Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F34: Thermal TransportFocus

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Sponsoring Units: DMP DCMP Chair: Olivier Delaire, Duke University Room: 297 
Tuesday, March 14, 2017 11:15AM  11:27AM 
F34.00001: Vibrational Properties of Phononic Crystals Ralf Meyer Phononic crystals are periodically structured, artificial materials that use Bragg reflection to manipulate the propagation of elastic waves. It has been shown theoretically and experimentally that nanoscale phononic crystals can have significantly reduced thermal conductivities. This makes them candidates for thermoelectric materials with high figures of merit $ZT$ and has generated interest in the properties of thermal phonons in these materials. In this work, the vibrational properties of nanoscale silicon phononic crystals are studied with molecular dynamics simulations as well as finite element method calculations. The molecular dynamics simulations account automatically for surface and interface effects that are important on the nanoscale. Comparison of the vibrational band structures makes it therefore possible to improve the computationally less demanding finite element model. Results are presented that show the character of the lowest nonacoustic bands and how the acoustic modes of the phononic crystal deviate from the bulk behavior for shorter wavelength. It is found that close to the Brillouin zone boundary a decoupling of the vibrations occurs in the phononic crystal. [Preview Abstract] 
Tuesday, March 14, 2017 11:27AM  11:39AM 
F34.00002: Phononfluid coupling and energy dissipation in singlewalled carbon nanotubes Subhadeep De, Narayana Aluru We elucidate the role of phonons, fluid and phononfluid coupling in the dissipation of high frequency carbon nanotube (CNT) resonators using molecular dynamics (MD) simulation. To investigate the dissipation mechanisms, we consider different simulation setups consisting of a singlewalled CNT and confined argon. First, we consider a flexible CNT in vacuum and show that the intrinsic dissipation due to phonons is governed by the Akhiezer theory. The parameters for the Akhiezer model  phonon relaxation times and the Grüneisen parameter are computed using Quasiharmonic methods. Next, we introduce a new formulation for viscous dissipation due to the fluid in terms of a forceresponse function using Linear response theory. The developed relation is validated for a rigid CNT (no phonons) with confined argon. Finally, we consider a flexible CNT with confined argon and incorporate the effect of phononfluid coupling on the Akhiezer model parameters and forceresponse function to explain the net dissipation. We observe reduction of the overall dissipation with increase in fluid density at low excitation frequencies. This counterintuitive behavior is shown to be a direct consequence of phononfluid coupling. [Preview Abstract] 
Tuesday, March 14, 2017 11:39AM  11:51AM 
F34.00003: Abinitio thermal properties of semiconductors with higher order anharmonicities Navaneetha K Ravichandran, David Broido The thermal properties of semiconductor materials are intimately connected to the anharmonicity of the crystal potential. In many such materials, calculations including only the thirdorder anharmonic processes involving three phonons are sufficient to accurately reproduce the measured thermal properties such as specific heat, thermal expansion and thermal conductivity. However, in highly anharmonic materials such as those used for thermoelectric applications, higher order anharmonicities in the crystal potential can become important [1, 2, 3]. In this talk, we describe an approach to calculate the abinitio thermal properties of highly anharmonic thermoelectric semiconductor materials, by including up to fourth order anharmonicity in the crystal potential. By evaluating higher order anharmonic contributions to the free energy, phonon line shifts and line widths, our work will inform accurate prediction of crystal stability, neutron scattering crosssections and thermal conductivity of several challenging anharmonic semiconductor crystals. [1] D. J. Ecsedy and P. G. Klemens, Phys. Rev. B 15, 5957 (1977) [2] Olle Hellman, Peter Steneteg, I. A. Abrikosov, and S. I. Simak, Phys. Rev. B 87, 104111 (2013) [3] Tianli Feng and Xiulin Ruan, Phys. Rev. B 93, 045202 (2016) [Preview Abstract] 
Tuesday, March 14, 2017 11:51AM  12:03PM 
