We report on transport measurements of YBa2Cu3O7-delta Single crystals with
different oxygen contents in the geometry B, J parallel to ab (J perpendic
ular toB). 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 o
f 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 betwee
n commensurate and incommensurate smectic states.