Vortex phase diagram as a function of oxygen deficiency in untwinned YBa2Cu3Oy

Magnetization and resistivity measurements were performed in untwinned YBa2Cu3 O-y (YBCO) single crystals with different oxygen content. We find that the vortex phase diagram is a strong function of the oxygen deficiency; (1) For fully oxidized YBCO (y similar or equal to 7, T-c similar or equal to 87.5 K), the first-order vortex lattice melting line T-m(H) is observed up to 30 T. (2) For slightly overdoped YBCO (T-c similar or equal to 92 K), T-m(H) and the vortex glass transition line T-m(H) terminate at the multicritical point and the field-driven disordering transition line separates the vortex solid phase into the Bragg glass and the vortex glass phases. (3) For slightly underdoped YBCO (T-c similar or equal to 92.3 K), T-g(H) terminates well below the critical point of T-m(H), indicating the existence of the vortex slush regime by the introduced oxygen deficiency.