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