What have we learned about the magnetic interactions in α-RuCl₃ from neutron scattering?

It has been almost 10 years since the first neutron scattering results from α-RuCl₃ were published starting in 2015 (see e.g. [1-3]). This was about a year after α-RuCl₃ was first described as a candidate material to realize the exactly solvable Kitaev model on a honeycomb lattice [4], as a related compound to the iridates Na₂IrO₃ and Li₂IrO₃. As of now we have however learned that none of these materials is a good realization of the Kitaev model. The model consists of bond directional nearest-neighbor magnetic interactions and since the interaction in neighboring spins connects orthogonal components, the model cannot generate long-range magnetic order and the ground state is a quantum spin liquid. The observation of collinear AFM magnetic order in α-RuCl₃ through neutron scattering immediately implies that the real material has additional magnetic couplings and the 3D ordering wavevector shows that interplane couplings are present. Inelastic neutron scattering (INS) has further revealed sharp modes expected for conventional magnon excitations superimposed on a broad continuum compatible with the presence of fractionalized excitations. Neutron scattering has also shown that the AFM order is suppressed by intermediate magnetic fields before a high-field ordered phase is entered. The minimal viable model that accounts for these observations includes up to 3rd neighbor couplings in the honeycomb plane and up to 2nd neighbor couplings perpendicular to the layers.

We will present new INS data together with models thereof that shed light on the spin excitations of this system indicating complex magnetic interactions. The data agrees with a pronounced anisotropy of the magnetic interaction but at the same time disagrees with a simple or strongly dominant Kitaev coupling. The comparison to 3D models reveals how interlayer interactions affect the magnetic excitations.

1. J. A. Sears et al., Phys. Rev. B 91, 144420 (2015).

2. A. Banerjee et al., Nat. Mat. 15, 733 (2016).

3. S.-H. Do et al., Nat. Phys. 13, 1079 (2017).

4. A. Kitaev, Annals of Physics 321, 2 (2006).