F34.00004: Origin of low thermal conductivity in organicinorganic thermoelectric materials Tomoyuki Hata, Giacomo Giorgi, Koichi Yamashita Hybrid organicinorganic halides have recently attracted attention as thermoelectric materials due to their low thermal conductivity, which still remains unexplained. There are many possible factors at the origin of phonon scattering, and the attribution of the low thermal properties to such factors is an inevitable issue to control and design the hybrid thermoelectric materials. In this study, we adopt CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ (hereafter MAPI) as organicinorganic halide species and perform the mentioned attribution by combining classical and \textit{abinitio} calculations. At first, we have developed an empirical potential for MAPI. Bond, angle, and dihedral potential functions are expanded to higherorder to include anharmonicity and fitted to the force trajectories of the \textit{abinitio} molecular dynamics calculations. By using our force fields, we calculate the thermal conductivity of MAPI by means of the reverse nonequilibrium molecular dynamics method. The results are in very good agreement with the experimental thermal conductivity. We investigate the origin of such transport properties by changing the degrees of freedom of the embedded MA molecular cations and by evaluating the relaxation paths based on the velocity autocorrelation functions. [Preview Abstract] 
Tuesday, March 14, 2017 12:03PM  12:15PM 
F34.00005: Transport Properties Of TypeI Sn Clathrates Peter Egbele, Daniel Joubert, Elvis Shoko The conversion of ‘waste’ heat into useful energy can contribute to the efficient use of available energy. This includes converting heat energy from internal combustion engines, conventional power plants and solar cells into usable energy. Thermoelectric devices can convert heat into an electric current and have immense potential for utilizing heat energy. One of the desired features of an efficient thermoelectric material is a low lattice thermal conductivity. In this study thermal transport properties of typeI Sn clathrates are investigated. We study the dynamics of the guest atoms Cs and K in the compound A$_{8}$ Sn$_{44}$ (A = Cs, K). We find that the guest atom are responsible for scattering of the heat in these systems, and hence responsible for the low thermal conductivity in these materials. These compounds are formed in a cubic lattice. A low thermal conductivity value of 0.17 and 0.18 W m$^{1}$ K$^{1}$ at 300 K respectively, was calculated for Cs$_{8}$ Sn$_{44}$ and K$_{8}$ Sn$_{44}$ . These are low values which makes these and similar materials attractive for further study. [Preview Abstract] 
Tuesday, March 14, 2017 12:15PM  12:27PM 
F34.00006: Ferroelectric Phase Transition and the Minimal Lattice Thermal Conductivity of (Pb,Ge)Te Alloys Ronan Murphy, Eamonn Murray, Stephen Fahy, Ivana Savic Exploiting the fascinating properties of materials near soft mode phase transitions is an emerging concept in the quest to increase thermoelectric efficiency [1,2]. The underlying idea is that soft phonons lead to intrinsically low thermal conductivity, while possibly preserving electronic transport properties. Here we investigate how tuning the proximity to the ferroelectric phase transition via chemical composition affects the lattice thermal conductivity of Pb$_{1x}$Ge$_{x}$Te alloys [3]. Using firstprinciples virtualcrystal simulations, we show that the anharmonic contribution to the lattice thermal conductivity is minimized at the phase transition due to the maximized acousticoptical interaction. The interplay between anharmonicity and mass disorder shifts the conductivity minimum towards the composition at which the scattering due to mass disorder is maximized. Our results suggest that tuning soft optical modes in Pb$_{1x}$Ge$_{x}$Te and similar alloys may be a promising strategy to enhance their thermoelectric efficiency. [1] D. T. Morelli, V. Jovovic, and J. P. Heremans, Phys. Rev. Lett. 101, 035901 (2008). [2] L.D. Zhao et al, Nature 508, 373 (2014). [3] R. M. Murphy, E. D. Murray, S. Fahy, and I. Savic, Phys. Rev. B 93, 104304 (2016). [Preview Abstract] 
Tuesday, March 14, 2017 12:27PM  12:39PM 
