Thermal fluctuation noise in a voltage biased superconducting transition edge thermometer

The current noise at the output of a microcalorimeter with a voltage biased superconducting transition edge thermometer is studied in detail. In addit ion to the two well-known noise sources: thermal fluctuation noise from the heat link to the bath and Johnson noise from the resistive thermometer, a third noise source strongly correlated with the steepness of the thermomete r is required to fit the measured noise spectra. Thermal fluctuation noise, originating in the thermometer itself, fully explains the additional noise . A simple model provides quantitative agreement between the observed and c alculated noise spectra for all bias points in the superconducting transiti on. (C) 2000 American Institute of Physics. [S0003-6951(01)03901-8].