ac magnetic response of mesoscopic type-II superconductors

Abstract

The response of mesoscopic superconductors to an ac magnetic field is numerically investigated on the basis of the time-dependent Ginzburg-Landau equations. We study the dependence with frequency ω and dc magnetic field Hdc of the linear ac susceptibility χ(Hdc,ω) in square samples with dimensions of the order of the London penetration depth. At Hdc=0 the behavior of χ as a function of ω agrees very well with the two-fluid model, and the imaginary part of the ac susceptibility, χ″(ω), shows a dissipative maximum at the frequency νo=c2/(4πσλ2). In the presence of a magnetic field a second dissipation maximum appears at a frequency ωp≪ν0. The most interesting behavior of mesoscopic superconductors can be observed in the χ(Hdc) curves obtained at a fixed frequency. At a fixed number of vortices, χ″(Hdc) continuously increases with increasing Hdc. We observe that the dissipation reaches a maximum for magnetic fields right below the vortex penetration fields. Then, after each vortex penetration event, there is a sudden suppression of the ac losses, showing discontinuities in χ″(Hdc) at several values of Hdc. We show that these discontinuities are typical of the mesoscopic scale and disappear in macroscopic samples, which have a continuous behavior of χ(Hdc). We argue that these discontinuities in χ(Hdc) are due to the effect of nascent vortices which cause a large variation of the amplitude of the order parameter near the surface before the entrance of vortices.

Source-reported materials — not catalogue approval

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