Measurement of vortex motion in a type-II superconductor: A novel use of the neutron spin-echo technique

We have used the neutron spin-echo technique to measure the small energy ch ange of neutrons which are diffracted by a moving vortex lattice in a low-p inning Nb-Ta superconducting sample. A transport current was passed in the mixed state to cause flux line movement. In the case of uniform motion, the flux velocity v(L) was given as expected by the values of electric and mag netic fields, via E = -v(L) boolean AND B. We show that with a nonuniformly moving vortex lattice, one can measure the dispersion of the velocities, o pening up new possibilities for investigating moving vortex lines.