Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B29: First-principles Modeling of Excited-State Phenomena in Materials II: Real-time TDDFTFocus
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Sponsoring Units: DCOMP DMP DCP DCMP Chair: Andre Schleife, University of Illinois at Urbana–Champaign Room: LACC 406A |
Monday, March 5, 2018 11:15AM - 11:51AM |
B29.00001: Recent advances in time-dependent density functional theory for applications to electronic excitations and non-adiabatic dynamics Invited Speaker: Efthimios Kaxiras Expert in Low-Dimensional Systems and TDDFT, which is part of our Focus Topic. |
Monday, March 5, 2018 11:51AM - 12:03PM |
B29.00002: Long Time Behavior of Time-Dependent Density Functional Theory Normand Modine, Cheng-Wei Lee, Andre Schleife We investigate whether real-time TDDFT calculations can capture internal equilibration of an electronic system by performing multi-picosecond TDDFT simulations of aluminum. We monitor the system by calculating the projections of the TDDFT states into the Kohn-Sham eigenvectors of the system. For a system of non-interacting Fermions at equilibrium, the ensemble average of these projections would follow a Fermi distribution, but for interacting systems (e.g., a Fermi liquid), the distribution can be modified. Using a recently published algorithm (Modine and Hatcher, JCP 142, 204111 (2014)), we construct initial TDDFT states whose average projections match a Fermi distribution. During TDDFT propagation of these states, the calculated projections are observed to evolve away from the Fermi distribution to a modified distribution with a sharper drop at the Fermi level and longer tails at high and low energies. |
Monday, March 5, 2018 12:03PM - 12:15PM |
B29.00003: Reproducibility in LR-TDDFT: The Importance of The Continuum of Unoccupied Orbitals Maxime Morinière, Luigi Genovese, Thierry Deutsch The reproducibility of DFT results was addressed recently [1], and other areas of computational physics and chemistry could benefit from the same type of study. As a method building on top of DFT, Linear-Response Time-Dependent DFT (LR-TDDFT) [2] is a potential candidate for such studies. In a typical LR-TDDFT calculation, the inclusion of the continuum of unoccupied orbitals is required, and we will see that this is not without raising difficulties when comparing the results of different codes. We will eventually show that the convergence of LR-TDDFT spectra is only possible over a finite range of energy that depends on the system under consideration. This conclusion allows for a meaningful study of the reproducibility of LR-TDDFT calculations. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B29.00004: Exponential Integrator Methods in Time-Dependent Density Functional Calculations Kalman Varga, Daniel Kidd, Cody Covington The integrating factor and exponential time differencing methods are implemented and tested within one-dimensional time-dependent density functional theory. Popular time propagation methods used in physics are also tested and compared to these exponential integrator methods. We determine an improvement in accuracy of multiple orders of magnitude when describing dynamics driven by nonlinear potentials using fourth-order Runge–Kutta-type exponential integrators. For cases of dynamics driven by a time-dependent external potential, the accuracy of the exponential integrator methods are less enhanced but still match or outperform the best of the conventional methods tested. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B29.00005: Real-time time-dependent density functional theory using higher-order finite-element methods Bikash Kanungo, Vikram Gavini We present a computationally efficient approach to solve the time-dependent Kohn-Sham (TDKS) equations in real-time using higher-order finite-element spatial discretization, applicable to both pseudopotential and all-electron calculations. To this end, we develop an a priori mesh adaption technique, based on semi-discrete error estimate on the time-dependent density, to construct a close to optimal finite-element discretization. We employ spectral finite-elements along with Gauss-Legendre-Lobatto quadrature to render the overlap matrix diagonal, thereby simplifying the inversion of the overlap matrix that features in the evaluation of the discrete propagator. We demonstrate a staggering reduction in the computational time afforded by higher-order finite-elements over linear finite-elements. We also perform a comparative study of the computational efficiency of the proposed method against finite difference (FD) based method and Gaussian basis for pseudopotential and all-electron calculations, respectively. Lastly, we demonstrate the capability and scalability of the proposed method on various large-scale systems. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B29.00006: Negative differential conductivity in liquid aluminum from real-time density functional theory Xavier Andrade, Sebastien Hamel, Alfredo Correa I this talk we will present our approach for the calculation of non-linear electronic conductivy based on time-dependent density functional theory (TDDFT). Based on these simulations, we predict that liquid aluminum exhibits negative-differential conductivity for current densities of the order of 1012−1013 A/cm2. We find that the changes in the conductivity, as the current increases, emerge from a competition between the accumulation of charge around the nuclei, that increases the scattering of the conduction electrons, and a decreasing scattering cross-section at high currents. |
Monday, March 5, 2018 12:51PM - 1:03PM |
B29.00007: Projectile's Core Electrons and Large Values of Electronic Stopping Rafi Ullah, Emilio Artacho, Alfredo Correa Light projectiles (H, He) shooting through solids lose energy at a rate of the order of a few eV/Ang, this value peaking at projectile velocities around 1% of the speed of light. These processes are well reproduced in simulations of electronic stopping using time-dependent density-functional theory in real time. Good agreement with non-trivial experimental results have already been obtained for a variety of hosts materials or liquids. Heavier projectiles as 3d transition metals, however, give rise to electronic stopping powers in the keV/Ang range, and the maximum stopping occurs at velocities several times higher. Here we present results of Ni projectiles shooting through bulk Ni using a plane-wave implementation of TD-DFT(t). The effect of core electrons is investigated, finding that the explicit and flexible consideration of deep core states of the projectile is crucial for an accurate description of the problem. Remarkably, instantaneous semilocal TD-DFT seems to capture the relevant physics in the electron excitation process relevant to electronic stopping. