Bulletin of the American Physical Society
APS March Meeting 2023
Las Vegas, Nevada (March 510)
Virtual (March 2022); Time Zone: Pacific Time
Session F61: Electrons, Phonons, ElectronPhonon Scattering and Phononics IIIFocus

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Sponsoring Units: DCOMP DFD Chair: Jennifer Coulter, Harvard University Room: Room 418 
Tuesday, March 7, 2023 8:00AM  8:36AM 
F61.00001: Vibrational dynamics driven by structural symmetries and complexities Invited Speaker: Lucas Lindsay Vibrational excitations and their influence on material structure and function often govern the potential for applications, such as thermoelectrics, or set limitations for these, such as spinphonon couplings in magnetic storage and processing devices. As such, developing deep insights into the rich physics of phonons and their quasiparticle interactions is critical to developing further fundamental insights toward design of improved or new material functionalities. Here, I will discuss recent advances in our understanding of vibrational behaviors of materials, particularly highlighting the role of structural complexities and symmetries in determining vibrational dynamics and transport. More specifically, I will discuss twist phase relations in materials with screw axes, translational phase relations between primitive and conventional geometries, how such relations manifest in measured spectra, and phonon scattering behaviors in layered structures. I will discuss this physics in the context of a variety of materials, including wideband gap compounds and superlattices [(Al/Ga)N], magnetic semiconductors [Mn(Te/Sb/Bi)], elemental metals (Te), and cleavable ferromagnets (CrCl_{3}). 
Tuesday, March 7, 2023 8:36AM  8:48AM 
F61.00002: Quartic Anharmonic Lattice Thermal Conductivity in 2D InS Monolayer: Selfconsistent Phonon Calculations Eesha Andharia There are many reports on low lattice thermal conductivity in 2D van der Waal’s based materials for thermoelectric applications using Density Functional Theory (DFT) and Molecular Dynamics simulations. Wickramaratne [1] et. al. have extensively studied the excellent thermoelectric properties of 2D group IIIVI materials such GaX and InX (X = S, Se) owing to the formation of ‘Mexican hat’ in the valence band of electronic structure. However, they have only focused on the electronic thermal conductivity and estimated the ZT using the upperbound value of κl. This gap in research was motivation for several studies focusing on phonon transport and thermal properties of 2D InS monolayer. All the above reports are limited to the calculations of 1) phonon dispersion relations using the harmonic approximation and 2) lattice thermal conductivity using third order interatomic force constants (IFC’s) by solving BTE. However, a more reliable calculation of lattice transport properties and the effect of finite temperature on phonon spectra requires the inclusion of higher order anharmonicity. Selfconsistent phonon theory based on quantum mechanical description of the phonons is an efficient nonperturbative way of including the anharmonicity. In this study, the abovementioned approach is used to study the phonon transport properties and the thermodynamic parameters using quartic anharmonicity by including the quartic IFCs of InS monolayer to find their significant effect on the value of lattice thermal conductivity. 
Tuesday, March 7, 2023 8:48AM  9:00AM 
F61.00003: Zonecenter chiral phonons from broken time reversal symmetry Shang Ren, John R Bonini, Massimiliano Stengel, Sinisa Coh, David Vanderbilt, Cyrus E Dreyer In conventional ab initio methodologies, phonons are calculated by solving equations of motion involving static interatomic force constants and atomic masses. However, this approach does not fully account for the effects of broken timereversal symmetry in systems with magnetic order. Recent attempts to rectify this involve the inclusion of the velocity dependence of the interatomic forces in the equations of motion, which is given by the nuclear Berry curvature. This can result in "chiral" phonon modes with nonzero angular momentum, even at the zone center. We show, via a novel densityfunctional theory methodology based on finite displacements, that the main contribution to this nuclear Berry curvature is from the spin rotations caused by the phonons. This requires spin and phonon degrees of freedom to be treated on the same footing. We propose a model involving Hessian matrices and Berry curvature tensors in terms of both spin and phonon degrees of freedom, and develop a firstprinciples methodology to calculate them. We present results on both ferromagnets and antiferromagnets, and the implication for splitting of chiral phonon modes is discussed. 
Tuesday, March 7, 2023 9:00AM  9:12AM 
F61.00004: Chiral phonons throughout reciprocal space Carl P Romao, Nicola A Spaldin The properties of chiral phonons (phonons with associated angular momentum and associated magnetic moments) have been studied mainly at the Γ point (where they can be created by circularly polarized excitations) and other highsymmetry points in the Brillouin zone. However, they can arise at arbitrary wavevectors in materials which lack a centre of symmetry. Thereby, phonon chirality can affect the bulk properties of solids, especially in cases where time reversal symmetry is broken, leading to, for example, the phonon thermal Hall effect. We have selected noncentrosymmetric materials in which chiral phononic effects are enhanced by structural and chemical factors, and calculated the distribution of chiral phonons throughout the Brillouin zone and their properties in order to predict chiral phononic responses to external perturbations (e.g. electromagnetic and thermal stimuli). 
