Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session X57: Excitonic and Photonic Behavior in 2D - IIFocus Live
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Sponsoring Units: DMP DCMP Chair: Prashant Padmanabhan, Los Alamos National Laboratory |
Friday, March 19, 2021 8:00AM - 8:12AM Live |
X57.00001: Ground and excited state exciton polarons in monolayer MoSe2 Jun Yan, Thomas A Goldstein, Yueh-Chun Wu, Shao-Yu Chen, Takashi Taniguchi, Kenji Watanabe, Kalman Varga The versatile experimentally accessible exciton species in monolayer transition metal dichalcogenide semiconductors offer an interesting platform for investigating the interaction between two-particle bound states and a Fermi sea of charges. Using hexagonal boron nitride encapsulated monolayer MoSe2, we study the impact of charge density tuning on the A and B series of exciton Rydberg states, including A:1s, A:2s, B:1s, and B:2s. The doping dependence of the A:2s state provides an opportunity to examine such interactions with greatly reduced exciton binding energy and more spatially diffuse structures, and we found that the impact of the Fermi sea becomes much more dramatic compared to the A:1s state. Using photoluminescence upconversion, we verify that the B:2s exciton state displays similar behavior when interacting with the Fermi sea despite being well above the bare bandgap in energy. Photoluminescence and reflection spectra of the A:1s state show clear evidence that the interaction of the exciton with a Fermi sea is best described by the exciton–polaron model, rather than a trion model. Our experimental results demonstrate that overall features of charge interaction are quite generic and highly robust, offering key insights into the dressed many-body states in a Fermi sea. |
Friday, March 19, 2021 8:12AM - 8:24AM Live |
X57.00002: Phonon-exciton Interactions in WSe2 under a quantizing magnetic field Shengnan Miao, Zhipeng Li, Tianmeng Wang, Yunmei Li, Chuanwei Zhang, Sufei Shi Strong many-body interaction in two-dimensional transitional metal dichalcogenides provides a unique platform to study the interplay between different quasiparticles, such as phonon replica emission and modified valley-selection rules. A large out-of-plane magnetic field is expected to modify the exciton-phonon interactions by quantizing excitons into discrete Landau levels. In this talk, I will discuss our exploration of the phonon–exciton interaction in monolayer WSe2 in the regime of Landau quantization. For the well-resolved PL peaks from different phonon replicas of the dark trion, the phonon-exciton interaction can lift the inter-Landau-level transition selection rules, manifested by a distinctively different Landau fan pattern compared to that of bright trions. The significant valley-Zeeman shift increase of Landau quantization suggests strong many-body effects on the phonon-exciton interaction. Our work demonstrates monolayer WSe2 as an intriguing playground to study phonon-exciton interactions and their interplay with charge, spin, and valley. |
Friday, March 19, 2021 8:24AM - 8:36AM Live |
X57.00003: Room temperature exciton modulation by surface acoustic wave in monolayer WSe2 Kanak Datta, Zidong Li, Zhengyang Lyu, Takashi Taniguchi, Kenji Watanabe, Parag Deotare The interaction of excitons with the travelling strain and piezoelectric field arising from a surface acoustic wave (SAW) remains an unexplored area of research. Here, we investigate the photoluminescence from monolayer WSe2 at room temperature under strong piezoelectric field generated by surface acoustic wave in lithium niobate (LiNbO3). The travelling piezoelectric field triggers strong dissociation of neutral and charged excitonic species accompanied by an efficient quasiparticle conversion process. From our measurement, we estimate the in-plane polarizability of neutral excitons in monolayer WSe2 under strong dielectric screening from polar LiNbO3 substrate to be (11±0.45)X10-6 Dm/V. We further demonstrate controlled exciton-acoustic wave interaction by using layered hexagonal boron nitride (hBN) spacer. |
Friday, March 19, 2021 8:36AM - 8:48AM Live |
