| dc.description.abstract | In part I, a numerical study of the mixed states in a mesoscopic type-II superconducting
cylinder is described. Steady-state configurations and transient behavior of
the magnetic vortices for various values of the applied magnetic field H are presented.
Transitions between different multi-vortex states as H is changed is demonstrated by
abrupt changes in vortex configurations and jumps in the B vs H plot. An efficient
scheme to determine the equilibrium vortex configuration in a mesoscopic system at
any given applied field, not limited to the symmetry of the system, is devised and
demonstrated.
In part II, a superconducting thin film is subject to a non-uniform magnetic field
from a vertical magnetic dipole, consisting of two magnetic monopoles of opposite
charges. For a film with constant thickness and with no pins, it has been found that
the film carries two pairs of vortex-antivortex in the steady state in the imposed
flux range of 2.15 < (Phi)+ < 2.90 (in units of flux quantum) and no vortex at all for
(Phi)+ <= 2.15. Transitions from a superconducting state with 3 pairs of vortex-antivortex
to one with 2 pairs, where a pair of vortex-antivortex annihilates, have been observed
in the pseudo-time sequence. With a perturbation with antidots (holes), vortexantivortex
pair has been created for lower magnetic fluxes down to (Phi)+ = 1.3.
In the sample of size 16(Xi) x 16(Xi), the attraction force between the vortex and
antivortex always dominates over the pinning force, so that they eventually come out
of pins, move toward each other, and annihilate each other. The annihilation rate,
measured with time taken for the annihilation, is reduced noticeably by the increase
of the distance between pins, or the increase in the pin size. A simulation of the
magnetic vortex pinning in the sample of size 32(Xi) x 32(Xi) suggests we are likely to
achieve pinning of the vortex-antivortex pair with the sample size around this and
vortex-antivortex separation of 22(Xi). Using this sample as a template, the maximum
density of pinned vortices achievable is calculates to be about 7.6 x 10^14 vortices/m2
for (Xi) =~ 1.6A°. | en |
