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

Abstract

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 $H(t)\ensuremath{\propto}sin(\ensuremath{\omega}t)$. However, as frequency $\ensuremath{\omega}$ 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 $\ensuremath{\omega}, I$ 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.