Cascade of Phase Transitions and Dirac Revivals in Magic Angle Graphene


  Uri Zondiner [1]  ,  Asaf Rozen [1]  ,  Daniel Rodan-Legrain [2]  ,  Yuan Cao [2]  ,  Raquel Queiroz [1]  ,  Takashi Taniguchi [3]  ,  Kenji Watanabe [3]  ,  Yuval Oreg [1]  ,  Felix von Oppen [4]  ,  Ady Stern [1]  ,  Erez Berg [1]  ,  Pablo Jarillo-Herrero [2]  ,  Shahal Ilani [1]  
[1] Weizmann Institute of Science, Israel [2] Massachusetts Institute of Technology, USA [3] National Institute for Materials Science, Japan [4] Freie Universität Berlin, Germany

In this talk, I will present our new local electronic compressibility measurements of twisted bilayer graphene near the 'magic angle'. Magic angle graphene has been shown to exhibit a variety of correlated electronic phases including insulating, magnetic, and superconducting. Our results reveal that these phases originate from a high-energy state with an unusual sequence of band populations. As carriers are added to the system, rather than filling all the four spin and valley flavors equally, we find that the population occurs through a sequence of sharp phase transitions, which appear as strong asymmetric jumps of the electronic compressibility near integer fillings of the moiré lattice. At each transition, a single spin/valley flavor takes all the carriers from its partially filled peers, "resetting" them back to the vicinity of the charge neutrality point. As a result, the Dirac-like character observed near the charge neutrality reappears after each integer filling. Measurement of the in-plane magnetic field dependence of the chemical potential near filling factor one reveals a large spontaneous magnetization, further substantiating this picture of a cascade of symmetry breakings. The sequence of phase transitions and Dirac revivals is observed at temperatures well above the onset of the superconducting and correlated insulating states. This indicates that the state we reveal here, with its strongly broken electronic flavor symmetry and revived Dirac-like electronic character, is a key player in the physics of magic angle graphene, forming the parent state out of which the more fragile superconducting and correlated insulating ground states emerge.