High-entropy alloys (HEAs) are a new class of materials which are being energetically studied around the world. HEAs are characterized by a multicomponent alloy in which five or more elements randomly occupy a crystallographic site. The conventional HEA concept has developed into simple crystal structures such as face-centered-cubic (fcc), body-centered-cubic (bcc) and hexagonal-closed packing (hcp) structures. The highly atomic-disordered state produces many superior mechanical or thermal properties. Superconductivity has been one of the topics of focus in the field of HEAs since the discovery of the bcc HEA superconductor in 2014. A characteristic of superconductivity is robustness against atomic disorder or extremely high pressure. The materials research on HEA superconductors has just begun, and there are open possibilities for unexpectedly finding new phenomena. The present review updates the research status of HEA superconductors. We survey bcc and hcp HEA superconductors and discuss the simple material design. The concept of HEA is extended to materials possessing multiple crystallographic sites; thus, we also introduce multisite HEA superconductors with the CsCl-type, α-Mn-type, A15, NaCl-type, σ-phase and layered structures and discuss the materials research on multisite HEA superconductors. Finally, we present the new perspectives of eutectic HEA superconductors and gum metal HEA superconductors.

Magnetic and transport properties of the cubic ternary compound ${\mathrm{Ce}}_3$ ${\mathrm{Pd}}_{20}$ ${\mathrm{Ge}}_6$ are herein reported. The electrical resistivity exhibits the Kondo lattice behavior and a large $T^2$ coefficient at low temperatures, indicating that this compound is a heavy-electron system with a Kondo temperature of a few degrees Kelvin. The low-temperature specific heat revealed two successive anomalies at $T_1=1.2\mathrm{}$ K and $T_2=0.7\mathrm{}$ K. The ${\Gamma }_8$ quartet crystalline-field ground state of Ce ions is conjectured from the thermal variation of magnetic entropy deduced from the specific-heat data. The anomaly at $T_1$ is probably a result of quadrupolar interaction between ${\Gamma }_8$ ground states, while the other one at $T_2$ is due to antiferromagnetic ordering.

Photoconductive antennas made on low-temperature-grown Be doped ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ $(0.45⩽x⩽0.53)$ have been investigated focusing on the terahertz emission properties. In the antenna of $x=0.45$ , the resistance as high as $3\mathrm{M}\Omega$ enabled us to increase the bias field up to $60\mathrm{kV}∕\mathrm{cm}$ , and the terahertz waves emitted from the antenna were significantly enhanced. In addition, terahertz waves with the spectral range over $2.5\mathrm{THz}$ and the peak to noise ratio of $45\mathrm{dB}$ were generated and detected using only $1.56\mu \mathrm{m}$ pulses.

The authors have investigated the dc and terahertz-detection characteristics of the photoconductive antennas made on low-temperature-grown (LTG) ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ $(0.4<x<0.53)$ . It was found that the resistivity of the LTG ${\mathrm{In}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{As}$ can be as high as $700\Omega \mathrm{cm}$ , with which the resistance of the antenna becomes higher than $3\mathrm{M}\Omega$ . Terahertz waves were detected by the antennas with the pulse excitation at $1.56\mu \mathrm{m}$ , with a spectral range exceeding $3\mathrm{THz}$ , and a dynamic range of about $55\mathrm{dB}$ . The results also indicate that the photocarrier dynamics depend on the In content.