A magnetic excitation linking quasi-1D Chevrel-type selenide and arsenide superconductors

The quasi-one-dimensional Chevrel phases, A_2Mo_6Se_6 (A = Tl, In, K, Rb, Cs), are of interest due to their atypical electronic properties. The Tl and In analogues undergo a superconducting transition whereas the alkali metal analogues undergo metal-to-insulator transitions, neither of which is fully understood. We report the results of inelastic neutron scattering on polycrystalline In_2Mo_6Se_6 (T_c = 2.85K) and Rb_2Mo_6Se_6 (T_MIT ≈170K) which reveal a highly dispersive column of intensity present in both compounds near Q = 1.0 Å^-1. The excitation is nearly indistinguishable from another excitation observed in the structurally related superconducting compound K_2Cr_3As_3, which has been interpreted as magnetic in origin and related to Fermi surface nesting. However, the calculated Fermi surface of K_2Cr_3As_3 differs substantially from the A_2Mo_6Se_6 compounds, and many consider them distinct classes of materials. Nevertheless, the new observation is most consistent with a magnetic origin, linking the physics of both classes. These findings suggest that the selenide and arsenide compounds are more closely related than originally thought, with properties falling on the same continuum.