Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A33: Excitons in 2D SemiconductorsFocus

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Sponsoring Units: DMP Chair: Andreas Stier, NHMFL Room: 296 
Monday, March 13, 2017 8:00AM  8:12AM 
A33.00001: Effective Mass Theory of 2D Excitons Revisited Joseph Gonzalez, Ivan Oleynik Twodimensional (2D) semiconducting materials possess an exceptionally unique set of electronic and excitonic properties due to the combined effects of quantum and dielectric confinement. Reliable determination of exciton binding energies from both firstprinciples manybody perturbation theory (GW/BSE) and experiment is very challenging due to the enormous computational expense as well as the tremendous technical difficulties in experiment.. Very recently, effective mass theories of 2D excitons have been developed as an attractive alternative for inexpensive and accurate evaluation of the exciton binding energies. In this presentation, we evaluate two effective mass theory approaches by Velizhanin \textit{et al} [1] and Olsen \textit{et al }[2] \quad in predicting exciton binding energies across a wide range of 2D materials. We specifically analyze the trends related to the varying screening lengths and exciton effective masses. We also extended the effective mass theory of 2D excitons to include effects of electron and hole mass anisotropies (m$_{\mathrm{x}}\ne $m$_{\mathrm{y}})$, the latter showing a substantial influence on exciton binding energies. The recent predictions of exciton binding energies being independent of the exciton effective mass and a linear correlation with the band gap of a specific material are also critically reexamined. 1. K. A. Velizhanin et al., Phys. Rev. B, \textbf{92}, 195305 (2015). 2. T. Olsen et al., Phys. Rev. Lett. \textbf{116}, 056401 (2016). [Preview Abstract] 
Monday, March 13, 2017 8:12AM  8:24AM 
A33.00002: Exciton transport phenomena in monolayer MoS$_{2}$ Masaru Onga, Yijin Zhang, Toshiya Ideue, Yoshihiro Iwasa Monolayer transition metal dichalcogenides exhibit unique optical phenomena owing to the twodimensional structure and valley degree of freedom. Many researchers have revealed that excitonic states play an important role in optical response, and have observed the diffusion transport of excitons in this system at room temperature [1, 2]. Here we report exciton transport phenomena in monolayer MoS$_{2}$ at low temperature through photoluminescence mapping. Our results can provide us a new platform for excitonbased optoelectronics with valley degrees of freedom. [1] S. Mouri et. al., PRB 90, 155449 (2014). [2] Q. Cui et. al., ACS nano 8, 2970 (2014). [Preview Abstract] 
Monday, March 13, 2017 8:24AM  8:36AM 
A33.00003: Nonequilibrium exciton dynamics in model systems Avinash Rustagi, Alexander Kemper Ultrashort laser pulses are used to observe time resolved dynamics in condensed matter systems. The typical time resolved measurements include reflection, transmission and absorption using pumpprobe spectroscopy where a pump pulse excites the system into a nonequilibrium state and the subsequent probe pulse measures the dynamical relaxation of the system as a function of a delay time. We study the twoparticle properties of a model interacting twoband Hamiltonian by solving the nonequilibrium BetheSalpeter equation for the correlation function. This allows us to study the dynamics of electronhole bound states i.e. excitons, in model systems where the time translational invariance is broken by the pump pulse. [Preview Abstract] 
Monday, March 13, 2017 8:36AM  8:48AM 
A33.00004: On hightemperature superfluidity of the twocomponent exciton gas in a TMDC bilayer Roman Ya. Kezerashvili, Oleg L. Berman The hightemperature superfluidity of twodimensional dipolar excitons in a TMDC bilayer was predicted. The exciton effective mass, energy spectrum of the collective excitations, the sound velocity and meanfield phase transition critical temperature were obtained for different TMDC materials. BoseEinstein condensation in the twocomponent weaklyinteracting gas of dipolar A and B excitons was studied. Within the Bogolubov approximation, the sound velocity in the twocomponent dilute exciton Bose gas is always larger than in any onecomponent one due to the fact that the sound velocity for a twocomponent dilute exciton gas system depends on the reduced mass of A and B excitons, which is always smaller than the individual effective mass of A or B exciton. Due to this difference between the reduced and individual effective exciton masses, the critical temperature for superfluidity for the twocomponent exciton system in a TMDC bilayer is about one order of magnitude higher than one in any onecomponent exciton system. The observation of the superfluidity of twodimensional dipolar excitons in a TMDC bilayer causes two opposite superconducting currents in each TMDC layer which can be observed experimentally. [Preview Abstract] 
Monday, March 13, 2017 8:48AM  9:00AM 
