Superconducting properties of Gd_xPb_1–xMo₆S₈ (x = 0.5, 0.7, 0.9) compounds

For the first time, the temperature dependences of the electrical resistivity ρ(T) were studied in Gd_xPb_1–xMo₆S₈ (x = 0.5, 0.7, 0.9) compounds in the temperature range 4–18 K and magnetic fields up to 14 T. It is shown that the superconducting transition temperature values T_c decrease from 14.6 K for a compound with x = 0.5 to 11.8 K for x = 0.9. Experimental dependences H_c2(T) were plotted and it was found that the theoretical dependence H_c2(T) within the microscopic theory of Werthamer–Helfand–Hohenberg poorly describes the experimental data for the Gd_0.9Pb_0.1Mo₆S₈ compound. At the same time, the fit of H_c2(T) within the Ginzburg–Landau theory is in good agreement with the experimental results for all the studied samples. An explanation of the dependence H_c2(T) of the studied samples from the gadolinium concentration was proposed. The differential resistance for the Gd_0.5Pb_0.5Mo₆S₈–Ag heterocontact with resistance RPC ≈ 2.6 Ω was obtained for 2.6 K using point-contact Andreev reflection spectroscopy. As a result, the superconducting gap for Gd_0.5Pb_0.5Mo₆S₈ was estimated for the first time to be Δ ≈ 1.95 meV at 2.6 K. The resulting ratio was 2Δ/kT_c ≈ 3.02, which is lower than the Bardeen– Cooper–Schrieffer value of 3.52 for conventional weakly coupled superconductors. (author)