arXiv subjects

Satoshi Nishimoto

Publications and source records attributed to Satoshi Nishimoto.

At least 19 recordsLinked to original sources

arXiv2026

Robust Spin-1 Haldane Topology under Itinerant Doping

We study itinerant hole doping of the spin-$1$ Haldane symmetry-protected topological (SPT) phase in two $t$--$J$ chains coupled by ferromagnetic (FM) Hund exchange. Density-matrix renormalization group calculations show that string order, a finite spin gap, and the Haldane entanglement structure coexist over a broad doping range. Combined with entanglement-entropy scaling consistent with a single gapless charge mode, these results support a metallic doped Haldane SPT with a $C1S0$ low-energy description. At low doping, short-range charge correlations reduce the density of spin-$1/2$ defect rungs, whereas at larger doping their positions become nearly uncorrelated while their spins are collectively incorporated into the correlated spin sector. The doped Haldane SPT terminates in a fully spin-polarized FM phase, which forms a reentrant pocket at high doping in the global phase diagram. Dilute-limit analyses trace the two FM boundaries to spin--charge factorization at weak Hund coupling and to rung-triplet binding with reduced pair mobility at strong Hund coupling. Our results show how mobile carriers reorganize the Haldane spin structure while preserving its characteristic signatures.

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arXiv2026

Nontrivial three-sublattice magnetization in the easy-axis spin-1/2 XXZ antiferromagnet on the triangular lattice

We investigate the ground-state magnetic structure of the spin-$1/2$ XXZ antiferromagnet on the triangular lattice in the easy-axis regime using the density-matrix renormalization group. By applying spiral boundary conditions, we exactly map finite $L\times L$ clusters onto one-dimensional chains while avoiding the spatial anisotropy inherent in cylindrical geometries. From symmetry-broken local magnetization profiles, we extract the three-sublattice moments and track their evolution with anisotropy. At the isotropic point, we obtain a positive sublattice moment of $0.217(3)$, consistent with previous numerical estimates. In the easy-axis regime ($\Delta=J_z/J_\perp>1$), the ordered moments remain close to a Y-like zero-magnetization three-sublattice state, whose $z$-component pattern is of the form $(2m,-m,-m)$, over a broad range of $\Delta$. Extrapolation in $1/\Delta$ shows that the positive sublattice moment stays well below the classical saturation value $1/2$, approaching $0.419(7)$ as $\Delta\to\infty$, while the magnitude of the negative sublattice moment approaches $0.209(4)$. We further compare the energies of the Y state and the up-down-down state and find that the Y state is favored at zero field. Independent thermodynamic-limit energy calculations, performed without assuming any particular ordered pattern, yield an energy consistent with the Y-state solution. These results show that the easy-axis ground state does not simply cross over to a trivially saturated collinear Ising state, but instead remains a nontrivial three-sublattice ordered state selected from the macroscopically degenerate Ising manifold by quantum fluctuations.

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arXiv2026

Ground State Properties of the Doped Kitaev-Heisenberg Chain: Topological Superconducting and Mott Insulating Phases Driven by Magnetic Frustration

We study the hole-doped Kitaev-Heisenberg chain using the density-matrix renormalization group. In the Kitaev-only limit, the bond-directional exchange itself promotes pairing, favoring spin-singlet and spin-triplet superconducting tendencies for antiferromagnetic and ferromagnetic Kitaev couplings, respectively, together with finite-size Majorana edge correlations suggestive of topological superconductivity. In the full Kitaev-Heisenberg chain, cooperative $J$ and $K$ exchanges broadly stabilize superconductivity, while competition between $J$ and $K$ induces a strong filling dependence and enables superconductivity even when both $J$ and $K$ are weak. At quarter filling, this competition produces a Mott insulator with spontaneous hopping dimerization. These results identify magnetic frustration as a common mechanism underlying superconducting and interaction-driven insulating phases in doped Kitaev systems.

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arXiv2026

Emergent Random Spin Singlets in Disordered Spin-1/2 perovskite BaCu$_{1/3}$Ta$_{2/3}$O$_3$

We investigate the disordered perovskite BaCu$_{1/3}$Ta$_{2/3}$O$_3$, where Cu (spin-1/2) and Ta randomly occupy a pseudo-cubic lattice. Synchrotron X-ray diffraction and X-ray absorption spectroscopy establish the local nature of the disorder, revealing the presence of structurally constrained magnetic exchange paths. No magnetic ordering or spin freezing is observed down to 0.1 K. The low-temperature magnetic and thermodynamic behavior is captured by a broad but non-singular distribution $P(J)$ of exchange couplings $J$. These results open the possibility of realizing a disordered quantum ground state where the exchange randomness is broad yet intrinsically bounded, departing from the conventional infinite-randomness fixed point driven random-singlet phase.

