Superconductivity in Ferromagnetic RuSr2GdCu2O8

TL;DR

This paper explores how superconductivity can coexist with ferromagnetism in RuSr2GdCu2O8 by analyzing its layered structure and magnetic interactions, suggesting the possibility of unconventional pairing states like FFLO.

Contribution

It provides a theoretical explanation for coexistence of superconductivity and ferromagnetism in RuSr2GdCu2O8 based on weak exchange splitting and layered electronic structure.

Findings

Weak exchange splitting allows singlet pairing.

Superconducting order may be of FFLO type.

π-phase formation is energetically favored.

Abstract

Relying on the inhomogeneous (layered) crystal, electronic, and magnetic structure, we show how superconductivity can coexist with the ferromagnetic phase of RuSr2GdCu2O8 as observed by Tallon and coworkers. Since the Cu d_{x^2-y^2} orbitals couple only to apical O p_x, p_y orbitals (and only weakly), which also couple only weakly to the magnetic Ru t_{2g} orbitals, there is sufficiently weak exchange splitting, especially of the symmetric CuO2 bilayer Fermi surface, to allow singlet pairing. The exchange splitting is calculated to be large enough that the superconducting order parameter may be of the Fulde-Ferrell-Larkin-Ovchinnikov type. We also note that \pi-phase formation is preferred by the magnetic characteristics of RuSr2GdCu2O8.