Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C29: Firstprinciples Modeling of ExcitedState Phenomena in Materials III: Phonons, Spins, DynamicsFocus

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Sponsoring Units: DCOMP DMP DCMP DCP Chair: Sivan Abramson Refaely, UC Berkeley Room: LACC 406A 
Monday, March 5, 2018 2:30PM  3:06PM 
C29.00001: BSE and time dependent DFT beyond the TammDancoff approximation: diagonalization versus time evolution Invited Speaker: Georg Kresse The Casida equation for time dependent DFT and the Bethe Salpether equation for many body response functions are tools commonly used to calculate optical properties in solids. Here an efficient algorithm to go beyond the commonly used TammDancoff approximation for periodic systems is discussed. The algorithm uses ideas from quantum chemistry and inversion symmetry to recast the Casida/BSE equation in a quadratic form. The final equation can be solve efficiently using standard linear algebra packages. For time dependent DFT, we compare this algorithm to time evolution, finding perfect agreement between spectra predicted from the Casida equation and the one obtained from the Fourier transformation of the polarization. Performance wise, we find that diagonalization of the Casida/BSE equation is extremely inefficient compared to the time evolution algorithms for density functionals, however, for hybrid functionals, the repeated application of the Fock operator at each time step, makes time evolution prohibitively expensive. 
Monday, March 5, 2018 3:06PM  3:18PM 
C29.00002: Treating chargeneutral excitations using geminalscreened electronhole interaction kernel in the BetheSalpeter and TDDFT formulations Mike Bayne, Arindam Chakraborty The electronhole or quasiparticle representation plays a central role in describing electronic excitations in manyelectron systems. Accurate determination of the electronhole interaction kernel remains a significant challenge for precise calculations of optical properties in both GW+BSE and TDDFT formalism. The inclusion of unoccupied states has long been recognized as the leading computational bottleneck that limits the application of this approach for large finite systems. In this work, we present the geminalscreened electronhole interaction kernel (GSIK) method that avoids using unoccupied states to construct the electronhole interaction kernel. Using diagrammatic techniques, we demonstrate that an infiniteorder summation of a subset of particlehole diagrams that appear in MBPT can be expressed as a renormalized twobody operator in realspace representation. The GSIK was combined with both BSE and linearresponse TDDFT methods to calculate excitation energies and exciton binding energies (where applicable) in atoms, molecules, clusters, and quantum dots. 
Monday, March 5, 2018 3:18PM  3:30PM 
C29.00003: A budget way to nonadiabatic molecular dynamics Thomas Niehaus Trajectory surface hopping combined with timedependent density functional theory (TDDFT) is a well established technique to investigate the nonadiabatic dynamics of small to medium sized molecules in the gas phase. Applications to complex materials have been hindered so far by the computational cost. In addition, TDDFT with conventional functionals suffers from erratic density localization and underestimated charge transfer excitations. Both problems become especially important in large systems. 
Monday, March 5, 2018 3:30PM  3:42PM 
C29.00004: Thermodynamic Properties of the Electron Gas from the Cumulant Green’s Function Approach Joshua Kas, John Rehr, Tyler Blanton, Tun Tan We present results for thermodynamic properties of the spin polarized electron gas based on the one electron cumulant Green's function approach and the GalitskiiMigdalKoltun sumrule. This approach was recently extended to finite temperature calculations for the unpolarized electron gas [1], and gives good agreement in comparison with results from quantum MonteCarlo (QMC) calculations of exchangecorrelation energies over a wide range of temperatures and densities. Excited state properties not readily available from QMC were also calculated. Here we describe extensions to include spinpolarization, the exchange correlation freeenergy and entropy, together with comparisons to QMC data and DFT fits. In addition, we extend the static COHSEX approximation of the selfenergy to finite temperature, and discuss the quality of this approximation along with possible improvements [2]. 
