Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F24: Focus Session: Computational Studies of Interactions between Electromagnetic Fields and Materials I |
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Sponsoring Units: DCOMP Chair: Carsten Ullrich, University of Missouri Room: 326 |
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F24.00001: Coherent phonon generation in time-dependent density functional theory Invited Speaker: George Bertsch We apply the time-dependent density functional theory (TDDFT) to the generation of coherent optical phonons in Si and Sb crystals. The computations are carried out by real-time evolution of the orbital wave functions on a coordinate-space mesh. The theory reproduces the main phenomena observed experimentally: dependence on polarization, strong growth at the direct band gap, and the change in phase from below to above the band gap. Comparing with more phenomenological models, we find that the TDDFT supports the impulsively stimulated Raman mechanism at low frequencies and the qualitative aspects of the displacive mechanism at higher frequencies. We also compare with the more detailed model of displacive excitation by Stevens, Kuhl, and Merlin. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F24.00002: Spectral Representation analysis of dielectric screening in solids and molecules Amandeep Kaur, Erik Ylvisaker, Deyu Lu, Tuan Anh Pham, Giulia Galli, Warren Pickett We propose a new approach to identify and rationalize the contribution of core electron polarization to dielectric screening, based on ab initio calculations of the dielectric matrix in its eigenpotential basis. We also present calculations of phonon frequencies, dielectric constants for alkali hydrides and quasiparticle energies of several sp bonded molecules, and we discuss the quantitative effect of including core polarization. We find that inclusion of semi-core (SC) electrons leads to new eigenmodes in the dielectric matrix with respect to those with valence electron only. These eigenmodes are highly localized in real space. Polarization arising from SC orbitals may contribute 4-6\% to the computed dielectric constants in alkali hydrides, and to differences in QP energies of $\sim$100 meV for sp bonded molecules. Our findings illustrate efficient ways of approximating the spectral decomposition of dielectric matrices used, e.g. in many body perturbation theory and dielectric constant calculations, with substantial computational gains for large systems composed of heavy atoms. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F24.00003: Real Time Dynamical Core-hole Effects in X-ray Spectra A.J. Lee, F.D. Vila, J.J. Kas, J.J. Rehr We present an extension of the real-time x-ray spectroscopy code RTXS\footnote{A. J. Lee \textit{et al.}, Phys. Rev. B \textbf{86}, 115107 (2012)} to introduce dynamic effects due to the sudden creation of a core hole in x-ray absorption (XAS) and emission (XES) spectra. RTXS is based on a local, time-correlation function approach using a real-time extension of the SIESTA code with a Crank-Nicolson time-evolution operator, and projector augmented wave (PAW) transition matrix elements. Originally RTXS used a statically screened core hole, an approximation equivalent to the final state rule as in $\Delta$SCF approaches. To introduce dynamic effects, we now start with the system in the ground state, suddenly introduce the core-hole, and then propagate the system in real time, again with the Crank-Nicolson approach. This implementation yields a generally applicable code that builds in full-potential electronic structure and dynamic core-hole screening. Illustrative examples are presented and compared with initial and final state rule approximations. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F24.00004: New method for calculating the optical absorption spectrum for solids using the transcorrelated method Masayuki Ochi, Shinji Tsuneyuki \textit{Ab initio} calculation of an accurate optical absorption spectrum for solids is a challenging problem, and various methods are proposed for this purpose, such as TDDFT using new functionals and GW+BSE. In this study, we propose a new method for calculating the optical spectra, using the transcorrelated (TC) method[1-6], which is one of the promising wave-function theories. In the TC method, the total wave function is approximated as the Jastrow-Slater-type wave function, and the many-body Hamiltonian is similarity-transformed by the Jastrow factor. Then we solve an SCF equation and optimize one-body orbitals in the Slater determinant with a relatively low computational cost.