Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C37: 2D Materials - Optics and Excitons IFocus
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Sponsoring Units: DMP Chair: Kristen Kaasbjerg, TU Denmark Room: LACC 411 |
Monday, March 5, 2018 2:30PM - 2:42PM |
C37.00001: Strongly bound excitons in Ruddlesden-Popper 2D perovskites Jean-Christophe Blancon, Andreas Steir, Wanyi Nie, Hsinhan Tsai, Constantinos Stoumpos, Scott Crooker, Mercouri Kanatzidis, Jared Crochet, Jacky Even, Aditya Mohite Ruddlesden-Popper halide perovskites are 2D solution-processed quantum wells with a general formula A2A’n-1MnX3n+1, where A, A’ are cations, M is a metal, X is a halide, and their physical properties can be tuned by varying the perovskite layer thickness (n value). They have recently emerged as efficient semiconductors for optoelectronics [1-3]. However, fundamental questions concerning the nature of optical resonances, their scaling with quantum well thickness, and the physics behind the exciton properties, remain unresolved. Here, using optical spectroscopy and 60-Tesla magneto-absorption supported by modelling, we unambiguously demonstrate that the optical resonances arise from tightly bound excitons with binding energies varying from 470 meV to 125 meV with increasing thickness from n=1 to 5 [4]. Comprehensive modelling of exciton states enable the understanding of dielectric confinement effects which prevail over quantum confinement in 2D perovskites. From these results we produce a general scaling behaviour for the binding energy of exciton states in Ruddlesden-Popper perovskites. |
Monday, March 5, 2018 2:42PM - 2:54PM |
C37.00002: Magneto-Optics of Exciton Rydberg States in a Monolayer Semiconductor, and Determination of Exciton Mass Scott Crooker, Andreas Stier, Nathan Wilson, Kirill Velizhanin, Junichiro Kono, Xiaodong Xu Historically, optical spectroscopy in high magnetic fields has provided an especially powerful way to identify and quantify excited Rydberg excitons in semiconductors, because each Rydberg state shifts very differently with increasing field. Crucially, these shifts can directly reveal fundamental parameters such as the exciton's size, mass, and spin -- essential information for benchmarking theoretical models. Here we report 65 tesla magneto-absorption spectroscopy of exciton Rydberg states in the archetypal monolayer semiconductor WSe2 [1]. The strongly field-dependent and distinct energy shifts of the 2s, 3s, and 4s excited neutral excitons not only permits their unambiguous identification but also allows for quantitative comparison with leading theoretical models. Both the sizes (via low-field diamagnetic shifts) and the energies of the ns exciton states agree remarkably well with detailed numerical simulations using the non-hydrogenic screened Keldysh potential for 2D semiconductors. Moreover, at the highest magnetic fields the nearly-linear diamagnetic shifts of the weakly-bound 3s and 4s excitons provide a direct and unambiguous experimental measure of the exciton’s reduced mass, mr = 0.20±0.01 m0. [1] Stier et al., arXiv:1709.00123 |
Monday, March 5, 2018 2:54PM - 3:06PM |
C37.00003: Control of Coherently Coupled Exciton Polaritons in Monolayer Tungsten Disulphide Xiaoze Liu, Wei Bao, Quanwei Li, Chad Ropp, Yuan Wang, Xiang Zhang Monolayer transition metal dichalcogenides (TMD) with confined 2D Wannier-Mott excitons are intriguing for the fundamental study of strong light-matter interactions and the exploration of exciton polaritons at high temperatures. However, the research of 2D exciton polaritons has been hindered because the polaritons in these atomically thin semiconductors discovered so far can hardly support strong nonlinear interactions and quantum coherence due to uncontrollable polariton dynamics and weakened coherent coupling. In this work, we demonstrate, for the first time, a precisely controlled hybrid composition with angular dependence and dispersion-correlated polariton emission by tuning the polariton dispersion in TMD over a broad temperature range of 110–230 K in a single cavity. This tamed polariton emission is achieved by the realization of robust coherent exciton-photon coupling in monolayer tungsten disulphide (WS2) with large splitting-to-linewidth ratios (>3.3). The unprecedented ability to manipulate the dispersion and correlated properties of TMD exciton polaritons at will offers new possibilities to explore important quantum phenomena such as inversionless lasing, Bose-Einstein condensation, and superfluidity. |
Monday, March 5, 2018 3:06PM - 3:18PM |
C37.00004: Substrate Renormalization of Quasiparticle Band Gaps and Exciton Binding Energies in Quasi-2D Materials Chin Shen Ong, Felipe da Jornada, Diana Qiu, Steven Louie Atomically thin quasi-two-dimensional (quasi-2D) materials, such as monolayer transition-metal dichalcogenides, display a much weaker electronic screening compared to their bulk counterparts as well as restricted geometry for the motion of the electrons. As a result, electron-electron and electron-hole interactions are enhanced. Owing to the atomic dimension of layer thickness, quasi-2D materials are sensitive to the screening environment produced by substrates, which allows one to dramatically tune their quasiparticle and optical properties. In this work, we extend a method recently developed in our group to incorporate substrate screening into the calculation of quasiparticle and optical properties of quasi-2D materials. We perform full-frequency ab initio GW and GW-Bethe Salpeter equation (GW-BSE) calculations to quantify the effect of the substrate on the electronic and optical gaps of quasi-2D systems. We find that a careful treatment of the dynamical effects of substrate polarizability is necessary to explain the effect of the renormalization on metallic substrates. |
