Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session T48: Electrons, Phonons, ElectronPhonon Scattering, and Phononics VIFocus Recordings Available

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Sponsoring Units: DCOMP DMP Chair: Austin Minnich, California Institute of Technology Room: McCormick Place W471A 
Thursday, March 17, 2022 11:30AM  12:06PM 
T48.00001: How Coherence of Thermal Phonons impacts Heat Transport Invited Speaker: Sebastian Volz Physics describes reality as excitations emerging from a collection of coupled oscillators and a typical example of this process is the apparition of thermal phonons. It will be explained how a thermally activated phonon mode involves a large population of elastic wavepackets. These excitations are characterized by a wide distribution of lifetimes and coherence times expressing particle and wavelike natures. In agreement with direct simulations, our theoretical derivation yields a generalized law for the decay of the phonon number taking into account coherent effects. Several examples of applications of this law will be presented. This work opens new horizons in the understanding of the origin and the treatment of thermal phonons. 
Thursday, March 17, 2022 12:06PM  12:18PM 
T48.00002: Warm electron tensor in nSi from first principles Benjamin Hatanpaa, Alexander Y Choi, Peishi Cheng, Austin J Minnich Abinitio calculations of lowfield charge transport properties such as mobility are now routine. However, few calculations beyond this regime have been reported although considerable experimental data exists for the warm electron tensor in Si. Here, we report a firstprinciples calculation of the warm electron conductivity tensor of electrons in Si. We find that the observed nonparallelism of current and electric field are accurately reproduced, but the decrease in mobility with electric field is overpredicted by around a factor of three. We show that this discrepancy can be mitigated by inclusion of onshell one electrontwo phonon scattering, suggesting that twophonon processes play an important role in the highfield properties of nSi. 
Thursday, March 17, 2022 12:18PM  12:30PM 
T48.00003: Twist Electron Dynamics Rinkle Juneja, Simon Thebaud, Lucas Lindsay Structural symmetries of crystals uniquely influence the quasiparticle behaviors. Here, I will discuss the consequences of twist symmetry on the electronic band structures of materials. We build a tight binding Hamiltonian from density functional theory inputs in terms of the underlying twist symmetry. This twist dynamics unravels symmetryenforced band degeneracies and nontrivial topologies in terms of pseudoangular momenta of the electronic bands. Inclusion of spinorbit coupling requires electron spin be included as a component of the total angular momentum. The dynamics and symmetry signatures will be illustrated in a variety of material classes. 
Thursday, March 17, 2022 12:30PM  12:42PM 
T48.00004: Firstprinciples prediction of outstanding thermoelectric performance in outofplane pdoped GeSe Daniel T Larson, Anderson S Chaves, Efthimios Kaxiras, Alex Antonelli The recordbreaking thermoelectric performance of tin selenide (SnSe) motivates the study of analog compounds such as germanium selenide (GeSe) that have the same crystal structure. Using extensive firstprinciples calculations of holeimpurity, holephonon, and phononphonon scattering, we investigate the thermoelectric transport properties of the orthorhombic (Pnma) phase of pdoped GeSe. Due to its ultralow total thermal conductivity, high Seebeck coefficients, extremely low Lorenz numbers, and relatively large band gap, we predict outstanding thermoelectric performance in GeSe crystals over a broad range of temperatures. In particular, the outofplane direction in GeSe exhibits comparable performance to SnSe at temperatures above 500 K. We extend the analysis to 900 K in order to determine the optimal hole doping to maximize zT. Our results, based on detailed firstprinciples calculations, are a strong motivation for continued experimental work focused on improving the efficiency of doping in GeSe to reach the optimal hole density. 
Thursday, March 17, 2022 12:42PM  12:54PM 
T48.00005: Freecarrier and defect absorption in gallium oxide Hartwin Peelaers, Arjan Singh, Okan Koksal, Nicholas Tanen, Jonathan McCandless, Debdeep Jena, Huili Grace Xing, Chris G Van de Walle, Farhan Rana Transparent conducting oxides (TCOs) are a technologically important class of materials used in optoelectronic devices, as TCOs balance two conflicting properties: transparency and conductivity. The requirement of transparency is typically tied to the band gap of the material being sufficiently large to prevent absorption of visible photons. This is a necessary but not sufficient condition: indeed, the high concentration of free carriers, required for conductivity, can also lead to optical absorption. This absorption can occur through direct absorption to higherlying conduction band states, or by an indirect process mediated by phonons or charged impurities. Defects can also lead to additional absorption. 
