Observation of Vortex Coalescence, Vortex Chains and Crossing Vortices in the Anisotropic Spin-Triplet Superconductor $Sr_2 Ru O_4$

TL;DR

This study uses scanning μSQUID microscopy to observe complex vortex behaviors in Sr₂RuO₄, revealing vortex coalescence, chains, and crossing vortices, which relate to the superconductor's anisotropic and chiral properties.

Contribution

First direct imaging of vortex coalescence, chains, and crossing vortices in Sr₂RuO₄, linking vortex structures to its anisotropic and chiral superconducting states.

Findings

Vortices carry single flux quantum at low fields.

Vortex coalescence occurs above 1 gauss.

Vortex chains form under in-plane magnetic fields.

Abstract

Scanning $\mu$SQUID force microscopy is used to study magnetic flux structures in single crystals of the layered spin triplet superconductor Sr$\_{2}$RuO$\_{4}$. Images of the magnetic flux configuration above the $\vec{a}\vec{b}$-face of the cleaved crystal are acquired, mostly after field-cooling the sample. For low applied magnetic fields, individual vortices are observed, each carrying a single quantum of flux. Above 1 gauss, coalescence of vortices is discovered. The coalescing vortices may indicate the presence of domains of a chiral order parameter. When the applied field is tilted from the $\vec{c}$-axis, we observe a gradual transition from vortex domains to vortex chains. The in-plane component of the applied magnetic field transforms the vortex domains to vortex chains by aligning them along the field direction.This behavior and the inter-chain distance varies in qualitative agreement with the Ginzburg Landau theory of anisotropic 3D superconductors. The effective mass anisotropy of Sr$\_{2}$RuO$\_{4}$, $\gamma$=20, is the highest observed in three dimensional superconductors. When the applied field is closely in plane, the vortex form flux channels confined between the crystal-layers. Residual Abrikosov vortices are pinned preferentially on these channels. Thus the in-plane vortices are decorated by crossing Abrikosov vortices: two vortex orientations are apparent simultaneously, one along the layers and the other perpendicular to the layers.