Observation of Vortex Coalescence, Vortex Chains and Crossing Vortices in the Anisotropic Spin-Triplet Superconductor Sr2RuO4

Abstract Scanning μSQUID 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 a → 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 G, 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 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 , γ = 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.