X57.00004: Waveguiding valley excitons in a homogeneous monolayer through dielectric-patterning of substrate Xu-Chen Yang, Hongyi Yu, Wang Yao In monolayers of the semiconducting transition metal dichalcogenides, the electron-hole exchange in an exciton couples exciton's valley pseudospin and centre-of-mass momentum. It splits the exciton dispersion into a massive transverse branch, and a longitudinal branch that has very light or even zero mass depending on the form of screened Coulomb interaction. Here we show that dielectric patterning of the substrate can be exploited to realize waveguide of the exciton in the longitudinal branch, leaving the massive transverse branch unaffected. At a lateral interface of different dielectric constant in the substrate, the longitudinal branch exciton's transmission and reflection obey the Snell-Descartes law of optical system. This dielectric heterostructure appears transparent to the transverse branch exciton. When placing the monolayer on a one-dimensional dielectric superlattice, the dispersion of the longitudinal branch exciton is changed signicantly, and the wavefunctions exhibit one-dimensional features. In contrast, the massive transverse branch excitons are not affected by the substrate dielectric pattern. |
Friday, March 19, 2021 8:48AM - 9:00AM Live |
X57.00005: Long-lived populations of momentum- and spin-indirect excitons in monolayer WSe2 Shao-Yu Chen, Maciej Pieczarka, Matthias Wurdack, Eliezer Estrecho, Takashi Taniguchi, Kenji Watanabe, Jun Yan, Elena Ostrovskaya, Michael S Fuhrer In monolayer WSe2, the ground-state exciton is dark (D exciton, spin-indirect), and the valley degeneracy allows low-energy dark momentum-indirect excitons (XK exciton) to form. Interactions between the dark excitons and the optically accessible bright exciton (X) are likely to determine the optical properties of X at high power and to limit the maximum achievable exciton densities. However, so far, little is known about these interactions. Here, we show long-lived dense populations of the XK and the D excitons by performing time-resolved photoluminescence spectroscopy using a streak camera. Our results uncover an efficient inter-state conversion between X to D excitons through the spin-flip process. Surprisingly, we also observe a prominent conversion between the dark excitons mediated by the exchange interaction (D + D ↔ XK + XK). This interaction behaves superlinearly to the exciton densities and responsible for the population of the XK exciton at high densities. Moreover, the long-lived XK exciton can induce a persistent redshift of X exciton, revealing non-trivial exciton-exciton interactions. Our results clarify the interactions between bright and dark excitons, and further envision the Bose-Einstein condensation of dark excitons. |
Friday, March 19, 2021 9:00AM - 9:12AM Live |
X57.00006: Ultrafast Pump-Probe Nano-Imaging of Coupled Exciton-Phonons Dynamics and Interfacial Substrate Dissipation of WSe2 Samuel C. Johnson, Jun Nishida, Peter A. Chabal, Markus Raschke Infrared vibrational scattering scanning near-field optical microscopy has advanced into a powerful nano-imaging and -spectroscopy technique. However, its extension to probe quantum dynamics with full spatial, temporal, and spectral resolution has remained challenging. Specifically, ultrafast nano-imaging 2D materials is particularly desirable to resolve exciton formation, lattice heating, and interfacial energy transport. We present ultrafast visible-pump, infrared-probe nano-imaging and -spectroscopy, of monolayer and bulk WSe2 on diamond, gold, and SiO2 substrates. We distinguish intralayer exciton dynamics on the few-ps time scale from interfacial energy dissipation and transport on the 100s of ps time scale. We resolve both the non-resonant exciton and Drude carrier response of WSe2 and the resonant substrate phonon frequency, which we use as a measure of the transient substrate lattice temperature evolution. We observe nano-volume substrate heating and expansion, WSe2 layer number dependence, and tip-controlled excitation and relaxation dynamics. These results reveal the complex interplay and competing relaxation pathways in TMD/substrate heterostructures, whose performance critically depends on exciton lifetimes, hot phonon-bottlenecks, and nanoscale thermal management. |
Friday, March 19, 2021 9:12AM - 9:24AM Live |
