Download EFFECT OF ZERO-FIELD (ANNIHILATION) LINES

EFFECT OF ZERO-FIELD (ANNIHILATION) LINES
ON AC RESPONSE IN SUPERCONDUCTORS
Leonid Feigel (Burlachkov) and Eli Shwartz
Department of Physics, Bar-Ilan University, Ramat-Gan, Israel
We show that the presence of the zero-field lines (where B=0) in a
superconducting sample can dramatically affect the flux motion and the ac response
of high-temperature superconductors. The same lines can be called “annihilation
lines” since (in the slab geometry) these are the places where the Abrikosov vortices
of different polarity annihilate each other. Such a annihilation line changes
significantly the distribution of currents in the sample and has a retardation effect on
the flux motion in the whole sample. The effect is mostly pronounced if the frequency
ω of the external field is relatively high, such that 1/ω is of the same order of lower
than the characteristic relaxation time of vortices in the sample in the flux-flow
regime. In this limit the Bean model can no longer be used for the description of
vortex motion in the sample [1], and one has to solve numerically the equation of flux
diffusion [2].
One of the results of the retardation effect of the zero-field lines is the
appearance of two maxima (per one half period of the external magnetic field) in the
voltage associated with the flux motion. One (usual) maximum is in phase with the
external field, the other one is out of phase with the field and its position is
determined by the zero-field lines appearance in the sample. The presence of two
maxima have been recently found out experimentally [3], and our analysis forms a
theoretical basis for understanding the effect. We study the dependence of the effect
on the amplitude of the magnetic field, its frequency as well as on the value of the
transport current flowing in the sample and get quite encouraging agreement between
the experimental data and theoretical analysis.
[1] G.P. Mikitik and E.H. Brandt, Phys. Rev. B 64, 92502 (2001).
[2] L. Burlachkov, D. Giller and R. Prozorov, Phys. Rev. B 58, 15067 (1998).
[3] G. Lukovsky et al, IEEE Trans. on Appl. Supercond. 17, 3137 (2007).
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