Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H24: ManyBody Perturbation Theory for Electronic Excitations: Excitonic PhenomenaFocus

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Sponsoring Units: DMP Chair: Serdar Ogut, University of Illinois at Chicago Room: 323 
Tuesday, March 15, 2016 2:30PM  3:06PM 
H24.00001: A direct approach to the calculation of manybody Green' s functions: quasiparticles and more Invited Speaker: Lucia Reining Manybody perturbation theory is a powerful approach to describe many properties of materials. Most often one uses Dyson equations with selfenergy kernels that are approximated to low order in the interaction. In Hedin's GW approximation, for example, the selfenergy is a product of the onebody Green's function and the screened Coulomb interaction. This is the stateofthe art method for bandstructure calculations in a wide range of materials. However, sometimes the GW approximation and related approaches are not sufficient, for example when one is interested in satellite structure beyond the quasiparticle peaks in the spectral function, or in the case of strong coupling, where the quasiparticle picture is no longer adequate. We explore an alternative route to the calculation of interacting electron Green's functions. It is based on a set of functional differential equations relating the onebody Green’s function to its functional derivative with respect to an external perturbing potential [1]. This set of equations can be used to generate the perturbation series. Here we will show that working directly with the differential equations yields precious insight concerning some fundamental questions, guidelines for practical calculations, and methods that lead to an improved description of spectra, in particular advanced versions of the cumulant expansion. Results will be illustrated on various levels of approximation starting from simple models [2], but with a focus on full ab inito calculations [3] and comparison with, and interpretation of, experiment. In particular, we will discuss various kinds of photoemission satellites, and also address questions linked to strong correlation. [1] L.P. Kadanoff and G. Baym, Quantum Statistical Mechanics (New York: Benjamin, 1962) [2] A. Stan et al., New J. Phys. 17, 093045 (2015) [3] M. Guzzo et al., Phys. Rev. Lett. 107, 166401 (2011); Phys. Rev. B 89, 085425 (2014) [Preview Abstract] 
Tuesday, March 15, 2016 3:06PM  3:18PM 
H24.00002: Multiple Exciton Generation in Semiconductor Nanostructures: DFTbased Computation Deyan Mihaylov, Andrei Kryjevski, Dmitri Kilin, Svetlana Kilina, Dayton Vogel Multiple exciton generation (MEG) in nmsized Hpassivated Si nanowires (NWs), and quasi 2D nanofilms depends strongly on the degree of the core structural disorder as shown by the perturbation theory calculations based on the DFT simulations. In perturbation theory, we work to the 2$^{nd}$ order in the electronphoton coupling and in the (approximate) RPAscreened Coulomb interaction. We also include the effect of excitons for which we solve BetheSalpeter Equation. To describe MEG we calculate excitontobiexciton as well as biexcitontoexciton rates and quantum efficiency (QE). We consider 3D arrays of Si29H36 quantum dots, NWs, and quasi 2D silicon nanofilms, all with both crystalline and amorphous core structures. Efficient MEG with QE of 1.3 up to 1.8 at the photon energy of about 3$E_{gap}$ is predicted in these nanoparticles except for the crystalline NW and film where QE$\simeq$1. MEG in the amorphous nanoparticles is enhanced by the electron localization due to structural disorder. The exciton effects significantly redshift QE vs. photon energy curves. Nmsized aSi NWs and films are predicted to have effective MEG within the solar spectrum range. Also, we find efficient MEG in the chiral singlewall Carbon nanotubes and in a perovskite nanostructure. [Preview Abstract] 
Tuesday, March 15, 2016 3:18PM  3:30PM 
