volumes ranging from 80$\mu $m$^{3}$ to 5000$\mu $m$^{3}$. In the smallest volume dielectrics we observe additional resonances which may be explained by CQED, despite the dielectric volume which is many orders of magnitude larger than Josephson junction barrier volumes. In addition to the volume dependence of the CQED resonances, we will report on the stability of the resonances in time and the phase noise. This research allows new fundamental studies on TLS phenomena in meso-volume amorphous dielectrics. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B25.00009: TLS-like temperature and power dependence for loss in superconducting coplanar resonators S. Gladchenko, M.J.A. Stoutimore, M. Khalil, K. D. Osborn Loss in 2D superconducting coplanar resonators and qubits is often limited by two-level systems thought to be on the metal and substrate surfaces. While these TLSs are thought to be similar to those found in amorphous dielectrics, their nature is generally different. In most experiments, loss in coplanar resonators shows power and temperature dependence which disagrees with TLS theory. Here we will show new data from high-quality Al on sapphire coplanar resonators which is in qualitative agreement with TLS theory, and discuss the quantitative differences to TLS theory. The data on surface TLS behavior will be compared to resonator measurements of ALD-grown thin films. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B25.00010: Non-equilibrium two-level system dynamics probed with a biased bridge resonator Moe S. Khalil, Sergiy Gladchenko, M.J.A. Stoutimore, F.C. Wellstood, K.D. Osborn We have designed a biased bridge resonator (BBR), which allows us to probe amorphous dielectric films by simultaneously applying a quasi-static electric bias field in addition to a microwave electric field. The BBR is made with a bridge arrangement of capacitors using superconducting aluminum electrodes and operated at millikelvin temperatures. Measurements of a universal amorphous dielectric film at high microwave amplitudes and a sufficiently fast bias field ramp reveals a non-equilibrium dielectric loss equal to its intrinsic steady state value. This phenomenon is explained by a theory which uses the dynamics of charged two-level systems undergoing Landau-Zener transitions to remain in their ground state. We will compare the experimental data to Monte Carlo simulations of the theory which allow for the separate extraction of the dipole moment and the spectral density of two-level systems. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B25.00011: Observation of the dynamics of two-state parametric fluctuators in superconducting flux qubits Adrian Lupascu, Mustafa Bal, Mohammad Ansari Spectroscopic measurements of a few persistent current qubit samples yield data in which the spectroscopic lines are doublets. The doublet splitting decreases with increasing qubit transition frequency. In three devices with a relatively low Josephson to charging energy ratio $E_J/E_c$, the maximum splitting ranges between 30 and 270 MHz. The splitting value is found to have variations over time scales of the order of days. The doublet structure was not observed in two other samples with larger $E_J/E_c$. Assuming a model in which the qubit experiences a parametric fluctuation that changes its frequency, we perform an experiment to probe the time scale of this fluctuation. We repeat a sequence in which the qubit is reset by energy relaxation, then driven with weak Rabi $\pi$ pulses on one of the spectroscopy lines, and finally measured. The time correlation of the series of measurement results displays an exponential decay, consistent with a telegraph noise component in the qubit frequency. The correlation does not depend on time if the qubit is either not excited or driven with a strong Rabi pulse. The transition rate was found to vary between 8 kHz and 38 kHz for temperatures between 43 and 165 mK. We discuss quasiparticle poisoning and other possible source of this effect. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B25.00012: Low-frequency two level systems and $1/f$ noise in Al/AlO$_{x}$/Al Josephson junctions for superconducting qubits: achieving a noiseless junction Christopher Nugroho, Vladimir Orlyanchik, Dale Van Harlingen The characterization of low-frequency two level systems (TLS) provides a connection between the generic $1/f$ noise in Josephson junctions to the TLSs observed in qubit energy spectroscopy. We present measurements of the tunneling-resistance noise in nanoscale Al/AlO$_{x}$/Al shadow evaporated junctions with areas $<(100~\textrm{nm})^{2}$. As the junction area or the temperature is decreased we observed a crossover from ensemble-averaged $1/f$ noise to a random telegraph noise from isolated TLSs. From the area threshold for the onset of non-gaussianity, we estimate a density of TLSs in the amorphous AlO$_{x}$ barrier consistent with the magnitude of $1/f$ noise in larger junctions and the density of high frequency TLSs from qubit spectroscopy. Furthermore we may deduce the potential landscape of the TLSs by characterizing the switching times and signal variance as a function of voltage bias and temperature. In some junctions no fluctuators are active, giving rise to immeasurably small noise signal. We discuss the implication of our findings to qubit coherence times. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B25.00013: Josephson Phase Qubits Incorporating Novel Coherent Materials U. Patel, Y. Gao, D. Hover, G. Ribeill, S. Sendelbach, R. McDermott The Josephson phase qubit is an attractive candidate for scalable quantum information processing in the solid state; however, qubit coherence is currently limited by coupling to spurious microscopic defects in the materials used to realize the circuit. Here we demonstrate that the incorporation of crystalline, defect-free dielectrics into the circuit leads to a dramatic enhancement of energy relaxation times. In addition we describe the realization of improved superconductor-insulator interfaces with extremely low levels of excess low-frequency flux noise, and we discuss efforts to incorporate these interfaces into the qubit circuit in order to extend pure dephasing times. We describe qubit fabrication and tomographic characterization and discuss ultimate limits to qubit coherence. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B25.00014: Optimization of Transmon Qubit Fabrication Josephine Chang, Mary Beth Rothwell, George Keefe Rapid advances in the field of superconducting transmon qubits have refined our understanding of the role that substrate and interfaces play in qubit decoherence. Here, we review strategies for enhancing coherence times in both 2D and 3D transmon qubits through substrate design, structural improvements, and process optimization. Results correlating processing techniques to decoherence times are presented, and some novel structures are proposed for further consideration. [Preview Abstract] |
Session B26: Focus Session: Quantum Characterization, Verification, and Validation I
Sponsoring Units: GQIChair: Charles Tahan, Laboratory for Physical Sciences
Room: 328
Monday, March 18, 2013 11:15AM - 11:51AM |
B26.00001: Using Compressed Sensing for Quantum Tomography Invited Speaker: Steve Flammia |
Monday, March 18, 2013 11:51AM - 12:03PM |
B26.00002: Analyzing quantum simulators efficiently: Scalable state tomography and quantifying entanglement with routine measurements Marcus Cramer, Tillmann Baumgratz, Oliver Marty, David Gross, Martin Plenio Conventional full state tomography reaches its limit already for a few qubits and hence novel methods for the verification and benchmarking of quantum devices are called for. We show how the complete reconstruction of density matrices is possible even if one relies only on local information about the state. This results in an experimental effort that is linear in the number of qubits and efficient post-processing -- in stark contrast to the exponential scaling of standard tomography. Whenever full tomography is not needed but instead less information required, one would expect that even fewer measurements suffice. Taking entanglement content of solid state samples and bosons in lattices as an example, we show how it may be quantified unconditionally using already routinely performed measurements only.\\ {\em Scalable reconstruction of density matrices}, T. Baumgratz, D. Gross, M. Cramer, and M.B. Plenio, arXiv:1207.0358.\\ {\em Efficient quantum state tomography}, M. Cramer, M.B. Plenio, S.T. Flammia, R. Somma, D. Gross, S.D. Bartlett, O. Landon-Cardinal, D. Poulin, and Y.-K. Liu, Nat. Commun. 1, 149 (2010).\\ {\em Measuring entanglement in condensed matter systems}, M. Cramer, M.B. Plenio, and H. Wunderlich, Phys. Rev. Lett. 106, 020401 (2011). [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B26.00003: Quantum Estimation, meet Computational Statistics; Computational Statistics, meet Quantum Estimation Chris Ferrie, Chris Granade, Joshua Combes Quantum estimation, that is, post processing data to obtain classical descriptions of quantum states and processes, is an intractable problem---scaling exponentially with the number of interacting systems. Thankfully there is an entire field, Computational Statistics, devoted to designing algorithms to estimate probabilities for seemingly intractable problems. So, why not look to the most advanced machine learning algorithms for quantum estimation tasks? We did. I'll describe how we adapted and combined machine learning methodologies to obtain an online learning algorithm designed to estimate quantum states and processes. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B26.00004: Direct characterization of any linear photonic device Alessandro Fedrizzi, Matthew Broome, Andrew White, Robert Fickler, Saleh Rahimi-Keshari, Timothy Ralph Linear photonic devices comprised of simple beamsplitters and phase shifters can implement any unitary operator for quantum information processing. The significant practical challenge is to characterize such an interferometric device once it is built. Performing quantum process tomography requires the full suite of quantum tools such as N-mode quantum state preparation and measurement, and is, despite progress on more efficient methods, slow and impractical for large interferometric devices. Here we introduce a simple technique to characterize the unitary matrix of a linear photonic device using standard laser sources and photodetectors, without the requirement for active locking or single-photon sources. Our method is precise and efficient, requiring only 2N-1 measurement configurations for a N-path network. We use it experimentally to characterise an integrated 3x3 fused-fibre coupler and highlight its precision by comparing measured quantum interference patterns with those predicted using the classically-estimated unitary. We observe excellent agreement between the two experimental methods. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B26.00005: Ultrafast Quantum Process Tomography via Continuous Measurement and Convex Optimization Charles Baldwin, Carlos Riofrio, Ivan Deutsch Quantum process tomography (QPT) is an essential tool to diagnose the implementation of a dynamical map. However, the standard protocol is extremely resource intensive. For a Hilbert space of dimension $d$, it requires $d^2$ different input preparations followed by state tomography via the estimation of the expectation values of $d^2-1$ orthogonal observables. We show that when the process is nearly unitary, we can dramatically improve the efficiency and robustness of QPT through a collective continuous measurement protocol on an ensemble of identically prepared systems. Given the measurement history we obtain the process matrix via a convex program that optimizes a desired cost function. We study two estimators: least-squares and compressive sensing. Both allow rapid QPT due to the condition of complete positivity of the map; this is a powerful constraint to force the process to be physical and consistent with the data. We apply the method to a real experimental implementation, where optimal control is used to perform a unitary map on a $d=8$ dimensional system of hyperfine levels in cesium atoms, and obtain the measurement record via Faraday spectroscopy of a laser probe. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 1:15PM |
B26.00006: Finding systematic errors in tomographic data: Characterising ion-trap quantum computers Invited Speaker: Thomas Monz Quantum state tomography has become a standard tool in quantum information processing to extract information about an unknown state. Several recipes exist to post-process the data and obtain a density matrix; for instance using maximum-likelihood estimation. These evaluations, and all conclusions taken from the density matrices, however, rely on valid data - meaning data that agrees both with the measurement model and a quantum model within statistical uncertainties. Given the wide span of possible discrepancies between laboratory and theory model, data ought to be tested for its validity prior to any subsequent evaluation. The presented talk will provide an overview of such tests which are easily implemented. These will then be applied onto tomographic data from an ion-trap quantum computer. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B26.00007: Adaptive quantum gate-set tomography Robin Blume-Kohout Quantum information hardware needs to be characterized and calibrated. This is the job of quantum state and process tomography, but standard tomographic methods have an Achilles heel: to characterize an unknown process, they rely on a set of absolutely calibrated measurements. But many technologies (e.g., solid-state qubits) admit only a single native measurement basis, and other bases are measured using unitary control. So tomography becomes circular -- tomographic protocols are using gates to calibrate themselves! Gate-set tomography confronts this problem head-on and resolves it by treating gates relationally. We abandon all assumptions about what a given gate operation does, and characterize entire universal gate sets from the ground up using only the observed statistics of an [unknown] 2-outcome measurement after various strings of [unknown] gate operations. The accuracy and reliability of the resulting estimate depends critically on which gate strings are used, and benefits greatly from adaptivity. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B26.00008: Quadratically faster state tomography using single-step adaptation Dylan Mahler, Lee Rozema, Ardavan Darabi, Christopher Ferrie, Robin Blume-Kohout, Aephraim Steinberg In quantum state tomography, an informationally complete set of measurements is made on N identically prepared quantum systems and from these measurements the quantum state can be determined. In the limit as $N\rightarrow\infty$ the estimate of the state converges on the true state. The rate at which this convergence occurs depends on both the state and the measurements used to probe the state. On the one hand, since nothing is known a priori about the state being probed, a set of maximally unbiased measurements should be made. On the other hand, if something was known about the state being measured a set of biased measurements would yield a more accurate estimate. It has been shown[1,2] that by adaptively choosing measurements optimal accuracy in the state estimate can be obtained regardless of the state being measured. Here we present an experimental demonstration of one-qubit adaptive tomography that achieves a rate of convergence of $1-O(\frac{1}{N})$ in the quantum state fidelity with only a single adaptive step and local measurements, as compared to $1-O(\frac{1}{\sqrt(N)})$ for standard tomography. Furthermore, we show how this protocol generalizes to arbitrarily entangled two-qubit systems. [1] Phys. Rev. Lett. 97, 130501 (2006) [2] Phys. Rev. A 85, 052120 (2012) [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B26.00009: Quantum process tomography of energy and phase relaxation through adaptive measurements Markku Stenberg, Frank Wilhelm Quantum process tomography tends to be very time consuming when multiple degrees of freedom are studied simultaneously. We propose a method of efficient quantum process tomography to estimate the energy and phase relaxation rates in qubits. The method applies Bayesian inference to adaptively choose measurements based on the previously obtained measurement outcomes. We adopt sequential Monte-Carlo approach to perform the Bayesian updates and make use of a fast numerical implementation of the algorithm. We compare the performance of our method to conventional offline (implemented after experimental data collection) strategies and illustrate how our method can speed up quantum process tomography. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B26.00010: A quantum neural network computes its own relative phase Elizabeth Behrman Complete characterization of the state of a quantum system made up of subsystems requires determination of relative phase, because of interference effects between the subsystems. For a system of qubits used as a quantum computer this is especially vital, because the entanglement, which is the basis for the quantum advantage in computing, depends intricately on phase. We present here a first step towards that determination, in which we use a two-qubit quantum system as a quantum neural network, which is trained to compute and output its own relative phase. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B26.00011: Modeling quantum noise for efficient testing of fault-tolerant circuits Easwar Magesan, Daniel Puzzuoli, Christopher E. Granade, David G. Cory Simulating fault-tolerant properties of quantum circuits is important for the design of large-scale quantum information processors. For general circuits and noise models, these simulations quickly become intractable in the size of the encoded circuit. We introduce methods for approximating a noise process by one which allows for efficient Monte Carlo simulation of properties of encoded circuits. The approximations are as close to the original process as possible without overestimating their ability to preserve quantum information, a key property for obtaining more honest estimates of threshold values. We numerically illustrate the method with physically relevant noise models. [Preview Abstract] |
Session B27: Focus Session: Adiabatic Quantum Computing II
Sponsoring Units: GQIChair: Sergio Boixo, University of Southern California
Room: 329
Monday, March 18, 2013 11:15AM - 11:51AM |
B27.00001: On optimal methods for adiabatic quantum state transformations Invited Speaker: Rolando Somma Many problems in science could be solved by preparing the low-energy quantum state (or any eigenstate) of a Hamiltonian. A common example is the Boolean satisfiability problem, where each clause can be mapped to the energy of an interacting many-body system, and the problem reduces to minimizing the energy. In quantum computing, adiabatic quantum state transformations (ASTs) provide a tool for preparing the quantum state. ASTs are conventionally implemented via slow or adiabatic perturbations to the Hamiltonian, relying on the quantum adiabatic theorem. Nevertheless, more efficient implementations of ASTs exist. In this talk I will review recently developed methods for ASTs that are more efficient and require less assumptions on the Hamiltonians than the conventional implementation [1,2]. Such methods involve measurements of the states along the evolution path and have a best-case implementation cost of L/G, where L is the length of the (evolved) state path and G is a lower bound to the spectral gap of the Hamiltonians. I will show that this cost is optimal [3] and comment on results of the gap amplification problem, where the goal is to reduce the cost by increasing G [4].\\[4pt] [1] S. Boixo, E. Knill, and R.D. Somma, ``Quantum state preparation by phase randomization,'' Quant. Inf. Comp. 9, 833 (2009).\\[0pt] [2] S. Boixo, E. Knill, and R.D. Somma, ``Fast quantum algorithms for traversing paths of eigenstates,'' e-print arXiv:1005.3034 (2010).\\[0pt] [3] R.D. Somma and S. Boixo, ``Necessary condition for the quantum adiabatic approximation,'' Phys. Rev. A 81, 032308 (2010).\\[0pt] [4] R.D. Somma and S. Boixo, ``Spectral gap amplification,'' SIAM J. Comp. (2012). [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B27.00002: Quantum Adiabatic Markovian Master Equations Tameem Albash, Sergio Boixo, Daniel Lidar, Paolo Zanardi We develop from first principles Markovian master equations suited for studying the time evolution of a system evolving adiabatically while coupled weakly to a thermal bath. We derive two sets of equations in the adiabatic limit, one using the rotating wave approximation that results in a master equation in Lindblad form, the other without the rotating wave approximation but not in Lindblad form. We use our formalism to study the evolution of Ising spin Hamiltonians and compare to experimental results from the D-Wave One Rainier chip. In particular, we study an Ising Hamiltonian that gives markedly different predictions for the ground state spectrum when solved using classical thermal annealing versus quantum annealing, and our master equations give qualitatively consistent results with the results of the D-Wave chip. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B27.00003: Quantum Simulation for Open-System Dynamics Dong-Sheng Wang, Marcos Cesar de Oliveira, Dominic Berry, Barry Sanders Simulations are essential for predicting and explaining properties of physical and mathematical systems yet so far have been restricted to classical and closed quantum systems [1,2]. Although forays have been made into open-system quantum simulation [3], the strict algorithmic aspect has not been explored yet is necessary to account fully for resource consumption to deliver bounded-error answers to computational questions. An open-system quantum simulator would encompass classical and closed-system simulation and also solve outstanding problems concerning, e.g. dynamical phase transitions in non-equilibrium systems, establishing long-range order via dissipation, verifying the simulatability of open-system dynamics on a quantum Turing machine. We construct an efficient autonomous algorithm for designing an efficient quantum circuit to simulate many-body open-system dynamics described by a local Hamiltonian plus decoherence due to separate baths for each particle. The execution time and number of gates for the quantum simulator both scale polynomially with the system size.\\[4pt] [1] S. Lloyd, Science 273, 1073 (1996).\\[0pt] [2] D. W. Berry et al, Comm. Math. Phys. 270, 359 (2007).\\[0pt] [3] M. Kliesch et al, Phys. Rev. Lett. 107, 120501 (2011). [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:51PM |
B27.00004: Complexity of the Quantum Adiabatic Algorithm Invited Speaker: Itay Hen The Quantum Adiabatic Algorithm (QAA) has been proposed as a mechanism for efficiently solving optimization problems on a quantum computer. Since adiabatic computation is analog in nature and does not require the design and use of quantum gates, it can be thought of as a simpler and perhaps more profound method for performing quantum computations that might also be easier to implement experimentally. While these features have generated substantial research in QAA, to date there is still a lack of solid evidence that the algorithm can outperform classical optimization algorihms. Here, we discuss several aspects of the quantum adiabatic algorithm: We analyze the efficiency of the algorithm on several ``hard'' (NP) computational problems. Studying the size dependence of the typical minimum energy gap of the Hamiltonians of these problems using quantum Monte Carlo methods, we find that while for most problems the minimum gap decreases exponentially with the size of the problem, indicating that the QAA is not more efficient than existing classical search algorithms, for other problems there is evidence to suggest that the gap may be polynomial near the phase transition. We also discuss applications of the QAA to ``real life'' problems and how they can be implemented on currently available (albeit prototypical) quantum hardware such as ``D-Wave One'', that impose serious restrictions as to which type of problems may be tested. Finally, we discuss different approaches to find improved implementations of the algorithm such as local adiabatic evolution, adaptive methods, local search in Hamiltonian space and others. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B27.00005: Power law scaling for the adiabatic algorithm for search engine ranking Adam Frees, John King Gamble, Kenneth Rudinger, Eric Bach, Mark Friesen, Robert Joynt, S. N. Coppersmith An important method for search engine result ranking works by finding the principal eigenvector of the ``Google matrix." Recently, a quantum algorithm for this problem and evidence of an exponential speedup for some scale-free networks were presented. Here, we show that the run-time depends on features of the graphs other than the degree distribution, and can be altered sufficiently to rule out a general exponential speedup. For a sample of graphs with degree distributions that more closely resemble the Web than in the previous work, the proposed algorithm does not appear to run exponentially faster than the classical one. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B27.00006: Frustration and ground state entanglement in 2D lattices Artur Garcia, Jose I. Latorre We investigate frustrated 2D lattice systems with an Ising-type interaction using exact diagonalization and Tensor Network techniques. The geometric frustration in these systems is controlled by the couplings of the Hamiltonian. We study the ground state entanglement for the combination of model parameters inducing a higher degree of frustrated interactions, showing relations between the frustration and the amount of quantum correlations present along different partitions of the lattice. Using the connection between ground state entanglement and the classical simulation of quantum systems, these results point to scenarios where simulating local systems is supposed to be hard. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B27.00007: Cycloid trajectory for a spin in a rotating magnetic field Sangchul Oh, Xuedong Hu A cycloid is a curve traced by a point on the rim of a circle rolling on a straight (or in general, a base) line. In classical mechanics, it is known as the solution of two famous problems: the brachistochrone (least-time) curve and tautochrone (equal-time) curve. Here we show that a cycloid is the quantum trajectory on the Bloch sphere when a spin is dragged along by a rotating magnetic field. Here an imaginary circle, whose radius is determined by how fast the magnetic field is rotating, rolls on the base line of the rotating magnetic field on the Bloch sphere. If the magnetic field rotates slower, the radius of the rolling circle shrinks (to a point at the adiabatic limit, when the trajectory traces a circle that spans a solid angle proportional to the Berry phase). We find that like classical cycloid curves, the curtate cycloid on a Bloch sphere is generated for initial states within a circle on the Bloch sphere surface, and a prolate cycloid results from initial states outside of this circle. If the initial state is given by the center of the circle, the quantum trajectory is a line of a constant latitude on the Bloch sphere, parallel to the curve of the rotating magnetic field. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B27.00008: Fibonacci wires Alexey Soluyanov, Matthias Troyer We show that models for one-dimensional quantum chains with local interactions can exhibit non-Abelian end modes, going beyond Kitaev's Majorana chain. We describe a model for a special case of $\rm{SU(2)}_k$ anyons, in particular Fibonacci anyons, and show how braiding of non-Abelian end modes can be done in networks of such chains. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B27.00009: Dynamical scaling in infinitely correlated many-body systems through a quantum phase transition Oscar Leonardo Acevedo, Luis Quiroga, Ferney Javier Rodriguez, Neil Johnson We assess dynamical scaling of many two-level systems (TLSs) infinitely correlated, either through a mediating radiation mode as in the Dicke Model, or through a direct interaction between TLSs as in the Lipkin-Meshkov-Glick model. Those models are characterized by the presence of a Quantum Phase Transition (QPT) in the thermodynamic limit, and they belong to the same universality class. The assessment is done by means of exact computational simulations of finite-size systems under linear rampings of the interaction parameter crossing the quantum critical point. Our results exhibit significant differences with respect to previous works on dynamical scaling across QPTs in the near-adiabatic regime, which have focused on spin-chain models where correlation lengths can be defined. We have confirmed that in infinitely correlated models an effective system size can play the role of the correlation length in traditional scaling arguments. However, due to the infinite correlation among TLSs, the standard Kibble-Zurek mechanism is not realized as the system cannot fully enter an adiabatic evolution during the ordered phase. Also, in the two-level approximation, a suitable deviation from the standard Landau-Zener protocol must be performed in order to obtain scaling collapse. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B27.00010: Phase transition detection using Renyi entropy in quantum and classical systems Stephen Inglis, Jason Iaconis, Ann Kallin, Roger Melko By extending the calculation of the Renyi entropy from quantum models [Phys. Rev. B 82, 100409(R) (2010)] to classical modes, we introduce a general procedure to calculate the Renyi mutual information in Monte Carlo simulations. Examining an array of quantum and classical models we show that the mutual information is able to detect general finite temperature phase transitions from different universality classes without knowledge of the specific order parameter or any special thermodynamic estimators. We demonstrate this technique on a standard symmetry breaking phase transition, the classical Ising model and anisotropic Heisenberg model, and a vortex-unbinding transition without a local order parameter, the classical and quantum XY model, and present the details necessary to implement this procedure on other models [arXiv:1210.2403]. [Preview Abstract] |
Session B28: Focus Session: Statistical Physics of Active Systems Away from Detailed Balance: Motors, Swimmers and All That
Sponsoring Units: GSNPChair: Alexander Grosberg, New York University
Room: 336
Monday, March 18, 2013 11:15AM - 11:51AM |
B28.00001: Mechanics and Stability of Healthy and Cancerous Tissues Invited Speaker: Thomas Risler We study the stability of the interface between a multilayered epithelium and its adjacent stroma. Treating the epithelium as a viscous fluid with cell division, we find a novel hydrodynamic instability that leads to the formation of fingering protrusions of the epithelium into the stroma [1]. Coupling cell division in the epithelium to the local concentration of nutrients diffusing from the stroma enhances the instability by a mechanism similar to that of the Mullins-Sekerka instability in single-diffusion processes of crystal growth [2]. This instability provides physical insight into a potential mechanism by which interfaces between epithelia and stroma undulate, and potentially by which tissue dysplasia leads to cancerous invasion. Later in the process of cancerous invasion, mechanics may also play an important part. We have recently proposed that one aspect of homeostasis is the regulation of tissues to preferred pressures, which can lead to a competition for space of purely mechanical origin and be an underlying mechanism for tumor growth. Surface and bulk contributions to growth lead to the existence of a critical size that must be overcome by metastases to nucleate macroscopic secondary tumors [3]. This property qualitatively explains the observed size distributions of metastases. Following these ideas, the influence of an externally applied osmotic stress on the long-term growth of cellular spheroids has been experimentally demonstrated [4].\\[4pt] In collaboration with M. Basan, F. Montel, M. Delarue, J. Elgeti, G. Cappello, J.-F. Joanny, Institut Curie, Centre de Recherche, UPMC Univ Paris 06, and CNRS, UMR 168, F-75005, Paris, France; and J. Prost, Institut Curie, Centre de Recherche, UPMC Univ Paris 06, CNRS, UMR 168, and 4ESPCI ParisTech, F-75005, Paris, France. \\[4pt] [1] M. Basan, J.-F. Joanny, J. Prost, and T. Risler, Phys. Rev. Lett., 106 (15), 158101 (2011).\\[0pt] [2] T. Risler and M. Basan, under review\\[0pt] [3] M. Basan, T. Risler, J.-F. Joanny, X. Sastre-Garau, and J. Prost, HFSP J., 3 (4), 265-272 (2009)\\[0pt] [4] F. Montel, M. Delarue, J. Elgeti, L. Malaquin, M. Basan, T. Risler, B. Cabane, D. Vignjevic, J. Prost, G. Cappello, and J.-F. Joanny, Phys. Rev. Lett., 107 (18), 188102 (2011). [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B28.00002: In-silico studies of the collective motility of cells crawling on a thick elastic substrate Aparna Baskaran, Arvind Gopinath, Michael Hagan Self-propelling cells crawling on elastic substrates are an example of a collective system that is driven away from equilibrium. Experiments show that such cells communicate with their neighbors by sensing the deformation of the underlying elastic substrate. We propose a minimal, over-damped Brownian dynamics simulation to mimic and study this emergent collective motility. The simulations incorporate intrinsic activity at the single cell level due to self-propulsion, noise and inter-cell interactions via the underlying elastic substrate. Elastic interactions are modeled on a pair-wise additive basis by treating each cell as a force dipole deforming the substrate. We find that self-propulsion, combined with elastic interaction is sufficient to generate the coordinated large scale streaming, migration, jamming and swirling motions observed in experiments. We extract the length and time scales characterizing these correlated motions and thresh out their dependence on activity and elastic interactions. The results are rationalized by deriving a mean-field hydrodynamic theory and studying the linear stability of the equations. Our results provide a unified picture of the patterns of collective migration resulting from mechanical interactions without overlying chemical cues. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B28.00003: Growth of Bacterial Colonies Mya Warren, Terence Hwa On hard agar gel, there is insufficient surface hydration for bacteria to swim or swarm. Instead, growth occurs in colonies of close-packed cells, which expand purely due to repulsive interactions: individual bacteria push each other out of the way through the force of their growth. In this way, bacterial colonies represent a new type of ``active'' granular matter. In this study, we investigate the physical, biochemical, and genetic elements that determine the static and dynamic aspects of this mode of bacterial growth for E. coli. We characterize the process of colony expansion empirically, and use discrete and continuum models to examine the extent to which our observations can be explained by the growth characteristics of non-communicating cells, coupled together by physical forces, nutrients, and waste products. Our results challenge the commonly accepted modes of bacterial colony growth and provide insight into sources of growth limitation in crowded bacterial communities. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B28.00004: Collective motion of squirmers in a quasi-2D geometry Andreas Z\"{o}ttl, Holger Stark Microorganisms like bacteria, algae or spermatozoa typically move in an aqueous environment where they interact via hydrodynamic flow fields. Recent experiments studied the collective motion of dense suspensions of bacteria where swarming and large-scale turbulence emerged. Moreover, spherical artificial microswimmers, so-called squirmers, have been constructed and studied in a quasi-2D geometry. Here we present a numerical study of the collective dynamics of squirmers confined in quasi-2D between two parallel walls. Because of their spherical shape the reorientation of squirmers is solely due to noise and hydrodynamic interactions via induced flow fields. This is in contrast to elongated swimmers like bacteria which locally align due to steric interactions. We study the collective motion of pushers, pullers and potential swimmers at different densities. At small densities the squirmers are oriented parallel to the walls and pairwise collisions determine the reorientation rate. In dense suspensions rotational diffusion is greatly enhanced and pushers, in particular, tend to orient perpendicular to the walls. This effects the dynamics of the emerging clusters. In very dense suspensions we observe active jamming and long-lived crystalline structures. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B28.00005: Identifying and quantifying interactions in a laboratory swarm James G. Puckett, Douglas H. Kelley, Nicholas T. Ouellette Emergent collective behavior, such as in flocks of birds or swarms of bees, is exhibited throughout the animal kingdom. Many models have been developed to describe swarming and flocking behavior using systems of self-propelled particles obeying simple rules or interacting via various potentials. However, due to experimental difficulties and constraints, little empirical data exists for characterizing the exact form of the biological interactions. We study laboratory swarms of flying {\it Chironomus riparius} midges, using stereoimaging and particle tracking techniques to record three-dimensional trajectories for all the individuals in the swarm. We describe methods to identify and quantify interactions by examining these trajectories, and report results on interaction magnitude, frequency, and mutuality. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B28.00006: Stochastic pattern transitions in large scale swarms Ira Schwartz, Brandon Lindley, Luis Mier-y-Teran We study the effects of time dependent noise and discrete, randomly distributed time delays on the dynamics of a large coupled system of self-propelling particles. Bifurcation analysis on a mean field approximation of the system reveals that the system possesses patterns with certain universal characteristics that depend on distinguished moments of the time delay distribution. We show both theoretically and numerically that although bifurcations of simple patterns, such as translations, change stability only as a function of the first moment of the time delay distribution, more complex bifurcating patterns depend on all of the moments of the delay distribution. In addition, we show that for sufficiently large values of the coupling strength and/or the mean time delay, there is a noise intensity threshold, dependent on the delay distribution width, that forces a transition of the swarm from a misaligned state into an aligned state. We show that this alignment transition exhibits hysteresis when the noise intensity is taken to be time dependent. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B28.00007: Concentrating Swimming Bacteria using Funnels: Connecting Simulation Results to Simple Random-Walk Models Yu-Guo Tao, Gary W. Slater Rectification of swimming bacteria has been observed when confined in a closed environment partitioned using porous walls with funnel shaped channels. Using Monte Carlo simulations that take into account the mechanical and thermodynamic properties of round-shape cells as well as the effect of noise on the run/tumble process, we show that the long-time behaviour of the system can be mapped onto a simple one-dimensional biased random-walk process. This implies that the many variables that are needed to describe the geometry of the system and the properties of the cells can be reduced to only two generalized variables plus the size of the system itself. We examine how these two variables depend on the initial variables and draw conclusions on the performance of the system when used as a tool to separate cells. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B28.00008: Hysteresis in transition between individual and collective behavior in suspension of swimming bacteria Andrey Sokolov, Igor Aranson We present a new method for control of motility and tumbling rate of swimming bacteria Bacillus Subtilis via precise and rapid control of temperature of the bacterial suspension. Transitions between individual and collective behaviors in a response to cyclical temperature change in a range of temperatures between 5C and 35C with the rates from 0.1C/s to 1C/s were investigated. Temperature decrease typically results in a decrease of bacterial motility while preserving low tumbling rates. The temperature increase above 20C triggers a ``heat shock'': a significant jump in tumbling rate resulting in temporal decrease of the average swimming speed and termination of collective motion. At temperatures below 20C due to relative low tumbling rates we discovered a hysteresis in the transition between individual and collective swimming: velocity correlation length vs. average swimming speed of bacteria exhibits hysteric behavior. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B28.00009: Flocking in Flow Nicholas Ouellette, Nidhi Khurana Models of active, self-propelled particles with simple interaction rules have long been shown to produce large-scale emergent behavior reminiscent of collective animal motion seen in nature. Such model flocks can be shown to be robust against random noise terms added to the equations. But real animals, such as birds, fish, or insects, live in fluid environments, where the background flow field is nonzero and is often turbulent. In this case, the fluctuations experienced by the individuals in the aggregation are not random, but rather are correlated in space and time. We explore the impact of such spatiotemporally correlated perturbations on flocking by numerically simulating the behavior of a simple flocking model in a turbulent-like flow field produced by a kinematic simulation. The introduction of flow strongly changes the flock formation dynamics. Additionally, we find that under some conditions the background flow tends to break stable flocks into smaller units. We study these clusters, and discuss their relation to the underlying flow field. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B28.00010: Pitfalls in mining active transport trajectories Kejia Chen, Bo Wang, Sung Chul Bae, Steve Granick Single particle tracking is useful in characterizing active motion. However, there are many pitfalls in mining such data, from separating the intermittently alternating active and passive motion to fitting a model to the motion. Using statistical tools, we carefully identified such pitfalls and developed new methods to avoid them. Applying this algorithm to endosomal active transport within living cells, imaged by fluorescence microscopy with nm resolution, we observed L\'{e}vy walk behavior in multiple cells lines and for different cargo types. This L\'{e}vy walk behavior could be easily missed without those statistical tools, which can be very useful in characterizing active motion and identifying regulators in other active systems. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B28.00011: Casimir Effect and Fluctuation-Induced Attractive Forces in Active Matter Cynthia Reichhardt, Lena Lopatina, Charles Reichhardt We consider the fluctuation-induced forces between plates in walls immersed in a bath of active matter, similar to the forces in the classical Casimir effect. The active matter could represent swimming bacteria. We find that the active matter causes a strong attractive force between two plates, whereas for strictly Brownian particles, there is little effect or no attraction between the plates. We discuss how the motion of the active particles, the breaking of detailed balance by the walls, and the geometry of the sample leads to a reduced particle density between the plates and produces a density-induced pressure on the plates. This result also indicates that for movable objects immersed in an active matter bath, larger objects will aggregate over time, suggesting that active matter could be used as the catalyst for a novel self-assembly method. Finally, we discuss other geometries that can produce a repulsive force between the walls, as well as the effect of flocking particles. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B28.00012: Pattern Formation in Growing Polar Bacteria Xingbo Yang, M. Cristina Marchetti, Davide Marenduzzo We analyze a continuum model of a bacterial suspension that includes motility suppression from steric repulsion, polar alignment, and bacteria reproduction and death. Using a combination of linear stability analysis and numerical solution of the nonlinear equations, we demonstrate that the model exhibits a rich variety of emergent structures, corresponding to generic patterns seen in experiments. Motility suppression in a crowded environment gives rise to a density phase separation, regulated by the growth/death of the bacteria, as demonstrated earlier by Cates et al. [PNAS 107, 11715--11720(2010)], with spherically symmetric patterns similar to those observed in \textit{S. typhimurium}. The addition of polar alignment yields new ring/band and swirl/spiral structures resembling those observed in \textit{E.coli } colonies. The stationary/traveling nature of the patterns and their symmetry is classified and summarized in a phase diagram. [Preview Abstract] |