F34.00007: Anharmonic phonon dynamics in superionic conductor CuCrSe$_2$ Jennifer Niedziela, Dipanshu Bansal, Andrew May, Georg Ehlers, Douglas Abernathy, Ayman Said, Olivier Delaire Efficient mechanisms to reduce thermal conductivity are critical in the search for new thermoelectric materials for energy applications. Here we investigate in detail the lattice dynamics of CuCrSe$_2$, a candidate thermoelectric material. CuCrSe$_2$ undergoes a superionic transition at 463 K, and exhibits extremely low lattice thermal conductivity. Using neutron and xray spectroscopy combined with first principles calculations, we show that the ultralow thermal conductivity arises from strongly anharmonic Cu vibrations, which broaden dramatically on heating and increase the scattering of heat carrying acoustic phonons. We find that these modes broaden at temperatures much below the superionic transition, signaling a dynamic precursor before the Cu sublattice melting at the superionic transition. The strongest anharmonic effects are confined to the quasi two dimensional lattice of copper ions, and the dynamics of the surrounding structure remain relatively intact, validating the physical picture of a partliquid/partsolid system. [Preview Abstract] 
Tuesday, March 14, 2017 12:39PM  12:51PM 
F34.00008: Phonon thermal transport in transition metal and rare earth nitride semiconductors from first principles David Broido, Chunhua Li The thermal properties of four transition metal and rare earth nitride compounds: ScN, YN, LaN and LuN, have been studied using a first principles approach. The phonon dispersions for the four compounds show large LOTO splittings and soft TO modes, which results in strong anharmonic scattering between acoustic and optic phonons that reduces the lattice thermal conductivities, $k_{L} $, of these compounds. The particularly soft TO mode at $\Gamma $ in LaN gives it a much lower $k_{L} $ than the other compounds. The room temperature $k_{L} $ value for LaN of only 6 Wm$^{\mathrm{1}}$K$^{\mathrm{1}}$ is four times smaller than that of LuN in spite of the latter having larger average atomic mass, similar acoustic phonon velocities, and similar mode averaged Gr\"{u}neisen parameters. Electronic structure calculations using the HSE06 hybrid functional including spinorbit coupling show that LaN has highly anisotropic conduction and valence bands in the vicinity of the band extrema. These features combined with its small $k_{L} $ make LaN a potentially viable thermoelectric material. [Preview Abstract] 
Tuesday, March 14, 2017 12:51PM  1:03PM 
F34.00009: Vibrational dynamics of $A$V$_{\mathrm{2}}$Al$_{\mathrm{20}}$ ($A \quad =$ Sc, La and Ce) cage compounds Andreas LeitheJasper, Marek Koza, Yuri Grin We report on the inelastic response of $A$V$_{\mathrm{2}}$Al$_{\mathrm{20}}$ (with $A \quad =$ Sc, La and Ce) probed by highresolution inelastic neutron scattering experiments [1]. Intense signals associated with the dynamics of Sc, La and Ce are identified in the lowenergy range at 614 meV in ScV$_{\mathrm{2}}$Al$_{\mathrm{20}}$ and at 816 meV in LaV$_{\mathrm{2}}$Al$_{\mathrm{20}}$ and CeV$_{\mathrm{2}}$Al$_{\mathrm{20}}$. Their response to temperature changes between 2 and 300 K reveals a very weak softening of the modes upon heating in LaV$_{\mathrm{2}}$Al$_{\mathrm{20\thinspace }}$and CeV$_{\mathrm{2}}$Al$_{\mathrm{20}}$ and a distinguished blue shift by about 2 meV in ScV$_{\mathrm{2}}$Al$_{\mathrm{20}}$. By means of density functional theory (DFT) and LDC we show that the unusual anharmonicity of the Scdominated modes is due to the local potential of Sc featured by a strong quartic term and compare it with anharmonic systems (Al,Ga)V$_{\mathrm{2}}$Al$_{\mathrm{20}}$ [2]. [1] M. M. Koza et al., Phys. Chem. Chem. Phys. \textbf{16,} 27119 (2014) [2] M. M. Koza et al., Phys. Chem. Chem. Phys. \textbf{17}, 24837 (2015) [Preview Abstract] 
Tuesday, March 14, 2017 1:03PM  1:39PM 
F34.00010: Thermal transport in Weyl, doubleWeyl, Dirac, and magnetically ordered systems with strong spinorbit coupling Invited Speaker: Gregory A. Fiete In this talk I will discuss some of our recent work on the thermal transport properties of Weyl and Dirac semimetals, doubleWeyl semimetals, and magnetically ordered insulators with strong spinorbit coupling. The thermal properties will be described primarily within the context of a Boltzmann transport theory. For the Weyl/Dirac systems we study the temperature, disorder, carrier density, and field (both magnetic and electric) dependent response. The doubleWeyl system is predicted to exhibit a spatially anisotropic response that would allow one to distinguish it from the single Weyl system in transport measurements. In addition, I will touch on some of our work related to electronic cooling by phonons in Dirac