Field-dependent specific heat of polycrystalline YBa2Cu3O7−x

Abstract

We have measured the specific heat of high-purity, sintered ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ powder. The sample was characterized by x-ray diffraction and Raman scattering. We measured its dc magnetic susceptibility as a function of magnetic field and temperature to verify its superconducting properties and found the transition temperature $T_c$ to be 90 K. The heat capacity was determined between 2 and 10 K in magnetic fields up to 3 T. The low-temperature specific heat exhibited a linear term (with a zero-field coefficient of 4.37 mJ/mol ${\mathrm{K}}^2$) and a cubic term (corresponding to a Debye temperature of 375 K) that varied with field. Raman-effect data obtained on our sample showed that less than one-third of the linear term could arise from the presence of ${\mathrm{BaCuO}}_2$. Using Ginzburg-Landau theory in the London limit, we calculated the field dependence of the specific heat of a uniaxially symmetric superconductor. We were unable to account for the reduction of the cubic term, and we were able to account for only part of the enhancement of the linear term with increasing field in terms of the kinetic energy of the electrons circulating around the magnetic vortices (fluxoids). Perhaps the reason that Ginzburg-Landau theory fails to account for our data is that the superconducting order parameter does not change slowly over a length characteristic of the atomic structure of this material.