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 P20: Electrons, Phonons, Electron-Phonon Scattering, and Phononics VIFocus Live
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Sponsoring Units: DCOMP DMP Chair: Olivier Delaire, Duke University |
Wednesday, March 17, 2021 3:00PM - 3:36PM Live |
P20.00001: Quantifying Uncertainty in First-Principles Predictions of Molecular Vibrational Frequencies, Phonon Properties, and Thermal Conductivity Invited Speaker: Alan McGaughey We present a robust method for quantifying the uncertainty in molecular vibrational frequencies, phonon properties, and thermal conductivities predicted from density functional theory calculations using the BEEF-vdW exchange-correlation (XC) functional. The procedure starts by displacing atoms in an equilibrium structure. BEEF-vdW generates an ensemble of energies for each perturbed structure as a computationally efficient post-processing step by perturbing the XC functional and solving for the energy non-self consistently. The energy ensembles of the perturbed structures are used with finite difference formulas to determine an ensemble of force constants, which is then used as input to lattice dynamics calculations and a solution of the Boltzmann transport equation. This procedure results in ensembles for the vibrational frequencies of a molecule or the phonon frequencies, group velocities, lifetimes, heat capacity, and thermal conductivity of a crystal, whose spreads can be used to quantify uncertainty. Results for molecules, molecular complexes from the S22 dataset, and silicon are presented and compared to predictions from XC functionals at the local density approximation and generalized gradient approximation levels. |
Wednesday, March 17, 2021 3:36PM - 3:48PM Live |
P20.00002: Electron-phonon drag enhancement of transport properties from fully coupled ab initio Boltzmann formalism Nakib Protik, Boris Kozinsky We present a combined and self-consistent treatment of electron and phonon transport by performing ab initio calculations of the fully coupled Boltzmann transport equations of the two carriers. We find that the presence of mutual drag between the two carriers causes the thermopower to be enhanced and dominated by the transport of phonons, rather than electrons as in the traditional semiconductor picture. Drag also strongly boosts the intrinsic electron mobility, thermal conductivity and the Lorenz number. Impurity scattering is seen to suppress the drag-enhancement of the thermal and electrical conductivities, while having weak effects on the enhancement of the Lorenz number and thermopower. We demonstrate these effects in n-doped 3C-SiC at room temperature, and explain their origins. This work establishes the roles of microscopic scattering mechanisms in the emergence of strong drag effects in the transport of the interacting electron-phonon gas. |
Wednesday, March 17, 2021 3:48PM - 4:00PM Live |
P20.00003: CDW phases of rare-earth Tritellurides from first principles Adam Denchfield, Hyowon Park, Peter Littlewood The rare-earth tri-tellurides, RTe3 exhibit many phases, such as antiferromagnetic and charge-density-wave (CDW) order at different temperatures. The phase boundary also depends sensitively on the size of the rare-earth ion. There has been debate as to whether or not the CDW order can be predicted by zero-temperature DFT band structure properties such as its Fermi surface nesting. In this work, we perform the Fermi surface, the charge susceptibility, and phonon calculations in RTe3 based on DFT and DFPT methods. One challenge of this work is the difficulty of performing DFT and DFPT on a very fine k-mesh. Here, we generate electron and phonon band structures on a very fine k-mesh by interpolating them using Maximally-Localized Wannier Functions obtained from first-principles, and diagonalizing them for billions of k-points using a GPU algorithm. We also compute the charge susceptibility dressed by the electron-phonon interaction obtained from the EPW package across different rare-earth ions (R=La-Lu) and analyze the progression of susceptibility peaks as a function of R, comparing with experiment. |
Wednesday, March 17, 2021 4:00PM - 4:12PM Live |
P20.00004: Phonon linewidths as measures of anharmonicity in different materials Claire Saunders, Brent Fultz Phonons are central to most thermophysical properties of materials. Understanding broad trends in thermophysical properties help inform our current understanding of types of anharmonicity in materials and guide research avenues. We report a comparative analysis of experimental phonon linewidths from literature and our own inelastic neutron scattering results for Si, NaBr, FeGe2, and Cu2O. The linewidths are indicative of phonon lifetimes owing to 3-phonon processes, and often vary substantially for different branches in the same material. Even though comparisons between materials are impaired by differences in experimental technique, many trends are large enough to see easily. The phonon linewidths of UO2 are approximately four times broader than those of Si [1]. Similar trends are seen when comparing Al and Si [2, 3]. In materials where the overall anharmonicity is not large, like Si, the anharmonicity can still dominate over the volume dependence of phonon frequencies for thermophysical properties such as thermal expansion [4, 5]. |
