Spontaneously gapped ground state in suspended bilayer graphene

TL;DR

This paper investigates the spontaneous formation of an insulating ground state in suspended bilayer graphene, revealing a new phase characterized by a zero-field gap and unique conductance properties.

Contribution

It demonstrates the existence of a spontaneously gapped insulating phase in suspended bilayer graphene, distinct from field-induced states, with detailed transport measurements.

Findings

Identification of a spontaneously gapped insulating state at zero magnetic field.

Observation of finite conductance in the insulating phase, indicating edge states.

Differentiation between two device types with distinct Landau level lifting behaviors.

Abstract

Bilayer graphene bears an eight-fold degeneracy due to spin, valley and layer symmetry, allowing for a wealth of broken symmetry states induced by magnetic or electric fields, by strain, or even spontaneously by interaction. We study the electrical transport in clean current annealed suspended bilayer graphene. We find two kind of devices. In bilayers of type B1 the eight-fold zero-energy Landau level (LL) is partially lifted above a threshold field revealing an insulating nu=0 quantum Hall state at the charge neutrality point (CNP). In bilayers of type B2 the LL lifting is full and a gap appears in the differential conductance even at zero magnetic field, suggesting an insulating spontaneously broken symmetry state. Unlike B1, the minimum conductance in B2 is not exponentially suppressed, but remains finite with a value G < e^2/h even in a large magnetic field. We suggest that this phase of B2 is insulating in the bulk and bound by compressible edge states.