Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session E20: Electrons, Phonons, Electron-Phonon Scattering, and Phononics IIFocus Live
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Sponsoring Units: DCOMP DMP Chair: Matthieu Verstraete, University of Liege |
Tuesday, March 16, 2021 8:00AM - 8:12AM Live |
E20.00001: Phonon-mediated Optical and Electronic Transport Properties of BAs Kyle Bushick, Kelsey Mengle, Sieun Chae, Zihao Deng, Emmanouil Kioupakis While boron arsenide (BAs) has attracted attention for its ultrahigh thermal conductivity, open questions remain about its use in semiconducting devices. To address this problem, we apply density functional and many body perturbation theory to understand its electronic and optical properties and guide device applications. We calculate the effect of strain on the band alignment and the electron and hole mobilities, and show that biaxial tensile strain increases the mobilities of both carriers by over 50%. We also determine the band offsets of BAs heterostructures with nearly lattice-matched ZnSnN2 and InGaN to guide heterostructure design. Finally, since BAs has an indirect band gap of approximately 2 eV and a direct gap of 4.1 eV, absorption of visible light is exclusively mediated by phonons. We therefore calculate the indirect and direct optical absorption spectra to assess its potential in photovoltaics and examine how excitonic effects alter the absorption. These results are in excellent agreement with experimental data. Our work elucidated a variety of the functional properties of BAs for technologically relevant device applications. |
Tuesday, March 16, 2021 8:12AM - 8:24AM Live |
E20.00002: Space-time dependent thermal conductivity in nonlocal thermal transport Chengyun Hua, Lucas Lindsay Solving the Peierls-Boltzmann transport equation (PBE) for a general space-time dependent problem remains a challenging task due to the high dimensionality of the integro-differential equation. In this work, we present a direct solution to the space-time dependent PBE with a linearized collision matrix using an eigendecomposition method. We show that there exists a generalized Fourier type relation that links heat flux to the local temperature, and this constitutive relation defines a thermal conductivity that depends on both time and space. Combining this approach with ab initio calculations of phonon properties, we demonstrate that the space-time dependent thermal conductivity gives rise to an oscillatory response in temperature in a transient grating geometry in high thermal conductivity materials. The present solution method will not only enable a more accurate interpretation of thermal measurements that observe nonlocal thermal transport but also enhance our physical understanding of nonlocal thermal transport in high thermal conductivity materials that are promising candidates for nanoscale thermal management applications. |
Tuesday, March 16, 2021 8:24AM - 8:36AM Live |
E20.00003: An atomistic S-Matrix method for computing boundary scattering amplitudes and predicting the mode-resolved phonon specularity parameter Zhun-Yong Ong A critical element in the reduced thermal conductivity of semiconducting nanowires and thin films is phonon momentum dissipation from boundary scattering, which varies with the specularity parameter and depends on the boundary structure and the incident phonon mode. |
Tuesday, March 16, 2021 8:36AM - 8:48AM Live |
E20.00004: Influence of spin-orbit coupling interaction on the electron-phonon renormalized electronic energy levels Véronique Brousseau-Couture, Xavier Gonze, Michel Cote Electron-phonon (e-p) interaction calculations from first-principles are well documented in the literature. The predominance of non-adiabatic effects in the zero-point renormalization (ZPR) of the band gap for polar materials has been recently assessed in the light of the Fröhlich interaction. |
Tuesday, March 16, 2021 8:48AM - 9:00AM Live |
E20.00005: Band convergence and thermoelectric transport properties of p-type PbTe Ransell DSouza, Jiang Cao, Jose Querales-Flores, Stephen Fahy, Ivana Savic The alignment of the valence band maxima at L and Σ (or “band convergence”) in p-type PbTe and related materials gives rise to a large thermoelectric figure of merit ZT [1]. It is still unclear why the band convergence improves ZT. Here we present a first principles model of electron-phonon scattering and thermoelectric transport in the L and Σ valleys of p-type PbTe. Our model accounts for the thermally induced renormalization of those valleys [2], which enables us to quantify the effects of the L and Σ valley convergence at ~620 K [3] on ZT. We find that even though the increased intervalley scattering due to the band convergence reduces the electrical conductivity, it significantly increases the Seebeck coefficient. The Σ valleys also provide additional transport channels which further improve the Seebeck coefficient. This results in the enhanced values of the thermoelectric power factor near the band convergence temperature and large ZT. Our findings thus support the idea that materials with high valley degeneracy may be good thermoelectrics. |
