Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K8: Electrons, Phonons, and Electron Phonon Scattering IIIFocus

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Sponsoring Units: DCOMP Chair: Oliver Albertini, Georgetown University Room: 267 
Wednesday, March 15, 2017 8:00AM  8:36AM 
K8.00001: Firstprinciples calculations of mobility Invited Speaker: Karthik Krishnaswamy Firstprinciples calculations can be a powerful predictive tool for studying, modeling and understanding the fundamental scattering mechanisms impacting carrier transport in materials. In the past, calculations have provided important qualitative insights, but numerical accuracy has been limited due to computational challenges. In this talk, we will discuss some of the challenges involved in calculating electronphonon scattering and carrier mobility, and outline approaches to overcome them. Topics will include the limitations of models for electronphonon interaction, the importance of grid sampling, and the use of Gaussian smearing to replace energyconserving delta functions. Using prototypical examples of oxides that are of technological importanceSrTiO$_{\mathrm{3}}$ [1], BaSnO$_{\mathrm{3}}$ [2], Ga$_{\mathrm{2}}$O$_{\mathrm{3}}$, and WO$_{\mathrm{3}}$we will demonstrate computational approaches to overcome these challenges and improve the accuracy. One approach that leads to a distinct improvement in the accuracy is the use of analytic functions for the band dispersion, which allows for an exact solution of the energyconserving delta function. For select cases, we also discuss direct quantitative comparisons with experimental results. The computational approaches and methodologies discussed in the talk are general and applicable to other materials, and greatly improve the numerical accuracy of the calculated transport properties, such as carrier mobility, conductivity and Seebeck coefficient. This work was performed in collaboration with B. Himmetoglu, Y. Kang, W. Wang, A. Janotti and C. G. Van de Walle, and supported by the LEAST Center, the ONR EXEDE MURI, and NSF. [1] B. Himmetoglu, A. Janotti, H. Peelaers, A. Alkauskas, and C.G. Van de Walle, \textit{Phys. Rev. B} \textbf{90}, 241204 (2014). [2] K. Krishnaswamy, B. Himmetoglu, Y. Kang, A. Janotti, C. G. Van de Walle, \textit{eprint arXiv}:1610.06253 (2016). [Preview Abstract] 
Wednesday, March 15, 2017 8:36AM  8:48AM 
K8.00002: Conditions for $T^{2}$ resistivity from electronelectron scattering Michael Swift, Chris G. Van de Walle Many complex oxides (including titanates, nickelates and cuprates) exhibit a carrier scattering mechanism with a power law dependence on temperature (resistivity $\rho \propto T^{2})$. By analogy to a similar phenomenon observed in metals at low temperature, this mechanism has often been identified as Fermiliquidlike electronelectron scattering (Baber scattering). However, other transport signatures in these materials have shown behavior that casts doubt on this simple picture. Careful investigation of electronelectron scattering (both direct and phononmediated) reveals that the Baber $T^{2}$ power law rests on several crucial assumptions. In some cases, these assumptions are not satisfied and removing them destroys the power law. We illustrate these issues with two case studies: sodium metal (in which electronelectron scattering gives $T^{2}$ resistivity) and strontium titanate (in which it does not). Our results suggest that an observation of $\rho \propto T^{2}$ is not sufficient evidence for electronelectron scattering. The power law observed in the complex oxides may instead be due to another, as yet undiscovered, mechanism. [Preview Abstract] 
Wednesday, March 15, 2017 8:48AM  9:00AM 
K8.00003: Using subKelvin thermal transport to determine electronphonon coupling in a metallic thin film Zachary Stegen, Daniel Queen, Matt Legro, John Pryzbyz, Sunny Bagga, Shaun Goodwin Steadystate thermal transport was measured in a thin film of the alloy Ti$_{0.1}$W$_{0.9}$ for temperatures ranging from approximately 100 mK to 500 mK. The electron temperature was measured using two normal metalinsulatorsuperconductor (NIS) junction thermometers. The temperature of the normal metal electrons was measured while changing the power applied to the normal metal thin film. The data were compared to the theoretically expected relationship, $P_{ep} = \Sigma\Omega(T_e^nT_p^n)$, where $\Sigma$ depends on the electronphonon coupling, and $n$ is effected by the electron meanfree path and the thermal phonon wavevector. [Preview Abstract] 
Wednesday, March 15, 2017 9:00AM  9:12AM 
K8.00004: Temperaturedependence of the forbidden (222) reflection in silicon Jean Paul Nery, Philip B. Allen Crystals with an $fcc$ lattice like silicon have Bragg scattering at $\mathbf{K}=2\pi(h,k,l)/a$ for integers $hkl$ all even or all odd. The twoatom basis of the diamond structure causes destructive interference whenever $h+k+l$ is an odd multiple of 2; for example, the (222) reflection is nominally forbidden. However, there is not total interference because of tetrahedral rather than spherical scattering symmetry. Such asymmetry arises from anharmonic vibrations and from bonding. Therefore, the weakly allowed (222) Xray reflection in silicon is useful for studying bond charge. Temperature variation of the (222) Xray intensity, beyond that expected from anharmonicity, has been measured [1] and studied [1,2]. Previous theories have been somewhat ad hoc, not dealing fully with electronphonon induced valence charge density thermal shifts. Our formulation of this shift uses full secondorder electronphonon perturbation theory. We include both Fan and DebyeWaller type terms, known to determine band gap thermal shifts. We compare with experiment and with previous theories. [1] J. B. Roberto, B. W. Batterman, and D. T. Keating, Phys. Rev. B $\mathbf{9}$, 2590 (1974). [2] J. R. Chelikowsky and M. L. Cohen, Phys. Rev. Lett. $\mathbf{33}$, 1339 (1974). [Preview Abstract] 
