We have developed a theory of quasiparticle and phonon energy downconversion in nonequilibrium superconductors following the absorption of an energetic photon. This stage of energy downconversion cascade is important for the production of quasiparticles and is shown to split into two phases. The first is controlled by the evolution of the phonon distribution while the second is dominated by quasiparticle downconversion. The relative durations of the two phases and hence the rates of quasiparticle generation depend on material parameters, and most common superconductors could be classified into three different groups. For typical superconductors used for x-ray detection the downconversion cascade was shown to be fast compared to various time scales in the tunneling regime.

Generic properties of elastic phonon transport at a disordered interface are studied. The results show that phonon transmittance is a strong function of frequency and the disorder correlation length. At frequencies lower than the van Hove singularity the transmittance at a given frequency increases as the correlation length decreases. At low frequencies, this is reflected by different power-laws for phonon conductance across correlated and uncorrelated disordered interfaces which are in approximate agreement with perturbation theory of an elastic continuum. These results can be understood in terms of simple mosaic and two-colour models of the interface.

We show that thermal activation of quasiparticles from local traps is responsible for the temperature variation of responsivity observed for some superconducting tunneling junction photon detectors. With this model, the depth of the local traps in two different proximized Ta structures was found to be the same, 0.20±0.02 meV.

We have produced a number of small format gallium arsenide (GaAs) arrays to address the material, electronic, and technological problems that need to be solved in order to develop mega pixel, Fano-limited spectroscopic x-ray imagers. Results will be presented of a series of x-ray measurements carried out on a prototype 5×5 array, fabricated from 40 μm thick epitaxial GaAs. The device has pixel sizes of 200×200 μm2 and pitch 250 μm. As a preliminary investigation of performance, two pixels have been instrumented. Measurements from 5.9 to 98 keV were carried out both in our laboratory and at the Hamburger Synchrotronstrahlungslabor research facility in Hamburg, Germany. Both pixels were found to be remarkably uniform, both in their spectral and spatial response to x-rays. The average nonlinearity in the spectral response is <1% across the energy range 5.9–98 keV. Using a 12 keV, 20×20 μm2 pencil beam, the spatial uniformity was found to be better than 98% over the entire pixel surfaces, consistent with the statistical precision of the measurement. The energy resolution at −40 °C is 400 eV full width at half maximum (FWHM) at 5.9 keV rising to 700 eV FWHM at 98 keV. No difference in energy resolution was found between full area and pencil beam illumination. An analysis of the resolution function has shown that the detector is dominated by electronic noise at low energies and Fano noise at energies above 30 keV. By best-fitting the expected resolution function to the entire data set, we derive a Fano factor of 0.140±0.05, together with a charge transport factor as low as 1.4×10−3. Further improvement in the resolution function has been achieved by replacing the conventional resistive feedback preamplifiers with a new resistorless design, which provides a lower component of electronic noise. In this case, a resolution of 266 eV FWHM at 5.9 keV has been achieved at room temperature (23 °C) and 219 eV FWHM with only modest cooling (−31 °C). The expected Fano noise at this energy is ∼140 eV.

This paper presents a general model for calculating the density of states and the Cooper pair potential in proximised superconducting bi-and trilayer films. It is valid for any kind of bilayer S 1 -S 2 , whatever the quality of the materials S 1 and S 2 , the quality of the S 1 -S 2 interface and the layer thicknesses. The trilayer model is valid for a thin S 3 layer, whereas the other two layers have arbitrary thicknesses. Although the equations of the dirty limit are used, it is argued that the model stays valid in clean bi-and trilayer films. The typical example of superconducting tunnel junctions is used to show that existing models, applying to very thin or very thick layers, or to perfectly transparent S 1 -S 2 interfaces, are too restrictive to apply to any bilayer. The new model is applied to existing junctions, with layer thicknesses intermediate between the 'thick' and the 'thin' approximation. 1 I INTRODUCTIONUnderstanding the proximity effect in superconducting films is important for the development of practical devices such as superconducting tunnel junctions (STJ's). Depositing a superconductor S 1 onto another S 2 modifies the properties of both S 1 and S 2 materials. If both superconductors are thick enough (typically thicker than 10 ξ S 1 (S 2 ) , with ξ S 1 (S 2 ) the coherence length of S 1 (S 2 )), the extremities of the bilayer behave as bulk materials obeying the BCS theory, though not necessarily like bulk S 1 and bulk S 2 . The intermediate region around the S 1 -S 2 interface is characterised by a relatively sharp transition between the two bulk-like regions, and can be pretty far from a BCS-like description. If the layers are relatively thin, any BCS-like behaviour can be absent from the structure. Finally, in the case where the layers are extremely thin, as described by McMillan [1], each layer behaves again like a BCS superconductor.The physical quantities affected in a proximised bulk superconductor, are the Cooper pair potential ∆, the density of states for the Cooper pairs, P , and the density of states for the quasiparticles, N. As the density of states in both superconductors is modified due to the proximity effect, the resultant bandgap ∆ g lies at an intermediate value between the bulk values for S 1 and S 2 , ∆ g,S 1 and ∆ g,S 2 respectively. This feature has been fully described in the specific case, of a thin, low bandgap material S 2 next to a thick, high bandgap material S 1 , with both superconductors in the dirty limit [2,3].The goal of the present paper is to present the need for, and develop a model of, the proximity effect, which is not restricted to this very specific case.In particular section V shows that there are many situations where this special case does not apply, and for which the simple BCS approach does not provide a satisfactory description.Specifically, in the case of STJ's used as photon detectors, a more general description of the proximity effect is required to adequately address such issues as device performance.The general conditions for an ...