Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D23: Focus Session: Theory and Simulation of Excited-State Phenomena in Semiconductors and Nanostructures I |
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Sponsoring Units: DCOMP Chair: Emmanouil Kioupakis, University of Michigan Room: 202B |
Monday, March 2, 2015 2:30PM - 3:06PM |
D23.00001: Modeling of Singlet Fission Kinetics for a Wide Range of Molecules Invited Speaker: Shane Yost Singlet exciton fission is a process that occurs in organic molecules where one high energy singlet exciton decays into two low energy triplet excitons. Over the years since its first discovery in the 1960s a number of different singlet fission materials have been discovered with a wide range of rates and yields. The mechanism for singlet fission in these materials is still not fully understood, and no method is able to accurately reproduce fission rates over a wide range of timescales. In order to gain a better understanding of the singlet fission mechanism a group of fission materials with vastly different crystal structures and fission rates were modeled. Using a first principles expression the rates were computed with constrained density functional theory with configuration interactions. The computed rates successfully predict the fission rates in the different materials studied. For the slow, weak intermolecular coupling materials singlet fission obeys Marcus theory, but for faster, larger intermolecular coupling materials the rate becomes diabatic in nature. This work alters the guidelines for tailoring molecular properties from a focus on crystal packing and intermolecular coupling to properties like solubility and energy level alignment while maintaining the high fission yield required for photovoltaic applications. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D23.00002: $GW$-BSE, self-consistency, and vertex corrections applied to group IB/IIB atoms and oxide molecules Linda Hung, Serdar Ogut Time-dependent density functional theory (TDDFT), the $GW$ approximation, and the Bethe-Salpeter equation (BSE) are often used for the first-principles calculation of excited-state properties of materials that contain transition metals. Accuracy is improved compared to mean-field theories such as Kohn-Sham DFT or Hartree-Fock; however, predicted quasiparticle levels and optical spectra can still differ from experiment. We model Cu, Zn, Ag, and Cd atoms and their oxide molecules to assess various approximations in many-body perturbation theory methods that contribute to these differences. In particular, we examine how self-consistent iterations and/or a two-point vertex function affect the predicted excitation energies, compared to ``one-shot" $G_0W_0$ calculations. Experimental measurements of optical spectra and ionization energies for charged and neutral atoms are widely available, and allow us to evaluate excitations from both $s$ and $d$ states. Differences between TDDFT and BSE spectra are also discussed. Calculations are performed with RGWBS, a software suite which uses a basis of transition space and quasiparticle wavefunctions. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D23.00003: Quasiparticle Band Gap Renormalization in Doped Two-Dimensional Materials Yufeng Liang, Li Yang Recently, atomically thin two-dimensional (2D) materials have emerged as new prototypes for a variety of electronic and optoelectronic devices, for which charge carrier doping is an effective approach for modifying their intrinsic properties. In the process of producing monolayer metal dichalcogenides, doping can occur naturally and may lead to exotic many-body phenomena as evidenced in recent optical experiments. Despite the common occurrence of doping in 2D structures, little knowledge has been obtained for the evolution of the band gap with the carrier concentration, which is key to harnessing the electronic properties and understanding more complicated many-body effects. Here, we investigate how the band gap changes with doping density in various 2D structures. Based on the conventional GW method for semiconductors, we devised and implemented an efficient calculation scheme to capture the unique dielectric screening arising from intraband transitions in low-dimensional structures, specifically MoS2 and MoSe2. We reveal that an enhanced band gap renormalization of a few hundred meV can be achieved and the band gap evolution displays an unusual nonlinear behavior with doping density. Our calculated band gap is in excellent agreement with the recent ARPES experiments on MoSe2. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D23.00004: Band Structures of Plasmonic Polarons Fabio Caruso, Henry Lambert, Feliciano Giustino In angle-resolved photoemission spectroscopy (ARPES), the acceleration of a photo-electron upon photon absorption may trigger shake-up excitations in the sample, leading to the emission of phonons, electron-hole pairs, and plasmons, the latter being collective charge-density fluctuations. Using state-of-the-art many-body calculations based on the `\textit{GW} plus cumulant' approach, we show that electron-plasmon interactions induce plasmonic polaron bands in group IV transition metal dichalcogenide monolayers (MoS$_2$, MoSe$_2$, WS$_2$, WSe$_2$). We find that the energy vs.\ momentum dispersion relations of these plasmonic structures closely follow the standard valence bands, although they appear broadened and blueshifted by the plasmon energy. Based on our results we identify general criteria for observing plasmonic polaron bands in the angle-resolved photoelectron spectra of solids. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 4:18PM |
