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_d=dV/dI in the vicinity of the order-disorder transition in NbSe₂ is investigated. r_d is large when measured by dc current. However, for a small I_ac on a dc bias, r_d decreases rapidly with frequency, even at a few hertz, 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_d are shown to result from a dynamic coexistence of two vortex matter phases rather than from the commonly assumed plastic depinning.