Superconductivity of one-unit-cell thick ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ thin film

We have found for the first time that superconductivity can occur in a one-unit-cell-thick layer of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ (YBCO). The layer is grown, while monitoring with RHEED oscillations, on a superconducting ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ (PrBCO) buffer layer and covered by a PrBCO layer by reactive evaporation. A reduction of the onset temperature is found to be mainly due to a decrease of the hole carriers but not due to the absence of interlayer couplings. This covering PrBCO layer has been found to provide the YBCO layer with carrier holes sufficient to induce superconductivity.