Composite Fermions and Broken Symmetries in Graphene

TL;DR

This paper reports the observation of fractional quantum Hall states in graphene's Landau levels, revealing broken valley symmetry and surprising spin behavior, advancing understanding of symmetry effects in graphene's electronic properties.

Contribution

It provides the first observation of composite-fermion FQH states in graphene's first two Landau levels and investigates symmetry breaking effects.

Findings

Fractional quantum Hall states observed at v<6 in graphene.

Evidence of broken valley symmetry from odd numerator fractions.

States in the first Landau level are not spin-polarized even at high magnetic fields.

Abstract

The electronic properties of graphene are described by a Dirac Hamiltonian with a fourfold symmetry of spin and valley. This symmetry may yield novel fractional quantum Hall (FQH) states at high magnetic field depending on the relative strength of symmetry breaking interactions. However, observing such states in transport remains challenging in graphene, as they are easily destroyed by disorder. In this work, we observe in the first two Landau levels (v<6) the composite-fermion sequences of FQH states at p/(2p+1) between each integer filling factor. In particular, odd numerator fractions appear between v=1 and v=2, suggesting a broken valley symmetry, consistent with our observation of a gap at charge neutrality and zero field. Contrary to our expectations, the evolution of gaps in a parallel magnetic field suggests that states in the first Landau level are not spin-polarized even up to very large out of plane fields.