Breaking through the quantum limit

In spite of years of intense investigation, low-temperature superconductors continue to hold surprises. These materials lose their resistance to electric currents and expel magnetic flux at temperatures close to absolute zero. It is widely believed that magnetic flux can only enter a superconductor in the form of vortices, each carrying a quantum of magnetic flux f = h/2e, where h is the Planck constant and e is the charge of an electron. However, this long-held assumption is wrong - as John Bardeen and Vitaly Ginzburg first pointed out in the 1960s. They showed theoretically that the amount of flux carried by vortices depends on their distance from the edge of the superconducting material, and that this flux can even be smaller than flux f close to the edge. But in the absence of any experimental evidence, the reduction in flux was considered an exotic or negligible effect. Now vortices that carry less than one quantum of magnetic flux have been observed in a thin superconducting film (A Geim et al 2000 Nature 407 55).