Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S22: Electrons, Phonons, Electron-Phonon Scattering and Phononics VIFocus
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Sponsoring Units: DCOMP DMP Chair: Yan Li, American Physical Society APS Room: BCEC 157C |
Thursday, March 7, 2019 11:15AM - 11:27AM |
S22.00001: An Ab-Initio Study of the Effect of Impurities on Electron-Phonon Energy Transfer in Niobium Nathan Sitaraman, James Maniscalco, Tomas Arias, Matthias Ulf Liepe A precise understanding of electron-phonon energy transfer is crucial to the optimization of superconducting radio frequency (SRF) cavities, photocathodes, and other material systems in which electrons are out of thermal equilibrium with phonons. The role played by interstitial impurities is a point of particular interest; SRF scientists have long believed that impurities affect electron-phonon energy transfer in niobium cavities, but it is difficult to distinguish this effect from other impurity-induced effects. An inability to precisely control impurity concentrations and directly measure electron temperature further complicates the problem. |
Thursday, March 7, 2019 11:27AM - 11:39AM |
S22.00002: Understanding the chemical enhancement mechanism of graphene-enhanced Raman Spectroscopy (GERS) – A first principles study. Kanchan Ulman, Su Ying Quek Surface-enhanced Raman spectroscopy (SERS), involving the enhancement of Raman signals for molecules on metal substrates, is a well-established field that has enabled the detection of trace concentrations of molecules. In recent years, graphene was found to be a candidate substrate for SERS, giving rise to a rapidly expanding field of graphene-enhanced Raman spectroscopy (GERS), and opening doors for other two-dimensional materials to be used for SERS. In conventional SERS, the Raman signal is enhanced by a combination of the dominant electromagnetic enhancement effect, and the smaller chemical enhancement effect, which stems from electron-phonon coupling in the system. The latter chemical enhancement effect is thought to play a dominating role in GERS, thus enabling detailed studies of electron-phonon coupling effects for organic-graphene interfaces. Yet, the details of the chemical enhancement effect in GERS are not well understood. Using first principles calculations, we study the GERS chemical enhancement mechanism using typical probe molecules such as pyridine adsorbed on graphene. We uncover a novel ground state enhancement mechanism that is distinct from the typical ground state charge transfer mechanism in conventional SERS. |
Thursday, March 7, 2019 11:39AM - 11:51AM |
S22.00003: Kagome modes, a new route to ultralow thermal conductivity? David Voneshen, Monica Ciomaga Hatnean, Toby Perring, Helen C Walker, Keith Refson, Geetha Balakrishnan, Jon Goff From next generation gas turbines to scavenging waste heat from car exhausts, finding new materials with ultra-low thermal conductivity (κ) has the potential to lead to large gains in device efficiency. Crystal structures with inherently low κ are consequently desirable, but candidate materials are rare and often difficult to make. Using first principles calculations and inelastic neutron scattering we have studied the pyrochlore La2Zr2O7 which has been proposed as a next generation thermal barrier. We find that there is a highly anharmonic, approximately flat, vibrational mode associated with the kagome planes. Our results suggest that this mode is responsible for the low thermal conductivity observed in the pyrochlores and that kagome compounds will be a fruitful place to search for other low κ materials. |
Thursday, March 7, 2019 11:51AM - 12:03PM |
S22.00004: Thermodynamics of Tin Using AFLOW-APL Mateo Ronquillo, Michael Mehl, Corey Oses, Stefano Curtarolo, Cormac Toher Tin is one of the more interesting metallic elements, with a 13.2oC phase transition from diamond-like "gray" (α-Sn) to "white" (the eponymous β-Sn). Density Functional Theory (DFT) finds a simple hexagonal phase (sh-Sn, hP1) nearly degenerate with the other phases, although it is never seen. sh-Sn is very close in energy to β-Sn at all volumes, as found by a variety of density functionals. While the simple hexagonal structure is a high pressure phase of Si, there it is not close to the ground-state structure at normal pressures. |
Thursday, March 7, 2019 12:03PM - 12:15PM |
S22.00005: Hybridization of Localized Surface Phonon Polaritons via Symmetry Breaking in Dolmen Nanostructures Swathi Iyer, Chase Ellis, Alexander Giles, Dmitry Chigrin, Richard Kasica, Michael Meeker, Joshua D Caldwell, Joseph G Tischler
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Thursday, March 7, 2019 12:15PM - 12:27PM |
