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.

Figure 1

(a) The measured $I(V)$ characteristics of the CorTES at different external magnetic fields denoted by the different symbols. The sensor itself is shielded by the Nb ground plane, and the $I(V)$ curve is not affected by the field. The Nb bias lines, however, are sensitive to the field and have a relatively small critical current due to difficulties in step coverage. The solid line corresponds to the fit with $\Lambda =1.25k_B{T}_c$. (b) An optical micrograph of the sensor. The radii of the center and outer contacts are 15 and 150 $\mu$m, respectively. (c) A diagram showing the layer order.

Figure 2

Transition steepness, $\alpha$, measured at different bath temperatures. A lower bath temperature corresponds to a larger bias dissipation and correspondingly larger biasing currents. However, the relative transition width remains almost unchanged. The inset shows the corresponding $R\mathrm{\text{−}}T$ curves calculated from the $I(V)$ curve.

Figure 3

The average rms current noise in a frequency band from 100 to 20 kHz (marked by open circles) measured at a bath temperature of 20 mK. The total modeled noise is the solid line, and it consists of TFN, JN, SN, ITFN, and fluctuation superconductivity noise (FSN). The dash-dotted line represents a model with only TFN, JN, and SN included. The dashed line represents a model with TFN, JN, SN, and ITFN.

Figure 4

Noise spectra measured at a bath temperature of 20 mK at bias voltages of (a) 0.75 $\mu$V, (b) 0.62 $\mu$V, and (c) 0.5 $\mu$V. The notation for the different modeled noise components is identical to that of Fig. 3.