Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session R29: Electrons, Phonons, Electron Phonon Scattering and Phononics IIIFocus

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Sponsoring Units: DCOMP DMP DCP DCMP Chair: Junqiao Wu, Univ of California  Berkeley Room: LACC 406A 
Thursday, March 8, 2018 8:00AM  8:12AM 
R29.00001: Optically excited structural transition in the In/Si(111) nanowire array at the quantum limit Stefan Wippermann, T. Frigge, B. Hafke, B. Krenzer, C. Streubühr, A. Samad Syed, V. Miksic Trontl, I Avigo, P. Zhou, M. Ligges, D. von der Linde, U. Bovensiepen, M. HornvonHoegen, A. Lücke, S. Sanna, U. Gerstmann, W. G. Schmidt 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. The transition is driven by a strong electronphonon coupling. We carried out constrained density functional theory calculations and ultrafast pumpprobe electron diffraction measurements. We demonstrate that an optical excitation breaks and creates specific bonds, leading to a nonthermal excitation of particular soft phonon modes. The excitation of these modes drive the structural transition in the limit of critically damped nuclear motion and allows to switch between metallic and insulating states within 350 fs. This finding demonstrates that carefully tuned electronic excitations can create nonequilibrium potential energy surfaces that drive structural dynamics at interfaces in a regime of deterministic and directed nuclear motion [Nature 544, 207 (2017)]. 
Thursday, March 8, 2018 8:12AM  8:24AM 
R29.00002: Ultrafast thermalization dynamics of hot electrons in titanium nitride Stefano Dal Forno, Johannes Lischner Titanium nitride (TiN) is a hard material with a high melting point and good electrical and thermal conductivity making TiN attractive for applications in hot carrier devices. We study the ultrafast thermalization dynamics of excited carriers in TiN due to their interaction with phonons. For this, we use the twotemperature model with parameters determined from ab initio densityfunctional theory calculations. We find that lifetimes of hot carrier populations in TiN are shorter than one picosecond. We also study the effect of oxygen defects on hot carrier thermalization. 
Thursday, March 8, 2018 8:24AM  8:36AM 
R29.00003: The important role of electronphonon scattering in photoemission from PbTe(111) Johannes Kevin Nangoi, Siddharth Karkare, Howard Padmore, Tomas Arias The state of the art in creating high quality electron beams for particle accelerator applications and next generation ultrafast electron diffractometers involves lasergenerated photoemission. In this context, a high quality beam requires that electrons emerge in a single direction with low mean transverse energy (MTE, the kinetic energy of the electrons parallel to the surface). Recent densityfunctional theory calculations by T. Li and W. A. Schroeder [arXiv:1704.00194v1 [physics.accph] (2017)] suggests that PbTe(111) is expected to produce lowMTE photoelectrons due to its electronic band structure. Based on this suggestion, we measured the distribution of photoelectrons from PbTe(111) and found the MTE to be 2 orders of magnitude larger than expected from the electronic band structure. In this talk, we present manybody photoemission calculations including electronphonon scattering. Our results are in far better agreement with the experiment and underscore the importance of electronphonon scattering in photoemission from PbTe(111). 
Thursday, March 8, 2018 8:36AM  8:48AM 
R29.00004: Nonequilibrium Electron Dynamics In PumpProbe Spectroscopy: Role Of Excited Phonon Populations Omadillo Abdurazakov, Avinash Rustagi, James Freericks, Alexander Kemper The time and angleresolved photoemission spectroscopy(trARPES) has emerged as a powerful tool to study the singleparticle as well as collective dynamics of elementary excitations. The dynamics allow for the measurement of energy transfer between degrees of freedom such as electrons and phonons. When significant energy is deposited by the pump, or when the phonons are resonantly excited, the effect of excited phonons on the electrons plays an important role. We examine the relaxation dynamics of electrons coupled to a bath of optical phonons when subjected to an ultrafast optical excitation be solving the equation of motion for the coupled system within a nonequilibrium Keldysh formalism. By comparing the decay rates of electron populations when interacting with the phonons of fixed properties to those of dynamically modified properties, we gain insight into the role of the excited phonon populations; they are found to significantly suppress the decay rates at different excitation energies and to induce strong time dependence. We will also show that the temporal changes in the phonon occupation number manifestly sets apart the population decay rates from the singleparticle scattering rates. 
Thursday, March 8, 2018 8:48AM  9:00AM 
R29.00005: The phonon softening due to melting of the ferromagnetic order in elemental iron Qiang Han, Turan Birol, Kristjan Haule We study the fundamental question of the lattice dynamics of a metallic ferromagnet in the regime where the static long range magnetic order is replaced by the fluctuating local moments embedded in a metallic host. 
