Spin-excitations in the superconducting states of K2Mo3As3 and K2Cr3As3 and indications of spin-triplet superconductivity

In the race to find new topological materials, superconductivity has found renewed interest as a potential host of the Majorana Fermion. This exotic solution to the relativistic wave-equation is predicted to arise as a quasiparticle in superconductors with either spin-triplet pairing or with topological Dirac-like dispersions in their band structure. Recently, a new family of quasi-1D superconductors A_1,2M₃As₃ (with A= Alkali metal and M = Cr or Mo) was discovered which is proposed to exhibit both of these rather rare properties. Here, we report the results of neutron scattering studies together with first principles calculations which reveal the presence of spin-excitations in both K₂Cr₃As₂ and K₂Mo₃As₃ and identify them as likely resulting form Fermi surface nesting between two quasi-one-dimensional sheets of the Fermi surface. Intriguingly, while these fluctuations gap below the superconducting transition in K₂Mo₃As₃, they do not in K₂Cr₃As₃. Considering the features of the Fermi surfaces the relative Fermi velocities and previous work on these materials we use these observations to strengthen the case for K₂Cr₃As₃ as a spin-triplet superconductor.