New superconductor with hallmark of unconventional superconductivity discovered

Tokyo, Japan – Researchers from Tokyo Metropolitan University have discovered a new superconducting material. They combined iron, nickel, and zirconium, to create a new transition metal zirconide with different ratios of iron to nickel. While both iron zirconide and nickel zirconide are not superconducting, the newly prepared mixtures are, exhibiting a “dome-shaped” phase diagram typical of so-called “unconventional superconductors,” a promising avenue for developing high temperature superconducting materials which can be more widely deployed in society.

Superconductors already play an active role in cutting-edge technologies, from superconducting magnets in medical devices and maglev systems to superconducting cables for power transmission. However, they generally rely on cooling to temperatures of around four Kelvin, a key roadblock in wider deployment of the technology. Scientists are on the lookout for materials which can show zero resistivity at higher temperatures, particularly the 77 Kelvin threshold at which liquid nitrogen can be used to cool the materials instead of liquid helium.

The good news is that promising candidates have begun to appear, like iron-based superconductors discovered in 2008. It is becoming increasingly clear that high-temperature superconductivity might follow a different mechanism from those of “conventional superconductors” that follow well-established theoretical frameworks, notably the BCS (Bardeen-Cooper-Schrieffer) theory. In particular, materials with magnetic elements, or those that exhibit “magnetic ordering,” have begun to emerge as being important for the emergence of “unconventional superconductivity.”

Now, a team of researchers led by Associate Professor Yoshikazu Mizuguchi from Tokyo Metropolitan University have conceived a new superconducting material containing a magnetic element. For the first time, they showed that a polycrystalline alloy of iron, nickel, and zirconium shows superconducting properties. Curiously, both iron zirconide and nickel zirconide are not superconducting in crystalline form. In experiments which began as an undergraduate student project, the team combined iron, nickel, and zirconium in different ratios using a method known as arc melting, confirming that the resulting alloy had the same crystal structure as tetragonal transition-metal zirconides, a family of promising superconducting materials. The lattice constants, or the lengths of repeating cells, were also found to change smoothly with the ratio of iron to nickel. Crucially, they found a region of compositions where the superconducting transition temperature rose, then fell again. This “dome-like” form is a promising hallmark of unconventional superconductivity.

Further experiments confirmed that the magnetization of nickel zirconide exhibits a magnetic-transition-like anomaly, suggesting a close relationship between their findings and the unconventional superconductivity arising from magnetic order suggested in other materials. They hope that their new platform for studying unconventional superconductivity might inspire new inroads into our understanding of its mechanism, as well as in the practical design of cutting-edge materials for the next generation of superconducting devices.

This work was supported by JSPS-KAKENHI Grant Number 23KK0088, a TMU Research Project for an Emergent Future Society, and a Tokyo Government-Advanced Research Grant (H31–1).

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