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B29.00008: Charge equilibration and electronic stopping for silicon projectiles in silicon Cheng-Wei Lee, Andre Schleife Understanding the effect of highly energetic particle radiation on semiconductors from first principles is important, e.g. for radiation hardness as well as ion implantation to create quantum bits. Previously we successfully used Ehrenfest molecular dynamics and real-time time-dependent density functional theory to describe electronic stopping during the early stages of radiation damage for light projectiles. Our recent results for heavier (silicon) projectiles traversing silicon bulk crystals show a pronounced dependence of electronic stopping on the initial condition, which we relate to the charge of the projectile. Not only was this effect absent for light projectiles, but on a femtosecond time scale this also depends on whether the target material is a metal or semiconductor. We analyze these recent results in terms of charge equilibration and contributions of core and valence electrons to electronic stopping. Developing a consistent picture from first principles is necessary to describe electronic friction in classical molecular dynamics with predictive accuracy. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B29.00009: Electronic stopping power of metallic clusters in TDDFT Ivan Maliyov, Jean-Paul Crocombette, Fabien Bruneval Irradiation damage in condensed matter is central to many technological fields: from nuclear energy production to medical physics. The interaction between an irradiating ion and a target material is characterized by the energy transfer between them, named the stopping power. Most of the energy transfer is mediated through the electron excitations of the target. This is the electronic stopping power that we aim at calculating from ab initio simulations here. |
Monday, March 5, 2018 1:27PM - 1:39PM |
B29.00010: Hot-electron mediated diffusion in proton-irradiated MgO Cheng-Wei Lee, Andre Schleife Ionizing radiation is known to give rise to enhanced defect diffusion in materials. However, existing models including "thermal spike'' and "ionization-enhanced diffusion'' focus on ion dynamics and neglect the influence of electronic excitations due to electronic stopping. We use time-dependent density functional theory (TDDFT) in Ehrenfest dynamics simulations to describe the first 30 fs after proton impact into MgO directly from first principles. Comparison to Born-Oppenheimer molecular dynamics clearly illustrates the importance of electronic excitations for the emerging ion dynamics. In order to quantify the effect of hot electrons and the resulting modified ion dynamics on migration barriers, we combine TDDFT with constrained DFT and the nudged-elastic band method. Our data shows that hot electrons need to be described explicitly and, during the early stages after particle impact, cannot be approximated by a Fermi temperature or an effective charge state of defects in the material. Due to the large computational cost, these simulations are restricted to short-time dynamics, however, we extract parameters that enable improved multi-scale simulations based on two-temperature molecular dynamics and migration barriers in kinetic Monte-Carlo studies. |
Monday, March 5, 2018 1:39PM - 1:51PM |
B29.00011: Electronic Response of Graphene to Ion Irradiation Alina Kononov, Andre Schleife Graphene and other two-dimensional materials have recently emerged as promising candidates for novel electronic devices. However, these applications often require high-resolution imaging and processing techniques, which typically employ focused ion beams. Thus, achieving finer control of the structure and properties of graphene necessitates a detailed understanding of the excited electron dynamics occurring in the material in response to ion irradiation. Using real-time time-dependent density functional theory and Ehrenfest dynamics, we simulate 10-80 keV protons traversing monolayer and trilayer graphene. We calculate the secondary electron yield, charge transfer, energy transfer, and equilibration time-scales after impact, and we investigate the dependence of these quantities on graphene thickness, projectile velocity, and projectile trajectory. We find that energy transfer is maximized with a proton energy near 80 keV, while electron emission and charge transfer are maximized with a proton energy near 25 keV. |
Monday, March 5, 2018 1:51PM - 2:03PM |
B29.00012: Quadratic response properties from TDDFT: trials and tribulations Shane Parker, Dmitrij Rappoport, Filipp Furche Nonlinear response theory is an increasingly important theoretical tool used to compute, for example, nonlinear optical properties needed to characterize complex materials or electronic couplings between excited states in time-dependent density functional theory (TDDFT). I will present an efficient implementation of the TDDFT quadratic response function, including the static and dynamic hyperpolarizability, two-photon absorption amplitudes, and excited-state absorption amplitudes. The successes and failures (with respect to the quadratic response function's recently detailed incorrect pole structure) of existing nonlinear response methods will be sketched out by computing the dynamic hyperpolarizability of several steroisomers of octupolar calix[4]arenes, the two-photon absorption spectra of a series of twisted conjugated porphyrins, and the excited-state absorption spectra during excimer formation in perylene diimide dimers. I will close with guidelines for practitioners on avoiding spurious resonances in applications, and by discussing possible remedies. |
Monday, March 5, 2018 2:03PM - 2:15PM |
B29.00013: Real-time TDDFT simulation of nonlinear effects in the absorption of intense soft X-rays in solid-state systems Sri Chaitanya Das Pemmaraju Synchrotron based X-ray spectroscopies are well described within linear response theory. The interaction with matter, of intense X-ray pulses from modern X-ray free electron lasers, can extend into the nonliner and non-perturbative regimes and requires theoretical descriptions that go beyond linear response. In this study the velocity gauge real-time TDDFT approach is employed to investigate nonlinear effects in the interaction of intense few femtoscond X-ray pulses with solid-state materials. In particular the role of light-induced transparency and stimulated emission in modulating the absorption signatures of XFEL pulses at light element K-edges and transition metal L-edges is explored through prototypical simulations on Silicon Carbide and metallic Cobalt. Absorption changes driven by nonlinear interactions are contrasted against those induced by lattice and electronic heating effects. Additionally, TDDFT results are compared to predictions from phenomenological optical bloch equation simulations. |
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