Field dependent magnetic excitations and possible topological transitions in α-RuCl₃

It is now well-known that when a magnetic field of approximately 7.5 T is applied in the honeycomb plane, the quantum magnet α-RuCl₃ displays a transition from zigzag magnetic order to a disordered state, believed to be a quantum spin liquid. This is reflected in the magnetic excitation spectrum as a disappearance of spin waves and a strengthening of a scattering continuum centered on the 2D Γ point that is interpreted as a signature of fractionalized excitations. Published measurements by Y. Kasahara et al (Nature 559, 227 (2018)) of the thermal Hall effect suggest a possible topological transition at an even higher field. Here I will discuss new high-resolution measurements of the excitations at fields spanning these two transitions. At the highest fields the broad scattering continuum is altered, and the response is seen to be dominated by a sharp peak at the lower bound, providing a strong indication that a new phase has been entered. The implications of these observations will be discussed in detail.