Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S29: Electrons, Phonons, Electron Phonon Scattering and Phononics IVFocus

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Sponsoring Units: DCOMP DMP Chair: Jiong Yang, Shanghai University Room: LACC 406A 
Thursday, March 8, 2018 11:15AM  11:51AM 
S29.00001: Rethinking Phonons Invited Speaker: Asegun Henry The current understanding of phonons treats them as plane waves/quasiparticles of atomic vibration that propagate and scatter. This concept of treating phonon transport like that of a gas is termed the phonon gas model (PGM), however, it only seems to rigorously hold true in the limit of a pure, infinite, perfect crystal. The problem is then that whenever any level of disorder is introduced or symmetry is broken, the character of vibrational modes changes, yet nearly all theoretical treatments continue to assume phonons are still waves. For example, the phonon contributions to alloy thermal conductivity rely on the PGM and are most often computed from the virtual crystal approximation. Good agreement is obtained in some cases, but there are many instances where it fails —both quantitatively and qualitatively. Similarly, for amorphous materials the PGM fails at describing the contributions to thermal conductivity, and new questions surrounding its validity at interfaces are also emerging – even when it is an interface between two crystalline materials. This talk will review a growing body of recent work that suggests that our understanding of phonons requires revision, because the critical assumption that all phonons/normal modes resemble plane waves with welldefined velocities is no longer valid when some form of disorder/symmetry breaking occurs. An emphasis will be placed on the new methodologies that have been developed to treat phonon transport more rigorously and generally. Additionally, some of the rather nonintuitive results/predictions that have emerged from applying these new approaches will be highlighted in an effort to identify some of the interesting questions that must be addressed in future work. 
Thursday, March 8, 2018 11:51AM  12:03PM 
S29.00002: Effect of Thermal Deformation on the Quasiparticle Gap and Exciton Binding Energies in Semiconductor Quantum Dots Jeremy Scher, Peter McLaughlin, Arindam Chakraborty Spectroscopic properties of semiconductor nanoparticles (NPs) exhibit strong dependence on NP shape and size. At finite temperatures, the NPs exists in an ensemble of structures and inclusion of these structures is essential for accurate theoretical investigation of the optical properties of these materials. However, performing ensembleaveraged calculations using conventional electronic structure theories prohibitive because of steep computational cost. In this work, we present a random matrix based statistical mechanical procedure for ensembleaveraged calculations of optical properties of semiconductor NPs. This approach is based on deriving an effective fluctuating potential for the NPs to account for the thermal fluctuations in the system. The method was applied to a series of CdSe quantum dots (Cd_{20}Se_{20}Cd_{200}Se_{200}) and was used for calculation of ensembleaveraged excitonic properties including quasiparticle gap, optical gap, exciton binding energies, and electronhole recombination probabilities. The results from these calculations highlight the importance of ensemble averaging and demonstrate the limitations of using a single optimized NP structure for computational prediction of optical properties of NPs. 
Thursday, March 8, 2018 12:03PM  12:15PM 
S29.00003: CurrentInduced Vibrational Instabilities in Molecular Wires with Broken Conjugation Giuseppe Foti, Héctor Vázquez The stability of molecular scale circuits in presence of an external bias is particularly sensitive to the subtle balance between heat generation and dissipation associated to the flow of electrical current [1]. In this talk I will present firstprinciples calculations based on density functional theory (DFT) and the nonequilibrium Green's function formalism (NEGF) of the currentinduced heating dynamics in a terphenylbased molecular junction [2]. 
Thursday, March 8, 2018 12:15PM  12:27PM 
S29.00004: Geometrodynamics of electrons in deforming crystals Liang Dong, Qian Niu Semiclassical dynamics of Bloch electrons in a slowly varying deforming crystal is developed in the geometric language of a lattice bundle. Berry curvatures and gradients of energy are introduced in terms of lattice covariant derivatives, with connections given by the gradient and rate of strain. A number of physical effects are discussed: an effective postNewtonian gravity, polarization induced by spatial gradient of strain, orbital magnetization induced by strain rate, and dissipationless electron viscosity. 
Thursday, March 8, 2018 12:27PM  12:39PM 
S29.00005: Understanding solitonic excitations in the charge density wave ordered ground state of the In/Si(111) nanowire array from phonon theory Samad Razzaq, TaeHwan Kim, Han Woong Yeom, Stefan Wippermann The Si(111)(4x1)/(8x2)In atomic wire array is an extremely popular model system for onedimensional electronic systems. It features a reversible temperatureinduced metalinsulator transition into a charge density wave (CDW) ordered ground state. Solitonic excitations of the CDW and associated topological edge states are presently the focus of increasing attention. We carried out a combined ab initio and scanning tunneling microscopy (STM) study of solitonic phase defects in the In/Si(111) atomic wire array. We show how the solitonic CDW excitations can be modeled in terms of collective excitations of particular phonon modes. In conjunction with STM measurements, this phonon expansion approach allows us for the first time to determine the atomistic structure of the solitonic excitations. Due to the topological properties of the solitons and a strongly nonlinear phononphonon coupling, these solitons interact in a deterministic way and are suitable for information processing. 
Thursday, March 8, 2018 12:39PM  12:51PM 
S29.00006: Vibrational and Configurational Entropy of Liquids and Glasses Hillary Smith, Marios Demetriou, Brent Fultz Over several decades there has been controversy about the specific heat absorbed as a glass transforms to a liquid—does this originate from vibrational entropy or configurational entropy? We report direct measurements of the total entropy and vibrational entropy of strong and fragile metallic glasses and the molecular glass GeO_{2} in the glass, liquid and crystalline states. Inelastic scattering methods were used to measure the vibrational spectra across the glass transition, and the total entropy assessed with calorimetry. These three materials differ fundamentally in their structure, bonding, and glassforming ability. The configurational disorder in oxide glasses is predominantly orientational, while metallic glasses are characterized by topological disorder. The excess vibrational entropy of the supercooled liquid over the crystalline phase will be compared for all three glassformers. [Smith, H. L., et al. Nature Phys. 13, 900905 (2017).] 
Thursday, March 8, 2018 12:51PM  1:03PM 
S29.00007: Molecular heat engines: Quantum coherence effects Feng Chen, Yi Gao, Michael Galperin Recent developments in nanoscale experimental techniques made it possible to utilize single molecule junctions as devices for electronics and energy transfer with quantum coherence playing an important role in their thermoelectric characteristics. Theoretical studies on the efficiency of nanoscale devices usually employ rate (Pauli) equations, which do not account for quantum coherence. Therefore, the question whether quantum coherence could improve the efficiency of a molecular device cannot be fully addressed within such considerations. Here, we employ a nonequilibrium Green function approach to study the effects of quantum coherence and dephasing on the thermoelectric performance of molecular heat engines. Within a generic bichromophoric donorbridgeacceptor junction model, we show that quantum coherence may increase efficiency compared to quasiclassical (rate equation) predictions and that pure dephasing and dissipation destroy this effect. 
(Author Not Attending)

