Heat transfer coefficient saturation in superconducting Nb tunnel junctions contacted to a NbTiN circuit and an Au energy relaxation layer

Abstract

International audienceIn this paper we present the experimental realization of a Nb tunnel junction connected to a high-gap superconducting NbTiN embedding circuit. We investigate relaxation of nonequilibrium quasiparticles in a small volume Au layer between the Nb tunnel junction and the NbTiN circuit. We find a saturation in the effective heat-transfer coefficient consistent with a simple theoretical model. This saturation is determined by the thickness of the Au layer. Our findings are important for the design of the ideal Au energy relaxation layer for practical SIS heterodyne mixers and we suggest two geometries, one, using a circular Au layer and, two, using a half-circular Au layer. Our work is concluded with an outlook of our future experiments

Terahertz radiation detectors: the state-of-the-art

“…The highest detectable frequency in the SIS mixer is set by the superconducting pair breaking energy 2Δ. For higher THz frequencies, for which hν>2Δ (ν∼700 GHz, Nb), the QP excitations in the embedding circuit are created due to breaking the Cooper pairs and large ohmic losses are observed that would significantly decrease the sensitivity of the mixer device [87].…”