Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session R7: First-Principles Modeling of Excited State Phenomena VII: Phonons and Electron DynamicsFocus
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Sponsoring Units: DCOMP DMP DCP Chair: Peihong Zhang, SUNY Buffalo Room: 266 |
Thursday, March 16, 2017 8:00AM - 8:12AM |
R7.00001: Asymmetry in the Ultrafast Dynamics of Excited Electrons and Holes in Gallium Nitride Vatsal Jhalani, Jin-Jian Zhou, Marco Bernardi Wurtzite GaN is the primary material for efficient light emission technology. The radiative processes in GaN are regulated by the dynamics of excited (or so-called “hot”) carriers, through microscopic processes not yet completely understood due to the ultrafast (fs – ps) timescales involved. We present ab initio calculations of hot carrier dynamics in GaN. We compute the electron-phonon (e-ph) interaction from first principles, and include the ab initio Fr\"{o}hlich correction due to the polar phonon modes. Using a novel numerical approach\footnote{J.J. Zhou and Marco Bernardi (2016). arXiv:1608.03514.}, we converge the e-ph relaxation times within 5 eV of the band edges, and find a significant asymmetry in the time scale for the electron and hole relaxation near the band edge. The dynamics of hot electrons and holes injected with up to 1 eV excess energy is investigated by solving the electronic Boltzmann equation, revealing the significant difference in the sub-ps relaxation times of electrons and holes in GaN.\footnote{V. Jhalani, J.J. Zhou, and M. Bernardi. Submitted for publication.} The talk will discuss the technological implications of our findings, as well as highlight the novel parallel algorithms that made these calculations possible. [Preview Abstract] |
Thursday, March 16, 2017 8:12AM - 8:24AM |
R7.00002: Lattice polarization contribution in the quasi-particle self-consistent GW approach. Walter R. L. Lambrecht, Churna Bhandari, Mark van Schilfgaarde The lattice polarization contribution to the screening of the screened Coulomb interaction $W$ of the $GW$ approach should in principle be included and is expected to occur only in the limit ${\bf q}\rightarrow 0$ and for frequencies well below the gap. This effect has been proposed by Botti and Marques [PRL,{\bf 110}, 226404(2013)] to be important for large gap insulators with strong-LO-TO splitting. We have implemented this effect in the QSGW approach using a muffin-tin-orbital basis set, for crystals of arbitrary symmetry. The ${\bf q}\rightarrow0$ contribution to the self-energy requires a careful treatment because of the integrable divergence of the Coulomb interaction, which is here done by means of the offset-$\Gamma$ approach. In the limit ${\bf q}\rightarrow0$, the ratio of the total (lattice + electronic) macroscopic dielectric constant to the electronic contribution only can be represented by a generalized Lyddane-Sachs-Teller relation. We will discuss results for ionic crystals, like MgO, NaCl, and others with large LO-TO splittings SrTiO$_3$ and V$_2$O$_5$. We find the effect to be somewhat smaller than previously proposed and sensitive to the q-point sampling. The question to be addressed is how fast this effect decays as function of $q$ near $q=0$. [Preview Abstract] |
Thursday, March 16, 2017 8:24AM - 8:36AM |
R7.00003: Strain-induced effects in the electronic and spin properties of a monolayer of ferromagnetic GdAg2 Matthieu Verstraete, Bin Xu, Lucia Vitali, Alejandro Correa We report on the structural, electronic and magnetic properties of a monolayer of GdAg2, forming a moiré pattern on Ag(111). Combining scanning tunneling microscopy and ab initio spin-polarized calcu- lations, we show that the electronic band structure can be shifted linearly via thermal dependent strain of the intra-layer atomic distance in a range between 1–7\%, leading to lateral hetero-structuring. Furthermore, the coupling of the incommensurable GdAg2 alloy layer to the Ag(111) substrate leads to spatially varying atomic relaxation causing subsurface layer buckling, texturing of the electronic and spin properties, and inhomogeneity of the magnetic anisotropy energy across the layer. These results provide perspectives for control of electronic properties and magnetic ordering in atomically-thin layers [Preview Abstract] |
Thursday, March 16, 2017 8:36AM - 8:48AM |
