Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H29: Firstprinciples Modeling of ExcitedState Phenomena in Materials VI: Solids and Layered MaterialsFocus

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Sponsoring Units: DCOMP DMP DCP DCMP Chair: Amanda J. Neukirch, Los Alamos National Laboratory Room: LACC 406A 
Tuesday, March 6, 2018 2:30PM  3:06PM 
H29.00001: New Insights into Single and MultiExciton Phenomena in Complex Materials from Ab Initio ManyBody Perturbation Theory Invited Speaker: Sivan RefaelyAbramson Theoretical predictions of excitedstate phenomena in complex materials can lead to better understanding of nanoscale energy conversion mechanisms, for instance in emerging photovoltaic and photocatalytic systems. In this talk, I will discuss recent studies using new ab initio manybody perturbation theory methods within the GW approximation and the Bethe Salpeter equation approach (GWBSE) to understand and uncover such mechanisms. In one example, I will present a new approach to calculate multiexciton generation processes in solids from first principles, without empirical input, used to study singlet fission in organic crystals. Applying this approach to crystalline pentacene, we discovered a new exciton—biexciton coupling channel, one that is purely Coulombic, with a predicted decay rate comparable to experiments; our results led to new understanding of the role of symmetry and structure in the singlet fission mechanism in the solid state. Additionally, I will discuss our recent progress in calculating excited state properties in complex systems of reduced dimensionality. Selected results will be presented for two materials: monolayer transition metal dichalcogenides with point defects, where the calculated GWBSE absorbance is strongly affected by the presence of localized defect states; and a new class of multilayered hybrid chalcogenides with 2Dlike excitonic behavior that is strongly coupled to their unique structure and chemistry. 
Tuesday, March 6, 2018 3:06PM  3:18PM 
H29.00002: GW and GWBSE Methods with Broken Time Reversal Symmetry and Their Applications in Magnetic Systems Meng Wu, Zhenglu Li, Steven Louie The ab initio GW and GWBSE methods based on manybody perturbation theory (MBPT) play an important role in understanding and predicting the electronic and optical properties of materials. However, broken timereversal symmetry in magnetic systems poses difficulties in both the formalism and its implementation. Moreover, a Hubbard onsite Coulomb interaction at the DFT level will introduce ambiguous starting point for MBPT calculations. In this work, we extend the GW and GWBSE methods to systems with broken timereversal symmetry and strong spinorbit coupling, and avoid the doublecounting issue from the onsite Coulomb potential. This method can be applied to complex magnetic systems or reduceddimensional systems such as 2D magnets. 
Tuesday, March 6, 2018 3:18PM  3:30PM 
H29.00003: Ordering of the Γ_{6}, Γ_{7}, and Γ_{8 }States, the Band Gap, and the Electron Effective Mass in βHgS via GW Calculations Bradford Barker, Steven Louie Materials with strong spinorbit coupling may have qualitatively different quasiparticle bandstructures than those calculated with density functional theory. One such solid is βHgS, the zincblende phase of HgS. Unlike HgSe and HgTe, βHgS has a band gap. However, the atomic character of the lowenergy states has been calculated within the GW approach with two different orderings of the Γ_{6}, Γ_{7}, and Γ_{8 }states. Previous calculations incorporating spinorbit coupling perturbatively find an ordering of 8, 7, then 6 (from lowest energy to highest), while previous calculations using the fullyrelativistic GW approach find an ordering of 6, 8, then 7. We present a bandstructure calculated within the fullyrelativistic GW approach that agrees with the ordering of 8, 7, then 6. We also calculate, with high agreement to experiment, the band gap and electron effective mass. 
Tuesday, March 6, 2018 3:30PM  3:42PM 
H29.00004: Layerresolved optical absorption in singleinorganic layer πconjugated 2D hybrid perovskites from first principles Joshua Leveillee, Amanda J. Neukirch, Andre Schleife, Sergei Tretiak Layered 2D hybrid organicinorganic perovskite materials have provided a more chemically stable and optically tunable alternative to the traditional 3D hybrid perovskites. More recently, attention has shifted towards optical engineering of the organic layer by introducing πconjugated molecules and chromophores to achieve excitedcarrier transfer from the perovskite to the organic groups. In the limit of alternating single inorganic and organic layers, joint optical response could be maximized. In this study, we employ density functional theory and the BetheSalpeter equation to predict the ionresolved optical response in ammoniumpropylimidazole (API)PbX_{4} (X=I, Br, Cl) materials. We determine in which layers electrons and holes are excited as a function of photon energy. This analysis reveals a strong preference for electron and hole confinement within the perovskite layer at visible photon energies and organicperovskite layer sharing of electrons and holes under nearUV photon energies. These results provide new insight into the limitations of simple πconjugated organic layer participation for visible light absorption in layered hybrid perovskite systems. 