S29.00008: Novel Insight into Thermally Activated Absorption, ChargeTransfer and Polaron Relaxation in Molecular Semiconductors from Theory and Experiment Frank Ortmann, Johannes Benduhn, Karl Schellhammer, Michel Panhans, Koen Vandewal Charge transport in molecular materials is governed by strong electronphonon coupling and polaronic effects. Such polaronic effects manifest in structural relaxation of molecules upon charging, which is quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular chargetransfer (CT) states formed at organic electron donor−acceptor interfaces. 
Thursday, March 8, 2018 1:15PM  1:27PM 
S29.00009: Polarons in CH_{3}NH_{3}.PBI_{3}: Formation, transport and recombination Jarvist Frost, Lucy Whalley, Jonathan Skelton, Pooya Azarhoosh, Scott McKechnie, Mark Schilfgaarde, Aron Walsh Hybrid halide perovskites are soft, polar, semiconductors[1]. We propose that low energy (9 meV) optical phonons limit room temperature mobility. We have written open source codes to solve the finitetemperature Feynman polaron state. This provides a temperaturedependent calculation of mobility[2], in good agreement with experiment. 
Thursday, March 8, 2018 1:27PM  1:39PM 
S29.00010: Study on thermal conductivity and electrical resistivity of AlCu alloys obtained by Boltzmann transport equation and firstprinciples simulation: Semiempirical approach Garam Choi, Hyoseok Kim, Won Bo Lee Boltzmann transport equation (BTE) and firstprinciple approaches are applied in the estimation of the thermal conductivity and electrical resistivity of AlCu alloys with varying AlCu ratios. The electronic transport properties of various AlCu alloys are predicted by solving the BTE using a relaxation time approximation (RTA). The theoretical results are estimated through fitting of experimental data. It is shown that the results correctly estimate transport property trends in terms of temperature and composition. The transport properties are also estimated using a semiempirical method based on experimental electrical resistivity data. It is believed that the primary reason for deviations between the experimental and theoretical values is computational limits arising from a lack of rigor in taking scattering effects and structural stability into account. This explanation for the deviation is also supported by the WF law and relaxation times (or scattering rate). 
Thursday, March 8, 2018 1:39PM  1:51PM 
S29.00011: Energy Waves in Harmonic Crystals: Phonon Interference and Apparent Dissipation Anant Raj, Jacob Eapen In this work, we probe possible relationships between phonon modes and energy transport in a harmonic crystal. We demonstrate theoretically that phonon modes or normal modes of displacements combine to produce energy wave packets. We further derive the condition that pairs of phonon modes interfere to produce waves of energy if and only if the threephonon interference/scattering law is satisfied even in the absence of phononphonon scattering. Further, we show that the frequency and decay of the energy normal modes are directly associated with the collective excitation of phonon modes. 
Thursday, March 8, 2018 1:51PM  2:03PM 
S29.00012: Extended Application of the Wittrick Model to Experimental Data of Bimetallic Disks Joe Zuchegno, Maureen Smith, Matthew Moelter, Nathan Heston

Thursday, March 8, 2018 2:03PM  2:15PM 
S29.00013: Transient Mass and Heat Transfer During Gas Adsorption into MOFs Hasan Babaei, Alan McGaughey, Christopher wilmer Using molecular dynamics simulations, we have investigated transient mass and heat transfer phenomena during gas adsorption into a metalorganic frameworks (MOFs). This study was motivated by the challenge of quickly dissipating heat generated in metalorganic frameworks (MOFs) during gas adsorption. 
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