Wednesday, March 17, 2021 4:12PM - 4:24PM Live |
P20.00005: Longitudinal and Transverse Dispersion in Liquids from Elementary Excitations Alexander Fullmer, Anant Raj, Jacob Eapen Phonons are quantized excitations of lattice vibrations. Although they are quasiparticles in the quantum framework, their properties and interactions in a crystal can be studied using the normal modes of vibrations in a periodic lattice. Extending a similar approach to model collective excitations in disordered systems is gaining traction in recent years. However, the absence of well-defined lattice sites hinders the definition of normal modes of atomic displacements in such systems. In this work, we extend our recent methodology of using elementary waves that can unambiguously identify phonons along a specified wavevector in an atomistic system to an aperiodic liquid system. Relaxing the periodic requirement, we employ our recent approach of using the projections of zero-time correlations of accelerations and velocities (ZTR-2) to circumvent the inherent inability to define atomic displacements about mean positions in a liquid. We show that our approach can successfully compute the normal mode dispersion in liquids from elementary atomic excitations using atomistic simulations. We conclude our presentation by discussing the possible applications for probing quasi-phonon modes in a wider class of materials, including supercooled liquids and glasses. |
Wednesday, March 17, 2021 4:24PM - 4:36PM Live |
P20.00006: Improvements to the Frequency Domain Perfectly Matched Layer (FDPML) method for fast and large scale phonon transport simulation in nanostructures Joseph Feser, Rohit R Kakodkar Recently, our group has presented a new method for predicting the interaction of phonons with general atomistic domains, called the frequency domain perfectly matched layer (FDPML) method, providing similar information to the phonon AGF method but on a wavevector-resolved basis and with a simpler, faster algorithm. We describe two improvements that now allow for larger computational domain sizes with reduced solution time (1) we describe an MPI approach that is scalable in both memory and time, allowing the method to be used for extremely large simulation domains, for which we show the example of composites with embedded nanoparticles as a useful application case. (2) we show that although the method is intrinsically wavevector-by-wavevector, it is possible to cheaply perform all isofrequency wavevector simulations simultaneously without increasing the computational requirements, significantly speeding transport property calculations requiring integrals over all wavevectors. |
Wednesday, March 17, 2021 4:36PM - 4:48PM Live |
P20.00007: State-of-the-Art Matrix-Product-State Methods for Lattice Models with Large Local Hilbert Spaces and Without Number Conservation Jan Stolpp, Thomas Koehler, Salvatore R Manmana, Eric Rene Jeckelmann, Fabian Heidrich-Meisner, Sebastian Paeckel Lattice models consisting of high-dimensional local degrees of freedom without global particle-number conservation constitute an important problem class in the field of strongly correlated quantum many-body systems. For instance, they are realized in electron-phonon models, cavities, atom-molecule resonance models or systems realizing superconductivity. In general, these systems elude a complete analytical treatment and need to be studied using numerical methods where matrix-product states (MPS) provide a flexible and generic ansatz class. Typically, MPS algorithms scale quadratic or even cubic in the dimension of the local Hilbert spaces. Hence, tailored methods, which truncate this dimension, are required to allow for efficient simulations. Here, we describe and compare three state-of-the-art MPS methods each of which exploits a different approach to tackle the computational complexity. We analyze the properties of these methods at the example of the Holstein model, performing high-precision calculations as well as a finite-size-scaling analysis of relevant ground-state obervables. The calculations are performed at different points in the phase diagram yielding a comprehensive picture of the different approaches. |
Wednesday, March 17, 2021 4:48PM - 5:00PM Live |
P20.00008: First principles simulations of phonon-assisted indirect optical properties of common SiC polytypes Xiao Zhang, Emmanouil Kioupakis Silicon carbide (SiC) is an important indirect-gap semiconductor used in many electronic/optoelectronic devices. Although being a well-studied material, first principles simulation of its phonon-assisted optical properties, dominating near the absorption onset, remains a challenge due to the high computational cost and lack of theoretical tools. In our study, we apply density functional perturbation theory and maximally localized Wannier functions to evaluate and interpolate the electron-phonon coupling matrix elements in order to investigate phonon-assisted optical absorption in five common SiC polytypes. We show that combined with the GW/Bethe-Salpeter equation approach, our simulated indirect optical spectra agree well with experimental measurements. Our work provides valuable foundation for the further theoretical investigation of phonon-mediated optical properties such as photoluminescence, as well as guidance on potential optoelectronic applications of the different polytypes. |
