High-Entropy Alloy Superconductors

**3**

**Chapter 1**

**Abstract**

Compounds

structural-dimensionality viewpoint.

**1.1 Superconducting materials**

**1. Introduction**

Superconductivity in HEA-Type

Since the discovery of superconductivity in a high-entropy alloy (HEA) Ti-Zr-Nb-Hf-Ta in 2014, the community of superconductor science has explored new HEA superconductors to find the merit of the HEA states on superconducting properties. Since 2018, we have developed "HEA-type" compounds as superconductors or thermoelectric materials. As well known, compounds like intermetallic compounds or layered compounds are composed of multi crystallographic sites. In a HEA-type compounds, one or more sites are alloyed and total mixing entropy satisfies with the criterion of HEA. Herein, we summarize the synthesis methods, the crystal structural variation and superconducting properties of the HEA-type compounds, which include NaCl-type metal tellurides, CuAl2-type transition metal zirconides, high-*T*<sup>c</sup> cuprates, and BiS2-based layered superconductors. The effects of the introduction of a HEA site in various kinds of complicated compounds are discussed from the

**Keywords:** superconductor, layered compounds, material design, high entropy alloy

Superconductivity is a quantum phenomenon, which is characterized by zero-resistivity states in electrical resistivity and exclusion of magnetic flux from a superconductor [1, 2]. Superconductivity has provided many exotic research topics not only in the field of science but also in application of superconductors. The zero-resistivity states can achieve large-scale electricity transport with ultra-low energy loss, very high magnetic fields, which has been used in various devices like a magnetic resonance imaging (MRI) and a superconducting Maglev train. Although superconductor devises look perfect, the use of superconductors are regulated by temperature in reality because superconducting states are observed only at temperatures below a superconducting transition temperature (*T*c), which is a parameter unique to the superconductor. To use superconducting devices, the system must be cooled down to low temperatures lower than the *T*c of its superconducting compo-

nents. Therefore, discovery of high-*T*c superconductors has been desired.

In 1986, superconductivity with a high *T*c in a Cu oxide (La,Ba)2CuO4 was discovered [3, 4]. Soon after the discovery, *T*c of the Cu-oxide superconductor (cuprate) family reached 90 K for *RE*Ba2Cu3O7-*d* (*RE*: rare earth) [5], which is higher than liquid nitrogen temperature, and finally reached 135 K in a Hg-Ba-Ca-Gu-O system [6]. After the discovery of the cuprate family, many layered compounds have been

*Yoshikazu Mizuguchi and Aichi Yamashita*

## **Chapter 1**