Session B29: Fluctuations in Non-Equilibrium Systems
Sponsoring Units: GSNPChair: Chris Jarzynski, University of Maryland
Room: 337
Monday, March 18, 2013 11:15AM - 11:27AM |
B29.00001: Large rare fluctuations in systems with delayed dissipation Mark Dykman, Ira Schwartz We study the probability distribution and the escape rate in noise-driven nonlinear systems with delayed dissipation. Accounting for the delay requires a significant modification of the conventional rare events theory. We develop the corresponding general formulation and find explicit results in the limiting cases. To logarithmic accuracy in the fluctuation intensity, the problem is reduced to a variational problem. It describes the most probable path followed by the system in the random rare event of interest. In contrast to Markov systems, the equations for the most probable paths are acausal due to the delay. If the dissipation and noise come from the coupling to a thermal bath, they are related by the fluctuation-dissipation relation, but our results are not limited to this case. In thermal equilibrium, the most probable path passing through a remote state has time reversal symmetry. However, again in contrast to Markov systems, one cannot uniquely define a path that starts from a state with given system coordinate and momentum. The corrections to the logarithm of the probability distribution and the escape activation energy for small dissipation delay and small noise correlation time are obtained in explicit form. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B29.00002: Thermal rectification in non-linear structures with bulk losses Martin Schmidt, Tsampikos Kottos A mechanism for thermal rectification based on the interplay between non-uniform bulk losses with nonlinearity is presented. We theoretically analyze the phenomenon using an anharmonic array of coupled oscillators coupled to the left and right with two Langevin reservoirs. A third probe thermostat (with temperature $T_B$) is placed in an asymmetric position in the bulk of the lattice thus breaking the translational symmetry and leading to rectification of heat flow. We note that for $T_B=0$ this Langevin term is equivalent to a simple friction. We find that an increase of the friction strength can increase both the asymmetry and heat flux. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B29.00003: An exactly solvable model of Maxwell's demon Dibyendu Mandal, Christopher Jarzynski The paradox of Maxwell's demon has stimulated numerous thought experiments, leading to discussions about the thermodynamic implications of information processing. However, the field has lacked a tangible example or model of an autonomous, mechanical system that reproduces the actions of the demon. To address this issue, we introduce an explicit model of a device that can deliver work to lift a mass against gravity by rectifying thermal fluctuations, while writing information to a memory register. We solve for the steady-state behavior of the model and construct its nonequilibrium phase diagram. In addition to the engine-like action described above, we identify a ``Landauer eraser'' region in the phase diagram where the model uses externally supplied work to remove information from the memory register. Our model offers a simple paradigm for investigating the thermodynamics of information processing by exposing a transparent mechanism of operation. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B29.00004: The thermodynamics of prediction Susanne Still, David Sivak, Anthony Bell, Gavin Crooks We expose the fundamental equivalence between model inefficiency and thermodynamic inefficiency, measured by dissipation. The dynamics of any system responding to a stochastic environmental signal can be interpreted as computing an implicit model of the driving signal. The system's state retains information about past environmental fluctuations, and a fraction of this information is predictive of future fluctuations. The remaining nonpredictive information reflects model complexity that does not improve predictive power, and thus represents the inefficiency of the model. We find that instantaneous nonpredictive information: 1) is proportional to the work dissipated due to environmental change; 2) provides a lower bound on the total average dissipated work when summed over the length of a driving protocol; 3) augments the lower bound on heat generated due to information erasure (Landauer's principle). Our results hold far from thermodynamic equilibrium and are thus applicable to a wide range of systems, including biomolecular machines. They highlight a profound connection between the effective use of information and efficient thermodynamic operation: any system constructed to keep memory about its environment and to operate with maximal energetic efficiency has to be predictive. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B29.00005: Geometry of thermodynamic control David Sivak, Patrick Zulkowski, Michael DeWeese, Gavin Crooks A fundamental problem in modern thermodynamics is how a molecular-scale machine performs useful work, while operating away from thermal equilibrium without excessive dissipation. We show that when a thermodynamic system is driven from equilibrium, in the linear response regime, the space of controllable parameters has a Riemannian geometry induced by a generalized friction tensor. This metric structure controls the dissipation of finite-time transformations, and bestows optimal protocols (geodesics on the Riemannian manifold) with many useful properties. We exploit this geometric insight to construct closed-form expressions for minimal-dissipation protocols for a model system of a particle diffusing in a one-dimensional harmonic potential, where the spring constant, inverse temperature, and trap location are adjusted simultaneously. This simple model has a surprisingly rich geometry, which we test via a numerical implementation of the Fokker-Planck equation. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B29.00006: Clinical application of fluctuation dissipation theory - Prediction of heart rate response to spontaneous breathing trial Liang R. Niestemski, Man Chen, Robert Prevost, Michael McRae, Sharath Cholleti, Gabriel Najarro, Timothy G. Buchman, Michael W. Deem Contrary to the traditional view of the healthy physiological state as being a single static state, variation in physiologic variables has more recently been suggested to be a key component of the healthy state. Indeed, aging and disease are characterized by a loss of such variability. We apply the conceptual framework of fluctuation-dissipation theory (FDT) to predict the response to a common clinical intervention from historical fluctuations in physiologic time series data. The non-equilibrium FDT relates the response of a system to a perturbation to natural fluctuations in the stationary state of the system. We seek to understand with the FDT a common clinical perturbation, the spontaneous breathing trial (SBT), in which mechanical ventilation is briefly suspended while the patient breathes freely for a period of time. As a stress upon the heart of the patient, the SBT can be characterized as a perturbation of heart rate dynamics. A non-equilibrium, but steady-state FDT allows us to predict the heart rate recovery after the SBT stress. We show that the responses of groups of similar patients to the spontaneous breathing trial can be predicted by this approach. This mathematical framework may serve as part of the basis for personalized critical care. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B29.00007: Cumulant generating function formula of heat transfer in ballistic systems with lead-lead coupling and general nonlinear systems Huanan Li Based on a two-time observation protocol, we consider heat transfer in a given time interval $t_M$ in a lead-junction-lead system taking coupling between the leads into account. In view of the two-time observation, consistency conditions are carefully verified in our specific family of quantum histories. Furthermore, its implication is briefly explored. Then using the nonequilibrium Green's function method, we obtain an exact formula for the cumulant generating function for heat transfer between the two leads, valid in both transient and steady-state regimes. Also, a compact formula for the cumulant generating function in the long-time limit is derived, for which the Gallavotti-Cohen fluctuation symmetry is explicitly verified. In addition, we briefly discuss Di Ventra's repartitioning trick regarding whether the repartitioning procedure of the total Hamiltonian affects the nonequilibrium steady-state current fluctuation. All kinds of properties of nonequilibrium current fluctuations, such as the fluctuation theorem in different time regimes, could be readily given according to these exact formulas. Finally a practical formalism dealing with cumulants of heat transfer across general nonlinear quantum systems is established based on field theoretical/algebraic method. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B29.00008: Fluctuation Relations for Current Components in Mesoscopic Electric Circuits Nikolai Sinitsyn, Sriram Ganeshan Discovery of Fluctuation Theorems (FTs) for non-equilibrium systems led to optimism that they might serve as universal laws that had long been missing from the study of nonequilibrium systems. Surprisingly, recent experimental work has shown that the FTs can fail in an electric circuit, but could be salvaged under the experimental conditions if the affinity parameter is suitably renormalized by a factor of 0.1. Motivated by this new experimental result we present a new class of fluctuation relations, to which we will refer as ``Fluctuation Relations for Current Components'' (FRCCs). Unlike standard fluctuation theorems, FRCCs follow from the seemingly trivial fact that to know statistics of particle currents, it is sufficient to know only statistics of single particle geometric trajectories while the information about time moments, at which particles make transitions along such trajectories, is irrelevant. We also show that FRCCs are robust in the sense that they do not depend on some basic types of electron interactions and some quantum coherence effects. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B29.00009: The Dependence of Heat Fluctuation Theorem on an Initial Distribution Kwangmoo Kim, Hyunggyu Park, Chulan Kwon The fluctuation theorem (FT) proven for work does not hold for heat even in the long time limit. As the two quantities differ by the change in energy at the initial and final times, we suspect that the memory of an initial distribution may remain in the heat production accumulated for a long time. We investigate the dependence of the large deviation function (LDF) and FT on the temperature of the initial equilibrium distribution for the motion of a Brownian particle in a harmonic potential dragged with a constant velocity. The conventional saddle point integration for the LDF used in van Zon and Cohen, Phys.\ Rev.\ Lett.\ {\bf 91}, 110601 (2003) is found to fail as the saddle point approaches asymptotically the singularity at the branch point in the long time limit. We develop a new mathematical method to resolve this problem and confirm it with numerical simulations. As a result, the tail of LDF, i.e., a region of rare events, is shown to depend remarkably on the initial temperature and also causes more types of modifications of FT's than the so called extended FT proposed by van Zon and Cohen. We expect that our method can be applied to the investigation of the dependence of initial memories in other nonequilibrium systems. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B29.00010: Threshold for everlasting initial memory for rare events in equilibration processes Jae Sung Lee, Chulan Kwon, Hyunggyu Park Conventional wisdom indicates that initial memory should decay away exponentially in time for general (noncritial) equilibration processes. In particular, time-integrated quantities such as heat are presumed to lose initial memory in a sufficiently long-time limit. However, we show that the large deviation function of time-integrated quantities may exhibit initial memory effect even in the infinite-time limit, if the system is initially prepared sufficiently far away from equilibrium. For a Brownian particle dynamics, as an example, we found a sharp finite threshold rigorously, beyond which the corresponding large deviation function contains everlasting initial memory. The physical origin for this phenomenon is explored with an intuitive argument and also from a toy model analysis. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B29.00011: Fluctuation theorems and entropy production with odd-parity variables Hyunggyu Park, Hyun Keun Lee, Chulan Kwon We show that the total entropy production in stochastic processes with odd-parity variables (under time reversal) is separated into three parts, only two of which satisfy the integral fluctuation theorems in general. One is the usual excess contribution, which can appear only transiently and is called non-adiabatic. Another one is attributed solely to the breakage of detailed balance. The last part not satisfying the fluctuation theorem comes from the steady-state distribution asymmetry for odd-parity variables, which is activated in a non-transient manner. The latter two parts combine together as the house-keeping (adiabatic) contribution, whose positivity is not guaranteed except when the excess contribution completely vanishes. Our finding reveals that the equilibrium requires the steady-state distribution symmetry for odd-parity variables independently, in addition to the usual detailed balance. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B29.00012: A novel nature in nonequilibrium entropy production with odd-parity variables Chulan Kwon, Hyunggyu Park, Joonhyun Yeo, Hyun Keun Lee We present our recent finding about a novel nature in nonequilibrium entropy production for systems with odd-parity variables under time reversal. In the presence of irreversible forces the entropy production $\Delta S_{env}$ transferred from system to environment is not equal to $Q/T$ where $Q$ is the heat transfer and $T$ the temperature of heat bath. We consider a dissipative force applied by external agent in addition to that given by heat bath. Then $\Delta S_{env}$ has extra contribution to $Q/T$ for which an appropriate physical explanation is still open. Another example for irreversible force is a form of $-A\cdot\vec{p}/m$ for antisymmetric matrix $A$ which is realized by a Lorentz force in a uniform magnetic field. In spite of no heat dissipation $\Delta S_{env}$ has a nonvanishing positive contribution. We find that it is due to a nonzero phase space current remaining through stochastic average, which is in fact a nonzero average force. Basically it plays the same role as a nonzero position space current observed in system with even variables only. We suppose interesting situations for different types of irreversible forces. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B29.00013: About the Equivalence of Phase Retrieval Methods Employed in Nonlinear Spectroscopy and Microscopy Laszlo Ujj, Rohan Hemasinha It is well known that the generalized Kramers-Kronig relationship is able to retrieve the phase of a signal from measured power spectra. This phase recovery is a critical procedure in nonlinear optical spectroscopy, e.g. coherent Raman time domain or frequency domain spectroscopy. Several other methods have been developed and being used in the past: notably, nonlinear fitting and maximum entropy method. A firm mathematical comparison of the methods including the effects of final signal sampling and their merit of fidelity will be presented. Attention is given to numerical implementation of the phase retrieval procedure to put it into practice in coherent anti-Stokes Raman microscopy. Phase retrieval examples using all the above methods are taken from earlier and recently recorded spectra. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B29.00014: Theory of zero-bias anomaly in low-temperature inelastic tunneling spectroscopy Yoshihiro Asai A small zero-bias anomaly (ZBA) in inelastic tunneling spectroscopy (IETS) through nonmagnetic quantum wires has been suggested experimentally at low temperatures [1,2]. Here, the mechanism is discussed theoretically with special attention paid to contributions from low energy phonons [3]. Our theoretical calculations, using an electron-phonon coupling model, predict the ZBA. While experimental information is still limited, our theoretical result agrees with existing experiments. The theory provides useful information, characterizing the ZBA in a nonmagnetic junction.\\[4pt] [1] L. F. Spietz, Ph.D. dissertation, Yale University, 2006.\\[0pt] [2] Y. Selzer, M. A. Cabassi, T. S. Mayer, and D. L. Allara, Nanotechnology 15, S483 (2004).\\[0pt] [3] Y. Asai, Phys. Rev. B Rapid Commun., in press. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B29.00015: Computation of Microcanonical Entropy Differences in Atomistic Computer Simulation Sergio Davis In this work, two alternative methods to compute thermodynamic entropy differences $\Delta S=S(E_2)-S(E_1)$ between two microcanonical states (produced via atomistic computer simulation, either deterministic or stochastic) at total energies $E_1$ y $E_2$ are presented. The first method is straightforward to implement, as it only needs potential energy samples from both simulations; however, it requires that fluctuations of potential energy are similar in magnitude to the energy difference $\Delta E$ between the states. It is therefore best suited for simulations in small systems (hundred of atoms). The second method, based on Bayesian probability and information theory, removes this limitation: it allows the computation of the entropy curve $S(E)$ for a wide range of energies and therefore is a viable alternative to methods such as Wang-Landau Monte Carlo. It is based on inferring the configurational density of states (CDOS) from potential energy samples. A simple model for the CDOS of embedded atom metals is presented and tested in Au and Cu by computing entropy and free energy differences. [Preview Abstract] |
Session B30: Colloids: Transitions and Structures
Sponsoring Units: DCMPChair: Piotr Habdas, St. Joseph's University
Room: 338
Monday, March 18, 2013 11:15AM - 11:27AM |
B30.00001: Coarsening of firefighting foams containing fluorinated hydrocarbon surfactants Matthew J. Kennedy, John A. Dougherty, Nicholas Otto, Michael W. Conroy, Bradley A. Williams, Ramagopal Ananth, James W. Fleming Diffusion of gas between bubbles in foam causes growth of large bubbles at the expense of small bubbles and leads to increasing mean bubble size with time thereby affecting drainage. Experimental data shows that the effective diffusivity of nitrogen gas in aqueous film forming foam (AFFF), which is widely used in firefighting against burning liquids, is several times smaller than in 1{\%} sodium dodecyl sulfate (SDS) foam based on time-series photographs of bubble size and weighing scale recordings of liquid drainage. Differences in foam structure arising from foam production might contribute to the apparent difference in the rates of coarsening. AFFF solution produces wetter foam with initially smaller bubbles than SDS solution due in part to the lower gas-liquid surface tension provided by the fluorosurfactants present in AFFF. Present method of foam production generates microbubble foam by high-speed co-injection of surfactant solution and gas into a tube of 3-mm diameter. These results contribute to our growing understanding of the coupling between foam liquid fraction, bubble size, surfactant chemistry, and coarsening. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B30.00002: Competition between phase separation and crystallization in attractive colloids Barbara Frisken, Arthur Bailey, Juan Sabin, Gabriel Espinosa We will present results from recent experiments on Earth and on the International Space Station investigating the interplay between phase separation and crystallization in samples prepared in the three-phase region (gas-liquid-crystal) of the phase diagram of a colloid-polymer mixture. On Earth, our samples first separate into a colloid-rich phase and a colloid-poor phase, with crystals forming in the colloid-rich phase. The denser phases sediment as expected. In microgravity, photographic images obtained in the BCAT-5 experiment reveal phase separation with crystal formation in the denser phase, where the phase separation continues normally until the dominant length scale is about 25\% of the cell thickness, at which point both phase separation and crystal growth are arrested before macroscopic phase separation can occur. We propose that this arrest occurs because a crystalline network forms in the liquid phase and that the gas-liquid surface tension is not sufficient to overcome the stiffness of this network. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B30.00003: Colloidal Wigner Crystals Near the Melting Transition Emily Russell, David Weitz We demonstrate the formation of colloidal ``Wigner'' crystals at low particle volume fraction. Particles are suspended in a nonpolar solvent and charged by the addition of a small amount of surfactant, generating a long-range interparticle repulsion which induces crystallization above a critical volume fraction of order 10\%. Confocal microscopy allows us to study in detail the three-dimensional structure and dynamics of these colloidal crystals as we vary the volume fraction, and we find a growing population of especially mobile particles with large local Lindemann parameter as we approach the critical volume fraction. We discuss our results and the implications of our findings to competing ideas of the mechanism of bulk crystal melting. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B30.00004: Visualization of colloidal liquid nucleation induced by Critical Casimir forces Duc Nguyen, Peter Schall We show that with precise temperature control of critical Casimir forces we achieve reversible control of colloidal gas-liquid. The exquisite temperature control of the potential allows us to even tune the degree of supersaturation of the liquid phase. We use a confocal microscopy to elucidate the nucleation process on the single particle level: We determine the Gibbs free energy, interfacial tension and chemical potential of the liquid aggregates directly from their size distribution. We estimate the interfacial tension of the aggregates at different degree of supersaturation directly from the particle potential and pair correlation function using Kirkwood and Buff theory. A good agreement between the two methods provides new insight into the gas-liquid transition. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B30.00005: Shear Driven Aggregation in Latex Colloids Suresh Ahuja Reynolds number is small in colloidal flow and therefore, colloidal volume fraction and Peclet number are important. AS the volume fraction and attractive coupling between particles increase, relaxation time and Weisenberg number become significant. Shear-induced aggregation of latex colloids is due to the interplay between the shear-induced formation and breakage of latex .particles. While particle size is limited by breakage, their number density increases with the shearing-time. Upon cessation of shear, the particles interconnect into an assembly held by grainy bonds. It results in increase in yield stress and dynamic modulus. A contact model enables aggregates maintaining their structures under low stress while being restructured under high stress. Modeling involves solution of Navier- Stokes equation with moving particles as boundary condition for the flow like using the Lattice Boltzmann approach or by using (accelerated) Stokesian Dynamics. Alternate approach is to model the fluid phase by soft repulsive particles with pair-wise noise and friction, known as dissipative particle dynamics (DPD). This method by construction produces full inertial hydrodynamics, but applying the correct fluid-particle boundary condition is non-trivial. Both particle to particle and particle to wall collisions can be considered using Johnson-Kendall- Roberts (JKR) analysis of collision dynamics of dissipative forces using a soft-sphere modeling technique. Our experimental work used emulsion polymerized latex that was subjected to steady and dynamic shear. Yield stress, dynamic modulus and relaxation time increased on shearing in conjunction with changes in aggregate size. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B30.00006: Domain, Stripe, and Pattern Formation for Colloids on Optical Trap Arrays Danielle McDermott, Jeffery Amelang, Lena Lopatina, Cynthia Reichhardt, Charles Reichhardt We examine pattern formation of colloids atop a square periodic substrate using large scale numerical simulations. The pins forming the substrate are modeled with a muffin-tin potential which is flat with localized traps. We show that with 4 colloids per pinning site the system has triangular ordering and with 5 colloids per site it has square ordering. We study intermediate fillings and identify a rich variety of distinct ordering regimes including disordered grain boundaries, crystalline stripe structures, superlattice orderings, and disordered patches of multiple phases. These different regimes are characterized with a Voronoi analysis, energy dispersion plots, and ordination of domains. We extend our studies to a wide range of other fillings which feature similar boundary formation patterns. Our results show that periodic substrates of muffin-tin potentials can be used to tailor grain boundary formation. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B30.00007: Anisotropic colloids for building complex molecular structures using critical Casimir effect Truc Anh Nguyen, Daniela Kraft, Sandra Veen, Peter Schall Here, we present a new way to build complex colloidal scale structures using critical Casimir forces on anisotropic colloids. These forces arise from the confinement of critical solvent fluctuations between the particle surfaces and allow temperature-control over the particle interactions. We use doublet particles made of polymethyl-methacrylate (PMMA) and exhibiting anisotropic surface charge densities, suspended in a binary liquid mixture. By controlling the applied temperatures of the system, we can tune the particle interactions of the two ends of the particles to observe different superstructures formed in time and space: at low temperature, the particles are randomly distributed and represent a gas phase; however, at higher temperatures, the particles form long chain-like structures and cubic crystal structures depending on the temperature difference to the solvent phase separation. This opens new opportunities to assemble complex building blocks for nano- and micro-devices. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B30.00008: Direct observation of the nucleation in colloidal solid-solid transitions Yi Peng, Feng Wang, Ziren Wang, Yilong Han Solid-solid phase transitions are ubiquitous in nature, but their microscopic mechanisms remain poorly understood. We employed thermally sensitive microgels to study the solid-solid transitions between square and triangular lattices in colloidal thin films. Two types of nucleation processes were directly observed by video microscopy and studied at the single-particle level. Under low flow rates, the nucleation is a two-step process: square lattice $\to$ liquid nucleus $\to$ triangle nucleus and its precursor is a local particle-exchange loop, whereas under high flow rates the nucleus of the triangle lattice forms directly from a dislocation pair by a martensitic mechanism. We measured the critical nucleus size, the energy barrier height and the hysteresis loop of the solid-solid transitions. Our results cast new light to solid-solid transitions in carbon systems, nano-crystals and geophysics. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B30.00009: Frustrated Ordering of Colloidal Crystals in Spatially Varying Potentials Vishal Soni, William T.M. Irvine Frustrated ordering processes are of wide interest in condensed matter systems. Experiments on interfacial colloidal systems have resulted in several recent insights into the two dimensional ordering of crystalline lattices frustrated by Gaussian curvature. We study the ordering of two-dimensional lattices of colloids frustrated by spatially varying dielectrophoretic forces. In particular, we investigate the role of topological defects in organizing the conformal-crystal like ground state and the defect dynamics that lead to equilibration as the applied dielectrophoretic force is increased. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B30.00010: Preparation of monodisperse microspheres from the Laplace pressure induced droplet formation in micromolds Chang-Hyung Choi, Jongmin Kim, Sung-Min Kang, Jinkee Lee, Chang-Soo Lee Monodisperse microspheres play critical roles in many applications such as micro-electromechanical systems (MEMS), chemical release systems, optical materials and various biological applications. Although microfluidic systems have been developed for producing monodisperse microspheres, it still definitely requires pressure driven flow for continuous fluid injection as well as use of surfactant to achieve their uniformity. Here, we present a novel molding method that generates monodisperse microspheres through surface-tension-induced flow. Two immiscible fluids that consist of photocurable monomer and hydrophobic oil are sequentially applied onto the mold. The mold geometry results in Laplace pressure induced droplet formation, and these droplets formed are individually localized into each micromold. Photopolymerization of the droplets allow for the formation of polymer microspheres with narrow size distribution (CV$=$1.9{\%}). We obtain the microspheres with diameter ranging from 20 to 300 $\mu $m by modulating mold dimensions. We provide a synthesis method to produce microspheres in micromolds for various reaction schemes: UV-polymerization, sol-gel reactions and colloidal assemblies. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B30.00011: Formation of Uniform Hollow Silica microcapsules Huan Yan, Chanjoong Kim Microcapsules are small containers with diameters in the range of 0.1 -- 100 $\mu$m. Mesoporous microcapsules with hollow morphologies possess unique properties such as low-density and high encapsulation capacity, while allowing controlled release by permeating substances with a specific size and chemistry. Our process is a one-step fabrication of monodisperse hollow silica capsules with a hierarchical pore structure and high size uniformity using double emulsion templates obtained by the glass-capillary microfluidic technique to encapsulate various active ingredients. These hollow silica microcapsules can be used as biomedical applications such as drug delivery and controlled release. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B30.00012: Interstitials in 2D colloidal crystals Lichao Yu, Sungcheol Kim, Alexandros Pertsinidis, Xinsheng Ling Point defects in crystalline solids are important in many areas of condensed matter physics, ranging from the mechanical properties of metals, to supersolidity in quantum solids, and most recently the magnetic properties of graphene. A key question to point defects is how they diffuse in the crystalline lattice. Colloidal crystals provide a perfect model system for studying the dynamics of point defects, since the kinetic pathways of diffusion can be identified in direct real-time video imaging experiments. Here we report an experimental study of another type of point defects: interstitials. We found that interstitial diffusion in a 2D colloidal crystal is also dominated by a dislocation pair unbinding-binding process. Similar to vacancies, interstitial diffusion exhibits strong memory effects. However, the contrast lies in the observation that the interstitials, as quasi-particles, diffuse faster than vacancies. We propose that higher diffusion constant of the interstitials is a result of the suppression of the Peierls barrier for the edge dislocations by the excess strain created by the extra particle(s). This work was supported by NSF-DMR. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B30.00013: Effects of Particle Shape on Growth Dynamics at Edges of Evaporating Drops of Colloidal Suspensions Peter J. Yunker, Matthew A. Lohr, Tim Still, Alexei Borodin, D.J. Durian, A.G. Yodh We study the influence of particle shape on growth processes at the edges of evaporating drops. Aqueous suspensions of colloidal particles evaporate on glass slides, and convective flows during evaporation carry particles from drop center to drop edge, where they accumulate. The resulting particle deposits grow inhomogeneously from the edge on the air-water interface in two-dimensions. The deposition front, or growth line, varies in space and time. Measurements of the fluctuations of the deposition front during evaporation enable us to identify distinct growth processes. Interestingly, three distinct growth processes were discovered in the evaporating colloidal suspensions by tuning particle shape-dependent capillary interactions and thus varying the microscopic rules of deposition. Sphere deposition exhibits a classic Poisson like growth process; deposition of slightly anisotropic particles, however, appears to belong to the Kardar-Parisi-Zhang (KPZ) universality class, and deposition of highly anisotropic ellipsoids appears to belong to a third universality class, characterized by KPZ fluctuations in the presence of quenched disorder. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B30.00014: Non-equilibrium Ionic Assemblies of Oppositely Charged Colloids Rui Zhang, Prateek Jha, Monica Olvera de la Cruz The structure and evolution kinetics of non-equilibrium clusters formed in a solution of oppositely charged colloids are analyzed by a kinetic Monte Carlo simulation scheme. A wide range of dynamic cluster configurations are obtained by varying the various external parameters controlling the interaction strength between colloids, screening length, and packing density of colloids. At low-salt concentrations, clusters with structures ranging from NaCl-type cubic aggregates to fibril-like chains are observed, while at high-salt concentrations, disordered compact clusters are observed. A chain-folding barrier model is proposed to explain the kinetically trapped fibril-like assemblies. In higher-density solutions, ionic clusters of bigger size and percolated gel structures are observed. Our work demonstrates the structural richness of non-equilibrium ionic assemblies of oppositely charged colloids and elucidates the effect of ionic correlations, not captured by mean field models such as the modified Poisson-Boltzmann approaches, in determining the structure of assemblies of oppositely charged colloids. These ``ionic composites" hold great promise in a variety of emerging applications such as templated polymerization of charged molecules and assembly of charged particles. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B30.00015: Directional Entropic Forces in Hard Colloids Greg van Anders, Khalid Ahmed, Ross Smith, Michael Engel, Sharon Glotzer Based on known results from the literature of hard particles we introduce the concept of entropically patchy particles -- particles that bind with angular specificity entirely due to their geometry via directional entropic forces or ``bonds''. Unlike ordinary patchy particles, in which ``valence'' vis-a-vis angular specificity is dictated by microscopic energetic considerations (sticky patches), entropic forces causing the binding of particles at entropic patch sites are emergent. Using basic examples we show both theoretically and computationally that we can alter the geometry of a particle to create an entropic patch and tune the resulting effective pair potential in such a way that it can lead to angularly specific binding, even in the absence of depletants. [Preview Abstract] |
Session B31: Focus Session: Nano to Meso-Scale Structure in Ordered Soft Matter: Liquid Crystal Structure, Dynamics and Function I
Sponsoring Units: DPOLYChair: Alberto Fernandez de las Nieves, Georgia Institute of Technology
Room: 339
Monday, March 18, 2013 11:15AM - 11:27AM |
B31.00001: Self-Assembly of Polyhedral Oligomeric Silsesquioxane-Based Giant Molecular Shape Amphiphiles Yiwen Li, Stephen Cheng A series of giant molecular shape amphiphiles based on functional polyhedral oligomeric silsesquioxane (POSS) particles was designed and synthesized. The supramolecualr structures of these assemblies along with the resulting ordered structures are fully investigated to determine their structure-property relationships. For example, functional POSS cages with different surface chemistry and sizes were employed to construct dumbbell- and snowman-like molecular Janus particles with various symmetry breakings. These particles could self-organize into hierarchically ordered supramolecular structures in the bulk. Another illustrating example is a series of novel giant surfactants, lipids and gemini surfactants possessing a hydrophilic POSS head and polymer or alkyl chain tails. Diverse architectures of this class of materials have been constructed and their self-assembly processes in solution and bulk state have been discussed. This set of research results not only has general implications in the basic physical principles underlying their self-assembly behaviors, but also create unique materials for developing advanced technologies by combining the properties of hybrid materials [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B31.00002: Nanoparticle Solubility in Liquid Crystalline Defects Jonathan K. Whitmer, Julio C. Armas-Perez, Abhijeet A. Joshi, Tyler F. Roberts, Juan J. de Pablo Liquid crystalline materials often incorporate regions (defects) where the orientational ordering present in the bulk phase is disrupted. These include point hedgehogs, line disclinations, and domain boundaries. Recently, it has been shown that defects will accumulate impurities such as small molecules, monomer subunits or nanoparticles. Such an effect is thought to be due to the alleviation of elastic stresses within the bulk phase, or to a solubility gap between a nematic phase and the isotropic defect core. This presents opportunities for encapsulation and sequestration of molecular species, in addition to the formation of novel structures within a nematic phase through polymerization and nanoparticle self-assembly. Here, we examine the solubility of nanoparticles within a coarse-grained liquid crystalline phase and demonstrate the effects of nanoparticle size and surface interactions in determining sequestration into defect regions. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B31.00003: Liquid Crystal Phase Transition driven three-dimensional Quantum Dot Organization Andrea L. Rodarte, R.J. Pandolfi, S. Ghosh, L.S. Hirst We use a nematic liquid crystal (LC) to create organized assemblies of CdSe/ZnS core/shell quantum dots (QDs). At the isotropic-nematic LC phase transition, ordered domains of nematic LC expel the majority of dispersed QDs into the isotropic domains. The final LC phase produces a series of three dimensional columnar QD assemblies that are situated at defect points in the LC volume. Within each assembly the QD emission is spectrally-red-shifted due to resonant energy transfer. We use this spectral shift as a measure of the inter-dot separation and find that the QDs are packed uniformly in these assemblies over distances of microns between the glass plates of a standard LC cell. In addition, because the QD clusters form at defects, we can deterministically control the location of the assemblies by seeding the LC cell with defect nucleation points. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B31.00004: Ordering of Lyotropic Chromonic Liquid Crystal Films In Cylindrical Micropost Arrays Marcello Cavallaro, Matthew Lohr, Daniel Beller, Laura Laderman, Kathleen Stebe, Randall Kamien, Peter Collings, Arjun Yodh The use of micropost arrays is explored as a means for controlling self-assembly and director alignment in nematic chromonic liquid crystal (CLC) films. Experiment and numerical solutions reveal that the micropost arrays induce bistable director alignment in the film, along either diagonal of a square micropost lattice. We demonstrate stabilization of large domains of a single director orientation by rubbing the substrate surface along a single diagonal, a procedure which biases planar CLC director alignment in the film. Additionally, by varying the rubbing angle we investigate the competition between alignment via micropost patterns versus substrate rubbing, and we find the resulting assemblies to be largely controlled by micropost geometry. Variation of micropost layout, spacing and dimensions leads to further interesting self-assembled patterns and defect geometries. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:39PM |
