SHEET SUPERCONDUCTIVITY IN TWIN WALLS - EXPERIMENTAL-EVIDENCE OF WO3-X

Single crystals of WO3 that contain ferroelastic twin walls have been chemically deoxygenated using a gas transport reaction with Na vapour. The reaction product consists of a non-superconducting matrix of tetr agonal WO3-x and superconducting twin boundaries. The superconducting transition temperature is 3 K. The upper critical field has been measu red; its temperature dependence shows BCS type behaviour at T < 2.5 K and a weak tail near T-c.