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 Y57: Excitonic and Photonic Behavior in 2D - IIIFocus Live
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Sponsoring Units: DMP Chair: Ling Miao, American Physical Society |
Friday, March 19, 2021 11:30AM - 12:06PM Live |
Y57.00001: Vallytronics and excitonics in 2D materials Invited Speaker: Tony Heinz Two-dimensional (2D) transion metal dichalcogenides (TMDCs) realize exceptional exciton dynamics with the new quantum index, valley index. Prof. Heinz group is one of the pioneering groups to realize the valley polarized excitons in 2D TMDC monolayers and has recently explored a whole range of physical properties of excitons in 2D TMDCs and their heterostructures. |
Friday, March 19, 2021 12:06PM - 12:18PM Live |
Y57.00002: Valley-selective Rydberg Excitons in Monolayer WSe2 Revealed by Magneto-photocurrent Spectroscopy Tianmeng Wang, Zhipeng Li, Yunmei Li, Zhengguang Lu, Shengnan Miao, Zhen Lian, Dmitry Smirnov, Chuanwei Zhang, Sufei Shi Monolayer transition metal dichalcogenides, with their greatly enhanced Coulomb interaction, are an ideal platform to host the Rydberg excitons in two dimensions. Here, we employ helicity-resolved magneto-photocurrent spectroscopy to identify Rydberg exciton states up to 11s in monolayer WSe2, from which the exciton binding energy and exciton radius can be extracted accurately. Photocurrent spectroscopy of monolayer WSe2 opens the path to study strong enhanced electron-electron interaction that leads to large nonlinearity such Rydberg exciton blockade, and it can be utilized for quantum information processing and quantum simulation. |
Friday, March 19, 2021 12:18PM - 12:30PM Live |
Y57.00003: Itinerant spin and valley ferromagnetism in rhombohedral trilayer graphene (Part I) Tian Xie, Haoxin Zhou, James Ehrets, Eric M Spanton, Takashi Taniguchi, Kenji Watanabe, Maksym Serbyn, Areg Ghazaryan, Erez Berg, Andrea Young In the following two talks we report the observation of ferromagnetism in rhombohedral (ABC-stacked) trilayer graphene. Under an applied electrical displacement field, rhombohedral trilayer graphene is a semiconductor with an unusually flat band minimum. Here we show that the resulting high density of states near the band edges leads to an instability towards ferromagnetic states in which one or more of the spin and valley symmetries are broken. Using capacitance measurements to probe the inverse compressibility as a function of carrier density and perpendicular electric field, we find a series of gate tuned first order transitions---demarcated by sharp negative compressibility features and change in the degeneracy of low-magnetic field quantum oscillations. In the first of the two talks, I will describe our fabrication process for building high mobility multi-layer heterostructures while preserving the metastable rhombohedral stacking order. I will also describe our capacitance measurement technique, and explain the basic features of the experimental phase diagram. |
Friday, March 19, 2021 12:30PM - 12:42PM Live |
Y57.00004: Itinerant spin and valley ferromagnetism in rhombohedral trilayer graphene (Part II) Haoxin Zhou, Tian Xie, James Ehrets, Eric M Spanton, Takashi Taniguchi, Kenji Watanabe, Maksym Serbyn, Areg Ghazaryan, Erez Berg, Andrea Young Under an applied electrical displacement field, rhombohedral (ABC-stacked) trilayer graphene is a semiconductor with an unusually flat band minimum. Here we show that the resulting high density of states near the band edges leads to an instability towards ferromagnetic states in which one or more of the spin and valley symmetries are broken. In this second of two presentations, we will focus on the phenomenology of the observed magnetism within the combined spin- and valley isospin. Using low magnet field quantum oscillations and tilted field magnetocapacitance, we classify the competing ferromagnetic states by their spin- and valley symmetry breaking. Remarkably, measurements of similarly fabricated devices in which the trilayer graphene is aligned to a hexagonal boron nitride substrate reveal a pattern of ferromagnetic phase transitions that is only weakly perturbed relative to the superlattice free case. Our measurements show that ABC trilayer graphene hosts rare high mobility, two dimensional “half” and “quarter” metal states, and provides insight into the origin of magnetism in moire systems. |
Friday, March 19, 2021 12:42PM - 12:54PM Live |
Y57.00005: Interferences of electrostatic moiré potentials and bichromatic superlattices of electrons and excitons in transition metal dichalcogenides Qingjun Tong, Mingxing Chen, Feiping Xiao, Hongyi Yu, Wang Yao Recent experimental progresses have demonstrated the great potential of electronic and excitonic moiré superlattices in transition metal dichalcogenides (TMDs) for quantum many-body simulations and quantum optics applications. Here we reveal that the moiré potential landscapes in the TMDs heterostructures have an electrostatic origin from the spontaneous charge transfer across the heterointerfaces dependent on the atomic registry. This allows engineering tunable multi-chromatic superlattices through the interference of moiré potentials from independently configurable heterointerfaces in multilayers. We show examples of bichromatic moiré potentials for valley electrons, holes, and interlayer trions in MX2/M'X'2/MX2 trilayers, which can be strain switched from multi-orbital periodic superlattices to quasi-periodic disordered landscape. The trilayer moiré also hosts two independently configurable triangular superlattices of neutral excitons with opposite electric dipoles. These findings greatly enrich the versatility and controllability of TMDs moiré as a quantum simulation platform. |