Tuesday, March 16, 2021 9:00AM - 9:12AM Live |
E20.00006: Understanding the Restoring Force From a Local Bonding Perspective: A First-Principles Picture of Phonons and Elasticity Ethan Ritz, Guru Khalsa, Hsin-Yu Ko, Robert Distasio, Nicole Benedek Though the calculation of bulk elastic quantities and phonon frequencies using modern ab-initio techniques is now routine, understanding the crystal chemical origins of these properties is still an open question. How do we relate the value of a specific bulk elastic constant or vibrational mode to chemically intuitive ideas about local structure and bonding, especially when our calculations are often done in a delocalized Bloch basis? Using a basis of maximally localized Wannier functions, we partition the total electronic energy onto real space representations of occupied states, as well of the curvature of that energy with respect to distortions such as mechanical strain or phonon excitation. By understanding how the energy of each individual bond changes with these distortions, we can obtain orbitally decomposed, chemically specific understanding of bulk properties. We use this approach to explore various perovskite oxides, exploring the chemical origins of elasticity, structural phase transitions, Grüneisen parameters, and thermal expansion properties, and discuss the possibility of enhancing or controlling these properties. |
Tuesday, March 16, 2021 9:12AM - 9:24AM Live |
E20.00007: Polaron effective mass and localization length in cubic materials: degenerate and anisotropic electronic bands Bogdan Guster, Pedro Melo, Matthieu Verstraete, Véronique Brousseau-Couture, Michel Cote, Matteo Giantomassi, Xavier Gonze The polaron notion is almost one century old, yet most works on polaron models, to understand their characteristics such as radius, effective mass, mobility and energy dispersion, have focused on the original Fröhlich model for large polarons, with a simple (non-degenerate) parabolic isotropic electronic band coupled to one dispersionless longitudinal optical phonon mode [1]. Real cubic materials have electronic band extrema that are often degenerate (e.g. 3-fold degeneracy of the valence band), or anisotropic (e.g. conduction bands at X or L)[2]. We go beyond the existing isotropic [3] and non-degeneracy hypotheses, and provide, for polaron effective masses (at the lowest order of perturbation theory), and for localization lengths (variational approach), with multiple phonon modes: (i) the analytical result for the case of anisotropic electronic energy dispersion, with two distinctive effective masses (uniaxial), (ii) an expression for the case of three distinctive axes (ellipsoidal), (iii) numerical simulations for the 3-band degenerate case, applied to III-V and II-VI semiconductor valence bands. |
Tuesday, March 16, 2021 9:24AM - 9:36AM Live |
E20.00008: Phonon anharmonicity and electron-phonon coupling in pure and doped VO2 Shan Yang, John D Budai, Dipanshu Bansal, Xing He, Michael E Manley, Chen Li, Jiawang Hong, Lynn A Boatner, Ayman Said, Olivier Delaire Important insights into the thermodynamics and mechanism of the VO2 MIT were achieved by probing lattice dynamics using IXS[1] and ultrafast pump-probe x-ray diffraction[2]. But detailed phonon dispersions for the insulating M1 phase have yet to be reported. In addition, abnormally low electronic thermal conductivity in Rutile VO2 requires further examination of phonon anharmonicity and electron-phonon coupling[3]. We report new measurements of phonon dispersions in VO2 and doped crystals, as well as DFT simulations of phonon dispersions and spectral functions. Our results explain the origin of strong phonon damping in Rutile VO2, compared to M1 VO2 and Rutile TiO2, and assess the failure of perturbation theory in predicting accurate phonon linewidths. Our simulations capture the phonon damping behavior beyond perturbation theory, providing critical insights into the unusual lattice thermal conductivity. |
Tuesday, March 16, 2021 9:36AM - 9:48AM Live |
E20.00009: Efficient ab initio Migdal-Eliashberg calculation considering the retardation effect in phonon-mediated superconductors Tianchun Wang, Takuya Nomoto, Yusuke Nomura, Hiroshi Shinaoka, Junya Otsuki, Takashi Koretsune, Ryotaro Arita We formulate an efficient scheme to perform Migdal-Eliashberg calculation considering the retardation effect from first principles[1]. While the conventional approach requires a huge number of Matsubara frequencies, we show that the intermediate representation (IR) of the Green's function[2] and the sparse sampling method[3] dramatically reduce the numerical cost to solve the linearized gap equation. Without introducing any empirical parameter, we demonstrate that we can reproduce the experimental superconducting transition temperature of elemental Nb (~ 10 K). The present result indicates that our approach has a superior performance for many superconductors for which Tc is lower than the order of 10 K. Our scheme based on IR reduces the computation time cost and memory footprint by 20 times, compared to the conventional method using 4000 Matsubara frequencies. |
