Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session A20: Heat Transport in Condensed Systems IFocus Live

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Sponsoring Units: DCOMP Chair: Nicola Marzari, Ecole Polytechnique Federale de Lausanne 
Monday, March 15, 2021 8:00AM  8:12AM Live 
A20.00001: Hydrodynamic heat transport in low thermal conductivity materials Kanka Ghosh, Andrzej Kusiak, JeanLuc Battaglia Phonon hydrodynamics was mostly identified for high thermal conductivity materials due to their dominant phononphonon scattering. However, we show that even a lowthermal conductivity material, such as chalcogenide GeTe can exhibit phonon hydrodynamics depending on the mutual influence of phononphonon and extrinsic scattering processes. Our study involves the firstprinciples density functional method along with the direct solution of linearized Boltzmann transport equations to analyze the thermal transport in crystalline GeTe for a wide range (4300 K) of temperatures. A phonon hydrodynamic regime is found to appear at low temperature analyzing the average scattering rates for normal, umklapp, and other resistive processes. Further, using the kineticcollective model, the Knudsen number is determined which suggests the consistent hydrodynamic behavior of phonon thermal transport for GeTe. Finally, grain size and vacancies are found strongly to modify the hydrodynamic window. 
Monday, March 15, 2021 8:12AM  8:24AM Live 
A20.00002: Koopmans' spectral functionals: an opensource periodicboundary implementation Nicola Colonna, Riccardo De Gennaro, Edward Linscott, Nicola Marzari Koopmans' spectral functionals aim to describe simultaneously ground state properties and charged excitations of atoms, molecules, nanostructures and periodic crystals[1,2]. This is achieved augmenting standard density functionals with simple but physically motivated orbitaldensitydependent corrections. These corrections act on a set of localized orbitals that, in periodic systems, resembles maximally localized Wannier function. At variance with a direct supercell implementation, we discuss here the complex but efficient formalism required for a periodicboundary code, using explicit Brillouin zone sampling and the calculation of the screened and unscreened response with densityfunctional perturbation theory. The implementation in the Quantum ESPRESSO distribution and the application to prototypical insulating and semiconducting systems are presented and discussed. 
Monday, March 15, 2021 8:24AM  8:36AM Live 
A20.00003: Fermi energy determination for advanced smearing techniques Flaviano dos Santos, Nicola Marzari Smearing techniques are widely used in firstprinciples calculations of metallic and magnetic systems, where they improve the accuracy of Brillouin zone sampling and lessen the impact of levelcrossing instabilities. Smearing works by introducing a fictitious electronic temperature that smooths the discontinuities of the integrands; consequently, a fictitious entropic term needs to be added to get the correct total free energy functional. MethfesselPaxton and cold smearing are two approaches that are constructed to make the system's total free energy temperatureindependent at least up to the third order. In doing so, these end up with nonmonotonic occupation functions (and, for MethfesselPaxton, not positive definite), which can result in the chemical potential not being uniquely defined. We explore this shortcoming in detail, and propose a protocol combining different rootfinding methods to implement a datadriven solution to determine the material's correct Fermi energy. We validate the method by calculating the Fermi energy of thousands of materials and comparing them with the results of previous approaches. 
Monday, March 15, 2021 8:36AM  9:12AM 
A20.00004: Bridging the difference between Fourier's law and NavierStokes equations Invited Speaker: Andrea Cepellotti Thermal transport often deviates from Fourier's law and in hydrodynamic conditions, as for example in layered and twodimensional materials, heat flux more closely resembles the flow of liquids rather than the diffusive flow predicted by Fourier's law. Here, we start from the linearized phonon Boltzmann transport equation to discuss how hydrodynamic heat transport in crystals arises from the propagation of both energy and crystal momentum fluxes. Microscopically, the two fluxes are distinguished by the parity of relaxons (eigenvectors of the scattering matrix), with odd relaxons contributing to energy transport, and even relaxons contributing to momentum transport. Macroscopically, the parity symmetry gives rise to two coupled equations, which we term viscous heat equations, describing thermal transport in terms of fields of temperature and phonon drift velocity. The energy and momentum diffusion is controlled by the coefficients of thermal conductivity and viscosity respectively. These viscous heat equations, which can be parametrized with abinitio simulations, extend Fourier's law to the hydrodynamic regime and represent the thermal counterpart of NavierStokes equations in the linear and laminar regime, extending the reach of mesoscopic models of heat conduction. 
Monday, March 15, 2021 9:12AM  9:24AM Live 
A20.00005: Heat transport in ordered and disordered solids within Wigner’s phasespace formulation Michele Simoncelli, Francesco Mauri, Nicola Marzari We explore the atomistic mechanisms of thermal transport in solids using Wigner’s [Phys. Rev. 40 (1932)] phasespace formulation of quantum mechanics, showing how this formalism allows to derive a heattransport equation that describes on an equal footing heat conduction in crystals, glasses, and anything in between [Simoncelli, Marzari, and Mauri, Nat. Phys., 15 (2019)]. We use this framework to shed light on formal aspects of the theory of thermal transport in solids, including the description of local equilibrium (i.e., the state associated to a spacedependent temperature) and the differences between Wigner’s [Nat. Phys., 15 (2019)] and Hardy’s [Phys. Rev. 132 (1963)] expressions for the heat flux. 
