Superconductivity in a unique type of copper oxide

Significance Superconductivity is one of the most mysterious phenomena in nature in that the materials can conduct electrical current without any resistance. The cuprates hold the record high superconducting temperature at room pressure so far, but understanding their superconducting mechanism remains one of the big challenges. Here, we report high- T c superconductivity in Ba 2 CuO 4- y with two unique features: an exceptionally compressed local octahedron and heavily overdoped hole carriers. These two features are in sharp contrast to the favorable criteria for all previously known cuprate superconductors. Thus, the discovery of high- T c superconductivity in Ba 2 CuO 4- y calls into question the widely accepted scenario of superconductivity in the cuprates. This discovery provides a direction to search for additional high- T c superconductors.