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_1−xNbO₂. 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 Li_1−xNbO₂ was ~77%, despite the large number of hole carriers exceeding 10²² per cubic centimeter. These results indicate that Li_1−xNbO₂ 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.