From Frustrated to Unfrustrated: Coupling two triangular-lattice itinerant Quantum Magnets

Motivated by systems that can be seen as composed of two frustrated sublattices combined into a less frustrated total lattice, we study the double-exchange model with nearest-neighbor (NN) and next--nearest-neighbor (NNN) couplings on the honeycomb lattice. When adding NN hopping and its resulting double exchange to the antiferromagnetic (AFM) Heisenberg coupling, the resulting phase diagram is quite different from that of purely Heisenberg-like magnetic models and strongly depends on electron filling. For half filling, patterns of AFM dimers dominate, where the effective electronic bands remain graphene-like with Dirac cones in all phases, from the FM to the $120^\circ$ limit. When the density of states at the Fermi level is sizable, we find non-coplanar incommensurate states as well as a small-vortex phase. Finally, a non-coplanar commensurate pattern realizes a Chern insulator at quarter filling. In the case of both NN and NNN hopping, the noncoplanar spin pattern inducing Chern insulators in triangular lattices is found to be quite stable under coupling into a honeycomb system. The resulting total phases are topologically nontrivial and either a Chern insulator with $C=2$ or a magnetic topological crystalline insulator protected by a combination or mirror-reflection and time-reversal symmetries arise.