[6] For excited-state calculations, we use configuration interaction singles (CIS) for the TC method, and will present an optical absorption spectrum of LiF calculated with this method. [1] S. F. Boys and N. C. Handy, Proc. R. Soc. London Ser. A 309, 209 (1969). [2] S. Ten-no, Chem. Phys. Lett. 330, 169 (2000). [3] N. Umezawa and S. Tsuneyuki, J. Chem. Phys. 119, 10015 (2003). [4] R. Sakuma and S. Tsuneyuki, J. Phys. Soc. Jpn. 75, 103705 (2006). [5] H. Luo, J. Chem. Phys. 133, 154109 (2010). [6] M. Ochi, K. Sodeyama, R. Sakuma, and S. Tsuneyuki, J. Chem. Phys. 136, 094108 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F24.00005: Computationally efficient dielectric calculations of molecular crystals Kathleen Schwarz, T.A. Arias The dielectric response is a key quantity for electronic materials such as organic semiconductors. Calculations of the dielectric response for molecular crystals are currently either expensive, or rely on extreme simplifications such as multipole expansions. We present an alternate approach using an analogue of the Clausius-Mossotti equation, which constructs the crystal's dielectric response from an eigenvalue decomposition of the molecular dielectric response. This method can be used to examine the effects of defects and surfaces on the dielectric properties of molecular crystals. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F24.00006: Exciton and trions binding energies in single-layer MoS$_2$: applications of the density-matrix time dependent density Alfredo Ram\'Irez-Torres, Volodymyr Turkowski, Talat S. Rahman Exciton and trion binding energies of a single layer of MoS$_2$ are studied using a time- dependent density-functional theory formalism. Kohn-Sham orbitals of the initial state were obtained using ab initio electronic structure calculations based on density functional theory. Several types of exchange-correlation (XC) kernels are implemented in our code to compare their performance. As expected our results depend crucially on the XC kernels used. In particular, the exchange-only adiabatic local density approximation kernel results in the binding energy about 0.1 eV, which is smaller than those obtained using the GW theory approximation ($\sim$ 0.9 eV) [1]. We have generalized the approach on the case of trion excitations, which gives the trion binding energy $\sim$ 0.3eV when one used the LDA approximation. On the other hand, we demonstrate that the results for the experimental binding energies can be reproduced by using phenomenological local and long-range XC kernels. [1] T. Cheiwchanchamnangij and W. R. L. Lambrecht, Phys. Rev. B \textbf{85}, 205302 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F24.00007: Landau-Zener Tunneling in 1-d periodic potential Jiajun Li, Jong Han Landau-Zener tunneling can be used to model the transition between energy bands of a particle in 1-d periodic potential [1-2]. It is pointed out that a specific model could be utilized to explain the transition driven by a uniform external force, between energy bands in a periodic lattice [3]. Here we examine the transition driven by an external force, in a sinusoidal periodic potential, by solving Schr\"odinger equation numerically. As an exact solution, all bands and transitions between them are included. By considering arbitrary crystal potential of any supercell size, we can approximate random potential scattering and examine how random elastic scattering modifies the inter-band transition and eventually the electron transport. Non-exponential decays and other patterns for different ranges of parameters will be presented. We will also make a connection between the numerical results and conventional Landau-Zener transition model, and show how a time-dependent periodic potential will change the nature of transition. Supported by NSF.\\[4pt] [1] Zener, C., 1932, Proc. Roy. Soc. London Ser. A 137, 696.\\[0pt] [2] C. Zener, A theory of the electrical breakdown of solid dielectrics, Proc. Royal Soc. A 145 (1934) 523.\\[0pt] [3] Q. Niu and M. G. Raizen, Phys. Rev. Lett. 80, 3491(1998) [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F24.00008: Efficient Numerical Modeling of Nonequilibrium Fluctuation Phenomena M. T. Homer Reid, Alejandro Rodriguez, Steven Johnson We present efficient numerical methods for computing non-equilibrium Casimir forces and radiative heat transfer between bodies of complex shapes and realistic material properties. Our methods borrow techniques from computational electromagnetism (specifically, surface integral equations and boundary-element methods) to describe fluctuations in \textit{fields} in terms of fluctuating \textit{sources} on the surfaces of material bodies. We obtain concise formulas expressing forces and heat-transfer rates in terms of traces of matrix products, where the elements of the matrices describe the interactions of tangential currents flowing on the surfaces of the interacting material bodies. Using our methods, we obtain new predictions of nonequilibrium phenomena in geometries that would be difficult or impossible to treat using other methods for modeling nonequilibrium fluctuations. