Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L34: Thermal Transport Modeling  Novel ApproachesFocus

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Sponsoring Units: DMP GERA DCOMP Chair: Maria Chan, Argonne National Laboratory Room: 297 
Wednesday, March 15, 2017 11:15AM  11:51AM 
L34.00001: Car and Parrinello meet Green and Kubo: simulating atomic heat transport from equilibrium ab initio molecular dynamics Invited Speaker: Stefano Baroni Modern simulation methods based on electronicstructure theory have long been deemed unfit to compute heat transport coefficients within the GreenKubo formalism. This is so because the quantummechanical energy density from which the heat flux is derived is inherently ill defined, thus allegedly hampering the use of the GreenKubo formula. While this objection would actually apply to classical systems as well, I will demonstrate that the thermal conductivity is indeed independent of the specific microscopic expression for the energy density and current from which it is derived. This fact results from a kind of \emph{gauge invariance} stemming from energy conservation and extensivity, which I will illustrate numerically for a classical LennardJones fluid. I will then introduce an expression for the adiabatic energy flux, derived within densityfunctional theory, that allows simulating atomic heat transport using equilibrium \emph{ab initio} molecular dynamics. The resulting methodology is demonstrated by comparing results from abinitio and classical moleculardynamics simulations of a model liquidArgon system, for which accurate interatomic potentials are derived by the forcematching method, and applied to compute the thermal conductivity of heavy water at ambient conditions. The problem of evaluating transport coefficients along with their accuracy from relatively short trajectories is finally addressed and discussed with a few representative examples. [Preview Abstract] 
Wednesday, March 15, 2017 11:51AM  12:03PM 
L34.00002: First principles calculations of thermal conductivity with out of equilibrium molecular dynamics simulations Marcello Puligheddu, Francois Gygi, Giulia Galli The prediction of the thermal properties of solids and liquids is central to numerous problems in condensed matter physics and materials science, including the study of thermal management of optoelectronic and energy conversion devices. We present [1] a method to compute the thermal conductivity of solids by performing ab initio molecular dynamics at non equilibrium conditions. Our formulation is based on a generalization of the approach to equilibrium technique, using sinusoidal temperature gradients, and it only requires calculations of first principles trajectories and atomic forces. We discuss results and computational requirements for a representative, simple oxide, MgO, and compare with experiments and data obtained with classical potentials. [1] M. Puligheddu et al. Submitted 2016 [Preview Abstract] 
Wednesday, March 15, 2017 12:03PM  12:15PM 
L34.00003: Ab initio thermal transport in strongly anharmonic materials Olle Hellman We present recent advances regarding the temperature dependent effective potential method (TDEP) and its application to thermoelectric materials. All orders of nonharmonic effects are implicitly included when calculating phonon dispersions, lattice thermal transport and finite temperature phase stability. Recent additions deal with thermal transport in disordered systems, complex crystal structures and numerical efficiency for highthroughput applications. [Preview Abstract] 
Wednesday, March 15, 2017 12:15PM  12:27PM 
L34.00004: Novel ElectronPhonon Relaxation Pathway in Graphite Revealed by TimeResolved Raman Scattering and AngleResolved Photoemission Spectroscopy JhihAn Yang, Stephen Parham, Daniel Dessau, Dmitry Reznik Ultrafast dynamics of photoexcited electronhole pairs has attracted a lot of attention in the last decade due to its importance for a number of technologies such as solar cells. We report dynamics of electronhole excitations as well as G phonons in graphite after an excitation by an intense laser pulse investigated by the combination of ultrafast pumpprobe Raman scattering and angleresolved photoemission spectroscopy. We found that the increase of the G phonon population occurs about 65 fs later than the prediction of the accepted twotemperature model. This timedelay is also evidenced by the absence of the socalled selfpumping for G phonons. The unusual pump fluence dependence also contradicts the twotemperature model. These experimental observations imply a new relaxation pathway which we call Anharmonic Phonon Model (APM): Instead of hot carriers transferring energy to Gphonons directly, the energy is first transferred to optical phonons near the zone boundary Kpoints, which then decay into Gphonons via phononphonon scattering. The simulation results based on the APM will also be presented. [Preview Abstract] 
