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Time dependent measurements of the electric field in BSCCO tapes carrying DC current and exposed to AC magnetic fields reveal new and rich details about the electric field behavior. In particular, E(t) curves obtained for currents above the engineering critical current exhibit "double peak" feature for each half-cycle of the external magnetic field. For currents lower than Ic, E(t) exhibits a usual sine ("single peak") with amplitude and frequency dependent phase shift relative to the external field. Similar behavior is obtained for currents above Ic for low frequencies and/or high amplitudes of the magnetic field. However, with the decrease of amplitude and/or increase of frequency, a "second peak" emerges and gradually becomes dominant. This "second peak" is in phase with the external magnetic field. We interpret the results by assuming a crossover between two regimes of flux dynamics affecting the mechanism for energy loss generation in the tape. In the "quasi-DC", low frequency - high-field regime, flux lines enter the tape from one side, cross it entirely and leave from the other side, causing a dissipation, which is in phase with the external field. In the high frequency - low-field regime, flux lines enter and leave the tape from the same side and some annihilation of vortex and anti-vortex occurs at the central region of the tape. In this regime the dissipation and its phase are frequency and amplitude dependent.

Local magnetic relaxation measurements in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} crystals show evidence for plastic vortex creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the {open_quote}{open_quote}fishtail{close_quote}{close_quote} magnetization. In this range the activation energy {ital U}{sub pl}, which decreases with field, drops below the elastic (collective) creep activation energy, {ital U}{sub el}, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}}. {copyright} {ital 1996 The American Physical Society.}

Numerical studies of the flux creep in superconductors show that the distribution of the magnetic field at any stage of the creep process can be well described by the condition of spatial constancy of the activation energy $U$ independently on the particular dependence of $U$ on the field B and current $j$. This results from a self-organization of the creep process in the undercritical state $j, Comment: 12 pages, 9 PS figures

Magnetic flux relaxation over the surface barrier in high temperature superconductors are investigated. Vortex dynamics controlled by the penetration both of pancake vortices and vortex lines are discussed. The penetration field Hp for pancakes decays exponentially with temperature. The size of the magnetization loop is determined by the decay of Hp during the process of relaxation, but its shape remains unchanged. The irreversibility line associated with the pancake penetration is given by Hirr ∞ exp(−2T/Tc), and may lie both above and below the melting line.