R7.00004: Phonon mediated ultrafast spin relaxation of a valley-polarized electron in monolayer MoS2 Dongbin Shin, HoSub Jin, Noejung Park The excited state of a particularly selected spin- and valley-polarized electron is gathering growing interest in terms of the coupling between different degrees of freedom and also in the perspective novel device functionality. The measurement of monolayer of MoS2 is quite much matured, and the time scales of spin relaxation, inter-valley scattering, intra-valley scattering, and electron-hole recombination have been analyzed through circularly polarized pump-prove experiments. The spin relaxation is believed to occur within 100 fs which is distinctly faster than all the other degrees of freedom. Here, we use the real-time propagation time-dependent density functional theory (rtp-TDDFT) method to investigate the microscopic origin of the spin dynamics. We present that the specific phonon, that breaks the mirror symmetry of 2H-phase of MoS2, sharply causes the precession of spins through the strong spin-orbit interaction. Thus the incoherent population of such phonons can cause the temperature-dependent relaxation of the spin polarization. We also discuss the general effect of oscillating magnetic field carried by phonons in the strong spin-orbit coupled solid system. [Preview Abstract] |
Thursday, March 16, 2017 8:48AM - 9:00AM |
R7.00005: Non-adiabatic electron-ion dynamics in proton irradiated aluminum sheets Alina Kononov, Andre Schleife Ion-irradiation of materials enables techniques like ion beam microscopy and can lead to material degradation in space and nuclear technology. Thin or two-dimensional materials respond to ion-irradiation differently than their bulk counterparts, and characterizing this pre-equilibrium response is essential for developing applications. We use Ehrenfest dynamics to simulate a \mbox{25 keV} proton traversing a \mbox{0.8\,--\,1.6 nm} thick aluminum sheet at a proton dose of \mbox{$2.4\times 10^{13}$ cm$^{-2}$}. We analyze the time-dependent electron density to obtain the entrance-side and exit-side secondary electron yields and the orbital occupations of the exiting projectile. From our results for position-dependent stopping in the target, we also compute the effective charge of the projectile inside the target. Finally, we consider the dependence of these quantities on target thickness. Our approach overcomes challenges posed by artificial interaction between entrance-side and exit-side secondary electron densities and integration error accumulated after propagating Kohn-Sham orbitals for thousands of time steps in large simulation volumes. [Preview Abstract] |
Thursday, March 16, 2017 9:00AM - 9:12AM |
R7.00006: Non-adiabatic simulations of terahertz manipulation of electronic excitations of Jahn-Teller active transition metal complexes using non-linear phononics. Oscar Granas Recent progress in laser technology has enabled the control of particular infra red (IR) active vibrational modes in molecules and solids. For cases when the activated IR modes couple to Raman active modes, it is possible to pump Jahn-Teller (JT) modes in compounds that where the electronic configuration is not JT active. Exciting the JT vibrational mode of transition metal complexes that are electronically JT active in the excited state, e.g. [Ni(H$_{2}$O)$_{6}$]$^{2+}$, facilitates a route to manipulate particular electronic excitations by enforcing a vibrational structure of the excited potential energy surface. We employ time-dependent density-functional theory in conjunction with Ehrenfest dynamics to simulate the process, describing ionic motion, external electromagnetic fields and evolution of the electronic wave-functions on the pico-second time-scale with realistic laser fluence.\footnote{G. Kolesov \emph{et al.} J. Chem. Theory Comput.,12, pp 466-476; doi: 10.1021/acs.jctc.5b00969 (2016)} The result sheds light on how coherent terahertz light influences the electronic degrees of freedom in open $d$-shell transition metal complexes. [Preview Abstract] |
Thursday, March 16, 2017 9:12AM - 9:48AM |
