U. Yaron et al., STRUCTURAL EVIDENCE FOR A 2-STEP PROCESS IN THE DEPINNING OF THE SUPERCONDUCTING FLUX-LINE-LATTICE, Nature, 376(6543), 1995, pp. 753-755
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.