We report on transport measurements of YBa 2Cu 3O 7- delta single crystals with different oxygen contents in the geometry B, J||ab (JperpB). Our data show that the vortices become confined between the Cu-O planes below a well-defined temperature at which the effective size 2 xi of the vortex core is approximately equal to the period of the Cu-O layers. This confinement strongly increases the vortex liquid freezing temperature. A new melting line is found separating a vortex liquid and a smectic phase, which shows an oscillatory field dependence reflecting differences between commensurate and incommensurate smectic states
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