Tuesday, March 6, 2018 3:42PM  3:54PM 
H29.00005: Optical properties of layered nonequilibrium ZnO from first principles Xiao Zhang, Andre Schleife The boronnitride (BN) phase has been reported for zinc oxide (ZnO) nanostructures and thin films. With the layered graphiticlike structure it exhibits, BNZnO has very different optical properties than the equilibrium wurtzite (WZ) phase of bulk ZnO. We use density functional theory to obtain the equilibrium structure of both polymorphs and we solve the BetheSalpeter equation to obtain accurate optical spectra. We found a larger band gap and larger optical anisotropy for BNZnO, compared to WZZnO. By performing Maxwell's equations simulations, we show that the anisotropy in BNZnO leads to clear differences at the absorption onset for thin films. Our results indicate that possible existence of BNZnO in nanostructures leads to increased transmission in ultraviolet region, that can be used to optically distinguish both phases. By comparing to experiment, we show that for ultrathin films, better agreement can be obtained when accounting for BNZnO, rather than pure WZZnO. 
Tuesday, March 6, 2018 3:54PM  4:06PM 
H29.00006: Ab Initio Radiative Lifetimes in Gallium Nitride Vatsal Jhalani, HsiaoYi Chen, Maurizia Palummo, Marco Bernardi Wurtzite GaN is the primary semiconductor for efficient solid state lighting. Light emission in materials is regulated by the dynamics of excited carriers, which is not completely understood in GaN. In particular, due to the ultrafast (fs – ps) timescales at play and to the presence of defects and interfaces in devices, the intrinsic radiative recombination rate is challenging to measure in GaN. Here, we present ab initio calculations of the radiative lifetime as a function of temperature in bulk GaN. We compute the exciton energies and wavefunctions using a combination density functional theory and the GWBethe Salpeter equation method. An equation for the ab initio temperature dependent radiative lifetime in a bulk crystal is derived using Fermi’s Golden rule and applied to GaN. Combined with previous first principles calculations of excited carrier relaxation in GaN [1], we can obtain from first principles key device parameters such as the hot carrier cooling time and the carrier diffusion lengths, with important technological implications. 
Tuesday, March 6, 2018 4:06PM  4:18PM 
H29.00007: First Principles Analysis on the Excitonic Properties of MoS_{2}/WS_{2} and MoSe_{2}/WSe_{2} Heterobilayers Engin Torun, Alejandro Sánchez, Henrique Miranda, Ludger Wirtz By using the method of many body perturbation theory (MBPT), we investigate the optical absorption spectra and the excitonic properties of MoS2/WS2 and MoSe2/WSe2 heterobilayers. We first correct the independent particle band gap by performing GW calculation on top of ground state density functional theory (DFT) calculations and the optical spectra of the bilayers are calculated by solving BetheSalpeter equation (BSE). Two different types of excitons present in this type of heterobilayers; the interlayer excitons (electron and hole locate in different layers) and the intralayer ones (electron and hole locate in the same layer). Relative spectral position of inter and intralayer excitons w.r.t. each other is determined by the band alignment of the constituent layers and binding energy of the excitons. The spectral position of the interlayer exciton w.r.t. intralayer one is crucial for the low energy optical response and the charge carrier dynamics of the heterobilayers. In this work, we show a detail analysis on inter and intralayer excitons in MoS_{2}/ WS_{2} and MoSe_{2}/WSe_{2} heterobilayers. We investigate their binding energy and origin as well as possible scenarios which might lead to alter the character of the lowest excitonic peak in the optical spectra of the bilayers. 
Tuesday, March 6, 2018 4:18PM  4:30PM 
H29.00008: Excitedstate Dynamics and the Role of ElectronPhonon Interactions in Quantum Materials Prineha Narang Integrated architectures at the atomicscale can be achieved via 2D materials and their corresponding van der Waals heterostructures with deterministic defect engineering. In this talk I will show several examples of ab initio designed quantum materials via an understanding of the electronphonon interactions in these. I will demonstrate our new carrier lifetimedriven approach to materials with an ab initio description of electronphonon and electronoptical interactions in a Feynman diagram manybody framework integrated with a nonequilibrium carrier transport theory method. Finally, I will give an outlook on theorydirected control of excited state and nonequilibrium phenomena to deliver integrated quantumengineered materials with diverse applications in quantum information science as well as quantum sensing and metrology. 
