Susceptibilitée magnéetique orbitale et conductance AC d'anneaux méesoscopiques isolées

Orbital Magnetic Susceptibility and AC Conductance of Isolated Mesoscopic Rings

Laboratoire de physique des solides, bât. 510, Universitée Paris-Sud, 91405 Orsay, France

Abstract

This work consist of a study of the complex AC conductance of an ensemble of isolated mesoscopic rings. As a matter of fact, such rings carry, in the presence of a magnetic field, a non dissipative persistent current. However they have a finite resistivity when they are connected to measurement wires, close to the bulk value. This paradox is both theoreticaly and experimentaly addressed by: i) the numerical computation of the average complex conductance of a disordered ring in the discrete spectrum limit, in the presence of magnetic field, inelastic scattering, at finite temperature and for the grand canonical and the canonical statistical ensembles. The magnetoconductance of the isolated system is shown, in some cases, to be much greater than the connected system's one and to reverse sign with respect to the temperature or the frequency; ii) the achievement of a very sensitive, well-suited detector: a superconducting bifilar micro-resonator ( MHz and Q=80 000 at 50 mK) strongly coupled to the rings. From the study of the temperature and magnetic field dependence of its resonance frequency and quality factor on can deduce the real and imaginary part of the conductance. Its performances have been optmised and its sensitivity checked, and the first measurement on isolated mesoscopic rings has been achieved, showing oscillations on both the dissipative and non dissipative part of the conductance.