Fluctuation-Limited noise in a superconducting transition-edge sensor

In order to investigate the origin of the until now unaccounted excess noise and to minimize the uncontrollable phenomena at the transition in x-ray microcalorimeters we have developed superconducting transition-edge sensors into an edgeless geometry, the so-called Corbino disk, with superconducting contacts in the center and at the outer perimeter. The measured rms current noise and its spectral density can be modeled as resistance noise resulting from fluctuations near the equilibrium superconductor-normal metal boundary.