Corbino geometry as a bulk probe in ultra-high mobility graphene.

In the quantum Hall regime, transport measurement of the edge states in a Hall bar geometry give an indirect signature of the bulk incompressible states, making the edge-bulk interplay a subject of continued importance. The Corbino geometry has the advantage of having no edges therefore providing a direct probe of bulk property. We report recent progress on designing and fabricating high quality graphene device with dual graphite gate, in a Corbino geometry. In addition to the usual fractional quantum Hall states associated with the 2-flux composite fermion sequence, we also resolve for the first time in graphene both 4-flux and 6-flux states , demonstrating the excellent quality of the Corbino devices. The dual gated geometry provides high charge carrier density, allowing us to access N=3 Landau level at magnetic field B~ 30 T. Here we observed features in conductance resembling reentrant integer quantum Hall effect for the first time in the N=3 Landau level, at filling fraction of around 10+1/4 and 10+3/4. Additionally, sdH measurements are performed to measure quantum life time in the low field limit, as a characterization of bulk disorder, and the result of such measurement is compared to Hall bar geometry.