Non-quantized penetration of magnetic field in the vortex state of superconductors

As first pointed out by Bardeen and Ginzburg in the early sixties(1,2), the amount of magnetic flux carried by vortices in superconducting materials d epends on their distance from the sample edge, and can be smaller than one flux quantum, phi(0) = h/2e (where h is Planck's constant and e is the elec tronic charge). In bulk superconductors, this reduction of flux becomes neg ligible at submicrometre distances from the edge, but in thin films the eff ect may survive much farther into the material(3,4). But the effect has not been observed experimentally, and it is often assumed that magnetic field enters type II superconductors in units of f0. Here we measure the amount o f flux introduced by individual vortices in a superconducting film, finding that the flux always differs substantially from f0. We have observed vorti ces that carry as little as 0.001 phi(0), as well as 'negative vortices', w hose penetration leads to the expulsion of magnetic field. We distinguish t wo phenomena responsible for non-quantized flux penetration: the finite-siz e effect(1-4) and a nonlinear screening of the magnetic field due to the pr esence of a surface barrier. The latter effect has not been considered prev iously, but is likely to cause non-quantized penetration in most cases.