Reentrant Superconductivity in HoMo 6 S 8

A simplified version of the s–f model, supplemented by the intersite Cooper pair interaction between conduction electrons with opposite spins, is used to investigate the competition between ferromagnetism and anisotropic superconductivity in a single‐domain ferromagnet. The calculations of the temperature dependence of the magnetizations m f (localized 4f electrons), m d (conduction electrons) and the superconducting order parameter show the existence of three critical temperatures: T C (Curie temperature), T S1 and T S2 (lower and upper superconducting transition temperatures). The critical temperatures fulfil the relation T S1 < T C < T S2 . For T ∈ [0, T S1 ] the system is ferromagnetic and normal, for T ∈ [ T S1 , T C ] ferromagnetic and superconducting, for T ∈ [ T C , T S2 ] paramagnetic and superconducting and for T > T S2 paramagnetic and normal. The superconducting order parameter possesses a distinct maximum between T S1 and T S2 and vanishes for T < T S1 and T > T S2 in agreement with the recent experimental results for HoMo 6 S 8 . The calculated temperature dependence of the dc resistivity shows a typical reentrant behaviour, characteristic for such ferromagnetic superconductors as ErRh 4 B 4 and HoMo 6 S 8 . Using a suitable choice of the model parameters we can obtain a reasonable fit of the recent experimental results for HoMo 6 S 8 , concerning the temperature dependence of the superconducting order parameter and dc resistivity.