Superconductivity in Isolated Single Copper Oxygen Plane

One of the central questions in cuprate superconductivity is if superconductivity can exist in an isolated single CuO$_2$ plane without any interlayer coupling. There have been numerous experimental efforts to answer this question, but it still has not been clearly resolved. Here we present a heterostructure system with an isolated half-unit-cell La$_{2-x}$Sr$_x$CuO$_4$ which has a single CuO$_2$ plane. Using in-situ angle-resolved photoemission spectroscopy, we measured the electronic and gap structures of a single CuO$_2$ plane. We observed a \textit{d}-wave-like gap which closes somewhat above the bulk T$_c$. Moreover, almost identical gap properties are seen for both single CuO$_2$ plane and bulk. These observations lead us to the conclusion that the d-wave superconductivity of cuprates also exists in a single CuO$_2$ plane. Our results demonstrate that cuprate superconductivity is essentially a two-dimensional phenomenon and provide a platform to study cuprate superconductivity in a purely two-dimensional system.