Spontaneous formation of kagome network and Dirac half-semimetal on a triangular lattice

TL;DR

This paper demonstrates that in a triangular lattice Kondo model, a spontaneous kagome network forms, leading to a Dirac half-semimetal state with potential for fractional charge excitations, controllable by magnetic fields.

Contribution

It reveals a novel spontaneous formation of a kagome network and Dirac half-semimetallic state in a triangular lattice Kondo system, including effects of Coulomb interactions and external magnetic fields.

Findings

Spontaneous kagome network formation at 5/8 filling.

Emergence of Dirac half-semimetallic electronic structure.

Charge order and fractional charge excitations potential.

Abstract

In spin-charge coupled systems, geometrical frustration of underlying lattice structures can give rise to nontrivial magnetic orders and electronic states. Here we explore such a possibility in the Kondo lattice model with classical localized spins on a triangular lattice by using a variational calculation and simulated annealing. We find that the system exhibits a four-sublattice collinear ferrimagnetic phase at 5/8 filling for a large Hund's-rule coupling. In this state, the system spontaneously differentiates into the up-spin kagome network and the isolated down-spin sites, which we call the kagome network formation. In the kagome network state, the system becomes Dirac half-semimetallic: The electronic structure shows a massless Dirac node at the Fermi level, and the Dirac electrons are almost fully spin polarized due to the large Hund's-rule coupling. We also study the effect of off-site Coulomb repulsion in the kagome network phase where the system is effectively regarded as a 1/3-filling spinless fermion system on the kagome lattice. We find that, at the level of the mean-field approximation, a $\sqrt{3} \times \sqrt{3}$-type charge order occurs in the kagome network state, implying the possibility of fractional charge excitations in this triangular lattice system. Moreover, we demonstrate that the kagome network formation with fully-polarized Dirac electrons are controllable by an external magnetic field.