Thermal Hall effect and spin nernst effect in a frustrated honeycomb magnet without DM interaction

We study thermal responses of magnons in a honeycomb lattice with frustration. While most of the previous studies on transverse thermal responses in magnets consider the effect of DM interaction, we propose that nonzero thermal Hall effect and spin Nernst effect arise without DM interaction once we incorporate frustration with next nearest neighbor coupling and asymmetry between sublattices. We determine spiral spin configurations that minimize the total energy classically and derive a magnon Hamiltonian from the obtained configuration using Holstein-Primakov transformation. We compute Berry curvature that characterize a nontrivial geometry of the magnon states and discuss thermal Hall effect and spin Nernst effect that originate from such nontrivial geometry.