Instantaneous dislocation velocities in iron at low temperature

We have presented measurements of instantaneous dislocation velocities in iron at and < 4.2K. These measurements cast serious doubts on a barrier mechanism controlling dislocation velocities in iron crystals at these temperatures; at the same time this is the very temperature range where such models are supposed to be most applicable. The measurements are consistent with inertial control of dislocation motion or quantum mechanical tunneling processes. The magnetic field dependence of V sub i should settle the question of whether inertia or tunneling determines the dislocation velocity; such measurements will be reported elsewhere. This research funded by NSf contract DMR-9703993 is gratefully acknowledged.