Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H22: Electrons, Phonons, Electron-Phonon Scattering and Phononics IFocus
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Sponsoring Units: DCOMP DMP Chair: Yan Li, American Physical Society APS Room: BCEC 157C |
Tuesday, March 5, 2019 2:30PM - 2:42PM |
H22.00001: High-Throughput Phonon Calculations from Ab Initio Molecular Dynamics Using the Real-Space Multigrid Method (RMG) Jiayong Zhang, Wenchang Lu, Emil Briggs, Yongqiang Cheng, Anibal J. Ramirez-Cuesta, Jerry Bernholc The real-space multigrid-based electronic structure code RMG (www.rmg.org) is highly parallel with excellent scalability to thousands of nodes and GPUs. It has accurate forces and is very suitable for high-throughput, large-scale ab initio molecular dynamics (AIMD). The AIMD results can be used to extract force constants that include anharmonic and finite temperature effects, and thus to compute phonon band structures and spectra that include these effects. We have performed phonon calculations for a number of different systems, including silicon, ZrH2, Carbazole and ZIF-8. Inelastic neutron scattering (INS) spectra can be rigorously calculated using the eigenvectors and eigenvalues from ab-initio phonon calculations; the quality of the theoretical predictions can be compared directly with experimental INS data. The reliability of the RMG code is verified by comparing the calculated results with other DFT codes and INS spectra measured by the VISION instrument at the Spallation Neutron Source at ORNL. We use the ALAMODE open-source software (sourceforge.net/projects/alamode) to extract the anharmonic force matrices from the dynamics and to obtain the phonon band structures. We present results comparing the AIMD-obtained phonon band structures to the experimental spectra. |
Tuesday, March 5, 2019 2:42PM - 2:54PM |
H22.00002: Phonons at finite temperature Olle Hellman We present recent developments using the temperature dependent effective potential technique (TDEP) to model strongly non-harmonic materials. The method employs model Hamiltonians that explicitly depend on temperature. I will present applications pertaining to thermal conductivity, inelastic neutron spectra and phase stabilities. In addition, we investigate non-adiabatic electron-phonon coupling and its influence on phonon spectra, and recent additions to that deal with nuclear quantum effects and efficient stochastic sampling. |
Tuesday, March 5, 2019 2:54PM - 3:06PM |
H22.00003: An optimal approach to computing phonons and their interactions via finite displacements Lyuwen Fu, Mordechai C Kornbluth, Zhengqian Cheng, Chris Marianetti Phonons and their interactions are critical to predicting a wide range of materials properties. Therefore, efficiently extracting a high resulotion Taylor series expansion of the Born-Oppeheimer surface from an arbitrary first-principles approach is of great importance. Here we present an optimal formalism to compute phonons and their interactions at arbitrary order and crystal dimension on a regular grid using finite displacements; yielding a Taylor series purely in terms of group theoretically irreducible derivatives. Building on a key theorem we derive, our approach ensures that a given derivative can always obtained from the smallest possible supercell dictated by the translation group. Our approach maximally exploits any derivatives the first-principles approach can efficiently deliver perturbatively (e.g. Hellman-Feynman forces, etc) to obtain higher order derivatives. We prove that our approach is superior to any single-supercell finite displacement approach. Applications are presented for graphene, computing and tabulating the irreducible derivatives up to 5th order. A number of critical tests are performed to demonstrate the fidelity of our results. |
Tuesday, March 5, 2019 3:06PM - 3:42PM |
H22.00004: New Thermal Transport Regime for Partial-Crystalline Partial-Liquid Materials Invited Speaker: Ming Hu Materials in partial-crystalline partial-liquid (PCPL) state are now widely used as thermoelectrics and battery electrodes, due to their low thermal conductivity and high ionic conductivity, respectively. However, the well-developed computational methods for pure crystalline materials such as anharmonic lattice dynamics coupled with Boltzmann transport equation cannot be used to study such systems. By performing first-principles and molecular dynamics simulations, we give a robust explanation of the thermal transport mechanism in PCPL material Li2S. At the temperature range where the system can be regarded as a solid, the large hopping of Li is found to be responsible for phonon thermal conductivity’s deviation from the traditional 1/T relationship. At high temperatures, the contribution of convection and liquid-phonon interaction increase significantly due to the fluidization of Li ions. Furthermore, there is an interplay between the enhanced phonon scattering and the increased force hopping between neighboring atoms as temperature arises, which results in a dip in the evolution of the virial term around 1200K. When the temperature is even higher, the virial term increases with temperature due to the contribution of vibrations with extremely short mean free path (diffusons). This point is validated by the evolution of the accumulative thermal conductivity with mean free path. At 1300 K, more than 46% of the heat carried by the S sublattice is contributed by the carriers with mean free path smaller than a few angstroms, which is the typical hopping distance. Our study [1] provides a clear physical map of the heat transport in PCPL materials and describes the key mechanism to guide the design of future thermoelectric materials and battery electrodes. |
