Vortex ratchet reversal: The role of interstitial vortices

TL;DR

This study investigates vortex dynamics in superconducting Nb films with Ni nanotriangle arrays, revealing how interstitial vortices cause ratchet reversal and influence collective pinning behavior across various magnetic fields and temperatures.

Contribution

It uncovers the role of interstitial vortices in vortex ratchet reversal within asymmetric pinning landscapes, advancing understanding of vortex motion control in superconductors.

Findings

Ratchet reversal occurs at all magnetic fields and temperatures.

Interstitial vortices coexist with pinned vortices, causing reversal.

Vortex configuration differs from the expected Abrikosov state.

Abstract

Triangular arrays of Ni nanotriangles embedded in superconducting Nb films exhibit unexpected dynamical vortex effects. Collective pinning with a vortex lattice configuration different from the expected fundamental triangular "Abrikosov state" is found. The vortex motion which prevails against the triangular periodic potential is produced by channelling effects between triangles. Interstitial vortices coexisting with pinned vortices in this asymmetric potential, lead to ratchet reversal, i.e. a DC output voltage which changes sign with the amplitude of an applied alternating drive current. In this landscape, ratchet reversal is always observed at all magnetic fields (all numbers of vortices) and at different temperatures. The ratchet reversal is unambiguously connected to the presence of two locations for the vortices: interstitial and above the artificial pinning sites.