Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C29: First-principles Modeling of Excited-State Phenomena in Materials III: Phonons, Spins, DynamicsFocus
|
Hide Abstracts |
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 Tamm-Dancoff 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 Tamm-Dancoff 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 charge-neutral excitations using geminal-screened electron-hole interaction kernel in the Bethe-Salpeter and TDDFT formulations Mike Bayne, Arindam Chakraborty The electron-hole or quasiparticle representation plays a central role in describing electronic excitations in many-electron systems. Accurate determination of the electron-hole 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 geminal-screened electron-hole interaction kernel (GSIK) method that avoids using unoccupied states to construct the electron-hole interaction kernel. Using diagrammatic techniques, we demonstrate that an infinite-order summation of a subset of particle-hole diagrams that appear in MBPT can be expressed as a renormalized two-body operator in real-space representation. The GSIK was combined with both BSE and linear-response 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 non-adiabatic molecular dynamics Thomas Niehaus Trajectory surface hopping combined with time-dependent density functional theory (TD-DFT) is a well established technique to investigate the non-adiabatic 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, TD-DFT 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 Galitskii-Migdal-Koltun sum-rule. 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 Monte-Carlo (QMC) calculations of exchange-correlation 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 spin-polarization, the exchange correlation free-energy and entropy, together with comparisons to QMC data and DFT fits. In addition, we extend the static COHSEX approximation of the self-energy 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 Refaely-Abramson, Jeffrey Neaton Singlet fission, a process in which a photoexcited singlet exciton rapidly decays to a bi-triplet exciton, is of recent interest for achieving beyond Shockley-Queissier 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 many-body perturbation theory within the GW approximation and the Bethe-Salpeter 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 TIPS-pentacene. 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: Phonon-driven spin dynamics in the valleys of TMDC Dongbin Shin, Hannes Huberner, Umberto Giovannini, Hosub Jin, Angel Rubio, Noejung Park Non-conventional device functionalities of two-dimensional materials, based on their extraordinary quantum mechanical properties, have fascinated many researchers in various fields. Among them, the exfoliated few-layer transition metal dichalcogenides have recently attracted focused attention owing to their valley degree of freedom. Here, using the time-dependent density functional theory, we investigated the phonon-driven dynamics of a spin state at the valleys of monolayer MoS2. 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 phonon-dressed spinors whose time-periodicity is defined by the phonon period. When the phonon is pumped with a circular polarity so as to break the system’s time-reversal symmetry, the dichroic responses of the phonon-coupled spins on both valleys lead to a net non-zero 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 wide-band limit Fabio Covito, Florian Eich, Riku Tuovinen, Michael Sentef, Angel Rubio Thanks to recent advances in ultra-fast pump-probe spectroscopies and nano-thermometry 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 wide-band limit is a commonly used approximation. Here we investigate the applicability of the wide-band 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 long-time steady-state charge and heat transport, it fails to characterize the short-time 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 magneto-optical Kerr effect of antiferromagnetic L10-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 first-principles calculations based on density functional theory and the spin spiral approximation to study the magnon dispersion of AFM L10-type MnPt. Néel temperatures computed using the mean-field/random-phase approximation are 1.12%/20.9% smaller than experimental results. For studying the magneto-optical 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 3.98µB 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 Many-Body Calculations Irene Aguilera, Christoph Friedrich, Stefan Bluegel We discuss topological phase transitions (TPTs) on the basis of relativistic self-consistent GW (QSGW) calculations where the spin-orbit coupling is incorporated into the self-energy. TPTs can be caused by the variation of the thickness of a sample, the spin-orbit 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 thickness-mediated TPT can also occur. This sheds light on the discrepancies about the topological or trivial character of bulk-like samples of Bi. Finally, we simulate Bi1-xSbx 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: Electron-Magnon Scattering in Elementary Ferromagnets from First Principles: Lifetime Broadening and Kinks Mathias C.T.D. Müller, Christoph Friedrich, Stefan Bluegel We study the electron-magnon scattering in bulk Fe, Co, and Ni within the framework of many-body perturbation theory implemented in the full-potential linearized augmented-plane-wave method. Starting from the GW approximation we obtain a Bethe-Salpeter equation for the two-particle (electron-hole) Green function, where single-particle Stoner excitations and collective spin-wave excitations (magnons) are treated on the same footing. We employ the solution of the Bethe-Salpeter equation to construct a self-energy that describes the scattering of electrons and magnons. The resulting renormalized electronic band structures exhibit strong spin-dependent 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 single-particle 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: Light-matter 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 ab-initio 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 ab-initio 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 photo-isomerization. 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. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700