Tuesday, March 16, 2021 9:48AM - 10:24AM Live |
E20.00010: Characterization of Thermal Effects in Wide Bandgap Semiconductor Materials and Devices Invited Speaker: Samuel Graham Wide bandgap electronics made from nitrides (e.g., Gallium Nitride (GaN)) and oxides (e.g., Gallium Oxide (Ga2O3)) are currently under development due to their potential to create some of the most advanced RF and power electronic devices in the world. However, the thermal response of these devices under applied electric fields can create large power densities (RF and power electronics) that must be understood. For many of these devices, the electrothermal response plays a strong role in both the acceptable operation or long-term failure and reliability of the devices. Thus, tools that can help elucidate these responses and provide a method to help design better devices is of critical need for this field. |
Tuesday, March 16, 2021 10:24AM - 10:36AM Live |
E20.00011: R1−xFex (R = Y, Ce, Nd, Sm, and Dy) compounds: Entropy-induced phase stabilization at finite temperature Guangzong Xing, Ishikawa Takahiro, Yoshio Miura, Takashi Miyake, Terumasa Tadano Up to now, phase stability problems of permanent magnets are still not yet entirely eliminated. Motivated by this, we report the dynamical and thermodynamic stability of the binary R1−xFex (0< x <1) compounds (R= Y, Ce, Nd, Sm, and Dy) at finite temperature predicted by first-principles calculation based on density functional theory (DFT). We first demonstrate that thermodynamic stability problems cannot be elucidated when using the static DFT energy at T = 0 K. By considering the entropy contribution, including electronic and vibrational free energies, we obtained convex hull plots at finite temperatures, which successfully explain the phase stability of known compounds. Our results show a trend of stabilizing many R1−xFex compounds studied here with increasing temperature. This entropy-induced stabilization mainly comes from the vibrational free energy, which decreases dramatically with temperature. We will discuss the origin of the stabilization and its rare-earth dependence. |
Tuesday, March 16, 2021 10:36AM - 10:48AM Live |
E20.00012: Exposing the hidden influence of selection rules on phonon-phonon scattering by pressure and temperature tuning David Broido, Navaneetha Krishnan Ravichandran Selection rules act to restrict intrinsic anharmonic interactions between phonons in all crystals. Yet their influence on phonon propagation is often hidden and so hard to interrogate experimentally. Using ab initio calculations, we show that the hidden influence of selection rules on three-phonon scattering can be exposed through anomalous signatures in the pressure (P) and temperature (T) dependence of the thermal conductivities, k, of certain compounds. Boron phosphide reveals such underlying behavior through an exceptionally sharp initial rise in k with increasing P, which may be the steepest of any material, and also a peak and decrease in k at high P. These features are in stark contrast to the measured behavior for many solids, and occur at experimentally accessible conditions. |
Tuesday, March 16, 2021 10:48AM - 11:00AM Live |
E20.00013: Polaronic Electron Transport in a Naphthalene Crystal from First Principles Benjamin Chang, Jin-Jian Zhou, Nien-En Lee, Marco Bernardi Charge transport is notoriously difficult to predict in organic molecular crystals due to their complex electron-phonon (e-ph) interactions. Two limit transport regimes in organic crystals are bandlike transport dominated by intermolecular phonons [1] and polaron hopping governed by intramolecular phonons. However, an intermediate regime is also possible in which both the bandlike and hopping descriptions break down; an accurate method to predict transport in the intermediate regime is still missing. In this talk, we show that electron transport in a naphthalene crystal in the in-plane molecular directions belongs to the intermediate transport regime. We employ our recently developed finite-temperature cumulant approach [2] to accurately predict the in-plane electron mobility, obtaining results in excellent agreement with experiments between 100-300 K temperatures. Our results demonstrate that the intermediate electron transport regime in organic crystals parallels the large polaron picture in oxides. Analysis of the computed electron spectral functions reveals an interplay between intermolecular and intramolecular phonons. |
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