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 Li1-x NbO2. The LiNbO2 epitaxial films with NbO2 sheets parallel to (111) plane of cubic MgAl2O4 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 Li1-x NbO2 was ~77%, despite the large number of hole carriers exceeding 1022 per cubic centimeter. These results indicate that Li1-x NbO2 is a previously unknown p-type transparent superconductor, in which strongly correlated electrons at the largely isolated Nb 4d z2 band play an important role for the high transparency.