Lattice planes that are in reflection position at rest must be rotated in order to keep them reflecting when the lattice is moving. This principle was applied to observe the motion of flux lines caused by a transport current in an almost ideal type-II NbTa superconductor. The periodic arrangement of flux lines was found to persist up to transport currents 20 times larger than the critical current, corresponding to a mean flux velocity of 1 m${\sec }^{-1\mathrm{}}$. The velocities determined by neutron diffraction are in agreement with the value $\frac{E}{B}$, where $E$ is the macroscopic electric field across a current-carrying superconductor and $B$ is the flux density.

• Received 1 May 1975

DOI:https://doi.org/10.1103/PhysRevB.12.1772