Flux in multiply connected superconductors

We study multiply connected superconductors by means of the magneto-optical technique. Our main aim is to explore the interesting physics and find interesting applications (e.g. a vortex transistor proposed in Phys. Rev. B 67 (2003) 212501).

Some interesting shapes, proposed by Chandran, were studied both experimentally and in simulations: Physica C 411 (2004) 1-10 we find that for these shapes the expected pattern is observed and not the one found by Chandran. For a nice movie (18Mb) click here

Flux penetration pattern in a square sample with a rectangular hole. See Physica C 411 (2004) 1-10Anisotropy of critical current induced in a circular sample with a square array of anti-dots.
See  Phys. Rev. B 67 (2003) 212501

Flux penetration in a superconducting ring and the corresponding field distribution across the superconducting material is discussed in: Phys. Rev B 64 (2001) 144505. This is especially relevant for the determination of the critical current by magnetization measurements on superconducting rings.

Our main activity, however, in multiply connected superconductors is on superconductors that are patterned with arrays of 'anti-dots' (holes in the superconductor) and with arrays of magnetic dots. In the latter case, the properties of the superconductor can be changed after fabrication of the film. We find:

That the critical current, even at low temperature, can be changed significantly in a reversible manner by changing the magnetization state of the dots: Phys Rev B 77 (2008) 054502.  
That an asymmetic flux penetration can be created by a ratchet potential and that this asymmetry is reversed in the case of avalanches: Phys Rev B 76 (2007) 184515. 
That both the flux penetration pattern and the avalanches are anisotropic for superconductors patterned with rectangular arrays of anti-dots: Physica C 437-438 (2006) 69-72 and Phys Rev B 71 (2005)104506. 
That resistivity and critical current display a fourfold symmetry, unexpected for a linear conductor: Phys. Rev. B 67 (2003) 212501. 
That the antidots contain a fixed maximum amount of flux and that some channels are preferred to others. See  Europhysics Lett. 54(2001) 682-687 where high relosution magneto-optics reveals the individual dots.