Superconducting phase diagram of single crystal MgB2
Résumé
Using magnetization, transport and single-crystal specific heat measurements we have determined the superconducting phase diagram of MgB2. A zero-temperature in-plane coherence length of 9.4 nm is found. The superconducting anisotropy c changes monotonously from a value around 2 near Tc to above 4.5 near 22 K. We present strong evidence for a surface superconducting state for Hkc which might account for the wide spread in reported values of the superconducting anisotropy.