B31.00005: Modelling liquid crystal elastomers and potential application as a reversibly switchable adhesive Invited Speaker: James Adams Liquid crystal elastomers (LCEs) are rubbery materials that composed of liquid crystalline polymers (LCPs) crosslinked into a network. The rod-like mesogens incorporated into the LCPs are have random orientations in the high temperature isotropic phase, but can adopt the canonical liquid crystalline phases as the temperature is lowered. Smectic liquid crystal elastomers have highly anisotropic mechanical behaviour. This arises in side chain smectic-A systems because the smectic layers behave as if they are embedded in the rubber matrix [1]. The macroscopic mechanical behaviour of these solids is sensitive to the buckling of the layers, so is a multiscale problem. A coarse grained free energy that includes the fine-scale buckling of the layers has been developed [2], which enables continuum modelling of these systems. In the first part of this talk I present a model of the mechanical behaviour of side chain smectic elastomers. The properties of nematic LCEs, such as their high loss tangent, and mechanical strain hardening, might enable them to be used as reversibly switchable pressure sensitive adhesive (PSA). PSAs are typically made from viscoelastic polymers. The quality of their adhesion can be measured by the \emph{tack energy}, which is the work required to separate two bodies. To obtain a high tack energy a PSA should be capable of a large strain. It should strain soften at low strain to produce crack blunting, and then strain harden at high strain to stiffen the fibrils formed late in the debonding process. I will present a model of the tack energy of weakly crosslinked nematic polymers. To describe the constitutive properties of this system the nematic dumbbell model of Maffettone \emph{et al.} was used [3]. This constutitive model was then combined with the block model of Yamaguchi \emph{et al.} describing PSAs [4]. It was found that the parallel orientation of the nematic has a higher tack energy than both the isotropic and the perpendicular director orientation [5].\\[4pt] [1] C. M. Spillmann et al, Phys. Rev. E 82, 031705, (2010).\\[0pt] [2] J. Adams, S. Conti and A. DeSimone, Mathematical Models and methods in Applied Sciences, 18, 1 (2008).\\[0pt] [3] P. L. Maffettone and G. Marrucci, Journal of Rheology 36 (8) 1547 (1992).\\[0pt] [4] T. Yamaguchi, H. Morita, and M. Doi, Eur. Phys. J. E 20, 7 (2006).\\[0pt] [5] D. R. Corbett and J. M. Adams, Soft Matter, DOI:10.1039/C2SM26868J (2012). [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B31.00006: Compliant random fields in gels formed from side-chain liquid crystalline polymers Paul Goldbart, Fangfu Ye, Bing Lu, Xiangjun Xing Localized polymer-chain backbones in gels formed from side-chain liquid crystalline polymers serve to create random fields that induce local orientational order of the nematogenic pendants of the side chains. ~These random fields differ, however, from conventional ones, in that they are compliant, and thus themselves undergo thermal fluctuations. ~We develop a free energy that describes local nematic ordering in presence of such compliant random fields. ~In particular, we show that, as a result of this compliance, the free energy has a qualitatively new structure, unattainable via truly static random fields. ~We discuss the physical implications this free energy, focusing on the consequences of the compliant nature of the random fields. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B31.00007: Phase Behavior of Semi-flexible-Coil Block Copolymers Studied by Monte Carlo Simulations Tao Wei, Robert Riggleman Semi-flexible/coil and rod/coil polymers have attracted increasing interest in the applications of organic electronics and biomaterials due to their novel supramolecular structures with nanoscale architecture and tunable domain size. The coupling of microphase separation and liquid-crystalline ordering, stemming from chain rigidity, yields complex phase behaviors. In this work, phase morphologies and phase diagram of semi-flexible/coil block copolymers were identified with efficient Theoretical informed coarse-grained Monte Carlo (TIMC) simulations, which tracks the local density of each grid, rather than computationally demanding pair-wise interactions. Besides the common Flory-Huggins interactions between dissimilar components, we incorporate anisotropic interactions through a Maier-Saup\'e potential. Due to the increased complexity of semi-flexible polymer, parameter number is significantly larger compared to fully flexible polymers. We will illustrate the TIMC method for semi-flexible/coil polymers and examine fluctuation effect on various phase diagrams. We demonstrate the influence of the relative strength of Maier-Saup\'e parameter to Flory-Huggins parameter, as well as the geometric factors that characterize the size of the semi-flexible block relative to the coil block. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B31.00008: Competition of Elasticity and Flexoelectricity for bistable alignment of nematics on patterned substrates Timothy Atherton, James Adler We show that patterned surfaces can promote bistable configurations of nematics for reasons other than the symmetry of the surface. Numerical and analytical calculations reveal that a nematic liquid crystal in contact with a striped surface is subject to the competing aligning influences of elastic anisotropy, differing energy cost of various types of deformation, and flexoelectricity, curvature-induced spontaneous polarization. These effects favor opposing ground states where the azimuthal alignment is, respectively, parallel or perpendicular to the stripes. Material parameters for which the effect might be observed lie within the range measured for bent-core nematogens. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B31.00009: Effect of ionic additives on elasticity of lyotropic chromonic liquid crystal Shuang Zhou, Adam J. Cervenka, Yogesh Singh, Luana T. Tortora, Carmen C. Almasan, Oleg D. Lavrentovich Using a magnetic Frederiks transition technique, we determine how the splay $K_{1}$ and bend $K_{3}$ elastic constants of lyotropic chromonic liquid crystal Sunset Yellow (SSY) depend on concentration of ionic additives, sodium chloride (NaCl) and magnesium sulfate (MgSO$_{4}$). Both salts increase the ratio $K_{1}/K_{3}$, by mainly increasing $K_{1}$ (MgSO$_{4}$) or mainly decreasing $K_{3}$ (NaCl). The effects are attributed to the screening of electrostatic repulsions of chromonic molecules, which is expected to increase the contour length (thus increasing $K_{1}$) and to decrease the persistence length (thus decreasing $K_{3}$) of the chromonic aggregates in which the molecules are stacked face-to-face. As in salt-free SSY, the ratio $K_{1}/K_{3}$ increases when the temperature decreases. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B31.00010: Tactoids and Defects in Nematic-Isotropic Phase Transition in Lyotropic Chromonic Liquid Crystal Young-Ki Kim, Oleg D. Lavrentovich We explore the structure of nuclei (tactoids) and topological defects (disclinations) in the first-order nematic-isotropic phase transition in self-assembled lyotropic chromonic liquid crystals. The shape of tactoids is determined by orientational elasticity of the liquid crystal, surface tension, and surface anchoring of the director. The positive tactoids (nuclei of the nematic phase) show two pointed ends (cusps). The negative tactoids (nuclei of the isotropic phase) show a variety of shapes, with one, two, or three cusps, depending on whether they nucleate at the core of disclinations of strength 1/2, in a homogeneous nematic, or at the core of a (-1/2) disclination, respectively. Zero-cusp and four-cusp formations are also possible at the core of stabilized disclinations of strength 1 and -1, respectively. The results demonstrate a profound role of surface tension and its anisotropy in the morphological dynamics of phase transitions in liquid crystals. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B31.00011: Liquid Crystal Switching Response by Localized Surface Plasmon Induced Electric Fields Zachary Nuno, Linda Hirst, Sayantani Ghosh We investigate the effect of electric fields induced by localized surface plasmons (LSPs) from gold nanoparticles (AuNPs) on the director of a nematic liquid crystal (LC). We deposit LC thin films on a self-assembled AuNP layer and excite the LSPs in the AuNPs using 530 nm excitation light. Using polarized optical microscopy we follow the birefringence of the LC film as the excitation is turned on and off and observe the homeotropic alignment of the LC change to planar. This realignment response is observed to be dependent on the excitation wavelength, excitation power, and temperature; occurring only within 1 degree Celsius of the LC phase transition from nematic to isotropic. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B31.00012: Tunable lithography masks using chiral nematic fluids Hyeon Su Jeong, Mohan Srinivasarao, Hee-Tae Jung We present a facile route for pattern formation using chiral nematic fluids as tunable masks in lithography process. The chiral nematic phase prepared by adding a chiral dopant (CB15) to 5CB acted as a set of parallel cylindrical lenses and as a polarization selective photomask for the preparation of periodic line patterns. The pitch of the helical twist was easily controlled by the concentration of chiral agent and the feature size of the resulting pattern was easily tuned. Because of the high mobility of the small liquid crystalline compound, the preparation of chiral nematic fluids based lithography masks requires only a few seconds. This approach has significant advantages including facility, range of surface ordering, and rate of forming periodic arrays. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B31.00013: Light sensitive liquid crystals: Focusing on surface and bulk transitions Petr Shibaev, Seth Bourg, Shannon Rosario, Daniel Bateman, Andrey Iljin The study of transitions in liquid crystalline matrix resulting from light-induced conformational changes in newly synthesized light sensitive molecules are studied and discussed. Light sensitive molecules (novel chiral and non-chiral azo dyes, spiropyrans) were either embedded in a polymer film serving as a container for liquid crystals or dissolved in a bulk of liquid crystals. In both cases light-induced re-orientation of director was observed in nematic liquid crystals. In chiral liquid crystals the family of regular domains with a different helical pitch was formed. One of the most ordered representatives of this family was observed earlier in [1]. Interestingly, the shape and structural characteristics of the domains were different in liquid crystalline droplets and in the films contained between glass plates. It is possible to freeze the regular structure of the domains in highly viscoelastic liquid crystals and obtain free standing films. The role of viscosity is discussed and a simple theoretical model of observed effects is presented. The studied films can be used in photonic devices and also as detectors of environmental changes. \\[4pt] [1] P. Shibaev. R. Sanford, D. Chiappetta, A. Genack and A. Bobrovsky Optics Express, Vol. 13, Issue 7, pp. 2358-2363 (2005) [Preview Abstract] |
Session B32: Focus Session: Polymer Crystallization and Morphology
Sponsoring Units: DPOLYChair: Xinfei Yu, NIST
Room: 340
Monday, March 18, 2013 11:15AM - 11:51AM |
B32.00001: Disentangled solid state and metastable polymer melt; a solvent free route to high-modulus high-strength tapes and films of UHMWPE Invited Speaker: Sanjay Rastogi Ultra High Molecular Weight Polyethylene (UHMWPE) having average molar mass greater than a million g/mol is an engineering polymer. Due to its light-weight, high abrasion resistance and biocompatibility it is used for demanding applications such as body armour, prostheses etc. At present, because of its high melt viscosity to achieve the uniaxial/biaxial properties in the form of fibers/films the polymer is processed via solution route where nearly 95wt{\%} of the solvent is used to process 5wt{\%} of the polymer. In past several attempts have been made to process the polymer without using any solvent. However, compared to the solvent processing route the achieved mechanical properties were rather poor. Here we show that by controlled synthesis it is feasible to obtain UHMWPE that could be processed free of solvent to make uniaxial tapes and biaxial films, having unprecedented mechanical properties, exceeding that of the solution spun fibers. We address some of the fundamental aspects of chemistry, physics, rheology and processing for the development of desired morphological features to achieve the ultimate mechanical properties in tapes and films. The paper will also address the metastable melt state obtained on melting of the disentangled crystals and its implication on rheology in linear and nonlinear viscoelastic region. Solid state NMR studies will be applied to establish disentangled state in solid state to the polymerisation conditions. References: Macromolecules 2011, 44(14), 5558-5568; Nature Materials 2005, 4, 635-641; Phys Rev Lett 2006, 96(21), 218303-218205. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B32.00002: A simple model for heterogeneous nucleation of isotactic polypropylene Michael Howard, Scott Milner Flow-induced crystallization (FIC) is of interest because of its relevance to processes such as injection molding. It has been suggested that flow increases the homogeneous nucleation rate by reducing the melt state entropy. However, commercial polypropylene (iPP) exhibits quiescent nucleation rates that are much too high to be consistent with homogeneous nucleation in carefully purified samples. This suggests that heterogeneous nucleation is dominant for typical samples used in FIC experiments. We describe a simple model for heterogeneous nucleation of iPP, in terms of a cylindrical nucleus on a flat surface with the critical size and barrier set by the contact angle. Analysis of quiescent crystallization data with this model gives reasonable values for the contact angle. We have also employed atomistic simulations of iPP crystals to determine surface energies with vacuum and with Hamaker-matched substrates, and find values consistent with the contact angles inferred from heterogeneous nucleation experiments. In future work, these results combined with calculations from melt rheology of entropy reduction due to flow can be used to estimate the heterogeneous nucleation barrier reduction due to flow, and hence the increase in nucleation rate due to FIC for commecial iPP. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B32.00003: Monte Carlo Simulations of Strain-induced Polymer Crystal Nucleation Wenbing Hu, Yijing Nie, Huanhuan Gao We performed dynamic Monte Carlo simulations of lattice polymer chains, to investigate primary crystal nucleation induced by a homogeneous stretching at high temperatures. We developed a new scheme to realize a homogeneous stretching of bulk polymer chains with their one chain ends fixed on a shifting plane and the other ends on a fixed plane. We observed a sudden decay of chain-folding probability in those newly emerged small crystallites, which indicated a transition of crystal nucleation from intramolecular mode to intermolecular mode. There exists a competition between two nucleation modes, as revealed by a theoretical fitting of the critical strains for mode transitions at various temperatures. The theoretical estimation is based on the classical nucleation theory. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B32.00004: Solid to solid beta to alpha form transition in crystalline structures of syndiotactic polystyrene (sPS) Tetsu Ouchi, Suguru Nagasaka, Atsushi Hotta The new solid to solid crystalline transition from beta to alpha forms in syndiotactic polystyrene (sPS) was discovered and analyzed. sPS has five crystalline structures: alpha, beta, gamma, delta, and epsilon forms. Among these crystalline structures, alpha and beta forms are two major crystalline structures due to their high mechanical properties. In this research, it was found that the transition was induced by mechanical strain well below the melting temperature of sPS (273 deg C) unlike the reverse transition from alpha to beta form observed at 260 deg C. The transition became more pronounced as testing temperature increased from 130 to 220 deg C. The transition should occur under the interaction of annealing process and mechanical strain, as the transition would not occur just by raising temperature. It was concluded from our experimental results that the destruction of the $\beta $ form first took place under mechanical strain, causing beta form to transform into not only alpha form but also mesomorphic alpha form. The hot environment had a great impact on the perfection and the promptness of the transition: testing temperature could accelerate the transition ending up with more perfect alpha form transformed from beta form through mesomorphic alpha form. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B32.00005: Molecular Structure of Semicrystalline Polyethylene Blends Studied by Broadband Coherent Anti-Stokes Raman Scattering Microscopy Young Jong Lee, Chad Snyder, Aaron Forster, Marcus Cicerone, Wen-li Wu Blends of polyolefins have been widely used to diversify and improve material properties and to avoid complications that exist in blending immiscible heterogeneous polymers. The properties of a blend quite often deviate from predicted ones with the linear combination of its constituent homopolymers or copolymers, likely due to microscopic phase separation and differing degrees of crystallization. However, the current measurement techniques for studying the structure of polyolefin blends are primarily based on bulk averaging methods such as calorimetry or neutron scattering or through solvent extraction of a lower melting component. As a result, development of new blends depends on mostly empirical approached, not based on microscopic structural information. We demonstrate here that a noninvasive imaging technique, based on coherent anti-Stokes Raman Scattering (CARS) microscopy, can provide microscopic structural information of a bimodal polyethylene blend. We discuss new findings of the spatial distribution of species with different molecular architectures and the orientation of their semicrystalline structures. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B32.00006: Enhanced segmental mobility of Poly(lactic acid) in presence of water Omkar Vyavahare, Shaw Hsu The objective of this work was to understand the effect of water on the segmental mobility of Poly(lactic acid) (PLA). When PLA was crystallized by soaking it in water, the crystallization temperature was lowered significantly, allowing PLA to crystallize even below the generally accepted glass transition temperature (T$_{\mathrm{g}})$. PLA crystallized below 90$^{\mathrm{o}}$C has a metastable $\alpha $' crystalline phase. However, in presence of water, it formed a mixture of $\alpha $', and stable $\alpha $ crystalline phases. Water also had a prominent effect on the physical aging of PLA, with the polymer rapidly undergoing densification in the glassy state compared to the dry conditions. Dielectric relaxation studies of PLA with 0{\%} and 13{\%} d-lactide content revealed contradictory influence of water on their respective sub-T$_{\mathrm{g}}$ ($\beta )$ transitions. The $\beta $ transition was suppressed for PLA with no d-lactide content, while it got enhanced for PLA with 13{\%} d-lactide. These experiments demonstrate that water promotes interchain interactions and enhances segmental mobility. This allows the chains to have a conformation which provides an easier pathway with lower energy barrier for the transformation among various polymorphic states. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B32.00007: Molecular engineering of high-performance elastomeric materials Shengwei Deng, Michael Falk Polyurethane is a typical elastomeric material and among the most versatile materials today. It is a linear block copolymer consisting of alternating soft and hard segments with phase separation due to thermodynamic segmental incompatibility. Inspired by the hierarchical structure of spider silk, this kind of block copolymer can be synthesized with two distinct blocks that can differ in their propensity to crystallize. Either the soft or hard segments can be amorphous or semicrystalline. Recent experiments indicate that crystallizable segments lead to higher tensile strength and that systems with crystalline hard segment exhibit better stiffness, strength and mechanical toughness. Here we implement molecular dynamics simulation to investigate the influence of block architectures on mechanical properties and molecular chain movement. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B32.00008: Tuning Properties of Semi-Crystalline Polymers at Constant Crystallinity: Adjusting Rigid Amorphous Fraction and Crystallization Conditions by Solid-State Shear Pulverization Philip Brunner, John Torkelson Semi-crystalline polymers consist of both crystalline and amorphous regions, the latter of which can be subdivided into rigid amorphous fraction (RAF) and mobile amorphous fraction. The RAF does not undergo a glass transition at the measured Tg but may remain rigid up to the melting temperature of the crystalline regions. This means that RAF can quantified by DSC measurements related to the change in heat capacity in going from the glassy to liquid state upon heating. We have discovered that RAF levels in some semi-crystalline polymers can be altered dramatically by solid-state shear pulverization although the crystallinity level remains constant. We take advantage of this to demonstrate how permeation characteristics and mechanical properties of semi-crystalline polymers may be significantly altered by SSSP while maintaining constant crystallinity levels. Examples include nylon 11 in which oxygen permeability can be decreased by 50{\%} due to an increase in RAF, with the permeability reduction caused by nearly equal effects of RAF on solubility and diffusivity. Additionally, major changes in tensile properties of nylon 11 and polycaprolactone can be correlated with changes in RAF at constant crystallinity. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B32.00009: Determining the Heat of Fusion and Crystallization Kinetics of Trogamid Bin Mao, Peggy Cebe Trogamid$^{\mathrm{TM}}$ is a high performance semi-crystalline polyamide with optical clarity, chemical resistance and high toughness. It also has much higher glass transition temperature than traditional polyamides, providing excellent thermal stability. We have used differential scanning calorimetry and real-time synchrotron wide and small angle X-ray scattering to study the isothermal crystallization of Trogamid from the melt at crystallization temperatures between T$_{\mathrm{c}} =$ 221 $^{\circ}$C and 233 $^{\circ}$C. Our goal is to correlate the endothermic area from heat flow measurements with the crystallinity index obtained from X-ray scattering, to provide fundamental thermal information about the heat of fusion of Trogamid, which has not been measured to date. Avrami analysis has also been performed over the same temperature range, and been correlated with spherulitic growth rate data obtained from polarizing optical microscopy. For isothermal melt crystallization, the Avrami exponent, n, ranged from 2.13 to 2.23 for T$_{\mathrm{c}}$ from 230 $^{\circ}$C to 233 $^{\circ}$C. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B32.00010: Unusual Temperature Dependence of the Growth Rate of a Bromine Substituted Polyethylene Rufina G. Alamo, Wei Zhang, Laura Santonja, Emine Boz, Kenneth B. Wagener Precisely halogenated polyethylenes are unique polyolefins with a halogen placed on and every ``n'' number of backbone carbons. Contrasting random analogs, precision systems are highly crystalline developing spherulitic morphologies due to a crystallization pattern similar to that of a homopolymer chain. The halogen is accommodated in the crystalline regions as a defect that strains the chain packing proportionally to the van der Waals radius of the halogen. In the present work, we have studied the temperature dependence of the linear growth rates of a bromine substituted polyethylene on each and every 21$^{\mathrm{st}}$ backbone carbon. The linear growth rates display a discrete minimum with decreasing temperature at a crystallization temperature of 64.5 $^{\circ}$C which is reminiscent of the minimum in crystallization rate observed in long chain $n$-alkanes. The spherulitic morphology and overall positive birefringence remains unchanged. The minimum in growth rate is analyzed on the basis of self-poisoning at the growth front resulting from frequent but unstable disordered chains depositions that accommodate the Br atoms. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B32.00011: A Fast Scanning Calorimetric Comparison Study of Crystallization Behavior between Semi-crystalline Polymers and Liquid Crystals Dongshan Zhou, Jing Jiang, Lai Wei, Zhijie Huang, Gi Xue Mesomorphic state with similar liquid crystal order was found to precede the crystallization in many polymers, so the study of nucleation and crystallization from a liquid crystal can provide reference for the study of polymers. The same procedure to study the nucleation and crystallization of semi-crystalline polymers was used to study 4-cyano-4'-octyloxy biphenyl-carbonitrile (8OCB). Different from metastable semi-crystalline polymers of multi-folded chains, whose melting temperature was basically continuously dependent on the crystallization temperature, melting temperature of 8OCB should have definite values, corresponding to disordering of four different polymorphism modifications at 309.0 K, 319.0 K, 325.0 K, and 327.0K, respectively. But, a lower temperature melting peak below 300K was found when 8OCB was annealed at temperature below 250K. More importantly, the peak temperature shifted positively with the increasing annealing temperature, just the same as that of semi-crystalline polymers. At the moment, we were not sure about the structure of the metamorphism and why small molecular liquid crystal showed similar melting behavior that was thought only inherited to chain like semi-crystalline polymers. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B32.00012: Unusual ``Twisting'' Morphology in Poly(3-hydroxybutyrate-co 3-hydroxyhexanoate) and Poly(bisphenol A hexane ether) Spherulites Jerold Schultz Polarized light images of poly(3- hydroxybutyrate-co 3-hydroxyhexanoate) spherulites grown from the melt exhibit the standard evidence of periodic twisting of lamellae. AFM images of lamellae growing from the melt, on the other hand, reveal a sudden change in orientation and a trowel-like morphology. Similarly, AFM images of poly(bisphenol A hexane ether) (BA-C6) lamellae growing from the melt show a sudden orthogonal change of orientation. It is suggested that chain extension in the melt near the propagating front forces the observed reorientation, possibly through creation of crystals with an orientation approximately orthogonal to that of the original crystals. A rudimentary model for this behavior is proposed. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B32.00013: Crystal Pattern and Orientation Structure of Poly Ethlyene Oxide at Surface Qi Liao We try to develop a new morphological method to estimate the orientation structure of polymer crystal at the surface quantitatively. The crystalline structures of PEO single crystals on PVPY substrates were studied in dependence on the degree of supercooling. We show that the diverse patterns could be explained by the difference of crystal orientation. The edge-on and flat-on structure, as well as the patterns in the cross-over states, could give the information of molecular structure. [Preview Abstract] |
Session B33: Focus Session: Dielectric and Ferroelectric Polymers for Electrical Applications: Ferroelectrics
Sponsoring Units: DPOLY DMPChair: Philip Taylor, Case Western Reserve University
Room: 341
Monday, March 18, 2013 11:15AM - 11:51AM |
B33.00001: Phase Transitions as a Novel Mechanism for High-Speed Energy Storage Invited Speaker: Jerry Bernholc In many energy applications there is an urgent need to store and quickly discharge large amounts of electrical energy. Since capacitors can be discharged far quicker than batteries and fuel cells, they have much higher power densities. At present, highly insulating polymers with large breakdown fields, such as polypropylene, are the dielectrics of choice in high-power capacitors. However, their energy densities are quite low because of small dielectric constants. Ferroelectric polymers from the PVDF family have significantly larger dielectric constants, yet their energy densities are still rather low. This can be traced to early saturation of their displacement fields with the applied electric field, and to somewhat lower breakdown fields. However, an admixture of a small amount of another polymer, such as CTFE, results in a dramatic increase in the stored energy [1]. We show that this highly non-linear increase in the energy density is due to the formation of disordered nanodomains with different copolymer concentrations, which undergo first-order non-polar to polar phase transitions with an increase of the applied field. The resulting energy density profile reproduces well the experimental data, while its variation with co-polymer concentration and distribution suggest avenues for additional substantial improvements in the stored energy [2]. Most recently, we have identified a low-activation-energy pathway for these successive phase transformations [3]. It provides further confirmation of the viability of the suggested energy storage mechanism and also enables fine-tuning of the kinetics of energy release by informed choices of suitable co-polymers.\\[4pt] [1] Chu et al, Science 313, 334 (2006).\\0pt] [2] V. Ranjan L. Yu, M. Buongiorno-Nardelli, and J. Bernholc, PRL 99, 047801 (2007).\\[0pt] [3] V. Ranjan, M. Buongiorno Nardelli, and J. Bernholc, PRL 108, 087802 (2012). [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B33.00002: Constrained Molecular Dynamics Modeling of Dielectric Response in Polar Polyethylene Analogs and Poly(vinylidene flouride) Jeffrey Calame A simplified molecular dynamics formalism for polymers, having united atoms with constrained bond lengths and bond angles along the backbone but allowing torsional motion, has been developed to model the dielectric response and ferroelectricity in polymers with permanent dipoles. Analytic relations existing on the backbone geometry and associated dihedral motion allow elimination of many dot and cross product evaluations. Also, constraint error correcting forces, symplectic integration with velocity prediction, random force excitation with damping and a momentum-conserving thermostat, and rapid neighbor list and long range force computation allow efficient computation and time steps as large as 20 fs to enable the study of relatively long time scale dielectric phenomena. Studies are performed on non-polar polyethylene for benchmarking, followed by a model system (polar polyethylene) which retains the molecular structure, dihedral potentials, and non-bonded interactions of polyethylene, except artificial partial charges are placed on the united atoms. The modeling is extended to poly(vinylidene fluoride) by changes to the molecular structure, potentials, and charges. Heterogeneous systems containing crystalline and amorphous arrangements of polymer chains are studied. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B33.00003: Crystal Orientation and Temperature Effects on the Double Hysteresis Loop Behavior of a PVDF-$g$-PS Graft Copolymer Lei Zhu, Lianyun Yang, Fangxiao Guan In a recent report, double hysteresis loop behavior is observed in a nanoconfined poly(vinylidene fluoride-\textit{co}-trifluoroethylene-\textit{co}-chlorotrifluoroethylene)-\textit{graft}-polystyrene [P(VDF-TrFE-CTFE)-$g$-PS] copolymer. It is considered that the PS grafts are capable of reducing the compensation polarization and thus the polarization electric field during the reverse poling process, resulting in the double hysteresis loop behavior. In this study, we further investigated crystal orientation and temperature effects on this novel ferroelectric behavior. It is observed that with increasing the orientation factor, the electric displacement-electric field (D-E) loop changes from linear for non-oriented film to double loop for the well-oriented film. With increasing the temperature, the double hysteresis loop is gradually replaced by the single and open loop, which is attributed to the impurity ion migrational loss in the sample. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B33.00004: Polarization Mapping in Ferroelectric Polymer Thin Films by Pyroelectric Scanning Microscopy Jingfeng Song, Stephen Ducharme High-resolution mapping of polarization distribution in P(VDF-TrFE) Langmuir-Blodgett film was carried out through pyroelectric scanning microscopy with a focused 405nm blue diode laser beam. A lateral resolution of 500 nm was achieved by modulating the laser power at high frequency. At frequencies above 1 MHz, the laser spot size, rather than the thermal diffusion, becomes the limiting factor in the lateral resolution. The experimental results were compared to computer models developed with the finite element method. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B33.00005: Interfacial polarization and internal electron tunneling effect on dielectric properties of multilayer polymer films Jung-Kai Tseng, Zheng Zhou, Matt Mackey, Joel Carr, Eric Baer, Lei Zhu Due to large contrasts in dielectric constant and volumetric conductivity, Maxwell-Wagner-Sillars interfacial polarization is observed in poly(vinylidene fluoride) (PVDF) based multilayer films. This interfacial polarization is helpful to enhance the breakdown strength of multilayer films, because they serve as electron traps to prevent hot electron thermal runaway. In this study, the relationship between volumetric resistivity and internal electron tunneling in polysulfone (PSF)/(PVDF) multilayer film is reported. In general, resistivity decreases with decreasing the thickness of the insulating PSF layer. This is attributed to the internal electron tunneling in thin PSF layers. As a result, the electron-hole neutralization via the PSF layer decreases the interfacial polarization in the PVDF layer, resulting in a lower volumetric resistivity. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B33.00006: Ab-initio study of high energy storage in polymers: PVDF-BTFE Rui Dong, V. Ranjan, M. Buongiorno-Nardelli, J. Bernholc Previous experiments [1] and our theoretical work [2] have indicated that introducing CTFE monomers in polyvinylidene fluoride (PVDF) in small concentration can lead to ultra high density capacitive energy storage. Our previous work indicates that this is due to (i) formations of domains with different impurity concentrations, and (ii) existence of a low energy barrier path connecting ground state non-polar phase to a polar phase. We are now investigating bromo-triflouroethylene (BTFE) in a PVDF-BTFE as a potential high energy density material. Our results show that PVDF-BTFE prefers the nonpolar phase up to a higher concentration of 33\%, as compared to PVDF-CTFE(17\%). This could lead to a higher proportion of PVDF sample being available for phase transition under the electric field. The calculated energy barriers for the electric-field-induced phase transition are also low and comparable to PVDF-CTFE. We will discuss the calculated phase equilibria and the potential of PVDF-BTFE for high density capacitive energy storage.\\[4pt] [1] B. Chu et al., Science 313, 334 (2006).\\[0pt] [2] V. Ranjan et al., PRL 108, 087802 (2012); PRL 99, 47801 (2007). [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B33.00007: Effect of crystal isomorphism on novel ferroelectric behaviors of P(VDF-TrFE)-based copolymers Lianyun Yang, Xinyu Li, Qiming Zhang, Lei Zhu Novel ferroelectric behaviors of poly(vinylidene fluoride-\textit{co}-trifluoroethylene) [P(VDF-TrFE)]-based copolymers, including relaxor ferroelectric and double hysteresis loop behaviors, have drawn great attention in research. Despite of a great amount of work have been done over the last two decades, the fundamental understanding of these behaviors is still lacking. In this work, the physics behind these novel ferroelectric behaviors are discussed based on the studies of P(VDF-TrFE)-based terpolymers and e-beam irradiated P(VDF-TrFE). We find that crystal isomorphism in P(VDF-TrFE)-based copolymers has a significant effect on the dielectric properties. This is achieved by pinning the polymer chains with structural defects. Consequently, nanodomains and easy dipole switching are responsible for the novel ferroelectric behaviors. This understanding will help us to further design new polymers with better dielectric/ferroelectric properties. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B33.00008: Ferroelectric Polymer Composite with Enhanced Breakdown Strength Kuo Han, Matthew Gadinski, Qing Wang Numerous efforts have been made in the past decades to improve the energy storage capability of dielectric capacitors by incorporating ceramic addictives into polymers. Ferroelectric polymers have been particularly interesting as matrix for dielectric composites because of their highest dielectric permittivity and energy density. However, most polymer composites suffer from significantly reduced breakdown strength, which compromises the potential gain in energy density. In this work, various metallic alkoxide were introduced into the functionalized ferroelectric poly(vinylidene fluoride-\textit{co}- chlorotrifluoroethylene), P(VDF-CTFE), via covalent bonding. The composite with the optimized composition exhibited the Weibull statistical breakdown strength of 504.8 MV/m, 67.6 {\%} higher than the pristine polymer. The enhanced breakdown strength was mainly ascribed to the cross-linking and the formation of deep traps, which effectively reduced the conduction and further lowered the energy loss. Additionally, the homogeneous dispersion of the inorganic phase and the small contrast in permittivity between the polymer and amorphous oxides also contribute to the improved dielectric strength. The dielectric spectra of the composites have been recorded at varied temperatures and frequencies, which revealed the presence of the interfacial polarization layer in the composites. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B33.00009: Effect of Polymer Blocking Layer and Processing Method on the Breakdown Strength and the Extractable Energy Density of Barium Titanate/poly(vinylidene fluoride-co-hexafluoropropylene) Nanocomposite Thin Film Capacitors Yunsang Kim, Mohanalingam Kathaperumal, O'Neil Smith, Ming-Jen Pan, Joseph Perry Polymer-metal oxide nanocomposites are of great interest because of their high energy density and easy processability, which make them candidate materials for energy storage applications. Although loading of high-k filler in polymer matrix is desirable to maximize energy density of nanocomposites, the decrease of breakdown strength at higher loading compromises a potential gain in energy density. In this work, we investigate the effect of a fluoropolymer (CYTOP) blocking layer in BaTiO$_{3}$/poly(vinylidene fluoride-co-hexafluoro propylene) nanocomposite films on the improvement of breakdown strength and energy storage density. The introduction of blocking layer may serve to prevent moisture absorption and charge injection from electrode, thereby decreasing the probability of catastrophic breakdown events. We also examine the influence of processing method, i.e. spin- or blade-casting, on the performance of bilayer films. The charge-discharge method shows about a twofold increase in extractable energy density (from 2 to 3.7 J/cm$^{3}$) of bilayer films fabricated by blade-casting compared to single layer film by spin-casting because of improved breakdown strength. The results will be discussed in regards to morphology, electric field distribution, and loss of bilayer films. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B33.00010: Relaxor Ferroelectric Behavior in Poly(vinylidene fluoride-co-bromotrifluoroethyene) Matthew Gadinski, Qing Wang Copolymers of vinylidene fluoride (VDF) and bromotrifluoroethylene (BTFE) were prepared over a composition range up to the disappearance of crystallinity ($\sim$ 9 mol {\%} BTFE). The resulting copolymers were characterized by $^{19}$F NMR to elucidate composition and quantify the linkage defects of the polymer chain. Chain conformations were analyzed by FTIR analysis and crystal structures were studied by DSC and WAXD. The dielectric properties were evaluated by dielectric spectroscopy as a function of frequency and temperature and at high fields to investigate the influence of BTFE on dielectric behavior. The results indicate that the P(VDF-BTFE) copolymers exhibit relaxor ferroelectric behavior similar to those reported in PVDF terpolymers resulting from a mixed $\alpha $/$\gamma $ crystalline phase. Effects of processing such as stretching and crosslinking have also been studied with respect to the relaxor ferroelectric properties of the copolymers. Stretching was found to improve the breakdown strength of the films along with enhancing both the stored and discharged energy density. Preliminary crosslinking results indicate that stretched and cross-linked polymer show a reduced remnant polarization. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B33.00011: BaTiO3 and polypropylene nanocomposites for capacitor applications Daxuan Dong, Longxiang Tang, Lei Zhu, Je Kyun Lee A novel strategy to uniformly disperse 70-nm BaTiO3 ferroelectric nanoparticles in a dielectric polypropylene (PP) matrix is developed in order to achieve high dielectric constant and high energy density for capacitor applications. By modifying BaTiO3 surface with a bis-phosphonic acid-terminated polyhedral oligomeric selsisquioxane (POSS), a nanocomposite with BaTiO3@POSS uniformly dispersed in PP matrix was achieved. The nanocomposite film containing a high nanoparticle content of 30 vol.{\%} exhibited a high dielectric constant of 32 and a breakdown voltage of 220 MV/m, but with a high energy loss. Improvement of this nanocomposite by understanding the interfacial polarization is carried out in this work. The dielectric constant difference between BaTiO3 and PP can generate interfacial polarization and subsequent internal conduction in BaTiO3 particles upon bipolar polarization. Reduction of this internal conduction mechanism will significantly reduce the hysteresis loss in polymer nanodielectrics. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B33.00012: Dielectric Bilayer Films Comprising Polar Cyanolated Silica Sol-Gel and Nanoscale Blocking Layer for Energy Storage Applications Mohanalingam Kathaperumal, Yunsang Kim, O'Neil Smith, Amir Dindar, Canek Fuentes-Hernandez, Do-Kyung Hwang, Ming-Jen Pan, Bernard Kippelen, Joseph Perry Organic-inorganic hybrid sol-gel containing polar groups, which can undergo orientational polarization under the influence of an electric field, provide a potential route to processable and rational design of materials for energy storage applications. However, the porous nature of sol-gel films, which significantly lowers breakdown strength, limits the potential of this material for energy storage particularly in high-field applications. In this work, we fabricate and characterize dielectric bilayer films comprising cyanolated silica sol-gel film prepared from 2-cyanoethyltrimethoxysilane (CNETMS) precursor and nanoscale blocking layers, which include amorphous fluoropolymer, SiO$_{2}$, Al$_{2}$O$_{3}$ and ZrO$_{2}$ deposited by spin casting, electron beam evaporation or atomic layer deposition (ALD). CNETMS films with 50 nm ZrO$_{2}$ blocking layer exhibit an extractable energy density of 13 J/cm$^{3}$, which is about a twofold enhancement compared to CNETMS films without blocking layer. The effect of the blocking layer will be discussed in terms of surface morphology, dielectric contrast, i.e. the ratio of relative permittivity between oxide layer and sol-gel film, electric field distribution, breakdown strength and statistics, bias polarity, and loss of the bilayer films. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B33.00013: A variational formulation of electrostatics for heterogeneous dielectric media Francisco Solis, Vikram Jadhao, Monica Olvera de la Cruz Many biological and synthetic soft matter systems involve fixed or mobile charges. The electrostatic interactions between these charges often play crucial roles in determining the structural properties and physical behavior of these systems. Coarse-graining of the properties of these systems often leads to consideration of free charges embedded in a medium with varying dielectric permittivities. Investigation of the behavior of these systems by theoretical or computational methods requires, therefore, formulations of their electrostatic properties that suitably address the properties of the medium. In this talk we present a new and powerful variational formulation of the electrostatics of charged particles in heterogenous media. Our formulation replaces the electric and polarization vector fields for the induced polarization charge density at interfaces. In addition, this variational principle has the property of evaluating to the true free energy of the system at its minimum; a property not found in many other variational formulations. We discuss the application of this functional to a variety of electrostatic problems and show how it allows the development of new algorithms for simulation of charged systems in heterogeneous media. [Preview Abstract] |
