To be or not to be

Nat. Commun. 8, 1152 (2017)

In an ordered spin system, the spectrum for collective excitations of the spin structure is discretized, the corresponding quasiparticles being magnons. However, magnets subject to strong spin–orbital coupling, such as α-RuCl₃, provide striking counterexamples. Their magnetic excitations form a continuum inconsistent with magnon descriptions. It is often thought that these are the coherent fractional excitations expected in the celebrated Kitaev spin liquid — a peculiar state displaying no long-range order down to zero temperature.

But now, Stephen Winter and colleagues have suggested that the observed continua could instead originate from non-Kitaev interactions naturally present in the low-symmetry crystalline environment of real materials. This scenario leads to incoherent excitations, and fully explains the inelastic magnetic response of α-RuCl₃ recently observed in neutron scattering experiments. The conditions under which the magnons break down due to these interactions were identified, revealing that nontrivial excitations persist well beyond the Kitaev spin liquid. As the knowledge of the underlying interactions is still incomplete, we still have a way to go before we can claim to have fully characterized the nature of excitations.