Wednesday, March 15, 2017 9:12AM  9:24AM 
K8.00005: Charge and energy transport at the nanoscale: A DFT perspective Florian Eich, Fabio Covito, Angel Rubio Understanding the interplay between charge and energy transport at the nanoscale paves the way for novel thermoelectric devices, which may prove useful for the development for sustainable energy sources. However, concepts, such as heat flow, temperature and entropy are only wellestablished at the macroscopic level for slow dynamics. This raises the question about whether these concepts can be employed for small length and short time scales. We will present our recent efforts to use a timedependent densityfunctional theory framework, dubbed thermal DFT, in order to generalize temperature and heat or energy flow to the microscopic regime. To this end we will highlight the analogy of the formally exact microscopic equations of motion for charge density and energy density in thermal DFT to the macroscopic equations of motion of hydrodynamics. Furthermore, we will present first result using our approach to compute transient energy energy currents induced by a temperature gradient and show that in the steadystate limit persistent temperature oscillations develop. [Preview Abstract] 
Wednesday, March 15, 2017 9:24AM  9:36AM 
K8.00006: Band gap renormalization and temperature dependence of acene crystals: the case of naphthalene F. BrownAltvater, T. Rangel, G. Antonius, M. Giantomassi, Y. Gillet, S. G. Louie, X. Gonze, J. B. Neaton The band gap is one of the defining properties for semiconductors and insulators. It determines the energy of absorbed light in photovoltaic devices, the color of light emitting materials, or the redox potential for electrochemical reactions. Being able to accurately predict the band gap (its value and the position of the band edge states with respect to the vacuum level) is thus paramount for the design of new optoelectronic materials. Electronic structure theory has made big leaps towards this goal with ever improving functionals within the density functional formalism and manybody theories. The renormalization of the band structure due to electronphonon interactions becomes equally important at finite as well as zero temperature. However, the computation of electronphonon renormalization in molecular crystals is challenging, due to the small dispersion of the bands. Through improved algorithms and taking advantage of symmetries, we calculate the electronphonon coupling in crystalline naphthalene from first principles using van der Waals density functionals, and determine the zeropoint renormalization and temperature dependence of the electronic eigenstates within the AllenHeineCardona theory. [Preview Abstract] 
Wednesday, March 15, 2017 9:36AM  9:48AM 
K8.00007: Firstprinciples anharmonic calculations and the dynamic JahnTeller effect Joseph Prentice, Bartomeu Monserrat, Richard Needs Firstprinciples density functional theory methods can be used to investigate the structural configurations, energetics and vibrational motions of solids, including anharmonicity, by using a vibrational selfconsistent field (VSCF) method. The possibility of calculating an anharmonic vibrational wavefunction using this method allows anharmonic effects such as the dynamic JahnTeller effect to be described accurately. In this work, we apply our VSCF method to an important example of a dynamic JahnTeller system, the neutral vacancy in diamond. Our calculations demonstrate that the dynamic JahnTeller distorted tetrahedral structure of the vacancy is more stable than the static JahnTeller distorted tetragonal structure, in agreement with experimental observations, across a wide range of temperatures. This work opens the way for firstprinciples treatments of dynamic JahnTeller systems in condensed matter. Further examples of systems our method can be applied to are considered as well. [Preview Abstract] 
Wednesday, March 15, 2017 9:48AM  10:00AM 
K8.00008: Temperature renormalization of the electronic and phonon properties of TiSe$_{2}$ Yanghao Chan, Peng Chen, TaiChang Chiang, MeiYin Chou 
Wednesday, March 15, 2017 10:00AM  10:12AM 
K8.00009: Abstract Withdrawn We report the impact of lattice vibrations on magnetic and electronic properties of paramagnetic bcc and fcc iron employing the disordered local moments molecular dynamics (DLMMD). Vibrations strongly affect the distribution of local magnetic moments and the electronic density of states in the paramagnetic regime. When the coupling between vibrations and magnetism is taken into account at the $\gamma$$\delta$ transition temperature (1662 K), the lattice distortions cause very similar mean magnetic moments and total electronic density of states of both bcc and fcc structures. Consequently, our simulations suggest that at the $\gamma$$\delta$ transition temperature, electronic and magnetic contributions to the Gibbs free energy are extremely similar in bcc and fcc Fe.[B. Alling et al. PRB 93, 224411 (2016) ] In the next step, going beyond the approximation of magnetism as an adiabatically fast degree of freedom, we study paramagnetic CrN using a combination of atomistic spin dynamics and abinitio molecular dynamics. We demonstrate how the relaxation time scales of the transverse spin dynamics and atomic vibrations are rather similar and study the impact of their explicit coupling on properties such as paircorrelation functions, potential energies, and trajectories. 
