The basic macroscopic properties of the high-$T_c$ superconductors ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_4$ (x=0.075–0.20) with bulk transition temperatures up to 37.5 K were studied. The upper and lower critical fields, specific heat, Meissner effect, and electric resistivity were measured and extreme type-II behavior with large upper-critical-field slopes is observed. The density of states at $E_F$ is caLculated from three different sets of experimental quantities (${\gamma }_{\mathrm{BCS}=6\mathrm{}\pm 1.5}$ mJ/mole? sup 2—). The critical temperatures of (La, Sr${)}_2$${\mathrm{CuO}}_4$ and Ba(Pb,Bi)${\mathrm{O}}_3$ are ∼3 times higher than those of other superconductors with comparable values of γ, which empirically distinguishes them as a separate class of superconductors.

• Received 3 February 1987

DOI:https://doi.org/10.1103/PhysRevB.35.5340