Vortex phase diagram near the lower critical point in untwinned YBa2Cu3Oy single crystals

In order to study the vortex phase diagram near the lower critical point Hlcp of the first-order melting transition, we have measured resistivity ρ(H, T) and magnetization M(H) in untwinned YBa2Cu3O7 single crystals. Below Hlcp, the resistive transition is described by the vortex glass theory and the relatively small value of the critical exponent can be explained by the recent theory of the vortex glass scaling which takes into account the pinning disorder strength. The partial loop measurements of M(H) with repeated field cycling provide evidence of the first-order transition at the onset of the peak effect H∗(T) between the Bragg glass and the vortex glass phases.