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 ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ may be such a superconductor. Using inelastic neutron scattering we probe the dynamic spin susceptibilities of ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ and ${\mathrm{K}}_2{\mathrm{Mo}}_3{\mathrm{As}}_3$ and show the presence of antiferromagnetic spin fluctuations in both compounds. Below the superconducting transition, these fluctuations gap in ${\mathrm{K}}_2{\mathrm{Mo}}_3{\mathrm{As}}_3$ but not in ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$. 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 ${\mathrm{K}}_2{\mathrm{Mo}}_3{\mathrm{As}}_3$ is a conventional superconductor, ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ is likely a spin triplet, and consequently a topological superconductor.

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• Fermi surface
• Spin-triplet pairing
• Topological superconductors
• 1-dimensional systems
• Superconductors
• Density functional theory
• Inelastic neutron scattering
• Neutron diffraction