Monday, March 5, 2018 3:42PM  3:54PM 
C29.00005: Lattice vibrational effects on single and multiexciton phenomena in organic crystals from a first principles Green’s function approach Jonah Haber, Sivan RefaelyAbramson, Jeffrey Neaton Singlet fission, a process in which a photoexcited singlet exciton rapidly decays to a bitriplet exciton, is of recent interest for achieving beyond ShockleyQueissier limit solar cells. Although this process was experimentally observed over 50 years ago in molecular crystals, mechanisms of singlet fission are still hotly debated. Here, we use a recently developed ab initio Green’s function method [1], based on manybody perturbation theory within the GW approximation and the BetheSalpeter equation approach in conjunction with density functional perturbation theory to examine the role of lattice vibrations in linear absorption and singlet fission processes in molecular crystals, including tetracene and TIPSpentacene. More specifically, inspired by recent work on “one shot” temperature calculations [2], we explore the coupling of specific modes to singlet and triplet excitons and how these might alter rates of singlet fission within the adiabatic and harmonic approximations. 
Monday, March 5, 2018 3:54PM  4:06PM 
C29.00006: Phonondriven spin dynamics in the valleys of TMDC Dongbin Shin, Hannes Huberner, Umberto Giovannini, Hosub Jin, Angel Rubio, Noejung Park Nonconventional device functionalities of twodimensional materials, based on their extraordinary quantum mechanical properties, have fascinated many researchers in various fields. Among them, the exfoliated fewlayer transition metal dichalcogenides have recently attracted focused attention owing to their valley degree of freedom. Here, using the timedependent density functional theory, we investigated the phonondriven dynamics of a spin state at the valleys of monolayer MoS_{2}. We show that the spin motion is strongly coupled to an optical phonon which breaks lattice mirror symmetry. We analyzed the Floquet spectrum of this phonondressed spinors whose timeperiodicity is defined by the phonon period. When the phonon is pumped with a circular polarity so as to break the system’s timereversal symmetry, the dichroic responses of the phononcoupled spins on both valleys lead to a net nonzero magnetic moment. We see that this emerging magnetism, as a result of the coupling of the valley spinors and the phonon, can be exploited as a novel spin manipulation method. 
Monday, March 5, 2018 4:06PM  4:18PM 
C29.00007: Transient charge and energy flow in the wideband limit Fabio Covito, Florian Eich, Riku Tuovinen, Michael Sentef, Angel Rubio Thanks to recent advances in ultrafast pumpprobe spectroscopies and nanothermometry it is possible to study charge and energy flow at atomic time and length scales. In order to analyze the transient dynamics of nanoscale devices theoretically, the wideband limit is a commonly used approximation. Here we investigate the applicability of the wideband limit to the study of charge and heat transport through nanojunctions exposed to voltage biases and temperature gradients. We find that while this approximation faithfully describes the longtime steadystate charge and heat transport, it fails to characterize the shorttime transient behavior of the junction. In particular, we find that the charge current flowing through the device shows a discontinuity when a temperature gradient is applied, while the energy flow is discontinuous when a voltage bias drives the dynamics and even diverges when the junction is exposed to both a temperature gradient and a voltage bias. We discuss this pathological behavior and propose two possible solutions. 
Monday, March 5, 2018 4:18PM  4:30PM 
C29.00008: Magnon dispersion and magnetooptical Kerr effect of antiferromagnetic L1_{0}type MnPt Kisung Kang, David Cahill, Andre Schleife Antiferromagnetic (AFM) materials are attracting attention due to weak response to external magnetic fields and fast spin dynamics in the THz frequency range. In particular, high Néel temperature AFM materials may be great candidates for future spintronic devices. We use firstprinciples calculations based on density functional theory and the spin spiral approximation to study the magnon dispersion of AFM L1_{0}type MnPt. Néel temperatures computed using the meanfield/randomphase approximation are 1.12%/20.9% smaller than experimental results. For studying the magnetooptical Kerr effect (MOKE), a fully relativistic band structure and dielectric tensor are computed. We simulate the change of the MOKE signal under magnetization on the order of 3.98µ_{B} per unit cell by spin flipping and tilting. Kerr rotation angles increase linearly with magnetization, corresponding to spin tilting, while there is no signal without spin tilting due to compensating AFM spin configuration. Our results show that Kerr rotation is three times larger in the UV region than for visible light. 
