The experimental discovery of the Richburg Mohr exciton has progressed

Xu Yang’s team from the Laboratory of Nanophysics and Devices of the Institute of Physics, Chinese Academy of Sciences reported for the first time the experimental observation of the Rydberg Mohr exciton, systematically demonstrating the controllable regulation and spatial binding of the Rydberg exciton, and providing a potential way to realize the application of the Rieedberg state in quantum science and technology based on solid-state systems. On June 30, the findings were published in Science under the title “Observation of Rydberg moiré excitons.”

The movie “Hulk” assumes that after people are subjected to strong radiation, they induce mysterious forces in their bodies and become Hulks with super power. What is difficult to achieve in reality can be achieved in solid by constructing delicate materials.

Atoms are the basic microscopic particles that make up matter. The electrons of atoms have the property of being arranged in layers. When electrons are excited into more outerly orbitals, the atoms formed are called Rydberg atoms. This excited atom is figuratively called the giant of the atomic world because of its larger “size”. The particles in semiconductor materials that are composed of positive and negative charges attracting each other are called excitons, and correspondingly, the excited state of the exciton is called the Rydberg exciton, which is also a giant in the exciton world. Like the “Hulk” has super strength, the exciton of the Rydberg state has many characteristics, such as being able to move freely in semiconductors and respond greatly to changes in the surrounding environment.

In the 50s of the 20th century, scientists first discovered an electron-hole pair in the excited state, the Rydberg exciton, in the semiconductor material Cu2O. Although such Rydberg excitons are more compatible with modern semiconductor technology, in three-dimensional solid systems, attempts to construct stable practical devices by manipulating the Rydberg exciton still face challenges such as easy absence of exciton states and few regulatory parameters. The Rydberg exciton in two-dimensional semiconductor materials provides a new direction for research due to the reduction of dimension and the enhancement of interface effects.

In the past few years, Xu Yang, a distinguished researcher in the N08 research group of the Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics, and his collaborators have developed a set of optical “Radberg exciton detection” methods. This method uses the Rydberg exciton state of the two-dimensional semiconductor WSe2 to be sensitive to the surrounding environment dielectric shielding, and realizes the effective detection of novel electron states in adjacent two-dimensional systems. Using this method, the charge ordinal state, also known as the generalized Wegener crystal state, which is widely present in WSe2/WS2 during fractional filling of the Mohr superlattice, is observed. The regulation of WSe2 band gap and exciton response by graphene/hexagonal boron nitride superlattice formation periodic dielectric environment was observed. The simulation and regulation of the two-band Hubbard model in the double-angle WSe2 were discussed. However, in these systems, the interaction between the Rydeburg exciton state and the surrounding dielectric layer is weak, and how to regulate the Rydberg exciton to form a strong coupling state and realize space confinement has become an urgent problem to be solved.

A two-dimensional material magic angle rotation method in the field of condensed matter physics brings new opportunities for manipulating the Rydberg exciton state. In recent years, under the guidance of Xu Yang, Hu Qianying, a doctoral student in the N08 group, prepared a two-dimensional van der Waals heterojunction device sample formed by monolayer WSe2 and corner graphene, and measured the exciton state in the system by low-temperature micro-reflectance spectroscopy/photoluminescence spectroscopy/photoluminescence spectroscopy, and studied the regulation of gate pressure doping. It is found that in the samples of large-angle corner graphene and magic angle graphene (~1.1°), the spectral signal of WSe2 is dominated by the “Rydberg exciton detection” mechanism, which mainly reflects the change of the intermediate electrical function of the system, such as the detection of a series of symmetry-breaking associated electron states in the sample of magic angle graphene (Figure 2). In a small-angle corner graphene sample (~0.6°, Moiré period 24 nm), the 2s Rydberg exciton state (about 7 nm in size) exhibits multiple cleavage and significant redshift with gate pressure regulation, which is called the Rydberg Mohr exciton state (Figure 3). Combined with the newly developed large-scale computational physics method in real space of Wuhan University, it is found that the spatial charge distribution in the Mohr superlattice adjusted with gate voltage may play a key role in the production of this experimental phenomenon. In this system, the periodic Moiré potential field generated in corner graphene is similar to the optical lattice in a cold atomic system, providing a highly adjustable bound potential field for the Rydberg exciton and bringing about electron-hole severely asymmetrical interlayer Coulomb interactions.

In addition, the strength of interlayer coupling in the system that evolves with the rotation angle (or Moiré cycle l) is studied (Figure 4). This coupling strength is directly reflected in the magnitude of the energy redshift of the Rydberg Mohr exciton and can be approximated by the ratio of the Mohr period l to the exciton radius rB (Figure 1). When l/rB is small, the effect of the Moiré potential field is weak, the cruising characteristics of excitons remain unchanged, and the optical signal is mainly dominated by the exciton detection mechanism. As l/rB increases, the system enters the strong coupling limit, and the Rydberg Mohr exciton spectrally exhibits multiple energy splitting peaks, non-monotonic redshift, and narrowed linewidth. These features become more pronounced as the Mohr period increases (the angle decreases), consistent with the physically image of the space-bound Rydberg exciton.

Just as Rydberg atoms can have strong interaction and sensitivity to external fields, and the light levitation arrays formed by them can be used for quantum simulation and quantum computing, the experimental findings of Rydberg Moir’s exciton state systematically demonstrate the controllable adjustment and spatial binding of Rydberg excitons, and provide a potential way to realize the application of Rydberg state in quantum science and technology and other directions based on solid-state systems.

The research work is supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences, the Huairou Comprehensive Extreme Conditions Experimental Device, and the Supercomputing Center of Wuhan University. Researchers from Nankai University and Wuhan University participated in the research. (Source: Institute of Physics, Chinese Academy of Sciences)

The experimental discovery of the Richburg Mohr exciton has progressed

Figure 1. Schematic diagram of the interaction between the Rydberg exciton and the Moiré superlattice

Figure 2. Device structure and optical response to WSe2 adjacent to 10° or 1.14° angle graphene

Figure 3. The Rydberg Moirer exciton in WSe2 adjacent to the 0.6° angle graphene and its grid pressure evolution law

Figure 4. The evolution of the Rydberg Mohr exciton state with rotation

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