Low-temperature specific heat of polycrystalline YBa2Cu3O6.70 in applied magnetic fields of 0, 1, 2, and 3 T

We have measured the specific heat of sintered YBa2Cu3O6.70 powder between 1.85 and 77 K in zero magnetic field, and between 1.75 and 9.5 K in applied fields up to 3 T. The sample was characterized by x-ray diffraction, scanning electron microscopy, iodometric titration, and magnetic susceptibility. The onset of the diamagnetic transition to the superconducting state occurred at Tc=67 K. For the specific heat in nonzero applied magnetic field, there is an increase in the magnitude of the term that is linear in temperature, and a decrease in the term that is cubic in temperature. These results agree with earlier results on the Tc=90 K phase of YBa2Cu3O7-. The data for nonzero applied fields were analyzed by using Ginzburg-Landau theory in the London limit for a uniaxially symmetric superconductor.