Monday, March 15, 2021 9:24AM  9:36AM 
A20.00006: Suppression of coherent thermal transport in quasiperiodic graphenehBN superlattice ribbons Isaac M Felix, Luiz Felipe Pereira Nanostructured superlattices are promising for novel electronic devices due to adjustable physical properties. Periodic superlattices facilitate coherent phonon transport due to constructive interference at the boundaries between materials. It is possible to induce a crossover from coherent to incoherent transport by adjusting the superlattice period. We have observed such crossover in periodic graphenehBN nanoribbons as the length of individual domains increased. In general, transport properties are dominated by translational symmetry and the presence of unconventional symmetries leads to unusual transport characteristics. We performed nonequilibrium molecular dynamics simulations to investigate phonon heat transport in graphenehBN superlattices following the Fibonacci quasiperiodic sequence. We show that the quasiperiodicity can suppress coherent phonon transport in these superlattices, and attribute this behavior to the increasing number of interfaces per unit cell as the generation increases, hindering phonon coherence. The suppression of coherent thermal transport enables a higher degree of control on heat conduction at the nanoscale, and shows potential for application in the design of novel thermal management devices. 
Monday, March 15, 2021 9:36AM  9:48AM Live 
A20.00007: Heat transport in water from Deep Neural Network potentials Davide Tisi, Linfeng Zhang, Roberto Car, Stefano Baroni The computation of heat transport coefficients from first principles has been made feasible by recent theoretical advances [1,2], but is still computationally extremely expensive, thus limiting the time and length scales of the simulations that one can afford. Neuralnetwork (NN) interatomic potentials hold the promise of combining the accuracy and transferability of ab initio simulations with the affordability of classical potentials. We develop a NN implementation of the energy flux based on the Deep PotentialSmooth Edition (DeepPotSE) NN model [3], which takes full advantage of the crucial role of gauge [1] and convective [2] invariances. Our methodology is first validated against DFTPBE ab initio results for liquid water [1], and then used to compute the thermal conductivity from a potential trained on accurate DFT data obtained from the SCAN functional over a broad range of physical conditions. 
Monday, March 15, 2021 9:48AM  10:00AM Live 
A20.00008: The wormLBM: enabling accurate ballisticdiffusive phonon transport René Hammer, Verena Fritz, Natalia BedoyaMartínez Direct discretization schemes for the Boltzmann transport equation (BTE) are plagued by numerical smearing, angular false scattering, and ray effect. The lattice Boltzmann method (LBM) has the advantage that it does not suffer from the first two problems. But, the limited angular resolution responsible for the ray effect hinders the application of conventional LBM in the ballistic regime. 
Monday, March 15, 2021 10:00AM  10:12AM Live 
A20.00009: Geometrybased atypical local energy current in semiclassical and quantum thermal transport Palak Dugar, ChihChun Chien We use the semiclassical quantum thermal bath approach to reconcile the classical and quantum realms for thermal transport. In this method, we use Langevin equations within the molecular dynamics approach but with the reservoirs being quantum mechanical with BoseEinstein statistics to calculate the local steady state thermal currents. We show that an atypical, local steadystate thermal current from cold to hot emerges in a classical harmonic system of Hookean springs and masses driven by two colored noise Langevin reservoirs at different temperatures in both the classical and quantum regimes for the system. In addition, for a full quantum treatment, we carry out the third quantization study of an equivalent quantum system and corroborate our results. 
Monday, March 15, 2021 10:12AM  10:24AM Live 
A20.00010: Influence of InPlane Liquid Ordering on the Kapitza Resistance at Liquid/Solid Interfaces Hiroki Kaifu, Sandra Troian Applications ranging from small scale avionics control to AI computing are becoming ever more reliant on efficient cooling of high power 3D integrated chips prone to hot spots and thermal runaway. Thermal extraction is now the limiting factor in information processing. As a result, convective cooling using fans and gases is being replaced by liquid cooled microfluidic channels which take advantage of the higher heat capacity of liquids. The decrease in channel dimension size also increases the importance of boundary effects. Here we rely on molecular dynamics simulations to examine the magnitude of the thermal jump in quiescent systems known to occur at liquid/solid interfaces. Most previous studies have examined the influence of liquid density stratification near the solid wall, wettability effects and wall symmetry on the magnitude of the thermal jump. Here we explore inplane ordering of the first liquid layer adjacent to the solid wall by tuning the local temperature, thermal flux and parameters controlling the intermolecular potentials. Our results, some intuitive and some not, yield a surprisingly general correlation for the thermal jump which reflects the collective response of the first liquid layer. 
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