Neutron scattering study of the 2D dipolar magnet ErBr3 and the 2D quantum spin liquid system YbBr3

In recent years, a lot of effort was made to investigate materials with a honeycomb lattice, experimentally as well as theoretically. Both electronic and magnetic honeycomb systems are studied to explore their rich variety of phenomena. Graphene is a good example for an electronic system that possess Dirac fermions and exhibits topological effects like the quantum Hall effect. In particular, the topological properties of materials are regarded as promising to result in novel technological applications. Magnetic equivalents to graphene also show topological properties. Among the topological states are some of the elusive quantum spin liquids that are regarded to hold potential for building fault-tolerant quantum computers. Here, I present the investigation of the two-dimensional van der Waals magnets ErBr3 and YbBr3 The materials are isostructural and form an undistorted honeycomb lattice of magnetic ions. ErBr3 is governed by dipolar interactions only and forms a continuously degenerate non-collinear ground-state with the spins restricted to the honeycomb plane. The ground-states correspond to specific spin structures what allows to reversibly manipulate the spin-wave dispersion between a magnetic phase with Dirac excitations and a phase with non-trivial valley Chern number. YbBr3, in contrast, is a collinear antiferromagnet with competing nearest and next-nearest neighbor interactions that remains only short-range correlated down to at least T = 100 mK. The spin-excitations show a broad continuum at the Brillouin zone boundary and are evidence for a quantum spin liquid phase near a quantum critical point. The results point towards fluctuations on the honeycomb plaquette as origin of the continuum