Gapless spin excitations in the superconducting state of a quasi-one-dimensional

Majorana zero modes form as intrinsic defects in an odd-orbital one-dimensional superconductor, thus motivating the search for such materials in the pursuit of Majorana physics. Here, we present combined experimental results and first-principles calculations which suggest that quasi-one-dimensional K2Cr3As3 may be such a superconductor. Using inelastic neutron scattering we probe the dynamic spin susceptibilities of K2Cr3As3 and K2Mo3As3 and show the presence of antiferromagnetic spin fluctuations in both compounds. Below the superconducting transition, these fluctuations gap in K2Mo3As3 but not in K2Cr3As3. Using first-principles calculations, we show that these fluctuations likely arise from nesting on one-dimensional features of the Fermi surface. Considering these results we propose that while K2Mo3As3 is a conventional superconductor, K2Cr3As3 is likely a spin triplet, and consequently a topological superconductor.