Capacitance-based tunneling spectroscopy of monolayer graphene

Traditional tunneling measurements do not function on insulating samples as there is no means to extract a tunneling current from them. We instead utilize a pulsed tunneling technique that exploits capacitive detection in a vertical sample geometry. Using this approach, we can tunnel into fully insulating materials such as the quantum Hall states of graphene. Moreover, the energy resolution of the method is limited only by the sample temperature. We measure the tunneling conductance between a graphite tunneling electrode and monolayer graphene through an atomically thin layer of hexagonal boron nitride in high magnetic fields and over a large range of carrier densities in the quantum Hall regime. We observe unexpected energy splittings in the Landau levels for filling factors that place the Fermi level in the cyclotron gap.