Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C34: Thermal and Thermoelectric Transport  Theory and ModelingFocus

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Sponsoring Units: DMP GERA DCOMP Chair: Marco Fornari Room: 297 
Monday, March 13, 2017 2:30PM  2:42PM 
C34.00001: Ultralow thermal conductivity in silicon membranes by surface oxidation and alloying Davide Donadio, Shiyun Xiong, Sanghamitra Neogi, Daniele Selli A detailed understanding of the relation between microscopic structure and phonon propagation at the nanoscale is essential to design nanomaterials with desired phononic and thermal properties. Here we show that native oxide layers that spontaneously grow at the surfaces of ultra thin silicon membranes effectively hamper the mean free path long wavelength phonons through surface resonances that hybridize with propagating modes. This mechanism can be combined with mass scattering in SiGe alloyed membranes to hinder heat carriers over the whole spectrum of frequencies, thus resulting in extremely low thermal conductivity, up to 100 times lower than the bulk. [Preview Abstract] 
Monday, March 13, 2017 2:42PM  2:54PM 
C34.00002: Thermal Transport Properties of Transition Metal Dichalcogenide Monolayers Haluk Yapicioglu, Arash Mobaraki, Ali Kandemir, Tahir Cagin, Oguz Gulseren, Cem Sevik The characterization of thermal transport in lowdimensional transition metal dichalcogenides (TMDs) is required for their efficient implementation, either for general overheating issues or specific applications in thermoelectric devices. In this study, the lattice thermal conductivities of singlelayer MoS$_{2}$, MoSe$_{2}$ WS$_{2}$, and WSe$_{2}$ are evaluated using classical molecular dynamics simulations. The interactions between atoms are defined by StillingerWeber type empirical potentials. In the parameterization of the potentials, a stochastic optimization algorithm, namely particle swarm optimization, is utilized. For the materials considered in this study, the final parameter sets produce quite consistent results with density functional theory in terms of lattice parameters, bond distances, elastic constants, and vibrational properties. The predicted thermal properties of all materials are in very good agreement with earlier first principles calculations. [Preview Abstract] 
Monday, March 13, 2017 2:54PM  3:06PM 
C34.00003: Phonon Optimized Potentials Andrew Rohskopf, Asegun Henry Atomistic simulations can be used to predict thermal properties with atomic insight, mainly via molecular dynamics (MD) simulations where the atomic forces are calculated every time step to predict atomic motions. Accurate dynamics requires an accurate method of sampling the potential energy surface (PES). For practical atomistic studies of thermal properties across relevant length ( \textgreater 10$^{3}$ nanometers) and time scales ( \textgreater 10$^{2}$ nanoseconds), a model of the PES with the following Requirements is needed: (1) PES sampling must be fast ( \textless 0.01 second/atom). (2) PES sampling must be chemically accurate ( \textless 1 kcal/mol). (3) Requirement 2 must be transferrable to thermally relevant regions of the PES. Quantum mechanical (QM) methods such as density functional theory (DFT) satisfy Requirements 2 and 3, but fail Requirement 1. Many attempts to bypass the computational costs of QM include empirical interatomic potentials (EIPs). Many EIPs fail Requirement 2 and 3, however. We have therefore created a machine learning program that can parameterize any EIP to satisfy Requirements 2 and 3. These phonon optimized potentials reproduce phonon dispersion relations, and therefore thermal properties, quite well. [Preview Abstract] 
Monday, March 13, 2017 3:06PM  3:18PM 
C34.00004: Thermal transport of IIIV semiconductor materials and superlattices based on molecular dynamics with optimized Tersoff potentials Song Mei, Irena Knezevic IIIV compound semiconductor materials are widely used in optoelectronics devices. IIIV superlattices (SLs) make the active core of quantum cascade lasers (QCLs). Achieving roomtemperature (RT), highpower, and continuouswave (CW) operation in QCLs hinges on the understanding and engineering of thermal transport in the layers and across the interfaces. Cations in IIIV ternary alloys differ a lot in mass and this effect on thermal transport is hard to capture using the scattering rates deduced from common perturbation theories. Molecular dynamics (MD) simulations can explicitly take the mass difference into consideration and are suitable for calculating the bulk thermal conductivity of IIIV ternary alloys. Furthermore, the morphology and anharmonic interactions at an interface are naturally captured in MD, leading to an accurate description of interfacial transport. We adopt the Tersofftype potentials for IIIV binaries and optimize them according to acoustic phonon dispersions in order to capture thermal properties. The optimized potential is then used to directly compute the thermal boundary resistance at a heterojunction interface, as well as the thermal conductivity in the SL as a whole. [Preview Abstract] 
