Vortex-matter phase diagram in YBa₂Cu₃Oy

This paper reviews the progressive results of the vortex-matter phase diagram for the high- T c superconductor YBa 2 Cu 3 O y in magnetic fields up to 30 T. Evidence of the first-order vortex-lattice melting transition and the second-order vortex-glass transition is demonstrated by resistivity and magnetization measurements. The feature of the novel second magnetization peak is summarized and the origin of the new line H *( T ), which is defined by the sharp onset of the second peak, is discussed on the basis of the field-driven disordering transition theory. Effects of the weak point disorder on the vortex-matter phase diagram are also studied by the irradiation of 2.5 MeV electrons. It is revealed that three transition lines of the vortex-lattice melting line H m ( T ), the vortex-glass transition line H g ( T ), and the field-driven disordering transition line H *( T ) terminate at the critical point and divide the vortex-matter phase into three different phases: the vortex liquid, the Bragg glass, and the vortex glass. Interesting similarities of the vortex matter phase diagram between YBa 2 Cu 3 O y and Bi 2 Sr 2 CaCu 2 O y are also discussed.