Dissipation and supercurrent fluctuations in a diffusive normal-conductor--superconductor ring
B. Dassonneville, M. Ferrier, S. Guéron, and H. Bouchiat
Accepted
A mesoscopic hybrid Normal/Superconducting (NS) ring is characterized by a dense Andreev spectrum with a flux dependent minigap. To probe the dynamics of such a ring we measure its linear response to a high frequency flux, in a wide frequency range, with a multimode superconducting resonator. We find that the current response contains, beside the well known dissipation-less Josephson contribution, a large dissipative component. At high frequency compared to the minigap and low temperature we find that the dissipation is due to transitions across the minigap. In contrast, at lower frequency there is a range of temperature for which dissipation is caused predominantly by the relaxation of the Andreev states' population. This dissipative response, related via the fluctuation dissipation theorem to a non intuitive zero frequency thermal noise of supercurrent, is characterized by a phase dependence dominated by its second harmonic, as predicted long ago \cite{limpitsky,martin96} but never observed so far.