arXiv subjects

Katsuhiro Morita

Publications and source records attributed to Katsuhiro Morita.

At least 19 recordsLinked to original sources

arXiv2026

Quantum magnetism of the spin-1 kagome-lattice antiferromagnet

We investigate the spin-1 kagome-lattice Heisenberg antiferromagnet using large-scale Lanczos diagonalization and the finite-temperature Lanczos method. The zero-temperature magnetization process exhibits plateaus at $m=0$, $1/3$, $7/9$, and $8/9$, where $m$ is the normalized magnetization. The $m=0$ plateau is identified as a trimer valence-bond-crystal state, while the high-field plateaus at $m=7/9$ and $8/9$ are identified as magnon crystals. In particular, the $m=8/9$ plateau corresponds to the exact localized-magnon crystal state. A smoothed zero-temperature magnetization curve constructed using the Gaussian-kernel smoothing method indicates magnetization jumps at the lower-field edge of the $m=1/3$ plateau and at the upper-field edges of the $m=7/9$ and $8/9$ plateaus. At finite temperatures, the specific heat exhibits a double-peak structure with peaks around $T/J\simeq0.1$ and $T/J\simeq1.1$, and the low-temperature peak may be related to trimer valence-bond-crystal ordering. The finite-temperature magnetization curves show that the $m=1/3$ plateau remains visible at low temperatures, whereas the high-field plateaus are rapidly smeared out by thermal effects. These results provide benchmark data for thermodynamic and high-field magnetization measurements in candidate spin-1 kagome-lattice materials.

cond-mat.str-el↗

arXiv2026

Finite-temperature quantum Krylov method from real-time overlaps

Accurately evaluating finite-temperature properties of quantum many-body systems remains a central challenge. Many existing quantum approaches typically require thermal-state preparation at each target temperature, making low-temperature calculations especially demanding in terms of circuit depth and accuracy. Here we introduce a distinct framework based only on the real-time overlap sequence $g_n=\langle ϕ|e^{-inτH}|ϕ\rangle$, which enables thermodynamic quantities to be obtained over a broad temperature range, without specifying a target temperature on the quantum device. For the one-dimensional spin-$\frac{1}{2}$ Heisenberg model with periodic boundary conditions, we obtain accurate specific heat, magnetic susceptibility, and entropy in the noiseless case. Magnetic susceptibility is also evaluated accurately without explicit symmetry-sector decomposition by employing pseudorandom vectors compatible with $S_{\mathrm{tot}}^{z}$ conservation. With suitable stabilization, the method further retains the main thermodynamic features under finite-shot statistical errors up to $σ\sim10^{-3}$. Our results establish real-time-overlap-based finite-temperature evaluation as a promising framework for finite-temperature computation on near-future quantum hardware.

quant-ph↗

arXiv2024

Valence Bond Crystal Ground State of the 1/9 Magnetization Plateau in the Spin-1/2 Kagome Lattice

We investigate the ground state of a spin-1/2 kagome antiferromagnet at the 1/9 magnetization plateau, focusing primarily on six types of valence bond crystal (VBC) distortions. Among six types of VBC distortions, type 1 consistently exhibits the lowest ground-state energy. Analysis of the second derivative of the energy with respect to the distortion strength $J_{\rm d}$ using exact diagonalization reveals that, as the system size approaches the thermodynamic limit, the type 1 VBC state remains stable down to $J_{\rm d} = 1$, corresponding to the undistorted kagome lattice. Perturbation theory further supports the stability of the type 1 VBC state, providing energy values that agree with exact diagonalization results within 3%. We also explore the possibility that other types of VBC distortions could become the ground state, but type 1 VBC still exhibited the lowest energy. These findings suggest that the ground state of the 1/9 plateau in the kagome lattice is the type 1 VBC state.

cond-mat.str-el↗

arXiv2024

Six magnetization plateau phases in a spin-1/2 distorted kagome antiferromagnet: application to $\rm{Y}_3\rm{Cu}_9(\rm{OH})_{19}\rm{Cl}_8$