H24.00003: Excitons in solids with timedependent densityfunctional theory: the bootstrap kernel and beyond YoungMoo Byun, Zenghui Yang, Carsten Ullrich Timedependent densityfunctional theory (TDDFT) is an efficient method to describe the optical properties of solids. Lately, a series of bootstraptype exchangecorrelation (xc) kernels have been reported to produce accurate excitons in solids, but different bootstraptype kernels exist in the literature, with mixed results. In this presentation, we reveal the origin of the confusion and show a new empirical TDDFT xc kernel to compute excitonic properties of semiconductors and insulators efficiently and accurately. Our method can be used for highthroughput screening calculations and large unit cell calculations. [Preview Abstract] 
Tuesday, March 15, 2016 3:30PM  3:42PM 
H24.00004: Stability of excitonic complexes in a multivalley/band semiconductor Hiroki Katow, Junko Usukura, Ryosuke Akashi, Kalman Varga, Shinji Tsuneyuki Whether bound states are present for fewparticle quantum systems is far from axiomatic and has been a hot topic for decades. For example, threepositronium and /hydrogen bound states are not present in the vacuum. On the other hand, it has also been proposed that three excitons can be bound with each other in multivalley/band semiconductors [J. S. Wang {\&} C. Kittel, Phys. Lett. 42A, No. 3 (1972)]. Indeed, an array of photoluminescence peaks have been recently observed in diamond [J. Omachi et al., Phys. Rev. Lett. 111, 026402(2013)], which could suggest the existence of possible multiexciton bound states. We theoretically examine if such bound states are possible by a variational method. For the electronhole Hamiltonian including the valley and band degrees of freedom, we expressed trial manybody wave function with the correlated Gaussian bases and optimized it with the stochastic variational method [J. Mitroy et al., Rev. of Mod. Phys., 85, 2013]. We have shown bound states for Nexciton systems with N more than two. In the talk, we discuss the dependence of the bound states on the model settings and its relation to the experimental observation. [Preview Abstract] 
Tuesday, March 15, 2016 3:42PM  3:54PM 
H24.00005: Exploring the nature of lowlying excitedstates in molecular crystals from manybody perturbation theory beyond the TammDancoff Approximation Tonatiuh Rangel, Sahar Sharifzadeh, Andre Rinn, Felipe H. da Jornada, Meiyue Shao, Gregor Witte, Chao Yang, Steven G. Louie, Sangaam Chatterjee, Leeor Kronik, Jeffrey B. Neaton Organic semiconductors have attracted attention due to their potential for optoelectronics and novel phenomena, such as singlet fission. Here, we use manybody perturbation theory to simulate neutral excitations in acene and perylene crystals. By diagonalizing the full BetheSalpether (BSE) Hamiltonian beyond the Tamm Dancoff approximation (TDA) [1], we find that both lowlying excitation energies and oscillator strengths are in improved agreement with experiments relative to the TDA. We characterize the lowlying excitons, focusing in the degree of chargetransfer and spatial delocalization, connecting their relevance to singlet fission. [2] For perylene, we find overall good agreement with absorption measurements, and we see evidence for the formation of an “excitonpolariton” band in $\beta$perylene. 1. da Jornada, F. H. et al., to be submitted. 2. Sharifzadeh. S. et al., J. Phys. Chem. Lett. 4, 2197 (2013). [Preview Abstract] 
Tuesday, March 15, 2016 3:54PM  4:06PM 
H24.00006: ABSTRACT WITHDRAWN 
Tuesday, March 15, 2016 4:06PM  4:18PM 
H24.00007: Effect of Crystal Packing on the Excitonic Properties of Rubrene Polymorphs Xiaopeng Wang, Taylor Garcia, Stephen Monaco, Bohdan Schatschneider, Noa Marom Singlet fission, the downconversion of one singlet exciton into two triplet excitons, has been recently observed in molecular crystals of rubrene. The orthorhombic form of rubrene is the most stable in ambient conditions. However, rubrene has two additional known polymorphs, a triclinic form and a monoclinic form. To investigate the relative stability of the three polymorphs under different temperature and pressure conditions we use dispersioninclusive density functional theory (DFT) with the pairwise TkatchenkoScheffler (TS) method and the manybody dispersion (MBD) method. Manybody perturbation theory is then employed to study the effect of crystal structure on the electronic and excitonic properties. Band structures are calculated within the GW approximation, where G is the oneparticle Green's function and W is the screened Coulomb interaction, and optical properties are calculated by solving the BetheSalpeter equation (BSE). We find that crystal packing affects the band gaps, band dispersion, optical gaps, singlettriplet gaps, and exciton localization in the three polymorphs of rubrene. Singlet fission efficiency may thus be improved by modulating the crystal packing. [Preview Abstract] 
