Electrostatic Control of the Evolution from a Superconducting Phase to an Insulating Phase in Ultrathin $\mathrmYBa_2\mathrmCu_3\mathrmO_7-x$ Films

The electrical transport properties of ultrathin YBa2Cu3O7-x films have been modified using an electric double layer transistor configuration employing an ionic liquid. A clear evolution from superconductor to insulator was observed in nominally 7 unit-cell-thick films. Using a finite size scaling analysis, curves of resistance versus temperature, R(T), over the temperature range from 6 to 22 K were found to collapse onto a single scaling function, which suggests the presence of a quantum critical point. However, the scaling fails at the lowest temperatures indicating the possible presence of an additional phase between the superconducting and insulating regimes.