Little–Parks effect
Little and Ronald D. Parks in experiments with empty and thin-walled superconducting cylinders subjected to a parallel magnetic field.
[2] The essence of the Little–Parks effect is slight suppression of the cylinder's superconductivity by persistent current.
More precisely, the Tc is such temperature at which the free energies of normal and superconducting electrons are equal, for a given magnetic field.
The Little–Parks effect is a result of collective quantum behavior of superconducting electrons.
It reflects the general fact that it is the fluxoid rather than the flux which is quantized in superconductors.
Electromagnetic theory implies that a particle with electric charge q travelling along some path P in a region with zero magnetic field B, but non-zero A (by
Therefore, the BCS condensate flowing around a closed path in a multiply connected superconducting sample acquires a phase difference Δφ determined by the magnetic flux ΦB through the area enclosed by the path (via Stokes' theorem and
[4][5][2] The challenge here is to separate Little–Parks oscillations from weak (anti-)localization, as in Altshuler et al. results, where authors observed the Aharonov–Bohm effect in a dirty metallic film.
Fritz London predicted that the fluxoid is quantized in a multiply connected superconductor.
Experimentally has been shown,[6] that the trapped magnetic flux existed only in discrete quantum units h/2e.
Deaver and Fairbank were able to achieve the accuracy 20–30% because of the wall thickness of the cylinder.
Little and Parks examined a "thin-walled" (Materials: Al, In, Pb, Sn and Sn–In alloys) cylinder (diameter was about 1 micron) at T very close to the transition temperature in an applied magnetic field in the axial direction.
What they actually measured was an infinitely small changes of resistance versus temperature for (different) constant magnetic field.
The figure to the right shows instead measurements of the resistance for varying applied magnetic field, which corresponds to varying magnetic flux, with the different colors (probably) representing different temperatures.
