Magnetic excitations in alpha-RuCl3 - a honeycomb-lattice quantum magnet with strong spin-orbit coupling - as a function of external magnetic field

It is known that strong spin-orbit coupling entangles the spin and orbital spaces, and leads to a rich variety of the low energy Hamiltonians. This gives way to ‘‘engineer’’ in such Mott insulators for example the exactly solvable spin model by Alexei Kitaev. The insulating magnetic material alpha-RuCl3 is comprised of van der Waals coupled honeycomb layers in which the Ru3+(4d5) ions are in the low spin state and form a honeycomb lattice. A strong cubic crystal field combined with spin-orbit coupling leads to a Kramer’s doublet, nearly perfect J=1/2 ground state thus satisfying the conditions necessary for producing the above mentioned Kitaev couplings in the low energy Hamiltonian.

Here, I will present neutron scattering investigations of the magnetic excitations and present how they drastically change under the application of an external magnetic field. We observe that, when a magnetic field of approximately 7.5 T is applied in the honeycomb plane, a transition from zigzag magnetic order to a disordered state, believed to be a quantum spin liquid, takes place.  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 gamma point that is interpreted as a signature of fractionalized excitations. Published measurements by Kasahara et al (Nature 559, 227 (2018)) of the thermal Hall effect suggest a possible topological transition at an even higher field. We have performed 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.