ac Magnetoconductance of isolated mesoscopic systems

We have measured dissipative (real) and non-dissipative (imaginary) magneto-conductance of arrays of disconnected GaAs-GaAlAs mesoscopic rings and square dots in the MHz-GHz range. The samples are coupled to a highly sensitive electromagnetic multimode superconducting micro-resonator and lead to a perturbation of the resonance frequency and quality factor. This experiment provides a tool for the investigation of the conductance of mesoscopic sytems without any connection to invasive probes. It can be confronted with recent theoretical predictions emphasizing the differences between isolated and connected geometries and the relation between ac conductance, orbital magnetism and electrical polarisability. The magneto-conductance measured is dominated by its imaginary component whose sign (corresponding to diamagnetism in zero field) and amplitude are analysed and compared to theoretical predictions on persistent currents and magneto-polarisability. The real component of the magneto-conductance is very different from the weak localisation signals observed in similar connected samples. It changes sign from positive to negative when increasing electronic density and frequency. We propose an interpretation of this phenomenon in relation to fundamental properties of energy level spacing statistics in the dots. (C) 1998 Academic Press Limited.