Structural evidence for a two-step process in the depinning of the superconducting flux-line lattice

A TYPE II superconductor in a magnetic field is penetrated by a hexagonal lattice of quantized flux lines. An applied current imposes a Lorentz force on these lines, but motion of the lattice will always be inhibited by pinning to material defects. Beyond a certain 'critical' current density, the lattice can break free of its pins and flow, dissipating energy and destroying superconductivity in the sample. The microscopic nature of this process is still poorly understood; in particular, little is known about the detailed structure of the flux-line lattice as it begins to depin and flow in response to the applied current. We have used small-angle neutron scattering^1-3 to image the structure of the flux lattice in NbSe² in the presence of a direct current, while also measuring the transport properties. Our observations of the structure of the flux lattice near the critical current verify theoretical predictions⁴ of the existence of three regimes as a function of increasing driving force (or current): first, no motion; then disordered, plastic motion; and finally, at high velocities, a coherently moving flux crystal.