Linear AC Response of Diffusive SNS Junctions

We explore the behavior of the ac admittance of superconductor-normal metal-superconductor (SNS) junctions as the phase difference of the order parameters between the superconductors is varied. We find three characteristic regimes, defined by comparing the driving frequency $\omega$ to the inelastic scattering rate $\Gamma$ and the Thouless energy $E_T$ of the junction (typically $\hbar\Gamma \ll E_T$). Only in the first regime $\omega \ll \Gamma$ the usual picture of the kinetic inductance holds. We show that the ac admittance can be used to directly access some of the characteristic quantities of the SNS junctions, in particular the phase dependent energy minigap and the typically phase dependent inelastic scattering rate. Our results partially explain the recent measurements of the linear response properties of SNS Superconducting Quantum Interference Devices (SQUIDs) and predict a number of new effects.