Session B34: Focus Session: Dynamics of Glassy Polymers Under Nanoscale Confinement
Sponsoring Units: DPOLYChair: Zahra Fakhraai, University of Pennsylvania
Room: 342
Monday, March 18, 2013 11:15AM - 11:27AM |
B34.00001: Self-Diffusion of Poly(isobutyl methacrylate) in Thin Films Joshua Katzenstein, Dustin Janes, Haley Hocker, Justin Chandler, Christopher Ellison While relevant to a variety of applications, such as nanocomposite intercalation and molecular transfer printing, the diffusion of polymers parallel to their confining interfaces has received limited experimental attention to date. A refinement of fluorescence recovery after patterned photobleaching (FRAPP) has been developed by our group as a versatile platform for understanding nanoconfined diffusion. Poly(isobutyl methacrylate) (PiBMA) is an ideal candidate for these studies because (in quartz or silicon wafer supported thin films) it exhibits a film thickness independent glass transition temperature (Tg). This is important because, according to the Rouse model, the diffusion coefficient does not depend simply on the absolute temperature, but on the distance from Tg. Therefore, in our systems the origin of the diffusion coefficient is possibly decoupled from Tg changes that are present in other polymer systems. In this talk, the effect of a variety of parameters, such as film thickness, diffusion temperature, and confining interfaces, will be discussed. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B34.00002: Viscosity of poly(methylmethacrylate) films on silicon Ophelia K. Tsui, Ranxing N. Li, Dongdong Peng Previously we showed that the viscosity of polystyrene films on silicon decreased noticeably with decreasing film thickness when the film thickness was decreased below about 10 nm. Moreover, the result could be explained by using a two-layer model presuming a hydrodynamic coupling between a mobile interfacial layer, located at the top, and the remaining, bulk-like layer underneath it. In this experiment, we study the viscosity of poly(methylmethacrylate) (PMMA) films supported by silicon. Contrary to the result found of the polystyrene films, the viscosity of the PMMA films increases with decreasing film thickness. The two-layer model still applies, but the interfacial layer has to be assumed to be slow and located at the substrate interface, beneath the bulk-like layer. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B34.00003: Relaxation of wrinkles: A new viscoelastic metrology Kamil Toga, Narayanan Menon, Thomas Russell The relaxation of a wrinkle pattern can be exploited as a viscoelastic metrology. We used spin-coated polystyrene (PS) films (thickness ranging from 69 to 299 nm) that were floated on the surface of water. Viscoelastic behavior is introduced to the film by depressing the glass transition of PS with a soluble plasticizer, dioctyl phthalate. Wrinkle patterns are formed by placing a small droplet (1$\mu $L) at the center of the floating disc. Due to the differential tension generated across the film, radial wrinkles form around the drop where the compressive axial force buckles the membrane. Thereafter, length of the wrinkles decays, and so does their wavelength. Stress and strain exerted by the droplet can be measured as a function of the size of the wrinkles. Hence, extensional slow-rate-viscosity is calculable. We have studied the relaxation of wrinkles as a function of confinement and plasticizer content. Unusual dynamic behavior due to confinement was observed. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B34.00004: Probing nano-rheology in thin polymer films Invited Speaker: Kari Dalnoki-Veress In this talk I will summarize our recent work on using stepped films to uncover some of the physics relevant to polymer rheology on length scales comparable to the size of polymer molecules. The work presented will focus on the efforts of a larger collaboration (Elie Raphael's theory group in Paris and James Forrest's group in Waterloo). The simple geometry of a polymer film on a substrate with a step at the free surface is unfavourable due to the excess interface induced by the step. Laplace pressure will drive flow within the film which can be studied with optical and atomic force microscopies. Because of the excellent agreement between theory and experiment when we probe ``bulk-like'' properties, these studies provide an opportunity to study how such systems transition from the bulk to confined. Starting with some of the results of levelling experiments on simple stepped films as well as the levelling of polymer droplets on thin films, I will finish with a discussion on our more recent efforts to elucidate confinement effects. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B34.00005: Structural relaxation of thin polymer films Bradley Frieberg, Emmanouil Glynos, Georgios Sakellariou, Peter Green Time-dependent structural relaxations, physical aging, of films with thicknesses in the range of 50 nm to 2 microns, of star-shaped polystyrene (SPS) macromolecules are dependent on film thickness, H. In contrast to linear chain PS (LPS) where the aging rate, R, is independent of molecular weight, M, R is dependent on the functionality, f, and on the molecular weight per arm, Marm for SPS macromolecules. For example, the aging rates decreased 15 percent, for f of 8, and 40 percent, for f of 16, in comparison to that of linear chains, for a given film thickness. The aging rates, R, of the SPS macromolecules moreover are appreciably slower than their linear chain analogs, for a given H. The aging rates of the linear chain and star-shaped polymer films may be reconciled in terms of a model that accounts for changes in the local glass transition of the polymer films as a function of distance from an interface. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B34.00006: Capillary-driven flow as a probe of enhanced surface mobility in glassy polymer films Yu Chai, Thomas Salez, Joshua D. McGraw, Elie Raphael, James A. Forrest We present the use of a novel experimental arrangement [\textit{McGraw et al. Soft Matter }\textbf{\textit{7}}\textit{, 7832 (2011)] }to directly distinguish the dynamical behavior and heterogeneity of polymer thin films above and below the glass transition temperature T$_{\mathrm{g}}$. In particular, by monitoring the capillary-driven evolution of a stepped thin polystyrene film over a temperature range encompassing the bulk T$_{\mathrm{g}}$ value, we find evidence suggesting enhanced surface mobility. Furthermore, by varying the initial aspect ratio of the sample we can examine the heterogeneity of the sample dynamics. The results of these experiments above T$_{\mathrm{g}}$ are consistent with homogenous viscous flow [\textit{McGraw et al. PRL }\textbf{\textit{109}}\textit{, 128303 (2012)],} whereas those below T$_{\mathrm{g}}$ indicate a localization of the flow over a thin surface layer only. We thus develop a linear thin film equation for superficial viscous flows, which is analogous to the surface diffusion model, and for which exact analytical solutions are known and in good agreement with the present experimental data. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B34.00007: Convergence to Self-Similar Regimes in Thin Polymer Films Michael Benzaquen, Thomas Salez, Elie Rapha\"el The surface of a thin liquid film with nonconstant curvature is unstable, as the Laplace pressure drives a flow mediated by viscosity. Recent experiments and theory applied to stepped polymer films have shown excellent agreement and provide a technique for the study of polymer confinement, the glass transition, and slip at the fluid substrate interface to name a few [1]. The thin film equation governs the evolution of the free surface profile in the lubrication approximation. Despite many efforts, this equation remains only partially solved. We present an analytical and numerical study of the thin film equation. Linearising this equation enables us to derive the Green's function of the problem and therefore obtain a complete set of solutions. We show that the solutions of the problem with equilibrium boundary conditions uniformly converge in time towards a first kind self-similar universal attractor. A numerical study enables us to extend our results to the nonlinear thin film equation.\\[4pt] [1] McGraw \textit{et al.} PRL \textbf {109} 128303 (2012). [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B34.00008: The Onset of Plasticity in thin Polymer Films Bekele J. Gurmessa, Andrew B. Croll Polymers are widely used materials because of their numerous advantageous mechanical properties, for example their high degree of toughness. Despite the fundamental importance of the onset of plastic deformation to many material processes, it is still relatively poorly defined in the literature. Here we employ a carefully designed experimental method in order to evaluate the point of onset of plasticity in thin, glassy polystyrene films. Essentially we utilize the residual stress caused by local bending in the thin film. We show that plastic failure is initiated at extremely low strains, of order 0.1\% for polystyrene. Not only is this critical strain small in comparison to bulk measurement, we also show that it is influenced by thin film confinement - leading to an increase in the critical strain for plastic failure as film thickness approaches zero. Finally, the same experimental method is used to investigate the response of confined block copolymer thin films in the ordered and disordered state. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B34.00009: Competitive effects in the dynamics of confined ultra-thin polymer films Chrysostomos Batistakis, Alexey Lyulin, Thijs Michels Fillers, as carbon black or silica, are widely used in polymer systems to improve mechanical properties. In high volume fractions, these fillers connect to each other through polymeric bridges and they create a percolation network inside the polymer matrix. The rigidity of this network depends on the filler volume fraction and is rapidly breaking down under loading. The scope of this work is to understand the polymer dynamical behavior of the interparticle polymeric bridges. For that purpose we have performed molecular-dynamics (MD) simulations on coarse-grained polymer films which are confined between two crystalline Lennard-Jones substrates for different substrate-substrate separations. Various polymer-substrate attraction strengths have been chosen. The polymer structure and segmental dynamics in different film layers has been analyzed. We found that increasing attraction strength leads to deceleration of the film dynamics due to a slowing down close to the substrates, but decrease of film thickness leads to an acceleration of these dynamics. We attribute this acceleration to finite-size scaling effects. For thick films an acceleration of dynamics in the middle takes place for sufficiently high attraction strengths due to the effective increase of the glassy layers thicknesses [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B34.00010: Insight into Polymer De-wetting: A Neutron Reflectivity Study of Three-Arm Polystyrene Stars in Polystyrene Thin films Thusitha Etampawala, Nampueng Pangpaiboon, Dvora Perahia, Candice Halbert, Jim Browning, Nisanart Traiphol, Rakchart Traiphol While polymeric coatings are ubiquitous, de-wetting remains a challenge. As both enthalpic and entropic contributions often affect the de-wetting process, small changes either compositional or in processing conditions are sufficient to impact the stability of thin films. We have recently shown that blending small amounts of three-arm polystyrene (PS) star polymers are sufficient to inhibit de-wetting of thin polystyrene thin films. The role of the three-arm star has been investigated using neutron reflectometry. We have followed the distribution of the three-arm PS stars in a thin film of d-PS as function of time as the temperature was raised above Tg of the PS. Films of d-PS/h-three-arm star PS were cast from toluene and the polymer profiles were determined as a function of time as the temperature was varied. The result show a clear migration of the three-arm stars to both interfaces, enhancing the number of chain ends at the interface. As the molecular weights of the star arm increases, it migrates slower to the interface. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B34.00011: Effects of Solvents on Confinement of Conjugated Polymer into Soft Nanoparticle Naresh Osti, Thusitha Etampawala, Umesh Shrestha, Sidath Wijesinghe, Dvora Perahia Conjugated polymers when collapsed into nano dimension form soft nanoparticles (poly-dots) without losing their electro-optical characteristics. The brightness together with the bio-compatibility of these nanoparticles has shown significant potential in intracellular fluorescence imaging as well as building blocks for light harvesting devices. The conformations of the polymers in the poly-dots are a key to their stability and optical properties. The current work investigates the structure and stability of poly-dots of \textit{di-alkyl} \textit{para polyphenyleneethynylene} (PPE) conjugated polymers in ethylene glycol and in water. Small angle neutron scattering (SANS) studies have shown that PPEs form spherical fuzzy poly-dots both in water and in ethylene glycol. In water, the poly-dots remain fairly stable up to a temperature of 80$^{0}$C. In ethylene glycol however the poly-dots swell with increasing temperature. The structure of the confined structure obtained from SANS is compared with fluorescence spectroscopy results where the intensity of the fluorescence is inversely proportional to the degree of confinement. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B34.00012: Probe of Dynamic Heterogeneity in Freeze-dried Polymer with similarities to thin film Jie Xu, Chao Teng, Gi Xue Understanding the dynamics of polymer chains in confined states is still crucial in the field of soft condensed matter. Dynamic heterogeneity is widespread in the confined system and could strongly alter the overall dynamics, such as in the experimental case of a free surface or a held fixed region. In this work, we show the dynamic heterogeneity in the freeze-dried polystyrene system through a combination of fluorescence nonradiative energy transfer (NRET) method, TMA, and PALS. The NRET data shows that the interchain distance could be altered by other the primitive solution concentration of the freeze-dried PS or the thickness of the free-standing film. Striking similarity of interchain packing density effect on the Tg is find the freeze-dried systems and free-standing films. The application of stress can also make a glass flow. We applied a uniaxial stress on the freeze-dried PS, a shear-induced flow is curried in the region with reduced interchain packing density. The polymer chains in this region show increased segmental mobility, which prompts the shear-induced solid-to-fluid transition to happen well below the bulk glass transition temperature. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B34.00013: Entanglement Density Changes in Free-Standing Thin Polymer Films Joseph Stanzione, Richard Wool The entanglement molecular weight M$_{\mathrm{e}}$ is obtained when a random walk chain crosses a plane three times to form a loop (R.P. Wool '83) such that for polymers with structure --CH$_{\mathrm{2}}$-CHX- where X is the side group, it is found that M$_{\mathrm{e}} =$ 31 C$_{\infty}$ M$_{\mathrm{o}}$/j, where C$_{\infty}$ is the characteristic ratio, M$_{\mathrm{o}}$ is the monomer mol weight and j$=$2 is the number of bonds per monomer. In thin films of thickness d \textless\ 2R$_{\mathrm{g}}$, M$_{\mathrm{e}}$ behaves as M$_{\mathrm{e}}$ $\sim$ d and this behavior is confirmed by computer simulation of random walks in thin films with reflecting boundary conditions. Thus, the entanglement density v $\sim$ 1/M$_{\mathrm{e}}$ increases as d decreases and rheological properties such as plateau modulus change as G$_{{\mathrm{N}}^{\mathrm{o}}}$ $\sim$ 1/d and plateau creep compliance J$_{\mathrm{o}}$ $\sim$ d. The mechanical stiffening of thin films is in accord with recent experiments of McKenna et al (2012). The results are also in accord with the Packing model (Lin, Kavassil, Fetters 1983) where M$_{\mathrm{e}} =$ 354 p$^3$ in which p $=$ M$_{\mathrm{o}}$ j/[C$_{\infty}$ b$_{\mathrm{o}}^2$]. The packing model is exactly derived from the Wool entanglement model for these polymers since C$_{\infty} =$ 1.36 [M$_{\mathrm{o}}$/j]$^{\mathrm{1/2}}$. The empirical packing model with its excellent data correlation M$_{\mathrm{e}}$ $\sim$ p$^3$ has been misinterpreted by many and such suggestions that v decreases due to nanoconfinement and G$_{{\mathrm{N}}^{\mathrm{o}}}$ $\sim$ d are incorrect. [Preview Abstract] |
Session B35: Superconductivity: Spin Properties
Sponsoring Units: DCMPChair: Yaroslaw Bazaliy, University of South Carolina
Room: 343
Monday, March 18, 2013 11:15AM - 11:27AM |
B35.00001: Magnetic excitations in the high-T$_{\mathrm{c}}$ superconductor HgBa$_2$CuO$_{4+d}$ at low doping Chelsey Dorow, M.K. Chan, Y. Tang, G. Yu, Yuan Li, N. Barisic, J. Park, O. Sobolev, A. Teichert, Y. Sidis, P. Steffens, D. Abernathy, X. Zhao, P. Bourges, M. Greven We report on the observation of magnetic excitations in the very underdoped regime of the high-T$_{\mathrm{c}}$ superconductor HgBa$_{2}$CuO$_{4+d}$ (Hg1201). Our previous inelastic neutron scattering measurements of optimally doped (T$_{\mathrm{c}}$ $\approx $ 95 K) and moderately underdoped (T$_{\mathrm{c}} \approx $ 65 K) samples revealed two novel, weakly-dispersive magnetic excitation branches below the pseudogap temperature T* [Y. Li et al., Nature 468, 283 (2010); Y. Li et al., Nature Phys. 8, 404 (2012)]. These excitations are associated with the translational symmetry preserving magnetic order previously established to be a universal property of the pseudogap phase [B. Fauqu\'{e} et al., Phys. Rev. Lett. 96, 197001 (2006); Y. Li et al., Nature 455, 372 (2008); Y. Li et al. Phys. Rev. B 84, 224508 (2011)]. In YBa$_{2}$Cu$_{3}$O$_{6+d}$, the strength of this order was found to decrease in very underdoped samples [V. Bal\'{e}dent et al. Phys. Rev. B 83, 104504 (2011)]. Indeed, we find no evidence of pseudogap excitations in very underdoped Hg1201 (T$_{\mathrm{c}} =$ 45 K), and instead we observe strong antiferromagnetic fluctuations over a large energy range (10 -150 meV). [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B35.00002: Unusual form factor of the novel pseudogap excitations in HgBa$_{2}$CuO$_{4+\delta}$ Mun Chan, C. Dorow, Y. Tang, G. Yu, M. Greven, N. Barisic, Y. Li, K. Hradil, R. Mole, P. Steffens, X. Zhao, Y. Sidis, P. Bourges Following the discovery of a universal novel magnetic order in the pseudogap phase of the cuprates [B. Fauqu\'{e} et al. PRL 96, 197001 (2006); Y. Li et al., Nature 455, 372 (2008)], our inelastic neutron scattering measurements of HgBa2CuO4$+\delta $ (Hg1201) revealed two weakly-dispersive excitation branches associated with this ordered state [Y. Li et al., Nature 468, 283 (2010); Y. Li et al., Nature Phys. 8, 404 (2012).]. The dependences of the mode intensities on the momentum transfer Q $=$ (HKL) (r.l.u.) are inconsistent with traditional magnetic or structural form factors. The intensity of the high-energy mode is zero when Q is parallel to the copper-oxygen planes (i.e., for L$=$0), peaks at L $=$ 8 (r.l.u.), and decreases again at large L. We observe the opposite behaviour for the low- energy mode, which is strongest when L$=$0. In combination with polarized inelastic neutron scattering results, this indicates possible dual magnetic and structural characteristics of the novel excitations. Work supported by DOE-BES. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B35.00003: Polarised neutron study of the ``even'' and ``odd'' magnetic excitations in YBa$_2$Cu$_3$O$_{6.9}$ Christopher Lester, Stephen Hayden, Jiri Kulda, David Cardwell, Nadendla Hari Babu On cooling through $T_c$, the spin excitation spectra of cuprate superconductors becomes dominated by the neutron spin resonance (NSR), a collective mode centred at $\mathbf{Q}_{AF}$. We have used polarized inelastic neutron scattering to measure the spin excitations of YBa$_2$Cu$_3$O$_{6.9}$ ($T_c=$93~K), unequivocally confirming the magnetic character of the NSR in both the odd and even channels. In the odd channel, the NSR is anisotropic in spin space, that is the out of plane ($c$) component of $\chi^{\prime\prime}(\mathbf{Q},\omega)$ is approximately 1.4 times larger than the in-plane ($a/b$) component. Conversely, the much weaker even channel resonance is isotropic to within experimental error, and the low energy response maintains a large gap (below $\sim$30~meV) in the normal state. While it is generally accepted that the NSR is ubiquitous in at least the hole-doped cuprates, recently two further collective modes have been observed in HgBa$_2$CuO$_{4+\delta}$. If these weakly-dispersive ``Ising-like'' modes were also universally present, then they might radically alter our view of the cuprate superconducting state. However, we find no evidence of this type of excitation in YBa$_2$Cu$_3$O$_{6.9}$, suggesting that these modes may in fact be unique to certain systems. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B35.00004: Effective $J_1$-$J_2$ model for the spin wave in the superconducting $\rm (Tl,Rb)_2Fe_4Se_5$ Songxue Chi, Feng Ye, Wei Bao, Andrei T. Savici, Matthew B. Stone, Randy S. Fishman, H.D. Wang, C.H. Dong, Minghu Fang Spin wave excitations in the superconducting state of $\rm (Tl,Rb)_2Fe_4Se_5$ were determined by inelastic neutron scattering measurements. Four doubly degenerate spin wave branches, one gapped acoustic and 3 optical, span an energy range of about 210 meV. The spin wave spectra were successfully described by a $J_1$-$J_2$ Heisenberg model which includes the in-plane nearest ($J_1$ and $J'_1$), next nearest neighbor ($J_2$ and $J'_2$) interactions within and between the 4-spin blocks, inter-plane interaction ($J_c$) and a single-ion anisotropy. The exchange coupling constants obtained indicate that the spin block order verges on a noncollinear in-plane-spin phase observed in Tl$_2$Fe$_4$Se$_5$. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B35.00005: Anisotropy of the Superconducting State in Sr$_2$RuO$_4$ M.R. Eskildsen, C. Rastovski, W.J. Gannon, C.D. Dewhurst, D. Peets, H. Takatsu, Y. Maeno Multiple experimental and theoretical studies provide compelling support for triplet pairing of electrons and an odd, $p$-wave order parameter symmetry in superconducting Sr$_2$RuO$_4$. However, seemingly contradictory experimental results have left important questions concerning the detailed structure and coupling of the orbital and spin parts of the order parameter in this compound unresolved. We have used small-angle neutron scattering to study the vortex lattice in Sr$_2$RuO$_4$ in order to measure the intrinsic anisotropy ($\Gamma_{ac}$) of the superconducting state between the the $c$ axis and the RuO basal plane. Up to fields of $1.2$ T and temperature of 800 mK, we found no variation of $\Gamma_{ac} \approx 60$. This is consistent with the Fermi velocity anisotropy on the $\beta$ Fermi-surface sheet, but greatly exceeds the upper critical field anisotropy $H_{c2}^{\perp c}/H_{c2}^{\parallel c} = 20$. This result poses significant constraints on the possible order parameter symmetry in Sr$_2$RuO$_4$. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B35.00006: Bulk Magnetization in the Superconducting State of UPt$_3$ William Gannon, William Halperin, Catherine Rastovski, Morten Eskildsen, Pengcheng Dai, Anne Stunault The unconventional superconductor UPt$_3$ has long been thought to have an odd parity orbital, and triplet spin state. An important signature of such a state is the temperature independence of the spin susceptibility across the superconducting transition temperature. Here, we report bulk measurements of the susceptibility of UPt$_3$ for magnetic fields along the crystal a-axis performed with polarized neutron diffraction. Temperature independence at all magnetic fields is observed, suggesting a spin triplet superconducting state for the entirety of the phase diagram, with equal spin pairs in the crystal basal plane. These results will be discussed in the context of existing theories for the superconducting state of this paradigm heavy fermion material [Graf et. al., PRB 62, 14393; Tsutsumi et. al., JPSJ 81, 074717 (2012)]. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B35.00007: Kerr effect studies of the heavy fermion superconductor URu$_2$Si$_2$ Elizabeth Schemm, Hovnatan Karapetyan, Eric Bauer, Aharon Kapitulnik In the heavy fermion metal URu$_2$Si$_2$, the very large entropy carried by the 5f electrons is released at $\sim 17.5$ K via a second-order phase transition to a ``hidden order" state. Below $\sim 1.5$ K superconductivity emerges with an as-yet unknown gap structure, adding to the mystery associated with this material. In this talk we present polar Kerr effect (PKE) measurements of URu$_2$Si$_2$ crystals using a Sagnac interferometer. PKE is sensitive to time-reversal symmetry (TRS) breaking since it measures the existence of an antisymmetric contribution to the real and imaginary parts of the frequency-dependent dielectric tensor. Such a contribution is necessarily absent if TRS is not broken in the material. We find a weak magnetic signal in the hidden order phase that seems to not influence superconductivity. The presence of a finite Kerr signal below $T_c$ provides strong evidence that time reversal symmetry is broken in the superconducting state. The relationship between the magnetic response in the hidden order phase and superconductivity is also studied. We further compare our results to other unconventional superconductors. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B35.00008: High resolution $^{17}$O Knight shift measurements of HgBa$_2$CuO$_{4+y}$ single crystals Andrew M. Mounce, Sangwon Oh, Jeongseop A. Lee, W.P. Halperin, A.P. Reyes, P.L. Kuhns, M. Chan, J. Li, D. Xia, X. Zhao, M. Greven The high superconducting transition temperature and the simple tetragonal structure of HgBa$_2$CuO$_{4+y}$ (Hg1201) makes this material an ideal candidate to study unconventional superconductivity in the cuprates[1]. Nuclear magnetic resonance has been performed on Hg1201 single crystals which have been annealed in an $^{17}$O atmosphere to achieve superconducting transition temperatures of underdoped 72 K and overdoped 76 K. Oxygen spectra are sufficiently narrow to resolve planar, apical, and dopant oxygen sites in addition to all satellite transitions of the planar and apical sites. The deconvolution of oxygen spin shifts into isotropic and axial shifts, for the underdoped crystal, shows temperature dependence in both the isotropic and axial components of the planar oxygen while the apical oxygen only has temperature dependence in the axial component. The rotational dependence of the apical oxygen shift does not indicate a predicted static local field component due to circulating orbital currents[2] which have been observed by neutron scattering.[3] [1] Barisic, N, PRB 78, 054518 (2008). [2] Lederer, S. and Kivelson, S. A., PRB 85, 155130 (2012). [3] Li, Y., et al, Nature 455, 372 (2008). [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B35.00009: NMR study of spin fluctuations and superconductivity in LaFeAsO$_{1-x}$H$_x$ Naoki Fujiwara, Ryosuke Sakurai, Soushi Iimura, Satoru Matsuishi, Hideo Hosono, Yoichi Yamakawa, Hiroshi Kontani We have performed NMR measurements in LaFeAsO$_{1-x}$H$_x$, an isomorphic compound of LaFeAsO$_{1-x}$F$_x$. LaFeAsO$_{1-x}$H$_x$ is most recently known for having double superconducting (SC) domes on H doping. LaFeAsO$_{1-x}$H$_x$ is an electron- doped system, and protons act as H$^{-1}$ as well as F$^{-1}$. The first SC dome is very similar between F and H doping, suggesting that H doping supplies the same amount of electrons as F doping. Interestingly, an excess amount of H up to x=0.5 can be replaced with O$^{2-}$. In the H-overdoped regime ($x > 0.2$), LaFeAsO$_{1-x}$H$_x$ undergoes the second superconducting state [1]. We measured the relaxation rate of LaFeAsO$_{1-x}$H$_x$ for x=0.2 and 0.4, and fond an anomalous electronic state; spin fluctuations measured from $1/T_1T$ is enhanced with increasing the doping level from $x=0.2$ to 0.4. The enhancement of spin fluctuations with increasing carrier doping is a new phenomenon that has not observed in LaFeAsO$_{1-x}$F$_x$ in which the upper limit of the doping level is at most $x=0.2$. We will discuss the phenomenon in relation to superconductivity.\\[4pt] [1] S. Iimura, $et. al.$, Nature Communications (2012) [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B35.00010: Electronic and Magnetic Properties of Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2}$ Studied by $^{55}$Mn and $^{75}$As-NMR S. Yeninas, A. Pandey, D.C. Johnston, Y. Furukawa BaMn$_{2}$As$_{2}$ (Mn$^{2+}$; S = 5/2) is a G-type antiferromagnetic (AF) semiconductor with N\'{e}el temperature $T_{N} \sim $ 625 K and a small band gap of $\sim $ 27 meV. Hole doping by substitution of Ba with K drives BaMn$_{2}$As$_{2}$ into a metallic state while maintaining the same AF spin structure with similar high $T_{N}$. In order to investigate hole doping effects on electronic and magnetic properties in Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2 }$from a microscopic point of view, we have conducted $^{55}$Mn and $^{75}$As-NMR spectra and spin-lattice relaxation measurements on single crystals of Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2}$ ($x$ = 0, 0.04, 0.4). The temperature ($T)$ dependence of 1/$T_{1}$ for $^{55}$Mn and $^{75}$As for the $x$=0 compound shows 1/$T_{1} \quad \sim \quad T ^{3}$ dependence for both nuclei, suggesting that 1/$T_{1}$ of the nuclei arises from interactions with magnon excitations in the local-moment AF state. On the other hand, the 1/$T_{1}$ of both nuclei is found to be proportional to $T$ (Korringa relation) in K-doped materials below $T_{N}$, which corresponds to the AF metallic state in Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2}$. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B35.00011: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 1:27PM - 1:39PM |
B35.00012: High Energy Magnetic Excitations in overdoped high Temperature Superconductors M. Le Tacon, G. Ghiringhelli, D.C. Peets, M. Moretti-Sala, S. Blanco-Canosa, M. Minola, V. Hinkov, R. Liang, D. Bonn, W. Hardy, C.T. Lin, T. Schmitt, L. Braicovich, B. Keimer Motivated by the search for the mechanism of high-temperature superconductivity, an intense research effort has been focused on the evolution of the spin excitation spectrum upon doping from the AF insulating to the superconducting (SC) states of the cuprates. Taking advantage of the recent developments of RIXS, we have shown that high energy magnetic excitations with dispersions and spectral weights similar to those of magnons in AF cuprates exist up to optimal doping. In the overdoped region, the normal state appears in many aspects similar to a Fermi liquid, and the available data on the magnetic excitations is rather limited. Inelastic neutron scattering work by Lipscombe et al. revealed the persistence of magnetic excitations up to 160 meV in an overdoped LSCO. This surprising result motivates us to investigate further the high energy magnetic excitations using RIXS in Ca-doped YBCO and Tl2201 compounds. We show that the high energy part of the excitation spectrum is essentially unaffected with hole doping, and that excitations up to 300 meV survive even at doping levels at which SC vanishes. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B35.00013: Two Dimensional Incommensurate Spin Excitations and Lattice Fluctuations in $La_{2-x}Ba_{x}CuO_{4}$ J.J. Wagman, J.P. Carlo, G. van Gastel, Y. Zhao, A.B. Kallin, E. Mazurek, H.A. Dabkowska, A. Savicii, G.E. Granroth, Z. Yamani, Z. Tun, B.D. Gaulin 'Hour-glass' shaped dispersions of antiferromagnetic (AF) spin fluctuations are a robust feature common to many high temperature superconductors. In 214 cuprates, these phenomena are well known to display a strong dependence on the concentration of holes that are introduced into the copper oxide planes by doping. The incommensurability (IC) of the two dimensional magnetic order in this system is sensitive to hole concentration. Here, we present a series of neutron scattering measurements on single crystals of $La_{2-x}Ba_{x}CuO_{4}$ (LBCO), with $0.035 \leq x \leq 0.095$, a doping range that spans the transition from diagonal to parallel IC ordering wavevectors, and from non-superconducting to superconducting ground states. Our measurements map out the evolution of the spin excitations for energies below $\sim$ 50 meV, and focus on an enhancement in the scattered intensity centered in the 17-20 meV at the AF IC positions. This regime corresponds to the approximate crossing of very dispersive spin excitations and weakly dispersive low lying optic phonons in LBCO. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B35.00014: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 2:03PM - 2:15PM |
B35.00015: Spin Susceptibility Enhancement in Superconductors Ben Rosemeyer, Anton Vorontsov We calculate electronic vector-dependent spin susceptibility tensor, $\chi_{\alpha\beta}(\bf{q})$, in the superconducting state, for a 2D Fermi surface. We investigate dependence of $\chi_{\alpha\beta}(\bf{q})$ on: a) magnetic ordering wave vector $\bf{q}$; b) symmetry of the order parameter, $\Delta(\bf{k})$; c) temperature; and d) effects of external Zeeman field. We find that under certain conditions longitudinal and transverse components of the susceptibility in the superconducting state can be enhanced compared to the normal state value, indicating effective attraction between magnetically ordered and superconducting phases. In particular, $d$-wave superconductors at low temperatures in strong magnetic field show increase of $\chi$ for $q=2k_f-\delta q$ ($\delta q/k_f\approx0.05$) for near-nodal direction of $\bf{q}$. We relate such enhancement or lack thereof to behavior of low-energy excitations in the system. These findings may be relevant to materials where magnetic and superconducting phases are close neighbors, such as heavy fermion CeCoIn$_5$, or Fe-based superconductors. [Preview Abstract] |
Session B36: HTSC: Transport Properties
Sponsoring Units: DMPChair: Carmen Almasan, Kent State University
Room: 344
Monday, March 18, 2013 11:15AM - 11:27AM |
B36.00001: Doping dependence of the upper critical field $H_{\rm c2}$ in the cuprate superconductor YBCO L. Taillefer, N. Doiron-Leyraud, D. LeBoeuf, B. Vignolle, C. Proust, B.J. Ramshaw, R. Liang, D.A. Bonn, W.N. Hardy It is generally thought that the upper critical field $H_{\rm c2}$ of underdoped cuprate superconductors is far greater than the vortex-solid melting field $H_{\rm vs}$ at which the state of zero resistance ends, even at $T = 0$ [1]. Here we compare electrical measurements of $H_{\rm vs}$ and thermal measurements of $H_{\rm c2}$ [2] in the cuprate YBCO and show that $H_{\rm c2} = H_{\rm vs}$ at $T \to 0$, strong evidence that there is no vortex liquid phase at $T = 0$. We then present extensive measurements of the electrical resistivity in high magnetic fields over a wide doping range, from which we obtain $H_{\rm c2}$ as a function of doping in YBCO. We find that $H_{\rm c2}$ collapses to remarkably low values in the underdoped regime, which we attribute to the competing effect of a phase with charge-density-wave order [3, 4], also responsible for a reconstruction of the Fermi surface [5, 6].\\[4pt] [1] T. Senthil and P.A. Lee, Phys. Rev. B {\bf 79}, 245116 (2009).\\[0pt] [2] See~APS~talk~by~G.~Grissonnanche.\\[0pt] [3] T. Wu \textit{et~al.}, Nature {\bf 477}, 191 (2011).\\[0pt] [4] G. Ghiringhelli \textit{et~al.}, Science {\bf 337}, 821 (2012).\\[0pt] [5] D. LeBoeuf \textit{et~al.}, Phys. Rev. B {\bf 83}, 054506 (2011).\\[0pt] [6] N. Doiron-Leyraud and L. Taillefer, Physica C {\bf 481}, 161 (2012). [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B36.00002: Thermal conductivity as a direct probe of the upper critical field Hc2 in cuprate superconductors G. Grissonnanche, O. Cyr-Choiniere, S. Dufour-Beausejour, A. Juneau-Fecteau, N. Doiron-Leyraud, L. Taillefer, B. Ramshaw, R. Liang, D. Bonn, W. Hardy, S. Kramer, D. Graf The value of the upper critical field $H_{\mathrm{c2}}$ in cuprate superconductors is an open question, subject to much debate [1]. Owing to its sensitivity to vortex scattering, the thermal conductivity is a powerful technique to directly measure the upper critical field $H_{\mathrm{c2}}$ in a clean type-II superconductor [2]. Here we report measurements of the thermal conductivity in the underdoped cuprate superconductor YBCO in magnetic fields up to 45 T, from which we can directly extract $H_{\mathrm{c2}}$. We find that $H_{\mathrm{c2}}$ is remarkably low at a doping $p \quad =$ 0.11, showing that quantum oscillations [3, 4] are observed above $H_{\mathrm{c2}}$, in a normal state without vortices.\\[4pt] [1] J. Chang \textit{et al}., Nat. Phys. \textbf{8}, 751 (2012).\\[0pt] [2] A. B. Vorontsov and I. Vekhter, Phys. Rev. B \textbf{75}, 224502 (2007).\\[0pt] [3] N. Doiron-Leyraud \textit{et al}., Nature \textbf{447}, 565 (2007).\\[0pt] [4] S. C. Riggs \textit{et al}., Nat. Phys. \textbf{7}, 332 (2011). [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B36.00003: Doping evolution of nodal quasiparticle velocities in cuprate superconductors S. Ren\'e de Cotret, J.-Ph. Reid, N. Doiron-Leyraud, L. Taillefer, B.J. Ramshaw, R. Liang, D.A. Bonn, W.N. Hardy The thermal conductivity of the cuprate superconductor YBa$_2$Cu$_3$O$_y$ was measured at temperatures down to $T \sim 50$~mK on high-quality single crystals with a hole doping ranging from $p = 0.06$ to $p = 0.18$. The residual linear term at $T \to 0$ is a direct measure of the ratio of nodal quasiparticle velocities. When combined with published data on Tl$_2$Ba$_2$CuO$_{6+\delta}$ [1], our data spans the full superconducting phase. The ratio of quasiparticle velocities agrees well with recent, high-resolution ARPES measurements of the Fermi velocity and gap velocity at the nodes as a function of doping, in the related cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ [2,3]. \\[4pt] [1] D.G. Hawthorn {\it et al.}, Phys. Rev. B {\bf 75}, 104518 (2007). \\[0pt] [2] I.M. Vishik {\it et al.}, Phys. Rev. Lett. {\bf 104}, 207002 (2010). \\[0pt] [3] I.M. Vishik {\it et al.}, ArXiv, 1209.6514 (2012). [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B36.00004: Hall and Nernst Coefficients of Underdoped HgBa$_2$CuO$_{4+\delta}$: Fermi-Surface Reconstruction in an Archetypal Cuprate Superconductor Nicolas Doiron-Leyraud, S. Lepault, O. Cyr-Choini\`ere, B. Vignolle, F. Lalibert\'e, J. Chang, N. Bari{\v s}i\'c, M.K. Chan, L. Ji, X. Zhao, Y. Li, M. Greven, C. Proust, Louis Taillefer The Hall coefficient $R_{\rm H}$ of underdoped HgBa$_2$CuO$_{4+\delta}$ (Hg1201) was measured at low temperature in magnetic fields large enough to suppress superconductivity. The normal-state $R_{\rm H}(T)$ is found to drop with decreasing temperature and become negative below 10~K, as also observed in the orthorhombic bi-layer cuprate YBa$_2$Cu$_3$O$_{y}$ (YBCO) at comparable doping. In YBCO, the negative $R_{\rm H}$ is the signature of a Fermi-surface reconstruction that produces a small electron pocket, attributed to the onset of charge-density wave order at low temperature. We infer that a similar Fermi-surface reconstruction occurs in the tetragonal single-layer material Hg1201. A striking similarity is also found in the normal-state Nernst coefficient $\nu(T)$, which drops below the pseudogap temperature $T^\star$, to reach a large negative value at low temperature, in both Hg1201 and YBCO. Our results are compelling evidence that the mechanisms responsible for Fermi-surface reconstruction and pseudogap formation in hole-doped cuprates are universal. Preprint reference: arXiv:1210.8411. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B36.00005: Doping dependence of the upper critical field in the electron-doped cuprate superconductor PCCO via the Nernst effect F. Lalibert\'e, F.F. Tafti, M. Dion, J. Gaudet, P. Fournier, L. Taillefer Superconducting fluctuations are known to persist above the critical temperature $T_{\rm c}$ and above the upper critical magnetic field $H_{\rm c2}$. The Nernst effect was shown to be a powerful probe of these fluctuations [1], in quantitative agreement with theory [2]. Here we report a detailed study of the Nernst effect in high-quality films [3] of the electron-doped cuprate superconductor PCCO, from which we extract $H_{\rm c2}$ as a function of doping. We find that $H_{\rm c2}$ follows the dome-like doping dependence of $T_{\rm c}$, revealing that the pairing strength decreases with underdoping, as it does in hole-doped cuprates [4].\\[4pt] [1] A. Pourret \textit{et al.}, Nat. Phys. {\bf 2}, 683 (2006)\\[0pt] [2] M. N. Serbyn \textit{et al.}, Phys. Rev. Lett. {\bf 102}, 067001 (2009); K. Michaeli and A. M. Finkel'stein, Europhys. Lett. {\bf 86}, 27007 (2009)\\[0pt] [3] G. Roberge \textit{et al.}, J. of Crystal Growth. {\bf 311}, 1340 (2009) \\[0pt] [4] J. Chang \textit{et al}., Nat. Phys. {\bf 8}, 751 (2012) [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B36.00006: Resistivity in the pseudogap phase of the underdoped cuprates Phillip Ashby, Jules Carbotte The pseudogap phase of the underdoped cuprates remains poorly understood. It exhibits many anomalous electronic properties. One example is the dc-resistivity which is metallic in the copper oxygen planes, while the c-axis response is insulating. We show how this can be understood within the pseudogap model of Yang, Rice, and Zhang (YRZ). The YRZ model naturally reconstructs the Fermi surface as a function of doping. This reconstruction places limits on the remaining quasiparticles allowed to participate in transport. As a result, the model is able to reproduce the qualitative experimental signatures, including the deviations from linear resistivity in the plane, as well as the insulating response along the c-axis. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B36.00007: Impurity Effects on Superconducting Properties coming from Nontrivial Nodal Structures in Order Parameters Heesang Kim, Hyunhee Chung, Nammee Kim Power-law behavior is expected in the temperature dependence of the specific heat in a superconductor whose order parameter has point- or line-nodes on its fermi surface. It is known that the dependence is $T^{2}$ for line-nodes and $T^{3}$ for point-nodes. However, the power-law behavior is different from what we expect in some cases such as $g$-wave and $g+s$-wave. We present the generalized power-law behavior in a superconductor with a nontrivial nodal structure in its order parameter. We also show how the presence of impurities wash out the power-law behavior. In the framework of the quasiclassical formalism, we focus on the density of states and the specific heat. We also present evolution of those quantities in the presence of impurities. The impurity effect is parameterized with two quantities, the scattering cross section $\sigma$ and impurity scattering rate $1/2\tau$, so that we can cover the whole range of the impurity effect from the Born limit to the unitary limit. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B36.00008: Conventional c-axis charge transport in the electron-doped cuprates Yangmu Li, N. Bari\v{s}i\'{c}, G. Yu, E.M. Motoyama, I.M. Vishik, S.T. Hannahs, M. Greven We have measured the interlayer (c-axis) magnetoresistivity of the electron-doped cuprate superconductor Nd$_{\mathrm{2-x}}$Ce$_{\mathrm{x}}$CuO$_{\mathrm{4+\delta}}$(NCCO) at and below optimal doping. In zero magnetic field, the low- and intermediate-temperature regimes are dominated by logarithmic and quadratic temperature dependences, respectively. The low-temperature logarithmic upturn indicates the onset of localization, whereas the quadratic dependence is attributed to Fermi-liquid behavior. Furthermore, the transverse c-axis magnetoresistivity exhibits H$^{\mathrm{2}}$ dependence, not only above the zero-field Tc, but also at lower temperature once a sufficiently large external field suppresses the superconductivity. These findings suggest that the out-of-plane conduction in the electron-doped cuprates is rather conventional. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B36.00009: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 1:03PM - 1:15PM |
