One Chromium Crystal Holds Three Different Superconducting States. That Is the Signature of a Rare Kind.

In an ordinary superconductor, electrons pair up with their spins pointing in opposite directions, adding to zero. In a spin-triplet superconductor, the pair's spins add to one. That difference is small to write down and enormous in consequence: a spin-triplet state has an internal degree of freedom an ordinary one lacks, which means it can support several distinct superconducting phases at once, and it can host Majorana bound states — quasiparticles that would let a quantum computer resist the errors that currently ruin its calculations.

The trouble has been finding one. Reporting in Physical Review Letters, a team at Okayama University led by physics professor Guo-qing Zheng says it has now identified three separate superconducting phases inside potassium chromium arsenide, K2Cr3As3, which is the behavior a spin-triplet superconductor is supposed to show and a conventional one cannot.

"Spin-triplet superconductors are rare," Zheng told Phys.org. "Although we previously discovered a spin-triplet state in the carrier-doped topological insulator CuxBi2Se3, and there are also suggestions of such states in some uranium-based compounds, several pieces needed to establish the field for applications are still missing. One of them is multiple phases due to the internal degree of freedom. The objective of our study was to find multiple phases in K2Cr3As3."

The material was a deliberate choice. It superconducts below 6.2 Kelvin and carries no magnetic order, meaning its magnetic moments never lock into a fixed pattern that would muddy the measurement. The team grew single crystals using high-temperature solution growth, then probed them with nuclear magnetic resonance, sweeping magnetic field strength and temperature across wide ranges with the field aligned along the crystal's c-axis. "In this study, nuclear magnetic resonance played a crucial role," Zheng said. "This technique is realistically the only method to probe spin susceptibility in the superconducting state."

Three regions emerged. In phase A, at relatively low field, the spin arrangement rotates with the momentum of the electrons while time-reversal symmetry stays intact. Cool further at low field and the material drops into phase B, which has a handed, momentum-dependent structure and breaks time-reversal symmetry. Push the field higher and phase C appears, in which the superconducting energy gap closes along a continuous line rather than at isolated points.

"By observing and elucidating the multiple superconducting phases, we definitely proved spin-triplet pairing in this compound with the highest transition temperature ever, above liquid helium temperature," Zheng said. "In addition, we clarified the topological nature of these phases, one of which can host the so-called Majorana excitation that can be used in fault-tolerant quantum computing."

Zheng's group now plans to apply the same NMR approach to other candidate materials, hunting for a spin-triplet superconductor that survives to a higher temperature.