Observation of Rydberg moiré excitons in 2D materials

Rydberg states have a wide presence across various physical platforms, including atoms, molecules, and solids. One fascinating manifestation of Rydberg states is observed in Rydberg excitons, which are highly excited electron-hole pairs bound by Coulomb forces. These excitons were first discovered in the semiconductor material Cu2O during the 1950s.

A group of scientists led by Dr. Xu Yang from the Institute of Physics of the Chinese Academy of Sciences (CAS) and Dr. Yuan Shengjun from Wuhan University recently published a noteworthy study in the journal Science. Their research focused on the observation of Rydberg moiré excitons, which are Rydberg excitons confined within a monolayer semiconductor (WSe2) placed adjacent to small-angle twisted bilayer graphene (TBG).

Rydberg excitons in solids possess several advantageous properties, such as large dipole moments, strong mutual interactions, and significantly enhanced interactions with their surroundings. These characteristics make them promising candidates for a wide range of applications in areas like sensing, quantum optics, and quantum simulation.

Despite the immense potential of Rydberg excitons, researchers have faced challenges in effectively trapping and manipulating them. A potential breakthrough lies in the emergence of two-dimensional (2D) moiré superlattices, which offer highly tunable periodic potentials. This development opens up new avenues for efficient trapping and control of Rydberg excitons.

Dr. Xu Yang and his collaborators have been actively investigating the applications of Rydberg excitons in 2D semiconducting transition metal dichalcogenides, specifically WSe2. They have made significant progress by developing a novel sensing technique based on Rydberg excitons. This technique leverages the sensitivity of Rydberg excitons to the dielectric environment to detect exotic phases in nearby 2D electronic systems.

Spectroscopic evidence of the Rydberg moiré exciton formation in WSe2 adjacent to 0.6° TBG and numerical calculations of the spatial charge distribution in TBG at different doping levels. Credit: IOP

Through the utilization of low-temperature optical spectroscopy measurements, the scientists conducted an investigation in which they made noteworthy discoveries regarding Rydberg moiré excitons. These excitons were observed to exhibit distinct characteristics in the reflectance spectra, including multiple energy splittings, a significant red shift, and a narrowed linewidth.

To explain these observations, the research team employed numerical calculations carried out by the Wuhan University group. Their analysis attributed these unique features to the spatially varying charge distribution present in twisted bilayer graphene (TBG), which in turn creates a periodic potential landscape known as a moiré potential. This moiré potential interacts with Rydberg excitons, resulting in the observed phenomena.

The confinement of Rydberg excitons is particularly strong in this system. This is achieved due to the highly disparate interlayer interactions between the constituent electron and hole of a Rydberg exciton. These interactions are influenced by the spatial accumulation of charges, predominantly located in the AA-stacked regions of TBG. Consequently, the Rydberg moiré excitons not only demonstrate electron-hole separation but also exhibit the characteristics of long-lived charge-transfer excitons.

Twist angle dependences and crossover to the strong-coupling regime. Credit: IOP

The researchers successfully demonstrated a groundbreaking approach to manipulate Rydberg excitons, a task that is typically challenging in bulk semiconductors. They achieved this by utilizing a long-wavelength moiré superlattice, which serves as an equivalent to optical lattices generated by standing-wave laser beams or arrays of optical tweezers commonly employed in trapping Rydberg atoms.

The key advantages of the moiré superlattice in this study include its tunable wavelengths, the ability to perform in-situ electrostatic gating, and an extended exciton lifetime. These factors collectively contribute to excellent controllability of the system, facilitating a strong interaction between light and matter. This allows for convenient optical excitation and readout of the Rydberg excitons.

The outcomes of this research hold significant promise and open up new prospects for advancing Rydberg-Rydberg interactions and coherent control of Rydberg states. These advancements are crucial for various applications, particularly in the realms of quantum information processing and quantum computation. The findings of this study offer exciting opportunities for further exploration in these fields.

Source: Chinese Academy of Sciences

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