Monday, March 5, 2018 3:18PM - 3:30PM |
C37.00005: Magnetic Control of Dark-Bright Exciton Splitting in Monolayer MoSe2 Zhengguang Lu, Daniel Rhodes, Jonathan Ludwig, Yuxuan Jiang, Seongphill Moon, Komalavalli Thirunavukkuarasu, Zhigang Jiang, James Hone, Dmitry Smirnov Excitons in monolayer transition metal dichalcogenides (TMDCs), depending on the spin configuration of the conduction and valence band, can be either optically bright or dark. Even though spin-forbidden transitions are optically dark, these dark excitons have been observed in tungsten based TMDCs by adding in-plane magnetic field or via near-field coupling to surface plasmon polaritons [1,2]. However, in MoSe2, the direct observation of dark excitons and the value of the bright-dark exciton splitting are still missing. We have performed low temperature magneto-photoluminescence (PL) and broadband reflection contrast measurements on BN encapsulated monolayer MoSe2. The experiments reveal the dark exciton brightening and energy shift of bright and dark exciton branches by in-plane magnetic field. Both branches follow B2 field dependence in magnetic fields up to about 14T as expected from a two-band model. By analyzing the field evolution of exciton branches, we extracted the value of zero-field bright-dark exciton splitting and excitons g-factors. |
Monday, March 5, 2018 3:30PM - 3:42PM |
C37.00006: Excitonic Effects in Hexagonal Boron Nitride Multilayer Systems Fulvio Paleari, Thomas Galvani, Marios Zacharias, Alejandro Molina-Sanchez, Hakim Amara, François Ducastelle, Feliciano Giustino, Ludger Wirtz Hexagonal boron nitride (hBN) is a layered material and an insulator displaying strong excitonic effects. |
Monday, March 5, 2018 3:42PM - 3:54PM |
C37.00007: Superior valley polarization and coherence of 2s excitons in monolayer WSe2 Jun Yan, Shao-Yu Chen, Thomas Goldstein, Takashi Taniguchi, Kenji Watanabe, Jiayue Tong We report on the fabrication and spectroscopic measurement of high-quality monolayer WSe2 crystals. The excitonic emission features in these samples exhibit narrow linewidth. We fully resolve the exchange-interaction-split trion doublet at low temperatures and observe the 2s exciton luminescence up to room temperature. From comparison with absorption spectra, both 1s and 2s luminescence radiation exhibit near-zero Stokes shift, confirming that disorders in these samples are minimal. Remarkably, under similar experimental conditions, the 2s luminescence is found to exhibit much better valley polarization and coherence than 1s. We explain our experimental results in the framework of exchange interaction induced intervalley scattering, the Maialle-Silva-Sham mechanism. |
Monday, March 5, 2018 3:54PM - 4:06PM |
C37.00008: Exciton Hall effect and valley-exciton transport Masaru Onga, Yijin Zhang, Toshiya Ideue, Yoshihiro Iwasa Spontaneous Hall effect caused by the Berry curvature (the internal magnetic field in momentum space) has recently attracted vast interest for investigating internal quantum degrees of freedom in solids. Here we report a new type of spontaneous Hall effect, exciton Hall effect (EHE), in monolayer MoS2[1]. |
Monday, March 5, 2018 4:06PM - 4:18PM |
C37.00009: Phonon-assisted Oscillatory Enhancement of Exciton Dynamics in Monolayer MoSe2 Colin Chow, Hongyi Yu, Aaron Jones, John Schaibley, Michael Koehler, David Mandrus, Roberto Merlin, Wang Yao, Xiaodong Xu Exciton-phonon interaction plays a major role in the relaxation dynamics of photocarriers in monolayer semiconducting transition metal dichalcogenides (TMDs). Despite a few experimental studies on phonon-limited excitonic relaxation, details behind which phonon modes and how they affect the formation and relaxation of excitons are still lacking. We observe an oscillatory enhancement of neutral exciton signal in monolayer MoSe2 in photoluminescence excitation (PLE), with a period matching the M-point longitudinal acoustic phonon, LA(M). Numerical fit to the emission lineshapes also reveals the oscillatory behavior, suggesting that the presence of LA(M) phonons significantly modifies the relaxation dynamics. This is verified by our observation of oscillatory exciton lifetime in time-resolved PLE. What is unusual here is the dominating role of acoustic phonons, rather than optical phonons as commonly expected. Our theory suggests the importance of Q valleys in the dynamics of excitons, although photocarriers are predominantly localized in K valleys. Reference: Chow et al., npj 2D Mater. Appl. 1, 33 (2017). |
Monday, March 5, 2018 4:18PM - 4:54PM |
