Electroplated Rhenium - A new high-enough T_c material⁸

We show that Re films, which normally have T_c in the range of 1.8-3 K, have a superconducting critical temperature of approximately 6 K when electroplated from aqueous solutions. Since these solutions are compatible with standard manufacturing processes and and Re is insoluble with many noble metals, this allows for integration of superconducting Re cores into, e.g., Au, Cu, and Ni multilayers. These superconducting multilayers can be usend a wide range of standard components for 4 K and below electronics, connectorized using either soldering or wirebonding. We find that potassium-based solutions result in films with high hydrogen content that evidence of stress above 150 nm thickness, while the Li-based solutions result in films stable to at least 600 nm. Magnetic response as a function of field at 1.8 K demonstrates type-II superconductivity, with an upper critical field of 2.5 T. Critical current densities on the order of 10⁸ A/m², comparable to Nb, were measured at liquid-helium temperature. Low-loss at RF was obtained for films on resonators made with Cu traces on commercial circuit boards. The relation between superconductivity and nano-structure will be investigated using x-ray diffraction and transmission electron microscopy.