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arXiv2025

Competing magnetic and topological orders in the spin-1 Kitaev-Heisenberg chain with single-ion anisotropy

We investigate the ground-state phase diagram of the spin-1 Kitaev--Heisenberg chain in the presence of uniaxial single-ion anisotropy (SIA) $D_z$ by density-matrix renormalization group (DMRG) calculations. By combining energy-curvature diagnostics on periodic $N=24$ clusters with a refined characterization based on order parameters and correlation functions for open chains up to $N=144$, we establish a comprehensive phase diagram in the $\phi$--$D_z$ plane. We identify four magnetically ordered phases -- FM-$z$, FM-$xy$, N\'eel-$z$, and a two-sublattice collinear LLRR2 state -- as well as magnetically disordered/critical regimes including N\'eel-$xy$, LLRR1, and two Kitaev spin-liquid (KSL) regions. A topological Haldane phase also emerges near the Heisenberg limit. Our results provide evidence that both AFM- and FM-KSL regimes acquire finite parameter widths in the spin-1 model, while the Haldane phase is fragile against Kitaev-type anisotropy, particularly for $D_z<0$. Increasing (decreasing) $D_z$ suppresses (enhances) magnetic order and expands (shrinks) the KSL and other magnetically disordered sectors. Also, at $D_z=0$, we identify an exactly solvable point at $\phi=\tan^{-1}(-2)$, which enforces a first-order transition between N\'eel-$z$ and LLRR2. We further contrast these findings with the spin-$1/2$ KH chain and with the spin-1 honeycomb KH model, highlighting the distinct roles of dimensionality and SIA in Kitaev-type magnets.

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arXiv2025

Systematic study of multi-magnon binding energies in the FM-AFM $J_1$-$J_2$ chain

We present a systematic study of multi-magnon bound states (MBSs) in the spin-$\tfrac{1}{2}$ FM-AFM $J_1$-$J_2$ chain under magnetic fields using the density-matrix renormalization group method. As a quantitative measure of stability, we compute the magnon binding energy $E_{\rm b}(M,p)$ for bound clusters of size $p$ over wide ranges of the frustration ratio $J_2/|J_1|$ and the normalized magnetization $M/M_{\rm s}$. Near saturation, we benchmark our data against the analytic two-magnon result and map out a clear hierarchy of $p$-magnon states, whose phase boundaries follow an empirical scaling $J_{2,{\rm c}}(p;p\!+\!1)/|J_1|\!\approx\!0.34\,p^{-2.3}$ for large $p$. We further quantify the relation between the most stable $p$ and the zero-field pitch angle $\theta$, verifying the conjectured inequality $1/p>\theta/\pi>1/(p+1)$ up to $p \lesssim 9$. The binding energy shows pronounced suppression as $J_2/|J_1|\!\to\!1/4^+$ and, for some frustration values, attains a maximum below full saturation, indicating that partial depolarization enhances bound-magnon mobility. Close to the FM instability, $E_{\rm b}(M_{\rm s},p)$ exhibits an empirical power-law vanishing consistent with a quantum-Lifshitz scenario. Our results provide a comprehensive, experimentally relevant map of MBS stability across field and frustration, offering concrete guidance for inelastic probes in quasi-one-dimensional magnets.

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arXiv2025

Instability of the Haldane Phase: Roles of Charge Fluctuations and Hund's Coupling

We systematically investigate the stability of the symmetry-protected topological (SPT) Haldane phase in spin-1/2 Heisenberg and half-filled Hubbard ladders coupled by ferromagnetic Hund's interactions. Using density-matrix renormalization group (DMRG) method, we analyze key signatures of the Haldane phase: long-range string order, finite spin gap, and characteristic entanglement spectrum degeneracies. In spin-only Heisenberg ladders, we find immediate onset and continuous strengthening of the Haldane phase with increasing Hund's coupling. In contrast, the inclusion of charge fluctuations in Hubbard ladders leads to a nontrivial stability regime, revealing a robust yet bounded region where SPT order persists despite significant charge fluctuations. We identify distinct boundaries separating a trivial insulating phase from the Haldane SPT phase, governed by both Coulomb repulsion and Hund's coupling. Our results highlight the subtle interplay of spin and charge degrees of freedom in correlated itinerant systems and establish essential criteria for observing Haldane physics experimentally in fermionic ladder materials.

