Possible coexistence of superconductivity and magnetism in layered compounds

Abstract

The possibility of a coexistent superconducting and magnetic phase in layered transition metal dichalcogenides of the type MX 2 A x is investigated (M =transition metal; X = S, Se; A = magnetic ion; x ≥ 0.25). Describing such systems with a model in which a highly anisotropic electron gas interacts with a quasi-two-dimensional Heisenberg magnet, we find the following conditions for coexistence: (1) The coupling between adjacent layers of ferromagnetically ordered magnetic ions needs to be antiferromagnetic (〈S z 〉 = 0) in order to have no pair-breaking internal fields. (2) The exchange interaction between conduction electrons and magnetic ions must be very small (≲ 0.003 eV), since otherwise, due to the high concentration of localized magnetic moments, spin-flip scattering processes and spin fluctuations would destroy superconductivity. The theoretical prediction for the persistence of superconductivity up to concentrations of x≈ 0.25 of magnetic ions is compared with recent experiments on Eu-intercalated TaS 2 and NbS 2 and related compounds showing a tendency for ferromagnetically ordered layers of Eu impurities and antiferromagnetic coupling between neighboring layers but no superconductivity for x exceeding a few percent. Reasons for the quick disappearance of superconductivity in these systems and criteria for possible observation of “high-magnetic-impurity-concentration superconductivity” in other layered compounds are given.