MICROSCOPIC DESCRIPTION OF THE VORTEX STATE NEAR THE UPPER CRITICAL-FIELD

We solve the BCS-Gor'kov equations for a pure, isotropic type-II superconductor near the upper critical field. Corrections to the semiclassical results lower the value of the upper critical field and cause H-c2(T) to vanish quadratically near T-c. This form leads to positive curvature in the critical field, which has been seen in a range of materials, and means that a type-II superconductor will convert to type I at a temperature close to T-c.