Antiferromagnetic superconductors with effective mass anisotropy in magnetic fields

TL;DR

This paper derives equations for the critical magnetic fields in antiferromagnetic superconductors with anisotropic effective mass, incorporating mechanisms like FISC, extended Jaccarino-Peter, and FFLO states, and compares with experimental data.

Contribution

It introduces a comprehensive theoretical framework for critical fields in anisotropic antiferromagnetic superconductors, including mechanisms and states not previously combined.

Findings

Reproduces experimental phase diagrams of kappa-(BETS)_2FeBr_4

Suggests the extended Jaccarino-Peter mechanism occurs in the studied compound

Predicts FFLO state may occur at low fields for certain pairings

Abstract

We derive critical field H_c2 equations for antiferromagnetic \textit{s}-wave, d_{x^2-y^2}-wave, and d_{xy}-wave superconductors with effective mass anisotropy in three dimensions, where we take into account (i) the Jaccarino-Peter mechanism of magnetic-field-induced superconductivity (FISC) at high fields, (ii) an extended Jaccarino-Peter mechanism that reduces the Pauli paramagnetic pair-breaking effect at low fields where superconductivity and an antiferromagnetic long-range order with a canted spin structure coexist, and (iii) the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO or LOFF) state. As an example, experimental phase diagrams observed in organic superconductor kappa-(BETS)_2FeBr_4 are theoretically reproduced. In particular, the upper critical field of low-field superconductivity is well reproduced without any additional fitting parameter other than those determined from the critical field curves of the FISC at high fields. Therefore, the extended Jaccarino-Peter mechanism seems to occur actually in the present compound. It is predicted that the FFLO state does not occur in the FISC at high fields in contrast to the compound lambda-(BETS)_2FeCl_4, but it may occur in low-field superconductivity for s-wave and d_{x^2-y^2}-wave pairings. We also briefly discuss a possibility of compounds that exhibit unconventional behaviors of upper critical fields.