A recently discovered kagome antiferromagnet $\rm{Y}_3\rm{Cu}_9(\rm{OH})_{19}\rm{Cl}_8$ has attracted significant interest due to its unique kagome lattice structure and magnetic properties. The kagome lattice has three types of exchange interactions: one hexagonal coupling and two different triangular couplings. Previous studies have shown that its ground state is significantly different from that predicted for the undistorted kagome lattice, forming a coplanar spin state with a commensurate magnetic wave vector ${\mathbf Q}=(1/3,1/3)$. Two separate studies have proposed distinct sets of exchange interaction parameters for this compound. In this study, we investigate the ground state of the spin-1/2 Heisenberg kagome model with three types of nearest-neighbor exchange interactions under a magnetic field by exact diagonalization using the Lanczos method. We reveal that clear magnetization plateaus at $M/M_{\rm sat}$=1/3, 5/9, and 7/9 are present under both parameter sets, which are identified as magnon crystal states based on their spin structures. Our findings suggest that these plateaus could potentially be experimentally confirmed with magnetization measurements on $\rm{Y}_3\rm{Cu}_9(\rm{OH})_{19}\rm{Cl}_8$ under a magnetic field of approximately 300 T, achievable with state-of-the-art magnetic field generators. In order to get a deeper understanding of magnetism of $\rm{Y}_3\rm{Cu}_9(\rm{OH})_{19}\rm{Cl}_8$, we perform additional calculations by varying these interactions. Consequently, we discover additional plateau phases at $M/M_{\rm sat}$=1/3, 5/9, and 7/9, each distinctly different from the magnon crystal states.

cond-mat.str-el↗

arXiv2023

Engineering of a Low-Entropy Quantum Simulator for Strongly Correlated Electrons Using SU($\mathcal{N}$)-Symmetric Cold Atom Mixtures

An advanced cooling scheme, incorporating entropy engineering, is vital for isolated artificial quantum systems designed to emulate the low-temperature physics of strongly correlated electron systems (SCESs). This study theoretically demonstrates a cooling method employing multi-component Fermi gases with SU($\mathcal{N}$)-symmetric interactions, focusing on the case of $^{173}$Yb atoms in a two-dimensional optical lattice. Adiabatically introducing a nonuniform state-selective laser gives rise to two distinct subsystems: a central low-temperature region, exclusively composed of two specific spin components, acts as a quantum simulator for SCESs, while the surrounding $\mathcal{N}$-component mixture retains a significant portion of the entropy of the system. The SU($\mathcal{N}$)-symmetric interactions ensure that the total particle numbers for each component become good quantum numbers, creating a sharp boundary for the two-component region. The cooling efficiency is assessed through extensive finite-temperature Lanczos calculations. The results lay the foundation for quantum simulations of two-dimensional systems of Hubbard or Heisenberg type, offering crucial insights into intriguing low-temperature phenomena in condensed-matter physics.

cond-mat.quant-gas↗

arXiv2023

Stability of the 1/3 magnetization plateau of the $J_1-J_2$ kagome Heisenberg model

In this study, we investigate the finite-temperature properties of the spin-1/2 $J_1-J_2$ Heisenberg model on the kagome lattice using the orthogonalized finite-temperature Lanczos method. Under a zero magnetic field, the specific heat exhibits a double-peak structure, as $|J_2|$ increases. Additionally, at approximately $J_2=0$, the magnetic entropy remains finite, even at low temperatures. The finite-temperature magnetization curve reveals the asymmetric melting behavior of the 1/3 plateau around $J_2=0$. As $|J_2|$ increases, the 1/3 plateau becomes more stable, exhibiting symmetric melting behavior. Specifically, for $J_2 > 0$, the $\bf Q=0$ up-up-down structure is stabilized, whereas for $J_2 < 0$, the $\sqrt{3} \times \sqrt{3}$ up-up-down structure is stabilized.

cond-mat.str-el↗

arXiv2023

A Series Expansion Study for Large Negative Quantum Renormalization of Magnon Spectra in the $S=1/2$ Kagome-Lattice Heisenberg Antiferromagnet Cs$_{2}$Cu$_{3}$SnF$_{12}$

