A specific heat and magnetization study of high T ceramic superconductors

From the temperature dependence of the specific heat of the semiconductor LaCuO and the high temperature superconductors LaSrCuO (T=37.2 K) and YBaKCuO (T=91.5 K) in the range 1.5-30 K, a strong similarity of the lowfrequency part of their phonon density of states with a peak around 10 meV could be inferred. In the case of LaSrCuO the thermodynamical critical field below T has been determined and using the Rutger's formula and the BCS model, a Sommerfeld coefficient γ=9 mJ·mol K was obtained, which, taking into account recent results of band structure calculations leads to an electron-phonon enhancement factor γ=1.3, value compatible with T=36 K when using McMillan's formula for T. A systematic study of the magnetization offered evidence for strong flux trapping effects at higher fields and for Meissner shielding by superconducting Josephson currents in fields below 6 mT at 4.2 K. © 1987 Springer-Verlag.