Superconducting Re-Mo Alloys from Electrodeposition
Authors: Liu Quanhong , Huang Qiang 
Published: December 2024

Rhenium, one of the rarest metals in the crust of earth, has many special properties and enables some unique applications, such as in high temperature high strength machines and in chemical and electrochemical catalysis. On the other hand, as a type I superconductor, rhenium is also of interest to interconnect applications for cryogenic quantum electronics. While the bulk crystalline rhenium has a superconducting transition temperature, or critical temperature (Tc), of 1.7 K, this temperature is strongly influenced by the form, stress, and preparation method of Re. For instance, tensile strain and compressive stress can increase and decrease the Tc of Re, respectively. This Tc increases to 2.7 K for powder rhenium, and to 6 K for electrodeposited amorphous Re films1,2. However, such an enhanced Tc deteriorates upon grain coarsening along thermal treatment. In an effort to stabilize the enhanced Tc or even further improve the Tc, doping elements can be introduced. For example, bulk tungsten-rhenium alloys exhibit superconductivity at 8 K3 and rhenium-molybdenum alloy (Re40Mo60) at above 10 K4. In this work, we report an effort to electrodeposit ReMo alloys from aqueous electrolytes and to study the properties of such films.
ReMo films are codeposit from highly concentrated acetate solutions containing citric acid as a complexing agent. ReMo films with different Mo content are obtained by adjusting the concentrations of metal cations and citric acid. Pure Re and Mo films are also deposited for comparison. The thickness of all samples is controlled between 200 and 300 nm to mitigate the impact of thickness variation. The films are characterized by X-ray diffraction, electron microscopy, as well as superconductivity measurements. The effects of thermal treatment are determined by annealing the films at various temperatures.
Figure 1 shows the superconducting transition of a set of as-deposit ReMo alloy films with various Mo contents, presented as 4-probe resistance measurements with temperature sweep. Pure Re film shows a transition at 6.4 K. As the Mo content increases up to 20 at.%, the transition temperature stays approximately unchanged with a small variation between 5.5 to 6.5 K. In addition, this enhanced Tc due to the amorphous grain structure remains unchanged upon thermal annealing up to 400 C. It is evidence that alloying Re with up to 20 at.% Mo is expected to stabilize the gain structure of electrodeposited Re while not compromising the superconductivity. A two-step transition is observed when Mo reaches 25 at.% in the alloy, indicating a discontinuous Re-rich phase with a Tc of 5.5 K and a second Re-poorer phase with a Tc at 3 K. (We need to discuss this last sentence when we meet.) Detailed discussion on the deposition and film characterization will be presented in the talk.
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Figure 1