Preparation, characterization, and superconducting properties of tetragonalLaBaCaCu3O7+δ

Samples of ${\mathrm{LaBaCaCu}}_3$ ${\mathrm{O}}_{7+\mathrm{\delta }}$ were prepared by standard solid-state reactions. X-ray powder diffraction data taken in the temperature range 6–300 K reveal no structural transition. Thermogravimetric analysis was used to determine the oxygen content δ. Both magnetization and resistivity measurements indicate sharp superconducting transitions (ΔT<4 K) at 78 K for samples with δ=0±0.05. Broad transitions were always observed for samples with δ<0, although the $T_c$ onsets remained essentially constant. A sample with excess oxygen (δ>0) achieved by high-pressure oxygen annealing showed a broad transition, as well as a $T_c$ which was depressed by 10 K. We consider these experimental results in the context of ordering of the cations La, Ba, and Ca as well as oxygen sublattice ordering. We estimate the electronic coefficient of specific heat γ and the Ginzburg-Landau parameters ${\xi }_{\mathrm{GL}}$, ${\lambda }_{\mathrm{GL}}$, and ${\kappa }_{\mathrm{GL}}$ from the measured temperature dependence of $H_{c2}$ and the Pauli susceptibility.