Use of hafnium-based superconducting tunnel junctions as high-resolution spectrometers for x-ray astronomy

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Abstract
The next generation of superconducting tunnel junctions based on lower critical temperature superconductors such as hafnium are now under development. Such a material with a bandgap well below a meV has the potential to provide very high wavelength resolution at soft x-ray wavelengths. In this paper we report the first results on the characteristics of hafnium thin films deposited on r-plane sapphire. The physical properties of these films together with the electrical and superconducting characteristics are described. Currently the electrical conductivity of these films are limited by scattering from the films columnar grain structure. The superconducting transition temperature has been found to vary from approximately 137-200 mK, somewhat higher than that in the bulk, while the critical magnetic field applied in a direction parallel to the film is determined to be approximately 750 gauss far, larger than that observed in bulk hafnium.

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