Electrostatic tuning of the superconductor to insulator transition of YBa₂Cu₃O_7-x using ionic liquids

Ultrathin YBa2Cu3O7-x (YBCO) films were grown on SrTiO3 (STO) substrates using a high-pressure oxygen sputtering system. The films were incorporated in a field effect transistor configuration to study the control of superconductivity by electrostatic charging. While devices using STO as both the substrate and gate dielectric have produced only small Tc shifts, a clear transition between superconducting and insulating behaviour was realized using an electronic double layer transistor employing the ionic liquid DEME-TFSI as a gate dielectric. Employing a finite size scaling analysis, curves of resistance vs. temperature were found to collapse into two branches over the temperature range from 6 K to 22K suggesting the existence of a quantum critical point. However the scaling failed at temperatures below this range, indicating the possible presence of an additional phase between the superconducting and insulating regimes. Further depletion of holes appears to result in the accumulation of electrons resulting in a change of the majority carriers from holes to electrons and the emergence of what appears to be very weak re-entrant superconductivity. By changing the polarity of the gate voltage, an underdoped film was tuned into the overdoped regime by accumulating additional holes. An unexpected two-step mechanism for electrostatic doping was revealed. Hall effect measurements exhibited anomalous features, which suggest the occurrence of an electronic phase transition near optimal doping.