Abstract
An extensive study of electrical resistivity, ac magnetic susceptibility, magnetization, specific heat, and NMR has been made on high purity samples of the spinel compounds and . The superconducting transitions occur at 4.70 K in and 3.48 K in . The magnetic susceptibilities show perfect diamagnetism in both compounds. Upper critical fields at T=0 are estimated to be 20.0 and 4.40 kOe, the lower critical fields at T=0 are 70 and 95 Oe, respectively. The thermodynamic critical fields at T=0 are 704 Oe () and 445 Oe (). These values are obtained from the temperature dependence of the free energy. These compounds are type-II superconductors with Ginzburg-Landau parameters κ=21 () and κ=6.6 () at T=0. The electron-phonon coupling constants, =0.68 () and =0.64 (), are obtained using the McMillan formula. The copper nuclear-spin-relaxation rate as a function of temperature shows a pronounced coherence peak just below for . Experimental results are analyzed on the basis of the BCS theory. These compounds exist between weak- and intermediate-coupling superconductors which are driven by electron-phonon interaction. The Debye temperatures FTHETA of these materials are 230 K () and 211 K ().
- Received 18 November 1994
DOI:https://doi.org/10.1103/PhysRevB.51.12673
©1995 American Physical Society