Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X29: Electrons, Phonons, Electron Phonon Scattering and Phononics VIFocus

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Sponsoring Units: DCOMP DMP Chair: Ruiqiang Guo, Caltech Room: LACC 406A 
Friday, March 9, 2018 8:00AM  8:36AM 
X29.00001: Phononic thermal properties of twodimensional materials Invited Speaker: Xiaokun Gu With many novel twodimensional (2D) materials beyond graphene emerging for various applications ranging from electronics, photonics, to thermal management and energy storage, phononic and thermal properties of 2D materials are of fundamental interest, for exploring both fundamental physics and practical applications. In this talk, we first summarize and compare the phonon properties, such as phonon dispersion and relaxation time, of pristine 2D materials with the single layer graphene to understand the role of crystal structure on their thermal conductivity. We then compare the phonon properties, between an idealized 2D crystal, realistic 2D crystals, and 3D crystals, and present the physical picture on how the thermal conductivity of 2D materials changes with sample sizes. The geometric effects, such as layer numbers and nanoribbon width, and other physical effects like defects, mechanical strains, and substrates, on the thermal properties of 2D materials are discussed. Intercalation could affect both the group velocities and phonon relaxation times of layered crystals, and thus tune the thermal conductivity along both the throughplane and basalplane directions. We also briefly discuss the challenges in theoretical and experimental studies of thermal transport in 2D materials. The rich and special phonon physics in 2D materials make them promising candidates for exploring novel phenomena like topological phonon effect and applications like phononic quantum devices, as discussed in the outlook. 
Friday, March 9, 2018 8:36AM  8:48AM 
X29.00002: Heat dissipation in the quasiballistic regime studied using Boltzmann equation in the spatial frequency domain Chengyun Hua, Austin Minnich Quasiballistic heat conduction, in which some phonons propagate ballistically over a thermal gradient, has recently become of intense interest. Most works assert that the thermal resistance associated with nanoscale heat sources is far larger than predicted by Fourier's law; however, recent experiments show that in certain cases the difference is negligible despite the heaters being far smaller than phonon mean free paths. In this work, we examine how thermal resistance depends on the heater geometry using analytical solutions of the Boltzmann equation. We show that the spatial frequencies of the heater pattern play the key role in setting the thermal resistance rather than any single geometric parameter, and that for many geometries the thermal resistance in the quasiballistic regime is no different than the Fourier prediction. We also demonstrate that selectively generating heat among phonon spectra could also affect thermal transport in quasiballistic regimes, which provides an alternative pathway to manipulate heat dissipation rate. Our work provides an intuitive link among heater geometry, heating profile, and the effective thermal resistance in the quasiballistic regime, a finding that could impact strategies for thermal management in electronics and other applications. 
Friday, March 9, 2018 8:48AM  9:00AM 
X29.00003: Phonon Hydrodynamic Heat Conduction and Knudsen Minimum in Graphite Zhiwei Ding, Jiawei Zhou, Bai Song, Vazrik Chiloyan, Mingda Li, TeHuan Liu, Gang Chen In the hydrodynamic regime, phonons drift with a nonzero collective velocity under a temperature gradient, reminiscent of viscous gas and fluid flow. The study of hydrodynamic phonon transport has spanned over half a century but has been mostly limited to cryogenic temperatures (~1 K) and more recently to lowdimensional materials. Here, we identify graphite as a threedimensional material that supports phonon hydrodynamics at significantly higher temperatures (~100 K) based on firstprinciples calculations. In particular, by solving the Boltzmann equation for phonon transport in graphite ribbons, we predict that phonon Poiseuille flow and Knudsen minimum can be experimentally observed above liquid nitrogen temperature. Further, we reveal the microscopic origin of these intriguing phenomena in terms of the dependence of the effective boundary scattering rate on momentumconserving phononphonon scattering processes and the collective motion of phonons. The significant hydrodynamic nature of phonon transport in graphite is attributed to its strong intralayer sp^{2} hybrid bonding and weak van der Waals interlayer interactions. 
Friday, March 9, 2018 9:00AM  9:12AM 
X29.00004: Mechanism of Coherence Heat Conduction in Phononic Crystal YUXUAN LIAO, Takuma Shiga, Makoto Kashiwagi, Junichiro Shiomi Heat in phononic crystals (PnCs) are carried by phonons, which can be divided to coherent (wavelike) and incoherent (particlelike) phonons. Many experiments have demonstrated reduction of thermal conductivity by PnCs at room temperature, however, comparison with theories suggest that it can be explained by surface and boundary scatterings, which not only backscatter phonons but also break their coherence. The logic here is that since average phonon wavelength at room temperature is only a few nanometers, the roughness at the surfaces and boundaries make the scattering diffusive (break the phase coherence of phonons), and thus only very long wavelength (low frequency) phonons with negligible contribute to total thermal conductivity remains coherent. Here, we will theoretically show that in a thin film PnCs, the lowfrequency coherent phonons could significantly contribute to thermal conductivity when assuming Klemens model for intrinsic scattering due to low dimensional nature of those phonons. Yet, further analysis shows the contribution of the low frequency coherent phonons are still negligible due to Akhieser damping. 