Tuesday, March 5, 2019 3:42PM - 3:54PM |
H22.00005: Observation of second sound in graphite at temperatures up to 100 K Ryan Duncan, Samuel Huberman, Ke Chen, Bai Song, Vazrik Chiloyan, Zhiwei Ding, Alexei Maznev, Gang Chen, Keith Adam Nelson Wavelike thermal transport in solids, referred to as second sound, has until now been an exotic phenomenon limited to a handful of materials at low temperatures. This has restricted interest in its occurrence and in its potential applications. Through time-resolved optical measurements of thermal transport on 5-20 μm length scales in graphite, we have made direct observations of second sound at temperatures above 100 K. The results are in qualitative agreement with ab initio calculations that predict wavelike phonon hydrodynamics on ~ 1-μm length scale up to almost room temperature. The results suggest an important role of second sound in microscale transient heat transport in two-dimensional and layered materials in a wide temperature range. |
Tuesday, March 5, 2019 3:54PM - 4:06PM |
H22.00006: First-principles study of lattice thermal conductivity in concentrated solid solution alloys Sai Mu, Lucas Lindsay, Raina Olsen, Bennett C Larson, George Malcolm Stocks Energy dissipation of concentrated solid solution alloys can be controlled by the number and types of alloying elements and has impacts on the defect formation and recombination after radiation. Here we use an ab-initio supercell method combined with a phonon unfolding technique to access the effects of disorder on the lattice-mediated thermal conductivity in a series of 2-4 component equiatomic alloys (e.g. NiFe, NiCo, NiCoCr, NiFeCo, NiFeCoCr) where the mass disorder is small while the force constant disorder is pronounced. We demonstrate that force constant disorder itself can efficiently reduce the thermal conductivity. We further show that the low-conductivity Cr-containing alloys present the largest force constant fluctuations across the studied alloys and that this is electronically driven. The results provide a new tuning parameter, based on the electronic structure, to manipulate the force constant disorder, thereby facilitating the design of materials with desirable thermal transport properties. |
Tuesday, March 5, 2019 4:06PM - 4:18PM |
H22.00007: The influence of interfacial structure and strain energy on phonon transport Riley Hanus, Jeff Snyder Phonon transport across interfaces is an inherently complex topic of great scientific and technological importance. Several experimental and theoretical results which aim to establish a fundamental understanding of heat transfer across interfaces will be presented. First, I will demonstrate how phonon diffraction and dimensionality crossover effects arise when the nanoscale structure and strain field of interfaces and grain boundaries (GBs) are considered. Next, an experimental study is presented where the thermal boundary resistance is measured on individual Si-Si twist GBs at different twist angles. The thermal boundary resistance at GBs, again, seems to be dominated by the interfacial strain field. Finally, it is shown how the thermal boundary resistance can be controlled by modifying the GB with 2D materials. Several layers of graphene were introduced into the GBs of skutterudite materials which dramatically increases the materials thermal boundary resistance, negligibly effecting electronic transport, resulting in a 24% improvement in measured thermoelectric device efficiency. |
Tuesday, March 5, 2019 4:18PM - 4:30PM |
H22.00008: Thermal properties of magnetic materials from first principles Matthew Heine, Olle Hellman, David A Broido In addition to representing an active area of physics research, magnetic materials are used in finite temperature applications such as computer memory technology. In this study we use the Temperature Dependent Effective Potential (TDEP) approach to calculate finite temperature properties of magnetic materials from first principles. In this approach, thermal disorder of the lattice and magnetic moments are incorporated self consistently to yield temperature-dependent properties. Calculations are performed within the framework of Density Functional Theory (DFT) using constrained, fully noncollinear magnetic moments. The effects of such thermal disorder are demonstrated across a range of temperatures spanning the magnetic transition temperature. |
