The vortex states of YBa2Cu3O7-delta: phase diagram; calorimetric and magnetic properties

We report measurements of both the specific heat (C) and the magnetization (M) of a naturally untwinned YBa2Cu3O7-delta(YBCO) single crystal in magnet ic field (H) up to 26 Tesla. The specific heat data determine the thermodyn amic nature of the melting and show its evolution with increasing H within a low-H region (0.1 less than or equal to H less than or equal to 10.5 Tesl a) in which it is first order, with a discontinuity in C and a hysteretic l atent heat, through a multicritical point, and into a high-H region (10.5 I H I 26 Tesla) in which it is second order, with only a discontinuity in C. The qualitative change in melting in that latter region seems to reflect n ot a change in the nature of the solid but rather that of the (reversible) liquid. The M data show the relation of irreversibility to melting and allo w to distinguish between the thermodynamic features at melting to disorder- dependent properties of the solid.