arXiv subjects

Daiki Sasamoto

Publications and source records attributed to Daiki Sasamoto.

6 recordsLinked to original sources

arXiv2026

Schwinger boson perturbation theory for spin-$S$ Kitaev-Heisenberg magnets: phase diagram and dynamical response near Kitaev spin liquids

We develop a Schwinger boson perturbative framework for the spin-$S$ Kitaev-Heisenberg model. The spin-liquid saddle point of the pure Kitaev model is used as the unperturbed state, and magnetic instabilities and dynamical spin correlations are evaluated around this saddle point. We decompose the Hamiltonian exactly into two parts by exploiting the Klein duality intrinsic to the Kitaev-Heisenberg model. We perform the random-phase approximation by taking the part invariant under the duality transformation as the unperturbed term and treating the remaining term, which changes sign under it, as the perturbation. We determine the phase boundaries separating the quantum spin-liquid regimes from the adjacent magnetically ordered phases for $S=1/2$, $1$, $3/2$, and $2$. The spin-liquid regions shrink rapidly with increasing $S$ and become very narrow at $S=2$. We also compute the spin dynamics of the spin-$S$ Kitaev-Heisenberg model, focusing on $S=1$, and find that the dressed dynamical spin structure factor retains a broad two-spinon continuum at finite energies, while the low-energy spectral weight softens at the ordering wave vectors of the adjacent magnetic phases. Our framework thus enables thermodynamic-limit calculations of spin dynamics near magnetic instabilities.

cond-mat.str-el↗

arXiv2026

Thermal Hall Signatures of Distinct Schwinger-Boson Flux Sectors on the Honeycomb Lattice

Thermal transport offers a bulk probe of charge-neutral excitations in frustrated magnets. It remains challenging, however, to determine whether such transport can distinguish different parton mean-field structures of the same spin system. This question is particularly relevant on the honeycomb lattice, where distinct $0$-flux, $\pi$-flux, and chiral $\pi/2$-flux sectors have been proposed. In this paper, we compare self-consistent saddle-point solutions in these three sectors for a frustrated $J_1-J_2$ Heisenberg model with a next-nearest-neighbor Dzyaloshinskii-Moriya interaction and a perpendicular magnetic field. Using finite-temperature Schwinger-boson mean-field theory and the Kubo formula for bosonic Bogoliubov-de Gennes systems, we evaluate the intrinsic spinon thermal Hall conductivity. At a common parameter point where all three branches remain gapped, the $0$-flux response is positive and the $\pi$-flux response is negative for $D>0$ and $h>0$ under the sign conventions used in this paper, whereas the $\pi/2$-flux response changes sign with increasing temperature. We further show that a projective symmetry combining a sixfold rotation with time reversal forces the zero-field response of a fixed chiral $\pi/2$-flux domain to vanish. Within the mean-field regime examined in this paper, the sign and temperature dependence of $\kappa_{xy}/T$ therefore provide a flux-sensitive transport signature.

cond-mat.str-el↗

arXiv2026

Chiral bosonic mean-field Ansatz and spin dynamics in spin-1 Kitaev magnets

The Kitaev model is a paradigmatic system for realizing quantum spin liquids, but its higher-spin extensions are not exactly solvable, and their spin dynamics is less well understood than in the spin-1/2 case. In this work, we reexamine a previously introduced triplet-pairing $\phi_t = \pi/2$ phase pattern for the antiferromagnetic $S = 1$ Kitaev model and extend the analysis to weak symmetric off-diagonal exchanges $\Gamma$ and $\Gamma'$. Using a bond-operator formulation of Schwinger-boson mean-field theory, we calculate the dynamical spin structure factor for the triplet 0-flux and triplet $\pi/2$-flux Ans\"atze with a spin-correlation scheme appropriate for Kitaev interactions. In the pure Kitaev limit, the $\pi/2$-flux Ansatz yields a flatter spectrum than the 0-flux Ansatz. The real-space spin correlations show that the $\pi/2$-flux Ansatz suppresses longer-distance correlations more strongly than the 0-flux Ansatz, yielding a correlation pattern closer to the short-ranged form expected in the Kitaev limit. This comparison shows that the flatness of $S(\boldsymbol{q}, \omega)$ is tied to short-ranged spin correlations and is therefore an important consistency check, although it is not, by itself, a diagnostic of time-reversal-symmetry breaking. We then study weak off-diagonal exchanges along $\Gamma' = \Gamma$ near the pure Kitaev limit, taking the same-sign relation from analyses of candidate spin-1 Kitaev materials. Gapped solutions are obtained within the constrained $\pi/2$-flux manifold, and the spectra share the qualitative energy- and momentum-space features found by finite-size exact diagonalization. Taken together, these results support the triplet $\pi/2$-flux chiral bosonic Ansatz as a useful mean-field description of spin dynamics near the antiferromagnetic $S = 1$ Kitaev limit with weak off-diagonal exchanges.

