V-I characteristics in the vicinity of the order-disorder transition in vortex matter

The shape of the V-I characteristics leading to a peak in the differential resistance r{sub d}=dV/dI in the vicinity of the order-disorder transition in NbSe2 is investigated. r{sub d} is large when measured by dc current. However, for a small Iac on a dc bias r{sub d} decreases rapidly with frequency, even at a few Hz, and displays a large out-of-phase signal. In contrast, the ac response increases with frequency in the absence of dc bias. These surprisingly opposite phenomena and the peak in r{sub d} are shown to result from a dynamic coexistence of two vortex matter phases rather than from the commonly assumed plastic depinning.