Three quantum phases in chromium-based material hint at a spin-triplet superconductor

09/09/2026
Multiple superconducting phases of K2Cr3As3 for B||c. Credit: Physical Review Letters (2026). DOI: 10.1103/kykd-2nj4.


Superconductors are materials that conduct electricity without electrical resistance when cooled below a specific critical temperature. These materials have proved promising for the development of various technologies, including medical imaging instruments, particle accelerators, ultrasensitive detectors and quantum processors.

Some superconductors are topological, which means their electronic states exhibit global patterns that remain unchanged under small disturbances. These materials can host Majorana bound states, localized quasiparticle excitations that could be leveraged to reduce errors made by quantum computers.

In recent years, some physicists have been trying to identify materials that could exhibit what is known as spin-triplet superconductivity. In this type of superconductivity, paired electrons have a combined spin of 1, as opposed to the spin of 0 observed in conventional superconductors.

Spin-triplet superconductors could, in principle, support multiple superconducting phases and host robust Majorana states. This would make them particularly advantageous for the study of exotic quantum phenomena and the development of fault-tolerant quantum processors.

Researchers at Okayama University recently uncovered three superconducting phases characterized by distinct spin arrangements in the chromium-based material potassium chromium arsenide (K2Cr3As3). Their paper, published in Physical Review Letters, offers hints that this material could be a spin-triplet superconductor.

"Spin-triplet superconductors are rare. 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," Guo-qing Zheng, senior author and professor of physics at Okayama University, told Phys.org.

"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."

A closer look at potassium chromium arsenide

Building on their earlier efforts to identify spin-triplet superconductors, Zheng and his colleagues set out to examine the superconducting phases of a known chromium-based material. They specifically examined K2Cr3As3, which has a superconducting transition temperature of 6.2 Kelvin and lacks magnetic order, meaning its magnetic moments do not settle into a fixed pattern.

First, the researchers grew single K2Cr3As3 crystals using a high-temperature solution-growth technique. Subsequently, they examined the crystals using nuclear magnetic resonance, a technique that relies on magnetic fields and radio waves to probe atomic nuclei, offering insight into their local electronic environment.

"By changing the strength of magnetic fields and temperature over wide ranges, we discovered three spin-triplet superconducting states with different spin configurations in K2Cr3As3," Zheng explained. "In this study, nuclear magnetic resonance played a crucial role. This technique is realistically the only method to probe spin susceptibility in the superconducting state."

The researchers identified three distinct superconducting phases in K2Cr3As3, which they dubbed phases A, B and C. These phases arose under different combinations of temperature and magnetic-field strength, with the field applied parallel to the crystal's c-axis.

Under relatively low magnetic fields, the material was found to enter phase A, a state in which the arrangement of spins changed direction with the momentum of electrons while preserving time-reversal symmetry.

When the material was cooled further under low fields, it transitioned into phase B. This state is characterized by a handed, momentum-dependent superconducting structure and breaks time-reversal symmetry.

Finally, under higher magnetic fields, K2Cr3As3 was found to enter phase C. In this phase, the superconducting gap vanished along a continuous line, rather than at isolated points.

Informing future research into topological quantum states

The team's observation of three distinct phases in K2Cr3As3 provides compelling evidence for spin-triplet superconductivity. Future studies could further examine the material for applications.

"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 (states), one of which can host the so-called Majorana excitation that can be used in fault-tolerant quantum computing."

The recent work by Zheng and his colleagues contributes to the ongoing search for spin-triplet superconductors. The researchers are now planning to use similar methods to study other candidate materials.

"We wish to explore the spin-triplet states in other materials, in the hope of raising the transition temperature (Tc)," Zheng added.



Source: https://tinyurl.com/56wpms8r via Phys.org