Fractional Quantum Hall Phase Transitions and Four-flux Composite Fermions in Graphene

TL;DR

This paper investigates fractional quantum Hall states in ultraclean graphene, revealing phase transitions and negative compressibility regions, and models these phenomena using crossing Landau levels of composite particles.

Contribution

It provides the first detailed local electronic compressibility measurements of fractional quantum Hall states in graphene, uncovering phase transitions and proposing a crossing Landau level model.

Findings

Observation of fractional quantum Hall states near v = -1/2 and -1/4

Detection of phase transitions with negative compressibility

Persistence of oscillations near v = -1/2

Abstract

Graphene and its multilayers have attracted considerable interest owing to the fourfold spin and valley degeneracy of their charge carriers, which enables the formation of a rich variety of broken-symmetry states and raises the prospect of controlled phase transitions among them. In especially clean samples, electron-electron interactions were recently shown to produce surprising patterns of symmetry breaking and phase transitions in the integer quantum Hall regime. Although a series of robust fractional quantum Hall states was also recently observed in graphene, their rich phase diagram and tunability have yet to be fully explored. Here we report local electronic compressibility measurements of ultraclean suspended graphene that reveal a multitude of fractional quantum Hall states surrounding filling factors v = -1/2 and -1/4. In several of these states, we observe phase transitions that indicate abrupt changes in the underlying order and are marked by a narrow region of negative compressibility that cuts across the incompressible peak. Remarkably, as filling factor approaches v = -1/2, we observe additional oscillations in compressibility that appear to be related to the phase transitions and persist to within 2.5% of v = -1/2. We use a simple model based on crossing Landau levels of composite particles with different internal degrees of freedom to explain many qualitative features of the experimental data. Our results add to the diverse array of correlated states observed in graphene and demonstrate substantial control over their order parameters, showing that graphene serves as an excellent platform to study correlated electron phases of matter.