The series expansion method is used to study magnon spectra of the kagome system with nearest-neighbor exchange interaction $J$ and out-of-plane Dzyaloshinskii-Moriya (DM) interaction $D^{\parallel}$, which is a minimal model for Cs$_{2}$Cu$_{3}$SnF$_{12}$. Compared to the magnon spectra by the linear spin wave (LSW) theory, we find that dispersions at high energy part suffer downward deformation, which is similar to the triangle lattice case, in addition to the reduction of the energy scale of about 40\% as pointed out in a neutron-scattering study by Ono {\it et al.} Using a reliable estimation $J=20.7$ meV in a previous study on the magnetic susceptibility of Cs$_{2}$Cu$_{3}$SnF$_{12}$, we use $D^{\parallel}$ as the fitting parameter to reproduce the experimental magnon spectra and obtain $D^{\parallel}=0.12J$. We also report that a roton-like minimum occurs at the M point and a maximum at points somewhat away from the $Γ$ point. Compression of the LSW magnon band is commonly seen in the kagome and triangular-lattice systems, which may be viewed as being pushed from above by the spinon continuum.

cond-mat.str-el↗

arXiv2022

Effects of bond-randomness and Dzyaloshinskii-Moriya interactions on the specific heat at low temperatures of a spherical kagomé cluster in {W$_{72}$V$_{30}$}

For the spin-1/2 spherical kagomé cluster, as well as for the 2D kagomé lattice, many low-energy singlet excitations have been expected to exist in the energy region below the spin gap, which has been actually confirmed by Kihara $et \ al.$ in their specific heat measurements up to 10K in {W$_{72}$V$_{30}$}, for which the exchange interaction was estimated as $J=115$K. However, the experimental result of the specific heat can not be reproduced by the theoretical result in the Heisenberg model. Although the theoretical result has a peak around 2 K, the experimental one does not. To elucidate this difference, we incorporate Dzyaloshinskii-Moriya (DM) interactions and bond-randomness into the model Hamiltonian for {W$_{72}$V$_{30}$} and calculate density of states, entropy, and specific heat at low temperatures by using the Lanczos method. We find that DM interactions do not significantly affect the energy distribution of about ten singlet states above the ground state, which are involved in the peak structure of the specific heat around 2K, while even 10% bond-randomness disperses this distribution to collapse the 2K peak. Kihara $et \ al.$ also reported experimental specific heats under magnetic fields up to 15T $(=0.17J)$, and found that the specific heats show almost no magnetic-field dependence, which strongly suggests that the bond randomness is much stronger than the magnetic fields. For example, our calculated specific heats with 50% randomness reproduce the experimental ones up to about 5K.

cond-mat.str-el↗

arXiv2022

Gapless chirality liquid with symmetry-protected edge spins

We report that a spin-1/2 tetrahedral Heisenberg chain realizes a gapless symmetry-protected topological (gSPT) phase characterized by the coexistence of the Tomonaga-Luttinger-liquid criticality due to chirality degrees of freedom and the symmetry-protected edge state due to spin degrees of freedom. This gSPT phase has an interesting feature that no symmetry forbids the trivial spin gap opening but a discrete symmetry, $\mathbb Z_3\times\mathbb Z_2^T$, forbids the unique gapped ground state. In the first part of the paper, we numerically show the coexistence of a critical entanglement entropy and a nontrivially degenerate entanglement spectrum based on the density-matrix renormalization group (DMRG) method.Next, we clarify that chirality degrees of freedom form the Tomonaga-Luttinger liquid while spin degrees of freedom form the spin-1 Haldane state based on a degenerate perturbation theory. Last but not least, we discuss the Lieb-Schultz-Mattis-type ingappability in the gSPT phase, using a local $\mathbb{Z}_3$ rotation. We can thus characterize our gSPT phase as a symmetry-protected critical phase protected by the $\mathbb{Z}_3$ on-site symmetry, the $\mathbb{Z}_2^T$ time-reversal symmetry, the lattice translation symmetry, and the U(1) spin-rotation symmetry.

cond-mat.str-el↗

arXiv2022

Isothermal and adiabatic magnetization processes of the spin-$\frac{1}{2}$ Heisenberg model on an anisotropic triangular lattice

