Insights on magnon topology in Heisenberg-Kitaev models for bilayer quantum magnets with Dzyaloshinskii-Moriya interaction and broken inversion symmetry
We present a comprehensive study on magnon topology in honeycomb bilayer quantum magnets with ferromagnetic (FM) and layered antiferromagnetic (LAFM) ground states. Several models are investigated to fully understand the separate and combined effects of Heisenberg exchange, Kitaev interaction, Dzyaloshinskii-Moriya interaction (DMI), and inversion symmetry breaking. Both ground states constitute rich platforms to realize several topological phases which can be tuned via experimentally controllable parameters. Nevertheless, LAFM bilayers are found to be more exotic: (i) the band gaps can close away from the Brillouin zone corners forming unconventional Dirac cones (UDCs) (ii) the UDCs play a fundamental role in shaping the topological phase diagram in LAFM bilayers and induces richer topology compared to their FM counterparts (iii) valley-polarized magnons can be excited in LAFM bilayers by Zeeman effect (iv) topological phase transitions in LAFM bilayers can reverse the signs of the magnon Hall and Nernst conductivities. We further set simple rules to unambiguously determine the nature of spin-orbit interactions (Kitaev versus DMI) in LAFM bilayers based on the profiles of their topological conductivities. We believe the present study provides important insights into the consequences of fundamental magnetic interactions on the topology, transport, and technological potentials of 2D magnons.