A33.00005: Amplitude(Higgs) modes in twodimensional spatially indirect exciton condensates Fei Xue, Fengcheng Wu, Allan MacDonald Higgs modes in condensed matter physics have drawn attentions in recent years because of close analogies that can be made in many cases to Higgs bosons that have been discovered in particle physics. Here we use a microscopic timedependent meanfield theory to study the collective mode spectra of twodimensional spatially indirect exciton (electronhole pair) Bose condensates. We apply linear response theory to identify a number of collective modes with a strong electronhole pairing amplitude(Higgslike) component. In the BEC limit, the Higgslike excitations correspond to removing an exciton from the condensate and exciting it to a higher energy boundpair state. We will discuss the relationship of this finding to the literature on Higgslike excitations of superconductors. [Preview Abstract] 
Monday, March 13, 2017 9:00AM  9:12AM 
A33.00006: Rotational symmetry breaking and topological phase transition in the excitonpolariton condensate of gapped 2D Dirac material KI HOON Lee, Changhee Lee, JaeSeung Jeong, Hongki Min, Suk Bum Chung For the quantum well in an optical microcavity, the interplay of the Coulomb interaction and the electronphoton coupling can lead to the emergence of bosonic quasiparticles consisting of the exciton and the cavity photon known as polariton, which can form the BoseEinstein condensate above a threshold density. Additional physics due to the nontrivial Berry phase comes into play when the quantum well consists of the gapped Dirac material such as the transition metal dichalcogenide (TMD) MoS2 or WTe2. Specifically, in forming excitons, the electronphoton coupling from the optical selection rule due to the Berry phase competes against, rather than cooperates with, the Coulomb interaction. We find that this competition gives rise to the spontaneous breaking of the rotational symmetry in the polariton condensate and also drives topological phase transition, both novel features in polariton condensation. We also investigate the possible detection of this competition through photoluminescence. [Preview Abstract] 
Monday, March 13, 2017 9:12AM  9:48AM 
A33.00007: Theory of dynamical screening of excitons in monolayer transitionmetal dichalcogenides Invited Speaker: Hanan Dery Exciton optical transitions in transitionmetal dichalcogenides offer unique opportunities to study rich manybody physics. Recent experiments in monolayer WSe$_2$ and WS$_2$ have shown that, while the lowtemperature absorption and photoluminescence from neutral excitons and threebody complexes is suppressed in the presence of elevated electron densities or strong photoexcitation, new dominant peaks emerge in the lowenergy side of the spectrum. I present a theory that elucidates the nature of these optical transitions showing the role of the intervalley Coulomb interaction and ensuing valley plasmons. Considering their signature in the selfenergy of electrons from the top spinsplit conduction valleys leads to the emergence of a correlationinduced virtual state in the band gap. This phenomenon sheds light on the origin of the luminescence in monolayer WSe$_2$ and WS$_2$ in the presence of pronounced manybody interactions. I will also present numerical results of the absorption spectrum calculated from the twoparticle Dyson Equation of the pair Green's function. Inclusion of dynamical screening in the potential is imperative to correctly describe the physics of excitons in gated structures. [Preview Abstract] 
Monday, March 13, 2017 9:48AM  10:00AM 
A33.00008: Coherent and incoherent coupling dynamics between neutral and charged excitons in monolayer MoSe2 Lixiang Xu, Kai Hao, Philipp Nagler, Akshay Singh, Kha Tran, Chandriker Kavir Dass, Christian Schuller, Tobias Korn, Xiaoqin Li, Galan Moody The optical properties of semiconducting transition metal dichalcogenides are dominated by both neutral excitons (electronhole pairs) and charged excitons (trions) that are stable even at room temperature. While trions directly influence charge transport properties in optoelectronic devices, excitons may be relevant through excitontrion coupling and conversion phenomena. In this work, we reveal the coherent and incoherent nature of excitontrion coupling and the relevant time scales in monolayer MoSe2 using optical twodimensional coherent spectroscopy. Coherent interaction between excitons and trions is definitively identified as quantum beating of cross peaks in the spectra that persists for a few hundred femtoseconds. For longer times up to 10 ps, surprisingly, the relative intensity of the cross peaks increases, which is attributed to incoherent energy transfer likely due to phononassisted upconversion and downconversion processes that are efficient even at cryogenic temperature. [Preview Abstract] 
Monday, March 13, 2017 10:00AM  10:12AM 
A33.00009: Splitting between Bright and Dark Excitons in Transition Metal Dichalcogenide Monolayers Iann Gerber, Juan Pablo Echeverry, Bernhard Urbaszek, Thierry Amand, Cedric Robert, Xavier Marie The optical properties of transition metal dichalcogenide monolayers such as the twodimensional semiconductors MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$ are dominated by excitons, Coulomb bound electronhole pairs [1]. The light emission yield depends on whether the electronhole transitions are optically allowed (bright) or forbidden (dark). By solving the BetheSalpeter equation on top of GW wave functions in density functional theory calculations, we determine the sign and amplitude of the splitting between bright and dark exciton states. We evaluate the influence of the spinorbit coupling on the optical spectra and clearly demonstrate the strong impact of the intravalley Coulomb exchange term on the darkbright exciton fine structure splitting [2].This paves the way for spinorbitengineering in Mo$_{\mathrm{(1x)}}$W$_{\mathrm{x}}$Se$_{\mathrm{2}}$ alloy monolayers for optoelectronics and applications based on spin and valleycontrol [3]. [1] G. Wang et al , Phys. Rev. Lett. \textbf{114}, 097403 (2015). [2] J. P. Echeverry et al, Phys. Rev. B \textbf{93}, 121107(R) (2016). [3] G. Wang et al, Nat. Commun. \textbf{6}, 10110 (2015). [Preview Abstract] 
