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 S20: Heat Transport in Condensed Systems IIFocus Session Live
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Sponsoring Units: DCOMP Chair: Elif Ertekin, University of Illinois at Urbana-Champaign |
Thursday, March 18, 2021 11:30AM - 12:06PM Live |
S20.00001: Thermal susceptibility -- the nonlocal temperature response to local heat input Invited Speaker: Philip Allen When a finite sample of a solid absorbs heat from an external source, the temperature response is interesting, especially in nanomaterials. Its understanding is important for heat management of circuit elements. Thermal susceptibility Θ(x-x’,t-t’) was defined by Allen and Perebeinos (2018) as the temperature rise at (x,t) per unit heat insertion at (x’,t’). This linear response function will be discussed for insulating crystals, where heat and temperature are described by phonons. For nanoscale studies, thermal susceptibility is a more useful and appropriate idea than thermal conductivity. It provides a more direct and visualizable understanding of the “ballistic to diffusive crossover”. Two particular issues will be discussed: (1) How can thermal susceptibility of nanoscale systems be studied by Boltzmann theory? (2) Are the results of Boltzmann theory reliable and useful for such systems? Can they help to interpret experiments and molecular dynamics simulations? A phonon Boltzmann theory appropriate for thermal susceptibility was given by Hua and Minnich (2014). The phonon distribution function N(Q) is driven not only by the usual terms, but also by external insertion of heat. This poses several interesting difficulties, which will be discussed. Numerical solutions are difficult unless the relaxation-time approximation is made. Computations will be discussed. These are being done in collaboration with Ali Kefayati and Vasili Perebeinos from University at Buffalo. |
Thursday, March 18, 2021 12:06PM - 12:18PM Live |
S20.00002: Quantized thermal and thermoelectric transport along single molecule junctions Andrea Gemma, Herve Dekkiche, Nico Mosso, Ute Drechsler, Sara Sangtarash, Michel Calame, Colin Lambert, Martin R. Bryce, Hatef Sadeghi, Bernd Gotsmann Molecules have proved to be extraordinary platforms to test quantum transport mechanisms, with predicted high Seebeck coefficient and thermoelectric efficiency, due to the discreetness of their energy levels and the tunability of their characteristics via a precise control on the chemical synthesis. Those features make molecules interesting systems to be studied as thermoelectric converters or energy harvesting devices. |
Thursday, March 18, 2021 12:18PM - 12:30PM Live |
S20.00003: The origin of the lattice thermal conductivity enhancement at the ferroelectric phase transition in GeTe Dorde Dangic, Olle Hellman, Stephen Fahy, Ivana Savic It is a general consensus that phonon anharmonicity increases near structural phase transitions (PT). Increased phonon anharmonicity leads to lower lattice thermal conductivity. However, the lattice thermal conductivity in germanium telluride (GeTe) increases near the ferroelectric PT. We use first-principles calculations coupled with the temperature-dependent effective potential method [1] and the Boltzmann transport equation (BTE) to elucidate this unexpected phenomenon. We find that, although anharmonicity increases at the PT in GeTe, the phonon group velocities increase as well, leading to an overall increase of the lattice thermal conductivity. The increased anharmonicity strongly affects the phonon spectral functions, leading to a giant softening of the peaks of the soft phonon power spectra at the PT. To account for these effects, we implement a novel method of calculating lattice thermal conductivity, which uses the information from the entire phonon power spectra. Using this approach, we find that the BTE underestimates the lattice thermal conductivity of GeTe at the phase transition. |
Thursday, March 18, 2021 12:30PM - 12:42PM Live |
S20.00004: Fast Simulations of Thermal Transport in Complex Materials using Machine Learning and Bayesian Force Fields Anders Johansson, Jonathan Vandermause, Andrea Cepellotti, Boris Kozinsky Controlling thermal conductivities of materials is important for a wide range of applications, from thermoelectrics for clean energy generation to electronic devices and thermal barrier coatings. The thermal conductivity is commonly estimated using molecular dynamics simulations within the Green-Kubo formulation. This requires a force field that is both 1) an accurate estimate of the interatomic interactions and 2) fast enough to allow simulations with sufficiently large length and time scales. Traditionally, only empirical force fields have fulfilled both of these requirements, which severely limits the applicability of the method. |
Thursday, March 18, 2021 12:42PM - 12:54PM Live |
S20.00005: Phonon-Defect Scattering: Success and Breakdown of the T-matrix Approximation Simon Thebaud, Carlos A Polanco, Lucas Lindsay, Tom Berlijn Understanding and predicting phonon transport in disordered compounds is critical to the design and developpment of new functional materials for thermal and energy applications. The T-matrix method, based on the single-scatterer approximation, has been used extensively to incorporate the influence of defects on phonon lifetimes and transport from first-principles. However, the validity of this approximation has not been investigated despite its use in strongly disordered materials such as maximally disordered alloys, for which multiple-impurity scatterings might be expected to play a role. Using the Chebyshev polynomials Green’s function method to unfold the phonon spectrum of large disordered supercells (tens of millions of atoms), we evaluate the phonon-defect scattering rate in mass-disordered alloys and two-dimensional systems featuring atomic vacancies.1 We explain the surprising success of the T-matrix approximation in predicting the thermal conductivity of even mass-disordered alloys, and find out-of-plane vibrations in monolayers to be especially sensitive to multiple-impurity scattering effects. |
Thursday, March 18, 2021 12:54PM - 1:06PM Live |
