Persistent currents in normal metal rings
Usually for an electrical current to flow in non-superconducting metal you have to supply enough energy to overcome the metal’s resistance. However, in a metal ring that is very small (about 1 μm diameter or less) we have to treat the dynamics of the electrons quantum mechanically, where electrons are represented mathematically as waves.
It can be shown that the electrons inside the metal should behave in much the same way as electrons orbiting an atomic nucleus without the need for a constant supply of energy. The ring size limitation for this to happen is dictated by the requirement that the electron will not encounter a certain kind of scattering event that would randomize the phase of its wave (for instance an electron-phonon scattering). The probability of these ‘phase- breaking’ events can be made very small in high grade aluminum and low temperatures. Recent precision measurements of mesoscopic persistent currents in normal-metal rings rely on the interaction between the magnetic moment generated by the current and a large applied magnetic field. Motivated by this technique, the theory of mesoscopic persistent currents has been extended to include the effect of the finite thickness of the ring and the resulting penetration of the large magnetic field. This includes both the sample-specific typical current and the ensemble-averaged current which is dominated by the effects of electron-electron interactions. It can be shown that the magnetic field strongly suppresses the interaction-induced persistent current and so provides direct access to the independent-electron contribution. Moreover, the technique allows for measurements of the entire distribution function of the persistent current.