Spectroscopic Evidence for a Spin- and Valley-Polarized Metallic State in a Nonmagic-AngleTwisted Bilayer Graphene

In the magic-angle twisted bilayer graphene (MATBG), strong electron−electron (e−e) correlations caused by the band-flattening lead to many exotic quantum phases such as superconductivity, correlated insulator, ferromagnetism, and quantum anomalous Hall effects, when its low-energy van Hove singularities (VHSs) are partially filled. We collaborated with the experimental group of Prof. Lin He from the Beijing Normal University on the twisted bilayer-graphene. The STM observations of He’s group showed that the 1.49o-twisted bilayer graphene would experience a phase transition at low temperature when the one van Hove peak would be split into four peaks. Simultaneously, the spatial symmetry of electronic states around the split VHSs is broken by the e−e correlation. Our analysis based on the continuum model suggests that such a one-to-four split of the VHS originates from the formation of an interaction-driven spin-valley-polarized metallic state near the VHS, which is a symmetry-breaking phase that has not been identified in the MA-TBG or in other systems.