R7.00007: Electron-phonon coupling from finite displacements: including electron correlation and higher order terms Invited Speaker: Bartomeu Monserrat The band gap is a central quantity determining many excited-state phenomena of materials, such as their optical properties. The effects of electron correlation on the band gap have been thoroughly investigated for some time, but the effects of electron-phonon coupling, which are fundamental for the study of temperature dependent properties, have received less attention. I will present recent developments in the study of electron-phonon coupling using the finite displacement method\footnote{J.H. Lloyd-Williams and B. Monserrat, PRB {\bf 92}, 184301 (2015)}$^,$\footnote{B. Monserrat, PRB {\bf 93}, 014302 (2016)} that allow us to study electron-phonon coupling beyond semilocal density functional theory and also including terms beyond the lowest order in the interaction. To illustrate the new approach, I will present three examples. First, I will describe the calculation of the temperature dependence of band gaps in semiconductors and insulators using the $GW$ method.\footnote{B. Monserrat, PRB {\bf 93}, 100301(R) (2016)} Second, I will discuss temperature induced band inversions in topological insulators, a description of which requires the inclusion of the spin-orbit interaction in the calculation of electron-phonon coupling.\footnote{B. Monserrat and D. Vanderbilt, arXiv:1608.00584 [cond-mat.mtrl-sci]} Third, I will show that higher-order terms in the electron-phonon interaction must be included for an accurate description of electron-phonon coupling in perovskite solar cell materials. [Preview Abstract] |
Thursday, March 16, 2017 9:48AM - 10:00AM |
R7.00008: Electron-Ion Dynamics in Semiconductors with Defects under Ion Irradiation Cheng-Wei Lee, Andr\' e Schleife Long-term stability is challenging for semiconductor devices under ion radiation such as solar panels in outer space. Exposure to ion radiation induces formation of defects that ultimately reduce solar cell efficiency. It is well-known in the literature that high-energy ion radiation transfers energy to the materials mostly via electronic excitation which is traditionally hard to model. Previously, we demonstrated that Ehrenfest molecular dynamics based on time-dependent density functional theory correctly describes electronic stopping of semiconductors. To better understand the effect of excited electrons on evolution of defects during ion irradiation, we further investigate the time-evolution of occupation number and found that it is correlated to long-term ion dynamics after passage of proton. Furthermore, we found that the presence of excited electrons significantly reduces the atomic diffusion barrier, indicating this effect is essential for the analysis of defect formation and ion dynamics under ion radiation conditions. [Preview Abstract] |
Thursday, March 16, 2017 10:00AM - 10:12AM |
R7.00009: Nonadiabatic dynamics of photo-induced proton-coupled electron transfer reactions via ring-polymer surface hopping. Farnaz Shakib, Pengfei Huo Photo-induced proton-coupled electron transfer reactions (PCET) are at the heart of energy conversion reactions in photocatalysis. Here, we apply the recently developed ring-polymer surface-hopping (RPSH) approach to simulate the nonadiabatic dynamics of photo-induced PCET. The RPSH method incorporates ring-polymer (RP) quantization of the proton into the fewest-switches surface-hopping (FSSH) approach. Using two diabatic electronic states, corresponding to the electron donor and acceptor states, we model photo-induced PCET with the proton described by a classical isomorphism RP. From the RPSH method, we obtain numerical results that are comparable to those obtained when the proton is treated quantum mechanically. This accuracy stems from incorporating exact quantum statistics, such as proton tunnelling, into approximate quantum dynamics. Additionally, RPSH offers the numerical accuracy along with the computational efficiency. Namely, compared to the FSSH approach in vibronic representation, there is no need to calculate a massive number of vibronic states explicitly. This approach opens up the possibility to accurately and efficiently simulate photo-induced PCET with multiple transferring protons or electrons. [Preview Abstract] |
Thursday, March 16, 2017 10:12AM - 10:24AM |
