Transport in insulating YBa₂Cu₃O_7-δ

We have measured the transport properties in field effect transistors that incorporate underdoped YBa2Cu3O7-δ as the conducting channel material. Once fabricated, devices are deoxygenated in argon gas at temperatures 250-350 °C. The devices are then vapor cooled above a bath of liquid helium and two-wire resistivity measurements are made between room temperature and 4.2 K. For zero applied gate voltage (Vg) and in sufficiently underdoped devices we observe two-dimensional (2D) Mott variable range hopping (VRH). As we remove more oxygen, we observe a transition to Efros-Shklovskii VRH. For Vg<O we generally observe 2D Mott VRH.