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F24.00009: Solution of electric-field-driven tight-binding lattice in contact with fermion reservoirs Jong Han Electrons in tight-binding lattice driven by DC uniform force field dissipate their energy through on-site fermionic thermostats. Due to the translational invariance in the transport direction, the problem can be block-diagonalized. We solve this time-dependent quadratic problem and demonstrate that the problem has an oscillatory steady-state. The steady-state occupation number shows that the Fermi surface disappears for any damping from the thermostats and any finite electric field. Despite the lack of momentum scattering, the conductivity takes the same form as the semi-classical Ohmic expression from the relaxation-time approximation. Despite the similarity of the Ohm's law with the Boltzmann transport, this solution does not support gradual shift of Fermi surface by drift velocity and, therefore, when used for many-body steady-state calculations, may lead to pathological effects. We discuss extensions of this model for more realistic dissipation mechanisms. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F24.00010: Investigation of quantum-confined Stark effect on exciton binding energy and electron-hole recombination in GaAs qdots Christopher Blanton, Arindam Chakraborty We present the field-dependent explicitly correlated full configuration interaction (XCFCI) method for calculating highly accurate electron-hole wavefunction in presence of external electric field. The XCFCI is a variational method which is based on performing FCI calculation using explicitly correlated reference wavefunction. Field-dependent basis functions were used and were constructed using a variational field-dependent coordinate transformation. A discussion between this method and the variational polaron transformation for spin-boson system will be presented. The exciton energy, exciton binding energy (EB), and electron-hole recombination probability (eh-RP) were computed using XCFCI and the analysis of the scaling laws will be presented. One of the key results is that EB and eh-RP follow very different scaling with respect to the field strength. It was found that for a 500 kV/cm change in the field reduces the EB and eh-RP by a factor of 2.6 and 166, respectively. The explicitly correlated term was found to be crucial for accurate computation of eh-RP and was also responsible for improving convergence of the XCFCI energy with respect to basis size. The field dependent basis functions were found to very important and comparison with field independent basis will be presented [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F24.00011: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F24.00012: A Computational Framework for Cavity Mediated Energy Transfer in Nanostructures Andrew Baczewski, Nicholas Miller, Daniel Dault, Carlo Piermarocchi, Balasubramaniam Shanker Cavity mediated energy transfer is vital to numerous technologies, such as systems that harvest/generate light, quantum information, and platforms for studying strongly coupled cavity QED. In these processes, the density of photonic states through which a donor and acceptor complex exchange energy is dramatically modified by a resonant structure such as a photonic crystal or a distributed Bragg reflector. The design and optimization of new systems of this nature is greatly facilitated by the development of high fidelity numerical methods for resolving the fields in structures not amenable to analytical methods. This is increasingly relevant at the nanoscale, wherein optically dense geometric features exist at or below the scale of the free space wavelength. To this end, we have implemented a nodal Discontinuous Galerkin discretization of the curl-curl Maxwell eigenproblem for the resolution of the spectrum of photonic modes in nanostructures. This framework delivers a high accuracy representation of light-matter coupling constants and optical eigenfrequencies that can be fed into quantum mechanical models of energy transfer. Details of our framework, implementation/validation, and applications germane to energy transfer between cavity-confined quantum dots will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F24.00013: Real-Time TDDFT simulation for coherent phonon generation in crystalline solids Yasushi Shinohara, Shunsuke A. Sato, Kazuhiro Yabana, Tomohito Otobe, Jun-Ichi Iwata, George F. Bertsch We have been developing a theoretical framework to describe electron dynamics in a crystalline solid under an ultrashort laser pulse. We rely upon the time-dependent density functional theory, solving the time-dependent Kohn-Sham equation in real-time and real-space. Using our method, it is possible to describe both linear and nonlinear light-matter interactions in a unified way. In my presentation, I will focus on the application to coherent phonon generation, a coherent atomic oscillation over a macroscopic volume. I will show applications to two material, semiconductor Si and semimetal Sb. For Si, we have found that the TDDFT is capable of describe two distinct mechanisms of the coherent phonon generation. When the laser frequency is below the direct bandgap, virtual electronic excitation induces impulsive force to atoms. When the laser frequency is above the gap, real electronic excitation causes the atomic motion. For Sb, we study the frequency dependence of the coherent phonon generation and compare our results with phenomenological theories. [Preview Abstract] |
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