SPECIFIC-HEAT UP TO 14 TESLA OF A YBA2CU3O6.92 SINGLE-CRYSTAL

A large porous YBa2Cu3O6.92 single crystal was synthesized using a recrystallisation technique. The superconducting transition at T(c) = 92.4 K could be narrowed down to 0.14 K by quenching. We present high resolution specific heat measurements in magnetic fields from 0 to 14 T applied along the principal crystallographic directions. Thermal fluctuations near the phase transition in zero field are analysed in terms of critical behaviour. The Gaussian approximation is found to be unsuitable inside of a temperature window \T - T(c)\ almost-equal-to 3 K. Measurements in magnetic fields indicate an anisotropy of is-a-member-of = 7 +/- 1 for H(c2), a positive curvature of the H(c2)(T) line near T(c), and a mixed-state linear term gamma(m)T increasing with H. The slope of the upper critical field at T(c), mu0partial-derivativeH(c2,c)/partial-derivativeT, is found to be approximately - 3.2 T/K both by direct determinations of T(c)(H), and by the analysis of Gaussian fluctuations in H = 0. The scaling behaviour predicted either by the low-field 3D-XY or by the high-field critical approaches are compared.