arXiv subjects

Ryo Makuta

Publications and source records attributed to Ryo Makuta.

3 recordsLinked to original sources

arXiv2026

Effective spin model with anisotropic exchange interactions for the spin-orbit coupled Hubbard model at half-filling

Spin-orbit coupling (SOC) in noncentrosymmetric materials is the source of incommensurate magnetic structures. In semiconductors, it drives the Rashba spin splitting and spin momentum locking, while in magnetic insulators based on transition metals, it induces anisotropic spin exchange interactions, like the Dzyaloshinskii-Moriya (DM) interaction which drives chiral magnetism and skyrmion formation. Here, we establish a direct connection between SOC and spin exchange interactions by deriving an effective spin model from the SOC Hubbard model at half-filling. Using a strong-coupling expansion up to fourth order by including the full set of terms, we identify Heisenberg, Ising-like, and ring exchange interactions, as well as a variety of four-body terms for realistic Hubbard parameters, which impose strong constraints on the relative strengths of the spin interactions. Our spin model shows excellent agreement in energy with the SOC Hubbard model down $U/t \sim 5$ near the metal-insulator transition point, providing insight to which kind of magnetic interactions relevant across this regime are responsible for the emergence of complex magnetic textures.

cond-mat.str-el↗

arXiv2024

Spin-orbit coupled Hubbard skyrmions

When the material phases exhibit topological quantum numbers, they host defects protected by the nontrivial topology. Magnetic skyrmions are such ``quantized" objects and although many of them are metals they had been most likely treated in theories as purely classical spin states. Here, we show that the electrons described by the Hubbard model with strong spin-orbit couplings can exhibit various nano- or flake-size skyrmions in their ground state. Importantly, the conducting electrons forming Fermi pockets themselves carry textured magnetic moments in both real and momentum space and on top of that, possess Chern numbers in their energy bands. This quantum and conducting skyrmion is related to small skyrmions observed in atomic-layered compounds. We clarify how the effective magnetic interactions and magnetic anisotropies are tied to the spin-orbit coupling, and how they influence the stability of skyrmions beyond the phenomenology.

cond-mat.str-el↗

arXiv2021

Dimensional reduction in quantum spin-1/2 system on a 1/7-depleted triangular lattice

We study the magnetism of a quantum spin-1/2 antiferromagnet on a maple-leaf lattice which is obtained by regularly depleting 1/7 of the sites of a triangular lattice. Although the interactions are set to be spatially uniform, the ground state shows a stripe Neel order and the temperature dependence of magnetic susceptibility follows that of the one-dimensional XXZ model with a finite spin gap. We examine the nature of frustration by mapping the low energy degenerate manifold of states to the fully packed loop-string model on a dual cluster-depleted honeycomb lattice, finding that the order-by-disorder due to quantum fluctuation characteristic of highly frustrated magnets is responsible for the emergent stripes. The excited magnons split into two spinons and propagate in the one-dimensional direction along the stripe which is reminiscent of the XXZ or Ising model in one dimension. Unlike most of the previously studied dimensional reduction effects, our case is purely spontaneous as the interactions of the Hamiltonian retains a spatially isotropic two-dimensional structure.

cond-mat.str-el↗