Normal metal-superconductor decoupling as a source of thermal fluctuation noise in transition-edge sensors

Abstract

We have studied the origin of excess noise in superconducting transition-edge sensors (TES) with several different detector designs. We show that most of the observed noise and complex impedance features can be explained by a thermal model consisting of three bodies. We suggest that one of the thermal blocks and the corresponding thermal fluctuation noise arises due to the high-frequency thermal decoupling of the normal and superconducting phase regions inside the TES film. Our results are also consistent with the prediction that in thin bilayer proximitized superconductors, the jump in heat capacity at the critical temperature is smaller than the universal BCS theory result.

Complex impedance, responsivity and noise of transition-edge sensors: Analytical solutions for two- and three-block thermal models

“…The derived theoretical formulas in their current formulation are mainly meant to be used in the analysis of TES experiments, and excellent agreement was already achieved in fitting real TES detector data. 8,9,36 However, for detector performance optimization one also needs to consider figures of merit such as noise equivalent power, energy…”

“…After the exhaustive discussion of the two-block models, we limit ourselves here to two examples of three-block models, which we have already used in analysis of real TES data, 8,9 see Fig. 9.…”

“…[7][8][9]14 The formalism is kept general so that we do not need to decide on the physical picture a priori.…”

“…The goal of the present work is thus to give an extensive set of equations for the responsivity, current noise and impedance in a compact and usable form, which can easily be used to analyze real noise and impedance data of TES detectors. 8,9 We also give many example plots to show how various thermal parameters affect the detector properties. Detailed discussion on the noise equivalent power, energy resolution and other figures of merit are left for future publications.…”

“…Sometimes this thermal model is fairly adequate in describing the detector response, 3 but some detector designs have been experimentally shown to behave in a more complex manner. [4][5][6][7][8][9] Therefore, theoretical modeling has been advanced in recent years to include more complex thermal circuits. 4,5,[10][11][12][13][14][15] One way to approach the problem is to generalize the problem fully to any number of heat capacity blocks and thermal conductances, and solve the obtained (large) linearized system of equations numerically.…”