Professor Michel Kenzelmann 

Quantum fluctuations and tunable magnetic excitations in the two-dimensional honeycomb materials YbBr3 and ErBr3

Low-dimensional magnetism on the honeycomb lattice is expected to lead to a range of novel phases, some of which feature quantum spin liquid correlations or topological degrees of freedom. We studied two different van-der Waals rare-earth trihalides, YbBr3 and ErBr3, using single-crystal neutron scattering to investigate primarily the magnetic correlations and their excitations. 

In YbBr3 we find no magnetic order down to the lowest temperatures surveyed of about 100mK [1]. The correlations remain short-ranged, but feature relatively well-defined magnetic excitations. At the zone boundary of the reciprocal cell, we find broad continuum scattering as a function of wave-vector and energy that is consistent with hexagon plaquette quantum fluctuations. The magnetic excitations are the result of competing nearest and next-nearest antiferromagnetic exchange interaction that place YbBr3 near a quantum critical point towards a quantum spin liquid phase.

In ErBr3, we find magnetic order below T=280mK and magnetic excitations that arise from purely dipolar magnetic interactions [2]. The magnetic ground state appears to include continuously degenerate non-collinear spin arrangements in the honeycomb plane. The magnon dispersion exhibit Dirac-like cones as long as time and inversion symmetry are respected, and a spin gap opening otherwise, with a finite Berry curvature near the Dirac points. These results illustrate that the spin-wave dispersion of magnetic dipoles on the honeycomb lattice may be reversibly controlled from a magnetic phase with Dirac cones to a topological phase with non-trivial magnon valley Chern numbers. In addition, we find evidence of a rare purely two-dimensional magnetic Kosterlitz-Thouless transition [3].

[1] C. Wessler et al. NPJ Quantum Materials 5, 85 (2020).

[2] C. Wessler, et al. Comm. Phys. 16, 5 (2022).

[3] C. Wessler, et al. unpublished.