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 γ increases from a value around 2 near T_c 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 that might account for the widespread in reported values of γ.