Wakeham, NicholasAdams, J. S.Bandler, S. R.Beaumont, S.Chervenak, J. A.Datesman, A. M.Eckart, M. E.Finkbeiner, F. M.Hummatov, R.Kelley, R. L.Kilbourne, C. A.Miniussi, AntoinePorter, F. S.Sadleir, J. E.Sakai, KazuhiroSmith, S. J.Wassell, E. J.2022-01-132022-01-132019-04-23"Wakeham, N.A. et al. ""Thermal fluctuation noise in Mo/Au superconducting transition-edge sensor microcalorimeters.: J. Appl. Phys. 125, 164503 (2019); https://doi.org/10.1063/1.5086045 Submitted: 17 December 2018 • Accepted: 04 April 2019 • Published Online: 23 April 2019"https://doi.org/10.1063/1.5086045http://hdl.handle.net/11603/23997In many superconducting transition-edge sensor (TES) microcalorimeters, the measured electrical noise exceeds theoretical estimates based on a thermal model of a single body thermally connected to a heat bath. Here, we report on noise and complex impedance measurements of a range of designs of TESs made with a Mo/Au bilayer. We have fitted the measured data using a two-body model, where the x-ray absorber and the TES are connected by an internal thermal conductance Gae. We find that the so-called excess noise measured in these devices is consistent with the noise generated from the internal thermal fluctuations between the x-ray absorber and the TES. Our fitted parameters are consistent with the origin of Gae being from the finite thermal conductance of the TES itself. These results suggest that even in these relatively low resistance Mo/Au TESs, the internal thermal conductance of the TES may add significant additional noise and could account for all the measured excess noise. Furthermore, we find that around regions of the superconducting transition with rapidly changing derivative of resistance with respect to temperature, an additional noise mechanism may dominate. These observations may lead to a greater understanding of TES devices and allow the design of TES microcalorimeters with improved performance.11 pagesen-USThis work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.Public Domain Mark 1.0https://creativecommons.org/publicdomain/mark/1.0/Text