Physics of low-temperature microcalorimeters

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This introductory tutorial includes a short description of the various types of thermal or phonon-mediated detectors and a discussion of optimal filtering, energy resolution calculations, and electrothermal feedback. We concentrate on the basic operation of simple equilibrium calorimeters with an ideal resistive thermometer. The thermometer technology could be doped semiconductor or superconducting transition edge. The calculations consider only Johnson noise in the thermometer and thermodynamic fluctuation noise between the calorimeter and the heat sink. For many practical devices, this is a good approximation and it provides a starting point for the discussion of other noise sources, various non-ideal effects, and non-linearity. Other types of thermometers may in practice have quite different limiting factors.