Vortex phase diagram of a weakly pinned YBa2Cu3O7−δ crystal for H || c

The vortex phase diagram in a weakly pinned crystal of YBa2Cu3O7−δ (YBCO) for H || c is reviewed in the light of a recent elucidation of the process of 'inverse melting' in a bismuth cuprate system and the imaging of an interface between the ordered and the disordered regions across the peak effect in 2H-NbSe2. In the given YBCO crystal, a clear distinction can be made between the second magnetization peak and the peak effect in between 65 K and 75 K. The field region between the peak fields of the second magnetization peak (Hmsmp) and the onset fields of the peak effect (Honpe) is not only continuously connected to the Bragg glass phase at lower fields, but it is also sandwiched between the higher temperature vortex liquid phase and the lower temperature vortex glass phase. Thus, an ordered vortex state between (Hmsmp) and (Honpe) can get transformed to the (disordered) vortex liquid state on heating as well as to the (disordered) vortex glass state on cooling, a situation analogous to the thermal melting and the inverse melting phenomenon seen in a bismuth cuprate.