Researchers believe they have developed a theory to explain how ultra-narrow wires (nanowires) show enhanced superconductivity when exposed to strong magnetic fields. The new theory, applying to wires only a few hundred atoms across, appears in Physical Review Letters. Magnetic fields are something of an enigma when it comes to superconductors. Generally, magnetic fields suppress a material's ability to exhibit superconductivity, but under some conditions, a magnetic field can actually boost superconductivity. Until now, there has been no satisfactory explanation for these sorts of behaviors.
"This phenomenon [of magnetic fields boosting superconductivity] is indeed curious," said researcher Alexey Bezryadin, given that magnetic fields have long been known to suppress superconductivity by raising the kinetic energy of the electrons and by influencing electron spin. Additionally, magnetic atoms, if present in the wires, also inhibit superconductivity.
But research by another scientist, Paul Goldbart, suggested that the enhancement observed by Bezyradin's group was due to magnetic moments in the wires. "Even though the two effects - magnetic fields and magnetic moments - work separately to diminish superconductivity; together one effect weakens the other, leading to an enhancement of the superconducting properties," Goldbart explained.
Working together, Bezryadin and Goldbart proposed that exposure of the wires to oxygen in the atmosphere causes magnetic moments to form on the wire surfaces. On their own, the moments weaken the superconductivity, but the magnetic field inhibits their ability to do this. This effect shows up in ultra-narrow wires because so many of their atoms lie near the surface, where the magnetic moments form.
"The results of this work may provide a key to explaining our previous findings that nanowires undergo an abrupt transition from superconductor to insulator as they get smaller," said Bezryadin.
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Source: University of Illinois at Urbana-Champaign
