News.hust.edu.cn (Correspodent Hu Hao) On September 23 - Nature published a paper of Professor Luo Yongkang of Wuhan National High Magnetic Field Center entitled Constraints on the superconducting order parameter in Sr₂RuO₄ from oxygen-17 nuclear magnetic resonance.

The result was jointly achieved by Professor Luo Yongkang, Professor Stuart Brown and Doctor Andrej Pustogow of UCLA, and the cooperators from Los Alamos National Laboratory, the Max Planck Institute in Dresden and Stanford University. Professor Luo Yongkang is the co-first author and corresponding author of the paper.

In condensed matter physics, the phase of matter is usually represented by the symmetry and its order parameter. However, it is very difficult to measure the symmetry and order parameters of some materials such as superconductive symmetry of Sr₂RuO₄. Since Sr₂RuO₄ was discovered (Y. Maeno, 1994), it has been known as a typical example of unconventional superconductors with odd symmetry, spin-triplet pairing, topological property, etc. for a long time. The preliminary study (Phys. Rev. X 9, 021044) of Professor Luo Yongkang et al has experimentally proved that strained Sr₂RuO₄ conforms to the characteristics of a van Hove singularity (vHs), which has laid a foundation for revealing superconductivity of Sr₂RuO₄. In one aspect, viewing from the state density angle, the Tc in a spin-singlet pairing state is more greatly affected by the state density. In another aspect, the Stoner factors are increased at the same time due to the van Hove singularity, that is, the system approaches ferromagnetic instability and thus the spin-triplet pairing strength is enhanced.

The left figure shows the measured knight shift and temperature curves of ¹⁷O NMR near a van Hove singularity in a uniaxial strained state. The upper illustration part shows the strain generator, and the lower illustration shows superconducting transition under the strain. The right figure shows the measured knight shift and radio frequency energy input curves of strain-free ¹⁷O NMR. By controlling and quantifying the radio frequency input energy, prove the changes of spin susceptibility in a strain-free state that take place before and after the superconductive phase change.

To deeply research the superconductive state properties of Sr₂RuO₄, Professor Luo Yongkang and his cooperators further measured and analyzed the ¹⁷O NMR of Sr₂RuO₄, which provided new enlightenment for revealing the superconductive pairing symmetry of Sr₂RuO₄. Their research findings: First, no matter strain exists or not, the spin susceptibility of Sr₂RuO₄ obviously changes before and after the superconductive phase change; second, these phenomena cannot be understood by using the conventional chiral p-wave representation and even all the superconductive order parameters parallel to the z direction are in doubt, which prompts people to re-examine the superconductive symmetry. The work does not completely eliminate all the p-wave symmetry possibilities. To prove or falsify them, the variation of the spin susceptibility before and after the superconductive phase change needs to be more accurately measured.

Professor Luo Yongkang is a high-level talent introduced from a foreign country by Wuhan National High Magnetic Field Center in August 2018. This work is another important achievement following the paper he published in Physical Review X in May 2019. In the recent years, Wuhan National High Magnetic Field Center constantly achieved high-level scientific research results as the operation efficiency of the national important scientific and technological infrastructure for pulsed high magnetic field was constantly improved. Wuhan National High Magnetic Field Center published 104 research papers in only the first half of 2019, including 29 Q1 papers. Many papers have been published in the top academic journals such as Nature, Nature Materials and Physical Review Letters. The scientific benefits of the great scientific facilities are now becoming more and more prominent.

Link to the paper: https://www.nature.com/articles/s41586-019-1596-2