| Jul 04, 2023 |
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(Nanowerk News) Rydberg states occur in a range of physical systems including atoms, molecules, and solids. In particular, Rydberg excitons, which are highly energetic electron-hole pairs, were first found in the Cu2O semiconductor material in the 1950s.
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Recent research published in Science (“Observation of Rydberg moiré excitons”) by Dr. XU Yang and his team from the Institute of Physics (IOP) of the Chinese Academy of Sciences (CAS), and a group led by Dr. YUAN Shengjun from Wuhan University, reports the observation of Rydberg moiré excitons. These are moiré-confined Rydberg excitons in the WSe2 monolayer semiconductor, next to small-angle twisted bilayer graphene (TBG).
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| An illustration showing the Rydberg moiré excitons in the WSe2/TBG heterostructure. (Image: IOP)
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The solid-state nature of Rydberg excitons, their significant dipole moments, robust mutual interactions, and heightened interactions with the environment suggest potential applications in sensing, quantum optics, and quantum simulation. However, the full capacity of Rydberg excitons has not been realized due to difficulties in efficiently trapping and manipulating them. The introduction of two-dimensional (2D) moiré superlattices with tunable periodic potentials could provide a solution.
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In recent years, Dr. XU Yang and his colleagues have been investigating the use of Rydberg excitons in 2D semiconducting transition metal dichalcogenides (like WSe2). They have developed a Rydberg sensing technique that leverages the sensitivity of Rydberg excitons to the dielectric environment for detecting exotic phases in nearby 2D electronic systems.
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In the study, the researchers used low-temperature optical spectroscopy measurements to detect Rydberg moiré excitons, as evidenced by multiple energy splittings, a notable red shift, and a narrowed linewidth in the reflectance spectra.
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Through numerical calculations by the team from Wuhan University, the findings were linked to the spatially varying charge distribution in TBG. This results in a periodic potential landscape (referred to as moiré potential) for interaction with Rydberg excitons.
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Strong confinement of Rydberg excitons was achieved due to unequal interlayer interactions of the constituent electron and hole of a Rydberg exciton. This was a result of spatially accumulated charges in the AA-stacked regions of TBG. This process leads to Rydberg moiré excitons exhibiting electron-hole separation and the properties of long-lived charge-transfer excitons.
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The team demonstrated a new method of manipulating Rydberg excitons, which is challenging in bulk semiconductors. The long-wavelength (tens of nm) moiré superlattice in this study is similar to optical lattices created by a standing-wave laser beam or optical tweezer arrays used for Rydberg atom trapping.
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Furthermore, the system’s control was improved due to tunable moiré wavelengths, in-situ electrostatic gating, and a longer lifetime. These features, combined with strong light-matter interaction, facilitate optical excitation and readout.
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This research could offer novel opportunities for further Rydberg-Rydberg interactions and coherent control of Rydberg states, potentially leading to applications in quantum information processing and quantum computation.
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