and Weyl semimetals. For the magnetically ordered insulators with strong spinorbit coupling, we study the KitaevHeisenberg model which exhibits 4 different ordered phases depending on the relative importance of the spinorbit coupling, and a model with DzyaloshinskiiMoriya interactions motivated by thin film pyrochlore iridates. We compute the thermal conductivity tensor and conclude that some properties of the magnetic order and its excitations, including topological magnon bands, can be inferred from the anisotropies and temperature dependence of the thermal conductivity. P. Laurell and G. A. Fiete, arxiv:1609.03612 Q. Chen and G. A. Fiete, Phys. Rev. B \textbf{93}, 155125 (2016), R. Lundgren and G. A. Fiete, Phys. Rev. B \textbf{92}, 125139 (2015), R. Lundgren, P. Laurell, and G. A. Fiete, Phys. Rev. B \textbf{90}, 165115 (2014). [Preview Abstract] 
Tuesday, March 14, 2017 1:39PM  1:51PM 
F34.00011: Studies Thermoelectric and Magneto Transport of the Phase Change material Ge2Sb2Te5 Ming Yin, Mohammed Abdi, Zibusisu Noimande, Godwin Mbamalu, Lei Wang, Timir Datta Phase Change materials (PCM) such as Ge2Sb2Te5 can be reversibly and rapidly switched between amorphous and crystalline phases by an electrical pulse. Hence PCM are used in flash memory applications. However, the thermoelectric properties of these materials are interesting. It is important to understand thermoelectric behavior of PCM for the optimal operation of these devices. But the low temperature experimental data is limited. Here we report measurements of Seebeck and magneto transport Coefficients at low temperature of GST225. In the temperature range from 9 K to 300 K, the Seebeck Coefficient of our sample increases with temperature increase from 1.5e6 V/K to 33e6 V/K. At low temperatures a positive magnetoresistance (MR) was observed for dc magnetic field (B) up to 9 Tesla. The temperature and field dependence of the electrical properties (MR) from 2.8 K to 20 K and 0 to 10 T will be reported. [Preview Abstract] 
(Author Not Attending)

F34.00012: Thermomagnetic mechanism for selfcooling cables Luca de' Medici A solidstate mechanism for cooling highcurrent cables is proposed based on the Ettingshausen effect, i.e., the transversethermoelectric cooling generated in magnetic fields. The intense current running in the cable generates a strong magnetic field around it that can be exploited by a small current running in a coating layer made out of a strong “thermomagnetic” material to induce a temperature difference between the cable core and the environment. Both analytical calculations and realistic numerical simulations for the steady state of bismuth coatings in typical magnetic fields are presented. The latter yield temperature drops $\simeq$60 K and $>$100 K for a single and doublelayer coating, respectively. These encouraging results should stimulate the search for better thermomagnetic materials in view of applications such as selfcooled superconducting cables working at room temperature. [Preview Abstract] 
Tuesday, March 14, 2017 2:03PM  2:15PM 
F34.00013: Localized magnetoplasmons in quantum dots: Magnetooptical absorption, Raman scattering, and inelastic electron scattering M.S. Kushwaha We investigate a onecomponent, quasizero dimensional, quantum plasma exposed to a parabolic potential and an applied magnetic field in the symmetric gauge. If the size of such a system as can be realized in the semiconducting quantum dots is on the order of the deBroglie wavelength, the electronic and optical properties become highly tunable. Then the quantum size effects challenge the observation of manyparticle phenomena such as the magnetooptical absorption, Raman intensity, and electronenergyloss spectrum. An exact analytical solution of the problem leads us to infer that these manyparticle phenomena are, in fact, dictated by the generalized Kohn's theorem in the longwavelength limit. Maneuvering the confinement and/or the magnetic field furnishes the resonance energy capable of being explored with the FIR, Raman, or electronenergyloss spectroscopy. This implies that either of these probes should be competent in observing the localized magnetoplasmons in the system. A deeper insight into the physics of quantum dots is paving the way for their implementation in such diverse fields as quantum computing and medical imaging. [See, e.g., M.S. Kushwaha, Europhys. Lett. {\bf 113}, 57005 (2016) [Preview Abstract] 
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