Spin-triplet superconductors are among the rare and intriguing quantum materials today. Unlike conventional superconductors, they allow electron pairs to retain an internal spin degree of freedom, giving rise to different superconducting states. Some topological spin-triplet superconducting phases can host Majorana excitations, which may have applications in fault-tolerant quantum computing. However, identifying and controlling genuine spin-triplet superconductivity has remained challenging. Previously studied candidates often have low transition temperatures or competing states that make their intrinsic behavior difficult to understand.
Addressing this challenge, a research team led by Professor Guo-qing Zheng, including a graduate student in the PhD program, Mr. Seiji Ogawa, from the Department of Physics, Okayama University, Japan, investigated the chromium-based superconductor K₂Cr₃As₃, a promising candidate with a superconducting transition temperature of at least 6.2 K and no long-range magnetic order. Using ⁷⁵As nuclear magnetic resonance, including Knight-shift and spin-lattice relaxation measurements, across temperatures and magnetic fields, the researchers tracked changes in its magnetic response. These measurements allowed them to determine how the orientation of the d(k)-vector associated with the paired spins and the superconducting gap structure changed. The study was made available online on August 19, 2026, and published in Volume 137, Issue 8 of the journal Physical Review Letters on August 21, 2026.
“Previously, we discovered a spin-triplet superconducting state in CuₓBi₂Se₃, but its transition temperature Tc was low. A spin-triplet superconducting state can also be obtained at the interface of an artificial junction, but the superconducting volume is small there ,” addressed Prof. Zheng. “ Our purpose was to find a real bulk spin-triplet superconductor with high Tc.” K₂Cr₃As₃ provides this opportunity while revealing a richer range of superconducting behavior.
The measurements revealed three distinct superconducting phases. At low magnetic fields and relatively high temperatures near the transition, K₂Cr₃As₃ enters Phase A, a helical state with a distinctive paired-spin arrangement. As the material is cooled further, it transforms into Phase B, a chiral state in which the d(k)-vector rotates by 90 degrees, accompanied by a change in the paired-spin orientation. Both phases have point nodes in their superconducting gaps, but their different spin configurations distinguish them as separate states.
At higher magnetic fields, the researchers identified a third state, Phase C. In this phase, the researchers found evidence for a line-nodal superconducting gap, different from the point-nodal gaps in Phases A and B. The transition between Phases B and C involves both a 90-degree reorientation of the d(k)-vector together with a change in the superconducting gap structure. Below about 7 T, the phase boundary splits, showing that the material undergoes two successive changes upon cooling: first a change in the gap structure and then a reorientation of the d(k)-vector.
“These results show that K₂Cr₃As₃ is not simply one type of spin-triplet superconductor,” said Prof. Zheng. “ Instead, temperature and magnetic field allow us to move between distinct superconducting phases with different spin structures and topological properties.”
The discovery is significant for topological quantum technologies. Phase B breaks time-reversal symmetry and is analogous to the superfluid helium-3 A phase, and its vortex cores may host Majorana excitations. Phase A is also topological and, in sufficiently thin films, could host Majorana states at their boundaries. This makes K₂Cr₃As₃ a platform for exploring topological quantum states and their potential relevance to future quantum technologies.
Overall, the discovery establishes K₂Cr₃As₃ as a rare bulk platform for studying and exploring multiple spin-triplet and topological superconducting phases. By demonstrating that magnetic field and temperature can tune the material between three distinct quantum states, the study provides a material platform for investigating Majorana excitations and a potential foundation for future fault-tolerant quantum computing technologies.
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Reference
Title of original paper: Multiple Phases in K₂Cr₃As₃: A Playground for Manipulating Topological Superconductivity
Journal: Physical Review Letters
DOI: 10.1103/kykd-2nj4
About Okayama University, Japan
As one of the leading universities in Japan, Okayama University aims to create and establish a new paradigm for the sustainable development of the world. Okayama University offers a wide range of academic fields, which become the basis of the integrated graduate schools. This not only allows us to conduct the most advanced and up-to-date research, but also provides an enriching educational experience.
Website: https://www.okayama-u.ac.jp/index_e.html
About Professor Guo-qing Zheng from Okayama University, Japan
Prof. Guo-qing Zheng is a Professor in the Department of Physics at Okayama University of Science, Japan. He received his PhD in Engineering from Osaka University in 1990. A condensed matter physicist, Prof. Zheng specializes in superconductivity and strongly correlated electron systems. Using advanced nuclear magnetic resonance techniques, he has conducted pioneering research on high-Tc cuprates, iron pnictides, heavy-fermion compounds, cobalt oxides, and non-centrosymmetric materials. He has also developed experimental methods for studying matter under high pressures, low temperatures, and intense magnetic fields. His current research explores topological phenomena and their potential for quantum technologies and computing.
Physical Review Letters
Experimental study
Not applicable
Multiple Phases in K₂Cr₃As₃: A Playground for Manipulating Topological Superconductivity
21-Aug-2026