S22.00006: Nonlinear Physics for the Engineering of Phononic Frequency Combs Adarsh Ganesan, Ashwin Seshia Phononic frequency combs (PFC) represent a new nonlinear phenomenon in the physical domain of micromechanical resonators [1]. The emergence of PFC is mediated by nonlinear modal coupling. Through a series of experiments with micromechanical resonators, various features of PFC have now been identified. These include drive parameters for comb operation, hysteresis for comb spectrum tailoring and evolving spectral envelopes of combs. Potential applications of PFC include high-precision sensing, spread spectrum information processing and other applications in timing and frequency control. However, for the successful realization of such applications, the nonlinear physics of micromechanical resonators underlying the evolution of PFC must be fully elucidated. Specific future directions include developing experimental protocols for engineering frequency combs at low drive power levels; resonant and off-resonant excitation; understanding comb frequency fluctuations; tailoring combs for broadband multi-octave spanning spectra; and modelling sensitivity to system parameters and physical perturbations. |
Thursday, March 7, 2019 12:27PM - 12:39PM |
S22.00007: Studying the correlations and solubility of hydrogen in niobium using Density Functional Theory calculations Arvind Ramachandran, Dr. Houlong Zhuang, Klaus S Lackner In this work, we present a Density Functional Theory (DFT) study of hydrogen correlations and solubility in niobium. Finding the preferred interstitial site for single hydrogen atoms, calculating the pair-wise hydrogen correlations, and the treatment of many hydrogen atoms in a niobium cell are parts of this work. By studying how the pair-wise hydrogen-hydrogen interaction energy varies as a function of their distance, we develop the theoretical counterpart of the empirical Westlake criterion, a rule that states that hydrogen atoms cannot simultaneously occupy pairs of interstitial sites closer than 0.21 nm. Based on this inference, we provide a systematic way of populating many (>3) hydrogens in the niobium lattice. Using the differential binding energies and vibrational frequencies of dissolved hydrogens at varying hydrogen concentrations, we estimate the solubility of hydrogen in niobium. Apart from the conventional way of calculating the entropy of interstitial hydrogens under the harmonic oscillator approximation, this work includes a new approximation that treats the hydrogens as a monatomic ideal gas. The solubility predictions are in good agreement with experimental data. |
Thursday, March 7, 2019 12:39PM - 12:51PM |
S22.00008: A simple method to determine lattice constants Chen Xiao-Fan A simple method to determine lattice constants is proposed. The method is based on the relation between electric conductivity and the lattice constants. A way to solve a puzzle in high-temperature superconductivity is also presented . |
Thursday, March 7, 2019 12:51PM - 1:03PM |
S22.00009: Purely electronic instabilities versus Peierls instabilities in semi-metallic 1D atomic chains Matteo Barborini, Sven Reichardt, Pier Luigi Cudazzo, Matteo Calandra, Francesco Mauri, Ludger Wirtz In 1929 Peierls hypothesized that a semi-metallic linear chain of equally spaced single electron atoms is unstable due to electron-phonon coupling at T=0: periodic lattice distortions along the longitudinal mode of vibration lead to the opening of an electronic band gap, lowering the symmetry of the lattice and stabilizing a semiconducting ground state. The origin of the symmetry breaking and of the formation of a charge density wave (CDW) state, was thus directly linked to the ionic displacement. |
Thursday, March 7, 2019 1:03PM - 1:15PM |
S22.00010: Magnetoelastic coupling, phonon and magnons in inverse spinel NiFe2O4 Qiang Zhang, Roshan Nepal, Ashfia Huq, Alexander I Kolesnikov, Stephen Nagler, Rongying Jin The inverse spinel NiFe2O4 has attracted enormous interest due to the potential applications in several important fields such as electronic devices and catalysis. NiFe2O4 exhibits a cation distribution of (Fe3+)A(Ni2+Fe3+)BO4, with Fe3+ occupying the tetrahedral(A) sites forming a diamond lattice, and Ni2+ and Fe3+ sharing the octahedral(B) sites forming a pyrochlore lattice. Powder neutron diffraction reveals the cubic spinel structure persisting down to 5 K. A collinear ferrimagnetic order with antiparallel moments between A and B sites is found below TN ≈ 860 K with a magnetoelastic coupling at TN. Magnetization measurements further reveal NiFe2O4 enters a state with the coexistence of a ferrimagnetic order and a spin glass like state below the freezing temperature Tf ≈ 40 K. A few branches of phonons and magnons with a crossing feature are observed via single-crystal inelastic neutron scattering at 100 K (Tf <T<TN) and 5 K (T< Tf). All these results and magnetic exchange constants will be compared to the well-known magnetite Fe3O4 to reveal the distinct spin, lattice and orbital degrees of freedom. |
