Evidence for Kitaev Physics in α-RuCl₃

There has been much interest recently in systems that may exhibit physics related to the Kitaev Model on a honeycomb lattice. Candidate materials should have J=1/2 ground states and anisotropic spin-spin interactions with the correct form. This can be realized in suitably structured materials with octahedrally coordinated magnetic ions with an electronic configuration nd5. The ion should be in the strong crystal field limit (i.e. low spin state) with spin-orbit coupling as the next most important energy. Most of the experimental effort to date has concentrated on iridate systems containing Ir⁴⁺ ions. It has recently become apparent that the material α-RuCl₃ is another possibility. It is a magnetic semiconductor composed of stacks of weakly coupled honeycomb lattices of octahedrally coordinated Ru³⁺ ions. This talk will discuss results of recent investigations of α-RuCl₃, focusing on neutron scattering measurements of the magnetic structure and excitations. Neutron diffraction reveals a low temperature ordered magnetic state corresponding to a "zig-zag" structure in the planes but with the full three-dimensional structure dependent on the stacking sequence of the layers. Time-of-flight inelastic neutron scattering reveals an excitation near 200 meV which is identified as a transition from the single ion J=1/2 ground state to the J=3/2 excited state, thus yielding a direct measurement of the spin orbit coupling energy. Higher resolution measurements reveal two collective modes at much lower energy scales. The results are compared with the theoretical expectations for excitations in the Heisenberg - Kitaev model on a honeycomb lattice, and show both that Kitaev interactions are important, and that there is evidence for quantum spin liquid behavior in the form of fractional magnetic excitations.