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

The quasi-one-dimensional Chevrel phases, A2Mo6Se6 (A=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 In2Mo6Se6 (Tc=2.85 K) and Rb2Mo6Se6 (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,E,T) behavior as one observed in the structurally related superconducting compounds A2Cr3As3 and A2Mo3As3 (A=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 A2Mo6Se6 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.