Tuesday, March 15, 2016 4:18PM  4:30PM 
H24.00008: Multiscale modeling of excitation dynamics in molecular materials with GWBSE/MM Bjoern Baumeier Processes involving electronic excitations govern the functionality of molecular materials in which the dynamics of excitons and charges is determined by an interplay of molecular electronic structure and morphological order. To understand, e.g., charge separation and recombination at donoracceptor heterojunctions in organic solar cells, knowledge about the microscopic details influencing these dynamics in the bulk and across the interface is required. For heterojunctions of smallmolecule donor materials with C60, we employ a hybrid QM/MM approach [JCTC 7, 3335 (2011)]$^{\mathrm{\thinspace }}$ linking densityfunctional and manybody Green's functions theory$^{\mathrm{\thinspace }}$[JCTC 8, 2790 (2012)] (DFT/GWBSE) to polarizable forcefields$^{\mathrm{\thinspace }}$[JCTC 10, 3140 (2014)] and analyze the charged and neutral electronic excitations therein. We develop models for both static and dynamic properties of the excitations, including (a) the diffusion of Frenkel excitons and (b) the relative energies of Frenkel and chargetransfer excitations at the donoracceptor interface and the resulting charge separation dynamics. Our simulations allow linking the molecular architecture of the donor material, its orientation on the fullerene substrate as well as mesoscale order [Nat. Mater. 14, 434 (2015)] to the solar cell performance. [Preview Abstract] 
Tuesday, March 15, 2016 4:30PM  4:42PM 
H24.00009: Excited Biexcitons in Transition Metal Dichalcogenides David Zhang Recently, experimental measurements and theoretical modeling have been in a disagreement concerning the binding energy of biexctions in transition metal dichalcogenides.\footnote{Y. You, X.X. Zhang, T. C. Berkelbach, M. S. Hybertsen, D. R. Reichman, T. F. Heinz, \textbf{Nat. Phys.} 11 477481 (2015).} While theory predicts a smaller binding energy ($\sim$20 meV) that is, as logically expected, lower than that of the trion, experiment finds values much larger ($\sim$60 meV), actually exceeding those for the trion. In this work, we show that there exists an excited state of the biexciton which yields binding energies that match well with experimental findings and thus gives a plausible explanation for the apparent discrepancy. Furthermore, it is shown that the electronhole correlation functions of the ground state biexciton and trion are remarkably similar, possibly explaining why a distinct signature of ground state biexcitons would not have been noticed experimentally.\footnote{D. K. Zhang, D. W. Kidd, K. Varga, \textbf{Nano. Lett.} Article ASAP (2015).} [Preview Abstract] 
Tuesday, March 15, 2016 4:42PM  4:54PM 
H24.00010: Exciton band structure in twodimensional materials Pier Luigi Cudazzo, Lorenzo Sponza, Christine Giorgetti, Lucia Reining, Francesco Sottile, Matteo Gatti In lowdimensional materials the screening of the Coulomb interaction is strongly reduced[1,2]. As a consequence, the binding energy of both Wannier and Frenkel excitons in the optical spectra is large and comparable in size. Therefore, contrarily to bulk materials, it cannot serve as a criterion to distinguish different kinds of excitons. Here we demonstrate that the exciton band structure, which can be accessed experimentally, instead provides a powerful way to identify the exciton character. By comparing the ab initio solution of the manybody BetheSalpeter equation for graphane and singlelayer hexagonal BN, we draw a general picture of the exciton dispersion in twodimensional materials, highlighting the different role played by the exchange electronhole interaction and by the hopping terms related to the electronic band structure. 1 Pierluigi Cudazzo et. al. Phys. Rev. Letter 104 226804 (2010) 2 Pierluigi Cudazzo et. al. Phys Rev. B 84, 085406 (2011) [Preview Abstract] 