Monday, March 13, 2017 3:18PM  3:30PM 
C34.00005: Uncovering the Role of Surfaces in CrossPlane Thermal Transport in Superlattices Abhinav Malhotra, Martin Maldovan Progress towards a deep fundamental understanding of nanoscale thermal transport shall remain elusive if the physical mechanisms behind phonon surface scattering are not completely uncovered. In semiconductor superlattices, the existence of multiple interfaces in the crossplane direction makes the surface scattering phenomena a critical aspect of thermal transport. Heat conduction in superlattices is a vital transport phenomena which if controlled effectively can lead to significant improvements in existing technologies such as quantum cascade lasers, thermoelectrics, and electronics. In this talk, we will discuss the physical mechanisms that control the crossplane thermal transport in Si/Ge and Si$_{\mathrm{1x}}$Ge$_{\mathrm{x}}$/Si$_{\mathrm{1y}}$Ge$_{\mathrm{y}}$ superlattices based on our recently developed rigorous quasiclassical treatment of phonon scattering mechanisms at rough interfaces [1,2]. The dependence of phonon surface scattering on incident phonon momentum, angle of incidence and surface properties including roughness and correlation length is established. This developed understanding will be used to fundamentally analyze the modifications in heat spectra in superlattices with the aim of moving towards a paradigm of rational design of thermal materials via the use of energy distributions among phonons with different frequencies and mean free paths. [1] Malhotra, A. and Maldovan, M. \textit{Sci Rep} \textbf{6}, 25818 (2016) [2] Malhotra, A. and Maldovan, M. \textit{J Appl Phys} \textbf{120} (2016) [Preview Abstract] 
Monday, March 13, 2017 3:30PM  3:42PM 
C34.00006: Cloaking of Thermoelectric Transport Troy Stedman, Lilia Woods Transformation optics techniques have proven to be a versatile approach to achieve unprecedented control of electromagnetic fields, heat currents, electric currents, and other physical phenomena. Most applications and designs thus far based on such techniques have targeted independent physical domains with specific functionalities. We present transformation optics applied to thermoelectricity by exploring the invariance of the governing and constitutive equations under coordinate transformations. We show that control of thermoelectrically coupled heat and electric currents is possible with highly anisotropic and inhomogeneous materials. Using layered structures, we design a thermoelectric cloak capable of hiding objects from thermoelectric flow. These proof of principle results constitute a significant step forward towards finding unexplored ways to control and manipulate coupled transport for thermoelectric applications. [Preview Abstract] 
Monday, March 13, 2017 3:42PM  4:18PM 
C34.00007: Phonon Scattering in Thermoelectrics: Thermal Transport, Strong Anharmonicity, and Emergent Quasiparticles Invited Speaker: Olivier Delaire Modern neutron and xray spectrometers can map phonon dispersions and scattering rates throughout reciprocal space, providing unique insights into microscopic scattering mechanisms, including anharmonicity, electronphonon coupling, or scattering by defects and nanostructures. In addition, firstprinciples simulations enable the rationalization of extensive experimental datasets. In particular, abinitio molecular dynamics simulations can capture striking effects of anharmonicity near lattice instabilities. A number of highperformance thermoelectric materials are found in the vicinity of lattice instabilities, including Pb chalcogenides PbX, SnSe, Cu2Se, among others. The large phonon anharmonicity found in such compounds suppresses the lattice thermal conductivity, enhancing their thermoelectric efficiency. In this presentation, I will present results from our investigations of phonons in these materials [14] using neutron and xray scattering combined with firstprinciples simulations, focusing on anharmonic effects near lattice instabilities. I will