cond-mat.str-el↗

arXiv2026

Dynamical spin correlations in kagome antiferromagnets: comparison of Abrikosov fermion and Schwinger boson approaches beyond mean field

Quantum spin liquids exhibit fractionalized spin excitations as a consequence of strong quantum many-body effects. The kagome antiferromagnetic Heisenberg model is a promising candidate for a quantum spin-liquid ground state; however, the nature of its excitation spectrum remains controversial, particularly regarding the presence of a spin gap and the gauge structure coupled to fractional quasiparticles. To address these issues, parton approaches have been extensively employed, where spin operators are represented in terms of fermionic or bosonic quasiparticles within the Abrikosov fermion and Schwinger boson frameworks. Thus far, these approaches have been pursued independently, and it has remained unclear how the results obtained from these frameworks compare, particularly with respect to the spin dynamics and gauge structure of the kagome antiferromagnet. Here, we investigate the dynamical spin structure factor of the antiferromagnetic Heisenberg model with a Dzyaloshinskii-Moriya interaction on the kagome lattice, relevant to herbertsmithite, by employing both approaches. We find that the dynamical spin structure factor obtained from the Abrikosov fermion mean-field theory exhibits dome-shaped features, and that its continuum structure significantly depends on the gauge structure of the spin-liquid ansatz. On the other hand, the Schwinger boson mean-field theory yields a concave-down structure in the low-energy region, distinct from that obtained using the Abrikosov fermion approach. Moreover, incorporating many-body effects beyond the mean-field approximation substantially reduces the low-energy gap and enhances the low-energy spectral weight, consistent with experimental observations. Our results suggest the importance of many-body effects in the Schwinger boson theory for capturing the low-energy spin dynamics of kagome antiferromagnets.

cond-mat.str-el↗

arXiv2025

Schwinger boson theory for $S=1$ Kitaev quantum spin liquids

The Kitaev model is an exactly solvable model with a quantum spin liquid ground state. While this model was originally proposed as an $S=1/2$ spin model on a honeycomb lattice, extensions to higher-spin systems have recently attracted attention. In contrast to the $S=1/2$ case, such higher-$S$ models are not exactly solvable and remain poorly understood, particularly for spin excitations at finite temperatures. Here, we focus on the $S=1$ Kitaev model, which is proposed to host bosonic quasiparticles. We investigate this model using Schwinger boson mean-field theory, introducing bosonic spinons as fractional quasiparticles by extending bond operators to address anisotropic spin interactions. We determine the mean-field parameters that realize a quantum spin liquid in both ferromagnetic and antiferromagnetic Kitaev models. Based on this ansatz, we calculate dynamical and equal-time spin structure factors. We find that the conventional scheme based on Wick decoupling with respect to spinons to calculate spin correlations, the resultant spin structure factors exhibit a momentum dependence that is not consistent with the sign structure expected from the exchange interaction. To resolve this issue, we propose an alternative evaluation based on decoupling with respect to bond operators. We demonstrate that, in our scheme, this discrepancy is removed, and the momentum dependence of the spin structure factors is consistent with the sign of the exchange constant. We also compute the temperature evolution of the dynamical spin structure factor and find that the zero-temperature continuum splits into two distinct structures as temperature increases, which can be understood in terms of the bandwidth narrowing of spinons. Finally, we clarify why the two decoupling schemes result in different momentum dependences and discuss their relationship to previous studies.

cond-mat.str-el↗

arXiv2023

General Formula for the Green's Function Approach to the Spin-1/2 Antiferromagnetic Heisenberg Model

A wide range of analytical and numerical methods are available to study quantum spin systems. However, the complexity of spin correlations and interactions limits their applicability to specific temperature ranges. The analytical approach utilizing Green's function has proved advantageous, as it allows for formulation without restrictions on the presence of long-range order and facilitates estimation of the spin excitation spectrum and thermodynamic quantities across the entire temperature range. In this work, we present a generalized formulation of the Green's function method that can be applied to diverse spin systems. As specific applications, we consider the hypercubic lattice and the $J_1$-$J_2$ model. For the cubic lattice case, the Green's function approach provides a good estimation for the transition temperature. Regarding the $J_1$-$J_2$ model, we include nematic correlations in the analysis and find no signature of such correlations, though accurate numerical calculations are required in the presence of strong frustration. Although our focus is on the spin one-half antiferromagnetic Heisenberg model on an arbitrary lattice, the Green's function approach can be generalized to incorporate other interactions and higher spin values.

cond-mat.str-el↗