Tuesday, March 15, 2016 4:54PM  5:06PM 
H24.00011: Effect of Lattice Screening on Excitonic and Optical Properties in CH$_3$NH$_3$PbI$_3$ Solar Cell Materials Joshua Leveillee, Andre Schleife Hybrid OrganoMetallic Perovskites have made great strides towards becoming a next generation solar cell material. Though high performing experimental devices have been constructed from these perovskites, the fundamental optical and electronic physics of these systems remains an active area of research. A large lattice dielectric constant in the Methylammonium(CH$_3$NH$_3$)Lead(Pb)Iodide(I$_3$) system potentially contributes to the screening of the electronhole interaction. The strongly increased dielectric screening due to lattice contributions has been suggested to reduce the exciton binding energy and strongly effects the optical band gap. In this study, we seek to understand, from first principles, the interplay between lattice screening and exciton binding energy. We use density functional theory for ground state calculations and the BetheSalpeter equation for the optical polarization function, from which we calculate optical spectra and excitonic properties. We will discuss differences between lattice and electronic screening and the effect on the optical properties of multiple CH$_3$NH$_3$PbI$_3$ phases. [Preview Abstract] 
Tuesday, March 15, 2016 5:06PM  5:18PM 
H24.00012: Substrateinduced renormalization of the quasiparticle and optical gaps in monolayer transition metal dichalcogenides from GW and GWBSE calculations. Felipe H. da Jornada, Chin Shen Ong, Diana Y. Qiu, Steven G. Louie There has been a considerable effort to experimentally characterize the electronic and optical properties of novel atomically thin 2D semiconductors, such as mono and fewlayer transition metal dichalcogenides (TMDs). However, the role that different substrates play in these experiments still remains unclear. From a theoretical perspective, it is hard to include the substrate in an \textit{ab initio} framework, while in experiments, it is often difficult to suspend these samples. Here, we present a new method to compute the substrate effect on the quasiparticle and optical properties of quasi2D materials based on stateoftheart \textit{ab initio} GW and GW plus BetheSalpeter equation (GWBSE) methods. We compute the effects of different metallic and semiconducting substrates, and show that the quasiparticle gap and exciton binding energy can be dramatically reduced even with semiconducting substrates. This work was supported by the National Science Foundation under Grant No. DMR151508412 and the DOE under Contract No. DEAC0205CH11231. [Preview Abstract] 
Tuesday, March 15, 2016 5:18PM  5:30PM 
H24.00013: Nonanalyticity, Valley Quantum Phases, and Massless Excitons in Monolayer Transition Metal Dichalcogenides Diana Y. Qiu, Ting Cao, Steven G. Louie Exciton dispersion as a function of centerofmass momentum \textbf{Q} is essential to the understanding of exciton dynamics, relaxation, and condensation. We use the ab initio GWBetheSalpeter equation(GWBSE) method to calculate the dispersion of excitons in monolayer MoS$_{2}$ and find a nonanalytic lightlike dispersion. This behavior arises from the interplay of an unusual $Q$term in both the intra and intervalley exchange of the electronhole interaction, which concurrently gives rise to a valley quantum phase of winding number two. We have derived a simple, effective Hamiltonian and analytic solutions, which quantitatively describe this physics, and we predict that signatures of this unusual dispersion can be measured with a linearly polarized optical beam tilted away from normal incidence. The existence of a nonanalytic exciton dispersion can be generalized to other 2D semiconductors with excitons whose amplitudes are localized in a small region of the Brillioun zone. [Preview Abstract] 
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