Controlling $T_c$ of Iridium Films Using the Proximity Effect

TL;DR

This paper demonstrates methods to precisely control the critical temperature of iridium films using the proximity effect, enabling their use in sensitive cryogenic detectors for advanced physics experiments.

Contribution

It introduces two fabrication recipes for Ir-based low-$T_c$ films with tunable transition temperatures, advancing the development of superconducting sensors.

Findings

Tunable $T_c$ range of 20-100 mK achieved.

Reproducible fabrication of Ir-based low-$T_c$ films.

Comparison of experimental results with theoretical models.

Abstract

A superconducting Transition-Edge Sensor (TES) with low-$T_c$ is essential in a high resolution calorimetric detection. With a motivation of developing sensitive calorimeters for applications in cryogenic neutrinoless double beta decay searches, we have been investigating methods to reduce the $T_c$ of an Ir film down to 20 mK. Utilizing the proximity effect between a superconductor and a normal metal, we found two room temperature fabrication recipes of making Ir-based low-$T_c$ films. In the first approach, an Ir film sandwiched between two Au films, a Au/Ir/Au trilayer, has a tunable $T_c$ in the range of 20-100 mK depending on the relative thicknesses. In the second approach, a paramagnetic Pt thin film is used to create Ir/Pt bilayer with a tunable $T_c$ in the same range. We present detailed study of fabrication and characterization of Ir-based low-$T_c$ films, and compare the experimental results to theoretical models. We show that Ir-based films with predictable and reproducible critical temperature can be consistently fabricated for use in large scale detector applications.