SPECIFIC-HEAT ANOMALIES ON THE VORTEX MELTING LINE IN YBA2CU3O7-DELTAUP TO B=23 TESLAS - OBSERVATION OF FIRST-ORDER AND 2ND-ORDER TRANSITIONS, EFFECT OF OXYGEN DOPING, FIELD ORIENTATION, CRYSTAL PURITY AND DETWINNING

Because strong fluctuations transform the upper critical field line B- c2(T) into a smooth crossover, vortex melting is now believed to be th e only true phase transition of high temperature superconductors in a magnetic field. We review recent work performed by the Geneva and Gren oble laboratories in the calorimetric determination of the melting lin e B-m(T) in YBa2Cu3Ox. This includes three crystals showing specific h eat steps upon crossing B-m(T), and three crystals showing specific he at peaks. Adiabatic calorimetry (up to 23 T) and oscillating dynamic m ethods (up to 7 T) are used. We investigate the effect of field orient ation (B//c and B perpendicular to c), oxygen concentration x (6.95-7) , and detwinning. These experiments document both processes that may o ccur on the B-m(T) line, depending on pinning properties: the melting of a vortex lattice with a first-order transition, and the melting of a vortex glass with a second-order or continuous transition.