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arXiv2025

Polarization-Driven Charge Frustration and Emergent Phases in the One-Dimensional Extended Hubbard Model

Frustration is a key driver of exotic quantum phases, yet its role in charge dynamics remains largely unexplored. We show that charge frustration - induced by electronic polarization effects - stabilizes unconventional insulating states in the one-dimensional extended Hubbard model. Using exact diagonalization and density-matrix renormalization group, we uncover a charge-disordered phase that remains insulating despite lacking long-range order and possessing an effectively attractive on-site interaction - a behavior reminiscent of gapful spin liquids in frustrated spin systems. We also identify a fragile ferroelectric phase and a charge-density-wave state with emergent eight-site periodicity. These findings establish charge frustration, driven by charge-dipole interactions, as a robust mechanism for realizing exotic phases in low-dimensional correlated systems, with implications for organic conductors, transition-metal oxides, and ultracold polar molecules.

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arXiv2024

Self-organised magnon condensation in quasi-1D edge-shared cuprates without external fields

Multimagnon bound states were predicted nearly a century ago and have since been a key topic in condensed matter physics due to their intriguing quantum properties. However, their realization in natural materials remains elusive, especially in low-dimensional quantum magnets, where stabilizing them is particularly challenging due to the traditionally required extreme external magnetic fields. Therefore, we introduce a novel mechanism that enables the stabilization of multimagnon bound states in quasi-one-dimensional edge-shared cuprates. Our theoretical framework, supported by numerical simulations and experimental data, demonstrates that small antiferromagnetic interchain couplings act as effective internal magnetic fields, promoting a collinear antiferromagnetic order and enabling magnon condensation even at zero external field. This intrinsic stabilisation mechanism eliminates the need for high external fields, offering a platform that is more accessible for experimental realization. We validate this concept by applying it to representative materials such as Li$_2$CuO$_2$, Ca$_2$Y$_2$Cu$5$O$_{10}$, LiCuSbO$_4$, and PbCuSO$_4$(OH)$_2$. Beyond its experimental feasibility, this mechanism could drive advancements in magnon-based quantum computing, low-power spintronic devices, and high-speed magnonic circuits. Moreover, our findings reveal that small interchain and/or interlayer couplings can generally unlock previously overlooked magnetic phenomena, redefining the nature of magnetically ordered states and expanding the frontiers of quantum magnetism.

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arXiv2024

Topological Phases in Half-Integer Higher Spin $J_1$-$J_2$ Heisenberg Chains

We study the ground state properties of antiferromagnetic $J_1$-$J_2$ chains with half-integer spins ranging from $S=\frac{3}{2}$ to $S=\frac{11}{2}$ using the density-matrix renormalization group method. We map out the ground-state phase diagrams as a function of $\frac{J_2}{J_1}$ containing topological phases with alternating $\frac{2S-1}{2}$ and $\frac{2S+1}{2}$ valence bonds. We identify these topological phases and their boundaries by calculating the string order parameter, the dimer order parameter, and the spin gap for those high-$S$ systems in thermodynamic limit (finite size scaling). We find that these topological regions narrow down inversely with $S$ and converge to a single point at $\frac{J_2}{J_1}=\frac{1}{4}$ in the classical limit -- a critical threshold between commensurate and incommensurate orders. In addition, we extend the discussion of the Majumder-Ghosh state, previously noted only for $S=\frac{1}{2}$, and speculate its possible presence as a ground state in half-integer high spin systems over a substantial range of $\frac{J_2}{J_1}$ values.

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arXiv2024

Relevance of on-site and intersite Coulomb interactions in the Kitaev-Heisenberg magnet Na$_3$Co$_2$SbO$_6$

The detection of considerable spin frustration in honeycomb cobalt oxide compounds indicates the presence of sizable Kitaev interactions in these systems, enlarging the pool of Kitaev spin liquid candidates. Several key questions remain to be answered, as basic as the mechanisms behind Kitaev couplings in Co$^{2+}$ $t_{2g}^5e_g^2$ magnets. Analyzing the quantum chemistry of interacting magnetic moments in Na$_3$Co$_2$SbO$_6$, a representative $LS$-coupled $t_{2g}^5e_g^2$ oxide, we find that the Kitaev and off-diagonal $\Gamma$ interactions are substantial and antiferromagnetic but somewhat weaker than the Heisenberg contribution. All nearest-neighbor couplings feature massive contributions from direct Coulomb exchange and/or on-site multiconfigurational dressing, mechanisms not considered so far in descriptive models of Kitaev-Heisenberg magnetism. These findings call for systematic wave-function quantum chemical studies in order to understand direct-indirect exchange synergies in Kitaev-Heisenberg magnets and how to possibly tune intersite couplings towards the Kitaev spin liquid ground state.

