Superconducting Transition and Phase Diagram of Single Crystal MgB2

The superconducting phase diagram of MgB2 was determined from magnetization, magneto-transport and the first single-crystal specific heat measurements. A zero-temperature in-plane coherence length of 8 nm is determined. The superconducting anisotropy increases from a value around 2 near Tc to above 4.5 at 22 K. For H||c a pronounced peak effect in the critical current occurs at the upper critical field. Evidence for a surface superconducting state is presented for H||c which might account for the wide spread in reported values of the anisotropy.