Dependence of transition width on current and critical current in transition-edge sensors
Kelsey M. Morgan, Christine G. Pappas, Douglas A. Bennett, Johnathon D. Gard, James P. Hays-Wehle, Gene C. Hilton, Carl D. Reintsema, Daniel R. Schmidt, Joel N. Ullom, Daniel S. Swetz
If transition-edge sensor (TES) X-ray detectors are to be useful in photon-rich environments,they must maintain high resolving power when pulse durations are engineered to be short, which is usually accomplished by increasing the thermal conductance (G)to the bath. This is challenging, because we find that as G increases for a TES, alpha (T/R dR/dT at constant I) drops, but beta (I/RdR/dI at constant T) does not, so energy resolution worsens. Using a two-fluid model for the TES transition, we show that the decrease in alpha can be attributed to the larger current(I0) necessary to bias the TES at a given point in the transition for high-G devices, resulting in a higher I0/Ic0 ratio (Ic0 is the films critical current at zero temperature). To recover higher alpha, we fabricated rectangular devices with varying numbers of normal-metal bars, while keeping G constant. These devices allow independent variation of both I0 and Ic0. We show that it is possible to manipulate alpha and G independently, thus enabling fast sensors with excellent energy resolution.