Yield and recovery of the flux line lattice in a type II superconductor

Sweep rate dependent hysteresis is observed in the voltage–current curves of a deformed niobium crystal at 4·2°K. This phenomenon is investigated quantitatively by observing the current necessary to maintain a constant voltage V as a function of time. Increases in the current with time (recovery) occur following the imposition of a current in the flux flow regime, a decrease in magnetic field from above Hc2 or a rotation of the crystal in the field. Recovery proceeds at a rate roughly proportional to V, and no recovery occurs if V is held at zero. Decreases in the current (yield) occur if the magnetic field is increased from zero before V is imposed. The magnitude of the current drop at yield is proportional to V at low V and the yield time is proportional to V −1, although it is much shorter than the recovery time. Yielding is thought to result from the multiplication of flux-carrying defects, specifically flux line dislocation (FLD) dipoles, in the flux line lattice (FLL). The voltage- and time -dependence of yield are in agreement with the predictions of a theory of FLD dipole dynamics. Recovery is tentatively attributed to the purging of sources of FLD dipoles from regions of low pin density within the crystal.