Thursday, March 7, 2019 1:15PM - 1:27PM |
S22.00011: ABSTRACT WITHDRAWN
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Thursday, March 7, 2019 1:27PM - 1:39PM |
S22.00012: Nonlinear Anomalous Thermoelectric Current Yang Gao, Di Xiao Using the concept of the orbital magnetic quadrupole, we derive a nonlinear anomalous thermoelectric current, i.e. a current quadratic in temperature gradient and perpendicular to its direction. In the system that breaks time reversal, inversion, and one mirror symmetry, but has the combined time reversal and inversion symmetry, the linear anomalous thermoelectric current vanishes but the nonlinear current we obtained does not vanish and hence can be used to probe such system. We demonstrate this nonlinear current in the loop-current model in the pseudogap phase. |
Thursday, March 7, 2019 1:39PM - 1:51PM |
S22.00013: Nanosecond Shape-driven Localized Surface Plasmon Resonance Switching in Silver Nanoparticles Venkatanarayana P Sandireddy, Krishna Koirala, Ramki Kalyanaraman The ability to tune the localized surface plasmon resonance (LSPR) in a fast and repeatable manner could form the basis for a hypothetical optical switch. Here we demonstrate a reversible change in the LSPR wavelength of Ag nanoparticles based on nanosecond melting induced shape effects. When Ag nanoparticles are melted by nanosecond laser pulses, the resulting contact angle is determined by the fluid environment during melting. For instance, Ag nanoparticles on quartz substrates melted in air are near hemispherical in shape (with contact angle ~ 96o), while those melted under water and glycerol have larger contact angles of ~127o and ~172o respectively. This shape or contact angle change significantly modifies the intensity and wavelength of the dipolar and quadrupolar plasmonic signals. We have utilized this shape change to switch the plasmonic signals over multiple cycles of irradiation and determined the fatigue limit of LSPR switching between air and glycerol environments. Such shape control and repeatable modification of the plasmonic wavelength could serve as a useful system for biological sensing and optical tuning applications. |
Thursday, March 7, 2019 1:51PM - 2:03PM |
S22.00014: Theory of time-resolved resonant inelastic x-ray scattering for studying material dynamics Yuan Chen, Yao Wang, Chunjing Jia, James Freericks, Andrij Shvaika, Brian Moritz, Thomas Devereaux With recent progress in the X-ray beamline and pump-probe instrumentation, time-resolved RIXS (tr-RIXS) promises to detect the nonequilibrium dynamics of collective modes, which play significant roles in quantum materials. Here we present a theory for evaluating the tr-RIXS cross-section. This time-domain theory is based on nonequilibrium linear response and can reproduce the common Kramers-Heisenberg formula at equilibrium. We numerically evaluated tr-RIXS cross-section for some example systems out of equilibrium, including a graphene nanoribbon and the 2D single-band Hubbard model. We show that tr-RIXS can capture the dynamics of multi-particle Floquet excitations in a momentum-resolved way, revealing underlying physics such as Floquet band renormalization, topological edge states, and pump-induced collective excitations. |
Thursday, March 7, 2019 2:03PM - 2:15PM |
S22.00015: Polarized Raman spectroscopy at the edges of exfoliated GeS and GeSe crystals Henrique Bucker Ribeiro, Sérgio L. L. de Moraes Ramos, Leandro Seixas Rocha, Christiano De Matos, Marcos Assunção Pimenta Germanium sulfide (GeS) and germanium selenide (GeSe) are layered crystals which structure bears a strong resemblance to that of black phosphorus (BP). As recently observed and reported, BP exhibits atomic rearrangements at crystal terminations, which lead to the activation of Raman modes that are otherwise forbidden by polarization selection rules. Considering the similarities in the crystalline structure of BP, GeS, and GeSe, the same behavior would be expected at the edges of these crystals. In this work, the Raman modes GeS and GeSe edges were experimentally studied using polarized Raman spectroscopy. We show, at the edges, the appearance of modes that are symmetry-forbidden. By carrying out DFT calculations, we conclude that the anomalous behavior is a consequence of edge atomic rearrangements. Such rearrangements, therefore, appear to be a general feature in lamellar crystals with the crystalline structure of BP. |
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