Persistent currents in normal metals

In the absence of an applied voltage, an induced electrical current rapidly decays thanks to the scattering of electrons from defects, phonons, and each other. But in a cold metal ring smaller than the electron’s coherence length, it’s possible to induce a dissipationless current, even if the metal is not superconducting. The trick, theorists predicted in the early 1980s, is to thread the ring with a magnetic field, which breaks time-reversal symmetry. The current is revealed only by its magnetic moment µ. And although researchers confirmed the effect early on, mostly using superconducting quantum interference devices (SQUIDs), complete agreement between theory and experiment, and even among experiments, has remained elusive. Jack Harris and colleagues from Yale University and the Free University of Berlin have now developed an elegantly simple alternative measurement scheme. The team deposited aluminum rings on a cantilever whose vibration frequency can be precisely monitored. In a magnetic field B, each ring’s current produces a torque ᴛ = µ × B, recorded as a shift in the cantilever’s resonance frequency of vibration. From that frequency shift, the researchers deduce the current with a precision two orders of magnitude greater than is possible using SQUIDs. For a magnetic flux threading the ring, the current exhibits an Aharonov--Bohm effect, measurable as oscillations, shown here, whose period corresponds to the addition of one flux quantum h/e through the ring. In experiments taken over a broad range of fields, temperatures, and ring sizes, Harris and coworkers find perfect agreement with a noninteracting electron model. (A. C. Bleszynski-Jayich et al., Science 326, 272, 2009 .)--R. Mark Wilson