Exploring the Proximity Effect in Mo/Au Bilayers

Abstract

We report on the sensitivity of superconducting transition temperature (Tc) to the individual layers' thickness in Mo/Au proximity bilayers to be used in Transition-Edge Sensors. The achieved good reproducibility and quality of the bilayers allow a clear determination of the superconducting critical temperature Tc as a function of the Mo and Au thicknesses. One objective of this work is to analyse the quality of the Mo-Au interface and to assess the possible effects of the double Au layer we use to fabricate these bilayers and TESs based on them. Experimental data are analysed on the base of Usadel equations using the model developed by Martinis and co-workers, in which the proximity effect in the bilayer is mainly governed by the interface transparency between the superconductor and the normal metal. We find that this model describes quite well the behaviour of Tc, even for quite thick Au layers, and that the double Au layer does not play any relevant role on the proximity effect.

Assessing the Aging Effect on Ti/Au Bilayers for Transition-Edge Sensor (TES) Detectors

“…The Ti/Au 90 nm/120 nm thick bilayer (green curve in Figure 4) presents a significant critical temperature reduction with respect to the Ti/Au 90 nm/60 nm thick bilayer (red in Figure 4). This behavior is consistent with the proximity effect theory, according to which an increase in the normal metal (Au) thickness leads to a strengthening of the proximity effect, i.e., a larger reduction in the titanium critical temperature [23]. Decreasing the titanium thickness from 90 nm (red curve in Figure 4) to 60 nm (purple curve in Figure 4), thus increasing the Au/Ti thickness ratio from 0.67 (red curve in Figure 4) to 1.5 (purple curve in Figure 4), leads to an even more pronounced shift in the critical temperature toward lower values.…”

Development of a transition-edge sensor bilayer process providing new modalities for critical temperature control

“…where n S and n M are the respective superconductor and normal metal density of states and λ f is the normal metal Fermi wavelength. A modification of the above equation is also presented for films outside the thin film limit; however, Fábrega et al found the difference to be negligible for Mo/Au bilayers with thicknesses comparable to our own, and thus the above equation will suffice for a first order approximation [33].…”

Point-Contact Spectroscopy of Mo/Si Superlattices

“…In this paper, we focus our attention on the molybdenum-silicon system, which has the highest critical temperature Tc among Mo-based superlattices. Since a bulk sample of bcc Mo is a poor superconductor with Tc  915 mK [4], and the cubic phase of Si manifests superconductivity only under extreme conditions (very low temperatures, extremely high pressures and heavy doping beyond the solubility limit [5]), the rise in Tc should be attributed to the multilayer nature. At the same time, theoretical investigations suggest emergence of superconductivity in doped elemental semiconductors like silicon and germanium under certain conditions [6,7] and recently, a superconducting phase with a high critical temperature above 10 K and an average superconducting energy gap of 2 meV was indeed found in moderately doped crystalline silicon samples under non-superconducting metallic point contacts [8].…”