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First spectroscopic evidence of strongly correlated states in graphene moiré superlattices. Recently, a new research result entitled "Spectroscopy signatures of electron correlations in a trilayer graphene/hBN moiré superlattice" was published in Science. This result is done by a collaborated team between Prof. Chen Guorui from the School of Physics and Astronomy, Shanghai Jiao Tong University and Prof. Ju Long from the Department of Physics, Massachusetts Institute of Technology. The research report the first spectroscopic evidence of a strong correlation phenomenon in a graphene moiré superlattice system.

Strong correlation (electron-electron interaction) is an important concept in condensed matter physics, which brings many important physical phenomena, such as magnetism, high temperature superconductivity, etc. In recent years, two-dimensional moiré superlattices (such as magic-angle graphene), which are prepared by stacking the same or different two-dimensional materials with a twist, were reported to host strong correlation phenomena including Mott insulators, superconductivity, orbital magnetism, Chen insulators, Wigner crystals, etc., rising many interesting problems and providing a new platform for the study of strongly correlated physics. Among different moiré superlattice systems, the ABC-trilayer graphene/boron nitride superlattice has unique controllability to achieve different quantum phases in a single device. People can tune the number of electrons, correlation strength, and topology of the system by simply adjust the gate voltages, leading to in situ realization of metals, Mott insulators, superconductors, and Chern insulators in ABC-trilayer graphene moiré superlattices. However, limited by the presence of the top gate metal, previous experimental work are mainly focused on electrical transport measurements, lacking important spectral information.

Figure 1. (A) Schematic of ABC-trilayer graphene/hBN moire superlattice device and FTIR photocurrent measurement setup; (B) Optical transition from valence band to conduction band；(C) Optical transition from LHB to UHB of a Mott insulator.

In order to directly detect the basic information such as the correlation strength in the Moiré superlattice system, in this work, the top gate of the device is designed as a nickel-chromium alloy with a certain transmittance in the infrared light, and the Fourier transform infrared (FTIR) photocurrent spectrum is used to measure the strongly correlated states (Mott insulating states) of ABC-trilayer graphene moiré superlattices. By measuring the optical transition spectra from the conduction band to the valence band with increasing the vertical electric field, a trend of decreasing band width of the trilayer graphene moiré superlattice was experimentally observed. A minimum band width of ~12 meV is observed, which is significantly smaller than the estimated electron-electron Coulomb energy of ~25 meV, supporting the theory that the correlation effect is dominant in the system. Further, by adjusting the number of electrons, the system was adjusted to n = 1/2 Mott insulating state, that is, two electrons in each moiré superlattice, and an optical transition with an energy of 18 meV was observed. It is found that this transition corresponds to the optical transition from the Lower Hubbard Band (LHB) to the Upper Hubbard Band (LHB), that is, the inter-electron Coulomb energy in the system, which is close to the estimated ~25 meV. And through further analysis, the possibility that the Mott insulating state at n = 1/2 is spin or valley polarization can be ruled out. This experimental result directly proves the existence of the strong correlation effect in the trilayer graphene moiré superlattice system, and experimentally gives the energy scale related to the Hubbard model describing the strong correlation system, which is very important for accurately describing the moiré superlattice. At the same time, this work demonstrates the unique advantages of the rich physics of ABC-trilayer graphene moiré superlattices and FTIR photocurrent spectroscopy in the measurement of similar systems.

Associate professor Chen Guorui is the co-first author of the paper, and the main collaborators are assistant professor Ju Long and Dr. Yang Jixiang and Dr. Han Tianyi from the Massachusetts Institute of Technology. The research was supported by National Key Research Program of China, NSF of China, and Shanghai Jiao Tong University.

Reference：https://www.science.org/doi/10.1126/science.abg3036