High-entropy alloy superconductors: Status, opportunities, and challenges

High entropy alloys (HEAs) have recently emerged as a new class of materials. Solids made from multiple transition metal elements, typically five or more, in equimolar or near equimolar ratios, they are stabilized by entropic contributions to the free energy. 3d-element-based HEAs have been widely studied in materials science. In addition to their promising mechanical properties, some HEAs have been reported to display superconductivity; these materials are largely based on 4d and 5d metals. In this update, we focus on the variations of HEA superconductors presently known and the progress made in studies of their properties, especially highlighting fundamental issues related to their superconducting transition temperatures. Some of the key factors that influence their characteristics include crystal structure, atomic makeup, valence electron count, molar volume and mixing entropy - all of these factors appearing to be influential in spite of the fact that the materials are most like metallic glasses on simple periodic lattices. Many opportunities and challenges remain for expanding our knowledge of HEA superconductors, finding new types of HEA superconductors, and their potential for applications.