Superconductivity of Electrodeposited Rhenium Alloys
Authors: Ma Zhefan , Huang Qiang 
Published: December 2024

Superconductivity of Electrodeposited Rhenium Alloys

Rhenium (Re) is a type I superconductor with an intrinsic superconducting transition temperature, or the critical temperature (Tc), at 1.7 K. Electrodeposited Re has been found to be in its amorphous state with an enhanced critical temperature of 6 K[1]. This enhancement enables superconductivity beyond the boiling point of liquid helium, i.e. 4.2K, facilitating the applications of Re in the connectivity for quantum devices. In addition, Re also maintains its ductility without undergoing ductile-to-brittle transition as temperature decreases, owing to its hexagonal close-packed (hcp) crystal structure[2], which further supports its accessibility to cryogenic electronic application, such as interconnects in quantum devices. However, this enhancement in T directly results from the amorphous or nanocrystalline grain structure and rapidly degrades upon Re recrystallization at elevated temperatures.

In order to improve the thermal stability of the T of nanocrystalline Re or even further increase the Tc, rhenium films doped with other elements have been electrodeposited. For example, while alloying Re with Fe inhibits the recrystallization, the superconductivity of ReFe is found significantly suppressed. On the other hand, while Co doping suppresses the superconductivity to a much less degree, it has no impact on the recrystallization. Between these two alloying elements, both Fe and Co are ferromagnetic and have similar atomic radius. Yet, they have completely different crystal structures and are also different with respect to the Re host.

The present work continues from previous studies to perform systematic comparison between dopants with various atomic radius, electronegativity, magnetism, and intrinsic crystallographic configurations to identify the determining factor for alloy superconductivity and the thermal stability. For example, Figure 1 shows the XRD patterns of as-deposited ReRu alloy films with controlled Ru contents as well as the same films after thermal annealing at various elevated temperatures. The electrodeposition is carried out on rotating Cu disk electrodes. The concentration of Re and Ru in electrolyte and the time of deposition are varied in order to achieve different composition and to keep film thickness between 300 and 400 nm. Film composition and thickness are measured using x-ray fluorescence spectroscopy (XRF). The film grain structure and superconductivity as well as their thermal stability will be discussed in detail in the presentation.

Reference

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Figure 1. XRD patterns of rhenium-ruthenium alloy films (a) before annealing, after annealing at (b) 150 °C, (c) 200 °C, (d) 300 °C. XRD standards of pure Re and Cu are included in (c) and (d).

Figure 1