In this study, we investigate the magnetic susceptibility, entropy, and isothermal magnetization curve of the spin-1/2 Heisenberg model on an anisotropic triangular lattice using the orthogonalized finite-temperature Lanczos method. In addition, we investigate the adiabatic magnetization curve and magnetocaloric effect. We estimate these physical quantities with sufficient accuracy in the thermodynamic limit, except at low temperatures. We observe a 1/3 magnetization plateau in the isothermal magnetization process, whereas the plateau is observed to have a slope in the adiabatic process. We show that the magnetocaloric effect can be used to detect the signature of phase transitions. We believe that these results will be useful for understanding the magnetism of anisotropic triangular lattice compounds through a comparison with experimental results in the future.

cond-mat.str-el↗

arXiv2021

Magnetic phase diagrams of the spin-$\frac{1}{2}$ Heisenberg model on a kagome-strip chain: Emergence of a Haldane phase

Frustrated one-dimensional quantum spin systems are known to exhibit a variety of quantum ground states due to the effects of quantum fluctuations and frustrations. In a spin-1/2 kagome-strip chain, which is one of the frustrated one-dimensional spin systems, many quantum phases have been found. However, the magnetic phase diagrams of the kagome-strip chain under magnetic field have not been fully understood. We construct magnetic phase diagrams at 0, 1/5, 3/10, 1/3, 2/5, 7/15, 3/5, and 4/5 magnetization ratio in the kagome-strip chain and investigate magnetic properties in each phase using the density matrix renormalization group method. We find fifteen magnetization-plateau phases, one of which is equivalent to the spin-1 Haldane phase.

cond-mat.str-el↗

arXiv2020

Resonating dimer-monomer liquid state in a magnetization plateau of a spin-$\frac{1}{2}$ kagome-strip Heisenberg chain

Highly frustrated spin systems such as the kagome lattice (KL) are a treasure trove of new quantum states with large entanglements. We thus study the spin-$\frac{1}{2}$ Heisenberg model on a kagome-strip chain (KSC), which is one-dimensional KL, using the density-matrix renormalization group (DMRG) method. Calculating central charge and entanglement spectrum for the KSC, we find a novel gapless spin liquid state with doubly degenerate entanglement spectra in a 1/5 magnetization plateau. We also obtain a gapless low-lying continuum in the dynamic spin structure calculated by dynamical DMRG method. We propose a resonating dimer-monomer liquid state that would meet these features.

cond-mat.str-el↗

arXiv2020

Finite-temperature properties of the Kitaev-Heisenberg models on kagome and triangular lattices studied by improved finite-temperature Lanczos methods

Frustrated quantum spin systems such as the Heisenberg and Kitaev models on various lattices, have been known to exhibit various exotic properties not only at zero temperature but also for finite temperatures. Inspired by the remarkable development of the quantum frustrated spin systems in recent years, we investigate the finite-temperature properties of the $S=1/2$ Kitaev-Heisenberg models on kagome and triangular lattices by means of finite-temperature Lanczos methods with improved accuracy. In both lattices, multiple peaks are confirmed in the specific heat. To find the origin of the multiple peaks, we calculate the static spin structure factor. The origin of the high-temperature peak of the specific heat is attributed to a crossover from the paramagnetic state to a short-range ordered state whose static spin structure factor has zigzag or linear intensity distributions in momentum space. In the triangular Kitaev model, the "order by disorder" due to quantum fluctuation occurs. On the other hand, in the kagome Kitaev model it does not occur even with both quantum and thermal fluctuations.

cond-mat.str-el↗

arXiv2019

Magnetic orders induced by RKKY interaction in Tsai-type quasicrystalline approximant Au-Al-Gd

Recent experimental study on Tsai-type quasicrystalline approximant Au-Al-Gd has revealed the presence of magnetic orders and phase transitions with changing the Au/Al concentration. Motivated by the experiment, we theoretically investigate whether a successive change of magnetic orders occurs in a minimal magnetic model including the RKKY interaction only. We find that the model induces multifarious magnetic orders depending on the Fermi wavenumber and gives a good starting point for understanding the experimental observation. In addition, we predict the presence of an undiscovered novel magnetic order called cuboc order at large Fermi wavenumber region.

