Fractional vortex lattice structures in spin triplet superconductors

TL;DR

This paper explores the vortex lattice structures in spin triplet superconductors, focusing on half-quantum vortices, their stability, and how various physical effects influence their formation and symmetry properties.

Contribution

It provides a detailed calculation of HQV lattice structures considering crystalline, Zeeman, and screening effects, and examines the impact of spin-orbit coupling on their stability in specific materials.

Findings

Half-quantum vortices can be topologically stable in certain spin triplet superconductors.

Crystalline lattice, Zeeman coupling, and Meissner screening significantly influence vortex lattice structures.

Spin-orbit coupling can break U(1)×U(1) symmetry, affecting HQV lattice stability.

Abstract

Motivated by recent interest in spin triplet superconductors, we investigate the vortex lattice structures for this class of unconventional superconductors. We discuss how the order parameter symmetry can give rise to U(1)$\times$U(1) symmetry in same sense as in spinor condensates, making half-quantum vortices (HQV) topologically stable. We then calculate the vortex lattice structure of HQV's, with particular attention on the roles of the crystalline lattice, the Zeeman coupling, and Meissner screening, all absent in spinor condensates. Finally, we consider how spin-orbit coupling leads to a breakdown of the U(1)$\times$U(1) symmetry in free energy and whether the HQV lattice survives this symmetry breaking. As examples, we examine simpler spin-triplet models proposed in the context of NaxCoO2$\cdot$yH2O and Bechgaard salts, as well as the better known and more complex model for Sr2RuO4.