  High entropy alloys (HEAs) are a recently-opened research area in materials
science and condensed matter physics. Although 3d-metal-based HEAs have already
been the subject of many investigations, studies of HEA superconductors, which
tend to be based on 4d metals, are relatively fewer. Here we provide a short
update of the progress made in studies of superconducting HEAs. We aim to
summarize their current status and describe some of the key factors that appear
to influence their superconducting transition temperatures and properties,
including crystal structure, atomic makeup, valence electron count, molar
volume and mixing entropy. Many opportunities and challenges remain for
expanding our knowledge of HEA superconductors, finding new types of HEA
superconductors, and their potential for applications; these are also briefly
discussed.
