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

A large porous YBa2Cu3O6.92 single crystal was synthesized using a rec rystallisation technique. The superconducting transition at T(c) = 92. 4 K could be narrowed down to 0.14 K by quenching. We present high res olution specific heat measurements in magnetic fields from 0 to 14 T a pplied along the principal crystallographic directions. Thermal fluctu ations near the phase transition in zero field are analysed in terms o f critical behaviour. The Gaussian approximation is found to be unsuit able inside of a temperature window \T - T(c)\ almost-equal-to 3 K. Me asurements 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 sl ope 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 di rect determinations of T(c)(H), and by the analysis of Gaussian fluctu ations in H = 0. The scaling behaviour predicted either by the low-fie ld 3D-XY or by the high-field critical approaches are compared.