image: (a) Phase diagrams of half-filling states as a function of displacement field and magnetic field. Here VP is the valley-polarized state, IVC is the intervalley coherence state, BZO are the Brown-Zak oscillations. (b) Quantum oscillations of resistance. view more
Credit: ©Science China Press
Graphene based moiré superlattice, stacked by two pieces of single or multilayer graphene with a twisted angle, is famous for hosting moiré flat bands and correlated states. Thus far, a new field of twistronics has emerged and attracted lots of attentions from various fields including materials science, theory, electronics and optoelectronics, and etc., since the discovery of the correlated insulators and superconductivity in twisted bilayer graphene (1+1). Compared to the 1+1 system, the band structure in twisted double bilayer graphene (2+2) can be further tuned by electric field, aside from the twisted angle, and thus it allows a tuning of flat bands and the correlation strength in situ. Recently, the spin-polarized and valley polarized correlated insulators have been observed when the moiré bands are half filled in 2+2. With its highly tunable nature, 2+2 offers a new platform for discovering novel exotic phases in the correlated insulating states.
Recently, a team led by Dr. Wei Yang and Dr. Guangyu Zhang (Institute of Physics, Chinese Academy of Sciences) reports the observation of anomalous quantum oscillations (QOs) of correlated insulators in twisted double bilayer graphene. The team has long been devoted to explore the quantum transport behaviors in moiré superlattices. Previously, they found that new correlated insulators with valley polarizations emerges at half fillings of energy bands, thanks to the orbital Zeeman effect in perpendicular magnetic field. To their surprise, recently, they found that the resistance of correlated insulators in 2+2 oscillates periodically with the inverse of magnetic field, similar to the Shubnikov de Haas oscillations in metal. Moreover, the oscillating periodicity of the insulating states is found tunable by electric field. To account for these anomalous phenomena, they built a phenomenological inverted band model. With the parameters extracted from experiments, calculations of the density of states from the model qualitatively reproduce the electric field tunable QOs of correlated insulators. The observation of QOs of insulators in this study builds an intimate connection to other strong correlated systems like Kondo insulators, topological insulators and excitonic insulators, and it highly suggests that more exotic phases are to be discovered in this system.
See the article:
Quantum oscillations in field-induced correlated insulators of a moiré superlattice
https://doi.org/10.1016/j.scib.2023.05.006
Journal
Science Bulletin