Tuesday, March 5, 2019 4:30PM - 4:42PM |
H22.00009: Nonperturbative modeling of high-frequency Holstein-coupled modes using matrix product states Benedikt Kloss, David Reichman, Roel Tempelaar Many nonequilibrium phenomena of current interest involve coupling of electronic coordinates to vibrational modes whose frequency and reorganization energy are high or comparable to other relevant energies. A proper theoretical account of such modes needs to be non-perturbative and non-classical. The commonly applied direct diagonalization approach rapidly becomes prohibitively expensive with increasing system sizes. We present an alternative approach based on tensor network states, merging concepts from the condensed matter physics and molecular quantum dynamics communities. Focusing on the one-particle (electronic) sector, relevant for exciton and polaron dynamics, we construct the vibronic wavefunction as a set of matrix product states, representing the vibrational degrees of freedom surrounding the particle. This approach allows for an exact treatment of Holstein-driven processes at an unprecedented scale. |
Tuesday, March 5, 2019 4:42PM - 4:54PM |
H22.00010: Accuracy of Lorenz number estimates based on transport data Aditya Putatunda, David Singh Determination of the thermal conductivity (κ) and its separation into its major constituent lattice and electronic parts is of importance in certain materials. In this work, we study the Wiedemann-Franz law which is used broadly by experimentalists to estimate these parts to investigate κ. For a group of known thermoelectric materials, we investigated the temperature and doping dependence of the Lorenz number (L). In relation to this, we examined the accuracy of an expression proposed by Kim et al. relevant in the literature which is used frequently to estimate L. Solving the Boltzmann transport equations for first-principles band-structure results, we find that this expression, despite being mathematically simple, captures the observed variations of L with satisfactory accuracy for wide band gap materials. It does not work as well for some other materials e.g. PbTe and Mg3Sb2 which primarily have non-parabolic band structures. |
Tuesday, March 5, 2019 4:54PM - 5:06PM |
H22.00011: Automatic ab initio calculations of electronic transport of semiconductors Henrique Miranda, Guillaume Brunin, Matteo Giantomassi, Gian-Marco Rignanese, Geoffroy Hautier We address the problem of calculating electronic transport quantities on semiconductors of technological interest (Si,GaAs,MoS2) with minimal user intervention. An important technical challenge is to accurately obtain quantities that depend on integrations on the band-structure. Several methods have been proposed based on Wannier functions or atomic orbitals with the drawback that they often require time-consuming user intervention. For automatic calculations, we use the Shankland-Koelling-Wood interpolation as implemented in the Boltztrap2[1] code. We will discuss some often seen interpolation artifacts, its limitation in reproducing band-crossings and how those affect the final results. We compare these with calculations done using a newly developed driver to obtain electron velocities within the Abinit[2] code on large k-point meshes integrated using the tetrahedron method. We will then show the importance of the accuracy of these quantities when performing calculations including the electronic lifetimes due to electron-phonon scattering. |
Tuesday, March 5, 2019 5:06PM - 5:18PM |
H22.00012: Evidence for the weak coupling scenario of the Peierls transition in the blue bronze K0.3MoO3 Bogdan Guster, Miguel Pruneda, Pablo Ordejon, Enric Canadell, Jean-Paul Pouget In this work we confirm the interband nesting mechanism for the Charge Density Wave in the blue bronze K0.3MoO3 by a direct calculation of the Lindhard function using the DFT band structure [1]. Furthermore, our calculation of the thermal dependence of the Lindhard function allows to quantitatively account for the standard weak coupling scenario of the Peierls transition. To the best of our knowledge, this type of validation of the weak coupling scenario based on actual data for a real material has never been reported in the literature [2]. |
Tuesday, March 5, 2019 5:18PM - 5:30PM |
H22.00013: A model to incorporate electron phonon coupling into classical molecular dynamics Artur Tamm, Magdalena Caro, Alfredo Caro, Alfredo A. Correa We have developed a new model to incorporate equilibration of ions and electrons at the corresponding timescales into classical molecular dynamics (MD) simulations. The model is based on Langevin dynamics where the stochastic description does not generate any linear momentum nor torque in the system. This is achieved by introducing spatial correlations into friction and random forces which have a range described by atomic electron density. |
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