Wednesday, March 15, 2017 10:12AM  10:24AM 
K8.00010: Band crossing driven by electron phonon coupling Mirko Moeller, George Sawatzky, Mona Berciu The coupling of charge carriers (electrons or holes) to phonons can lead to the formation of a polaron, a coherent quasiparticle consisting of the charge carrier and the cloud of phonons surrounding it and moving coherently with it. Polarons have been studied extensively in the Holstein model and to a lesser extent in the SSH model, both of which are single band models. However, for many of the materials in which polarons are the lowenergy excitations a description with multiband models is more appropriate. Here we present results obtained with the highly accurate momentum average approximation for the single polaron properties of a two dimensional, threeband model. The model is inspired by the perovskite BaBiO$_3$ and the coupling to phonons modifies the hopping integrals. We find that the electron phonon coupling changes the ground state momentum from $\mathbf{k}=(\pi,\pi)$ to $\mathbf{k}=(\pi,0)$. Furthermore it can lead to the formation of a tilted band crossing point (BCP) and/or shift the location of existing BCPs in the Brillouin zone. These findings are of interest in the light of Dirac or Weyl materials in which BCPs play an important role. [Preview Abstract] 
Wednesday, March 15, 2017 10:24AM  10:36AM 
K8.00011: Lattice dynamics and electronphonon coupling on Mn$_{1x}$Fe$_{x}$Si: effect of magnetism Paola Gonzalez Castelazo, Omar De la Peña Seaman, Rolf Heid, KlausPeter Bohnen We have studied the electronic, lattice dynamics, and electronphonon (eph) coupling properties of the Mn$_{1x}$Fe$_x$Si alloy. This system have been analyzed within the framework of density functional perturbation theory, using a mixedbasis pseudopotential method and the virtual crystal approximation (VCA) for modeling the alloy. In particular, the electronic density of states (DOS), the fullphonon dispersion, as well as the electronphonon coupling ($\lambda$) and the phonon linewidth ($\gamma$) have been calculated with and without the inclusion of spin polarization. While for FeSi is very well known that the effects of magnetism on the lattice dynamics are observed trough the phonon linewidths for specific regions on the zone boundary, on MnSi such detail analysis has not been addressed so far. Thus, the evolution of phonon frequencies and linewidths as a function of Fecontent are presented and discussed in detail, paying special attention the effect of spinpolarization on such properties for the magnetic region on the phase diagram $x < 0.16$. [Preview Abstract] 
Wednesday, March 15, 2017 10:36AM  10:48AM 
K8.00012: Phononinduced superlattice structures in titaniumoxypnictides superconductors Kenta Hongo, Kousuke Nakano, Ryo Maezono Firstprinciples electronic and phonon simulations have been carried out within density functional theory (DFT) for layered titaniumoxypnictides, BaTi$_2Pn_2$O ($Pn$ = As, Sb, Bi). We have found a new possibility of orthorhombic $2\times2\times1$ superlattice structure for BaTi$_2$As$_2$O, while that of tetragonal $\sqrt{2}\times\sqrt{2}\times1$ for BaTi$_2$Sb$_2$O and BaTi$_2$Bi$_2$O [1]. It was found that their phonon dispersons and changes of nesting vectors in Fermi surfaces can account for such varieties of superlattice structures even starting with the common undistorted structure when without the charge ordering. This new finding can naturally resolve a discrepancy between experiments and theoretical predictions on the charge ordering of the compounds without any relying on complicated unconventional mechanism proposed recently, which could also affect the understanding of superconductivity on the compounds. [1] K. Nakano, K. Hongo, and R. Maezono, Sci. Rep. 6, 29661 (2016). [Preview Abstract] 
Wednesday, March 15, 2017 10:48AM  11:00AM 
K8.00013: Effect of local correlation on electron phonon coupling in $La_2CuO_4$ using LDA+DMFT Julien Groulx, Paul Boulanger, Michel C\^ot\'e ARPES measurements show a kink in the electron dispersion of $La_2CuO_4$ around 80 meV. The cause of this kink is still under debate. The phonon spectrum is consistent with a kink at that energy but previous studies have demonstrated that the response of the electronphonon coupling calculated in the framework of density functional theory (DFT) with a GGA functional is too weak to account for the observed kink. However, other works have shown that the electronphonon coupling can be underestimated by the treatment of LDA/GGA functionals. For this reason, we investigate the effect of the strong local correlation caused by the "d" electrons of Cu on the calculation of electronphonon matrix elements. The local correlation is added using the dynamic meanfield theory (DMFT) approach within a DFT method. Specific electronphonon matrix elements are calculated using the Frozen Phonon method. [Preview Abstract] 
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