Orientation of a Moving Vortex Lattice in an Amorphous Mo1−xGex Superconducting Film

The orientation of a vortex lattice in the flux-flow state of an amorphous Mo1−xGex superconducting film is studied by mode-lock experiments. The formation of a hexagonal moving lattice is observed for a wide range of vortex velocity up to ∼20 m/s, which is about 1/10 of the critical velocity (∼ 200 m/s) at which the flux-flow instability occurs. The orientation of the hexagonal lattice is observed to be either parallel or perpendicular to the flow direction and a reorientation occurs at a characteristic velocity (∼ 1 m/s), separating the perpendicular orientation for low velocity from the parallel orientation for high velocity.