Spontaneously Gapped Ground State in Suspended Bilayer Graphene

Abstract

Bilayer graphene bears an eightfold 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 kinds of devices. In bilayers of type B1 the eightfold zero-energy Landau level is partially lifted above a threshold field revealing an insulating ν=0 quantum-Hall state at the charge neutrality point. In bilayers of type B2 the Landau level 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 is < or approximately equall to 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.

Wannier function approach to realistic Coulomb interactions in layered materials and heterostructures

“…Theoretical studies have predicted that charge-and spin-density waves (CDW or SDW), quantum spin hall states (QSH), nematic, superconducting and excitonic insulator states could emerge in the bilayer [34][35][36][37][38][39][40] . Different experiments have addressed the issue of symmetry broken ground states in bilayer graphene [41][42][43][44][45][46] but the issue remains controversial also from the experimental point of view and it is e.g. unclear whether or not the ground state exhibits a finite electronic excitation gap.…”

Evidence for a spin phase transition at charge neutrality in bilayer graphene

“…In particular, a quantum spin Hall analogue has been predicted at ν = 0 in bilayer graphene if the ground state is a spin ferromagnet 8,9 . Previous studies have demonstrated that the bilayer ν = 0 state is an insulator in a perpendicular magnetic field [10][11][12][13][14][15][16] , though the exact nature of this state has not been identified. Here we present measurements of the ν = 0 state in a dual-gated bilayer graphene device in tilted magnetic field.…”

“…The application of an in-plane magnetic field and perpendicular electric field allows us to map out a full phase diagram of the ν = 0 state as a function of experimentally tunable parameters. At large in-plane magnetic field we observe a quantum phase transition to a metallic state with conductance of order 4e 2 /h, consistent with predictions for the ferromagnet.Under a strong perpendicular magnetic field, bilayer graphene (BLG) develops a ν = 0 quantum Hall (QH) state at the charge neutrality point (CNP) which displays anomalous insulating behavior [10][11][12][13][14][15][16] . Transport studies in a dual-gated geometry 12-14 indicate that this gapped state results from an interaction-driven spontaneous symmetry breaking in the valley-spin space 17 .…”

Transport gap in suspended bilayer graphene at zero magnetic field

“…2,3 Recent advances in obtaining suspended bilayer graphene devices with charge carrier mobility exceeding μ > 10 000 cm 2 V −1 s −1 gave access to the investigation of many-body phenomena in clean bilayer graphene at low charge carrier concentration (n < 10 10 cm −2 ). [4][5][6][7][8][9][10][11] Due to the nonvanishing density of states at the charge neutrality point (CNP), bilayer graphene is predicted to have a variety of ground states triggered by electron-electron interaction. There are two competing theories describing the ground state of BLG: a transition (i) to a gapped layer-polarized state [12][13][14][15][16][17] (excitonic instability) or (ii) to a gapless nematic phase.…”