Electronic phase separation in topological surface states of rhombohedral graphite

TL;DR

This study investigates the topological surface states of rhombohedral graphite, revealing robust quantum Hall effects, phase transitions, and evidence of electronic phase separation driven by strong correlations.

Contribution

It provides the first detailed experimental analysis of topological surface states in rhombohedral graphite, including phase transitions and spontaneous gap opening due to electronic correlations.

Findings

Observation of quantum Hall effect in RG surface states

Detection of phase transitions between semimetallic and quantum spin Hall phases

Evidence of electronic phase separation in thin RG films

Abstract

Of the two stable forms of graphite, hexagonal (HG) and rhombohedral (RG), the former is more common and has been studied extensively. RG is less stable, which so far precluded its detailed investigation, despite many theoretical predictions about the abundance of exotic interaction-induced physics. Advances in van der Waals heterostructure technology have now allowed us to make high-quality RG films up to 50 graphene layers thick and study their transport properties. We find that the bulk electronic states in such RG are gapped and, at low temperatures, electron transport is dominated by surface states. Because of topological protection, the surface states are robust and of high quality, allowing the observation of the quantum Hall effect, where RG exhibits phase transitions between gapless semimetallic phase and gapped quantum spin Hall phase with giant Berry curvature. An energy gap can also be opened in the surface states by breaking their inversion symmetry via applying a perpendicular electric field. Moreover, in RG films thinner than 4 nm, a gap is present even without an external electric field. This spontaneous gap opening shows pronounced hysteresis and other signatures characteristic of electronic phase separation, which we attribute to emergence of strongly-correlated electronic surface states.