Superconductivity in spinel-type compounds CuRh2S4 and CuRh2Se4

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 CuRh2S4 and CuRh2Se4. The superconducting transitions occur at 4.70 K in CuRh2S4 and 3.48 K in CuRh2Se4. 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 (CuRh2S4) and 445 Oe (CuRh2Se4). These values are obtained from the temperature dependence of the free energy. These compounds are type-II superconductors with Ginzburg-Landau parameters κ=21 (CuRh2S4) and κ=6.6 (CuRh2Se4) at T=0. The electron-phonon coupling constants, λep=0.68 (CuRh2S4) and λep=0.64 (CuRh2Se4), are obtained using the McMillan formula. The copper nuclear-spin-relaxation rate as a function of temperature shows a pronounced coherence peak just below Tc for CuRh2S4. 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 Θ of these materials are 230 K (CuRh2S4) and 211 K (CuRh2Se4)