Nonlinear vortex transport in mesoscopic channels of amorphous NbGe
Amorphous Nb0.7Ge0.3, a high-k type-II superconductor with very low pinning, allows for measurements in the flux-flow regime over large parts of the B-T-phase diagram. When a transport current is driven through a narrow wire (width 250 nm) connected to remote voltage probes via a perpendicular channel (length 2 µm) in presence of an external (out-of-plane) magnetic field, the Transversal Flux Transformer Effect can be used to produce a nonlocal voltage drop on the remote contacts caused by vortex motion in the channel. In the simplest picture, the Lorentz force acting on the vortices in the local wire creates a pressure on the vortices in the channel, such that the mutual vortex repulsion can explain the nonlocal vortex motion. However, detailed measurements of nonlocal DC voltage-current characteristics taken across the whole B-T-plane show several new aspects, including abrupt sign reversals of the vortex motion. This can be understood in terms of an interplay between Lorentz force (low currents) an Nernst effect via local electron heating (high currents) for T''Tc, and between the Lorentz force (low currents) and a force due to the local suppression of the superconducting gap (high currents) for T close to Tc.