On page 144 of this issue, Geim and colleagues present measurements made on superconducting aluminium discs as small as 0.3 μm across. They show that some superconductors have a peculiar attraction for magnetic fields. When metals become superconducting, not only does their resistance drop to zero, but their magnetic properties also change markedly: magnetic fields are expelled from the material's interior. Often this phenomenon is more readily measurable and more revealing about the nature of superconductivity than a material's electrical properties. Magnetic fields may still penetrate into so-called type-II superconductors, in vortices of circulating supercurrent (the resistance-free current carried by paired electrons). Nevertheless, the magnetic field is always diminished inside a superconductor relative to the external field—that is, superconductors are diamagnetic.
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