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

TL;DR

This study experimentally investigates heat transfer saturation in Nb tunnel junctions with NbTiN circuits and Au layers, revealing a saturation behavior influenced by Au layer thickness, crucial for designing SIS heterodyne mixers.

Contribution

The paper demonstrates the experimental realization and analysis of heat transfer saturation in Nb tunnel junctions with a NbTiN circuit and Au energy relaxation layer, providing new insights for device design.

Findings

Heat transfer coefficient saturates with Au layer thickness.

Saturation behavior aligns with a simple theoretical model.

Proposed geometries for optimal Au energy relaxation layers.

Abstract

In 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.