X57.00007: Fermi Liquid Theory Sheds Light on “Hot” EHL in 1L-MoS2 Ryan Wilmington, Hossein Ardekani, Avinash Rustagi, Alexander W Bataller, Alexander F Kemper, Robert Younts, Kenan Gundogdu 2D transition metal dichalcogenides (TMDCs) exhibit an electron-hole liquid phase transition at unusually high temperatures. Because these materials are atomically thin, optical excitation leads to material expansion. As a result, during the EHL phase transition the electronic band structure evolves due to both material thermal expansion and renormalization of the bands under high excitation densities. Specifically, these effects lead to indirect gap electronic band structure with a valence band maximum located at the Γ valley. In this work we developed a methodology for analyzing the spectral evolution of the photoluminescence of suspended 1L-MoS2 during the EHL phase transition by using Fermi liquid theory. The resulting analysis reveals valley-specific carrier densities, radiative recombination efficiencies, and intraband carrier relaxation kinetics in 1L-MoS2. More broadly, the results outline a methodology for predicting critical EHL parameters, shedding light onto the EHL phase transition in 2D TDMCs. |
Friday, March 19, 2021 9:24AM - 10:00AM Live |
X57.00008: Multiparticle Excitations, Spin-orbit Coupling, and Magnetism in the Photophysics of Quasi 2D Semiconductors Invited Speaker: Steven G Louie Strong electron-hole interaction and spin-orbit coupling, as well as magnetic ordering, can give rise to novel features in the photophysics of 2D semiconductors. Using the ab initio many-body interacting Green’s function approach, we investigate manifestations of these interactions in several atomically thin 2D materials. We show that electron-hole exchange interaction in monolayer transition metal dichalcogenides (TMDs) mixes the prominent A and B excitons that heretofore were believed to be completely independent of each other, since they had been viewed as derived from inter-band transitions between different pairs of spin-polarized bands. We clarify the physical origin of giant excitonic and magneto-optical responses in monolayer CrI3, a 2D ferromagnetic semiconductor, showing that these properties are dominated by extended exciton states. Finally, with a newly developed approach, we perform ab initio calculations on the properties of trions and bi-excitons in monolayer TMDs, elucidating the energetics and the wavefunction characters of these strongly bounded correlated 3-particle and 4-paricle excitations in 2D semiconductors. |
Friday, March 19, 2021 10:00AM - 10:12AM Live |
X57.00009: Enhanced multiexciton formation by an electron-hole plasma in 2D semiconductors Matthew Strasbourg, Cory Johns, Zoe E Noble, Emanuil Yanev, Thomas Darlington, P. James Schuck, James Hone, Nicholas Borys Transition metal dichalcogenide semiconductors are layered van der Walls materials that exhibit exceptional optoelectronic properties in monolayer form. Their atomically thin nature and reduced long-range dielectric screening make them ideal systems in which to study a rich suite of many-body electronic states that emerges from intense coulomb interactions between quantum-confined charge carriers in a truly 2D system. Using photoluminescence action spectroscopy of monolayer WSe2, we find an enhancement of multiexciton formation with increasing excitation energy. This enhancement is attributed to the formation of excitons from a high-energy electron-hole plasma and generates 200% more multiexciton states than lower-energy excitation. The enhancement does not affect relaxation dynamics and its onset coincides with the energy of the quasiparticle bandgap, corroborating the role of the electron-hole plasma and highlighting how the formation of excited states can be uniquely manipulated in 2D semiconductors. Understanding these formation and relaxation dynamics of the rich manifold of exciton states is critical for leveraging this new class of 2D semiconductors for advanced technologies. |
Friday, March 19, 2021 10:12AM - 10:24AM Live |
