Direct Imaging of Transition-Edge Sensors with Scanning SQUID Microscopy

TL;DR

This study uses scanning SQUID microscopy to image and analyze the local magnetic response of transition-edge sensors, revealing how device dimensions and proximity effects influence their superconducting transition and resistance behavior.

Contribution

It introduces magnetic imaging with SSM as a novel method for local characterization of TES devices, providing new insights into their transition physics and design considerations.

Findings

Excess diamagnetic response near Nb contacts at higher temperatures.

Proximity effect between Nb and Al-Mn influences transition width.

Device dimensions affect the temperature dependence of resistance.

Abstract

Significant advancements have been made in understanding the physics of transition-edge sensors (TESs) over the past decade. However, key questions remain, particularly a detailed understanding of the current-dependent resistance of these detectors when biased within their superconducting transition. We use scanning superconducting quantum interference device (SQUID) microscopy (SSM) to image the local diamagnetic response of aluminum-manganese alloy (Al-Mn) transition-edge sensors (TESs) near their critical temperature of approximately 175 mK. By doing so, we gain insights into how the device dimensions influence TES transition width, which in turn affects device operation and informs optimal device design. Our images reveal that the Al-Mn thin film near the niobium (Nb) leads exhibits an excess diamagnetic response at temperatures several milli-Kelvin (mK) higher than the bulk of the film farther from the contacts. A possible origin of this behavior is a longitudinal proximity effect between the Nb and Al-Mn where the TES acts as a weak link between superconducting leads. We discuss how this effect shapes the temperature dependence of the resistance as the spacing between the leads decreases. This work demonstrates that magnetic imaging with SSM is a powerful tool for local characterization of superconducting detectors.