Direct Imaging of Transition-Edge Sensors With Scanning SQUID Microscopy

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.

I. Introduction

Superconducting detectors have been of immense interest to the astronomy community over the past twenty years. These detectors utilize phenomena in superconductivity to enable photon-noise-, or background-noise-, limited performance and good optical efficiency. These types of detectors have demonstrated powerful capabilities for observing electromagnetic radiation across wavelengths spanning many orders of magnitude, from millimeter (mm) to gamma-ray wavelengths. The transition-edge sensor (TES) bolometer is a highly sensitive and stable superconducting detector used to measure incident radiation through heating of a strongly temperature-dependent resistor [1], in this case a superconducting film biased in its superconducting transition. Through voltage-biasing the TES for negative electrothermal feedback, device operation is well-stabilized in the superconducting transition, even after power absorption. Large arrays of these detectors have significantly advanced our understanding of the Universe's history and its contents through measurements of the cosmic microwave background (CMB) at mm wavelengths (e.g. [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]).