Insulating Behavior at the Neutrality Point in Single-Layer Graphene

The fate of the low-temperature conductance at the charge-neutrality (Dirac) point in a single sheet of graphene is investigated down to 20 mK. As the temperature is lowered, the peak resistivity diverges with a power-law behavior and becomes as high as several Megohms per square at the lowest temperature, in contrast with the commonly observed saturation of the conductivity. As a perpendicular magnetic field is applied, our device remains insulating and directly transitions to the broken-valley-symmetry, ν=0 quantum Hall state, indicating that the insulating behavior we observe at zero magnetic field is a result of broken valley symmetry. Finally we discuss the possible origins of this effect. The ability to create electronic devices in graphene has made it possible to study 2D Dirac fermions in the solid state [1]. Transport measurements in a large magnetic field display quantum Hall plateaus with unconventional values of conductance, a signature of the Dirac equation describing electrons in graphene [1, 2]. When the cy-clotron gap becomes larger than disorder-induced fluctu