Two-phase/two-gap scenario in heavily reduced superconducting ITO films revealed by point-contact spectroscopy

We present the first point-contact measurements of heavily reduced indium tin oxide (ITO) films in the superconducting state, which simultaneously demonstrate high transparency in the visible light spectrum. Analysis of the differential conductance spectrum indicates the presence of two superconducting phases, the main one with a critical temperature T_c of about 4.6 K and an additional phase with T_c ∼ 10 K, most likely localized near the surface. The phase separation is indirectly confirmed by resistance measurements of the layers and ab initio calculations of the doped ITO electronic structure. The results obtained give hope for the creation of transparent superconducting films with critical temperatures of about 10 K.