Spin-triplet superconductivity in K2Cr3As3

A spin-triplet superconductor with ferromagnetic spin fluctuation superconducts at a critical temperature of 6.5 K. A spin-triplet superconductor can harbor Majorana bound states that can be used in topological quantum computing. Recently, K 2 Cr 3 As 3 and its variants with critical temperature T c as high as 8 kelvin have emerged as a new class of superconductors with ferromagnetic spin fluctuations. Here, we report a discovery in K 2 Cr 3 As 3 single crystal that the spin susceptibility measured by 75 As Knight shift below T c is unchanged with the magnetic field H 0 applied in the ab plane but vanishes toward zero temperature when H 0 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 point nodal gap, we show that K 2 Cr 3 As 3 is a new platform for the study of topological superconductivity and its possible technical application.