Relationship between surface superconductivity, bulk superconductivity and the peak effect in MgB₂ single crystals

Utilizing a systematic study of transport measurements, we constructed a detailed H-T phase diagram of an MgB₂ single crystal. There was no tricritical point of the surface superconductivity, the bulk superconductivity and the peak effect, in contrast to the existence of this point in Nb single crystals, as obtained from the magnetic susceptibility. We found that the surface effect was still strong up to the zero-field superconducting transition temperature and that the peak effect did not disappear on the H_c2 line because a disordered flux flow was present just below H_c2. The disappearance of the peak effect is closely related to thermal fluctuations.