Itinerant magnetism and magnetic polarons in the triangular lattice Hubbard model

Abstract: We use density matrix renormalization group to investigate the phase diagram of the Fermi Hubbard model on a triangular lattice with densities above half-filling, $1 \leq n < 2$. We discuss the important role of kinetic magnetism and magnetic polarons. For strong interactions and low doublon dopings, attractive interaction between polarons results in phase separation between the fully polarized state at finite doping and the commensurate spin-density wave state at half-filling. For intermediate interaction strength and small doping, competition between antiferromagnetic superexchange and kinetic magnetism gives rise to the incommensurate spin density wave (I-SDW) phase. Fully polarized ferromagnetic (FPF) phase for weak interactions is limited to dopings close to the van Hove singularity in the density of states. With increasing interactions the FPF phase expands to lower dopings. For strong interactions it reaches the low doping regime and is better understood as arising from proliferation of Nagaoka-type ferromagnetic polarons. Other phases that we find include a partially polarized phase, another type of I-SDW at high densities, and M\"uller-Hartmann ferromagnetism close to the band insulating regime.