Tuesday, March 7, 2023 9:12AM  9:24AM 
F61.00005: Entropy contributions to explain thermal expansion: thermodynamics of the Invar effect Stefan Lohaus, Pedro Guzman, Camille M BernalChoban, Claire N Saunders, Brent T Fultz The anomalously low thermal expansion observed in some metals, known as the Invar effect, has long been associated with magnetism. Its microscopic underpinnings, however, are still an open scientific question, long after C. E. Guillaume received the 1920 physics Nobel Prize for its discovery. In particular the role of phonons, their contribution to the thermal expansion, and their dependence on magnetism has not been previously quantified. We explore a new method for obtaining the thermal expansion from the pressure dependence of the entropy. By combining two nuclear xray scattering techniques, suitable for pressure experiments in diamondanvil cells, we probe contributions from phonons and spins to the entropy of FeNi Invar. We show that the Invar behavior stems from a competition between phonons and spins, resulting in a cancellation of their entropy contributions. Ab initio phonon calculations reveal a spinlattice coupling at the root of the Invar anomaly. Such couplings of excitations from phonons and spins go beyond the case of Invar, and could be the cause of the thermal expansion behavior in other magnetic materials. 
Tuesday, March 7, 2023 9:24AM  9:36AM 
F61.00006: Electronic density of states of bodycenteredcubic Fe under phonon excitations, boron doping and amorphization Zhihao Jiang, Axel Hoffmann, Andre Schleife It has been shown that the intrinsic magnon damping in metallic ferromagnetic materials is proportional to the electronic density of states (EDOS) at Fermi level. This is theoretically described by the breathing Fermi surface model. Understanding how the EDOS at the Fermi level is affected by different factors has potential value for experimentalists to optimize lowdamping magnetic materials for applications in spintronics and magnonics. To investigate this question, we consider a paradigmatic ferromagnetic material, the bodycenteredcubic iron (Fe). We calculate the EDOS of Fe based on the density functional theory (DFT) using the Vienna AbInitio Simulation Package (VASP). Three different scenarios are considered: 1) Fe with thermal lattice disorder, 2) Fe doped by boron (B) and 3) Fe in an amorphous state. We have observed that, while the EDOS at Fermi level is not much affected by phonons up to the temperature of 500K, it is significantly reduced by B doping. Lowered EDOS at Fermi level can also be achieved in the amorphous Fe and FeB. 
Tuesday, March 7, 2023 9:36AM  9:48AM 
F61.00007: Quantum nuclear vibrations and the electronic properties of molecular crystals Arpan Kundu, Giulia Galli We present a study of the electronic properties of molecular crystals, with a focus on the effect of nuclear quantum motion and anharmonicity on their band gap. We consider a system composed of relatively rigid molecules, a diamondoid crystal, and one composed of floppier molecules, NAIDMAC, a thermally activated delayed fluorescence compound. We compute the electronic properties at the DFT level of theory, by coupling first principles molecular dynamics with a nuclear quantum thermostat, following a protocol previously applied to diamond and amorphous carbon [1]. We use the Qbox code (http://qboxcode.org/) coupled with iPI (http://ipicode.org/). We find a sizable zeropointrenormalization (ZPR) of the band gaps, which is much larger in the case of diamondoids (~ 0.6 eV) than for NAIDMAC (~ 0.22 eV). We show that when using the frozen phonon (FP) approximation, which neglects intermolecular anharmonic effects, we introduce a large error (~ 50%) in the calculation of the band gap ZPR. Instead, when using a stochastic method [2], we obtain results in good agreement with those of our quantum simulations for the diamondoid crystal. However, the agreement is worse for NAIDMAC where intramolecular anharmonicities largely contribute to the ZPR. Our results highlight the importance of accurately including nuclear and anharmonic quantum effects to predict the electronic properties of molecular crystals. 
Tuesday, March 7, 2023 9:48AM  10:00AM 
F61.00008: Impacts of Shortrange Order on Thermal conductivity of SiGeSn Alloys Shunda Chen, Tianshu Li SiGeSn alloys are siliconbased, versatile materials for electronic, photonic, and topological quantum applications. Thermal management in such applications is an important issue but has not been well addressed. As our recent studies^{13} predicted the complex shortrange order behaviors in SiGeSn alloys, it remains to be understood how thermal transport is affected by the nonrandom distribution of alloying elements. Here, by computing lattice thermal conductivity from firstprinciples calculations, we find that the thermal conductivity of SiGeSn alloys shows strong variation with atomic ordering. The underlying mechanisms are explored and discussed. Our preliminary results suggest that shortrange order may provide a new route for engineering the thermal properties of Group IV alloys. 