B36.00010: Universal sheet resistance of the cuprate superconductors N. Bari\v{s}i\'{c}, M.K. Chan, G. Yu, Y. Li, X. Zhao, M. Dressel, A. Smontara, M. Greven Upon introducing charge carriers into the underlying copper-oxygen sheets of the cuprates, the parent insulator evolves into a superconductor and eventually into a seemingly conventional Fermi liquid. Much has remained elusive about the nature of this evolution, and about the peculiar metallic state at intermediate hole-carrier concentrations ($p)$, where the planar resistivity exhibits a linear temperature dependence ($\rho \propto T)$ that is disrupted upon cooling toward the superconducting state by the opening of a `pseudogap' along the Fermi surface. Here we demonstrate for the quintessential compound HgBa$_{\mathrm{2}}$CuO$_{\mathrm{4+\delta }}$ a purely Fermi-liquid-like resistivity ($\rho \propto T^{\mathrm{2}})$ deep in the pseudogap regime. Our result when combined with select prior work for other compounds reveals the fundamental resistance per copper-oxygen sheet in both the linear ($\rho_{\mathrm{S}}=A_{\mathrm{1S}}T)$ and quadratic ($\rho_{\mathrm{S}}=A_{\mathrm{2S}}T^{\mathrm{2}})$ regimes, with $A_{\mathrm{1S\thinspace }}\propto \quad A_{\mathrm{2S\thinspace }}\propto $ 1/$p$. Theoretical models for the cuprates can now be benchmarked against this remarkably simple universal behavior. \textit{Preprint: arXiv:1207.1504. }Work supported by DOE-BES. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B36.00011: Magnetotransport of La$_{(2-x)}$Sr$_x$CuO$_4$ with nearly continuous doping in intense magnetic fields Zachary Stegen, Greg Boebinger, Jie Wu, Ivan Bozovic, Fedor Balakirev, Albert Migliori Pulsed magnetic fields of up to 57 T were used to measure the Hall resistivity and longitudinal magnetoresistance of La$_{(2-x)}$Sr$_x$CuO$_4$ to low temperatures by suppressing the superconducting state. The samples were grown using Combinatorial Molecular Beam Epitaxy (COMBE) where the Sr concentration -- and hence carrier doping, p -- changes continuously across the sample. Performing 30 simultaneous transport measurements on a single growth allows for unprecedented resolution in doping ($\Delta p \approx 0.0002$). We examine the phase diagram of the resistive state in this hole-doped cuprate by measuring a series of COMBE samples. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B36.00012: Transport and contact-free investigation of REBCO thin film temperature dependent pinning landscapes John Sinclair, Jan Jaroszynski, Xinbo Hu, Michael Santos Studies of the pinning mechanisms and landscapes of REBa$_{2}$Cu$_{3}$O$_{x}$ (RE=rare earth elements) thin films have been a topic of study in recent years due to, among other reasons, their ability to introduce nonsuperconducting phases and defects. Here we will focus on REBCO thin films with BaZrO$_3$ nanocolumns and other isotropic defects. The evolution of the dominant pinning mechanisms seems to change as a function of temperature even to the point that samples with similar critical current density properties at high temperatures can have distinctly different properties at low temperatures. Earlier work focused on the angular selectivity of the current density profile, though other properties (such as alpha values) can evolve as well. Characteristic results accentuating this evolution of current density properties will be presented. Challenges exist in evaluating these low temperature properties in high magnetic fields, therefore both transport and contact-free results were be presented to compliment the work. Support for this work is provided by the NHMFL via NSF DRM 0654118. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B36.00013: Contact free transport characterization of recent REBCO films in very high magnetic field Jan Jaroszynski, John Sinclair, Valeria Braccini, Xinbo Hu Studies on pinning mechanisms in high temperature superconductors often require detailed knowledge of critical current density $J_c$ as a function of magnetic field orientation as well as field strength and temperature. However, recent REBa$_{2}$Cu$_{3}$O$_{x}$ (RE=rare earth elements) thin films can achieve remarkably high critical current density values by the incorporation of nonsuperconducting nanoparticles, that often pose problems with $J_c$ measurements, due to extremely high Lorentz force, Joule heating etc. in the limited space of high field magnets. Here we demonstrate the angularly dependent $J_c$ measured by means of contact-free vector magnetization measurements in a vibrating sample magnetometer equipped with both longitudinal and transverse pickup coils. The studies complement traditional transport techniques and are readily extended to conditions of fields and temperatures where the current density is very large and transport methods become difficult. Our results clearly show an evolution of pinning from strongly correlated effects seen at high temperatures to significant contributions from dense but weak pins that thermal fluctuations render ineffective at high temperatures but which become strong at lower temperatures [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B36.00014: Linear and quadratic temperature dependence of electronic specific heat for cuprates P. Salas, F.J. Sevilla, M.A. Solis We model cuprate superconductors as an infinite layered lattice structure which contains a fluid of paired and unpaired fermions (electrons or holes). Paired fermions, which are the superconducting carriers, are considered as noninteracting zero spin bosons (cuasi-particles) with a linear dispersion relation, which coexist with the unpaired fermions in a series of almost two dimensional slabs stacked in their perpendicular direction. The inter-slab penetrable planes are simulated by a Dirac comb potential in the direction in which the slabs are stacked, while paired and unpaired electrons (or holes) are free to move parallel to the planes. Paired fermions condense at a BEC critical temperature exhibiting a jump in their specific heat, which are taken as the experimental values of the superconducting critical temperature and the specific heat jump of YBaCuO$_{7-x}$, to fix our model parameters: the plane impenetrability and the fraction of superconducting charge carrier. We straightforwardly obtain, near and under the superconducting temperature $T_c$, the linear ($\gamma_e T$) and the quadratic ($\alpha T^2$) electronic specific heat terms, with $\gamma_e$ and $\alpha$ in agreement with the latest experimental values reported. [Preview Abstract] |
Session B37: Focus Session: Fe-based Superconductors: Impurity Effects
Sponsoring Units: DMP DCOMPChair: Ruslan Prozorov, Ames Lab
Room: 345/346
Monday, March 18, 2013 11:15AM - 11:51AM |
B37.00001: Effect of heavy-ion and electron irradiation on properties of Fe-based superconductors Invited Speaker: Marcin Konczykowski The introduction of defects by particle irradiation is used to reveal the role of disorder in matter, which is unavoidable in all crystalline solids. In superconductors defects introduce flux pinning, controlling critical current, $J_c$; as well as pair-breaking scattering, limiting the critical temperature, $T_c$. To elucidate defect related properties of Fe-based superconductors (FBS) we precede in two types of irradiation: heavy ion (6GeV Pb) to create disorder in the form of amorphous tracks and low temperature electron irradiation (2.5MeV at 20K) to create point like defects. Substantial increase of irreversible magnetization and an upward shift of the irreversibility line are observed after heavy ion irradiation of all FBS investigated to date. In $BaK122$, signatures of a Bose-glass vortex state; angular dependence and variable-range hopping flux creep are revealed. Remarkably, heavy ion irradiation does not depress $T_c $, however, point-like disorder introduced by electron irradiation does substantially. In isovalently substituted $Ba(FeAs_{1-x}P_{x})_{2}$ and $Ba(Fe_{1-x}Ru_{x}As)_{2}$ crystals, $T_c $ decreases linearly with dose. Suppression to $40\%$ of initial value of $T_c $ was achieved in $Ba(FeAs_{1-x}P_{x})_2$. An increase of normal state resistivity is observed and correlated to depression of $T_c $. Change of superconducting gap structure with disorder was determined from penetration depth measurements, $\lambda(T)$ dependence, at various stages of irradiation. Linear in $T$ variation of pristine samples, indicative of the presence of nodes in gap, turned at low irradiation dose to exponential $T$ variation, indicative of a fully gaped state. $T^{2}$ variation of $\lambda$ is observed at higher doses. This behaviour is incompatible with symmetry-imposed nodes of d-wave pairing but consistent with $S+/-$, $S+/+$ mechanisms. This is the first observation of the impurity-induced node lifting expected in anisotropic s-wave superconductors [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B37.00002: Impurity-induced changes in the superconducting order parameter in iron-based superconductors BaFe$_2$(As$_{1-x}$P$_x)_2$ Yuta Mizukami, Y. Kawamoto, K. Hashimoto, S. Kasahara, M. Konczykowski, C.J. van der Beek, B. Boizot, R. Prozorov, Y. Wang, A. Kreisel, P.J. Hirschfeld, V. Mishra, Y. Matsuda, T. Shibauchi To determine the symmetry and structure of superconducting order parameter in iron-based superconductors is one of the prime challenges in strongly correlated electron systems. A systematic study on the effect of impurity scattering on the structure of superconducting order parameter can be used to distinguish S$++$ and S$+$- symmetry. We introduced the point defects on iron-based superconductors BaFe$_2$(As$_{1-x}$P$_x)_2$ by electron irradiation which does not change lattice constants and carrier density, and performed magnetic penetration depth measurements on those samples. Here, we report on the suppression of the critical temperature and the change of the superconducting gap structure in iron-based superconductors BaFe$_2$(As$_{1-x}$P$_x)_2$ with increasing the defect density, from which we discuss the superconducting symmetry in this system. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B37.00003: Theoretical study of impurity effects in iron-based superconductors Maria Navarro Gastiasoro, Peter Hirschfeld, Brian Andersen Several open questions remain unanswered for the iron-based superconductors (FeSC), including the importance of electronic correlations and the symmetry of the superconducting order parameter. Motivated by recent STM experiments which show a fascinating variety of resonant defect states in FeSC, we adopt a realistic five-band model including electronic Coulomb correlations to study local effects of disorder in the FeSC. In order to minimize the number of free parameters, we use the pairing interactions obtained from spin-fluctuation exchange to determine the homogeneous superconducting state. The ability of local impurity potentials to induce resonant states depends on their scattering strength $V_{imp}$; in addition, for appropriate $V_{imp}$, such states are associated with local orbital- and magnetic order. We investigate the density of states near such impurities and show how tunneling experiments may be used to probe local induced order. In the SDW phase, we show how C2 symmetry-breaking dimers are naturally formed around impurities which also form cigar-like (pi,pi) structures embedded in the (pi,0) magnetic bulk phase. Such electronic dimers have been shown to be candidates for explaining the so-called nematogens observed previously by QPI in Co-doped CaFe$_2$As$_2$. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B37.00004: Enhancement of $T_c$ by impurity scattering in underdoped iron-arsenide superconductors Maxim G. Vavilov, Rafael M. Fernandes, Andrey V. Chubukov When analyzing the effects of disorder on the superconducting transition temperature $T_c$ of the iron pnictides, the conventional wisdom is that inter-band impurity scattering is quite harmful to the $s^{\pm}$ state. In this talk, we show that this is the case only in the overdoped region of the phase diagram. In the underdoped region, impurity scattering gives rise to two opposite effects due to the competition between superconductivity and a pre-existing magnetic state. The first effect is the direct reduction of $T_c$ due to the pair-breaking contribution coming from inter-band impurity scattering. The second effect is an indirect increase in $T_c$ due to the suppression of long-range magnetic order by both intra-band and inter-band impurity scattering. We show that for a wide range of parameters the second effect overcomes the first, leading to an overall enhancement of $T_c$ by disorder. Our results explain recent puzzling experimental observations on the impact of disorder on $T_c$ of the iron pnictides, providing further evidence in favor of an $s^{\pm}$ pairing state. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B37.00005: Using controlled disorder to distinguish $s_\pm$ and $s_{++}$ gap structure in Fe-based superconductors Yan Wang, Andreas Kreisel, Peter Hirschfeld, Vivek Mishra We reconsider the effect of disorder on the properties of a superconductor characterized by a sign-changing order parameter appropriate for Fe-based materials. Within a simple two band model, we calculate simultaneously $T_c$, the change in residual resistivity $\Delta \rho_0$, and the zero-energy density of states, and show how these results change for various types of gap structures and assumptions regarding the impurity scattering. The rate of $T_c$ suppression is shown to vary dramatically according to details of the impurity model considered. We search therefore for a practical, experimentally oriented signature of a gap of the $s_\pm$ type, and propose that observation of particular evolution of the penetration depth, thermal conductivity or NMR temperature dependence with disorder would suffice. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B37.00006: Single nonmagnetic impurity resonance in FeSe-based 122-type superconductors as a probe for pairing symmetry Qian-En Wang, Zi-Jian Yao, Fu-Chun Zhang The effect of a single nonmagnetic impurity in A$_{y}$Fe$_{2-x}$Se$_{2}$ (A=K, Rb, or Cs) superconductors has been studied based on a three-orbital model. The local density of states on and near the impurity site has been calculated by solving the Bogoliubov-de Gennes equations self-consistently. Both repulsive and attractive impurity scattering potential are considered in our calculations. The impurity-induced in-gap bound states are found only for attractive scattering potential in the state of $d_{x^2-y^2}$ wave pairing, and it turns out that they are very sensitive to the magnitude of the scattering potential. The emergence of the impurity-induced bound states in the vicinity of the Fermi level demonstrates a strong violation of the electron-hole symmetry which is originated from the nodeless $d_{x^2-y^2}$ wave pairing state. The results obtained in our calculation, which simulate the doping of Co and Ni in FeSe-based 122-type superconductors, as an approach to examine the pairing symmetry of this novel superconducting material, can be a proposal of STM observation. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B37.00007: Effect of electron irradiation on superconductivity in isovalently substituted Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ Ruslan Prozorov, M.A. Tanatar, A. Thaler, S.L. Bud'ko, P.C. Canfield, M. Konczykowski, T. Shibauchi Single crystals of isovalently substituted Ba(Fe$_{1-x}$Ru$_{x}$)$_{2}% $As$_{2}$ were irradiated at $23$ K by $2.5$ MeV electrons with a total fluence up to $2\times10^{19}$ electrons per cm$^{2}$. The resistance was measured both in situ at $23$ K during irradiation, and as a function of temperature in a separate set-up, between the irradiation runs while the sample warmed to room temperature. We found that $\Delta\rho_{0}/\rho_{0}\approx0.2$ change in the residual resistivity, reached at the maximum irradiation dose, led to about a $\Delta T_{c}/T_{c0}\approx0.35$ decrease of $T_{c}$. This trend is universal in samples of different doping levels with different initial $T_{c0}$. The in-situ measurements also allowed us to understand the effects of room temperature annealing on the point-like defects induced by irradiation. The annealing results in a decrease of about a 20\% of the total increase in resistance achieved due to irradiation. However, residual 80\% remain stable at least one month after irradiation. We compare our results with theoretical predictions for different pairing scenarios, including extended $s_{\pm}$. Work in Ames was supported by the Department of Energy Office of Science, Basic Energy Sciences under Contract No. DE-AC02-O7CH11358. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B37.00008: Theoretical visualization of atomic-scale impurity states in Fe-based superconductors Peayush Choubey, Peter Hirschfeld, Tom Berlijn, Chao Cao We study the impurity induced local density of states (LDOS) in Fe-based superconductors, incorporating Wannier functions to obtain a higher resolution derived from a downfolding of density functional theory bands onto a 10-Fe tight-binding model. This enables us to compare our results with those obtained experimentally using STM. We solve the ten orbital Bogoliubov-de Gennes (BdG) equations for the single impurity problem and obtain the superconducting state lattice space Green's function, which is then transformed to the Wannier basis. The utility and limitations of this approximation are discussed. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B37.00009: Effects of disordered Ru substitution in BaFe$_2$As$_2$: possible superdiffusion mechanism in real materials Limin Wang, Tom Berlijn, Yan Wang, Chia-Hui Lin, Peter Hirschfeld, Wei Ku An unexpected insensitivity of the Fermi surface to impurity scattering is found in Ru substituted BaFe$_2$As$_2$ from first-principles theory, offering a natural explanation of the unusual resilience of transport and superconductivity to a high level of disordered substitution in this material. This robustness is shown to originate from a coherent interference of correlated on-site and inter-site impurity scattering, similar in spirit to the microscopic mechanism of superdiffusion in one dimension. Our result also demonstrates a strong substitution dependence of the Fermi surface and carrier concentration, and provides a natural resolution to current discrepancies in recent photoelectron spectroscopy. These effects offer a natural explanation of the diminishing long-range magnetic, orbital and superconducting order with high substitution. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B37.00010: Electron irradiation of iron-based superconductors S. Demirdis, C.J. van der Beek, M. Konczykowski, S. Kasahara, T. Terashima, R. Okazaki, T. Shibauchi, Y. Matsuda, D. Colson, P. Gierlowski, R. Prozorov The premise of $\mbox{s}_{\pm } $ superconductivity in the multiband iron-based superconductors, with a sign-changing order parameter between the electron-like and hole-like Fermi-surface sheets, has raised the question of the effect of atomic-scale point-like disorder on superconductivity in these materials. In this contribution, we compare the effect of the controlled introduction of point-like defects in different 122-type iron-based superconductors by 2.5 MeV electron irradiation at 20 K. Preliminary data reveal that the effect point-like defects on the critical temperature of isovalently doped materials vastly outweighs that on the charge-doped compounds. The weak collective contribution to $J_{c}$ in Co-doped 122 compounds is found to clearly increase. Moreover this contribution appears after irradiation of the P-doped compound in which it was previously absent. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B37.00011: Impurity substitution effect on Dirac cone in Ba(FeAs)$_2$ studied by magnetoresistance Takahiro Urata, Yoichi Tanabe, Khuong Huynh, Satoshi Heguri, Hidetoshi Oguro, Kazuo Watanabe, Katsumi Tanigaki In iron pnictide superconductors, the three dimensional Dirac cones (DCs) are created as a node of spin-density-wave gap [1]. Due to the pseudospin chirality, these DCs are robust for both nonmagnetic and magnetic impurities. Here we report Ru and Mn substitution effects on DCs in Ba(FeAs)$_{2}$ studied by magnetoresistance [2]. We assume that Ru is the nonmagnetic impurity while Mn is the magnetic one due to the stability of the Mn$^{\mathrm{2+}}$(3d$^{5})$ state. A linear magnetoresistance (LMR) against magnetic field (B) was observed above the certain magnetic field strength of B* for both cases. LMR in Ba(Fe$_{\mathrm{1-x}}$Ru$_{\mathrm{x}}$As)$_{2}$ is consistent with the quantum limit of DC [3,4]. Temperature (T) dependence of B* deviated from the theoretical model at low T for Ba(Fe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As)$_{2}$. This can be understood in term of the decrease of Fermi velocity of DCs, being associated with the magnetic scattering effect on DCs. [1] Y. Ran et al., Phys. Rev. B 79, 014505 (2009).[2]Y. Tanabe et al., Phys. Rev. B 84, 100508(R) (2011). /Phys. Rev. B 86, 094510 (2012). [3]A. A. Abrikosov, Phys. Rev. B 58, 2788 (1998).[4]K. K. Huynh et al., Phys. Rev. Lett. 106, 217004 (2011). [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B37.00012: Effect of electron irradiation on superconductivity in isovalently substituted Ba(Fe$_{1-x}$Ru$_{x}$)$_{2}$As$_2$ and SrFe$_2$(As$_{1-x}$P$_x$)$_2$ C.P. Strehlow, A. Thaler, M.A. Tanatar, S.L. Bud'ko, P.C. Canfield, R. Prozorov, M. Koczykowski, S. Miyasaka Single crystals of isovalently substituted Ba(Fe$_{1-x}$Ru$_{x}$)$_{2}$ As$_{2}$ and SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ were irradiated at $23$ K by $2.5$ MeV electrons with a total fluence up to $2\times10^{19}$ electrons per cm$^{2}$. Both the resistivity and Hall coefficient were measured before and after irradiation using the van der Pauw method. Irreversible vortex properties were probed using miniature Hall-probe arrays. We correlate the change in resistivity due to irradiation with changes in flux pinning, relaxation rate and irreversibility line. We compare the results with theoretical predictions for different pairing scenarios, including extended $s_{\pm}$. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B37.00013: Understanding the role of disorder in Fe-arsenide superconductors James Analytis, Hsueh-Hui Kuo, Ian Fisher Disorder has a profound affect on iron-pnictide superconductors, changing the transport anisotropy, the magneto-elastic coupling, and the properties of the superconductivity itself. In the 122 structural motifs, the parent compounds have a folded band structure with compensated hole and electron pockets. Some of these pockets are thought to be protected from disorder by the topological properties of the band structure. However, the influence of disorder on each pocket is in general very difficult to reveal because transport properties will in general measure an average of all Fermi surfaces, and other Fermi surface probes (such as ARPES) will not be sensitive to subtle changes in the dynamical properties of each Fermi surface. We present results of detailed quantum oscillation studies which aim to understand how the dynamics of each Fermi surface pocket is affected by disorder. [Preview Abstract] |
Session B38: Focus Session: Building a Thriving Undergraduate Physics Program
Sponsoring Units: FEdChair: Ted Hodapp, American Physical Society
Room: 347
Monday, March 18, 2013 11:15AM - 11:51AM |
B38.00001: From Near Extinction to Academic Excellence: The University of Wisconsin-La Crosse Physics Program Invited Speaker: Gubbi Sudhakaran A physics department that was on the brink of extinction has been successfully resuscitated into a nationally recognized program at the University of Wisconsin-La Crosse (UW-L). The revitalization efforts included sweeping curricular reforms, aggressive recruitment, and retention of students and faculty. The reforms included the introduction of new academic programs for the majors, new courses for non-majors, a dual-degree program in Physics and Engineering, and opportunities for undergraduate research. The department uses several recruitment techniques which include contacting high school seniors in the region and conducting outreach activities to attract students to the program. In order to sustain and enhance the quality of the program, the department carries out comprehensive assessment of its programmatic goals on a regular basis. The department is also very successful in placing students with bachelor's degrees in physics in STEM careers at an exceptional rate. The success of the program in recruiting, retention, and career placement can be attributed to a combination of aggressive advising and flexible options designed to meet the needs and career goals of each student. The retention rate in the program is high due to one-on-one advising, involving students in undergraduate research at an early stage, and a very vibrant student society. Due to these initiatives, the department has maintained its growth over the years with 160 majors currently, and 29 majors graduating during the 2011- 2012 academic year. Recently, the UW-L Physics Program was selected to receive the 2013 American Physical Society (APS) ``Improving Undergraduate Physics Education Award''. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B38.00002: Increasing student success Gay Stewart, John Stewart A more scientifically literate society benefits all STEM disciplines, as well as society as a whole. It is best realized by better serving all undergraduate STEM students. In better-serving all students, a physics department also benefits. The University of Arkansas, Fayetteville physics department has seen a drastic change in number of majors, the number of students active in research and the number of graduates pursuing graduate work, while also increasing the number of majors who decide to teach. Prior to our involvement with the Physics Teacher Education Coalition, graduation rates had increased by more than a factor of 4 in 4 years. After the increased efforts when we became a part of PhysTEC (http://PhysTEC.org) our graduation numbers doubled again. Specific attention to class policy to impact student learning in our introductory courses and strong preparation of the graduate teaching assistants, and quality advising were our primary areas of emphasis. What worked to build these numbers and strengthen these resources at Arkansas will be discussed. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B38.00003: SPIN-UP Regional Workshops: Enhancing Undergraduate Physics Programs Robert Hilborn, Ruth Howes, Kenneth Krane Through a grant from the National Science Foundation Division of Undergraduate Education (0741560), the American Association of Physics Teachers has been hosting a series of regional workshops for teams of faculty members from physics departments across the country. The goal of the program is to help departments develop and implement plans to enhance their undergraduate programs for both majors and non-majors. We give a brief overview of the Strategic Plans for Innovations in Undergraduate Physics (SPIN-UP) effort, the characteristics of ``thriving undergraduate physics programs'' articulated in the SPIN-UP report, and the six regional workshops. We provide data on physics majors' enrollment and graduation data at the participating departments to assess the impact of the program. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B38.00004: SPIN-UP Regional Workshops: Texas Physics Programs and Physics Programs at HBCUs Beth Cunningham, Paul Gueye, Michael Marder, James Stith, Quinton Williams As part of the broader SPIN-UP Regional Workshops program, the American Association of Physics Teachers organized two workshops directed at specific audiences. In May 2011, Hampton University hosted a SPIN-UP workshop focusing on physics programs at Historically Black Colleges and Universities. In May 2012, the University of Texas at Austin hosted a workshop focusing on physics programs in Texas, many of which were affected by a decision of the Texas Higher Education Coordinating Board to eliminate degree programs (in all fields) that produced fewer than five majors per year averaged over the most recent three-year period. We will summarize the discussions at these meetings and what is being done to respond to the challenges faced by the physics departments attending the workshops. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B38.00005: Learning by doing at the Colorado School of Mines Thomas E. Furtak, Todd G. Ruskell With over 260 majors, the undergraduate physics program at CSM is among the largest in the country. An underlying theme in this success is experiential learning, starting with a studio teaching method in the introductory calculus-based physics courses. After their second year students complete a 6-week full-time summer course devoted to hands-on practical knowledge and skills, including machine shop techniques, high-vacuum technology, applied optics, electronic control systems, and computational tools. This precedes a two-semester laboratory sequence that can be taught at an advanced level because of the students' experience. The required capstone senior course is a year-long open-ended challenge in which students partner with members of the faculty to work on authentic research projects, teaming with grad students or post-docs as contributing members to the department's externally funded scholarship. All of these features are important components of our B.S. degree, Engineering Physics, which is officially accredited by ABET. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B38.00006: Physics Teacher Preparation as a Means for Growth Ron Henderson Physics departments across the country are experiencing pressures to increase the number of graduates. One response is to improve marketing and recruiting efforts to add students to existing pipelines. A more innovative approach is to create new pathways tied to career paths that are alternatives to graduate school. One occupation that currently needs more graduates than physics departments are supplying is physics teaching. About 3 years ago, MTSU began implementing a strategy to prepare physics majors for careers in high school teaching. These efforts included developing coursework specifically related to physics teaching, creating relationships with the college of education, moving to pedagogies that reflect physics education research (PER)-validated best practices, hiring a tenure-track PER expert, implementing new ways to reach potential majors, and seeking external funding. The cumulative result has not only added a number of physics teaching majors to our roles, but has affected our existing programs in a manner that has yielded further growth. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B38.00007: Positive Aspects and Challenges Associated with Program Growth in Towson University's Physics Department David Schaefer Towson University's physics department has experienced dramatic growth over the past five years. Many directed and strategic initiatives have been implemented to increase student enrollment and retention. This has resulted in an increase from approximately 60 majors in 2007 to 115 in 2012. Graduation numbers have also seen a corresponding increase. This presentation will discuss efforts taken to produce these results as well as information related to the positive and negative aspects of growth. Future directions and plans to deal with challenges encountered will be discussed. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B38.00008: Retention at Departments of Physics Rafael Muller, Luis Rosa A thriving physics department is the end result of many actions, taken over time, that results in the development of a sense of community between the faculty and the students. As part of this sense of community, gifted students must receive special attention and innovative ideas must be incorporated to successfully accommodate the needs of these students. We have found that the best retention strategy for gifted undergraduates is the total involvement of them in undergraduate research projects and also the development of leadership in extracurricular activities within the department. A careful employment strategy is needed to secure a faculty committed to the goals of the community. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B38.00009: Biological Physics major as a means to stimulate an undergraduate physics program Herbert Jaeger, Khalid Eid, Jan Yarrison-Rice In an effort to stress the cross-disciplinary nature of modern physics we added a Biological Physics major. Drawing from coursework in physics, biology, chemistry, mathematics, and related disciplines, it combines a broad curriculum with physical and mathematical rigor in preparation for careers in biophysics, medical physics, and biomedical engineering. Biological Physics offers a new path of studies to a large pool of life science students. We hope to grow our physics majors from 70-80 to more than 100 students and boost our graduation rate from the mid-teens to the mid-twenties. The new major brought about a revision of our sophomore curriculum to make room for modern topics without sidelining fundamentals. As a result, we split our 1-semester long Contemporary Physics course (4 cr hrs) into a year-long sequence Contemporary Physics Foundations and Contemporary Physics Frontiers (both 3 cr hrs). Foundations starts with relativity, then focuses on 4 quantum mechanics topics: wells, spin 1/2, oscillators, and hydrogen. Throughout the course applications are woven in whenever the opportunity arises, e.g. magnetism and NMR with spin 1/2. The following semester Frontiers explores scientific principles and technological advances that make quantum science and resulting technologies different from the large scale. Frontiers covers enabling techniques from atomic, molecular, condensed matter, and particle physics, as well as advances in nanotechnology, quantum optics, and biophysics. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B38.00010: Design of an Experimental Contemporary Physics Course which Develops the Full Experience of Scientific Research and Highlights Current Faculty Research Jan M. Yarrison-Rice, Herbert Jaeger, Khalid F. Eid From background literature searches and reading, to conducting experiments, to presenting results and writing a journal manuscript, Miami University has revised its second-year Experimental Contemporary Physics Course, Phy293, to follow a basic research model. We examined research that faculty were conducting and chose experiments which were strongly related to understanding the ongoing research in the Department, while being based in fundamental quantum mechanics and recent 21st century physics. Experiments often had common instrumentation and data analysis techniques which allowed for grouping them into 3 basic categories: 1) Spectroscopy of gases and solids, 2) Characterization of contemporary samples, and 3) Quantized systems in electronic, magnetic and nuclear physics. These experiments also supported our secondary goal of preparing students to enter our research laboratories. At Miami, we generally have between 25-35 second year students, so the laboratory course must be managed to maintain groups of 2-3 for the best student learning outcomes. We will report on course logistics, the grouping of experiments, and methods for assessing students' learning. Having run the revised, full experimental format of Phy293 a 3rd time, we feel confident stating that this course demonstrates to students ``how physics research in the 21st century is actually conducted!'' [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B38.00011: Development of the Future Physicists of Florida A. Wade, C. Weatherford, P. Cottle, S. Fannin, W. Roberts, M. Fauerbach, L. Ponti, J. Sear We present the development of the ``Future Physicists of Florida'' (FPF) comprised of Florida university physics professors, middle and high school science teachers, and backed by the Florida Legislature. Our purpose is to address the lack of incoming college freshmen ready and willing to become physics majors. We will discuss the building of FPF and the development of a pipeline for middle and high school students predicted to produce the optimal number of bachelor's degrees in STEM. We will also discuss our use of community-building activities to educate the students, and their parents and teachers about the educational value of taking physics before going to college and potential careers in physics, to entertain them with fun physics related activities in order to peak their interest in physics, and to ultimately inspire the students to become physicists. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B38.00012: A Thriving and Innovative Undergraduate Experiential Physics Program Bahram Roughani The thriving physics program at Kettering University has experienced a three-fold increase in the number of physics majors since 2002. Our unique physics program requires students alternate between on-campus academic terms and off-campus co-op work terms on a three months rotation format to complete their degree in 4.5 years that includes summer as either school or co-op term. Students complete a minimum of five terms ($\sim$15 months) of cooperative work terms, and two terms ($\sim$6 months) of senior thesis work. The IP of the thesis work done at a co-op site belongs to the company. This has attracted co-op sponsors for our program by removing the IP concerns. The cooperative and experiential education part of our program is required for graduation, without any credits assigned to it. At the end of every co-op term students' work performance is evaluated by their co-op supervisor, which should match expected performance standards. In addition to co-op and thesis, our programs include a senior capstone design project course, concentrations within physics (Acoustics, Optics, and Materials), a required technical sequence outside physics, as well as entrepreneurship across curriculum. The success of our student securing the highest paid jobs for undergraduate physics majors in the nation plus their success in graduate studies are the main ``Pull Factors'' that has lead to three fold increase the physics majors since 2002. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B38.00013: A capstone research experience for physics majors David Jackson Dickinson College is a small liberal arts college with a thriving physics program. For years, one of the key features of our program has been a year-long senior research project that was required for each student. Unfortunately, as our number of majors increased, it became more and more difficult to supervise such a large number of senior research projects. To deal with this growing challenge, we developed a capstone research experience that involves a larger number of students working together on an independent group project. In this talk I will give a broad overview of our new senior research model and provide a few examples of projects that have been carried out over the past few years. I will also briefly describe the positive and negative aspects of this model from the perspective of faculty and students. [Preview Abstract] |
