Josephson vortex lattices with modulation perpendicular to an in-plane magnetic field in layered superconductor

TL;DR

This paper studies Josephson vortex lattices with FFLO-type modulation perpendicular to an in-plane magnetic field in layered superconductors, revealing field-induced structural transitions and implications for experimental observations.

Contribution

It introduces the formation of higher Landau level Josephson vortex lattices with nodal planes and analyzes their field-induced transitions and experimental signatures.

Findings

Higher LL Josephson vortex lattices with nodal planes form under in-plane fields.

Structural transitions involve changes in nodal plane orientation.

Transition from modulated state to normal phase can be second order or discontinuous depending on paramagnetic effects.

Abstract

In quasi low dimensional superconductors under {\it parallel} magnetic fields applied along a conducting direction, vortex lattices with a modulation of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type {\it perpendicular} to the field may occur due to an enhanced paramagnetic depairing. As the strength of an in-plane field is varied in a Q2D material, the Josephson vortex lattices accompanied by nodal planes are formed in higher Landau level (LL) modes of the superconducting (SC) order parameter and show field-induced structural transitions. A change of orientation of nodal planes induced by these transitions should be observed in transport measurements for an out-of-plane current in real superconductors with point disorder effective on the SC layers. Further, the $H_{c2}$-transition from this higher LL state to the normal phase is of second order for moderately strong paramagnetic effects but, in the case with a strong enough paramagnetic effect, becomes discontinuous as well as the transition between this modulated state and an ordinary Abrikosov vortex lattice in the lowest LL. Relevance of these results to recent observations in organic superconductors suggesting the presence of an FFLO state are discussed.