Manifestation of triplet superconductivity in superconductor-ferromagnet structures

TL;DR

This paper investigates how triplet superconductivity manifests in superconductor-ferromagnet multilayer structures, revealing long-range triplet components and their effects on Josephson coupling and density of states.

Contribution

It demonstrates the emergence of long-range triplet superconductivity in noncollinear magnetization configurations within S/F structures, a novel insight into proximity effects.

Findings

Triplet component penetrates F layers over long distances independent of exchange energy J.

Josephson coupling can be mediated solely by triplet components in thick F layers.

Density of states measurements can detect the presence of triplet superconductivity.

Abstract

We study proximity effects in a multilayered superconductor/ferromagnet (S/F) structure with arbitrary relative directions of the magnetization ${\bf M}$. If the magnetizations of different layers are collinear the superconducting condensate function induced in the F layers has only a singlet component and a triplet one with a zero projection of the total magnetic moment of the Cooper pairs on the ${\bf M}$ direction. In this case the condensate penetrates the F layers over a short length $\xi_J$ determined by the exchange energy $J$. If the magnetizations ${\bf M}$ are not collinear the triplet component has, in addition to the zero projection, the projections $\pm1$. The latter component is even in the momentum, odd in the Matsubara frequency and penetrates the F layers over a long distance that increases with decreasing temperature and does not depend on $J$ (spin-orbit interaction limits this length). If the thickness of the F layers is much larger than $\xi_J$, the Josephson coupling between neighboring S layers is provided only by the triplet component, so that a new type of superconductivity arises in the transverse direction of the structure. The Josephson critical current is positive (negative) for the case of a positive (negative) chirality of the vector ${\bf M}$. We demonstrate that this type of the triplet condensate can be detected also by measuring the density of states in F/S/F structures.