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arXiv2024

Comparing quantum fluctuations in the spin-$\frac{1}{2}$ and spin-$1$ XXZ Heisenberg models on square and honeycomb lattices

We present a detailed investigation of the XXZ Heisenberg model for spin-$1/2$ and spin-$1$ systems on square and honeycomb lattices. Utilizing the density-matrix renormalization group (DMRG) method, complemented by Spiral Boundary Conditions (SBC) for mapping two-dimensional (2D) clusters onto one-dimensional (1D) chains, we meticulously explore the evolution of staggered magnetization and spin gaps across a broad spectrum of easy-axis anisotropies. Our study reveals that, despite the lower site coordination number of honeycomb lattice, which intuitively suggests increased quantum fluctuations in its N\'eel phase compared to the square lattice, the staggered magnetization in the honeycomb structure exhibits only a marginal reduction. Furthermore, our analysis demonstrates that the dependence of staggered magnetization on the XXZ anisotropy $\Delta$, except in close proximity to $\Delta=1$, aligns with series expansion predictions up to the 12th order. Notably, for the $S=1/2$ honeycomb lattice, deviations from the 10th order series expansion predictions near the isotropic Heisenberg limit emphasize the critical influence of quantum fluctuations on the spin excitation in its N\'eel state. Additionally, our findings are numerically consistent with the singular behavior of the spin gap near the isotropic Heisenberg limit as forecasted by spin-wave theory. The successful implementation of SBC marks a methodological advancement, streamlining the computational complexity involved in analyzing 2D models and paving the way for more precise determinations of physical properties in complex lattice systems.

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arXiv2023

Emergent Quadrupolar Order in the Spin-$1/2$ Kitaev-Heisenberg Model

Motivated by the largely unexplored domain of multi-polar ordered spin states in the Kitaev-Heisenberg (KH) systems we investigate the ground state dynamics of the spin-$\frac{1}{2}$ KH model, focusing on quadrupolar (QP) order in 2-leg ladder and two-dimensional honeycomb lattice geometries. Employing exact diagonalization and density-matrix renormalization group methods, we analyze the QP order parameter and correlation functions. Our findings reveal a robust QP order across a wide range of the phase diagram, influenced by the interplay between Heisenberg and Kitaev interactions. Notably, we observe an enhancement of QP order near Kitaev quantum spin liquid (QSL) phases, despite the absence of long-range spin-spin correlations. This highlights a complex relationship between QP order and QSLs, offering new insights into quantum magnetism in low-dimensional systems. Our findings provide a rational explanation for the observed nonlinear magnetic susceptibility in $\alpha$-RuCl$_3$.

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arXiv2023

Emergence of vortex state in the $S=1$ Kitaev-Heisenberg model with single-ion anisotropy

The search for Kitaev spin liquid states has recently broadened to include a number of honeycomb materials with integer spin moments. The qualitative difference with their spin-1/2 counterparts is the presence of single-ion anisotropy (SIA). This motivates our investigation of the effects of SIA on the ground state of the spin-$1$ Kitaev-Heisenberg (KH) model using the density-matrix renormalization group which allows construction of detailed phase diagrams around the Kitaev points. We demonstrate that positive out-of-plane SIA induces an in-plane vortex state without the need for off-diagonal interactions. Conversely, negative SIA facilitates the emergence of a ferromagnetic state in presence of antiferromagnetic Heisenberg interactions, while a Nèel state can emerge for ferromagnetic Heisenberg coupling. These findings, pertinent even for weak SIA, not only enhance our theoretical understanding of the spin-1 KH model but also propose experimental prospects for observing these novel magnetic states in material realizations.