Wednesday, March 15, 2017 12:27PM  12:39PM 
L34.00005: FirstPrinciples Simulations of NonEquilibrium Phonon Dynamics in IIIV Materials Sridhar Sadasivam, Yi Xia, Maria Chan, Pierre Darancet Understanding nonequilibrium energy transfer between hot electrons and phonons is important to improving the efficiency of solarenergy conversion and nanoelectronic devices. As photo or electricallyexcited electrons excite nonthermal phonons, they generate transient and/or steadystate nonequilibrium phonon distributions known as "phonon bottlenecks", impacting thermal transport and structural properties. In this work, we develop a firstprinciples method that captures these effects, by computing the transient dynamics of nonequilibrium phonon distribution using the BlochBoltzmannPeierls equations along with firstprinciples calculations of electronphonon and phononphonon interactions. We apply our method to the description of phonon thermalization in IIIV semiconductors. We examine the effects of polarity and phononphonon scattering phase space on the thermalization time, and discuss the deviation from simple twotemperature models that are commonly used to interpret ultrafastlaser experiments. [Preview Abstract] 
Wednesday, March 15, 2017 12:39PM  12:51PM 
L34.00006: Nonlinear thermoelectric nanodevice with electronphonon interactions Bradley Nartowt, Selman Hershfield, Khandker Muttalib We consider electron transport through a single (tightbinding Hamiltonian) site and a localized phonon, about which are placed two leads at disparate chemical potentials due to being at disparate temperatures (the usual thermoelectric regime). In a calculation patterned after the large body of zerotemperature work done in the driven regime (where an external agent maintains the chemical potential difference), nonequilibrium Green functions are used to obtain the nonlinear currentvoltage characteristics. The Green functions are calculated using the selfconsistent Born approximation to incorporate (at the HartreeFockdiagram level) interactions between the itinerant electrons and localized phononmode. In the thermoelectric regime, we evaluate the power and efficiency of the device as a function of the electronphonon coupling at various temperature differences. In addition, we will report studies of a new regime where an external driving agent and a temperaturedifference are both responsible for the chemical potential difference: the partiallydriven thermoelectric regime. [Preview Abstract] 
Wednesday, March 15, 2017 12:51PM  1:03PM 
L34.00007: Estimation of effective electron relaxation times in real thermoelectric materials Yukari Katsura, Kaoru Kimura Good samples of thermoelectric materials should be PhononGlassElectronCrystal (PGEC) [1], which scatters phonons without scattering electrons. However, the actual values of electron relaxation time $\tau_{\mathrm{el}}$ are not well investigated, and $\tau _{\mathrm{el}}$ is simply set as 10$^{\mathrm{14}}$ s in many firstprinciples calculations of thermoelectric properties. In this study, we attempted to estimate the effective values of $\tau _{\mathrm{el}}$ of various thermoelectric materials, by fitting the calculation results by WIEN2k [2] and BoltzTraP [3] with experimental Seebeck coefficient and electrical conductivity. When we fitted experimental thermoelectric properties of Si$_{\mathrm{0.8}}$Ge$_{\mathrm{0.2}}$ samples [4][5], $\tau_{\mathrm{el}}$'s at 300 K were 9x10$^{\mathrm{15}}$ s in a highly crystalline sample [4], and 56x10$^{\mathrm{15}}$ s in ballmilled polycrystalline samples [5]. Temperature dependences of $\tau _{\mathrm{el}}^{\mathrm{1}}$ were also different between samples, implying the difference in dominant electron scattering mechanisms. Various samples of thermoelectric materials with fairly high figures of merit exhibited $\tau_{\mathrm{el}}$ of several 10$^{\mathrm{15\thinspace }}$s at 300 K. [1] G. A. Slack, \textit{CRC Handbook of Thermoelectrics}, 407 (1995). [2] P. Blaha, \textit{et al}., \textit{WIEN2k Users' Guide}, Vienna Univ. of Technology, Austria (2001). [3] G.K.H. Madsen et al., \textit{Comp. Phys. Comm.} 175, 67 (2006). [4] J.P. Dismukes et al., \textit{J. Appl. Phys.} 10 2899 (1976). [5] C.B. Vining et al., \textit{J. Appl. Phys.} 69 4333 (1991). [Preview Abstract] 
Wednesday, March 15, 2017 1:03PM  1:39PM 
