A spin-triplet superconductor can harbor Majorana bound states that can be used in topological quantum computing. Recently, K2Cr3As3 and its variants with critical temperature Tc as high as 8 K have emerged as a new class of superconductors with ferromagnetic spin fluctuations. Here we report a discovery in K2Cr3As3 single crystal that, the spin susceptibility measured by 75As Knight shift below Tc is unchanged with the magnetic field H0 applied in the ab plane, but vanishes toward zero temperature when H0 is along the c axis, which unambiguously establishes this compound as a spin-triplet superconductor described by a vector order-parameter d parallel to the c axis. Combining with points-nodal gap we show that K2Cr3As3 is a new platform for the study of topological superconductivity and its possible technical application.
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