SPACE-TIME IMAGE OF THE MAGNETIC-FLUX PENETRATING INTO TYPE-II SUPERCONDUCTORS IN AN APPLIED OSCILLATING MAGNETIC-FIELD

We study the nonlinear diffusion of magnetic flux#in soft superconduct ors in situations where linearization of the diffusion equations for t he magnetic induction is impossible. Evolution of the magnetic flux di stributions, which begins once a low-frequency applied magnetic field (H(a)>H(c1)) is switched on, is considered. For H(a)(t>0)=H-1sin(omega t+phi), it is shown that the flux, which penetrated into the supercond uctor after the field was turned on, then gradually flows out of it. I n the case of a semi-infinite sample, the time asymptote of the distri bution front is shown to be x(f) is-proportional-to t1/4, the full flu x averaged over periods of the applied field being m is-proportional-t o t-1/4. The value and the sign of the penetrating flux are determined by the phase phi of the applied field at the switching moment. We sho w that, in general, an applied periodic magnetic field may induce a co nstant magnetic induction B(infinity) not-equal 0 deep inside the bulk , even if the constant component of the external field is zero. The ap plied field H-0 + H-1sin omegat produces B(infinity) >> H-0 if H(c2) > H-1 >> H-0 > H(c1).