Electrostatic Control of the Evolution from Superconductor to Insulator in Ultrathin Films of Yttrium Barium Copper Oxide

The electrical transport properties of ultrathin YBCO films have been modified using an electric double layer transistor configuration employing an ionic liquid. The films were grown on SrTiO3 substrates using high pressure oxygen sputtering. 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 6K to 22K were found to collapse onto a single scaling function, which suggests the the presence of a quantum critical point. However the scaling failed at the lowest temperatures suggesting the presence of an additional phase between the superconducting and insulating regimes.