cond-mat.str-el↗

arXiv2019

Magnetization plateau and supersolid phases in the spin-1/2 antiferromagnetic Heisenberg model on a tetragonally distorted fcc lattice

The Heisenberg model on a face center cubic (fcc) lattice is a typical three-dimensional frustrated spin system expected to have magnetization plateaus and supersolid phases. There are model compounds $A_2$CoTeO$_6$ ($A$ = Ca, Sr, Pb) for the fcc lattice but with lattice distortions. Motivated by the presence of the compounds, we investigate the ground state of the spin-1/2 antiferromagnetic Heisenberg model on a tetragonally distorted fcc lattice in the magnetic field using a large-size cluster mean-field method for the sake of finding new supersolid phases. We find five supersolid phases in the model, indicating possibility to observe supersolid phases in these compounds. We also find that one of the supersolid phases is similar to the nonclassical coplanar phase obtained in the XXZ model on the triangular lattice.

cond-mat.str-el↗

arXiv2018

Ground-state phase diagram of the Kitaev-Heisenberg model on a kagome lattice

The Kitaev-Heisenberg model on the honeycomb lattice has been studied for the purpose of finding exotic states such as quantum spin liquid and topological orders. On the kagome lattice, in spite of a spin-liquid ground state in the Heisenberg model, the stability of the spin-liquid state has hardly been studied in the presence of the Kitaev interaction. Therefore, we investigate the ground state of the classical and quantum spin systems of the kagome Kitaev-Heisenberg model. In the classical system, we obtain an exact phase diagram that has an eight-fold degenerated canted ferromagnetic phase and a subextensive degenerated Kitaev antiferromagnetic phase. In the quantum system, using the Lanczos-type exact diagnalization and cluster mean-field methods, we obtain two quantum spin-liquid phases, an eight-fold degenerated canted ferromagnetic phase similar to the classical spin system, and an eight-fold degenerated $\bf q=0$ $120^\circ$ ordered phase induced by quantum fluctuation. These results may provide a crucial clue to recently observed magnetic structures of the rare-earth-based kagome lattice compounds $A_2$RE$_3$Sb$_3$O$_{14}$ ($A$ = Mg, Zn; RE = Pr, Nd, Gd, Tb, Dy, Ho, Er, Yb).

cond-mat.str-el↗

arXiv2018

Cluster-Based Haldane States in Spin-1/2 Cluster Chains

The Haldane state is a typical quantum and topological state of matter, which exhibits an edge state corresponding to symmetry-protected topological order in a one-dimensional integer spin chain. This edge state can be utilized for a processing unit of quantum computation. Its realization, however, has difficulties with synthesis of integer spin compounds. In contrast, one-half spin systems are more designable due to recent progress on intended synthesis of organic materials, quantum dots, and optical lattices. Here we propose a concept to design the Haldane state with one-half spins by making use of a chain composed of one-half spin clusters. If the clusters contains two spins, the ground state of a chain corresponds to the Affleck--Kennedy--Lieb--Tasaki state. In the case of an odd number of spins in the clusters, we propose a concrete procedure to construct a field-induced Haldane state. We illustrate the procedure with a 5-spin cluster chain.

cond-mat.str-el↗

arXiv2018

Ground state phase diagram of Kitaev-Heisenberg model on honeycomb-triangular lattice

The Kitaev-Heisenberg model defined on both honeycomb and triangular lattices has been studied intensively in recent years as a possible model to describe spin-orbital physics in iridium oxides. In the model, there are many phases characteristic for each lattice. However, there is no study how the phases in the two lattices merge each other when geometry changes from honeycomb lattice to triangular lattice. We investigate the ground state of the Kitaev-Heisenberg model defined on the system connecting the honeycomb and triangular lattices, named a honeycomb-triangular lattice. We obtain a ground state phase diagram of this model with classical spins by using the Luttinger-Tisza method and classical Monte Carlo simulation. In addition to known phases in the honeycomb and triangular lattices, we find coexisting phases consisting of the known phases. Based on the exact diagonalization and density-matrix renormalization group calculations for the model with quantum spins, we find that quantum fluctuations less affect on the phase diagram.

cond-mat.str-el↗