The Rydberg state is common in a range of physical platforms like molecules, atoms, and solids.
A cartoon showing the Rydberg moiré excitons in the WSe2/TBG heterostructure. Image Credit: IOP
Especially, Rydberg excitons are known to be highly excited Coulomb-bound states of electron–hole pairs, initially discovered in the semiconductor material Cu2O in the 1950s.
In a study published in the journal Science, Dr. Yang Xu and his collaborators from the Institute of Physics (IOP) of the Chinese Academy of Sciences (CAS), in partnership with scientists headed by Dr. Shengjun Yuan of Wuhan University, have reported seeing Rydberg moiré excitons, which are moiré-trapped Rydberg excitons in the monolayer semiconductor WSe2 that is adjacent to small-angle twisted bilayer graphene (TBG).
The solid-state nature of Rydberg excitons, integrated with their big dipole moments, powerful mutual interactions, and greatly improved interactions with the surroundings, hold a guarantee for an extensive range of applications in quantum optics, sensing, and quantum simulation.
But scientists have not completely exploited the ability of Rydberg excitons. One of the primary hindrances comes under the complexity of effectively trapping and manipulating Rydberg excitons. The increase of two-dimensional (2D) moiré superlattices with greatly tunable periodic potentials offers a possible step ahead.
In the past few years, Dr. Yang Xu and his partners have worked on examining the application of Rydberg excitons in 2D semiconducting transition metal dichalcogenides (like WSe2).
They have come up with a new Rydberg sensing method that uses the sensitivity of Rydberg excitons to the dielectric environment to discover the strange phases in a nearby 2D electronic system.
In this study performed, making use of low-temperature optical spectroscopy measurements, the scientists initially discovered the Rydberg moiré excitons manifesting as several energy splittings, a pronounced red shift, and also a narrower linewidth in the reflectance spectra.
With the help of numerical calculations executed by the group from Wuhan University, the scientists attributed such observations to the spatially altering charge distribution in TBG. This is responsible for creating a periodic potential landscape (alleged moiré potential) for cooperating with Rydberg excitons.
The powerful confinement of Rydberg excitons has been obtained by the mostly uneven interlayer interactions of the constituent electron and hole of a Rydberg exciton as a result of the spatially accumulated charges centered in the AA-stacked regions of TBG. Hence, the Rydberg moiré excitons identify electron–hole separation and display the character of long-lived charge-transfer excitons.
A new technique of manipulating Rydberg excitons was illustrated by the scientists, which is hard to be achieved in heavy semiconductors. In this study performed the long-wavelength (tens of nm) moiré superlattice acts as an analog to the optical lattices made by a standing-wave laser beam or arrays of optical tweezers that have been utilized for Rydberg atom trapping.
Besides, tunable moiré wavelengths, in-situ electrostatic gating, and a longer lifetime all guarantee huge controllability of the system, with a powerful light-matter interaction for comfortable optical excitation and readout.
This study performed might offer new chances for identifying the next step in coherent control of Rydberg states and Rydberg–Rydberg interactions, with feasible applications in quantum computation and quantum information processing.