We have synthesized high-entropy-alloy-type (HEA-type) RE123 superconductors with a superconducting transition temperature (T c ) exceeding 90 K. From comparison between HEA-type and conventional RE123 samples, we found that the mixing entropy at the rare earth site does not affect T c and critical current density (J c ) of the RE123 samples. For all the examined samples, orthorhombicity was found to be an essential parameter for T c and J c . In the regime near orthorhombic-tetragonal boundary, J c for the HEA-type sample was larger than that of conventional-type samples, which suggests that HEA-type RE123 may be useful to improve J c in the structure with low orthorhombicity.HEA superconductor Ti-Zr-Hf-Nb-Ta [2], which is the firstly-reported HEA superconductor Ti-Zr-Hf-Nb-Ta with a transition temperature (T c ) of 7.3 K. A metal site, which is in a HEA state, is drawn with five different colors, indicating that five different elements randomly occupy the site with occupations indicated by the coloring ratio. Namely, the HEA superconductor is composed of a metal site, which is randomly occupied with five different elements of Ti, Zr, Hf, Nb, and Ta. Although the observed superconductivity in most HEA superconductors discovered so far has been characterized as a conventional phonon-mediated type, the field of HEA superconductors has been growing because of their notable features and possible material development. For example, high pressure measurements revealed that the superconductivity states in a HEA superconductor is robust at pressure under 190 GPa [5]. This fact suggests that HEA superconductors may be useful under extreme conditions. Superconductivity was recently observed in CsCl-type HEAs, Sc-Zr-Nb-Rh-Pd and Sc-Zr-Nb-Ta-Rh-Pd, as well [6]. Since a CsCl-type compound has two crystallographic sites. The observation of superconductivity in a CsCl-type system suggests that the concept of HEA can be expanded into superconducting compounds, which are not a simple alloy with single crystallographic site. However, so far, there are a few examples of HEA-type compounds which show superconductivity. Therefore, for further development of HEA-type compounds, understanding the influence of the presence of a HEA site in the crystal structure to the superconducting characteristics is quite important.Recently, we discovered a superconductor AgInSnPbBiTe 5 with a NaCl-type structure [7]. As shown in Fig. 1B, AgInSnPbBiTe 5 is composed of a metal (M) site (cationic site) occupied with M = Ag, In, Sn, Pb, and Bi and a Te site (anionic site). Therefore, this material can be regarded as a compound containing both HEA site and non-HEA site. Since, the metal telluride has covalent M-Te bonds with ionic characteristics of M 2+ and Te 2-, HEA-like M-Te bond lengths are expected to be included in the telluride. Namely, highly disordered superconductor can be

RE Ba 2 Cu 3 O 7- δ ( RE 123, RE : rare earth) is one of the high-temperature superconductors with a transition temperature ( T c ) exceeding 90 K. Because of its high- T c and large critical current density ( J c ) under magnetic fields, RE 123 superconductors have been expected to play a key role in superconductivity application. To accelerate application researches on RE 123-based devices, further improvements of J c characteristics have been desired. In this study, we investigated the effects of high-entropy alloying at the RE site on the superconducting properties, through the measurements of local (intra-grain) J c ( J c local ) by a remanent magnetization method. We found that J c local shows a trend to be improved when four or five RE elements are mixed at the RE site, which results in high configurational entropy of mixing (Δ S mix ). All samples exhibited an order of few MA cm −2 which is a criterion for practical application and the highest J c local resulted in a value of around 7.0 MA cm −2 at T = 2.0 K. Because high-entropy alloying can improve J c local of RE 123 superconductors, our entropy-engineering strategy introduced here would be useful for the development of RE 123 superconducting materials available under high magnetic fields.