Current distribution and transition width in superconducting transition-edge sensors

TL;DR

This paper introduces a model for current distribution in superconducting transition-edge sensors (TESs), supported by experimental data showing how device geometry and magnetic fields influence transition sharpness and width.

Contribution

It presents a new model with one free parameter for current distribution in TESs, validated by experiments with different geometries and magnetic field considerations.

Findings

Current meanders through TES in low-resistance state

Normal-metal features influence current flow by 40% of normal resistance

Self-induced magnetic fields significantly affect transition width

Abstract

Present models of the superconducting-to-normal transition in transition-edge sensors (TESs) do not describe the current distribution within a biased TES. This distribution is complicated by normal-metal features that are integral to TES design. We present a model with one free parameter that describes the evolution of the current distribution with bias. To probe the current distribution experimentally, we fabricated TES devices with different current return geometries. Devices where the current return geometry mirrors current flow within the device have sharper transitions, thus allowing for a direct test of the current-flow model. Measurements from these devices show that current meanders through a TES low in the resistive transition but flows across the normal-metal features by 40% of the normal-state resistance. Comparison of transition sharpness between device designs reveals that self-induced magnetic fields play an important role in determining the width of the superconducting transition. [http://dx.doi.org/10.1063/1.4771984]