in a transverse field*

BaCo₂(AsO₄)₂ is a honeycomb quantum magnet where an antiferromagnetic third nearest neighbor interaction frustrates nearest neighbor ferromagnetic easy plane interactions which may place the material close to a quantum spin-liquid phase. In particular, a field applied transverse to the honeycomb plane should enhance quantum fluctuations and might be able to induce a quantum spin-liquid.

To explore this possibility, we performed inelastic neutron scattering measurements in a large transverse field. By tilting the c-axis by five degrees relative to a large vertical field, a small in-plane field was applied along the nearest-neighbor bond direction. Even with 10.9 tesla along c and 1 tesla along a, commensurate antiferromagnetic order and coherent, gapped spin wave exciations were found at T=1.8 K. Analysis of the corresponding anisotropic dispersion relation was used to further constrain the spin Hamiltonian for BaCo₂(AsO₄)₂, and gauge opportunities for realizing spin liquid phases in the honeycomb cobaltate.