Evidence for localized high-T-C superconducting regions on the surface of Na-doped WO3

Scanning tunneling spectroscopy along with magnetization measurements were employed in the study of Na-doped WO3. The magnetization data suggest that a superconducting phase is formed below T-C = 91 K. Tunneling experiments r eveal islands, 20-150 nm in size, having a superconductor-like density of s tates. These islands cover about 10% of the surface, the rest of which is i nsulating, and exhibit spectroscopic gaps having values up to 2 Delta/kT(C) similar to 4. Therefore this material seems to be a non-cuprate supercondu ctor with a high critical temperature.