Session B39: Focus Session: Imaging & Modifying Materials Under Extreme Conditions of Radiation, Temperature, and at the Limits of Space and Time Resolution
Sponsoring Units: DMP GIMSRoom: 348
Monday, March 18, 2013 11:15AM - 11:51AM |
B39.00001: Quantifying transient dynamics in materials using time resolved \textit{in situ} TEM Invited Speaker: Geoffrey Campbell The dynamic transmission electron microscope (DTEM) is a standard TEM that has been modified such that the electron beam can be operated with a single intense pulse of electrons (\textgreater\ 10$^9$ e$^{-})$ with a pulse duration of just 15 ns. The short pulse of electrons is created via photoemission at the microscope cathode and enables time resolved observations of \textit{in situ} experiments. However, it can also be operated in thermionic emission mode for normal operation of the microscope for alignment and experiment setup. Additional modifications have also been made to the optical design of the condenser lens system. The \textit{in situ} experiments currently use a second laser to initiate the dynamic response of interest in the specimen. The relative timing of the pulses from the two laser systems sets the time of the observation relative to the initiation of the event under study. The DTEM has been used to investigate a number of rapid phenomena in materials We have studied the rapid nucleation and growth at the nanoscale of crystalline phases from an initially amorphous metal alloy parent phase and in amorphous Ge. DTEM has also been used to study reactive multilayer films of Ni and Al that sustain a reaction front speed greater than 10 m/s. We have also investigated rapid solidification of nanoscale films of liquid Al-Cu alloys. This work performed under the auspices of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B39.00002: Studying dynamic processes in liquids by TEM/STEM/DTEM Patricia Abellan, James E. Evans, Taylor J. Woehl, Katherine L. Jungjohann, Lucas R. Parent, Ilke Arslan, William D. Ristenpart, Nigel D. Browning In order to study dynamic phenomena such as corrosion or catalysis, extreme environmental conditions must be reproduced around the specimen - these include high-temperatures, high-pressures, specific oxidizing/reducing atmospheres or a liquid environment. The use of environmental stages specifically designed to fit in any transmission electron microscope (TEM) allows us to apply the distinct capabilities of each instrument to study dynamic processes. Localized gas/fluid conditions are created around the sample and separated from the high vacuum inside the microscope using hermetically sealed windowed-cells. Advanced capabilities of these techniques include spatial resolutions of $\sim $1 Angstrom or better in aberration corrected instruments or temporal resolutions in the microsecond-nanosecond range in a dynamic TEM (DTEM). Here, unique qualities of the DTEM that benefit the \textit{in-situ} experiments with gas/fluid environmental cells will be discussed. We also present our results with a liquid stage allowing atomic resolution imaging of nanomaterials in a colloidal suspension, core EEL spectra acquisition, continuous flow, controlled growth of nanocrystals and systematic calibration of the effect of the electron dose on silver nuclei formation. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B39.00003: Imaging Lead Dendrite Formation and Ion Diffusion in Aqueous Solution with Scanning Transmission Electron Microscopy Edward White, Scott Singer, Veronica Augustyn, William Hubbard, Matthew Mecklenburg, Bruce Dunn, B. C. Regan Using a scanning transmission electron microscope, we image the formation of lead dendrites and the local Pb$^{2+}$ concentration in an electrochemical cell containing a saturated solution of lead(II) nitrate. We control the morphology of the lead deposits with the rate of potential change, which can result in dendrites or compact layers. The processes are reversible and can be repeated. During lead stripping and plating the local Pb$^{2+}$ concentration can be measured as an increase or decrease in signal intensity, respectively, as ions come into and out of solution. Quantitative digital image analysis reveals excellent correlation between changes in the Pb$^{2+}$ concentration, the rate of lead deposition, and the current passed by the electrochemical cell. Furthermore imaging the ionic concentration as a function of time and distance from the electrode provides a measurement of the diffusion coefficient of the Pb$^{2+}$ ion. Real-time electron microscopy of dendritic growth dynamics and the associated local ionic concentrations can provide new insight into the functional electrochemistry of batteries and related energy storage technologies. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:51PM |
B39.00004: Imaging and measuring the evolution of solid density within a thermal explosion Invited Speaker: Laura Smilowitz Explosives have been used for millennia. All materials are energetic, but high explosives have the ability to release their stored energy in a very short period of time- nanoseconds in the case of detonations. Many explosives have an as-designed behavior that is well understood and controlled. However, the off-nominal behavior, such as would occur in an accident scenario, is typically much less understood. The subject of our research has been the energy release mechanisms for secondary high explosives heated to thermal explosion. The study of thermal explosions poses several difficulties including extreme temperature, pressure, and rate of change. In addition, thermal explosions pose the difficulty of being spontaneous dynamic events with limited ability to predict the time of the event. Typically, event durations are tens of microseconds and timing jitter is tens of seconds- essentially a one in a million duty cycle. These difficulties have precluded the use of many standard laboratory diagnostics to the study of the phenomena. In the past years, we have developed diagnostics which can survive the extremes of the thermal explosion with sufficient response time and the ability to remain armed and be triggered by the onset of the spontaneous event. In addition to microsecond temporal resolution, the diagnostics need to be spatially resolved with 100 micron spatial resolution and centimeter field of view in order to capture the spatial heterogeneity of the event. Our work has focused on the important secondary high explosive PBX 9501 which is a formulation of the organic crystalline nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). Our evolving understanding of this material has enabled us to develop a table-top x-ray imaging experiment providing millisecond time resolution with duration of minutes and sensitivity to density changes of better than 1{\%}. This quasistatic regime provides images of material thermal expansion, phase transitions, and thermal decomposition leading to the onset of thermal ignition. A second technique provides microsecond scale time resolution with duration of milliseconds and contrast sensitivities of a few percent. This technique allows us to observe the propagation of ignition which determines the overall violence of the thermal explosion. In this talk, I will describe our current understanding of thermal explosions, and the evolution of the radiographic diagnostics that we have developed to study thermal explosions. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B39.00005: Radiographic imaging of solidification in Al-Cu alloys Jason Cooley, Amy Clarke, Seth Imhoff, Brian Patterson, Wah-keat Lee, Kamel Fezzaa, Alexander Deriy, Tim Tucker, Martha Barker, Kester Clarke, Robert Field, Dan Thoma, David Teter Until the advent of third generation synchrotrons the ability to image the microstructure of metals during solidification was non-existent. Today's sources have sufficient energy and flux to perform real time radiographic imaging of solidification in thin samples with resolution sufficient to image dendrites, eutectic lamellae, and the density change across the solidification front. Feedback control of the solidification interface is also possible. We report on the radiographic imagining of Al-Cu eutectic alloys during solidification at the Argonne National Laboratory Advanced Photon Source. Cooling rates of up 10 degrees C / sec and, temperature gradients of up to 150 degrees C / cm were used to control the solidification. The samples were $\sim$ 100 microns thick and the field of view was $\sim$ 1.4 x 1.7 mm. The experimentally accessible phase space included both plane front and cellular growth regimes. The experimental resolution in the micron range was adequate to quantify cellular radii, cellular interface angles, lamellar interface angles, and lamellar spacing. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B39.00006: Pressure-induced antiferrodistortive phase transition and phonon softening in SrTiO$_3$ Shih-Chang Weng, Ruqing Xu, Ayman Said, Shih-Lin Chang, Tai-Chang Chiang SrTiO$_3$, at room temperature, undergoes an antiferrodistortive transition under pressure with a critical pressure of P$_c \sim 9.6$ GPa. This transition is accompanied by a cubic--to--tetragonal structural distortion, and the same distortion can be induced at ambient pressure by lowering the sample temperature to below T$_c \sim 105$ K. The temperature-induced transition is known to involve a soft phonon at the R point in the Brillouin zone based on neutron scattering, inelastic x-ray scattering, and thermal diffuse scattering studies. The same soft mode is expected for the pressure induced transition, and we report herein the first direct measurement using inelastic x-ray scattering and a diamond-anvil pressure cell. The phonon softening behavior follows a power law and is accompanied by a central peak. The results are analyzed theoretically and correlated with those for temperature-induced transition. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B39.00007: Femtosecond laser fabrication of micro/nano-channel array devices for parallelized fluorescence detection Brian Canfield, William Hofmeister, Lloyd Davis Cost-effective pharmaceutical drug discovery depends on increasing assay throughput while reducing reagent needs. Ultrasensitive, highly parallelized fluorescence-based platforms that incorporate a nano/micro-fluidic chip with an array of closely spaced channels would meet this need. We discuss the use of direct femtosecond laser machining to fabricate prototype fluidic chips with arrays of more than one hundred closely spaced channels. Traditional machining techniques involve overlapping focal spots from many laser pulses while scanning the substrate in order to create channels. However, this procedure is not only lengthy but may allow thermal effects to accumulate that degrade the quality of both the channel profile and surrounding substrate material. We are developing a different method for machining a line with just a single pulse, using a combination of cylindrical lenses and an aspheric lens to reshape a near-Gaussian beam into a tight line focus. Channels on the order of 1 micron wide, 5 microns deep, and nearly 2000 microns long may be made this way. We also address the critical issue of mitigating the high autofluorescence responses that arise from the creation of defects by fs-laser machining in fused silica. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B39.00008: Bond dissociation of small molecules on the silver tip under the influence of local electric field Haiyan He, Mayukh Banik, Vartkess Apkarian, Ruqian Wu The manipulation of chemical bonds at metallic nano-junctions, such as at scanning tunneling junctions, and under laser irradiation is currently of great interest, motivated by both fundamental considerations and applications in nanoeletronics, nanophotonics and nanocatalysis. In this work, we systematically investigate bond formation and dissociation of small molecules (e.g., oxygen and carbon monoxide) at the junction of two silver (111) tipped surfaces, through first principles molecular dynamics simulations. The electronic structures and vibrational frequencies are a sensitive function of the gap size, and significantly modified by the local electric fields. The calculated results are compared with recent experiments. \textbf{Acknowledgement.} This work was supported by the National Science Foundation under CHE-0802913 and computing time at XSEDE. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B39.00009: Understanding the ultrafast electron photoemission process, from simulation to experiment Jenni Portman, He Zhang, Zhensheng Tao, Chong-yu Ruan, Martin Berz, Philip Duxbury The ongoing efforts to develop a reliable ultrafast electron diffraction and imaging system require a stable source of photoemitted electrons and an understanding of how the properties of the generated bunch depend on the photocathode. In order to gain more understanding of this process, we combine the three-step photoemission model with N-particle electron simulations. By using the Fast Multipole Method to treat space charge effects, we are able to follow the time evolution of pulses containing over $10^6$ electrons and investigate the role of laser fluence and extraction field on the total number of electrons that escape the surface. The results of these simulations are compared to experimental images of the photoemission process collected using the shadow imaging technique. We are able to show good quantitative agreement both for the number of electrons generated and the pulse parameters. We also see evidence of a virtual cathode limit, which gives an upper limit to the number of electrons that is is possible to extract. The extension of these results to various extraction fields, laser pulse shapes and photocathode material parameters, represents a very interesting future development, allowing to better optimize the materials used in electron pulse generation. [Preview Abstract] |
Session B40: Surfaces, Interfaces, and Thin Films: Electronic and Magnetic Properties
Sponsoring Units: DCMPChair: Michael Horn von Hoegen, Universitaet Duisburg-Essen
Room: 349
Monday, March 18, 2013 11:15AM - 11:27AM |
B40.00001: OAM and spin structure of Cu(111) and Au(111) surface state bands Beomyoung Kim, Panjin Kim, Wonsig Jung, Yeongkwan Kim, Yoonyoung Koh, Changyoung Kim, Masashi Arita, Kenya Shimada, Hirofumi Namatame, Masaki Taniguchi, Choong H. Kim, Jaejun Yu We performed angle-resolved photoemission studies on Cu(111) and Au(111) surface states with circularly polarized light to investigate local orbital angular momentum (OAM) structures. Existence of OAM is confirmed, as predicted, to exist in systems with an inversion symmetry breaking. Cu(111) surface state bands are found to have chiral OAM in spite of very small spin-orbit coupling, consistent with the theoretical prediction. As for Au(111), we observe split bands for which OAM for the inner and outer bands are parallel, unlike the Bi2Se3 case. We also performed first-principles calculations and the results are found to be consistent with experimental results. Moreover, the majority of OAM is found to have $d$orbital origin while a small contribution comes from $p$orbitals. An effective Hamiltonian that incorporates the role of OAM is derived and is used to extract the spin and OAM structures. We discuss the evolution of angular momentum structures from a pure OAM system to a strongly spin-orbit-entangled state. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B40.00002: Electronic properties of precious-metal coated W tips in STM: Role of spin-orbit coupling T. Yamashita, T. Akiyama, K. Nakamura, T. Ito, S.H. Rhim, A.J. Freeman Scanning tunneling microscopy (STM) has proved a versatile tool invigorating many physics at an atomic scale, where chemical identity and shape of the probe tip greatly affect resolution and sensitivity. There have been many efforts to functionalize STM tips: coating W tips with organic molecules and {\em 3d} transition metals, which facilitate the selective imaging with enhanced tunneling current. In this work, we model W(110) tips coated by precious metals such as Au, Ag, and Pt, in which large spin-orbit coupling significantly influences the electronic structure of the STM probe. Furthermore, we argue that this spin-orbit coupling can be used as a spin detecting STM probe without additional bias switching. The stability of the W(110) apex atom for each metal coating is also discussed. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B40.00003: Ion bombardment of Ni(110) studied with inverse photoemission, LEED, and simulations Benjamin Young, Jim Warner, David Heskett Inverse Photoemission Spectroscopy (IPES) performed on clean Ni(110) reveals an unoccupied electronic surface state $\sim$2eV above the Fermi level at the \={Y} point of the surface Brillouin Zone. Ion bombardment (sputtering) of the sample creates vacancies and adatoms, which reduce the intensity of the representative state peak in IPES spectra. While the intensity of this IPES peak decreases with sputtering, well-defined diffraction spots in the surface LEED pattern give way to more diffuse ones with higher background intensity. Quantization of these permits analysis of their intensity profiles. Results of these techniques are presented for various sputtering conditions with 1keV Ne$\ ^{+}$ and compared to previous results for 500eV Ar$\ ^{+}$ on the same sample. Finally, we connect sputtering trends in the IPES and LEED data to Monte Carlo simulations of the sputtering process. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B40.00004: Measurement of the Spectral Distribution of Low Energy Electrons Emitted as a Result of M$_{2,3}$VV Auger Transitions in Cu(100) and the N$_{2,3}$VV transition in Ag Prasad Joglekar, Suman Satyal, Karthik Shastry, Steven Hulbert, Alexander Weiss Auger Photoelectron Coincidence Spectroscopy (APECS) was used to investigate the physics of electron emission in the Low Energy Tail (LET) of the MVV and NVV Auger spectra obtained from Cu(100) and Ag(100) surfaces, respectively. A beam of 200eV photons (180 eV in the case of Ag) was used to probe the Cu (Ag) sample. Two Cylindrical Mirror Analyzers (CMAs) were used to select the energy of electrons emitted from the sample. Auger electrons were detected in coincidence with the 3p $_{3/2}$ photoemission peak in the case Cu and the 4p photoemission peak in the case of Ag. A set of coincidence measurements were made with the fixed analyzer set at a series of energies between the core and the valence band in order to obtain an estimate of the background due to the inelastic scattering of the valence band electrons. This background was then subtracted yielding a spectrum consisting only of electrons emitted as a result of the Auger transition process. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B40.00005: Transient Exciton at Ag(111) Surface Cong Wang, Xuefeng Cui, Adam John Argondizzo, Sean Garrett-Roe, Hrovje Petek We investigate the surface states on Ag(111) by means of multi-photon photoemission using ultrashort laser pulses. The angle-resolved photoemission spectra at the non-resonant range are consistent with the well-known structures of Shockley states and image potential states. But when we tune the wavelength to the resonant range by two photon, the spectra is dominated by a non-dispersive feature, which should correspond to a localized state, and we assign it to transient exciton. Then we do time-resolved measurements and take Fourier Transformation with respect to the delay-axis. The dominant response of the Ag(111) sample is the driving frequency, which is unexpected because there is no one-photon resonant transition in the excitation scheme. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B40.00006: Phonon spectra on ultrathin Pb films with scanning tunneling spectroscopy Hyoungdo Nam, Chih-Kang Shih After Blatt and Thomson's prediction [Phys. Rev. Lett. 10, 332 (1963)], several groups have reported the quantum size effect on transition temperature(Tc) as a function of thicknesses of atomically flat ultrathin Pb film. In those cases, Tc oscillation related to film thickness was attributed to oscillation of the density of states (DOS) near the Fermi energy. However, the Tc oscillation amplitude is much smaller than that derived from the DOS oscillation. One therefore would ask: What is the role of electron-phonon interaction? Also as reported by Qin, et. al. [Science 324, 1314 (2009)], when the film is only 2ML thick, the pseduomorphically strained film has lower Tc than the unstrained one, suggesting that interfacial phonons may play a role. To answer to above question, we perform layer-dependent scanning tunneling spectroscopy of Pb films on Si(111) at 2.3 K to observe the phonon related features in the tunneling spectra. Detailed analysis of thickness dependence of photon spectra will be reported. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B40.00007: An STM and STS study on Iridium modified Si(111) Surface Nuri Oncel, Dylan Nicholls The structure of Si(111) $\sqrt 7 \times \sqrt 7 \,R\,19.1^{0}-Ir$ reconstructed surface have been investigated with the help of scanning tunneling microscopy/spectroscopy and low energy electron diffraction. We propose a model based on the experimental data. The model defines a unit cell containing one surface substitutional iridium atom centered under six silicon ad-atoms. Once the sample is annealed at 1200 $^{\circ}$C, a low density lattice gas of these ring clusters forms on top of an impurity stabilized $'1\times 1'$ domains. These ring clusters and $'1\times 1'$ domains co-exist with $7\times 7$ domains of clean Si(111) surface. The local density of states graphs measured on Si(111) $\sqrt 7 \times \sqrt 7 \,R\,19.1^{0}-Ir$ reconstructed surface contains an asymmetric peak at the edge of the valence band suggesting that there is a surface state exhibiting a Rashba type spin-orbit coupling. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B40.00008: Surface Electronic Excitations of Quantum Confined Mg Films on Si(111) Ao Teng, Krzysztof Kempa, Xiaoguang Li, Mustafa Ozer, Saban Hus, Paul Snijders, Geunseop Lee, Hanno Weitering We have investigated surface electronic excitations at atomically-smooth ultrathin Mg(0001) films on a Si(111)-7x7 substrate using high-resolution electron energy loss spectroscopy. The monopole and multipole surface plasmons of bulk Mg have their counterparts in the thin film regime. The dispersion of the monopole mode, as well as the relative intensity of the multipole mode, exhibit interesting thickness dependencies that are directly associated with quantum size effects in the Mg films. Additionally, we present the first clear observation of a photo-threshold excitation not seen at the surface of bulk Mg. Its intensity is also thickness dependent and anti-correlates with the multipole mode intensity. The results can be modeled with an effective jellium model in which the local Wigner-Seitz radius follows the thickness-dependent variation of the ground-state charge density at the surface. The results are a clear manifestation of quantum-size phenomena in the collective plasmon response of ultrathin metal films. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B40.00009: Oscillation of conductivity in layer-by-layer growth of Bi thin film phase Yasunori Fujikawa, Eiji Saitoh Thin film growth of Bi and related compounds has been attracted much attention because of their exotic properties originating in the large spin-orbit interaction of Bi. Growth of its simple substance is known to result in the formation of a thin-film phase in the initial stage, which is taken over by the bulk growth when the coverage exceeds several monolayers (ML). [1] With typical growth conditions, this transition takes place before the completion of the thin-film layer, which tends to agglomerate to form 4-ML thick islands, making it difficult to measure the intrinsic property of the thin-film phase. In this work, Bi growth on Si(111)-7x7 has been performed in a multi-probe VT-STM system, which provides wide-ranging opportunity of kinetic control and in-situ transport measurement during the thin film growth. By tuning the kinetic condition of the growth, it becomes possible to grow the thin-film phase uniformly covering the substrate in layer-by-layer mode. In-situ transport measurement has been performed during the layer-by-layer growth of the Bi thin-film phase, distinguishing the conductivity of each growth unit. It oscillates with a period of 2 ML, which reflects the atomic structure of the thin-film phase. [1] Nagao et al., Phys. Rev. Lett. 93, 105501 (2004). [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B40.00010: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 1:15PM - 1:27PM |
B40.00011: Measurement of bandgap and doping effects in ultrathin MoS2 layers using scanning tunneling spectroscopy. Chih-Pin Lu, Guohong Li, Ivan Skachko, Eva Andrei Molybdenum disulfide MoS$_{2}$, a semiconductor in the layered transition-metal dichalcogenide family of materials which is composed of weakly interacting layers held together by van der Waals interactions, offers an attractive possibility as a field effect transistor in low-power switching devices. We studied ultrathin MoS2 samples, ranging from single to several layers in thickness, that were extracted by mechanical exfoliation from the bulk material. Using a device geometry which allows varying the carrier density by gating across a 300nm insulating layer of SiO$_{2}$, together with low temperature Scanning Tunneling Microscopy and Spectroscopy, we investigated the bandgap and its dependence on doping and number of layers. For few layer samples we observe a well resolved atomic structure and a band gap of $\sim$ 1.1eV which is a little small than bulk band gap of 1.2eV. In addition we observe that electron doping shifts the Fermi energy into the conduction band. In single layer samples the measured bandgap is about $\sim$ 1.8eV in agreement with photoluminescence measurements and can change by backgate voltage. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B40.00012: Probing the Effects of Interface Band Structure Using Ballistic Electron Emission Microscopy Robert Balsano, Vincent LaBella Ballistic electron emission microscopy (BEEM) is a scanning tunneling microscopy (STM) technique that can measure transport of hot electrons through materials and interfaces with high spatial and energetic resolution. Using this technique an attenuation length for electrons in the film can be extracted from the relationship between film thickness and the number of hot electrons transmitted through the film. The behavior of the attenuation lengths of carriers with energies just above the Schottky barrier height is indicative of the interface band structure. BEEM requires an additional contact to ground the metal base layer of a metal semiconductor junction. Performing BEEM in situ with the sample fabrication greatly increases the through put for these types of measurements. This presentation will detail our data on electron transport through metals and across different interfaces and also highlight our work to develop a special silicon substrate that has the extra contact and oxide hard mask built in to enable in situ BEEM without modifications to the STM. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B40.00013: Interface States in the metal-CdSe interfaces Michelle Tomasik, Jeffrey Grossman, Varadharajan Srinivasan CdSe, a potential material for hybrid solar cells, has a well known reconstruction at the surface which removes the surface states. Using Density Functional Theory (DFT) we explore what happens to the now-removed surface states when CdSe is interfaced with two different metals, Al and Au. We compare and contrast this with the interfaces of a pristine unreconstructed CdSe surface with the two metals. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B40.00014: Calculated Stability and Band Offsets for Compensated and Abrupt Polar Si/Zn(S,Se) (111) Interfaces David Foster, Guenter Schneider Heterovalent semiconductor interfaces, particularly in the non-symmetrizable (111) and (0001) directions, present computational challenges that must be addressed in order to predict properties such as band offsets and interface energies. We perform first principles GGA+$U$ calculations of interface energies and band offsets for the nominally polar interfaces Si/Zn(S,Se) (111). Such wide-gap/narrow-gap heterostructures have been proposed as a possible means for altering the relaxation channel branching ratios for the decay of high energy photoelectrons (blue to UV) in favor of impact ionization (two carrier pairs from one photon). Examining configurations with one and two substitutional defect layers, we find the expected trend that compensated interfaces typically have lower energies than abrupt interfaces. The valence band offset ($-0.8 \pm 0.1$ eV) for the lowest energy abrupt Si/ZnS interfaces agrees well with the experimentally determined value of $-0.7$ eV. We examine methods to address the ambiguities that arise from both finite size induced inter-interface charge transfer and the non-symmetrizability of (111) oriented supercells. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B40.00015: Atom probe characterization of an AlN interlayer within HEMT structures grown by molecular beam epitaxy and metal-organic chemical vapor deposition Baishakhi Mazumder, Stephen W. Kaun, Jing Lu, Stacia Keller, Umesh K. Mishra, James S. Speck An AlN interlayer is introduced in a conventional AlGaN/GaN HEMT to enhance the density and mobility of the two dimensional electron gas (2DEG). MBE and MOCVD are two competitive and proven techniques to grow high quality AlN, but a chemical characterization technique is desired to investigate the purity of the AlN interlayer. Amongst nanoanalyzing techniques, atom probe tomography (APT) is unique for its spatial resolution and 3-D compositional images (\textless\ 0.2nm) with analytical sensitivity (10appm). In this work, plasma assisted MBE(PAMBE) and MOCVD techniques were employed to grow AlGaN/AlN/GaN heterostructures. Detailed compositional data from atom probe shows that a pure AlN layer was grown by PAMBE. From Hall measurements, the carrier density (sheet resistance) was found to be 1.65 $\times$ 10$^{13}$cm$^{-3}$(188$\Omega $/sq). The MOCVD structures do not form a pure AlN layer but that of Al$_{0.45}$Ga$_{0.55}$N layer. The carrier density was found to be 1.15 $\times$ 10$^{13}$cm$^{-3}$ (425 $\Omega $/sq). This work showed that MBE technique is more suitable than MOCVD for growing pure AlN interlayers and that APT can provide valuable nano scale information for further optimization of growth structures, thereby improving device performance. [Preview Abstract] |
Session B41: Non-equilibrium Physics with Cold Atoms II
Sponsoring Units: DAMOPChair: Lode Pollet, Ludwig Maximilian University
Room: 350
Monday, March 18, 2013 11:15AM - 11:27AM |
B41.00001: Time dependent impurity in ultracold fermions: orthogonality catastrophe and beyond Michael Knap, Aditya Shashi, Yusuke Nishida, Adilet Imambekov, Dmitry A. Abanin, Eugene Demler The physics of impurities in metals and mesoscopic structures provided a deeper understanding of electrical and thermal transport properties, guided the development of new mathematical techniques, and gave useful insights into the behavior of more complicated strongly correlated materials. Ensembles of ultracold atoms offer new opportunities to study impurity physics in a well isolated, coherent setting with relatively slow time scales, that can be faithfully determined by a small number of precisely controllable parameters. In this talk, we outline a program of how to explore quantum impurity problems with ultracold atoms. In particular, we reconsider the problem of the orthogonality catastrophe (OC), which describes the dynamics of a localized impurity in a Fermi sea, and show that techniques from atomic physics, such as Ramsey pulses, spin-echo, and RF-spectroscopy, can be used to probe the OC in both time and energy domains. We present the complete solution of the OC using a combination of analytical and numerical techniques and discover new qualitative features which could not be observed in metallic systems. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B41.00002: Topological charge pumping in a one-dimensional optical lattice Lei Wang, Matthias Troyer, Xi Dai A topological charge pump transfers charge in a quantized fashion. The quantization is stable against the detailed form of the pumping protocols and external noises. Such a quantum pump shares the same topological origin as the quantum Hall effect. We propose an experiment setup to realize the topological charge pumping of cold atoms in a one-dimensional optical lattice. The quantization of the pumped charge is confirmed by first-principle simulations of the dynamics of uniform and trapped systems. Quantum effects are shown to be crucial for the topological protection of the charge quantization. Finite-temperature and non-adiabatic effect on the experimental observables are discussed. Realization of such a topological charge pump servers as a firm step towards exploring topological nontrivial phases and non-equilibrium dynamics using cold atoms. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B41.00003: Heat and spin transport in a cold atomic fermi gas Hyungwon Kim, David Huse Motivated by recent experiments measuring the spin transport in ultracold unitary atomic Fermi gases [Sommer et al., Nature (London) 472, 201 (2011); Sommer et al., New J. Phys. 13, 055009 (2011)], we explore the theory of spin and heat transport in a three-dimensional spin-polarized atomic Fermi gas. We develop estimates of spin and thermal diffusivities and discuss magnetocaloric effects, namely the the spin Seebeck and spin Peltier effects. We estimate these transport coefficients using a Boltzmann kinetic equation in the classical regime and present experimentally accessible signatures of the spin Seebeck effect. We study an exactly solvable model that illustrates the role of momentum-dependent scattering in the magnetocaloric effects. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B41.00004: Non-equilibrium steady states in quenched s-wave superfluids Maxim Dzero, Emil Yuzbashyan, Victor Gurarie Nature and microscopic structure of the non-equilibrium many-body states in strongly interacting quantum systems remains one of most active research areas in physics. In this work, we study the steady states, which appear in a s-wave superfluid at zero temperature following a quench of the pairing strength. We use the BCS Hamiltonian which we solve exactly in the thermodynamic limit using classical integrability. We obtain a generic phase diagram of the resulting steady states for quenches corresponding to an arbitrary change of the pairing strength. We calculate single particle distribution function for each of the steady states that we find. In addition, we determine the asymptotic behavior of the pairing amplitude at long times. The experimental signatures of the steady states will also be discussed. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B41.00005: Mean-field description of non-equilibrium dynamics of a 1D Bose gas in a weak optical lattice potential Juan Carrasquilla, Aaron Reinhard, Laura Zundel, Jean-Felix Riou, David Weiss, Marcos Rigol We study the expansion of a large array of one-dimensional Bose gases subject to a weak optical lattice potential using Gutzwiller mean-field calculations aimed at describing a recent experiment with ultracold atoms. We calculate the evolution of the density profile, the quasimomentum distribution, and the density profile after a band-mapping protocol followed in experiments with ultracold atoms designed to measure the quasimomentum distribution. We find that a large fraction of bosons remains trapped at the center of the lattice. Furthermore, interactions during the expansion dramatically change the momentum distribution. Our simulations qualitatively capture most aspects of the experiment. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B41.00006: Influence of the Amplitude in Lattice Modulation Spectroscopy Andreas Dirks, Karlis Mikelsons, Jim Freericks, H.R. Krishnamurthy Within the Mott-insulating phase of the Hubbard model, linear-response calculations for a periodically modulated optical lattice depth clearly predict a resonance when modulated at a frequency equal to the Hubbard repulsion U. In this work we examine the effect of the amplitude of the lattice depth modulation on the threshold for excitation. Based on a recently developed strong-coupling approach to the non-equilibrium Hubbard model, we report results on the nonlinear regime and discuss effects of the amplitude as compared to the frequency for driving excitations into the upper Hubbard band. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B41.00007: Decoherence and heating of two species fermions in optical lattices Saubhik Sarkar, Johannes Schachenmayer, Stephan Langer, Andrew J. Daley Experiments with ultracold fermionic atoms in optical lattices present a unique way to study strongly interacting many-body quantum systems, including the Fermi-Hubbard model, in a microscopically well-understood environment. A key challenge to explore many interesting quantum phases is to reach sufficiently low temperatures and therefore it is necessary to charecterise and control competing heating processes in experiments. Incoherent scattering of light from the lasers that form the lattices can contribute significantly to the heating. We study the robustness of many-body states to this mechanism, deriving a many-body master equation for two-component fermions and investigating how the heating is influenced by choices in the atomic physics and how it depends on the parameteres in the many-body Hamiltonian. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B41.00008: Diffusive Spin Transport of Lattice Fermions in One Dimension Andrew Snyder, Theja De Silva We study the long-time spin transport of fermions moving diffusively in a one dimensional lattice due to a directly introduced population imbalance and harmonic trapping potential. We combine the thermodynamic Bethe anzatz technique with the local density approximation to calculate local quantities such as magnetization and polarization. Utilizing Fick's Law, we are able to calculate the ratio of spin current to spin diffusion coefficient for both the weak and strong coupling cases that is driven by the population imbalance. We find spin current is characterized by magnetization moving from regions of low magnetization to high, with spin current being zero through insulating regions. Further, in the weak coupling limit, utilizing the linear response theory and calculating current-current correlation, we calculate local spin diffusion coefficient. The local spin diffusion coefficient shows maxima at all the insulating regions. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B41.00009: Interaction-induced transport of ultra-cold atoms in 1D optical lattices Daniel Gruss, Chih-Chun Chien, Massimiliano Di Ventra, Michael Zwolak The study of time-dependent, many-body transport phenomena is increasingly within reach of ultra-cold atom experiments. These systems not only allow experimental emulation of solid state systems, but allow us to probe the dynamics of transport at a previously unreachable level of detail. We will discuss computational results for the dynamics of electronic/atomic transport and, in particular, simulation of interacting fermionic atoms via a micro-canonical transport formalism using approximations that go beyond mean-field. We will discuss applications of this in terms of simulating particle currents under the influence of applied current and potentials, differing spin-spin interactions, and inhomogeneous lattice impurities. Finally, we will discuss these results in the context of present-day cold atom experiments.\footnote{C.C. Chien, D. Gruss, M. Di Ventra, and M. Zwolak. \\ arXiv:1203.5094v2, 2012.} [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B41.00010: Effect of quantum fluctuations on classical motion near a separatrix in a weakly anharmonic lattice Rafael Hipolito, Vadim Oganesyan We investigate the role of quantum fluctuations in the relaxation of a nonequilibrium interacting system for which the phase space curve of the corresponding classical dynamics lies near a separatrix. Such a system may be realized, for example, in a weakly interacting bosonic system if we initially excite a normal mode which lies in the low quasimomentum sector for which the the system is nearly dispersionless but of nondecay type ($\omega''(q)\la 0$). As an example of such a system, we consider the case of a weakly anharmonic lattice in one dimension, where our results have some relevance to the famous Fermi-Pasta-Ulam problem. In the regime considered, we show that the classical dynamics is effectively dominated by just two normal modes which can be mapped into a single particle problem whose phase space curve lies near a separatrix. We show that for the quantum system the initial number of quanta plays the role of effective $\hbar$. Quantum fluctuations have a dramatic effect on the classical trajectory, causing the system to relax into a steady state where both the time scales associated with the relaxation and the steady state itself are strongly dependent on effective $\hbar$. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B41.00011: Emergence of long distance pair coherence through incoherent local environmental coupling Jean-Sebastien Bernier, Peter Barmettler, Dario Poletti, Corinna Kollath We demonstrate that the interplay between a purely local incoherent environmental coupling, effectively heating up the system, and Hamiltonian dynamics generates quantum coherence. For a repulsively interacting fermionic lattice gas initially prepared in a Mott insulating state, coupling a noise field to the local spin density produces coherent fermionic pairs. We show that the formation of pair coherence is approximately diffusive with distance, and is experimentally observed in the pair momentum distribution as the formation of a sharp feature at the zone boundary. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B41.00012: Dynamics of spin-1 bosons in an optical lattice Khan W. Mahmud, Eite Tiesinga We study spin-mixing and collapse and revival dynamics of spin-1 atoms in an optical lattice. Starting with the ferromagnetic or anti-ferromagnetic superfluid ground state - a sudden raising of the lattice depth creates a non-equilibrium state. Analysis of the oscillations in atom numbers in different spin states and the collapse and revivals in visibility reveals details about the system parameters and the initial superfluid state. For example, in situ number oscillations reveal the spin-dependent interactions, and visibility oscillations reveal the ratio of on-site and spin-dependent interactions, and thus the various scattering lengths in different channels can be determined. To study the interplay of superlfuidity and magnetism, we also examine the oscillations in various observables in the presence of an external magnetic field in the form of quadratic Zeeman energy. The frequency spectrum of the oscillations reveals the discrete energy levels and relative importance of different Fock states in the initial superfluid and magnetic states. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B41.00013: Fluctuation-induced dissipation in non-equilibrium moving systems Mohammad Maghrebi, Ramin Golestanian, Robert Jaffe, Mehran Kardar Quantum fluctuations in moving systems lead to nontrivial effects such as dissipation and radiation. We consider moving bodies---a single rotating object or multiple objects in relative motion---and derive the frictional force by using techniques from non-equilibrium statistical physics as well as quantum optics. The radiation to the environment is obtained as a general expression in terms of the scattering matrix which is a powerful analytical tool. We apply our general formulas to several examples of systems out of equilibrium due to their motion. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B41.00014: Dynamical Entanglement Growth and Measurement with Cold Atoms or Ions Johannes Schachenmayer, Hannes Pichler, Peter Zoller, Ben Lanyon, Andrew J. Daley Systems of cold atoms in optical lattices or a string of ions in a linear trap offer the possibility to experimentally study non-equilibrium dynamics of 1D many-body quantum systems with interactions of varying range in a controlled environment. Entanglement is a basic feature of these systems, and the increase of the entanglement entropy between different blocks of a many-body state as a function of time determines whether the long-time evolution of the system can be efficiently simulated on a classical computer. Correspondingly, states with large-scale entanglement offer regimes where quantum simulators could be used to outperform classical simulation. Thus, there is a great interest to produce large-scale entanglement in these types of experiments. Here we present analytical and numerical results on the entanglement entropy growth behavior in 1D lattice systems after a sudden quench of a model parameter, and the dependence of this growth on the range of the interactions. Furthermore, we present how bipartite R\'enyi entropies can be measured solely by using tunnel couplings and local measurements, tools which are both available in recent experiments with bosons in optical lattices. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B41.00015: Non-equilibrium scaling, response and coarsening in the quantum large N vector model Anushya Chandran, Vedika Khemani, Arun Nanduri, S. S. Gubser, S. L. Sondhi The out-of-equilibrium dynamics of a quantum system that is suddenly or slowly driven in the vicinity of critical point is conjectured to be universal and can be described in a scaling framework. The long time tails of scaling functions for a quench from the disordered to the ordered phase are of particular experimental interest. We theoretically investigate this in the $O(N)$ vector model as $N\rightarrow \infty$ for different spatial dimensions. We demonstrate that the quartic operator that is irrelevant to the equilibrium physics above the upper critical dimension is dangerously irrelevant to the long time dynamics in the scaling limit. We also observe a quantum analogue of the classical process of coarsening in which a correlation length diverges at long times in the thermodynamic limit. Suitably defined linear response measurements offer the tantalizing possibility of directly observing the non-equilibrium scaling functions; we explore these in classical models and Chern insulators as well. [Preview Abstract] |
