Oscillatory melting temperature of the vortex smectic phase in layered superconductors

We report on transport measurements of YBa2Cu3O7-delta Single crystals with different oxygen contents in the geometry B, J parallel to ab (J perpendicular 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 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.