Calorimetric study of the transitions between the different vortex states in YBa{sub 2}Cu{sub 3}O{sub 7}.

We have studied the vortex phase diagram of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) in very strong magnetic field (0-26 Tesla) by a.c. calorimetry. We describe the anomalies associated with the transitions between the different vortex states (solid, liquid, and glass), with special emphasis on the first order flux lattice melting.