CRITICAL FIELDS OF MESOSCOPIC SUPERCONDUCTORS

Recent measurements have shown oscillations in the upper critical fiel d of simply connected mesoscopic superconductors. A quantitative theor y of these effects is given here on the basis of a Ginzburg-Landau des cription. I;or small fields, the H - T phase boundary exhibits a cusp where the screening currents change sign for the first time thus defin ing a lower critical field H-c1. In the limit where many flux quanta a re threading the sample, nucleation occurs at the boundary and the upp er critical field becomes identical with the surface critical field H- c3.