Cascade of Phase Transitions and Dirac Revivals in Magic Angle Graphene

TL;DR

This paper uncovers a sequence of phase transitions and Dirac revivals in magic angle graphene, revealing a high-energy state with asymmetric flavor population and symmetry breaking, which underpins its complex correlated phases.

Contribution

It demonstrates a novel cascade of flavor polarization transitions and Dirac revivals in magic angle graphene, highlighting their role as the parent state for superconductivity and insulating phases.

Findings

Sequence of sharp phase transitions with asymmetric flavor filling

Reappearance of Dirac-like electronic character after each transition

Large spontaneous magnetization indicating symmetry breaking

Abstract

Twisted bilayer graphene near the magic angle exhibits remarkably rich electron correlation physics, displaying insulating, magnetic, and superconducting phases. Here, using measurements of the local electronic compressibility, we 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 moire 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.