Friday, March 19, 2021 12:54PM - 1:06PM Live |
Y57.00006: Voltage-controlled long-range propagation of indirect excitons in MoSe2/WSe2 heterostructure Lewis Fowler-Gerace, Darius Choksy, Leonid V Butov Indirect excitons (IXs) can form the medium for excitonic devices based on controlled propagation of excitons. Proof of principle excitonic devices are demonstrated in GaAs structures but limited to low temperatures due to the low IX binding energy [1]. IXs in transition-metal dichalcogenide (TMD) heterostructures are characterized by high binding energies offering the possibility for room-temperature operation of excitonic devices. However, IX propagation is fundamentally different between GaAs and TMD heterostructures due to the presence of moiré superlattice potentials in TMDs, reaching tens of meV [2,3] and forming an obstacle for IX propagation. We present long-range IX propagation with 1/e IX luminescence decay distances reaching 13 microns in MoSe2/WSe2 heterostructure despite predicted moiré potentials. We directly measure long-range IX propagation by tracing the IX luminescence through the device and control it by voltage in an excitonic transistor. |
Friday, March 19, 2021 1:06PM - 1:42PM Live |
Y57.00007: Probing correlated insulating states in twisted transition metal dichalcogenide heterostructures Invited Speaker: Yang Xu The extended Coulomb interactions between quantum particles on a lattice can drive the system into a rich variety of quantum many-body ground states that challenge theory. The emergence of transition metal dichalcogenide moiré heterostructures provides a highly controllable platform to study long-range electronic correlations. Here we report an observation of nearly two-dozen correlated insulating states at fractional fillings of a WSe2/WS2 moiré heterostructure through electrostatic gating. The discovery is enabled by a new optical sensing technique that is built on the sensitivity to dielectric environment of the exciton excited states in single-layer semiconductor WSe2. The cascade of insulating states exhibits an energy ordering which is nearly symmetric about filling factor of half electron (or hole) per superlattice site. We propose a series of charge-ordered states at commensurate filling fractions that range from generalized Wigner crystals to charge density waves. Our study lays the groundwork for utilizing moiré superlattices to simulate a wealth of quantum many-body problems that are described by the two-dimensional t-V model or spin models with long-range charge-charge and exchange interactions. |
Friday, March 19, 2021 1:42PM - 1:54PM Live |
Y57.00008: Valley-selective optical Stark effect of exciton-polaritons in monolayer WS2 Trevor LaMountain, Jovan Nelson, Erik J Lenferink, Samuel H Amsterdam, Akshay A Murthy, Tobin J Marks, Vinayak D. Dravid, Mark C Hersam, Nathaniel Stern Light provides a high-speed coherent medium for measurement and manipulation of electronic quantum states. Exploiting the optical selection rules of transition metal dicalchogenide monolayers (TMDs), the optical Stark effect allows for valley-selective control of energy levels using sub-resonant optical pulses. Recent discoveries have shown that microcavity exciton-polaritons in TMDs preserve valley features while also incorporating properties of light that can enhance valley properties. Here, we demonstrate valley-selective control of polariton energies in WS2 using the optical Stark effect. Transient reflectance measurements reveal polariton spectra with strong polarization contrast originating from valley-selective energy shifts. The shifts are well-understood using a transfer matrix model of the coupled exciton-cavity structure. This robust, valley-selective control over TMD polaritons establishes a powerful new approach for coherent manipulation of hybrid light-matter states with valley sensitivity. |
Friday, March 19, 2021 1:54PM - 2:30PM Live |
Y57.00009: Excitonic effects in optical-field-driven quasi 2D materials from time-dependent GW approach Invited Speaker: Yang-hao Chan Atomically thin quasi two-dimensional (2D) insulating materials exhibit novel exciton physics due to ineffective screening, quantum confinement, and topological effects. Such exciton physics has recently been studied in details experimentally and theoretically. Going beyond near-equilibrium, one expects that excitonic effects also dominate the responses of out-of-equilibrium systems and can lead to interesting phenomena in optically-driven 2D materials. Using a newly developed real-time, non-equilibrium Green function method within the adiabatic GW approximation, we show that, for non-centrosymmetric 2D semiconductors, excitonic effects give rise to a strong DC current, the so-called shift current, upon even sub-bandgap frequency CW light illumination through a second-order nonlinear optical process. The frequency-dependent shift current coefficients can be enhanced by orders of magnitude by the strong e-h interactions, producing a bulk photovoltaic effect (i.e., without having to have a p-n junction) of promise for applications with appropriate materials. Furthermore, we show that, in optical-field-driven angle-resolved photoemission spectroscopy (ARPES) experiments, the energy and wavefunction of excitons may be measured directly under achievable laboratory conditions. With optical pump frequencies close to the resonance frequency for exciton excitations, distinct excitonic features manifest as modulated replicas of the involved valence band states. Alos, at higher pump intensity, the quasiparticle band energies are renormalized due to the driving optical fields. |
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