Vortex ratchet reversal: Role of interstitial vortices

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 channeling effects between triangles. Interstitial vortices coexisting with pinned vortices in this asymmetric potential lead to ratchet reversal, i.e., a dc output voltage that 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.