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arXiv2023

Existence of two distinct valence bond solid states in the dimerized frustrated ferromagnetic $J_1$-$J_1'$-$J_2$ chain

We study the frustrated and dimerized ferromagnetic-antiferromagnetic $J_1$-$J_1'$-$J_2$ chain using the density-matrix renormalization group (DMRG) method. Based on numerical calculations of the second derivative of energy, spin gap, spin-spin correlations, string order parameter (SOP), and entanglement spectrum (ES), we obtain the ground-state phase diagram for a wide range of $J_1'/J_1$ and $J_2/|J_1|$ values. This phase diagram reveals a ferromagnetic phase and two distinct valence-bond-solid (VBS) phases. The first VBS phase, referred to as $\mathcal{D}_3$-VBS, is typified by the formation of valence bonds between third-neighbor spin-1/2's, persisting as a continuation from the $J_1'/J_1=1$ limit. Alternatively, the second VBS phase, referred to as mixed-VBS, exhibits a coexistence of both second- and third-neighbor valence bonds, interpreted as a continuation from the $J_1'/J_1=0$ case. Remarkably, both VBS states are identified as being of Haldane-type, marked by a finite SOP and 2-fold ES degeneracy. Unexpectedly, our analysis uncovers a significant enhancement of the valence bond stability at the boundary of the two VBS phases. This study provides the first empirical demonstration of a nontrivial quantum phase transition between different topological VBS states in spin-1/2 chains. Moreover, we find that the ground state of the relevant quasi-one-dimensional material LiCuSbO$_4$ is classified as the $\mathcal{D}_3$-VBS state. Collectively, these results mark a substantial stride forward in our comprehension of quantum phase transitions and topological states.

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arXiv2023

NaRuO$_2$: Kitaev-Heisenberg exchange in triangular-lattice setting

Kitaev exchange, a new paradigm in quantum magnetism research, occurs for 90$^{\circ}$ metal-ligand-metal links, $t_{2g}^5$ transition ions, and sizable spin-orbit coupling. It is being studied in honeycomb compounds but also on triangular lattices. While for the former it is known by now that the Kitaev intersite couplings are ferromagnetic, for the latter the situation is unclear. Here we pin down the exchange mechanisms and determine the effective coupling constants in the $t_{2g}^5$ triangular-lattice material NaRuO$_2$, recently found to host a quantum spin liquid ground state. We show that, compared to honeycomb compounds, the characteristic triangular-lattice cation surroundings dramatically affect exchange paths and effective coupling parameters, changing the Kitaev interactions to antiferromagnetic. The quantum chemical analysis and subsequent effective spin model computations provide perspective onto the nature of the experimentally observed quantum spin liquid -- it seemingly implies finite longer-range exchange, and the atypical proximity to ferromagnetic order is related to sizable ferromagnetic Heisenberg nearest-neighbor couplings.

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arXiv2023

Phase diagram of the Kitaev-Heisenberg model using various finite-size clusters

We estimate phase boundaries of four ordered and two spin-liquid phases for the spin-$\frac{1}{2}$ Kitaev-Heisenberg (KH) model using four kinds of relatively-small clusters, based on the second derivative of ground-state energy. The estimated values are compared between the clusters as well as the previous iPEPS results. We thus find that the boundaries can be accurately estimated within limited-size clusters. The used clusters may appropriate to study higher-$S$ KH models having less fluctuations.

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arXiv2023

Disorder effects in the Kitaev-Heisenberg model

We study the interplay of disorder and Heisenberg interactions in Kitaev model on honeycomb lattice. The effect of disorder on the transition between Kitaev spin liquid and magnetic ordered states as well as the stability of magnetic ordering is investigated. Using Lanczos exact diagonalization we discuss the consequences of two types of disorder: (i) random-coupling disorder and (ii) singular-coupling disorder. They exhibit qualitatively similar effects in the pure Kitaev-Heisenberg model without long-range interactions. The range of spin liquid phases is reduced and the transition to magnetic ordered phases becomes more crossover-like. Furthermore, the long-range zigzag and stripy orderings in the clean system are replaced by their three domains with different ordering direction. Especially in the crossover range the coexistence of magnetically ordered and Kitaev spin-liquid domains is possible. With increasing the disorder strength the area of domains becomes smaller and the system goes into a spin-glass state. However, the disorder effect is different in magnetically ordered phases caused by long-range interactions. The stability of such magnetic ordering is diminished by singular-coupling disorder, and accordingly, the range of spin-liquid regime is extended. This mechanism may be relevant to materials like $α$-RuCl$_3$ and H$_3$LiIr$_2$O$_6$ where the zigzag ground state is stabilized by weak long-range interactions. We also find that the flux gap closes at a critical disorder strength and vortices appears in the flux arrangement. Interestingly, the vortices tend to form kinds of commensurate ordering.

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