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Halogen control of magnetic competition in Kitaev candidate Ru$X_3$ ($X =$ Cl, Br)
Authors:
Ryuta Iwazaki,
Shinnosuke Koyama,
Takashi Koretsune,
Shintaro Hoshino,
Joji Nasu
Abstract:
The spin-orbital Mott insulators Ru$X_3$ ($X =$ Cl, Br) have attracted considerable attention as promising candidate materials for realizing a Kitaev spin liquid. In this study, we construct effective pseudospin models from multiorbital Hubbard models derived from first-principles calculations and investigate the magnetic states of RuCl$_3$ and RuBr$_3$. From the constructed effective models, we f…
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The spin-orbital Mott insulators Ru$X_3$ ($X =$ Cl, Br) have attracted considerable attention as promising candidate materials for realizing a Kitaev spin liquid. In this study, we construct effective pseudospin models from multiorbital Hubbard models derived from first-principles calculations and investigate the magnetic states of RuCl$_3$ and RuBr$_3$. From the constructed effective models, we find that RuBr$_3$ has more extended Wannier orbitals and stronger interlayer exchange interactions than RuCl$_3$. These interactions enhance three-dimensional correlations, consistent with the stronger antiferromagnetic tendency experimentally inferred for RuBr$_3$. Orbital-dependent Coulomb anisotropy further reduces the energy difference between ferromagnetic and zigzag states. Our results clarify how halogen substitution controls magnetic competition in Ru$X_3$ through interlayer exchange interactions and effects of orbital-dependent Coulomb interactions.
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Submitted 14 July, 2026;
originally announced July 2026.
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Three-dimensional zigzag correlations in the van der Waals Kitaev magnet RuBr$_3$
Authors:
H. Gretarsson,
R. Iwazaki,
F. Sato,
H. Gotou,
S. Francoual,
J. Nasu,
Y. Imai,
K. Ohgushi,
J. Chaloupka,
B. Keimer,
H. Suzuki
Abstract:
Ruthenium trihalides Ru$X_3$ ($X$ = Cl, Br, I) provide a tunable platform for Kitaev magnetism in two-dimensional van der Waals materials. Despite their similar crystal structures and zigzag antiferromagnetic order, RuBr$_3$ exhibits a higher Néel temperature ($T_N$) than RuCl$_3$, suggesting their distinct proximity to the Kitaev quantum spin liquid phase. Using Ru $L_3$-edge resonant x-ray scatt…
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Ruthenium trihalides Ru$X_3$ ($X$ = Cl, Br, I) provide a tunable platform for Kitaev magnetism in two-dimensional van der Waals materials. Despite their similar crystal structures and zigzag antiferromagnetic order, RuBr$_3$ exhibits a higher Néel temperature ($T_N$) than RuCl$_3$, suggesting their distinct proximity to the Kitaev quantum spin liquid phase. Using Ru $L_3$-edge resonant x-ray scattering, we show that, while the long-range zigzag order in RuBr$_3$ disappears at $T_N$, the zigzag correlations that persist well above $T_N$ show a pronounced spectral weight redistribution along the interlayer direction. These results suggest that the enhanced interlayer magnetic interactions driven by the extended Br 4$p$ orbitals stabilize three-dimensional zigzag correlations in RuBr$_3$.
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Submitted 6 April, 2026;
originally announced April 2026.
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Material-based analysis of spin-orbital Mott insulators
Authors:
Ryuta Iwazaki,
Hiroshi Shinaoka,
Shintaro Hoshino
Abstract:
We present a framework for analyzing Mott insulators using a material-based tight-binding model. We start with a realistic multiorbital Hubbard model and derive an effective model for the localized electrons through the second-order perturbation theory with respect to intersite hopping. This effective model, known as the Kugel-Khomskii model, is described by SU($N$) generators, where $N$ is the nu…
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We present a framework for analyzing Mott insulators using a material-based tight-binding model. We start with a realistic multiorbital Hubbard model and derive an effective model for the localized electrons through the second-order perturbation theory with respect to intersite hopping. This effective model, known as the Kugel-Khomskii model, is described by SU($N$) generators, where $N$ is the number of localized states. We solve this model by the mean-field theory that takes local correlations into account and reveal spin-orbital ordered states. To include spatial correlations, we apply the classical Monte Carlo based on the path-integral approach with SU($N$) coherent states, and also derive the equation of motion for spin-orbital degrees of freedom. Our approach is applicable to any Mott insulator with reasonable computational cost. The $5d$-pyrochlore oxide is used here as demonstration.
