Okayama University: Researchers Find New Material Promising for Quantum Computing Advances

Ogawa and colleagues at the University of Tokyo have recently published findings concerning the intriguing superconducting properties of K2Cr3As3.

Mapping electron pairing in K2Cr3As3 using arsenic-75 nuclear magnetic resonance

Nuclear magnetic resonance (NMR) proved key in discerning the subtle changes occurring within K2Cr3As3 as it transitioned between superconducting states. NMR is a spectroscopic technique that exploits the magnetic properties of atomic nuclei to provide detailed information about the material’s structure and dynamics. It detects the nuclei of atoms, revealing their magnetic environment and thus providing insights into electron behaviour. The 75As isotope was utilised, owing to its sensitivity to local spin susceptibility, to map the arrangement of paired electrons. Arsenic-75 possesses a nuclear spin of I = 3/2, making it particularly well-suited for NMR studies of magnetic materials. Its sensitivity allowed investigation of the material’s superconducting properties, offering a detailed view of electron interactions. The technique relies on applying a radiofrequency pulse to the nuclei in a strong magnetic field and observing the frequency at which they resonate, which is affected by the local magnetic environment. This allows researchers to probe the electronic structure and pairing symmetry of the superconductor.

K2Cr3As3 exhibits superconductivity up to 6.2 Kelvin, a relatively high transition temperature for a Chromium-based material, and lacks long-range magnetic order which simplifies analysis. This is significant because many candidate materials for topological superconductivity suffer from the presence of competing magnetic orders that obscure the superconducting signal and complicate the interpretation of experimental results. NMR was favoured over techniques like nuclear quadrupole resonance (NQR) as it directly probes the spin state of the superconducting electrons, confirming a spin-triplet pairing mechanism. In spin-triplet pairing, the paired electrons have a net spin of 1, unlike conventional spin-singlet superconductors where the paired electrons have a net spin of 0. This difference is crucial for the potential hosting of Majorana bound states. The absence of competing magnetic orders allows for clearer observation of electron behaviour, a significant advantage over previous uranium-based superconductors, which often suffered from interference. U-based compounds, while also investigated for topological superconductivity, typically exhibit complex magnetic behaviour and lower transition temperatures, hindering detailed analysis.

Superconducting phase transitions and d-vector rotation in K2Cr3As3

K2Cr3As3 demonstrates a transition temperature of 8.6 Kelvin, exceeding that of previous uranium-based superconductors hampered by low temperatures and competing magnetic orders. This higher transition temperature is a crucial step towards potential applications, as it reduces the cooling requirements for operating superconducting devices. Three distinct superconducting phases, A, B, and C, were identified, each characterised by a unique orientation of electron spins, revealing a substantial degree of tunability within a single material. This tunability is highly desirable for tailoring the properties of the superconductor for specific applications. NMR measurements revealed that the direction of these paired electron spins, known as the d(k)-vector, rotates from being aligned within the material’s plane to perpendicular to it as the material transitions between phases A and B. Subsequently, it lies again within the plane in phase C. The d(k)-vector represents the direction of the spin component of the Cooper pairs, and its rotation indicates a change in the pairing symmetry. Analysis of the Knight shift revealed a transition temperature of 5.3 Kelvin, alongside a secondary temperature of 4.5 Kelvin where the spin alignment begins to change, demonstrating a complex interaction of magnetic and superconducting properties. The Knight shift is a measure of the change in the NMR signal due to the presence of magnetic moments, and its analysis provides information about the spin susceptibility of the material. Measurements of the relaxation rate, 1/T1, indicated that the superconducting gap possesses line nodes at a magnetic field of 13 Tesla, transitioning to point nodes at lower fields, confirming a tunable topological state. The presence of nodes in the superconducting gap is related to the topological properties of the superconductor and is a key characteristic of materials that can host Majorana bound states. Further work is needed to substantially raise the 6.2 Kelvin transition temperature for practical applications, potentially unlocking wider technological uses. Increasing the transition temperature would significantly reduce the cost and complexity of operating superconducting devices.

Tunable superconductivity in K2Cr3As3 offers promise for Majorana bound state investigation

Establishing multiple, tunable superconducting phases within K2Cr3As3 represents a strong step forward in the pursuit of materials suitable for quantum technologies. The ability to control the orientation of the d(k)-vector through external parameters, such as magnetic field or pressure, is crucial for manipulating the superconducting properties.

Establishing evidence of spin-triplet superconductivity in K2Cr3As3 is significant because this type of superconductivity can potentially host Majorana bound states for use in fault-tolerant quantum computing. Researchers identified three distinct superconducting phases within the material, observing a rotation of the paired-spin direction as the material cooled. Nuclear magnetic resonance measurements confirmed a transition temperature of 6.2 Kelvin and revealed the evolution of these spin pairings. The authors suggest further research is needed to increase this transition temperature and fully explore the material’s potential.