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 CuxBi2Se3 with a critical temperature Tc~3.5K was unambiguously proved to be in such state . Recently, A2Cr3As3 (A = Na, K, Rb, Cs) superconductors with Tc 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 (Tc=6.5 K) that, the spin susceptibility measured by 75As Knight shift remains unchanged across Tc 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 . 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.
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