show how strong anharmonicity can lead to emergent quasiparticles qualitatively different from harmonic phonons, which we probe in our measurements and simulations of the phonon selfenergy. Commonalities between systems will be highlighted, including connections between strong anharmonicity and the electronic structure. [1] O. Delaire, J. Ma, K. Marty, A. F. May, M. A. McGuire, M.H. Du, D. J. Singh, A. Podlesnyak, G. Ehlers, M. Lumsden, B. C. Sales, Nature Materials 10, 614 (2011). [2] J. Ma*, O. Delaire*, A. F. May, C. E. Carlton, M. A. McGuire, L. H. VanBebber, D. L. Abernathy, G. Ehlers, Tao Hong, A. Huq, Wei Tian, V. M. Keppens, Y. ShaoHorn, and B. C. Sales, Nature Nanotechnology 8, 445 (2013). [3] C.W. Li, O. Hellman, J. Ma, A.F. May, H.B. Cao, X. Chen, A.D. Christianson, G. Ehlers, D.J. Singh, B.C. Sales, and O. Delaire, Physical Review Letters (2014). [4] C.W. Li,* J. Hong,* A.F. May, D. Bansal, S. Chi, T. Hong, G. Ehlers and O. Delaire, Nature Physics 11, 1063 (2015). [5] D. Bansal, J. Hong, C.W. Li, A.F. May, W. Porter, M.Y. Hu, D.L. Abernathy, and O. Delaire, Phys. Rev. B 94, 054307 (2016) [Preview Abstract] 
Monday, March 13, 2017 4:18PM  4:30PM 
C34.00008: Understanding the Origins of Large Negative Thermal Expansion in Ferroelectric Perovskites from First Principles Ethan Ritz, Nicole Benedek Many of the functional properties of ABO$_{\mathrm{3}}$ perovskite oxides (for example, ferroelectricity) are strongly linked to particular phonon modes in the material. In addition, in many cases it is possible to formulate simple guidelines or `rules of thumb' that link crystal structure and chemistry to specific lattice dynamical characteristics. The thermal transport properties of perovskites are thus potentially highly tunable and dynamically controllable with external fields. We use firstprinciples density functional theory to reveal new details related to the origin of the large negative thermal expansion (NTE) observed for ferroelectric PbTiO$_{\mathrm{3}}$. Although the origin of NTE in this material is often ascribed to ferroelectricity (which arises from the freezing in of a soft, zonecenter optical phonon), our results suggest that zoneboundary modes play a major role in driving NTE. In addition, hybridization between different electronic states has a significant effect on the lattice dynamics of PbTiO$_{\mathrm{3}}$ in general, and its NTE behavior in particular. Our work has implications for the understanding of, discovery and design of NTE in perovskites and other families of inorganic materials. [Preview Abstract] 
Monday, March 13, 2017 4:30PM  4:42PM 
C34.00009: A firstprinciples study of thickness dependent thermoelectric power in topological insulating thinfilms: Bi$_{2}$Te$_{3}$, Bi$_{2}$Se$_{3}$, and Sb$_{2}$Te$_{3}$ MyungSoo Lim, SeungHoon Jhi Threedimensional topological insulator Bi$_{2}$Te$_{3}$, Bi$_{2}$Se$_{3}$ and Sb$_{2}$Te$_{3}$ are good thermoelectric materials. We study the semiclassical thermoelectric properties of this topological insulating thin films with the 410 quintuplelayer thickness using firstprinciples calculations and the twochannel model combined with the Boltzmann transport equations. We observe the thickness and material dependent Seebeck coefficients of n and ptype doped films which is associated the bulk and surface gap, surface to bulk ratio and relative position of surface states with respect to bulk states. [Preview Abstract] 
Monday, March 13, 2017 4:42PM  4:54PM 
C34.00010: FirstPrinciples Computational Study of the Properties of the Clathrate Compounds A$_{\mathrm{8}}$Al$_{\mathrm{8}}$Si$_{\mathrm{128\thinspace }}$(A$=$Na,Rb,Cs) Dong Xue, Charles Myles Our recent study [1] of the properties of the TypeII binary clathrate compounds A$_{\mathrm{x}}$Si$_{\mathrm{136}}$, A$_{\mathrm{x}}$Ge$_{\mathrm{136}}$ and A$_{\mathrm{x}}$Sn$_{\mathrm{136}}$ (A$=$Na, K, Rb, Cs; 0$\le $x$\le $24) has increased the understanding of the role of the guest atoms and their composition x in determining the properties of these materials. For Na$_{\mathrm{x}}$Si$_{\mathrm{136}}$, our results have helped to explain the observed dependence of the lattice constant on x, which has a very unusual, distinct minimum at x $=$ 8. In this case, we have also found a ``Mexicanhat'' shaped effective potential curve for the Na atom motion in the large, (28atom) Si cages. Motivated by these fascinating results for Na$_{\mathrm{x}}$Si$_{\mathrm{136}}$, we have extended our study of the effects of alkali guests on the properties of the TypeII clathrates to include their effects on the properties of