Anomalous out-of-phase magnetic ac response in superconducting wires

We describe and explain two competing regimes of ac magnetic response in current-carrying type-II superconducting wires which were observed experimentally. In the usual regime, voltage V(t), induced by vortex motion across the wire, is "in phase"with the external magnetic field H(t)∝sin(ωt). However, as frequency ω grows up or transport current I decreases, an anomalous, "out-of-phase"peak in V(t) appears. If these two regimes coexist, then two peaks in voltage are observed per half period of H(t) in both experiment and numerical simulations. At certain combinations of ω, I and the amplitude of the external field, the out-of-phase mechanism even overwhelms the usual, in-phase one. It is shown that the out-of-phase maximum in V(t) is due to the inhibition effect of zero-field (annihilation) lines on flux motion. Such lines, if present in the sample, significantly decelerate magnetic relaxation and dramatically affect the induced voltage. A phase diagram enabling one to distinguish between the in-phase and out-of-phase regimes is constructed.