Magnetic excitation linking quasi-one-dimensional Chevrel-type selenide and arsenide superconductors

The quasi-one-dimensional Chevrel phases, ${A}_{2}{\mathrm{Mo}}_{6}{\mathrm{Se}}_{6}$ ($A=\mathrm{Tl}$, In, K, Rb, Cs), are of interest due to their atypical electronic properties. The Tl and In analogs undergo a superconducting transition whereas the alkali metal analogs show charge gapping of another, not well understood type. We report the results of inelastic neutron scattering on polycrystalline ${\mathrm{In}}_{2}{\mathrm{Mo}}_{6}{\mathrm{Se}}_{6}$ (${T}_{\mathrm{c}}=2.85$ K) and ${\mathrm{Rb}}_{2}{\mathrm{Mo}}_{6}{\mathrm{Se}}_{6}$ (nonsuperconducting) samples, which reveal a column of intensity with linear dispersion from [0 0 1/2] to [0 0 1] in both compounds. The observed temperature and $|Q|$ independence together suggest the presence of unconventional carriers with a spin contribution to the excitation. This is contrary to the prevailing model for these materials, which is that they are nonmagnetic. The excitation has similar dispersion and $S(Q,\phantom{\rule{0.16em}{0ex}}\mathrm{E},\phantom{\rule{0.16em}{0ex}}T)$ behavior as one observed in the structurally related superconducting compounds ${A}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ and ${A}_{2}{\mathrm{Mo}}_{3}{\mathrm{As}}_{3}$ ($A=\mathrm{K}$, Rb, Cs), which has been interpreted as magnetic in origin and related to Fermi surface nesting. The connection is unexpected because the calculated Fermi surface of the arsenides differs substantially from the ${A}_{2}{\mathrm{Mo}}_{6}{\mathrm{Se}}_{6}$ compounds, and many consider them distinct classes of materials. This observation suggests a hidden link in the physics between both classes of superconductors, perhaps originating from their quasi-low-dimensional character.