D23.00005: Recent Progress in GW-based Methods for Excited-State Calculations of Reduced Dimensional Systems Invited Speaker: Felipe H. da Jornada \textit{Ab initio} calculations of excited-state phenomena within the GW and GW-Bethe-Salpeter equation (GW-BSE) approaches allow one to accurately study the electronic and optical properties of various materials, including systems with reduced dimensionality. However, several challenges arise when dealing with complicated nanostructures where the electronic screening is strongly spatially and directionally dependent. In this talk, we discuss some recent developments to address these issues. First, we turn to the slow convergence of quasiparticle energies and exciton binding energies with respect to k-point sampling. This is very effectively dealt with using a new hybrid sampling scheme, which results in savings of several orders of magnitude in computation time. A new \textit{ab initio} method is also developed to incorporate substrate screening into GW and GW-BSE calculations. These two methods have been applied to mono- and few-layer MoSe${}_2$, and yielded strong environmental dependent behaviors in good agreement with experiment. Other issues that arise in confined systems and materials with reduced dimensionality, such as the effect of the Tamm-Dancoff approximation to GW-BSE, and the calculation of non-radiative exciton lifetime, are also addressed. These developments have been efficiently implemented and successfully applied to real systems in an \textit{ab initio} framework using the BerkeleyGW package. \\\\ I would like to acknowledge collaborations with Diana Y. Qiu, Steven G. Louie, Meiyue Shao, Chao Yang, and the experimental groups of M. Crommie and F. Wang. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D23.00006: Ultrafast Hot Carrier Scattering and Generation from Surface Plasmons in Noble Metals Marco Bernardi, Jamal Mustafa, Jeffrey B. Neaton, Steven G. Louie Non-equilibrium \lq\lq hot\rq\rq carriers in materials are challenging to study experimentally as they thermalize at subpicosecond time and nanometer length scale. Recent experiments employed hot carriers generated by light absorption or surface plasmon annihilation in noble metals (e.g., Au and Ag) for catalysis and solar cells. The energy distribution and transport of the generated hot carriers play a key role in these experiments. We present ab initio calculations of the energy distribution of hot carriers generated by surface plasmons in noble metals, and the relaxation time and mean free path of the hot carriers along different crystal directions within 5 eV of the Fermi energy. Our calculations show the interplay of the noble metal $s$ and $d$ bands in determining the damping rate of the plasmon and the mean free path of the hot carriers. The trends we find as a function of surface plasmon momentum and frequency allow us to define optimal experimental conditions for hot carrier generation and extraction. Our approach combines density functional theory, GW, and electron-phonon calculations. Our work provides microscopic insight into hot carriers in noble metals, and their ultrafast dynamics in the presence of surface plasmons. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D23.00007: First-Principles Simulation of Hot Electron Dynamics at Silicon-Molecule Interfaces Lesheng Li, Yosuke Kanai Hot carrier relaxation process at an interface between semiconductor and molecular ligands is of great importance for a number of technological applications ranging from photo-electrochemical cells to quantum-dot light emitting diodes. Although a number of spectroscopic experiments suggest important role of molecular ligands at surface in the hot carrier relaxation, a quantitative understanding has not been developed. We investigate the hot electron relaxation process through synergetic use of first-principles molecular dynamics (FPMD), fewest switch surface hopping (FSSH) algorithm, and GW calculations. Using FSSH stochastic dynamics simulation based on non-adiabatic couplings from FPMD and quasi-particle energy level alignment at the interface, we investigate the role of molecular passivation at silicon (111) surface as a representative example. We will discuss how different types of molecules influence the relaxation process and elucidate important factors controlling the relaxation time scale. