Research report: One superconductor, three phases: A new route to tunable quantum states

Physicists at Okayama University have uncovered something unusual hiding inside a single crystal: three distinct superconducting phases within one material, each with its own spin structure and topology. The compound, K₂Cr₃As₃, a chromium-based superconductor with a transition temperature of at least 6.2 kelvin, has long been suspected of hosting exotic spin-triplet superconductivity. Now, using nuclear magnetic resonance measurements, a team led by Professor Guo-qing Zheng, together with graduate student Seiji Ogawa of the Department of Physics, has mapped how temperature and magnetic field switch the material between three separate quantum states, each carrying different implications for topological physics.

The findings, published in Physical Review Letters on August 21, 2026, in Volume 137, Issue 8, and made available online on August 19, 2026, rest on ⁷⁵As nuclear magnetic resonance, specifically Knight-shift and spin-lattice relaxation measurements taken across a range of temperatures and magnetic fields. These techniques probe the local magnetic environment of arsenic nuclei, allowing the researchers to track how the orientation of the d(k)-vector, the internal direction associated with the paired electron spins, changed alongside the structure of the superconducting energy gap. Together, these two quantities define the character of a superconducting state, and in K₂Cr₃As₃ both proved to be tunable knobs.

Spin-triplet superconductors are a rare breed among quantum materials. In conventional superconductors, electrons pair up with opposite spins in a way that erases their internal spin degree of freedom. Spin-triplet pairing preserves that freedom, opening the door to a richer family of superconducting states, some of which are topological.

Topological superconducting phases can host Majorana excitations, quasiparticles that behave as their own antiparticles and are considered promising building blocks for fault-tolerant quantum computing. The catch has been that identifying genuine spin-triplet superconductivity in real materials has proven difficult, and previously studied candidates often suffer from low transition temperatures or competing orders that obscure their intrinsic behavior. Professor Zheng's motivation reflects that struggle directly.

In earlier work, his team identified a spin-triplet superconducting state in CuₓBi₂Se₃, but its transition temperature was low. Spin-triplet superconductivity can also be engineered at the interface of an artificial junction, but the superconducting volume there is small. What the field needed, he explained, was a real bulk spin-triplet superconductor with a high transition temperature.

K₂Cr₃As₃ offers exactly that opportunity, and the new measurements reveal that it holds an unexpectedly rich range of superconducting behavior rather than a single state. The first two phases emerge as the material is cooled at low magnetic fields. Near the superconducting transition temperature, K₂Cr₃As₃ enters Phase A, a helical state with a distinctive paired-spin arrangement.

Cooling further transforms it into Phase B, a chiral state in which the d(k)-vector rotates by 90 degrees, accompanied by a corresponding change in the orientation of the paired spins. Both phases share point nodes in their superconducting gaps, meaning the energy gap vanishes only at isolated points on the Fermi surface, but their different spin configurations mark them out as genuinely separate states rather than minor variations of one another. The third phase appears only under higher magnetic fields.

Phase C shows evidence for a line-nodal superconducting gap, in which the gap vanishes along lines rather than at points, a fundamentally different gap topology from the point nodes of Phases A and B. The transition between Phases B and C involves both a 90-degree reorientation of the d(k)-vector and a change in the gap structure occurring together. Intriguingly, below about 7 tesla the phase boundary splits into two, revealing that on cooling the material undergoes two successive changes: first a change in the gap structure, then a reorientation of the d(k)-vector. That separation of what might have seemed like a single transition into two distinct steps is one of the study's most striking observations.

These results show that K₂Cr₃As₃ is not simply one type of spin-triplet superconductor, Zheng noted. Instead, temperature and magnetic field act as control parameters that move the material between distinct superconducting phases with different spin structures and topological properties. In effect, a single compound functions as a tunable playground where the geometry of electron pairing can be switched on demand, something that has been extraordinarily difficult to achieve in other candidate materials.

The topological implications are considerable. Phase B breaks time-reversal symmetry and is analogous to the A phase of superfluid helium-3, a celebrated topological quantum fluid. Its vortex cores may host Majorana excitations, the localized quasiparticle states that quantum computing researchers hope to exploit for topologically protected information storage.

Phase A is also topological, and in sufficiently thin films it could host Majorana states at its boundaries. The prospect of accessing two different Majorana-hosting regimes within one bulk crystal, simply by adjusting temperature and field, makes K₂Cr₃As₃ a compelling platform for exploring topological quantum states and their potential relevance to future quantum technologies. What distinguishes K₂Cr₃As₃ among spin-triplet candidates is the combination of a relatively high transition temperature of at least 6.2 kelvin and the absence of long-range magnetic order.

Competing magnetic phases have complicated the interpretation of other triplet candidates, muddying whether the superconductivity itself is genuinely triplet. Overall, the study establishes K₂Cr₃As₃ as a rare bulk platform for studying multiple spin-triplet and topological superconducting phases within a single material. By demonstrating that magnetic field and temperature can tune the compound between three distinct quantum states, the work provides an experimental foundation for investigating Majorana excitations in a controlled setting and points toward a potential materials basis for future fault-tolerant quantum computing technologies.

Whether the promise of Majorana-based quantum information processing will be realized in this compound remains an open question for future experiments, but the ability to navigate between topological phases in one crystal marks a significant step forward for the field.

Subject of Research: Three tunable spin-triplet superconducting phases in the chromium-based superconductor K₂Cr₃As₃

Article Title: One superconductor, three phases: A new route to tunable quantum states

Article References: One superconductor, three phases: A new route to tunable quantum states. Original publication (Bibliographic details unavailable.)

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Keywords: spin-triplet superconductivity, K2Cr3As3, topological superconductivity, Majorana excitations, nuclear magnetic resonance, superconducting phases, phase diagram, quantum computing, Okayama University, Physical Review Letters, d-vector, time-reversal symmetry