Coulomb-driven broken-symmetry states in doubly gated suspended bilayer graphene

TL;DR

This study investigates how electron interactions in suspended bilayer graphene lead to various symmetry-breaking states in the quantum Hall regime, revealing three distinct ordered phases at low carrier densities.

Contribution

It provides the first experimental identification of three different broken-symmetry states in bilayer graphene, including one with potential spontaneous time-reversal or rotational symmetry breaking.

Findings

Identification of three distinct broken-symmetry states at low densities

Evidence of a state with spontaneous time-reversal or rotational symmetry breaking

Controlled manipulation of spin and sublattice symmetries to determine order parameters

Abstract

The non-interacting energy spectrum of graphene and its bilayer counterpart consists of multiple degeneracies owing to the inherent spin, valley and layer symmetries. Interactions among charge carriers are expected to spontaneously break these symmetries, leading to gapped ordered states. In the quantum Hall regime these states are predicted to be ferromagnetic in nature whereby the system becomes spin polarized, layer polarized or both. In bilayer graphene, due to its parabolic dispersion, interaction-induced symmetry breaking is already expected at zero magnetic field. In this work, the underlying order of the various broken-symmetry states is investigated in bilayer graphene that is suspended between top and bottom gate electrodes. By controllably breaking the spin and sublattice symmetries we are able to deduce the order parameter of the various quantum Hall ferromagnetic states. At small carrier densities, we identify for the first time three distinct broken symmetry states, one of which is consistent with either spontaneously broken time-reversal symmetry or spontaneously broken rotational symmetry.