Mesoscopic SNS junctions and their application to SQUIDs and millimeter-wave detectors

Abstract

Mesoscopic SNS junctions have been studied both in the ballistic and diffusive regimes. SNS junctions in the ballistic regime behave as an ideal Fermion oscillator which is to be compared with the Boson oscillator or the Planck theory of blackbody radiation. The current of mesoscopic SNS junctions in the diffusive regime has the same phase dependence as that of dirty-limit short weak links derived by a transport equation. Recent theories of mesoscopic SNS junctions have successfully unified the theories of the tunnel Josephson junction, the clean-limit short weak link and the dirty-limit short weak link which look very different conceptionally. We can even observe transitions among the three types of junctions when we change the transmission coefficients of the barriers between the superconducting electrodes experimentally. We looked experimentally for the optimum transmission coefficient which gives the minimum low-frequency telegraph noise in order to make a low-noise SQUID magnetometor for brain science. We have observed signals of 5 fT from human brains with a good signal-to-noise ratio using the SQUID magnetometor of the SNS junctions. The 64-channel SQUID magnetometer of SNS junctions has confirmed that mesoscopic SNS junctions are important not only theoretically but also practically. These data could encourage people studying SNS junctions of high-Tc superconductors.