Transparent superconductors.

Superconductivity and optical transparency are generally considered to be mutually exclusive properties. However, it has been found that some oxides can be both transparent and superconducting. Using indium tin oxide (ITO) films, we demonstrate a methodological approach to creating superconductors with controllable transparency and critical temperature. Commercial or sputtered ITO films with a thickness of 250-500 nm electrochemically reduced with various aqueous electrolytes show tunable superconducting transitions with T_c > 5 K and variable transparency. The density of states extracted from the measured Pauli susceptibility and T_c exhibit the same dome-shaped behavior as a function of electron concentration. The relationship between processing parameters and spatial variations of superconducting properties in the films were examined by analysing temperature and angular dependences of the upper critical field extracted from resistance-versus-temperature measurements. An electrochemical route to creating transparent superconductors and the mechanism behind the normal-to-superconducting transition are discussed. Our work shows the promise of ITO films for on-chip hybridization of photonic and superconducting devices and new optimized single-photon detectors.