L34.00008: Emergent phenomena in phonon thermal transport Invited Speaker: Andrea Cepellotti The thermal conductivity of insulating crystals originates from the energy transfer through lattice vibrations, as commonly described by the phonon Boltzmann transport equation. Since the pioneering work of Peierls, the prevalent hypothesis is that phonons are the heat carriers. However, it has long been known that this picture has shortcomings. For example, in materials of reduced dimensionality or at cryogenic temperatures, the scattering dynamics is dominated by momentum conserving  normal  processes, as opposed to momentum dissipating  Umklapp  processes. In these circumstances, heat flux is not lost at every scattering event. Instead, scattering shuttles heat flux through multiple phonon states, coupling them. As a result, collective phonon excitations arise, causing exotic phenomena: in 2D materials, they are responsible of high thermal conductivities and hydrodynamic behaviors, such as second sound, where temperature propagates as a damped wave, a phenomenon hitherto observed only in a few materials at cryogenic temperatures. I will show how these properties can be rationalized by introducing a gas of collective phonon excitations, called 'relaxons' [1]. Defined as the eigenvectors of the scattering matrix, relaxons allow for a simple  yet exact  interpretation of thermal conductivity in terms of a kinetic gas theory, where the relevant gas is made of such relaxons, the true heat carriers, and not phonons. These considerations provide a new explanation of the high conductivities of 2D materials, revise the time evolution of thermalization processes [2], correct the relevant time and length scale of heat flux dissipation, and provide a new viewpoint on semiclassical transport theories. \newline \newline [1] A. Cepellotti, and N. Marzari Phys. Rev. X 6, 041013 (2016) \newline [2] J. Hu, et al., PNAS 113, 43, E6555 (2016) [Preview Abstract] 
Wednesday, March 15, 2017 1:39PM  1:51PM 
L34.00009: Thermal insulator transition induced by interface scattering Brian Slovick, Srini Krishnamurthy The classic Bruggeman percolation model of thermal conductivity for composite materials is generalized to include the effects of interfacial scattering [1]. This generalized model accurately explains the measured variation of the composite thermal conductivity with loading and particle size. At high loadings, this model further predicts that strong interface scattering leads to a sharp decrease in thermal conductivity, or an insulator transition, when conduction through the matrix is restricted and heat is forced to diffuse through particles with large interface resistance. The closed form and accuracy of the model, and its ability to predict transitions between insulating and conducting states, suggest it can be a useful tool for designing composite materials with low or high thermal conductivity for a number of applications. [1] B. A. Slovick and S. Krishnamurthy, Appl. Phys. Lett. 109, 141905 (2016). [Preview Abstract] 
Wednesday, March 15, 2017 1:51PM  2:03PM 
L34.00010: Nanoscale Interface Scattering of Phonons in Multilayered Nanostructures Kartik Kothari, Martin Maldovan Nanostructures can be employed to control thermal and electrical properties and increase thermoelectric energy conversion efficiency. An insight into surface and interface scattering in nanostructures is essential in manipulating these properties, which in turn affect the thermoelectric figure of merit ZT. An understanding of the surface conditions is imperative for predicting the amount of specular reflection and transmission of phonons. A crucial challenge is to incorporate this along with different physical properties of constituents across an interface. We employ an extension of the electromagnetic wave scattering theory for rough surfaces developed by P. Beckmann and A. Spizzichino to account for thermal phonon interface scattering. This is supplemented with the FuchsSondheimer theory to formulate thermal transport in layered nanostructures. A rigorous analysis incorporating complete layer coupling, reflection and refraction conditions, and surface shadowing effects is presented. Thermal conductivities of superlattices, bilayers and sandwichlayered structures made of different constituent materials including SiGe, AlAsGaAs and SiSi$_{\mathrm{x}}$Ge$_{\mathrm{1x}}$ are calculated. An evaluation of the amount of heat flow confined within a constituent, and extended over multiple layers is performed. A detailed analysis of the heat spectrum is also presented which allows to predict the amount of heat carried by phonons of different frequencies and mean free paths. The proposed accurate description of phonon surface scattering and prediction of heat spectra would allow for the design of nanoengineered materials and devices with improved thermoelectric properties. [Preview Abstract] 
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