Monday, March 5, 2018 4:30PM  4:42PM 
C29.00009: Topological Phase Transitions from Relativistic ManyBody Calculations Irene Aguilera, Christoph Friedrich, Stefan Bluegel We discuss topological phase transitions (TPTs) on the basis of relativistic selfconsistent GW (QSGW) calculations where the spinorbit coupling is incorporated into the selfenergy. TPTs can be caused by the variation of the thickness of a sample, the spinorbit strength (alloying [1]), by strain, etc. The well known underestimation of band gaps in standard DFT translates into an overestimation of the inverted band gaps. This indicates that standard DFT is unable to provide the correct critical points of TPTs. As practical examples, we concentrate on semimetals Bi and Sb. In addition to the TPT that Bi undergoes under strain [2], we discuss that a thicknessmediated TPT can also occur. This sheds light on the discrepancies about the topological or trivial character of bulklike samples of Bi. Finally, we simulate Bi_{1x}Sb_{x} alloys varying the Sb concentration to find the critical concentration for which the system becomes a topological insulator. 
Monday, March 5, 2018 4:42PM  4:54PM 
C29.00010: ElectronMagnon Scattering in Elementary Ferromagnets from First Principles: Lifetime Broadening and Kinks Mathias C.T.D. Müller, Christoph Friedrich, Stefan Bluegel We study the electronmagnon scattering in bulk Fe, Co, and Ni within the framework of manybody perturbation theory implemented in the fullpotential linearized augmentedplanewave method. Starting from the GW approximation we obtain a BetheSalpeter equation for the twoparticle (electronhole) Green function, where singleparticle Stoner excitations and collective spinwave excitations (magnons) are treated on the same footing. We employ the solution of the BetheSalpeter equation to construct a selfenergy that describes the scattering of electrons and magnons. The resulting renormalized electronic band structures exhibit strong spindependent lifetime effects close to the Fermi energy, which are strongest in Fe. In the case of Co and Ni, the renormalization gives rise to kinks in the electronic band dispersion at low binding energies, which we attribute to electron scattering with spatially extended spin waves. Furthermore, we find a band anomaly at larger binding energies in iron, which results from a coupling of the quasihole with singleparticle excitations that form a peak in the Stoner continuum. This band anomaly has, in fact, been observed in recent photoemission experiments at the same energy and momentum. 
Monday, March 5, 2018 4:54PM  5:06PM 
C29.00011: Lightmatter interactions in optical cavities beyond the classical Maxwell description Norah Hoffmann, Aaron Kelly, Niko Säkkinen, Christian Schaefer, Angel Rubio, Neepa Maitra, Heiko Appel In common methods for the abinitio description of photoinduced processes typically the classical Maxwell's equations are employed to describe the propagation of light. The applicability of these equations has been demonstrated since decades. However, considering the ultimate limit of single molecules interacting with a few photons, the classical description of the electromagnetic field does not suffice anymore. In this case the quantum nature of the electromagnetic field has to be taken into account and therefore existing abinitio approaches have to be extended. 
Monday, March 5, 2018 5:06PM  5:18PM 
C29.00012: Conceptual insights to the interaction of the electric field and molecule in spectroscopy to further our understanding of molecular dynamics such as isomerization Rachel Glenn Molecular isomerization is one of the most complex and difficult topics in theoretical chemistry to develop accurate models to help interpret molecular spectroscopy. Isomerization is sensitive to the excitation wavelength of the pulse, pulse power, and the molecular environment of a solvent or membrane. From the experimental perspective, it is important to develop conceptual ideas that can further our methods for understanding photoisomerization. My goal is describe conceptual methods and ideas to help advance both theoretical models and experimental methods to better understand molecular dynamics such as isomerization. 
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