Wednesday, March 17, 2021 5:00PM - 5:12PM Live |
P20.00009: Quasi-harmonic temperature dependent elastic constants from first principles Cristiano Malica, Andrea Dal Corso We present our implementation of the temperature dependent elastic constants (EC) in the thermo_pw software, a driver of the Quantum ESPRESSO routines for the calculation of material properties. The calculation can be done within either the quasi-static or the quasi-harmonic approximations (QHA) and can provide both the isothermal and isoentropic EC. Within the QHA, isothermal EC are computed from second derivatives with respect to strain of the Helmholtz free energy, derived from the phonon frequencies computed via Density Functional Perturbation theory for several strained configurations. |
Wednesday, March 17, 2021 5:12PM - 5:24PM Live |
P20.00010: Lattice dynamics of body-centered cubic Zr and FeTi from scratch Adrian De la Rocha Galán, Vanessa Judith Meraz, Armando Garcia, Bethuel Khamala, Yu-Hang Tang, Wibe A De Jong, Jorge Munoz Linear fits to forces as a function of atomic displacements were performed for each time step of a quantum molecular dynamics simulation of body-centered cubic (bcc) zirconium (Zr) and the equiatomic bcc-based iron-titanium (FeTi) intermetallic alloy to determine interatomic force constants. The distributions of force constants between pairs of atoms are Gaussian with variances that depend on the atomic configuration. The symmetry operations of the bcc crystallographic point group were applied to wrap the force constants into their primitive unit cell representation. The means of the distributions were used to fit a Born-von Kármán (BvK) lattice dynamics model from which phonon dispersion curves were computed in the harmonic approximation. Phonon dispersion curves provide the energy of particular atomic vibration patterns. Several thermodynamic variables were derived from the dispersion curves and compared to experiments. |
Wednesday, March 17, 2021 5:24PM - 5:36PM Live |
P20.00011: Spin-lattice coupling in the classical invar alloy from first principles Matthew Heine, Olle Hellman, David Broido We present a method for treating magnetic systems and use it to investigate the anomalously low thermal expansion of the classical Invar alloy, Fe0.65Ni0.35. Results at and below room temperature are consistent with the experimentally observed low thermal expansion of this Invar alloy. Phonon dispersions are also calculated, and renormalization of the phonon spectra by magnetic thermal disorder is found to be significant. The calculated phonon dispersions are in excellent agreement with measurement. |
Wednesday, March 17, 2021 5:36PM - 5:48PM Live |
P20.00012: Search for high thermal conductivity materials among boron/carbon/nitrogen compounds Chunhua Li, Matthew Heine, David Broido The Slack guidelines [1] suggest that materials with complex crystal structures should not have high thermal conductivity. However, an exception to this rule was recently identified in BC2N, which was predicted to achieve a room temperature thermal conductivity of around 1000-2000 W m-1 K-1 in the trigonal structure [2, 3]. Moreover, recent work suggests that ultrahigh thermal conductivity may not be possible in compounds that do not contain boron (B), carbon (C) or nitrogen (N) [4]. Motivated by these findings, we present results of a first principles-based computational search for high thermal conductivity among several hundred compounds composed of combinations of B, C, and N. The relatively low thermal conductivities of the studied compounds highlight the challenges in satisfying the stringent high thermal conductivity criteria among real materials. |
Wednesday, March 17, 2021 5:48PM - 6:00PM Live |
P20.00013: Implementation, validation and applications of electron-phonon calculations to large systems Han Yang, Marco Govoni, Giulia Galli We generalize to extended systems a method recently developed [1] to carry out electron-phonon calculations, in which the dielectric matrix is represented in terms of dielectric eigenpotentials [2]. The latter are utilized for the evaluation of G0W0 quasi-particle energies, as well as dynamical and electron-phonon coupling matrices. The implementation is part of the WEST code [3] (www.west-code.org) and does not require summation over virtual electronic states or self-consistent density-functional-perturbation-theory calculations, thus making it scalable to large systems. Validation of the approach and applications to point defects in semiconductors are presented. |
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