p-type transparent superconductivity in a layered oxide.

Development of p-type transparent conducting materials has been a challenging issue. The known p-type transparent conductors unsatisfy both of high transparency and high conductivity nor exhibit superconductivity. Here, we report on epitaxial synthesis, excellent p-type transparent conductivity, and two-dimensional superconductivity of Li NbO. The LiNbO epitaxial films with NbO sheets parallel to (111) plane of cubic MgAlO substrates were stabilized by heating amorphous films. The hole doping associated with Li ion deintercalation triggered superconductivity below 4.2 kelvin. Optical measurements revealed that the averaged transmittance to the visible light of ~100-nanometer-thick Li NbO was ~77%, despite the large number of hole carriers exceeding 10 per cubic centimeter. These results indicate that Li NbO is a previously unknown p-type transparent superconductor, in which strongly correlated electrons at the largely isolated Nb 4d band play an important role for the high transparency.