S20.00006: Integration and conservation conditions in thermal conductivity calculations Lucas Lindsay A variety of advanced software packages for calculating phonon thermal transport from Peierls-Boltzmann and density functional theory methods are now openly available. This has led to a proliferation of materials thermal conductivity studies from fully first principles methods. Here I will discuss some of the nuances behind thermal conductivity calculations as relating to varying methods of integration and coupling with energy and momentum conserving delta functions. These concepts are examined with regards to examining vibrational properties and transport in different 2D and bulk materials. |
Thursday, March 18, 2021 1:06PM - 1:18PM Live |
S20.00007: Controlling phonon thermal conduction at low temperatures using pillar phononic crystals Tatu Korkiamaki, Tuomas Puurtinen, Ilmo Raisanen, Ilari Maasilta Phononic crystals (PnC) are periodic structures which can have a strong impact on the phonon dispersions and thus group velocities and the density of states, if coherence is maintained. We have shown before [1,2] that such coherent modification of thermal conductance was possible at sub-Kelvin temperature range, using periodic arrays of holes etched into 2D silicon membranes. Here, we consider an alternative geometry for the PnC structure, that of a periodic array of superconducting pillars on an unperforated SiN membrane. Somewhat unintuitively, even if the membrane itself is left untouched, our coherent calculations still predict a strong effect on the thermal conductance. We have also performed sub-Kelvin experiments with two different pillar PnC structures with lattice constants 1 µm and 5 µm, both of which produced a sizeable reduction in thermal conductance, up to an order of magnitude, with the shorter period producing a stronger effect contrary to the coherent simulations. This possibly indicates a partial destruction of the coherence for the larger period structure due to scattering from the rough pillar edges. |
Thursday, March 18, 2021 1:18PM - 1:30PM Live |
S20.00008: Impact of Dimensional Crossover on Phonon Transport in Van der Waals Materials: A Case Study of Graphite and Graphene Patrick Strongman, Jesse Maassen Two-dimensional materials are the focus of intense research in part due to their unique thermal properties. With layered van der Waals materials, from which often 2D materials are isolated, the difference between bulk and monolayer originates from weak inter-layer coupling. Thus, 2D materials should retain some of the bulk character, and vice versa. In this talk, we present theoretical work investigating how phonon transport evolves in van der Waals materials when going from 3D to 2D, using graphite/graphene as a case study. The results are obtained using density functional theory combined with the phonon Boltzmann equation. To model the transition from 3D to 2D the inter-layer distance of graphite is gradually increased, to show how the phonon dispersion and scattering properties continuously evolve. Changes in the phonon dispersion, specifically the formation of low-energy optical phonons, play a significant role in the lower thermal conductivity of graphite, versus graphene. Interestingly, the 3-phonon scattering properties of graphite display similar behavior to graphene, in which selection rules restrict certain 3-phonon processes, due to the weak inter-layer coupling. Lastly, we attempt to identify generic features likely shared with other van der Waals materials. |
Thursday, March 18, 2021 1:30PM - 1:42PM Live |
S20.00009: Sub-Planckian thermal diffusivity in a classical model with lattice dynamics Huan-Kuang Wu, Jay Sau Planckian transport, characterized by a relaxation timescale of τH=hbar/kBT, has been suggested to appear in many correlated fermionic systems in the electrical conductivity. This is an interesting regime because the relaxation of excitations appears to be larger than the excitation energy of the excitations themselves, indicating a breakdown of the Boltzmann approach. More recently, this concept has been extended to thermal transport properties of insulators where it is suggested that a similar lower bound appears in the thermal diffusivity Dth that is also characterized by the Planckian time Dth≥DH=vs2τH. |
Thursday, March 18, 2021 1:42PM - 1:54PM Live |
S20.00010: Nonlocal thermal transport modeling using thermal susceptibility Ali Kefayati, Vasili Perebeinos, Philip Allen If heat current in crystals is probed on length scales smaller than phonon mean free paths, the current j at point r is not a local function of the local temperature, T(r), or its gradient, grad T(r). Ballistic heat transport depends on non-locally on thermal behavior at other points, r’. Thermal susceptibility, Θ(r,r’), is the temperature response of the system at point r to the heat input at point r’. The non-local effects have to be considered rigorously for an accurate interpretation of experimental measurements and theoretical investigations. Here, we investigate the ability of the thermal susceptibility in taking into account the non-local effects by using Boltzmann theory. Our results illustrate that thermal transport modeling using the thermal susceptibility function accurately reveals the nonlocal effects from ballistic to diffusive transport regimes. |
Thursday, March 18, 2021 1:54PM - 2:06PM Live |
S20.00011: The impact of twin boundaries on thermal transport in Bi2Te3 Aoife Lucid, Javier Fernandez Troncoso, Jorge Kohanoff, Stephen Fahy, Ivana Savic Advancing beyond the current state-of-the-art nanostructured thermoelectric materials requires a detailed understanding of the impact of interfaces on their thermal properties. In this work, we utilise reverse non-equilibrium molecular dynamics (rNEMD) simulations, with a recently developed empirical interatomic potential [1], to elucidate the impact of specific twin boundary structures on thermal transport in bismuth telluride (Bi2Te3). Three basal plane twin boundaries and the 60° twin boundary [2] are investigated. The interfacial thermal resistance is calculated, along with an analysis of the structural changes observed at interfaces. We find that interfacial thermal resistance increases with decreasing stability of the interface, suggesting that the most effective interfaces may require more effort to fabricate. A comparison of the properties of these four twin boundaries is carried out, enabling us to identify those that may be the most impactful in terms of suppressing lattice thermal conductivity at room temperature. |
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