Thursday, March 8, 2018 9:00AM  9:12AM 
R29.00006: Scalebridging Simulations of Electronic Relaxation from Master Equation with firstprinciplesderived Rates Peter Kratzer, Maedeh Zahedifar The lifetime of excited electrons in metals is limited by both electronelectron and electronphonon scattering. Quantum well states (QWS) in thin metals films offer an ideal probing ground for exploring the competition between both relaxation mechanisms. We performed firstprinciples densityfunctional (DFT) calculations adressing the electronic band structure of fewatomiclayer Pb films on Si(111). In addition, phonon spectra and matrix elements for electronphonon coupling within deformation potential theory were obtained from DFT calculations. This enables us to calculate statespecific rate constants for the electronphonon scattering in particular QWS. The contribution of impact ionization processes to the lifetime can be estimated from the imaginary part of the electronic selfenergy calculated in the GW approximation. By combining both channels and numerically solving rate equations for the electronic occupations coupled to a phononic heat bath, we are able to follow the dissipation of the electronic excitation energy to the Pb lattice vibrations over long time. The time scales extracted from the simulations are compared to experimental data from timeresolved pumpprobe experiments. 
Thursday, March 8, 2018 9:12AM  9:48AM 
R29.00007: Structure, structural evolution and Superconductivity of high pressure hydrogenrich alloys Invited Speaker: John Tse Pressure has an enormous effect in altering the structure, physical and chemical properties of matters. In general, for elemental metals, compression led to loosely bound valence electrons redistributed into the interstitial vacancies and often resulted in novel open 2D and 3D structures. Incidentally, when mixed with H_{2,} which has an electronegativity similar to group 13 and 14 elements, charge transfer ZintlKlemen type compounds can be formed. This phenomenon helps to explain the structural trend in hydrogenrich alloys predicted by First Principle methods. Under suitable conditions, these alloys may even become superconductors and, in some cases, with very high critical temperature (T_{c}). Analysis of the functional derivative of Eliashberg spectral functions show that efficient electronphonon coupling over the entire vibrational spectrum can be achieved on crystal structures when the stretch and bend vibrations of weakly linked hydrogen network atoms are strongly mixed. 
Thursday, March 8, 2018 9:48AM  10:00AM 
R29.00008: Crossover from Polarons to Fermi Liquids in Doped Oxides: Ab Initio ManyBody Calculations Using the Cumulant Expansion Approach Carla Verdi, Fabio Caruso, Feliciano Giustino The coupling of infraredactive phonons to electrons (Fröhlich interaction) can lead to the formation of polarons, i.e. electrons dressed by a phonon cloud. Conducting oxides represent an ideal playground to investigate this phenomenon. In angleresolved photoemission spectroscopy (ARPES) the signature of polarons is the appearance of spectral satellites below the conduction band. Recent ARPES experiments reported a transition of the charge carriers from polarons to a weaklycoupled Fermi liquid with increasing doping [1,2]. Here we calculate ARPES spectra from first principles by combining accurate ab initio calculations of the electronphonon coupling with the cumulant expansion method [3]. For the paradigmatic example anatase TiO_{2}, we show that the transition observed in the experiments originates from nonadiabatic polar electronphonon coupling. We show that this is a universal mechanism, and in particular it also applies to the ferromagnetic semiconductor EuO. 
Thursday, March 8, 2018 10:00AM  10:12AM 
R29.00009: Superconductivity, chargedensity waves, antiferromagnetism, and phase separation in the HubbardHolstein model Seher Karakuzu, Luca Fausto Tocchio, Sandro Sorella, Federico Becca By using variational wave functions and quantum Monte Carlo techniques, we investigate the interplay between electronelectron and electronphonon interactions in the twodimensional HubbardHolstein model. Here, the groundstate phase diagram is triggered by several energy scales, i.e., the electron hopping t, the onsite electronelectron interaction U, the phonon energy ω_{0}, and the electronphonon coupling g. At half filling, the ground state is an antiferromagnetic insulator for U ≥ 2g^{2}/ω_{0}, while it is a chargedensitywave (or bipolaronic) insulator for U < 2g^{2}/ω_{0}. In addition to these phases, we find a superconducting phase that intrudes between them. For ω_{0}/t=1, superconductivity emerges when both U/t and 2g^{2}/tω_{0} are small; then, by increasing the value of the phonon energy ω_{0}, it extends along the transition line between antiferromagnetic and chargedensitywave insulators. Away from half filling, phase separation occurs when doping the chargedensitywave insulator, while a uniform (superconducting) ground state is found when doping the superconducting phase. In the analysis of finitesize effects, it is extremely important to average over twisted boundary conditions, especially in the weakcoupling limit and in the doped case. 
