Critical temperature of superconducting bilayers: Theory and experiment

Abstract

A generalized model for the critical temperature T c of superconducting bilayers is presented, which is valid with no restrictions to film thicknesses, T c of the layers and interface resistivity. The model is verified experimentally on a series of Nb-Al and Ta-Al bilayers with Nb, Ta layer thicknesses of 100 nm and Al layer thicknesses ranging from 5 nm to 200 nm. Excellent agreement between theory and experiment was found for the energy gap and the T c of bilayers. The results are important for designing practical superconducting devices.

Fabrication of robust superconducting granular aluminium/palladium bilayer microbolometers with sub-nanosecond response

“…Measurements of the bolometer resistance-current calibration curves for a discrete set of bath temperatures ( Figure 2 shows one at T=1.759 K) have been collected using a 4-wire technique, and the transition temperature is near 1.8 K, in agreement with previous investigations of the superconductivity of GA alone [13]. The Pd capping layer is apparently sufficiently thin that there is no substantial suppression of the SC transition of the underlying GA layer due to the proximity effect [14]. Figure 3 shows the bolometer response (at two different temperatures and bias currents) to photoexcitation with an attenuated nanosecond-pulsewidth JDS NanoGreen NG-10320-010 mini-YAG laser; measured τ's are <0.74 and 2.7 ns, respectively.…”

Energy-dependent kinetic model of photon absorption by superconducting tunnel junctions

“…All three quantities can be calculated with the proximity effect theory. 3,4,15 Here, x is the direction perpendicular to the barrier and is the quasiparticle energy. Figure 2 shows the real ͓density of states ͑DOS͔͒ and the imaginary part of the Green's function of a Nb-Al bilayer with 100 nm of Nb and 120 nm of Al at four different positions in the bilayer.…”

“…1,2 Over the last few years both the quality of the devices fabricated and the understanding of the relevant physical processes have improved greatly. Notable among the latter are the proximity effect, which determines the properties of the bilayer electrodes commonly used, [3][4][5] the various diffusion and loss mechanisms which limit the energy resolution of a STJ, 6,7 and the details of the quasiparticle interactions through which the overall performance of the detector is modeled. [8][9][10] In this article, we describe a major advance in the treatment of quasiparticle dynamics, which is essential for modeling the latest generation of low-gap, multitunneling STJs designed to operate at mK temperatures.…”

Evaluation of the specific boundary resistance of superconducting/weakly ferromagnetic hybrids by critical temperature measurements

“…This number scales well with the values obtained, with different methods, for Nb=Cu 1Àx Ni x hybrids for different Ni concentrations of the ferromagnetic alloy. 10,15,[31][32][33][34] We did not try to make any attempt to fit these data since in this case, due to very small values of d F , the dependence of q F on d F should have been explicitly taken into account 35 going beyond the aim of this paper.…”

“…As already pointed out, in fact, the microscopic parameters describing the F-layer drastically depend on the thickness for d F 10 nm. 35 The values used for T S , n S , and E ex are the same as above. We see that, even if a certain dispersion is present in the experimental data, they can be reproduced by the curve corresponding to the pair ðn F ¼ 6:8 nm, c b ¼ 0:1260:06Þ.…”