Thursday, March 17, 2022 12:54PM  1:06PM 
T48.00006: Flattening of the acoustic phonon branches in the clathrate Ba_{8}Ga_{16}Ge_{30} Susmita Roy, Tyler C Sterling, Dan Parshall, Eric Toberer, Mogens Christensen, Devashibhai T Adroja, Dmitry Reznik In the search for highperformance thermoelectric materials, phononglass electron crystal (PGEC) materials such as clathrates have drawn attention due to having both glasslike low phonon thermal conductivity and crystallike high electrical conductivity [1]. Ba_{8}Ga_{16}Ge_{30} (BGG) has a guest Ba atom trapped inside Ga/Ge cage structures is known for avoided crossings between acoustic phonons and the flat guest atom modes proposed to be the source of the low lattice thermal conductivity [2,3]. Ga/Ge site disorder with Ga and Ge exchanging places in different unit cells has also been reported [4]. We used timeofflight neutron scattering to measure the complete phonon spectrum in a large single crystal of BGG and compared these results with predictions of density functional theory. Calculations assuming the structure where Ga/Ge atoms occupy their nominal sites, as well as a disordered configuration showed that the latter agrees much better with the data. Disorder strongly reduces phonon group velocities despite nearly identical masses of Ga and Ge, which accounts for exceptionally low thermal conductivity. Our work points at a new path towards optimizing thermoelectrics. 
Thursday, March 17, 2022 1:06PM  1:18PM 
T48.00007: NonAbelian braiding of phonons from first principles Bo Peng NonAbelian braiding of quasiparticles can encode information immune from environmental noise with the potential to realize topological quantum computation. Here we propose that phonons, a bosonic excitation of lattice vibrations, can carry nonAbelian charges in their band structures that can be braided using external stimuli. Taking some earthly abundant materials such as silicates and aluminium oxide as representative examples, we demonstrate that an external electric field or electrostatic doping can give rise to phonon band inversions that induce the redistribution of nonAbelian charges, leading to nonAbelian braiding of phonons. We show that phonons can be a primary platform to study nonAbelian braiding in the reciprocal space, and we expand the toolset to study such braiding processes. 
Thursday, March 17, 2022 1:18PM  1:30PM 
T48.00008: Investigating the Role of Phonons in the Phase Stability of Uranium based Laves phases Erik Nykwest, Zachary Brubaker, Ashley Shields, Andrew Miskowiec, Jennifer Niedziela Laves phases are intermetallic alloys which form in one of three specific structural arrangements, denoted C14 (P63/mmc), C15 (Fd3mS), and C36 (P63/mmc), and comprise interconnected tetrahedra that facilitate high electrical and thermal conductivity. The factors governing the formation of a Laves phase into one structure over another remains an open question. In particular, the role of phonons, which may provide information about phase transition mechanisms, transition temperatures, and vibrational contributions to entropy, has been widely overlooked. 
Thursday, March 17, 2022 1:30PM  1:42PM 
T48.00009: Vibrational properties of solid molecular hydrogen through quantum correlators Tommaso Morresi, Michele Casula, Rodolphe Vuilleumier In the complex phase diagram of solid hydrogen at high pressures, phase transitions are indirectly deduced from the analysis of infrared and Raman spectra by comparison of experimental data with theoretical predictions. However, anharmonic and nuclear quantum effects (NQE) are notoriously large in hydrogen and hydrogenbased materials, and pose a formidable challenge for theory predictions. In this work we propose a generalization of the method presented in Ref. [1], in order to accurately describe anharmonicity and NQE in molecular crystals where the interplay between molecular and lattice degrees of freedom plays a pivotal role to determine their vibrational properties. We show that our approach is able to clearly distinguish between different competing candidates for hydrogen phase III, thanks to an accurate determination of vibronic frequencies. Our scheme therefore represents a valuable tool to identify highpressure molecular structures from first principles through their vibronic characterization. 