Tuesday, March 7, 2023 10:00AM  10:12AM 
F61.00009: Interfacial thermal transport in Bi_{2}Te_{3} Aoife K. Lucid, Javier F Troncoso, Jorge Kohanoff, Stephen B Fahy, Ivana Savic Highly efficient, roomtemperature thermoelectrics are key to the future of environmental energy harvesting and solidstate cooling, among many other applications. To advance beyond the current stateoftheart roomtemperature thermoelectrics it is critical to understand the impact of interfaces on thermal transport in currently utilised materials, such as Bi2Te3, at an atomistic level. The importance of this understanding is highlighted even further considering the prevalence of nanostructured (often polycrystalline) materials. In this work, we employ reverse nonequilibrium molecular dynamics simulations (rNEMD) with a classical twobody interatomic potential (IP) [1], to examine the effect of specific interfacial structures on thermal transport in Bi2Te3. The interfacial thermal resistance (Kapitza resistance) and interfacial structures are compared across a number of twin boundaries. In addition, we will examine finitelength effects in rNEMD simulations of interfaces; often these are overlooked. We will also discuss the limitations of existing IPs and consider the development of machinelearned IPs for Bi2Te3. 
Tuesday, March 7, 2023 10:12AM  10:24AM 
F61.00010: Anomalous thermoelectric transport phenomena from interband electronphonon scattering Boris Kozinsky, Natalya S Fedorova, Andrea Cepellotti The Seebeck coefficient and electrical conductivity are two central quantities to be optimized simultaneously in designing thermoelectric materials, and they are determined by the dynamics of carrier scattering. We uncover a new regime where the presence of multiple electron bands with different effective masses, crossing near the Fermi level, leads to strongly energydependent carrier lifetimes due to intrinsic electronphonon scattering. In this anomalous regime, electrical conductivity decreases with carrier concentration, Seebeck coefficient reverses sign even at high doping, and power factor exhibits an unusual second peak. We explain the origin and magnitude of this effect using a general simplified model as well as firstprinciples Boltzmann transport calculations in recently discovered halfHeusler alloys. We identify general design rules for using this paradigm to engineer enhanced performance in thermoelectric materials. 
Tuesday, March 7, 2023 10:24AM  10:36AM 
F61.00011: A Study Of The Heusler Alloy (Fe_{2}V_{0.8}W_{0.2}Al) Ming Yin, Darnell Coicous, Krystin Ferguson, Dania Collieand, ostonya thomas, Timir Datta Thermoelectric materials offer a great potential for directly converting heat into electricity and are essential for widescale renewable energy application. For practical thermoelectric generation applications, a large figure of merit (ZT) is beneficial. The goal of our research is to search for large figure of merit (ZT) thermoelectric materials. Because of their reasonably large ZT values Fe_{2}V_{0.8}W_{0.2}Al and related Heusler systems are potential thermoelectric materials. Here we describe synthesis, as well as experimental details and numerical results on thermal and electrical properties of bulk Fe_{2}V_{0.8}W_{0.2}Al. Briefly, we find that both electrical resistivity, r and thermal conductivity, k increases with temperature. For instance, around room temperature over 100K, both r and K increase by ~10% . Also, Seebeck coefficient is about 40 μV/K around room temperature and mildly increases with increasing temperature The implication of these results will be discussed. 
Tuesday, March 7, 2023 10:36AM  10:48AM 
F61.00012: Quasiharmonic approximation via irreducible derivatives: low symmetry crystals Mark Mathis, Chris A Marianetti The quasiharmonic approximation is a standard approach to include anharmonicity when evaluating temperaturedependent vibrational observables, yet computing arbitrary observables is nontrivial. Here we execute the irreducible derivative approach to the quasiharmonic approximation, greatly reducing the computational cost and facilitating the study of lower symmetry crystals. Specifically, we study PbTiO$_3$ using density functional theory within various exchangecorrelation functionals, computing the thermal expansion and the full elastic constant tensor as a function of temperature. The BornOppenheimer potential and the irreducible components of the dynamical matrix are parametrized by a Taylor series expansion in symmetrized strain, allowing for the systematic study of successively higher order truncations of the quasiharmonic potential. Additionally, we explore the validity of the quasiharmonic approximation in metals, including ZrN. Results are compared to existing experimental measurements. 
Tuesday, March 7, 2023 10:48AM  11:00AM 
F61.00013: FirstPrinciples Simulation of Lattice Thermal Conductivity: Uncertainties from Different Flavors of TemperatureDependent Force Constants Zhi Li Firstprinciples simulations of lattice thermal conductivity, κ_{L}, of highly anharmonic crystals have long been challenging in condensed matter physics. With recent theoretical advances, the calculation of κ_{L} has evolved into a sophisticated process requiring the consideration of highorder phononphonon scattering, anharmonic phonon renormalization, and the temperaturedependent potential energy surface. Interatomic force constants (IFCs), however, as a shared pillar of the above concepts, are ambiguously implemented in this process, resulting in nonnegligible discrepancies between different studies. Here, we revisit the ultralow κ_{L} of Tl_{3}VSe_{4} and make a rigorous comparison using different flavors of IFCs. We find that the phonon spectrum obtained with 0 K IFCs is prone to get over stiffened after anharmonic renormalization, which leads to a doubled κ_{L} even with up to fourphonon scattering considered. Moreover, the different flavors of fitting 2nd, 3rd, and 4thorder IFCs at finite temperatures using forcedisplacement data leads to significantly dispersive κ_{L}. Our work illuminates how choosing different IFCs alters κ_{L}, which contributes to a better understanding of lattice dynamics simulation based on the perturbation theory. 
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