High-entropy alloys for new superconductors 16.01.2021

Researchers at Tokyo Metropolitan University have developed a new high-entropy alloy superconductor using extensive data on simple superconducting substances with a specific crystal structure. High-entropy alloys are known to retain superconducting characteristics up to extremely high pressures. The new superconductor Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2 has a superconducting transition at 8K, which is a relatively high temperature for alloys. The team's approach can be applied to the discovery of new superconducting materials with specific desirable properties.

More than a hundred years have passed since the discovery of superconductivity, when it was found that some materials suddenly exhibit minimal resistance to electric currents below the transition temperature. As we explore ways to eliminate energy waste, a way to significantly reduce transmission losses is an exciting prospect. But the widespread use of superconductivity is constrained by the requirements of existing superconductors, especially the required low temperatures. Scientists need a way to discover new superconducting materials without brute trial and error and tweak key properties.

The team, led by Associate Professor Yoshikazu Mizuguchi of Tokyo Metropolitan University, has created a "discovery platform" that has already led to the creation of many new superconducting substances. Their method is based on high-entropy alloys, where certain positions in simple crystal structures can be occupied by five or more elements. After being applied to heat-resistant materials and medical devices, some high-entropy alloys have been found to have superconducting properties with some exceptional characteristics, in particular maintaining zero resistivity under extreme pressures. The team scours material databases and cutting-edge research and finds a range of superconducting materials with a common crystal structure but different elements in specific locations. They then mix and create a structure containing many of these elements; throughout the crystal, these "nodes of high-entropy alloys" are occupied by one of the mixed elements. They have already succeeded in creating high-entropy variants of layered superconductors of bismuth sulfide and telluride compounds with the crystal structure of sodium chloride.

The scientists focused on the structure of copper aluminide (CuAl2). Compounds combining a transition metal element (Tr) and zirconium (Zr) in TrZr2 with this structure are known to be superconductive, where Tr can be Sc, Fe, Co, Ni, Cu, Ga, Rh, Pd, Ta, or Ir. The team combined a "cocktail" of these elements using arc melting to create a new high-entropy alloy type compound, Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2, which showed superconducting properties. They looked at both resistivity and electronic heat capacity, the amount of energy used by electrons in a material to raise the temperature, and determined a transition temperature of 8,0 K. Not only is this a relatively high value for a high-entropy alloy-type superconductor, they confirmed that this material has signs of "bulk" superconductivity.

The most exciting aspect of this is the wide range of other transition metals and ratios that can be tried and tweaked to achieve higher transition temperatures and other desired properties, all without changing the underlying crystal structure. The team hopes that their success will lead to more discoveries of new superconductors based on high-entropy alloys in the near future.