Friday, March 9, 2018 9:12AM  9:24AM 
X29.00005: Experimental evidence of chiral phonons in 2D materials Hanyu Zhu, Jun Yi, MingYang Li, Jun Xiao, Lifa Zhang, ChihWen Yang, Yuan Wang, Robert Kaindl, LainJong Li, Xiang Zhang Chirality reveals symmetry breaking of the fundamental interaction of elementary particles. In condensed matter, the chirality of electrons governs many unconventional transport phenomena such as the quantum Hall effect. However, chiral phonons exhibiting intrinsic collective atomic rotation, as opposed to superpostion of degenerate linear oscillation, have never been observed without external magnetic fields. Here, we report an experimental evidence of phonons with intrinsic chirality in 2D materials. The broken inversion symmetry of the lattice lifts the degeneracy of clockwise and counterclockwise phonon modes at the corners of the Brillouin zone. We identified the chirality of phonons by the selection rules of holephonon interactions based on pseudoangular momentum (PAM) conservation. We measured the linear momentum, PAM and energy of the phonon, and the results agree well with the theory. The discovery of phonon chirality is important for electronphonon coupling in solids, phonondriven topological states and energyefficient information processing. 
Friday, March 9, 2018 9:24AM  9:36AM 
X29.00006: An optimal approach to computing phonons and their interactions via finite difference Lyuwen Fu, Mordechai Kornbluth, Chris Marianetti Phonons and their interactions are critical to predicting a wide range of materials properties. Therefore, efficiently extracting a high resolution Taylor series expansion of the BornOppenheimer surface from an arbitrary firstprinciples approach is of great importance. Here we present an optimal formalism to compute phonons and their interactions at arbitrary order on a uniform grid using finite difference. Our approach ensures that a given derivative is always obtained from the smallest possible supercell dictated by the translation group, in addition to the smallest number of runs dictated by space group symmetry. We demonstrate that our approach is superior to any singlesupercell finite difference approach, which is commonplace in the literature for phonons and cubic interactions. Applications are presented for graphene and PbTe, providing phonons and interactions up to 5th order at an unprecedented qspace resolution with minimal discretization errors. Additionally, we present the phonons of the rareearth nickelates, demonstrating the utility of our approach for large complex unit cells. 
Friday, March 9, 2018 9:36AM  9:48AM 
X29.00007: Collective Goldstone Mode Induced Ultralow Lattice Thermal Conductivity in Snfilled Skutterudite SnFe_{4}Sb_{12} Yuhao Fu, Xin He, Lijun Zhang, David Singh We demonstrate that the concept of Goldstone bosons can be exploited for phonon control and thermal conductivity reduction of materials. By studying lattice dynamics of the Sn filled skutterudite SnFe_{4}Sb_{12}, we find Sn offcenters in its coordination cage in contrast to common rare earth fillers. This leads to lowfrequency optical phonons even below 1 THz associated mainly with Sn motions, but with strong collective motion of other atoms in the host skutterudite lattice. The optical modes transverse to the Sn offcentering direction are identified as Goldstone type modes in association with a threedimensional Mexican hatlike potential energy surface. The interaction of these collective Goldstone modes with the host heatcarrying phonons leads to ultralow thermal conductivities. This provides a novel mechanism to reduce thermal conductivity by exploiting the complex physics of the lattice dynamics of solids. 
Friday, March 9, 2018 9:48AM  10:00AM 
X29.00008: Thermal Expansion of GeTe from First Principles Djordje Dangic, Eamonn Murray, Stephen Fahy, Ivana Savic GeTe, a wellknown ferroelectric and thermoelectric material, undergoes a structural phase transition from a rhombohedral to the rocksalt structure at ~600700 K. We model this phase transition using density functional theory by minimizing the Helmholtz free energy using the elastic and quasiharmonic approximations and Gruneisen theory. By accounting for up to the fourth order elastic constants and their temperature dependence, we obtain the temperature variation of the structural parameters of rhombohedral GeTe (the lattice constant, the angle between the primitive lattice vectors and the internal atomic displacement) in good agreement with experiment [1]. From the calculated temperature dependence of the transverse optical (TO) mode, which is the primary order parameter for this phase transition, we extracted the critical temperature of 701 K and the critical exponent of 0.17, which are in good agreement with experiment [2]. We find that the divergence of the thermal expansion coefficients near the phase transition in GeTe is induced by acoustic phonon coupling to soft TO modes. 
