Electric field effect on insulating cuprate planes

We have studied field effect doping of nearly insulating p-type CuO2 planes in single crystal transistor heterostructures. By using a high epsilon(r) epitaxial SrTiO3 dielectric layer, a wide range of doping control is obtained, from -0.40 to 0.15 carriers/Cu (or similar to10(14) carriers/cm(2)). While a considerable field effect is observed for carrier depletion, the induced holes are completely localized even up to carrier density levels far beyond the bulk insulator-to-superconductor transition value. This implies that large induced carrier density and single crystalline interface is not a sufficient condition for electric field induced insulator-to-superconductor transition for cuprates. We show that the induced carriers are almost confined to the top single CuO2 plane and propose that two-dimensional confinement introduces this localization. Understanding and overcoming this localization behavior is a serious challenge to any attempt to use electric field to induce superconductivity in insulating cuprates.