Transport gap in suspended bilayer graphene at zero magnetic field

We report a change of three orders of magnitude in the resistance of a suspended bilayer graphene flake which varies from a few kΩ in the high-carrier-density regime to several MΩ around the charge neutrality point (CNP). The corresponding transport gap is 8 meV at 0.3 K. The sequence of quantum Hall plateaus appearing at filling factor ν=2 followed by ν=1 suggests that the observed gap is caused by the symmetry breaking of the lowest Landau level. Investigation of the gap in a tilted magnetic fields indicates that the resistance at the CNP shows a weak linear decrease for increasing total magnetic field. Those observations are in agreement with a spontaneous valley splitting at zero magnetic field followed by splitting of the spins originating from different valleys with increasing magnetic field. Both the transport gap and B field response point toward the spin-polarized layer-antiferromagnetic state as the ground state in the bilayer graphene sample. The observed nontrivial dependence of the gap value on the normal component of B suggests possible exchange mechanisms in the system.