Abstract
Magnetization and resistivity measurements on untwinned YBa 2Cu 3O y single crystals are performed in high magnetic fields up to 30T. The upper critical point H ucp at which three different phases (the vortex liquid, the vortex glass, and the Bragg glass) meet strongly depends on the oxygen content y. For fully oxidized YBa 2Cu 3O y ( y ⋍ 7, T c = 87.5 K), the first-order vortex lattice melting line H m( T) is observed up to 30 T. In the low field region below 5 T, on the other hand, H m( T) changes to the second peak and the entropy jump disappears; the results imply the existence of the lower critical point. For optimally doped YBa 2Cu 3O y ( y ⋍ 6.92, T c ⋍ 93 K), the vortex glass transition line H g ( T) approaches to the first-order transition line well below H acp, indicating the new vortex state such as a vortex slush (a pinned liquid) regime.
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