Session B42: Quantum Hall Effect: Edges, Interferometry, & nu = 5/2
Sponsoring Units: FIAPChair: Sriram Ganeshan, University of Maryland
Room: Hilton Baltimore Holiday Ballroom 3
Monday, March 18, 2013 11:15AM - 11:27AM |
B42.00001: Spin and charge distribution symmetry dependence of stripe phases in two-dimensional electron systems confined to wide quantum wells Yang Liu, Dobromir Kamburov, Mansour Shayegan, Loren Pfeiffer, Ken West, Kirk Baldwin When a spin-split $N\le 2$ Landau level is half filled, the two-dimensional electron system (2DES) is expected to break the rotational symmetry by forming a unidirectional charge density wave, the so-called stripe phase. The stripes are known to rotate from the ``normal'' ([110]) direction to the ``abnormal'' ($[1\bar{1}0]$) direction when the 2DES density is raised above a critical density. We report a study of the evolution of the stripe phase orientation near Landau level filling factors $\nu = 13/2$ and 15/2 when $E_F$ lies in the two, spin-split, $N = 2$ Landau levels of the symmetric subband (the S2$\uparrow$ and S2$\downarrow$ levels) while the $N = 0$ Landau levels of the antisymmetric subband are fully occupied. We find that when $E_F$ lies in S2$\downarrow$ the stripes are always formed along the ``normal'' direction. But, when $E_F$ lies in the S2$\uparrow$ level, the orientation of the stripes can rotate to be along the ``abnormal'' direction at high densities. At a density where the stripe phase at $\nu = 13/2$ is along the ``abnormal'' direction, we can rotate it back to the normal direction by making the charge distribution asymmetric while keeping the density fixed. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B42.00002: Distinguishing Particle-Hole Conjugated Fractional Quantum Hall States Using Quantum Dot Mediated Edge Transport Hsin-Hua Lai, Kun Yang We first study the edge transport in the $\nu=1/3$ and $\nu=2/3$ Fractional Quantum Hall bars mediated by a $\nu=1$ quantum dot. We conclude that the $\nu=1/3$ and $\nu=2/3$ systems show different $1/3$-charged quasi-particle tunneling exponents. When the quantum dot becomes large, its edge states join those of the original Hall bar to reconstruct the edge state configurations. In the disorder-irrelevant phase, the two-terminal conductance of the original $\nu=1/3$ system vanishes at zero temperature, while that of the $\nu=2/3$ case is finite. In the disorder-dominated phase, the two-terminal conductance of $\nu=1/3$ system is $(1/5)e^2/h$ while that of $\nu=2/3$ system is $(1/2)e^2/h$. We further apply the same idea to the $\nu=5/2$ system which realizes either Pfaffian or anti-Pfaffian states. By engineering a central $\nu=3$ quantum dot in the $\nu=5/2$ Hall bar, we study the charged quasi-particle tunneling effects and conclude that the Pfaffian and anti-Pfaffian states show different quasi-particle tunneling exponents. If the quantum dot is large enough for its edge states joining with those of the original Hall bar, the two-terminal conductance of Pfaffian state can be $G_{Pf}\rightarrow 2 e^2/h$ while that of anti-Pfaffian state is higher, $G_{aPf} > 2 e^2/h$. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B42.00003: Intrinsic edge dipole moment of incompressible fractional quantum Hall ground states YeJe Park, F.D.M. Haldane The edges of incompressible fractional quantum Hall (FQH) fluids have a characteristic dipole moment related to their Hall viscosity, which can be split into two separate contributions: a (trivial) contribution from the Landau orbit (common to all FQH fluids in the same Landau level, and a (non-trivial) guiding-center contribution that depends on the FQH state. Using the model wave functions for (fermonic and bosonic) Laughlin states ($\nu$ = 1/2, 1/3, 1/4), and Moore-Read states ($\nu$ = 2/2, 2/4) expressed as Jack polynomials, we obtained the guiding-center occupation number distributions $n(k)$ of ``Landau-gauge" basis states with $k$ near the edge $``\nu^{-1}k_F"$ of a FQH fluid in cylindrical geometries of various circumferences, and verified the ``Luttinger" and ``edge-dipole" sum rule. The edge-dipole moments of the FQH fluids were expressed as a combination of quantized quantities: electric charge e, ``guiding center spin" s and number of fluxes per ``composite boson'' q in [F. D. M. Haldane, arXiv:0906.1854 (2009)]. Our work provides a numerical verification of the prediction. The edge dipole experiences a force due to the gradient of electric field perpendicular to the edge, and the force is balanced by stress from the ``guiding center Hall viscosity". [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B42.00004: FQHE interferometer in strong tunneling regime: The role of compactness of edge fields Sriram Ganeshan, Alexander Abanov, Dmitri Averin The defining feature of quantum Hall states is the existence of topologically protected massless edge states. These states are believed to be effectively described by a theory of chiral bosons also known as the one-dimensional chiral Luttinger Liquid. The tunneling experiments provide one of the natural ways to probe these edge states. In this work, we consider multiple-point tunneling in the interferometers formed between edges of electron liquids with in general different filling factors in the regime of the Fractional Quantum Hall effect (FQHE). We derive an effective matrix Caldeira-Leggett model for the multiple tunneling contacts connecting the chiral single-mode FQHE edges. We show that the compactness of the Wen-Fr\"ohlich chiral boson fields describing the FQHE edge modes plays a crucial role in defining strong (quasiparticle) tunneling regime. We also show that the compactness condition results in electron periodicity for quasiparticle tunneling with respect to adiabatic variation of flux. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B42.00005: Edge properties of principal fractional quantum Hall states in the cylinder geometry Paul Soule, Thierry Jolicoeur We study fractional quantum Hall states in the cylinder geometry with open boundaries. We focus on principal fermionic $\nu = 1/3$ and bosonic $\nu = 1/2$ fractions in the case of hard-core interactions. The gap behavior as a function of the cylinder radius is analyzed. By adding enough orbitals to allow for edge modes, we show that it is possible to measure the Luttinger parameter of the nonchiral liquid formed by the combination of the two counterpropagating edges when we add a small confining potential. Although we measure a Luttinger exponent consistent with the chiral Luttinger theory prediction for the full hard-core interaction, the exponent remains nontrivial in the Tao-Thouless limit as well as for simple truncated states that can be constructed on the cylinder. If the radius of the cylinder is taken to infinity, the problem becomes a Tonks-Girardeau one-dimensional interacting gas in Fermi and Bose cases. Finally, we show that the Tao-Thouless and truncated states have an edge electron propagator, which decays spatially with a Fermi-liquid exponent, even if the energy spectrum can still be described by a nontrivial Luttinger parameter. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B42.00006: Edge spin excitations and reconstructions of spin-polarized and spin-unpolarized quantum Hall liquids Yuhui Zhang, Kun Yang We study the effect of electron-electron interaction on the charge and spin structures at the edge of quantum Hall liquids, under three different kinds of confining potentials. Our exact diagonalization calculation for small systems indicates that the low energy excitations of $\nu=1$ ferromagnetic state are bosonic edge spin waves. Instabilities of $\nu=1$ ferromagnetic state with altering confinement strength result from the softening of these edge spin waves, and formation of edge spin textures. In $\nu\la 2$ regime, exact diagonalization on edge electron systems indicates that compact Hartree-Fock states with different total spin always become ground states in some regions of parameter space, and the ground states appear in between two compact states are their edge spin waves. The initial $\nu=2$ instability is toward the compact state with total spin $1$. Larger systems are studied using a microscopic trial wave functions, and some quantitative predictions on the edge instabilities for a certain type of confining potential are reached in the thermodynamic limit. In fractional quantum Hall regime, $\nu= 1/3$ polarized and $\nu=2/3$, $2/5$ unpolarized states' low energy edge states are also obtained by exact diagonalization for small systems. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B42.00007: Quantum Hall Line Junctions under In-Plane Magnetic Fields P. Jiang, I. Yang, W.-H. Wang, S.-C. Yu, L.N. Pfeiffer, K.W. West, K.W. Baldwin, W. Kang Study of tunneling between two antiparallel quantum Hall edge states under the influence of in-plane magnetic field will be presented. Previously quantum Hall line junctions were shown to have highly correlated behavior consistent with formation of coupled Luttinger liquids. Power-law energy dependence observed in the tunneling conductance supports realization of the Luttinger-liquid correlation of the coupled edge states. Under the presence of in-plane magnetic field, the tunnel spectrum is found to evolve with emergence of novel features. Systematic evolution of these features are studied with in-plane fields either parallel or perpendicular to the line junction. We discuss these results in terms of the presence of additional low-excitation modes detected through momentum-resolved tunneling. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B42.00008: X-ray edge singularity in the visibility of the Aharonov-Bohm oscillations in a quantum Hall interferometer Iurii Chernii, Ivan Levkivskyi, Eugene Sukhorukov We consider a quantum dot strongly interacting with several quantum Hall edge channels. One of the channels is an arm of an electronic Mach Zender interferometer, and another one is coupled to the dot via weak tunneling. Fluctuations of the charge in the quantum dot lead to dephasing of the interfering electrons. Such processes have been studied extensively, however the effects of backaction were either not considered at all, or taken into account perturbatively in the interaction strength. We show that there are regimes where tunneling itself is mainly induced by the non-equilibrium noise in the interferometer at finite bias. Importantly, this backaction effect is non-perturbative and can not be neglected. The problem of tunneling induced by the non-equilibrium noise demonstrates equivalence to the X-Ray edge singularity problem, and the tunneling rates are found to be a power-low functions of the detuning between the dot energy level and the Fermi energy. Consequently, the visibility of the interference pattern shows a crossover between the two lorentzian-type functions with different effective temperatures at small and large energies. The two temperatures are proportional to the noise temperature with a coefficients depending on the interaction strength. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B42.00009: Backaction Dephasing Induced by a Quantum Dot Detector Toshihiro Kubo, Yasuhiro Tokura We theoretically investigate the backaction dephasing by a quantum dot detector (QDD) that couples to the quantum dot embedded in one arm of Aharonov-Bohm (AB) interferometer. We employ the nonequilibrium second-order perturbation theory and provide an analytical expression for the backaction dephasing rate, which characterizes the disturbance induced by coupling with an environment containing QDD. We show that the origin of backaction dephasing is a charge noise of QDD. In the linear transport regime through a QDD, this backaction dephasing induced by charge noise can be explained as a relaxation by an inelastic electron-electron scattering within the framework of Fermi liquid theory. In the low bias voltage regime, the increase or decrease of dephasing rate depends on the QDD energy level, the linewidth functions, and how to apply the bias voltage. Unlike quantum point contact detector, the dephasing rate would be insensitive to the bias voltage in a high bias voltage regime since the charge noise of a QDD is saturated. Moreover, such behaviors can be verified in terms of the visibility of AB oscillations by changing the bias voltage across the QDD. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B42.00010: Visibility recovery by strong interaction in an electronic Mach-Zehnder interferometer Soo-Yong Lee, Hyun-Woo Lee, Heung-Sun Sim We study the evolution of a single-electron packet of Lorentzian shape along an edge of the integer quantum Hall regime or in a Mach-Zehnder interferometer, considering a capacitive Coulomb interaction and using a bosonization approach. When the packet propagates along a chiral quantum Hall edge, we find that its electron density profile becomes more distorted from Lorentzian due to the generation of electron-hole excitations, as the interaction strength increases yet stays in a weak interaction regime. However, as the interaction strength becomes larger and enters a strong interaction regime, the distortion becomes weaker and eventually the Lorentzian packet shape is recovered. The recovery of the packet shape leads to an interesting feature of the interference visibility of the symmetric Mach-Zehnder interferometer whose two arms have the same interaction strength. As the interaction strength increases, the visibility decreases from the maximum value in the weak interaction regime, and then increases to the maximum value in the strong interaction regime. We argue that this counter-intuitive result also occurs under other types of interactions. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B42.00011: Coupling a quantum Hall droplet to a microwave transmission line Jennifer Cano, Chetan Nayak Electromagnetically coupling a quantum Hall droplet to a microwave transmission line establishes a realm of new experiments that might provide a more direct measurement of certain physical properties. Specifically, peaks in the absorption spectrum would occur at multiples of the ratio of the edge velocity to the perimeter of the droplet, potentially offering a more precise measurement of the velocity of edge modes than the few existing measurements. If the droplet is at filling fraction 5/2 and deformed to allowing tunneling between edges, additional peaks would emerge corresponding to the velocity of the neutral mode, which has never before been measured. In addition, the set-up could be used as an interferometer in fractional quantum Hall states by observing shifts in the magnitude of the absorption peak at fixed frequency as the number of quasiparticles is varied via the magnetic field. This would be complementary to existing interferometry measurements of fractional statistics. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B42.00012: Interactions in quantum Hall edge channels at filling fraction 2 Pascal Degiovanni, Erwann Bocquillon, Vincent Freulon, Charles Grenier, Jean-Marc Berroir, Bernard Pla\c{c}ais, Antonella Cavanna, Yong Jin, Gwendal F\`eve Coulomb interactions play a major role in one dimensional electronic transport. They modify the nature of the elementary excitations from Landau quasiparticles in higher dimensions to collective excitations in 1D. We report here on the direct observation of the collective neutral and charge modes of the two chiral co-propagating edge channels of opposite spins of the quantum Hall effect at filling factor $\nu=2$. Generating a charge density wave at frequency $f$ in the outer channel, we measure the current induced by inter-channel Coulomb interaction in the inner channel after a 3 microns propagation length. Varying the driving frequency from 0.7 to 11 GHz, we observe damped oscillations in the induced current that results from the phase shift between the fast charge and slow neutral eigenmodes. Measuring the dispersion relation and dissipation of the neutral mode from provides quantitative information on the scattering of quantum edge magnetoplasmons. We will then comment on the consequences of these results on quasi-particle relaxation and decoherence in the $\nu=2$ quantum Hall edge channel system. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B42.00013: Coherent Terahertz Magneto-Spectroscopy of High-Mobility Two-Dimensional Electron Gases Qi Zhang, Takashi Arikawa, Wei Pan, John Reno, John Watson, Michael Manfra, Junichiro Kono Landau-quantized high-mobility two-dimensional electron gases (2DEG) in GaAs quantum wells provide an ideal platform for studying and controlling the coherence of many-electron states. Here, we study the coherent dynamics of cyclotron resonance (CR) in a 2DEGin the terahertz range. It is well known that Kohn's theorem protects the CR frequency from the influence of electron-electron interactions, but how the coherence of CR decays via electron-electron interactions is an open question. Since the 1980s, studies have focused on CR decoherence time measurements, primarily using incoherent far-infrared spectroscopy, which fails to obtain the true CR linewidth due to the `saturation effect' in high-mobility systems. By using coherent time-domain magneto-terahertz spectroscopy, we have systematically studied the CR decoherence time in an ultrahigh-mobility 2DEG as a function of both temperature and magnetic field. These results show a clear saturation of the CR decoherence time at low temperature, which decreases monotonically with increasing magnetic field. No filling-factor-dependent oscillations of CR dephasing time have been observed. Possible CR decoherence mechanisms will be discussed in light of these new findings. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B42.00014: Influence of Device Geometry on Tunneling in $\nu$=5/2 Quantum Hall Liquid Guang Yang, Dmitri Feldman Two recent experiments [1,2] measured the temperature and voltage dependence of the tunneling current through a constriction in the $\nu$=5/2 quantum Hall liquid. The results led to conflicting conclusions about the nature of the 5/2 quantum Hall state. The results of Ref. [1] were interpreted as supporting the anti-Pfaffian non-Abelian state while the results of Ref. [2] suggested that the Abelian 331 state was more likely. Several different constriction geometries were used in Refs. [1,2]. We argue that in some of those geometries there is significant unscreened electrostatic interaction between segments of the quantum Hall edge on the different sides of the constriction. The Coulomb interaction affects the tunneling current. After the Coulomb corrections are taken into account, the results from all geometries agree and support the same 5/2 state.\\[4pt] [1] I. P. Radu, J. B. Miller, C. M. Marcus, M. A. Kastner, L. N. Pfeiffer, and K. W. West, Science 320, 899 (2008).\\[0pt] [2] X. Lin, C. Dillard, M. A. Kastner, L. N. Pfeiffer, and K. W. West, Phys. Rev. B 85, 165321 (2012). [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B42.00015: Anomalous density dependence of the activation gap of $\nu =$5/2 fractional quantum Hall state at extremely large Landau level mixing Nodar Samkharadze, Michael Manfra, Loren Pfeiffer, Ken West, Gabor Csathy We have conducted a study of the density dependence of $\nu =$5/2 fractional quantum Hall state (FQHS) in the regime of extremely low densities, down to n$=$4.9x10\textasciicircum 10 cm\textasciicircum -2. In the density range accessed in our sample, the Landau level mixing parameter $\kappa $ spans the so far unexplored range 2.52\textless $\kappa $ \textless 2.82. Here we observe an anomalous dependence of the activation gap of $\nu =$5/2 FQHS on the carrier density. We discuss the possible origins of this unexpected behavior. N.S. and G.C. were supported by the NSF grant DMR-0907172 and DMR-1207375. K. West and L. Pfeiffer acknowledge the support of the Princeton NSF-MRSEC and the Moore Foundation. [Preview Abstract] |
Session B43: Focus Session: Multiscale modeling--Coarse-graining in Space and Time II
Sponsoring Units: DCPChair: Garegin Papoian, University of Maryland at College Park
Room: Hilton Baltimore Holiday Ballroom 2
Monday, March 18, 2013 11:15AM - 11:51AM |
B43.00001: Multiscale simulations of ion channel opening and closing provide insights into the molecular mechanisms of gating Invited Speaker: Gerhard Hummer We develop and implement a multiscale molecular simulation approach to study the opening and closing of a ligand-gated ion channel at atomic resolution. Ligand-gated channels are essential in biological signaling pathways that range from chemical sensing in bacteria to the firing of neurons in humans. On the basis of recently determined crystal structures and with the help of multiscale molecular simulations we study the conformational changes associated with GLIC ion channel gating transition. Starting from a coarse-grained transition pathway constructed on the basis of a multistate elastic network model, we perform string-method molecular dynamics simulations to refine the pathway at full atomic resolution. We find that the channel closes in an iris-like fashion as a result of a two-stage tilting of the pore lining helices. Water plays a central role in the gating transition. We find that the hydrophobic gate of the pore undergoes highly cooperative transitions between a densely filled and an empty state. The subtle tilting of the helices shifts the balance to the dry state, in which a 1.5 nm long hydrophobic stretch of the pore completely empties. By calculating the ionic conductance and the underlying free energy surface, we quantitatively demonstrate that this drying of the hydrophobic constriction, not sterics, is the major determinant of ion conductivity in the GLIC pentameric ion channel. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B43.00002: Multiscale modeling of macromolecular dynamics Invited Speaker: Cecilia Clementi The understanding of emerging collective behaviors in biomolecular complexes represents a major challenge in modern biophysics. As a first step toward the study of such processes we have applied multi-resolution nonlinear dimensionality reduction and diffusion analysis to obtain reliable low-dimensional representations and models for the dynamics of apparently high-dimensional complex systems such as proteins in a biological environment. The results clearly show that the proposed methods can efficiently find low dimensional representations of complex processes such as protein folding, and suggest strategies to simplify significantly the study of such processes. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 1:03PM |
B43.00003: Transferability of Coarse Grained Models: a Challenge for Simulation of Phase Transitions or Phase Separation Processes Invited Speaker: Christine Peter Upon developing a coarse grained (CG) model, representability and transferability limitations are a problem that is inherent to the process of reducing the number of degrees of freedom. In this context, representability refers to the question which structural or thermodynamic properties of a higher resolution reference are reproduced by the CG model, and transferability refers to the question to which extent a CG model is applicable at a state-point that differs from the one where it was parametrized. This is naturally a highly relevant problem in simulations that involve phase transitions or structure formation processes driven by phase separation, for example in liquid crystalline systems or in biomolecular aggregation. I will show with a few examples how one can achieve and rationalize state-point transferability for CG models that have been parameterized in a bottom-up procedure from atomistic reference simulations, for example by choosing an appropriate reference state point. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:39PM |
B43.00004: Mesoscopic Dynamics of Biopolymers and Protein Molecular Machines Invited Speaker: Raymond Kapral The dynamics of biopolymers in solution and in crowded molecular environments, which mimic some features of the interior of a biochemical cell, will be discussed. In particular, the dynamics of protein machines that utilize chemical energy to effect cyclic conformational changes to carry out their catalytic functions will be described. The investigation of the dynamics of such complex systems requires knowledge of the time evolution on physically relevant long distance and time scales. This often necessitates a coarse grained or mesoscopic treatment of the dynamics. A hybrid particle-based mesoscopic dynamical method, which combines molecular dynamics for a coarse-grain model of the proteins with multiparticle collision dynamics for the solvent, will be described and utilized to study the dynamics of such systems. See, C. Echeverria, Y. Togashi, A. S. Mikhailov, and R. Kapral, Phys. Chem. Chem. Phys 13, 10527 (2011); C. Echeverria and R. Kapral, Phys. Chem. Chem. Phys., 14, 6755 (2012); J. M. Schofield, P. Inder and R. Kapral, J. Chem. Phys. 136, 205101 (2012). [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B43.00005: Multiscale Modeling of Deformation of Glassy Polymers Thomas Rosch, John Brennan, Sergei Izvekov, Jan Andzelm We examine the ability of chemically informed coarse-grained (CG) models to quantitatively describe correct mechanical properties of glassy polymer systems. The force-matching and the structure-matching procedures were used to obtain CG potentials at different levels of resolution. Equilibrium molecular dynamics simulations of amorphous polymers modeled at the all-atom level provided the necessary reference data. This work explores what characteristics are necessary for quantitative agreement of stress-strain curves between scales. For large coarse-graining (17 atoms per CG site of polystyrene) the force-matching procedure produces a potential that does not contain enough attraction to predict the correct elastic properties. Systematic methods were employed to match mechanical properties and their effects on polymer structure were examined. Higher resolution coarse-graining (5-11 atoms per CG site) is better able to reproduce atomistic mechanical data. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B43.00006: Force fields for describing the solution-phase synthesis of shape-selective metal nanoparticles Ya Zhou, Wissam Al-Saidi, Kristen Fichthorn Polyvinylpyrrolidone (PVP) and polyethylene oxide (PEO) are structure-directing agents that exhibit different performance in the polyol synthesis of Ag nanostructures. The success of these structure-directing agents in selective nanostructure synthesis is often attributed to their selective binding to Ag(100) facets. We use first-principles, density-functional theory (DFT) calculations in a vacuum environment to show that PVP has a stronger preference to bind to Ag(100) than to Ag(111), whereas PEO exhibits much weaker selectivity. To understand the role of solvent in the surface-sensitive binding, we develop classical force fields to describe the interactions of the structure-directing (PVP and PEO) and solvent (ethylene glycol) molecules with various Ag substrates. We parameterize the force fields through force-and-energy matching to DFT results using simulated annealing. We validate the force fields by comparisons to DFT and experimental binding energies. Our force fields reproduce the surface-sensitive binding predicted by DFT calculations. Molecular dynamics simulations based on these force fields can be used to reveal the role of solvent, polymer chain length, and polymer concentration in the selective synthesis of Ag nanostructures. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B43.00007: Enhancing and reversing the electric field at liquid/liquid interfaces Yufei Jing, Guillermo Guerrero Garcia, Monica Olvera de la Cruz The ion distribution at the interface between two immiscible electrolyte solutions determines the macroscopic properties of these liquid interfaces. The classical Poisson-Boltzmann theory has been widely used to describe it, even though it neglects the polarization and ion correlations typical of these ionic solutions. Here, we provide an enhanced description of a liquid/liquid interface in the presence of an electric field from first principles--that is, without needing any fitting parameter--including ion correlations, image charges and realistic ion-sizes in Monte Carlo simulations. Our data agree well with experimental excess surface tension measurements for a wide range of electrolyte concentrations, contrasting with the results of the classical Poisson-Boltzmann theory. More importantly, we observe that, in the vicinity of the point of zero charge, the electric field can increase significantly in strength near the liquid interface, or it can even reverse locally, at high salt concentration. [Preview Abstract] |
Session B44: Focus Session: Population and Evolutionary Dynamics II
Sponsoring Units: DBIO GSNPChair: Uwe Tauber, Virginia Tech
Room: Hilton Baltimore Holiday Ballroom 1
Monday, March 18, 2013 11:15AM - 11:51AM |
B44.00001: The statistics of genetic diversity in rapidly adapting populations. Invited Speaker: Michael Desai Evolutionary adaptation is driven by the accumulation of beneficial mutations, but the sequence-level dynamics of this process are poorly understood. The traditional view is that adaptation is dominated by rare beneficial ``driver'' mutations that occur sporadically and then rapidly increase in frequency until they fix (a ``selective sweep''). Yet in microbial populations, multiple beneficial mutations are often present simultaneously. Selection cannot act on each mutation independently, but only on linked combinations. This means that the fate of any mutation depends on a complex interplay between its own fitness effect, the genomic background in which it arises, and the rest of the sequence variation in the population. The balance between these factors determines which mutations fix, the patterns of sequence diversity within populations, and the degree to which evolution in replicate populations will follow parallel (or divergent) trajectories at the sequence level. Earlier work has uncovered signatures of these effects, but the dynamics of genomic sequence evolution in adapting microbial populations have not yet been directly observed. In this talk, I will describe how full-genome whole-population sequencing can be used to provide a detailed view of these dynamics at high temporal resolution over 1000 generations in 40 adapting \textit{Saccharomyces cerevisiae }populations. This data shows how patterns of sequence evolution are driven by a balance between chance interference and hitchhiking effects, which increase stochastic variation in evolutionary outcomes, and the deterministic action of selection on individual mutations, which favors parallel solutions in replicate populations. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B44.00002: Metastability and Anomalous Fixation in Evolutionary Games on Scale-Free Networks Michael Assaf, Mauro Mobilia We study the influence of complex graphs on the metastability and fixation properties of a set of evolutionary processes. In the framework of evolutionary game theory, where the fitness and selection are frequency dependent and vary with the population composition, we analyze the dynamics of snowdrift games (characterized by a long-lived metastable coexistence state) on scale-free networks. Using an effective diffusion theory valid in the weak selection limit, we demonstrate how the scale-free structure affects the system's metastable state and leads to anomalous fixation. In particular, we analytically and numerically show that the probability and mean time to fixation are characterized by stretched-exponential behaviors with exponents depending on the network's degree distribution.\\[4pt] M. Assaf* and M. Mobilia*, PRL 109, 188701 (2012) (* - equal contribution) [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B44.00003: Unveiling adaptation using high-resolution lineage tracking Jamie Blundell, Sasha Levy, Daniel Fisher, Dmitri Petrov, Gavin Sherlock Human diseases such as cancer and microbial infections are adaptive processes inside the human body with enormous population sizes: between $10^6 - 10^{12}$ cells. In spite of this our understanding of adaptation in large populations is limited. The key problem is the difficulty in identifying anything more than a handful of rare, large-effect beneficial mutations. The development and use of molecular barcodes allows us to uniquely tag hundreds of thousands of cells and enable us to track tens of thousands of adaptive mutations in large yeast populations. We use this system to test some of the key theories on which our understanding of adaptation in large populations is based. We (i) measure the fitness distribution in an evolving population at different times, (ii) identify when an appreciable fraction of clones in the population have at most a single adaptive mutation and isolate a large number of clones with independent single adaptive mutations, and (iii) use this clone collection to determine the distribution of fitness effects of single beneficial mutations. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B44.00004: Evolutionary dynamics of fluctuating populations with strong mutualism Thiparat Chotibut, David Nelson Evolutionary game theory with finite interacting populations is receiving increased attention, including subtle phenomena associated with number fluctuations, i.e., ``genetic drift.'' Models of cooperation and competition often utilize a simplified Moran model, with a strictly fixed total population size. We explore a more general evolutionary model with \textit{independent} fluctuations in the numbers of two distinct species [1], in a regime characterized by ``strong mutualism.'' The model has two absorbing states, each corresponding to fixation of one of the two species, and allows exploration of the interplay between growth, competition, and mutualism. When mutualism is favored, number fluctuations eventually drive the system away from a stable fixed point, characterized by cooperation, to one of the absorbing states. Well-mixed populations will thus be taken over by a single species in a finite time, despite the bias towards cooperation. We calculate both the fixation probability and the mean fixation time as a function of the initial conditions and carrying capacities in the strong mutualism regime, using the method of matched asymptotic expansions. Our results are compared to computer simulations.[1] S. Pigolotti et al., http://arxiv.org/abs/1208.4973 [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B44.00005: Range expansions favor the evolution of cooperation in an experimental microbial metapopulation Manoshi Datta, Kirill Korolev, Ivana Cvijovic, Carmel Dudley, Jeff Gore Natural populations frequently undergo range expansions in response to changes in the environment. Recent work suggests that range expansions can have a strong effect on evolution, even leading to the fixation of deleterious alleles that would normally be outcompeted in the absence of migration. However, little is known about how range expansions might influence alleles under frequency- or density-dependent selection. Moreover, there is very little experimental evidence to complement existing theory, since expanding populations are difficult to study in nature. In this study, we have used a yeast experimental system to explore the effect of range expansions on the evolution of cooperative behaviors, which commonly display frequency- and density-dependent selection and are widespread in nature. We found that range expansions favor the evolution of cooperation in two ways: (1) through the enrichment of cooperators at the front of the expanding population, and (2) by allowing cooperators to ``outrun'' an invading wave of defectors. In this system, cooperation is enhanced through the coupling of population ecology and evolutionary dynamics in expanding populations, providing experimental evidence for a novel mechanism through which cooperative behaviors could be maintained in nature. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B44.00006: Slower recovery in space before collapse of connected populations Lei Dai, Kirill Korolev, Jeff Gore Slower recovery from perturbations near a tipping point and its indirect signatures in fluctuation patterns have been suggested to alert catastrophes in a wide variety of systems. Recent studies of populations in the field and in the laboratory have used time-series data to confirm some of the theoretically predicted early warning indicators, such as an increase in recovery time or in the size and timescale of fluctuations. However, the performance of warning signals in spatially extended systems remains to be examined empirically. Here we use spatially extended yeast populations, an experimental system displaying a fold bifurcation, to evaluate early warning signals based on spatio-temporal fluctuations and to identify a novel warning indicator in space. We found that two leading indicators based on fluctuations increased before collapse of connected populations; however, the magnitude of increase was smaller than that observed in isolated populations, possibly because local variation is reduced by dispersal. Furthermore, we propose a generic indicator based on deterministic spatial patterns, ``recovery length''. As the spatial counterpart of recovery time, recovery length is defined as the distance for connected populations to recover from perturbations in space (e.g. a region of poor quality). In our experiments, recovery length increased substantially before population collapse, suggesting that the spatial scale of recovery can provide a superior warning signal before tipping points in spatially extended systems. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B44.00007: Competitive Exclusion in Microbial Communities Charles Fisher, Pankaj Mehta The competitive exclusion principle of ecology suggests that two or more species cannot coexist in a community while living off of the same resources. Therefore, only species that occupy different niches can coexist. The process of community assembly is also heavily influenced by neutral drift due to stochastic birth, death and immigration of species. Currently, there is no consensus on the relative importance of ``niche'' and ``neutral'' processes in community assembly. We develop a stochastic birth-death-immigration model with competition for resources to examine the relative importance of these processes in microbial communities, and search for signatures of competitive exclusion in a large dataset of microbial community compositions containing relative species abundance data for thousands of environments. In addition, we discuss the role of metabolism in defining microbial niches. