The temperature dependence of fluxoid quantization in a superconducting loop. The sensitivity of the device is expected to surpass that of other super-conducting-based bolometric devices, such as superconducting transition-edge sensors and superconducting nanowire devices. Just as important, the proposed device has advantages in sample fabrication. Two challenges of transition edge sensor fabrication are the reproducibility of the superconducting transition temperature, Tc , and the sharpness of the transition. In the proposed device, unlike in other devices, the sample would remain in the superconducting state at all times during operation. That is to say it would be maintained at an absolute temperature, T, below its superconducting

Tc. Thus, the sharpness of the transition does not directly come into play. Also, the device can operate over a relatively wide temperature span of about 0.70 Tc to 0.95 Tc. Therefore, reproducibility of Tc is not important from sample to sample. These two advantages eliminate major challenges in device fabrication.

The proposal is based on the theory of fluxoid quantization in a superconducting loop (see figure) with a track width (w) less than the temperature-dependent characteristic depth of supercurrent penetration (λ) of the material. The theory has been shown to lead to the following equation:

where Is is the temperature-dependent supercurrent, t is the thickness of the superconducting ring, Φ0 is the magnetic-flux quantum, n is an integer denoting the number of fluxoid quanta, ΦA is the magnetic flux applied to the ring, r is the radius of the ring, and λ0 is the characteristic depth of penetration of supercurrent at absolute zero temperature.

The applied magnetic flux (ΦA) would serve as a bias that could be adjusted to select the mode of operation. This flux could be generated by any convenient means — such as those used to flux bias DC-SQUIDS. To obtain one of two distinct modes of operation, one would adjust ΦA toobtain nΦ0 – ΦA ≈ Φ/2, placing the device in the middle of the n-fluxoid quanta branch. This mode would be a radiometric one, in which the device would function similarly to a superconducting-transition-edge sensor (TES) bolometer. Assuming the proposed device would be mounted on a low-thermal-conductance membrane similar to that used for TES bolometers, it has been estimated that the responsivity would be an order of magnitude greater than that of a typical TES bolometer.

A Superconducting Ring would support a temperature-dependent supercurrent Is in the presence of an applied magnetic flux ΦA.

To obtain the other distinct mode of operation, one would adjust ΦA to place the device extremely close to the transition between the n-and (n – 1)-fluxoid-quanta branches. In this mode, the device would function as a threshold-type sensor having potential utility in applications that involve photon counting and other counting-type detections.

This work was done by Joseph A. Bonetti, Matthew E. Kenyon, Henry G. Leduc, and Peter K. Day of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at under the Physical Sciences category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-45655, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).
Bolometric Device Based on Fluxoid Quantization

(reference NPO-45655) is currently available for download from the TSP library.

Don't have an account? Sign up here.