Tuesday, March 6, 2018 4:30PM  4:42PM 
H29.00009: Picosecond Electronic and Structural Dynamics in Photoexcited Monolayer MoSe_{2} Lindsay Bassman, Aravind Krishnamoorthy, Rajiv Kalia, Aiichiro Nakano, Priya Vashishta, Hiroyuki Kumazoe, Masaaki Misawa, Fuyuki Shimojo Monolayers of semiconducting transition metal dichalcogenides (TMDC) are emerging as strong candidate materials for next generation electronic and optoelectronic devices. Prior studies have demonstrated strong lightmatter interactions in these materials which suggests optical control of material properties is a promising route for their functionalization. However, the electronic and structural dynamics in response to electronic excitation have not yet been fully elucidated. In this work, we use nonadiabatic quantum molecular dynamics based on timedependent density functional theory to study lattice dynamics of a model TMDC monolayer of MoSe_{2} after electronic excitation and explore the dependence of dynamics on photogenerated electronhole density. We observe phonon mode softening induced by Fermisurface nesting, as well as increasing lattice disorder, as measured by the DebyeWaller factor (DWF), with increasing excitation. Furthermore, we find a transition from singleexponential to biexponential decay of the DWF at higher electronhole densities. 
Tuesday, March 6, 2018 4:42PM  4:54PM 
H29.00010: Simulation of ultrafast spin and valley dynamics in twodimensional materials Alejandro MolinaSanchez, Davide Sangalli, Stefano Dal Conte, Giulio Cerullo, Ludger Wirtz, Andrea Marini We present simulations of how optical experiments probe the spin and valley dynamics in twodimensional materials  such as singlelayer transition metal dichalcogenides (MoS_{2}, WS_{2} and WSe_{2}). For the simulations, we have developed an ab initio implementation of timedependent manybody perturbation theory. Simulations include the photogeneration of electrons and holes and the subsequent carrier dynamics, including the scattering mechanisms induced by electronphonon and electronelectron interactions. We show simulations of pumpprobe ultrafast spectroscopy experiments such as timedependent Kerr rotation [1] and twocolour helicityresolved pumpprobe spectroscopy [2]. We can simulate the selective optical excitation of electron and holes in different valleys of the Brillouin zone. Our simulations shed light on the understanding of the mechanisms driving the intravalley and intervalley spin relaxation dynamics and the valley polarization dynamics in singlelayer transition metal dichalcogenides. We demonstrate that the dynamics is largely driven by electronphonon coupling and obtain good quantitative agreement with the experiments. 
Tuesday, March 6, 2018 4:54PM  5:06PM 
H29.00011: Ab Initio Carrier Dynamics in Twisted 2D Heterostructures Christopher Ciccarino, Ravishankar Sundararaman, Prineha Narang Stacked 2D materials known as van der Waals (vdW) heterostructures are of significant prominence in quantum materials research. Despite a recent surge in exploration of these heterostructures, their fundamental dynamics, particularly the impact of twist angle, are still poorly understood. The promise of these materials is partially rooted in the vast landscape of parameter space available in materials choice for the heterostructure stack. Transition metal dichalcogenides (TMDs), such as MoSe_{2 }and MoS_{2}, have received significant attention in particular for their wide array of optoelectronic properties. In this talk, we will show an ab initio Feynman diagram manybody description of dynamics and optoelectronic response of twisted TMD heterostructures. This diagrambased theoretical approach enables understanding of 2D heterostructures and their electronelectron and electronphonon lifetimes. We will show comparison of our predictions with pumpprobe experiments of 2D heterostructures with an emphasis on electronphonon dynamics. 
Tuesday, March 6, 2018 5:06PM  5:18PM 
H29.00012: Ab Initio Radiative Lifetimes and Angular Dependence of the Photoluminescence in TwoDimensional Transition Metal Dichalcogenides HsiaoYi Chen, Maurizia Palummo, Davide Sangalli, Marco Bernardi We compute from firstprinciples the radiative lifetimes and the photoluminescence (PL) in twodimensional (2D) transition metal dichalcogenides (TMDCs) with chemical formula MX2 (M=Mo, W and X=S, Se, Te). We first compute excitons in TMDC monolayers with the GWBethe Salpeter equation method. We then derive a new equation for the radiative lifetimes that extends a previous treatment [1] to correctly include the angular dependence of the PL. Besides refining the values of the temperaturedependent radiative lifetimes, our formalism can explain the angular dependence of the PL observed experimentally under linearly polarized light excitation, thus enabling ab initio computations of polar plots of the PL in excellent agreement with experiment. We further discuss how decoherence due to electronphonon and electronelectron scattering modifies the angular dependence of the PL, and how nonparabolic exciton dispersions are expected to change the radiative lifetimes. Our work presents a direct connection between microscopic exciton states in TMDCs and PL measurements, thus providing computational tools to advance understanding of carrier and exciton dynamics in 2DTMDCs. 
Tuesday, March 6, 2018 5:18PM  5:30PM 
H29.00013: Atomiclike highharmonic generation from twodimensional materials Nicolas TancogneDejean, Angel Rubio The generation of highorder harmonics from atomic and molecular gases enables the production of highenergy photons and ultrashort isolated pulses. 
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