the TypeII the ternary clathrate compounds A$_{\mathrm{8}}$Al$_{\mathrm{8}}$Si$_{\mathrm{128\thinspace }}$(A$=$Na,Rb,Cs). An earlier study of similar compounds, (Rb,Cs)$_{\mathrm{8}}$Ga$_{\mathrm{8}}$Si$_{\mathrm{128}}$, has found low frequency guest rattling modes (\textasciitilde 4151 cm$^{\mathrm{1}})$. Here, we report the results of a firstprinciples computational study of the electronic, structural and vibrational properties of the compounds A$_{\mathrm{8}}$Al$_{\mathrm{8}}$Si$_{\mathrm{128\thinspace }}$(A$=$Na,Rb,Cs). We contrast our results with those for (Rb,Cs)$_{\mathrm{8}}$Ga$_{\mathrm{8}}$Si$_{\mathrm{128\thinspace }}$, as well as with our recent results for the binary clathrates A$_{\mathrm{x}}$Si$_{\mathrm{136\thinspace }}$(A$=$Na,Rb,Cs). Our calculations are based on the Local Density Approximation to Density Functional Theory, as implemented by the VASP code. [1] D. Xue, C. Myles, C. Higgins; Materials 2016, \textbf{9}(8), 691. [Preview Abstract] 
Monday, March 13, 2017 4:54PM  5:06PM 
C34.00011: Electronic structure and band alignments of halfHeusler semiconductors Abhishek Sharan, Zhigang Gui, Anderson Janotti HalfHeuslers compounds with 18 valence electrons are promising materials for electronic and thermoelectric devices. However, data on basic parameters, such as their electronic structure, electron and hole effective masses, and the position of their valence and conduction bands with respect to those of other semiconductors are scarcely available. Here we explore a few members of this large family of compounds, TiCoSb and TiNiSn, which can be grown on conventional IIIV semiconductors and could potentially be integrated in IIIVbased devices. We present results of firstprinciples calculations of electronic structure and band alignments between these materials and with respect to conventional IIIV semiconductors. Electronic structures are calculated using density functional theory within both the generalized gradient approximation and the screened hybrid functional HSE06. The results are discussed and compared to the available experimental data. [Preview Abstract] 
Monday, March 13, 2017 5:06PM  5:18PM 
C34.00012: Electronic Structures and Band Alignments of Mg$_{\mathrm{2}}$(Si,Ge,Sn) Thermoelectric Materials Byungki Ryu, EunAe Choi, Sudong Park Mg$_{\mathrm{2}}$Si, Mg$_{\mathrm{2}}$Ge, Mg$_{\mathrm{2}}$Sn, and their alloys are considered as ecofriendly and low cost thermoelectric materials. The low energy band structures are responsible for the electronrelated thermoelectric transport, such as electrical conductivity, Seebeck coefficient, and electric part of thermal conductivity. Here, by using first principles density functional theory (DFT) calculations, we investigate the electronic structures and the band alignments of Mg$_{\mathrm{2}}$Si, Mg$_{\mathrm{2}}$Ge, Mg$_{\mathrm{2}}$Sn. In DFT, the band gaps are severely underestimated compared to the experimental results. By adopting beyondDFT calculations such as hybridDFT and manybody GW calculations, we obtain the reliable band gaps. We also calculate the band alignments using various calculation schemes. Based on the calculation results, we discuss the thermoelectric properties of inhomogeneous Mg$_{\mathrm{2}}$(Si,Ge,Sn) materials. [Preview Abstract] 
Monday, March 13, 2017 5:18PM  5:30PM 
C34.00013: Electronic transport simulations in nanocrystalline materials for enhanced thermoelectric power factors Neophytos Neophytou, Mischa Thesberg Nanocrystalline thermoelectric materials have shown the potential to provide much larger Seebeck coefficients compared to pristine materials. In this work we perform electronic transport simulations to study thermoelectric transport through 2D nanocrystalline channels. In these channels, the grain boundaries act as barriers for energy filtering, which provides large improvements to the Seebeck coefficient. On the other hand, the grain geometry, doping, and the underlying electrostatic potential can be engineered in such a way as to compensate for the reduction of the electrical conductivity that the barriers on the grain boundaries cause. Thus, optimal designs can be achieved, in which the power factor is improved compared to pristine materials. We discuss in addition the influence that the variation in the local thermal conductivity of the grains compared to that of the grain boundaries has on providing further improvements to the Seebeck coefficient, as well as the influence of parameter variations on degrading the power factor. [Preview Abstract] 
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