Superconductors are not the only materials in which current can flow virtually without attenuation. Theories have predicted, and recent experiments have now indicated, that isolated rings of normal metals threaded by a magnetic flux can support persistent equilibrium currents. But this surprising phenomenon can occur only when the wavefunctions of the electrons remain in phase around the circumference and the electrons do not become localized in part of the ring. This condition requires the phase coherence length to exceed the ring's circumference and restricts experiments to micronsized rings even for the cleanest of metals. Even then the signal—a magnetic response induced by the current—is expected to be remarkably faint, less than 10−7 times the superconducting flux quantum φs which is 2.07×10−7gauss‐cm2.

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