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Submitted 24 January, 2023;
originally announced January 2023.
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Spin-orbital model for fullerides
Authors:
Ryuta Iwazaki,
Shintaro Hoshino
Abstract:
The multiorbital Hubbard model in the strong coupling limit is analyzed for the effectively antiferromagnetic Hund's coupling relevant to fulleride superconductors with three orbitals per molecule. The localized spin-orbital model describes the thermodynamics of the half-filled (three-electron) state with total spin-1/2, composed of singlon and doublon placed on the two of three orbitals. The mode…
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The multiorbital Hubbard model in the strong coupling limit is analyzed for the effectively antiferromagnetic Hund's coupling relevant to fulleride superconductors with three orbitals per molecule. The localized spin-orbital model describes the thermodynamics of the half-filled (three-electron) state with total spin-1/2, composed of singlon and doublon placed on the two of three orbitals. The model is solved using the mean-field approximation and magnetic and electric ordered states are clarified through the temperature dependences of the order parameters. Combining the model with the band structure from {\it ab initio} calculation, we also semi-quantitavely analyze the realistic model and the corresponding physical quantities. In the A15-structure fulleride model, there is an antiferromagnetic ordered state, and subsequently the two orbital ordered state appears at lower temperatures. It is argued that the origin of these orbital orders is related to the $T_h$ point group symmetry. As for the fcc-fulleride model, the time-reversal broken orbital ordered state is identified. Whereas the spin degeneracy remains in our treatment for the geometrically frustrated lattice, it is expected to be lifted by some magnetic ordering or quantum fluctuations, but not by the spin-orbital coupling which is effectively zero for fullerides in the strong-coupling regime.
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Submitted 8 March, 2021;
originally announced March 2021.
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Exact Thermodynamic Properties of (1,1/2) Mixed Diamond Chains with Strong Single-Site Anisotropy
Authors:
Ryuta Iwazaki,
Kazuo Hida
Abstract:
The ground states and finite-temperature properties of mixed diamond chains with spins 1 and 1/2 are investigated in the limit of strong easy-axis anisotropy on the spin-1 sites. Magnetization curves, entropy, specific heat and magnetic susceptibility are exactly calculated using the method of Čanovà et al. [J. Phys.: Condens. Matter 18 4967 (2006)].
The ground states and finite-temperature properties of mixed diamond chains with spins 1 and 1/2 are investigated in the limit of strong easy-axis anisotropy on the spin-1 sites. Magnetization curves, entropy, specific heat and magnetic susceptibility are exactly calculated using the method of Čanovà et al. [J. Phys.: Condens. Matter 18 4967 (2006)].
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Submitted 24 July, 2019;
originally announced July 2019.
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Nature of superconducting fluctuation in photo-excited systems
Authors:
Ryuta Iwazaki,
Naoto Tsuji,
Shintaro Hoshino
Abstract:
The photo-excited state associated with superconducting fluctuation above the superconducting critical temperature $T_c$ is studied based on the time-dependent Ginzburg-Laundau approach. The excited state is created by an electric-field pulse and is probed by a weak secondary external field, which is treated by the linear response theory mimicking pump-probe spectroscopy experiments. The behavior…
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The photo-excited state associated with superconducting fluctuation above the superconducting critical temperature $T_c$ is studied based on the time-dependent Ginzburg-Laundau approach. The excited state is created by an electric-field pulse and is probed by a weak secondary external field, which is treated by the linear response theory mimicking pump-probe spectroscopy experiments. The behavior is basically controlled by two relaxation rates: one is $γ_1$ proportional to the temperature measured from the critical point $T - T_c$ and the other is $γ_2$ proportional to the excitation intensity of the pump pulse. The excited state approaches the equilibrium state exponentially in a long time $t \gg γ_1^{-1}$, while in the intermediate time domain we find a power-law or logarithmic decay with different exponents for $t\ll γ_2^{-1}$ and $γ_2^{-1} \ll t \ll γ_1^{-1}$, even though the system is located away from the critical point. This is interpreted as the critical point in equilibrium being extended to a finite region in the excited situation. The parameter dependences on both the pump and probe currents are also systematically studied in all dimensions.
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Submitted 11 April, 2019;
originally announced April 2019.