R7.00010: Phonon contribution to RIXS spectra calculated with a cumulant expansion for the quasiparticle Green's function Keith Gilmore, Andrey Geondzhian, Josh Kas Much of the effort in many-body techniques for going beyond standard density functional theory seeks to improve the accuracy of quasiparticle energies, particularly for large or complex systems. A quantity that is sometimes overlooked is the quasiparticle spectral function. Accurately calculating satellite features due to boson excitations is essential for providing a meaningful interpretation of many experimental results, particularly for X-ray spectroscopies. Resonant inelastic x-ray scattering (RIXS) is a relatively new experimental probe of the coupling of electronic states to various excitations in a material such as plasmons, magnons and phonons. The localized nature of the core hole in X-ray spectroscopies allows one to use linked-cluster formulations, as in the seminal work of Nozieres [1], that express the electron Green's function as a cumulant expansion rather than via a Dyson equation. Kas et al. have recently used this approach for electron-plasmon coupling in X-ray photoemission [2] and X-ray absorption [3]. We perform analogous work for the case of coupling to phonons, with a particular focus on RIXS. RIXS is increasingly used to study electron-phonon coupling in unconventional superconductors and it is essential to improve our interpretation of these spectra. TiO2, for which high energy resolution RIXS data was recently reported, serves as our test case. [1] Nozieres and Dominicis, Phys Rev 178, 1097 (1969). [2] Kas et al., Phys Rev B 91, 121112R (2015). [3] Kas et al., Phys Rev B 94, 035156 (2016). [Preview Abstract] |
Thursday, March 16, 2017 10:24AM - 10:36AM |
R7.00011: Dependence of Non-adiabatic Couplings, Energy Levels, and Lattice Ion Movements on Exchange-Correlation Approximations Lesheng Li, Jian Cheng Wong, Yosuke Kanai Dependence on nonadiabatic couplings (NACs), single-particle energy levels, and lattice ion movements on approximated exchange-correlation (XC) functional is examined for modeling excited electron dynamics using fewest switches surface hopping simulations approach. A representative interface between boron nitride and Li$^{\mathrm{\thinspace }}$ion was considered because of its strong charge transfer character, and wave function localization can be quite sensitive to the approximated XC potential. Generalized gradient approximation (PBE) and its hybrid (PBE0) are considered in this work. We investigate the extent to which the excited electron dynamics is influenced by XC approximation and how, through examining NAC, energy levels, and lattice ion movements. [Preview Abstract] |
Thursday, March 16, 2017 10:36AM - 10:48AM |
R7.00012: Resonance Raman intensities including excitonic effects from first-principles: application to 2D materials Yuanxi Wang, Amber McCreary, Jeffrey Simpson, Daniel Rhodes, Luis Balicas, Mauricio Terrones, Angela Walker, Vincent Crespi Recent advances in Resonance Raman (RR) spectroscopy on 2D semiconductors such as monolayer MoS$_2$ and ReS$_2$ discovered a rich variety of frequency- and polarization-dependent Raman signatures. Although the theory for RR has been well developed for the case of graphene, strong excitonic effects present in 2D semiconductors call for further scrutiny on Raman intensities based on first-principle calculations. Compared to existing methods of calculating RR intensities based on finite differences where the Bethe-Salpeter equation (BSE) was solved twice for each degree of freedom, we present a perturbational approach in which the BSE is only solved once statically and can be implemented to model first- and second-order resonance Raman processes. Comparisons with experimental results are discussed, as well as the connection between the finite differences and perturbational approaches. [Preview Abstract] |
Thursday, March 16, 2017 10:48AM - 11:00AM |
R7.00013: Size Dependence of Hot Electron Relaxation Dynamics in Silicon Quantum Dots Jian Cheng Wong, Lesheng Li, Yosuke Kanai The size dependence on the dynamics of hot electron relaxation in fluorine-passivated silicon quantum dots is investigated via first-principles simulations. Using the fewest switches surface hopping simulation based on non-adiabatic couplings and energies from density functional theory calculations, we investigate the extent to which the quantum dot size influences the hot electron dynamics in comparison to the surface passivation. We will also discuss influences of decoherence on the excited electron dynamics. [Preview Abstract] |
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