Origin of Optical Transparency in a Transparent Superconductor LiTi₂O₄

We discuss the transparent conducting mechanism of LiTi2O4. In the Bardeen-Cooper-Schrieffer (BCS) framework, achieving a high superconducting transition temperature (Tc) requires high carrier density at the Fermi energy. This requirement suppresses the emergence of transparent superconductivity at high temperature because the high carrier density results in optical absorption in the visible region. In this paper, we here demonstrate transparent superconducting properties with Tc exceeding 13 K for epitaxial thin films of the spinel LiTi2O4(111). Although LiTi2O4 has a very high carrier density in excess of 1022 cm-3, the film exhibits high transmittance in the visible region, which cannot be explained by the model for conventional transparent conductors. We show that a large effective mass plays a key role in the optical transparency in the visible region. These studies provide the material design principles for transparent conductors that would enable the development of highly efficient optoelectronic devices.