STRUCTURAL EVIDENCE FOR A 2-STEP PROCESS IN THE DEPINNING OF THE SUPERCONDUCTING FLUX-LINE-LATTICE

A TYPE II superconductor in a magnetic field is penetrated by a hexago nal lattice of quantized flux lines. An applied current imposes a Lore ntz force on these lines, but motion of the lattice will always be inh ibited by pinning to material defects. Beyond a certain 'critical' cur rent density, the lattice can break free of its pins and flow, dissipa ting energy and destroying superconductivity in the sample. The micros copic nature of this process is still poorly understood; in particular , little is known about the detailed structure of the flux-line lattic e as it begins to depin and flow in response to the applied current. W e 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, whi le also measuring the transport properties. Our observations of the st ructure of the flux lattice near the critical current verify theoretic al predictions(4) of the existence of three regimes as a function of i ncreasing driving force (or current): first, no motion; then disordere d, plastic motion; and finally, at high velocities, a coherently movin g flux crystal.