Grain growth and superconductivity of rhenium electrodeposited from water-in-salt electrolytes

The relationship between superconductivity and the film preparation conditions, i.e., the thickness and annealing process, in electrodeposited rhenium (Re) films is studied in order to understand the effect of grain size and impurities on the film's superconducting transition temperature, T _c . A water-in-salt electrolyte was used to mitigate embrittlement by reducing hydrogen evolution at the cathode where Re is deposited. The as-deposited films exhibit a highly disordered atomic structure and superconductivity up to a temperature of T _c > 5.8 K, consistent with that expected from amorphous films. A reduction of the critical temperature of superconductivity is found to accompany grain growth. However, for film thicknesses less than 300 nm the grain growth is inhibited. This leads to a retention of the critical temperature upon annealing these thin films. A reduction of impurities in Re films is found to accompany annealing, and significant grain growth is found to proceed rapidly at temperatures of 220 °C in inert atmospheres. The introduction of hydrogen in the annealing ambient further facilitates this grain growth.