Localized high-T-c superconductivity on the surface of Na-doped WO3

WO3 crystals were doped with Na on the surface to similar to7% nominal atom ic concentration. Scanning tunneling microscopy and spectroscopy were emplo yed, together with magnetization measurements, in the study of these crysta ls. Tunneling experiments reveal superconducting islands, 20-150 nm in size , covering about 10% of the surface, the rest of which is insulating. Magne tization measurements show that the superconducting phase formed at the sur face has a critical temperature of 91 K, while tunneling spectroscopy yield s superconducting gaps having values up to 2 Delta /kT(c) similar to 4. Pre sumably most of the sodium is concentrated in these islands and therefore t hey are metallic in nature, above 30% in atomic concentration. This materia l is therefore a noncuprate superconductor with a high critical temperature .