Thursday, March 8, 2018 10:12AM  10:24AM 
R29.00010: Quasiparticles and phonon satellites in spectral functions of semiconductors and insulators: Cumulants applied to full first principles theory and Fröhlich polaron. Jean Paul Nery, Philip Allen, Gabriel Antonius, Lucia Reining, Anna Miglio, Xavier Gonze The electronphonon interaction causes zeropoint renormalization of electron quasiparticle (QP) energies ε_{kn} and broadening of QP peaks. Sidebands might also appear, as in the case of materials with infrared active phonons. All these features are captured by the spectral function A(kn,ω)=ImG_{R}(kn,ω)/π. Here we consider LiF and MgO and determine their nonadiabatic Migdal self energy. The spectral function obtained from the Dyson equation makes errors in the position and weight of both the QP peak and the sideband. Only one phonon satellite appears, with an unphysically large energy difference with respect to the QP peak. In contrast, a cumulant treatment [1],[2] gives an accurate QP energy and several satellites separated by the LO phonon energy. For the Fröhlich Hamiltonian, the positions of the QP peak and the first satellite agree closely with Monte Carlo results of Ref. [3]. We provide a detailed comparison between the firstprinciples MgO and LiF results and those of the Fröhlich Hamiltonian. 
Thursday, March 8, 2018 10:24AM  10:36AM 
R29.00011: Vibrational Entropy from ThermallyDriven Electronic Topological Transitions Fred Yang, Jorge Munoz, Olle Hellman, Matt Lucas, Brent Fultz A novel thermallydriven electronic topological transition was discovered in the B2ordered intermetallic compound FeTi. Abinitio molecular dynamics, supported by inelastic neutron scattering and nuclear resonant inelastic xray scattering, showed an anomalously large thermal softening of the M_{5}^{–} phonon mode that could not be explained by phononphonon interactions or electronphonon interactions calculated at low temperatures. The softening is caused by an adiabatic electronphonon interaction with an unusual temperature dependence. This interaction arises from the appearance of new features of the Fermi surface at elevated temperatures. This electronic topological transition (Lifshitz transition) was analyzed by electronic band unfolding, Fermi surface visualization, and enumerating the electronphonon spanning vectors. These methodologies have also shown that there is a temperaturedependent evolution of the spanning vector distributions that is linked to anomalous thermal shifts in phonon energies in the A15 compound V_{3}Ge. 
Thursday, March 8, 2018 10:36AM  10:48AM 
R29.00012: The role of electronphonon coupling on optoelectronic properties of crystalline naphthalene from first principles Florian BrownAltvater, Gabriel Antonius, Tonatiuh Rangel Gordillo, Matteo Giantomassi, Steven Louie, Claudia Draxl, Xavier Gonze, Jeffrey Neaton Organic molecular crystals, periodic arrays of noncovalently bound molecules held together by electrostatic and dispersion forces, are chemically diverse platforms for fundamental studies and use in optoelectronic applications. Using density functional perturbation theory, we compute the finitetemperature electronic structure of naphthalene, a prototypical molecular crystal. Due to electronphonon interactions, the direct gap decreases by 0.2 eV at room temperature. We also compute the corresponding broadening and lifetimes of singleparticle excitations relevant to transport properties. To account for the dispersive forces that play a large role in these systems, we use van der Waals density functionals to calculate the crystal structure and phonon dispersion. We compare our calculations to experiments and discuss the consequences on optoelectronic properties of naphthalene and other organic crystals. 
Thursday, March 8, 2018 10:48AM  11:00AM 
R29.00013: Modelling Electron Trapping in Titanium Dioxide Keith McKenna, Razak ElMaslmane The trapping of electrons in titanium dioxide (TiO_{2}) underpins a diverse range of applications in areas such as solar energy generation, catalysis, gas sensing and nanoelectronics. Predictive modelling of charge trapping using density functional theory remains challenging owing to selfinteraction errors present in many widely used approximations. Here, we present a number of practical approaches to predictively model electron trapping in TiO2 and apply them to model the polaronic trapping of electrons and holes in bulk TiO_{2} [1] as well as electron trapping at twodimensional defects such as surfaces, heterointerfaces and grain boundaries [25]. 
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