Sheet superconductivity in twin walls: experimental evidence of

Single crystals of 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 tetragonal and superconducting twin boundaries. The superconducting transition temperature is 3 K. The upper critical field has been measured; its temperature dependence shows BCS type behaviour at T < 2.5 K and a weak tail near .