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

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.