Thursday, March 17, 2022 1:42PM  1:54PM 
T48.00010: Revisiting the electronic properties of diamondlike amorphous carbon Arpan Kundu, Yunxiang Song, Giulia Galli We present results for the electronic properties of diamondlike amorphous carbon obtained using first principles molecular dynamics (FPMD) and including nuclear quantum effects (NQEs) by coupling FPMD with a quantum thermostat. We carry out simulations with a protocol recently applied to crystalline diamond and molecular carbon [1]. We show that NQEs lead to a substantial renormalization of the fundamental and mobility gaps below 500 K, due to electronphonon interaction, and that their inclusion in first principles calculations is critical to obtain a reliable physical picture of the electronic structure of diamondlike carbon. Surprisingly, despite the presence of localized electronic states near the HOMOLUMO gap, in diamondlike carbon the electronphonon renormalization can be as large as in crystalline diamond, showing great variability depending on the specific atomistic structure (e.g. sp^{3} to sp^{2} bond ratio) of the sample. 
Thursday, March 17, 2022 1:54PM  2:06PM 
T48.00011: First principles deformation potential extraction and computationally efficient mobility calculations Zhen Li, Patrizio Graziosi, Neophytos Neophytou Based on density functional theory (DFT) and density functional perturbation theory (DFPT), we have developed a firstprinciples framework to extract acoustic, optical, and intervalley deformation potentials from the shortrange electronphonon (eph) matrix elements, for incorporation with the Boltzmann transport equation (BTE). Using the BTE based on a numerical simulator that allows for the incorporation of eph scattering and ionized impurity scattering (IIS), we are able to compute a comparable mobility with results from advanced first principles calculations for Si. The method we present would be the middle ground computationally between the constant relaxation time (CRT) approximation and ab initio relaxation time extraction with ultradense grids, while providing first principles accuracy. We describe the method for Si, but it can be generalized and applied to other solidstate semiconductors and insulators, with much higher computational efficiency compared to fully ab initio simulations. 
Thursday, March 17, 2022 2:06PM  2:18PM 
T48.00012: FourDimensional Imaging of Lattice Dynamics using Inelastic Scattering Navdeep Rana, Aditya Prasad Roy, Dipanshu Bansal, Gopal Dixit Timeresolved mapping of lattice dynamics in real and momentumspace is essential to better understand several ubiquitous phenomena. In this work, we demonstrate that inelastic scattering methods, are competent to provide similar information as one could obtain from the timeresolved diffraction and imaging measurements. To illustrate the robustness of the proposed method, our simulated result of lattice dynamics in germanium is in excellent agreement with the timeresolved xray diffuse scattering measurement performed using xray freeelectron laser. For a given inelastic scattering data in energy and momentum space, the proposed method is useful to image insitu lattice dynamics under different environmental conditions. Few tens of femtoseconds temporal and ~ 1 Å spatial resolutions can be achieved during the reconstruction of lattice dynamics by utilizing the superior energy and momentum resolutions of inelastic scattering measurements. Our proposed theoretical method allows for direct imaging of lattice dynamics and enables us to extract the lifetime of selective phonon modes. The technique will profoundly impact where timeresolved diffraction within the pump–probe setup is not feasible, for instance, in inelastic neutron scattering. 
Thursday, March 17, 2022 2:18PM  2:30PM 
T48.00013: Unified Model of Lattice Thermal Conductivity in Crystalline Cellulose Iβ Zhiyu Liu, Peter W Chung Cellulose is one of the most abundant natural polymers and is a sustainable energy source. Under delignification and densification, cellulose was found to be having outstanding radiative cooling properties. Thus, phonons, being the dominant heat carrier in crystalline cellulose, are of great importance in understanding the energy transfer mechanism. In this work, we investigate the thermal transport in crystalline Cellulose Iβ using molecular dynamics, lattice dynamics with ReaxFF that has been modified for the study of phonons. Different thermal transport models including GreenKubo (GK) formula, AllenFeldman, CahillWatsonPohl, the ideal phonon gas models, and the contribution from the offdiagonal terms in the heatflux operator are applied to calculate the thermal conductivity in cellulose. Our results indicate that the unified model including both diagonal and offdiagonal contribution of heatflux operator shows the best agreement with GK and experiments among all the theoretical models. Compared with the phonon gas model, the leading contribution of the coherences' terms to thermal conductivity indicates its predominant role in thermal transport in cellulose. Our study provides insights into the understanding of thermal transport mechanisms in complex molecular crystals. 
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