Orbital magnetism and Fermi surface reconstructions near half filling in twisted bilayer graphene

Magic-angle twisted bilayer graphene (MATBG) exhibits a wide variety of correlated phases, spanning from insulating to superconducting and magnetic states, favored by the flat bands. The degeneracy among closely competing ground states can be lifted by polarizing spin and valley degrees of freedom; hence, the four-fold degeneracy of the low-energy electrons has a significant impact on the underlying mechanism governing the correlated phases at different band fillings. The overall phase diagram of MATBG is remarkably sensitive to external perturbations such as carrier density, electromagnetic field, pressure, temperature, and dielectric environments. Despite this unprecedented tunability, a complete understanding of the observed phases has remained elusive. In our recent study, we conducted magneto-transport measurements on MATBG proximitized by a layer of tungsten diselenide, thereby introducing finite spin-orbit coupling into the system. Our findings unveiled an anomalous Hall effect in the vicinity of half-filling (ν = 2), accompanied by an abrupt switching of magnetization that can be fine-tuned by varying the carrier density. Such a reversal of hysteresis suggests a ferromagnetic ground state that is orbital in nature. Additionally, near ν = 2, we observed a series of Lifshitz transitions in the zero-magnetic field limit, indicating Fermi surface reconstructions. As we increased the magnetic field, a perfectly quantized Chern insulator was observed exactly at ν = 2. These intriguing results collectively suggest the presence of valley-polarized ground states near ν = 2, which are stabilized by the inclusion of spin-orbit coupling.





Funding acknowledgment:

U.C. acknowledges funding from SERB via SPG/2020/ 000164 and WEA/2021/000005.