Superconducting transition and phase diagram of single-crystal MgB2

The superconducting phase diagram of MgB2 was determined from magnetization, magnetotransport, and single-crystal specific-heat measurements. A zero-temperature in-plane coherence length of 9.4 nm is determined. The superconducting anisotropy gamma increases from a value around 2 near T-c to above 4.5 at 22 K. For Hparallel toc a pronounced peak effect in the critical current occurs at the upper critical field. Evidence for a surface superconducting state is presented for Hparallel toc that might account for the widespread in reported values of gamma.