Monday, March 13, 2017 10:12AM  10:24AM 
A33.00010: Theoretical study of luminescence spectrum of exciton in monolayer transition metal dichalcogenides; The role of intervalley Coulomb interaction and dynamical screening. Dinh Van Tuan, Hanan Dery We investigate the luminescence properties of excitons in monolayer transition metal dichalcogenides in order to elucidatethe experimental results and the differences in the spectrum of MoSe2 and WSe2. We find that the experimental results can be explained only when incorporating dynamical screening and intervalley Coulomb interaction in the BetheSalpeter Equation (BSE) of the electronhole pair Green's function. In the first step of our numerical scheme, the pair Green's function is evaluated at Matsubara frequencies, followed in the second step, by analytical continuation to the real axis using a Pade approximation. The obtained results agree far better with the experimental data compared with the solution of the BSE when using a statically screened potential. [Preview Abstract] 
Monday, March 13, 2017 10:24AM  10:36AM 
A33.00011: Probing ManyBody Interactions in Monolayer TransitionMetal Dichalcogenides Benedikt Scharf, Zefang Wang, Dinh Van Tuan, Jie Shan, Kin Fai Mak, Igor Zutic, Hanan Dery Manybody interactions in monolayer transitionmetal dichalcogenides are strongly affected by their unique band structure. We study these interactions by measuring the energy shift of neutral excitons (bound electronhole pairs) in gated WSe$_{2}$ and MoSe$_{2}$. The gateinduced charge density screens the electronhole Coulomb attraction and renormalizes the bandgap energy via exchange and correlation interactions. We compute the energy shift of neutral excitons as a function of charge density with the BetheSalpeter equation. Moreover, we study the effects of dynamical screening in the lowest order. We resolve the contributions of the bandgap renormalization and dynamically screened potential by comparing the results of WSe$_{2}$ and MoSe$_{2}$, due to their distinct spinsplit conduction bands. [Preview Abstract] 
Monday, March 13, 2017 10:36AM  10:48AM 
A33.00012: ExcitonPolariton Dynamics of a Monolayer Semiconductor Coupled to a Microcavity YenJung Chen, Teodor K. Stanev, Nathaniel P. Stern, Jeffrey D. Cain, Vinayak P. Dravid Strong lightmatter interactions, evidenced by excitonpolariton states, have been observed in the twodimensional limit with monolayer transition metal dichalcogenides (TMDs) embedded in a microcavity\footnote{~X. Liu, \textit{et al}. \textit{Nature Photon.} \textbf{9}, 30 (2015)}. Because of the valley degree of freedom in monolayer TMDs, these hybrid lightmatter states can exhibit valley polarization as in a bare monolayer, with stronglycoupled dynamics determined by the relative rates of exciton relaxation and intervalley scattering, which can be highly modified in onresonant cavities\footnote{~YJ. Chen, \textit{et al}. \textit{CLEO: Science and Innovations.} STu3F2 (2016)}. Here, we test this intuitive picture of the polarized excitonpolariton dynamics with monolayer MoS$_2$ coupled to detuned cavities. Upper and lower polariton branches exhibit distinct decay rates indicative of different cavity dynamics. As with onresonant, stronglycoupled excitonpolaritons, the weaklycoupled regime causes excitonpolariton valley polarization to persist at room temperature, demonstrating that dynamics of valleypolarized excitations can be controlled by engineering lightmatter interactions. [Preview Abstract] 
Monday, March 13, 2017 10:48AM  11:00AM 
A33.00013: Electrically tunable strong lightmatter coupling in a transition metal dichalcogenide monolayer embedded in a plasmonic crystal cavity Giovanni Scuri, You Zhou, Alexander High, Alan Dibos, Kristiaan De Greve, Mark Polking, Luis Juaregui, Dominik Wild, Andrew Joe, Kateryna Pistunova, Mikhail Lukin, Philip Kim, Hongkun Park Twodimensional transitionmetal dichalcogenide (TMDC) monolayers exhibit direct bandgap excitons with large binding energy. The optical response of TMDCs is electrically tunable over a broad wavelength range, making these 2D materials promising candidates for optoelectronic devices. In this work, we enhance excitonplasmon coupling by embedding a single layer of tungsten diselenide (WSe2) into a plasmonic crystal cavity, which confines surface plasmon polaritons in an analogous manner to photonic crystal cavities. We observe strong lightmatter interactions and the formation of microcavity polaritons when the cavity mode is on resonance with the exciton absorption in WSe2. Using the electrostatically controllable response of such excitons, we also demonstrate tunable vacuum Rabi splitting in such a system. [Preview Abstract] 
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