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D23.00008: Improved Description of Electron-Plasmon Coupling In Green's Function Calculations Jianqiang Zhou, Lucia Reining Green's function (GF) methods have been very successful for describing one- or two-particle excitations in solids. The GW approximation [1] is a well established approach for describing quasi-particle peaks in the spectral function. Beyond GW, the cumulant expansion, which is based on a hole-boson coupling model, gives a better description of plasmon satellites [2,3]. However, this traditional time-ordered cumulant (TOC) is only valid far from the Fermi level. Recent development [4] of a generalized cumulant (GC) improves the spectral function close to the Fermi level, but a framework for systematically improving is still missing. Here we show how GW, TOC and GC can be derived as a successive series of approximations in a unified way, and how one can go beyond today's state-of-the-art methods. Results for spectral functions and total energies of an exactly solvable model show that systematic improvement is obtained.\\[4pt] [1] L. Hedin, Phys. Rev. 139 A796 (1965).\\[0pt] [2] F. Aryasetiawan, L. Hedin and K. Karlsson, Phys. Rev. Lett. 77 112268 (1996).\\[0pt] [3] M. Guzzo et al, Phys. Rev. Lett. 107 166401 (2011).\\[0pt] [4] J. J. Kas, J. J. Rehr, and L. Reining, Phys. Rev. B 90 085112 (2014). [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D23.00009: Plasmon Pole Approximation within the GW Lanczos approach Vincent Gosselin, Bruno Rousseau, Michel Cote The well-known DFT gap problem is adressed by computational methods that are more ressource intensive both in terms of memory and time requirements. Amongst other methods, the GW approach has known great success in the field of electronic structure calculations. Addressing the main bottlenecks impeding one shot GW calculations, a sum over all conduction states and an integral over all frequencies must be carried. Within an implementation of the GW method based on the Lanczos algorithm, the sum over conduction states is treated with a Sternheimer method whereas the frequency integral is carried out numerically. In this talk, I will present an implementation of a plasmon-pole model combined with the Lanczos method that allows a treatement of this integral that is computationally favorable. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D23.00010: Applications of the Retarded Cumulant Expansion to Realistic Systems J.J. Kas, J.J. Rehr The cumulant expansion of the one-electron Green's function has proved extremely useful in describing electron correlation in materials beyond the one-electron approximation.\footnote{Matteo Guzzo et al. Phys. Rev. Lett. {\bf 107}, 166401 (2011) and Phys. Rev. B {\bf 89}, 085425 (2014).} For example, the approach improves on the GW approximation, accounting for multiple satellites in the spectral function and x-ray photoemission spectra. Previous implementations based on the time ordered representation ignore diagrams which lead to partial occupations and satellite features in the spectral function above and below the Fermi surface. Recently, we have shown that these difficulties can be overcome with a cumulant expansion of the retarded Green's function.\footnote{J. J. Kas, J. J. Rehr, and L. Reining, Phys. Rev. B {\bf 90}, 085112 (2014).} This model was tested on the homogeneous electron gas, giving good results for the spectral function, correlation energies, and occupation numbers. In this follow-up, we discuss the extension of the approach to realistic condensed matter systems, using GW calculations of the self-energy to approximate the cumulant. Results are presented for the spectral function and occupation numbers, and compared to experimental XPS data. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D23.00011: Photoelectron spectra of copper oxide (Cu$_x$O$_y^-$, $x=1-2$, $y=1-4$) clusters from first principles Bin Shi, Shira Weissman, Linda Hung, Leeor Kronik, Serdar Ogut Copper oxide clusters are systems of both technological and fundamental interest. They have unique electronic and optical properties due to the exchange and correlation effects of their $d$ electrons, which also make their modeling from first principles computationally demanding. We optimize the ground-state structures of copper oxide Cu$_x$O$_y^-$ ($x=1-2$ and $y=1-4$) cluster anions using density functional theory (DFT). We compare photoelectron spectra determined at two levels of theory: DFT and the $GW$ approximation. DFT calculations use Perdew-Burke-Ernzerhof (PBE), hybrid, and range-separated exchange-correlation functionals. The calculated photoelectron spectra are compared with available experimental measurements to identify the nature of the observed electronic excitations. [Preview Abstract] |
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