C37.00010: Valley Exciton Polaritons Invited Speaker: Vinod Menon The valley degree of freedom in exciton-polaritons (half-light half-matter quasiparticles) provide a largely unexplored control parameter for all-optical manipulation of polaritons. Owing to the large exciton binding energy and oscillator strength in two-dimensional (2D) transition metal dichalcogenides (TMDs), exciton-polariton formation at room temperature has been reported in variety of cavity architectures. One of the important distinctions of polaritons formed in 2D TMDs is their valley polarization property arising from the quantum mechanically distinct valley origin of their excitonic component. We will discuss the formation and optical control of the valley exciton-polaritons in 2D WS2 embedded in a microcavity. These valley polaritons show in-plane momentum dependent helicity arising from their exciton-photon composite nature. Interestingly, even when the polariton state is optically excited at energies below the exciton reservoir, the polariton emission shows valley polarization. This unambiguously shows that the distinct valley origin of the exciton component is imprinted onto the hybrid polariton state, and that they can be addressed via their photonic component. Following this, we will briefly discuss the possibility to electrically control the exciton polaritons achieved through gating in a field effect transistor geometry. The realization of half-light half-matter valley polaritons at room temperature presents an attractive approach towards optical control and manipulation of valley degree of freedom in 2D semiconductors. |
Monday, March 5, 2018 4:54PM - 5:06PM |
C37.00011: Enhanced exciton-phonon scattering from monolayer to bilayer WS2 Archana Raja, Malte Selig, Gunnar Berghäuser, Jaeeun Yu, Heather Hill, Albert Rigosi, Louis Brus, Andreas Knorr, Ermin Malic, Tony Heinz, Alexey Chernikov Layered 2D transition metal dichalcogenides like WS2 exhibit the emergence of a direct bandgap when the thickness is reduced down to a monolayer, along with the appearance of pronounced excitonic effects. The coherence lifetime of these tightly-bound excitons is fundamental to optoelectronic properties of the material. In high-quality samples, scattering with phonons is the chief mechanism that limits the coherence lifetime and defines the linewidth of excitonic transitions. While there has been much focus on the evolution of the band structure with thickness, the change in exciton coherence lifetime as the material transitions from a direct to an indirect gap semiconductor is not well understood. Here, we address this question with systematic measurements of temperature-dependent exciton linewidths in mono- and bilayer WS2. We find a significant increase in the A exciton linewidth in the bilayer compared to the monolayer, with room temperature linewidths ~50 meV broader in the former than in the latter, indicative of new scattering channels that produce dephasing on the 10 fs timescale. We will discuss these experimental results in terms of theoretical modeling of exciton phonon scattering processes present in the indirect-gap bilayer system, but absent in the monolayer. |
Monday, March 5, 2018 5:06PM - 5:18PM |
C37.00012: Strain Tuning of the Excitons of Monolayer WSe2 Ozgur Burak Aslan, Minda Deng, Tony Heinz Excitons dominate the optical spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs) due to their high binding energy and oscillator strength. In this work, we apply tensile strain to monolayer (1L) WSe2, to manipulate and investigate its excitonic properties. At 2.1% strain, we achieve a redshift of 100 meV in the optical gap and a decrease of 25 meV in the binding energy of the A exciton, as revealed by exciton Rydberg spectroscopy, corresponding to a decrease of 125 meV in the quasiparticle band gap. The B-A exciton splitting increases by 20 meV, mainly due to an increase in the spin-orbit coupling. Surprisingly, the spectral linewidth of the A exciton decreases by almost a factor of two under strain, from 42 to 24 meV at room temperature. We explain this effect as the result of suppression of phonon-mediated exciton scattering associated with the relative shift under strain of a secondary valley in the conduction band that is nearly degenerate with the K valley involved in the A exciton. Our results help us to understand the excitonic properties of 1L TMDCs and what contributes to the linewidths of these features. In particular, we show that the excitonic linewidths can be strongly manipulated by mechanical means. |
Monday, March 5, 2018 5:18PM - 5:30PM |
C37.00013: Excitonic structuring of the Optical Conductivity in MoS2 Monolayers Emilia Ridolfi, Vitor Pereira, Caio Lewenkopf We revisit the excitonic spectrum in monolayer MoS2 and how, in comparison with a single-particle picture, it strongly restructures the optical absorption spectrum over a large range of energies. Our expedited approach takes into account the anomalous screening in 2D and the presence of a substrate by the effective Keldysh potential. By solving the Bethe-Salpeter equation with a Slater-Koster parameterization of the first-principles single particle bandstructure, we obtain quantitative accuracy in relation to existing measurements. Our results capture well the absolute magnitude of the experimental optical conductivity and we scrutinize the details of the so-called C resonant excitons that provide a large spectral weight well within the continuum. The contributions to the C-excitons arise not only from the vicinity of the Gamma point, but from a set of rather broad portions of the Brillouin zone, consistent with their resonant nature and with recent experiments1-2. We also study the effect of the contributions of higher energy bands and spin-flipping terms in the Hamiltonian, on the main features of the experimental optical absorption. |
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