Superconductivity in uranium ferromagnets

Abstract

The theoretical description and the survey of physical properties of superconducting states in the uranium ferromagnetic materials are presented. On the basis of microscopic theory is shown that the coupling between the electrons in these ferromagnetic metals by means of magnetization fluctuations gives rise the triplet pairing superconducting state and the general form of the order parameter dictated by the symmetry is established. The theory allows to explain some specific observations including peculiar phenomenon of reentrant superconductivity in URhGe in magnetic field perpendicular to the direction of spontaneous magnetization.In addition we describe several particular topics relating to uranium superconducting ferromagnets: (i) critical magnetic relaxation in dual localized-itinerant ferromagnets, (ii) phase transition to ferromagnetic state in Fermi liquid and UGe2, (iii) superconducting ordering in ferromagnetic metals without inversion symmetry.

Pairing mechanism in the ferromagnetic superconductor UCoGe

“…32 successfully explains the positive curvature of H c2 in UCoGe for H // c , as well as the anomalous anisotropy between c -axis and the ( a , b ) plane. This theory includes no element specific to UCoGe, and should apply equally to URhGe.…”

“…1b). References 17,321732 proposed an earlier theoretical approach based on a general and well defined framework. It had predicted that when pairing is mediated by ferromagnetic spin fluctuations, the field dependence of the magnetization or of the Curie temperature drives the field dependence of the superconducting coupling strength.…”

“…17 (extended in the more recent ref. 32), a field dependence of the pairing strength is predicted from a general framework of the ferromagnetic state (Landau approach), valid both in the itinerant and localized limits. It has been derived in the weak coupling limit for superconductivity: comparing this theory to our experimental results implicitly assumes that the same field dependence is preserved in the intermediate coupling regime, and shows up in normal state quantities like γ ( H ).…”

“…Reference 3232 gives expressions for the susceptibilities χ ij ( T , M , q ), with a wave vector ( q ) dependence arising from the exchange terms in the Landau free energy. The pairing strength for a p -wave order parameter (called ‘ g ' instead of ‘ λ ' in ref.…”

“…The pairing strength for a p -wave order parameter (called ‘ g ' instead of ‘ λ ' in ref. 32) is calculated in a form which can be cast as:…”