Okayama University researchers have identified three distinct superconducting phases within the material K₂Cr₃As₃, a chromium-based compound exhibiting superconductivity at a relatively high 6.2 K and lacking long-range magnetic order. These phases offer a path for exploring topological superconductivity and the potential Majorana excitations vital to advanced quantum computing.
Professor Guo-qing Zheng explained the team’s aim was “to find a real bulk spin-triplet superconductor with high Tc,” and K₂Cr₃As₃ delivers this opportunity alongside a surprising range of behaviors revealed through detailed magnetic response measurements. The findings, published in Physical Review Letters, map how paired electron spins and energy gaps change with both temperature and magnetic field.
K₂Cr₃As₃ Exhibits Three Distinct Spin-Triplet Superconducting Phases
K₂Cr₃As₃ achieves a superconducting transition at 6.2 Kelvin, a comparatively high temperature for materials exhibiting spin-triplet superconductivity and notable because the material demonstrably lacks long-range magnetic order, a condition often complicating research in this area. This absence of magnetic order simplifies the analysis of the superconducting states, allowing researchers to more clearly isolate and study the properties of paired electrons. The ability to achieve superconductivity at this temperature, combined with the lack of complicating magnetic effects, positions K₂Cr₃As₃ as a promising candidate for further investigation into topological superconductivity.
Measurements revealed not one, but three distinct superconducting phases within K₂Cr₃As₃, offering what researchers describe as a means to transition between different quantum states. The discovery allows for the application of both temperature and magnetic fields to transition between these phases, each possessing unique spin structures and topological properties, a level of control previously difficult to achieve in similar materials.
Phase B, for example, breaks time-reversal symmetry and shares characteristics with superfluid helium-3 A, with its vortex cores potentially hosting Majorana excitations, particles considered crucial for building fault-tolerant quantum computers. Phase An is also topological and, when formed in sufficiently thin films, could support Majorana states at its boundaries. Seiji Ogawa, a graduate student at Okayama University’s Department of Physics, contributed to the research, highlighting the importance of student involvement in advancing the field of quantum materials.
The ability to reliably switch between these phases opens new avenues for exploring the fundamental physics of topological superconductivity and could accelerate the development of quantum technologies reliant on Majorana excitations. Overall, the discovery establishes K₂Cr₃As₃ as a rare bulk platform for studying and exploring multiple spin-triplet and topological superconducting phases.
“These results show that K₂Cr₃As₃ is not simply one type of spin-triplet superconductor,”
Prof. Guo-qing Zheng, Professor at Okayama University
⁷⁵As NMR Maps Tunable Gap Structures & d(k)-Vector Orientation
⁷⁵As nuclear magnetic resonance measurements have revealed how the superconducting gap structure and d(k)-vector orientation change within K₂Cr₃As₃, detailing the transitions between its three distinct superconducting phases. Researchers tracked the material’s magnetic response across varying temperatures and magnetic fields, establishing a detailed map of these phase changes and their associated spin configurations. This precise mapping is important because it demonstrates how external stimuli can be used to manipulate the material’s quantum state, a capability previously difficult to achieve in similar compounds.
The team identified that as K₂Cr₃As₃ cools from a relatively high transition temperature of 6.2 K, it shifts from Phase A, a helical state, to Phase B, a chiral state. This transition involves a 90-degree rotation of the d(k)-vector, indicating a fundamental rearrangement of the paired-spin orientation.
Further application of magnetic fields induces a transition to Phase C, characterized by a line-nodal superconducting gap, differing from the point-nodal gaps observed in Phases An and B. Below 7 Tesla, the phase boundary splits, revealing that cooling induces two successive changes: first, a modification of the gap structure, then a reorientation of the d(k)-vector. The study detailing these findings appeared in Volume 137, Issue 8 of Physical Review Letters on August 21, 2026, following its online availability on August 19, 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,”
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