X57.00010: Determination of scattering lifetime of the bright excitonic state in twisted bilayer graphene via resonant Raman spectroscopy Matthew DeCapua, Yueh-Chun Wu, Jun Yan The band structure of bilayer graphene has been shown to be tunable by introducing a relative twist angle between the two layers, unlocking exotic phases, such as superconductor and Mott insulator, and providing a fertile ground for new physics. At intermediate twist angles around 10°, highly degenerate electronic transitions hybridize to form excitonic states, resulting in a resonant condition that can be observed by Raman spectroscopy. We use a reflection contrast technique to quickly identify this resonance, then follow up with Raman scattering measurements to track the evolution of the intensity of the graphene Raman G peak, corresponding to the E2G phonon. The intensity profile features broadening due to the finite lifetime of the scattered carriers. For a sample with twist angle 8.6°, we report a temperature dependent broadening ≈ 0.07 eV, putting the lifetime of the bright exciton on the order of 10 fs. |
Friday, March 19, 2021 10:24AM - 10:36AM Live |
X57.00011: Dynamic Exciton Spectra in WSe2-MoSe2 Moire Crystals Jacob Embley, Junho Choi, Kha Tran, Dong Seob Kim, Takashi Taniguchi, Kenji Watanabe, Keiji Ueno, Xiaoqin (Elaine) Li In van de Waals heterostructures, exciton resonances are strongly modified by the periodic moire potential. While much attention has focused on how interlayer excitons in WSe2-MoSe2 bilayers are modified by the moire potential, intralayer exciton resonances also undergo modification. In particular, a splitting of intralayer exciton resonances is observed for WSe2-MoSe2 with a twist angle of ~ 1 degree. These resonances exhibit distinct dynamics, suggesting that the moiré potential leads to different electron and hole dynamics. |
Friday, March 19, 2021 10:36AM - 10:48AM Live |
X57.00012: Direct measurement of interlayer exciton absorption in MoSe2/WSe2 heterostructures Elyse Barre, Ouri Karni, Aidan Luke O'beirne, Henrique Ribeiro, Bumho Kim, Kenji Watanabe, Takashi Taniguchi, Katayun Barmak, Felipe Da Jornada, Sivan Refaely-Abramson, Tony Heinz Direct coupling of light and interlayer excitons (ILX) in transition metal dichalcogenides heterostructures has been sought after for both fundamental and applied purposes because of the interlayer excitons’ sensitivity to the interface details, electrical tunability [1,2], and long lifetimes [2]. However, since ILX absorption is weak, most of its investigations have used photoluminescence [1,2] or inferred absorption data from excitation spectra [3,4]. In this work, we present direct measurements of the ILX absorption resonances in MoSe2/WSe2 heterostructures for different twist angles using an electromodulation technique. We determine the energies, electric-dipole moments, and oscillator strengths of interlayer exciton states. We compare the absorption spectra with emission spectra from ILXs to first-principle calculations to assign the observed features. |
Friday, March 19, 2021 10:48AM - 11:00AM Live |
X57.00013: Enhanced Photoluminescence in Encapsulated TFSI treated MoS2 Kurt Tyson, James Godfrey, James M Fraser, Robert Knobel Semiconducting transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) are 2D materials that possess a direct bandgap at a single-layer thickness. Their bandgap makes them ideal for optical sensing and switching applications. However, the function of as-fabricated MoS2 optoelectronic devices is throttled by their extremely low photoluminescence quantum yield (PLQY) of 0.01%-0.6% [1]. Surprisingly, recent literature has demonstrated methods to drastically increase PLQY using chemical treatments and electrostatic doping [1]. Here we demonstrate significantly enhanced photoluminescence (PL) up to an order of magnitude in MoS2 using trifluoromethansulfonimide (TFSI) acid treatment. We observe a suppression of a charged exciton feature suggesting a reduction in non-radiative recombination via charge-neutralization. Additionally, as the PL enhancing effects of TFSI-acid deteriorate after exposure to standard lithography solutions, we attempt to remedy this using hexagonal boron nitride (hBN) encapsulation. While TFSI-acid treated MoS2 demonstrates increased PL efficiency, hBN encapsulation is mixed for protecting the enhanced PLQY. |
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