Spin-triplet superconductivity is a rare quantum phenomenon and is particularly fascinating from the topological perspective, as such novel superconductors can host Majorana bound states that can be used in topological quantum computing. Although spin-triplet state has long been sought, concrete evidence had been lacking until 2016 when Cu_xBi₂Se₃ with a critical temperature T_c~3.5K was unambiguously proved to be in such state [1]. Recently, A₂Cr₃As₃ (A = Na, K, Rb, Cs) superconductors with T_c as high as 8 K have emerged as a new class of materials with ferromagnetic spin correlations [2, 3]. In this presentation, we show in K₂Cr₃As₃ single crystal (T_c=6.5 K) that, the spin susceptibility measured by ⁷⁵As Knight shift remains unchanged across T_c with the magnetic field applied in the ab plane, but is reduced in the superconducting state when the field is along the c axis, vanishing toward zero temperature [4]. Such spontaneous emergence of spin nematicity unambiguously indicates that K₂Cr₃As₃ is a spin-triplet superconductor described by a vector order parameter d that is parallel to the c axis. We further find that the interaction dictating the d-vector direction is less than 13 T. We discuss the multiple-phases feature of the superconducting state, and show that K₂Cr₃As₃ is a new platform for the study of topological superconductivity at the highest temperature ever.

References
[1] K. Matano, M. Kriener, K. Segawa, Y. Ando, G.-q. Zheng, Nature Physics 12, 852 (2016).
[2] J. Yang et al., Phys. Rev. Lett. 114, 147002 (2015).
[3] J. Luo et al., Phys. Rev. Lett. 123 047001 (2019).
[4] J. Yang, J. Luo, C. Yi, Y. Shi, Y. Zhou, and G.-q. Zheng, Sci. Adv. 7, eabl4432 (2021).