Friday, March 9, 2018 10:00AM  10:12AM 
X29.00009: A General Framework for Understanding Negative Thermal Expansion in Complex Oxides from First Principles Ethan Ritz, Nicole Benedek Many of the functional properties of ABO_{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 formulate a general framework for understanding negative thermal expansion (NTE) in complex oxides, which we demonstrate for the test case of PbTiO_{3}. Although the origin of NTE is often ascribed to negative Grüneisen parameters or rigid unit modes (RUMS), our results suggest that neither are necessary conditions for NTE. We find that hybridization between different electronic states has a significant effect on both the elastic properties and lattice dynamics of PbTiO_{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. 
Friday, March 9, 2018 10:12AM  10:24AM 
X29.00010: Ab initio study of the phononic origin of negative thermal expansion Uri Argaman, Guy Makov, Eitan Eidelstein, Ohad Levy <div style="direction: ltr;">Negative thermal expansion is an uncommon phenomenon of theoretical interest. Multiple hypotheses regarding its microscopic origins have been suggested. In this paper, the thermal expansion of a representative semiconductor, Si, and a representative metal, Ti, are calculated ab initio using densityfunctional perturbation theory. The phonon modes' contributions to the thermal expansion are analyzed and the negative thermal expansion is shown to be dominated by negative mode Grüneisen parameters at specific points on the Brillouin zone boundaries. Thus, the elastic (Debye) theory for negative thermal expansion is shown to be irrelevant for these phenomena. The anomalous behavior of these modes in Ti is shown to be unaffected by an electronic topological transition as previously suggested, instead it arises from complex interplay of atomic displacements of the anomalous mode.</div><div style="direction: ltr;">Phys. Rev. B 94, 174305 (2016).</div> 
Friday, March 9, 2018 10:24AM  10:36AM 
X29.00011: Anomalous lattice vibrations of orthorhombic black phosphorus Cesar Enrique Perez Villegas, Alexandre Rocha, Andrea Marini Fewlayer Black phosphorus (BP) has been extensively studied, over the last years, due to a vast number of peculiar physical properties that are strongly related to its structural anisotropy. While most investigated properties have focused on its electronic and optical properties, there are still some vibrational phenomena, observed experimentally, that are not fully understood from the atomistic point of view. 
Friday, March 9, 2018 10:36AM  10:48AM 
X29.00012: Phononinterference Resonance Effects of Nanoparticles Embedded in a Matrix Lei Feng, Takuma Shiga, Haoxue Han, Shenghong Ju, Yuriy A. Kosevich, Junichiro Shiomi We report an unambiguous phonon resonance effect originating from germanium nanoparticles embedded in silicon matrix. Our approach features the combination of phonon wavepacket method with atomistic dynamics and finite element method rooted in continuum theory. We find multimodal phonon resonance, caused by destructive interference of coherent lattice waves propagating through and around the nanoparticle, gives rise to significantly sharp transmittance dips, blocking phonon transport in the lowend frequency range that is hardly diminished by other nanostructures. The resonance is sensitive to the phonon coherent length, where the finiteness of wave packet width weakens the transmittance dip even when the coherent length is larger than the particle size. Further strengthening of transmittance dips is achieved by arraying multiple nanoparticles that leads to collective resonant mode. Finally, atomistic Green's function demonstrates that these resonance effects can significantly reduce thermal conductance in the lowend frequency range. 
Friday, March 9, 2018 10:48AM  11:00AM 
X29.00013: Effect of Extreme Disorder on the Lattice Dynamics and Phonon Scattering in Concentrated Solid Solution Alloys Sai Mu, Raina Olsen, Biswanath Dutta, German Samolyuk, Tom Berlijn, Lucas Lindsay, Tilmann Hickel, Bennett Larson, G. Malcolm Stocks Composition complexity has a profound influence on lattice dynamics and phonon scattering of solid solution alloys. Here we combine inelastic neutron/Xray scattering measurements and theoretical calculations based on the itinerant coherent potential approximation (ICPA) and the phonon unfolding method (PUM) to study phonon spectrum of a series of equiatomic solid–solution alloys: NiCo, NiFe, NiFeCo, NiCoCr, NiFeCoCr. Because phonons in these alloys are dominated by force constant disorder they present a considerable challenge to theory. We find that the phonon dispersion relationships are similar across all alloys, however, the disorder induced linewidths are very qdependent and vary considerably between different alloys. We find excellent overall agreement between experiment and the PUM for dispersions and linewidths and that the broadening reflects strong fluctuations in the pairwise interactions. Furthermore, an underestimation by ICPA of linewidths in NiFe results from neglect of fluctuations in the speciesdependent pairwise interactions. 
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