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B44.00008: Quantifying genetic diversity under a broad spectrum of deleterious mutations Benjamin Good, Michael Desai Recent studies have shown that selection against deleterious mutations may play a major role in shaping observed patterns of sequence variation in natural populations. However, our understanding of these patterns remains limited, since selection creates correlations along the genome that are difficult to disentangle from each other. Previous theoretical work has focused on the qualitative effects of selection on sequence diversity, using simplified models in which all selected mutations have the same fitness cost. Yet is known that deleterious mutations follow a wide distribution in most organisms, so it is necessary to extend our theoretical predictions to this more general case before we can make quantitative connections with existing data. The evolutionary dynamics of this regime are complicated: extant mutant lineages represent large, correlated fluctuations away from the background expectation, which hinders efforts to apply existing methods based on deterministic or ``mean-field'' approximations. Here, we will describe recent progress towards this goal, which is based on a ``coarse-graining'' of the underlying distribution of fitnesses in the population. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B44.00009: Cooperative Antibiotic Resistance in a Multi-Drug Environment Eugene Yurtsev, Lei Dai, Jeff Gore The emergence of antibiotic resistance in bacteria is a significant health concern. A frequent mechanism of antibiotic resistance involves the production of an enzyme which inactivates the antibiotic. By inactivating the antibiotic, resistant cells can ``share" their resistance with other cells in the bacterial population, suggesting that it may be possible to observe cooperation between strains that inactivate different antibiotics. Here, we experimentally track the population dynamics of two \emph{E. coli} strains in the presence of two different antibiotics. We find that together the strains are able to grow in antibiotic concentrations that inhibit growth of either of the strains individually. We observe that even when there is stable coexistence between the two strains, the population size of each strain can undergo large oscillations. We expect that our results will provide insight into the evolution of antibiotic resistance and the evolutionary origin of phenotypic diversity and cooperative behaviors. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B44.00010: Stochastic Loss of an Occasionally-Essential Function Elizabeth Jerison, Michael Desai Many biological functions are useful only in specific circumstances. For example, hundreds of single-gene deletions in yeast increase growth rate in some laboratory conditions. During periods of disuse, these genes are vulnerable to disruption or loss via random mutation and genetic drift. Yet they are maintained in natural populations, suggesting that they must be useful at least occasionally. Here we quantify the risk of loss of such occasionally-important functions. We focus on predicting how the statistics of environmental change determine the mean time to loss of the function. Our results suggest a refinement to the Savageau 'use-it-or-lose-it' principle of regulation, and put theoretical lower bounds on how often these functions must be necessary to the organism, in order to be maintained. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B44.00011: Rarely clicking Muller's ratchets Stephan Eule, Jakob Metzger In populations of finite size, weakly deleterious mutations can fix by chance. This phenomenon has been termed Muller's ratchet and one click of the ratchet refers to the loss of the fittest class of individuals with the fewest mutations. Despite the simplicity of the classical mathematical model of Muller's ratchet, surprisingly little is known in the biologically relevant regime where a click of the ratchet is a rare event. Here we show numerically that in this regime the rate of the ratchet strongly depends on the applied microscopic formulation (Wiright-Fisher/Moran) of the model, thus challenging the widely used diffusion approximation. Furthermore by employing a WKB-approximation in a simplified model, we obtain analytical results for the click rate, which agree well with the click rate of the full ratchet of the corresponding microscopical model. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B44.00012: Evolution of Bacterial Suicide Martin Tchernookov, Ilya Nemenman While active, controlled cellular suicide (autolysis) in bacteria is commonly observed, it has been hard to argue that autolysis can be beneficial to an \textit{individual} who commits it. We propose a theoretical model that predicts that bacterial autolysis is evolutionarily advantageous to an \textit{individual }and would fixate in physically structured environments for stationary phase colonies. We perform spatially resolved agent-based simulations of the model, which predict that lower mixing in the environment results in fixation of a higher autolysis rate from a single mutated cell, regardless of the colony's genetic diversity. We argue that quorum sensing will fixate as well, even if initially rare, if it is coupled to controlling the autolysis rate. The model does not predict a strong additional competitive advantage for cells where autolysis is controlled by quorum sensing systems that distinguish self from nonself. These predictions are broadly supported by recent experimental results in \textit{B. subtilis }and \textit{S. pneumoniae.} [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B44.00013: Experimental Insights into Collective Effects in Eukaryotic Cell Proliferation in Dilute Suspensions Carl Franck, Igor Segota, Ariana Strandburg-Peshkin, Xiao-Qiao S. Zhou, Archana Rachakonda, Benjamin Yavitt, Catherine J. Lussenhop, Sungsu Lee, Kevin Tharratt, Amrish Deshmukh, Elisabeth Sebesta, Myron Zhang, Sharon Lau, Sarah Bennedsen, David Franck, Viyath Fernando, Junseok Oh Physicists can look to dilute suspensions of apparently solitary cells in suspension for elegant realizations of multicellular behavior. In contrast to our earlier work (Phys. Rev. E v. 77, 041905 (2008)) with the amoeba Dictyostelium discoideum we are discovering that the vital intercellular communications responsible for the well-known but poorly understood slow to fast transition in a growing culture as a function of time might be due to the passage of chemical messages between transient cell clusters or throughout the entire system as opposed to binary collisions. In considering the observed variation in proliferation rates we have been surprised to discover that for best growth cultures are much more dependent on incubator geometry than previously suspected. [Preview Abstract] |
Session B45: Focus Session: Structure and Dynamics of Biomembranes II
Sponsoring Units: DBIOChair: Mu-Ping Nieh, University of Connecticut
Room: Hilton Baltimore Holiday Ballroom 4
Monday, March 18, 2013 11:15AM - 11:51AM |
B45.00001: Effect of Protein Crowding: Multivalent Protein Binding Induces a New Phase State in Lipid Membranes Invited Speaker: Tonya Kuhl It is well known that lipid membrane properties change as a function of composition and phase state, and that protein-lipid interaction can induce changes in the membrane's properties and biochemical response. This talk demonstrates that multivalent binding of proteins to putative membrane receptors can induce structure changes and a new phase state in lipid membranes. These molecular level changes are precisely characterized using grazing incidence X-ray diffraction. Protein binding is shown to perturb lipid packing within lipid monolayers and bilayers resulting in topological defects and the emergence of a new orientationally textured lipid phase. In bilayers this altered lipid order is transmitted from the receptor laden exterior membrane leaflet to the inner leaflet, representing a potential mechanism for lipid mediated outside-in signaling by multivalent protein binding. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B45.00002: Stabilization of composition fluctuations in mixed membranes by hybrid lipids Samuel Safran, Benoit Palmieri A ternary mixture model is proposed to describe composition fluctuations in mixed membranes composed of saturated, unsaturated and hybrid lipids. The asymmetric hybrid lipid has one saturated and one unsaturated hydrocarbon chain and it can reduce the packing incompatibility between saturated and unsaturated lipids. A methodology to recast the free-energy of the lattice in terms of a continuous isotropic field theory is proposed and used to analyze composition fluctuations above the critical temperature. The effect of hybrid lipids on fluctuations domains rich in saturated/unsaturated lipids is predicted. The correlation length of such fluctuations decreases significantly with increasing amounts of hybrids even if the temperature is maintained close to the critical temperature. This provides an upper bound for the domain sizes expected in rafts stabilized by hybrids, above the critical temperature. When the hybrid composition of the membrane is increased further, a crossover value is found above which ``stripe-like'' fluctuations are observed. The wavelength of these fluctuations decreases with increasing hybrid fraction and tends toward a molecular size in a membrane that contains only hybrids. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B45.00003: Self-assembly of colloidal rafts Prerna Sharma, Thoams Gibaud, Andrew Ward, Zvonimir Dogic Interactions between nanometer-sized particles or molecules suspended in a bulk fluid are well understood. However, when such particles are embedded in a membrane, the inter-particle potential is significantly modified by membrane mediated forces and gives rise to novel phase behavior. Visualizing and manipulating such inclusions in a lipid bilayer is difficult due to the nanometer length scales involved. Here, we use a model system of micron sized colloidal membranes doped with molecules shorter or longer than that of the bulk. Surprisingly, the dopant molecules form self-limited finite size clusters. These clusters further self-organize into a wide variety of higher order structures such as hexagonal and square lattice arrays, lamellar patterns and saddle shaped surfaces. Understanding the phase behavior and measuring repulsive forces between such clusters may have implications for the similar mechanisms that operate in conventional lipid bilayers. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B45.00004: Mechanism of lipid bilayer penetration by mixed monolayer-protected gold nanoparticles Reid Van Lehn, Prabhani Atukorale, Randy Carney, Francesco Stellacci, Darrell Irvine, Alfredo Alexander-Katz Recently, gold nanoparticles (AuNPs) protected by a binary mixture of hydrophobic and hydrophilic alkanethiol ligands were observed to spontaneously penetrate cellular membranes via a non-specific mechanism. Penetration was observed even at low temperatures and in the presence of endocytotic inhibitors, implying that AuNPs crossed the membrane by a non-endocytotic process. Furthermore, penetration was shown to depend on the amphiphilicity and nanoscale morphology of the protecting monolayer. In this work, we use a variety of simulation techniques to elucidate the mechanism of lipid bilayer penetration and compare our results to experiments with lipid vesicles. We show that these AuNPs can stably embed within lipid bilayers by ``snorkeling'' charges out of the bilayer core; the stability of such a state is a function of particle size, the composition of the protecting monolayer, and other environmental conditions. We use detailed simulations to analyze structural changes in the surrounding lipids and show that the energy barrier for embedding is considerably reduced in the presence of bilayer defects. We expect that these results will enable the design of novel drug delivery carriers and biosensors. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B45.00005: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 12:39PM - 12:51PM |
B45.00006: Dynamics and Self-Assembly of Nanoparticles on Biomembranes Rupak Bhattacharya, Vishal Maingi, Subbarao Kanchi, Bagul Rahul Suresh, N Jayaraman, Prabal Maity, K.G Ayappa, Jaydeep Basu We have recently been investigating the diffusion mediated self-assembly of various types of Dendrimers on supported DMPC lipid bilayer. Atomic Force Microscopy is used to study the pattern formation for PETIM dendrimers of different core composition as well as of generations. Extensive studies have been carried out using different concentration and different packing of lipid molecules constituting the lipid bilayer. Interestingly Oxygen Core dendrimer forms regular circular patterns on membranes whereas the Nitrogen Core dendrimer do not. A fully atomistic Molecular Dynamics simulation with implicit water clearly shows the evidence of domain formation for O-core dendrimers on bilayer, which is absent in the other one. Different generation for Oxygen core dendrimers forms patterns with a pore inside. The reduction of the diameter of these patterns with decreasing packing of lipid molecules indicates the possible role of lipid molecules in aggregation process. Further study using Confocal Fluorescence Correlation Spectroscopy is underway to correlate this type of membrane mediated pattern formation with underlying lipid diffusion. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B45.00007: Cholesterol Translocation in a Phospholipid Membrane Amit Choubey, Rajiv Kalia, Noah Malmstadt, Aiichiro Nakano, Priya Vashistha Cholesterol (CHOL) molecules play a key role in modulating the rigidity of cell membranes, and controlling intracellular transport and signal transduction. Using all-atom molecular dynamics and the parallel replica approach, we study the process of CHOL interleaflet transport (flip-flop) in a dipalmitoylphosphatidycholine (DPPC)--CHOL bilayer, the effect of this process on mechanical stress across the bilayer, and the role of CHOL in inducing molecular order in the respective bilayer leaflets. The simulations are carried out at physiologically relevant CHOL concentration (30{\%}), temperature 323 K and pressure 1 bar. CHOL flip-flop events are observed with a rate constant of 3$\times $10$^{4}$ s$^{-1}$. Once a flip-flop event is triggered, a CHOL molecule takes an average of 73 nanoseconds to migrate from one bilayer leaflet to the other. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B45.00008: Simulating liquid-liquid phase separation and lipid transport on the Anton special purpose machine Edward Lyman, Logan Sandar, Alexader Sodt, Richard W. Pastor We present simulation data for a bilayer composed of a ternary mixture of cholesterol, dioloeoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine. The composition is chosen to be in the two-phase region and the temperature in the vicinity of the miscibility transition. Using the Anton special purpose computer to generate continuous trajectories longer the ten microseconds---which admits complete mixing of the lipids---we observe robust liquid-liquid phase coexistence. The time-and ensemble-averaged mean squared displacement (MSD) displays anomalous scaling on timescales less than 50 nsec and normal diffusion on longer timescales. The short-time anomalous scaling is explained by a mode-coupling argument[Flenner et al Phys Rev E 79:011907(2009)]. The per-lipid MSD's suggest that a few lipids remain associated with the liquid ordered domain for the duration of the simulation, suggesting a possible mechanism for anomalous transport on experimentally accessible timescales. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B45.00009: Interaction of Ionic Liquids with Lipid Biomembrane: Implication from Supramolecular Assembly to Cytotoxicity Benxin Jing, Nan Lan, Y. Elaine Zhu An explosion in the research activities using ionic liquids (ILs) as new ``green'' chemicals in several chemical and biomedical processes has resulted in the urgent need to understand their impact in term of their transport and toxicity towards aquatic organisms. Though a few experimental toxicology studies have reported that some ionic liquids are toxic with increased hydrophobicity of ILs while others are not, our understanding of the molecular level mechanism of IL toxicity remains poorly understood. In this talk, we will discuss our recent study of the interaction of ionic liquids with model cell membranes. We have found that the ILs could induce morphological change of lipid bilayers when a critical concentration is exceeded, leading to the swelling and tube-like formation of lipid bilayers. The critical concentration shows a strong dependence on the length of hydrocarbon tails and hydrophobic counterions. By SAXS, Langmuir-Blodgett (LB) and fluorescence microscopic measurement, we have confirmed that tube-like lipid complexes result from the insertion of ILs with long hydrocarbon chains to minimize the hydrophobic interaction with aqueous media. This finding could give insight to the modification and adoption of ILs for the engineering of micro-organisms. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B45.00010: The Effect of Tension on Phase Transitions and Domains in Phospholipid Membranes Maria Santore, Dong Chen The relevance phase transitions in phospholipid membranes to the effect of confinement on phase transitions and to the structure-function relationship in biological membranes has driven decades of scientific study of the behavior of model membranes. A primary focus of these studies has been the impact of temperature. We argue here, however, that tension can have a profound impact on transitions, suppressing domain formation, or shifting the nature of the domains themselves. While Clausius-Clapeyron predicts depression of a melting transition as small as 1/3 C for every mN/m of applied tension, the presence of a triple point or similar features can lead to the formation of different domains altogether. We provide here dramatic demonstrations of these behaviors in the form of fluorescence microscopy images in systems with controlled tension. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B45.00011: Confinement of 5CB Between Lyotropic Bilayers Cory Dolbashian, Rizwan Mahmood, Tommaso Bellini, Noel Clark We report phase behavior of mixtures of 5CB (4-Cyano-4'-Pentyl-1, 1'-biphenyl), a calamitic thermotropic liquid crystal, with mixtures of the lyotropic double tailed cationic surfactant DDAB (diodecyldimethylammonium-bromide) and water. These mixtures had a fixed ratio of DDAB to water (75{\%} / 25{\%}) and 5CB concentrations ranging from 10{\%} to 85{\%}. Our preliminary phase diagram suggests transition from isotropic to lamellar phase having higher birefringence at higher DDAB concentration. We have also observed low vale of birefringence at lower DDAB concentration suggesting swelling of bilayers. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B45.00012: Bilayer thickness mismatch controls domain size in biomimetic membranes Frederick A. Heberle, Robin S. Petruzielo, Jianjun Pan, Paul Drazba, Norbert Ku\v{c}erka, Robert F. Standaert, Gerald W. Feigenson, John Katsara In order to promote functionality, cells may alter the spatial organization of membrane lipids and proteins, including separation of liquid phases into distinct domains. In model membranes, domain size and morphology depend strongly on composition and temperature, but the physicochemical mechanisms controlling them are poorly understood. Theoretical work suggests a role for interfacial energy at domain boundaries, which may be driven in part by thickness mismatch between a domain and its surrounding bilayer. However, no direct evidence linking thickness mismatch to domain size in free-standing bilayers has been reported. We describe the use of Small Angle Neutron Scattering (SANS) to detect domains in simplified lipid-only models that mimic the composition of plasma membrane. We find that domain size is controlled by the degree of acyl chain unsaturation of low-melting temperature lipids, and that this size transition is correlated to changes in the thickness mismatch between coexisting liquid phases. [Preview Abstract] |
Session B46: SPS Undergraduate II
Sponsoring Units: SPSChair: Crystal Bailey, American Physical Society
Room: Hilton Baltimore Holiday Ballroom 5
Monday, March 18, 2013 11:15AM - 11:27AM |
B46.00001: Redesign of an AC Magnetic Susceptometer for Measurements in Smaller Samples Andres Vargas, Ryan Fukuda, Smitha Sunny, Pei-Chun Ho A new AC magnetic susceptometer was created for the purpose of measuring the magnetic properties of smaller samples, such as nanoparticles that are currently being synthesized in our lab. The susceptometer consists of a primary coil, a secondary coil, and a sample holder. The primary coil is the outer component of the susceptometer, which provides a magnetic field when current is applied due to Ampere's Law. Inside of the primary coil lies the secondary coil, which has two oppositely wound solenoids; they are oppositely wound to reduce background signal. The sample holder lies inside of the secondary coil with the sample. All of these go inside of a beryllium copper casing for protection. We tested the susceptometer by looking for the ferromagnetic phase transition of an 11 mg Gd sample. A $\sim 100 \mu$A AC current was applied to the primary coil, which created a magnetic field that polarized the magnetic moments in the sample. This induced a voltage on the secondary coil, which is proportional to the magnetic susceptibility. We measured the temperature dependency of the induced voltage from 10 K to 300 K. The results showed a sharp increase in the induced voltage around 293K, which agrees with the known ferromagnetic transition of Gd. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B46.00002: Electrospun fibers of PLA/P3HT blends for device and sensor applications William Serrano, Nicholas Pinto The thermoplastic aliphatic polyester, poly (lactic acid) (PLA) is a biodegradable polymer that is sometimes used in implant screws for bone repair. Our focus was to fabricate fibers of this polymer and its blends with p-doped poly (3-hexylthiophene)-(P3HT) in order to extend its use to devices and/or sensors. PLA/P3HT fibers were prepared in air at room temperature using the electrospinning technique that is cheap, fast and reliable. Scanning Electron Microscope images of the fibers reveal that the presence of P3HT does not affect the fabrication of PLA fibers at low or high polymer concentrations in chloroform, retaining the same morphological structure of pure PLA fibers. The fiber diameters were in the range 1-10 microns. A slight increase in fiber formation results with the addition of P3HT, most likely due to a reduction of the solution surface tension. Results of the electrical characterization of this material will be presented. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B46.00003: AC Circuit Measurements with a Differential Hall Element Magnetometer Matthew W. Calkins, B. Scott Nicks, Pedro A. Quintero, Mark W. Meisel As the biomedical field grows, there is an increasing need to quickly and efficiently characterize more samples at room temperature. An automated magnetometer was commissioned to do these room temperature magnetic characterizations. This magnetometer, which is inspired by a Differential Hall Element Magnetometer,\footnote{Yongquing Li\textit{ et al}., Appl. Phys. Lett. \textbf{80 }(2002) 4644.} uses two commercially available Hall elements wired in series. One Hall element measures the external magnetic field of a 9 T superconducting magnet and the other measures the same external field plus the field due to the magnetization of the sample that sits on top of the Hall element. The difference between these two Hall elements is taken while a linear stepper motor sweeps through the external magnetic field. The linear motor and data acquisition are controlled by a LabVIEW program. Recently, the system was outfitted for AC circuit measurements and these data will be compared to DC circuit data. In addition, the lowest signal to noise ratio will be found in order to deduce the smallest amount of sample needed to register an accurate coercive field. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B46.00004: Low Temperature Probe for Measuring Anisotropic Magnetotransport Galin Dragiev, Daniel Grant, Amlan Biswas Certain materials display a change in resistance when a magnetic field is applied on them. This resistance change is called magnetoresistance (MR). The value of MR may also depend on the direction of the magnetic field relative to the crystal structure of the material, which is called anisotropic MR (AMR). We built a probe which allows us to measure the AMR of a sample in a temperature range of 1.2 K to 300K in magnetic fields of up to 9 tesla. The probe design allows the angle between the magnetic field and a particular direction of the sample to be changed over almost the entire solid angle of 4$\pi $. In particular, this probe lets us measure the AMR of a sample with magnetic anisotropy when the magnetic field is applied either along the hard or easy axes, or somewhere in between. The probe allows us to change the orientation of the sample while it is inside the low temperature cryostat. We will present our data on hole-doped manganese oxide (manganite) thin films and discuss the possible origins of AMR in these materials. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B46.00005: Probing Quantum Turbulence in He II with a MEMS Oscillator Aleksander Levental, Josh Bauer, Miguel Gonzalez, Pan Zheng, Yoonseok Lee, Ho Bun Chan Micrometer scale mechanical oscillators based on MEMS technology have been developed for the study of quantum fluids and have been tested successfully at ultra low temperatures. Our recent low temperature test [1] in which the device was immersed in the superfluid phase of $^{4}$He revealed striking behavior below 400 mK: nonlinear and hysteretic resonance at high excitations. The observed phenomenon is thought to be related to vortices and quantum turbulence and warrants a systematic investigation for better understanding. We constructed an experimental set-up that allows us to cool a MEMS device in liquid $^{4}$He down to 50 mK at pressures up to 25 bar. We will discuss our new set-up and present our preliminary results performed at saturated vapor pressure. \\[4pt] [1] M. Gonzalez, B. Moon, P. Zheng, E. Garcell, H. B. Chan, and Y. Lee. \textit{Journal of Low Temperature Physics, Online First}$^{TM}$\textit{, 22 August 2012}, DOI: 10.1007/s10909-012-0682-8. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B46.00006: Exploration of Quartz Tuning Forks as Potential Magnetometers for Nanomagnets B. Scott Nicks, Matthew W. Calkins, Pedro A. Quintero, Mark W. Meisel A change in the resonance frequency, $f_0 \approx 32$ kHz, of quartz tuning forks is expected when nano-sized magnetic particles or films are applied to a fork that is then exposed to a variable magnetic field. This work explores the feasibility of using these forks, once removed from their protective canisters, as potentially inexpensive magnetometers operating at room temperature in fields up to 2~T, and eventually up to 9~T, by analyzing the responses of loaded forks in such a field. However, the forks are also dependent on subtle variations of the ambient temperature, and the magnetic leads may present a background signal that must be subtracted. Preliminary results are encouraging, but better understanding of the noise sources must be made for these forks to be used as envisioned. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B46.00007: Toward CN-VFET logic circuits Stephen Gilbert, Bo Liu, Mitchell McCarthy, Evan Donoghue, Andrew Rinzler Gate field modulation of the Fermi level in the low density of electronic states carbon nanotubes provides a new control mechanism for modulating the Schottky barrier between the nanotubes and a semiconductor to control charge injection across their interface. This has been exploited in the recently developed carbon nanotube-enabled vertical field effect transistor (CN-VFET) comprised of a bottom gate, dielectric layer, dilute nanotube source electrode, semiconducting channel layer, and drain electrode situated in a collinear, vertical stack. Since the channel length in this architecture is simply the thickness of a thin film, the naturally short channel lengths can overcome the relatively low mobility of organic semiconductors to source higher on-state currents or potentially improve operating speeds. Prototype logic gates using such organic transistors have yet to be demonstrated. As a step in this direction we have fabricated organic CMOS inverters utilizing a p-type and an n-type CN-VFET. The device fabrication, materials used, performance and progress toward a CN-VFET ring oscillator will be discussed. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B46.00008: Pressure Dependence of MEMS Oscillator Quality Factor Joshua Bauer, Sarah Geiger, Miguel Gonzalez, Pan Zheng, Yoonseok Lee This paper details a study in which the pressure dependence of the quality factor and resonance frequency of a micro-electro-mechanical device is examined. The results obtained will aid in the understanding of the effects of slide film damping in various gasses on oscillators operating at micrometer length scales. The device utilized was a capacitively driven plate oscillator positioned 1.25$\mu$m above a silicon substrate. The dominant damping mechanism for this geometry is slide film damping from the gaseous film between the oscillating plate and substrate. The mechanical resonance of the device was characterized as a function of pressure from 6 mTorr to 1 atm in air. We observed three distinct damping regimes in the quality factor. In addition to the characterization performed in air, pressure dependences in helium and argon were also examined at pressure ranges of 6.5 mTorr to 5 Torr and 750mTorr to 760 Torr, respectively. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B46.00009: Intrinsic Localized Modes in nonlinear two-dimensional electrical lattices J.F. Stormes, L.Q. English, F. Palmero, P.G. Kevrekidis We report on the generation of stationary and traveling intrinsic localized modes (ILMs), also called discrete breathers or discrete solitons, in two dimensions in damped-driven electrical lattices. ILMs are spatially localized eigenmodes that arise due to the nonlinearity of the system, not due to spatial impurities. Since solitons are generally unstable in two dimensions, the existence of these ILMs relies on the discreteness of the lattice. We show experimentally that depending on the frequency and amplitude of the spatially uniform driving, different numbers of ILMs can be induced in both square and hexagonal lattices. In lattices that allow ILM motion, we furthermore study the interaction of such modes. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B46.00010: Thermal effects of laser illumination on coated quartz crystal microbalance surfaces Benjamin Keller, Keeley Stevens, Liming Pan, Jacqueline Krim Prior work on the thermal sensitivity of quartz crystal microbalances (QCM) has shown them to be powerful tools, capable of measuring milli-Kelvin temperature impulses while also presenting a well-understood response to steady state heating [1]. This has been demonstrated for physical contact to the QCM surface via a STM tip with a temperature differential [2]; here we present a novel application wherein a laser is focused onto the coated QCM, thus applying a non-contact thermal pulse. By applying variable length (second to minute) exposures from a laser source we can isolate the thermal shock, time decay and gross heating effects. The system is sensitive to the coating used, showing significant differences in heating for absorbative and reflective coatings. This method is unique in that the QCM measures energy lost into the substrate, unlike standard techniques which focus primarily on material efficiency. This has potential to characterize various coatings used in solar cells and thermal collectors, as well as in photovoltaic materials.\\[4pt] [1] Wolsky, S.\ P.\ and Zdanuk, E.\ J., editors. \textit{Ultra Micro Weight Determination in Controlled Environments} {\bf 1969}.\\[0pt] [2] Pan, L.\ and Krim, J.\ {\it Rev.\ Sci.\ Instr.\ }{\bf 2012}, in press. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B46.00011: Spectrum, symmetries, and dynamics of Heisenberg spin-1/2 chains Kira Joel, Davida Kollmar, Lea Santos Quantum spin chains are prototype quantum many-body systems. They are employed in the description of various complex physical phenomena. Here we provide an introduction to the subject by focusing on the time evolution of Heisenberg spin-1/2 chains with couplings between nearest-neighbor sites only. We study how the anisotropy parameter and the symmetries of the model affect its time evolution. Our predictions are based on the analysis of the eigenvalues and eigenstates of the system and then confirmed with actual numerical results. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B46.00012: High temperature series expansion and the exact solution study of the 1/5 depleted square lattice Ising model Simeon Hanks, Trinanjan Datta, Jaan Oitmaa The critical behavior of the 1/5 depleted square-lattice Ising model with nearest neighbor ferromagnetic interaction has been investigated by means of both a high-temperature series expansion and an exact solution. The critical point in the coupling constant has been accurately determined with a series expansion up to order eighteen in the high temperature expansion parameter. For the exact solution we use a set of decoration transformations to recast the original model in terms of a set of nearest neighbor, next-nearest neighbor, and four spin interaction Ising model. This is followed by a transformation to a staggered 8-vertex model. As the vertex weights satisfy the free-fermion condition the free energy and critical point are obtainable by standard methods. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B46.00013: Developing an Embedded Atom Method Potential for Copper Ben Stortenbecker, Brian Demaske, Vasily Zhakhovsky, Ivan Oleynik A new embedded-atom method (EAM) interatomic potential for copper has been developed in order to improve upon the predictive power of atomistic simulations under extremes of pressures and temperatures induced by shock compression and ultrashort laser irradiation. Several candidate potentials were fit to a database consisting of \textit{ab initio} cold pressure tensor components calculated for a wide range of hydrostatic and uniaxial deformations as well as experimental properties near equilibrium conditions. The close relationship between the stress tensor and interatomic forces under naturally-occurring material states ensures the accuracy of the potential without the need for a large number of fitting points. After fitting, the candidates were then screened against the experimental melting point in order to select a single best potential. This final potential will be verified against the experimental melting line, liquid-vapor coexistence curve, and the shock Hugoniot. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B46.00014: Surface-induced reduction of the spin coherence times of nitrogen-vacancy centers in diamond Jeffrey M. Moore, Michael E. Flatt\'e The exceptionally long room-temperature spin coherence times of nitrogen-vacancy (NV) centers in diamond indicate their potential utility for quantum information processing. The remarkable sensitivity of the spin dynamics of NV centers to electric and magnetic fields, and to strain, also suggests these centers can be used in novel sensors. The sensitivity and spatial resolution of such a sensor will depend on the depth of the NV center below the diamond surface. Local relaxation of the atomic positions near the diamond surface, however, will strain the NV center and consequently reduce its spin coherence time. We evaluate this effect by calculating the strain near a (001) diamond surface using density functional theory. The strain for a specific NV-center depth was evaluated using the linearized augmented plane wave (LAPW) method and the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional within the WIEN2k density functional code. The effect of the resulting strain values on the spin coherence times were determined using a low-energy effective Hamiltonian for the NV-center energies and wave functions, and their strain dependence. This work was supported by an AFOSR MURI. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B46.00015: Density of States of Type-II Superconductors in High Magnetic Field and Low Temperatures Renzo Villazon, Owen Lehmer, Julian Irwin, Sasha Dukan In high magnetic fields and at low temperatures, electronic energies are quantized in the form of Landau levels. The inclusion of Landau level quantization in the superconducting pairing (both diagonal and off-diagonal) leads to gapless points on the Fermi surface. Within this theory, the density of states of a type-II superconductor in the range of magnetic fields 0.2B$_{c2}$ \textless\ B \textless\ B$_{c2}$ is calculated. The influence of disorder on the density of states is investigated for a range of impurity concentrations and scattering potential strengths. We compare our theoretical predictions to experimental results for superconductor YNi$_{2}$B$_{2}$C and find that our model is reliable at high magnetic fields but has limited applicability at lower fields. [Preview Abstract] |
Session B47: Invited Session: Physical Organizing Principles of Biomineral Formation
Sponsoring Units: DBIO DMPChair: Susan N. Coppersmith, University of Wisconsin
Room: Hilton Baltimore Holiday Ballroom 6
Monday, March 18, 2013 11:15AM - 11:51AM |
B47.00001: Phase transitions and their energetics in calcite biominerals Invited Speaker: Pupa Gilbert Biominerals include mollusk shells and the skeletons of algae, sponges, corals, sea urchins and most other animals. The function of biominerals are diverse: mechanical support, attack, defense, grinding, biting, and chewing, gravitational and magnetic field sensing, light focusing, and many others. The exquisite nanostructure of biominerals is directly controlled by the organisms, which have evolved to master the chemico-physical aspects of mineralization. By controlling the inorganic precursor nanoparticle size, packing, and phase transitions, organisms efficiently fill space, produce tough and hard structures, with micro- or macroscopic morphology optimized for their functions. Specifically, this talk will address two key questions: Q: How are the beautiful biomineral morphologies achieved? A: Using amorphous precursor phases, with phase transitions kinetically regulated (retarded) by proteins. Q: How do organisms co-orient their single-crystalline biominerals? A: Controlling the propagation of crystallinity one nanoparticle at a time, not atom-by-atom. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B47.00002: Bottom-up molecular models of hierarchical mineralized tissues: Structure, mechanics, biology Invited Speaker: Markus J. Buehler Biological materials are intriguing examples of advanced materials, which are synthesized, controlled and used for an astonishing variety of purposes—structural support, force generation, mass transport, catalysis, or energy conversion. By incorporating concepts from biology and engineering, computational modeling has led the way in identifying the core principles that link the molecular structure of biomaterials at scales of nanometers to macroscopic scales through hierarchical structures. Here we review case studies of a range of mineralized tissues, focused on bottom-up models and analyses of the structure and mechanics of mineralized tissues. We report an atomistic model of collagen, bone and describe the process of mineralization and the interplay of different hierarchical levels. Combined with experimental studies, such \textit{in silico} models allow us to simulate disease, understand catastrophic failure of tissues, and enable us to translate concepts from the living world into material designs that blur the distinction between the living and non-living systems. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 1:03PM |
B47.00003: Reverse engineering biological crystal growth Invited Speaker: Derk Joester |
Monday, March 18, 2013 1:03PM - 1:39PM |
B47.00004: Bio-Inspired Approaches to Crystals with Composite Structures Invited Speaker: Fiona Meldrum Advances in technology demand an ever-increasing degree of control over material structure, properties and function. As the properties of monolithic materials are necessary limited, one route to extending them is to create a composite by combining contrasting materials. The potential of this approach is beautifully illustrated by the formation of biominerals where organic macromolecules are combined with brittle minerals such as calcite to create crystals with considerable fracture toughness. This talk will discuss how bio-inspired approaches can be used to generate single crystals with composite crystals through a simple one-pot method. By precipitating calcite crystals in the presence of ``occlusion species'' ranging from latex particles, to organic and inorganic nanoparticles and finally small molecules we demonstrate that high amounts of foreign species can be incorporated through control over the additive surface chemistry, and that this can lead to an enhancement of the mechanical properties of the calcite. Occlusion of 20 nm anionic diblock copolymer micelles was achieved at levels of over 13 wt{\%}, and the properties of the resuktant composite calcite crystals were measured using a range of techniques including IR spectroscopy, high resolution powder XRD and high resolution TEM. Incorporation of these macromolecules leads to crystals with structures and mechanical properties similar to those of biominerals. With sizes in the range of some intracrystalline proteins, the micelles act as ``pseudo-proteins'', thereby providing an excellent model system for investigation of the mechanism of macromolecule insertion within biominerals. Extension of these studies to the incorporation of small molecules (amino acids) again demonstrated high levels of incorporation without any change in the crystal morphology. Further, occlusion of these small molecules within the calcite lattice again resulted in a significant increase in the hardness of the calcite, a result which appears to derive from an increase in lattice strain on molecular occlusion. Finally, the generality of this strategy is demonstrated by its extension to the incorporation of inorganic particles such as magnetite and gold within calcite, leading to the formation of inorganic-inorganic composites. [Preview Abstract] |
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