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論文

Magnetic excitation in the =1/2 Ising-like antiferromagnetic chain CsCoCl $_{3}$ in longitudinal magnetic fields studied by high-field ESR measurements

木村尚次郎*; 大西弘明; 奥西巧一*; 赤木暢*; 鳴海康雄*; 萩原政幸*; 金道浩一*; 菊池彦光*

Journal of the Physical Society of Japan, 92(9), p.094701_1 - 094701_9, 2023/09

https://doi.org/10.7566/JPSJ.92.094701

被引用回数：0 パーセンタイル：0(Physics, Multidisciplinary)

High-field electron spin resonance (ESR) measurements were performed on the =1/2 Ising-like antiferromagnetic chain CsCoCl $_{3}$ in the frequency range up to 4.4THz and magnetic fields for up to 53T. We observed softening of the spinon excitation, which drives the spin chain into the quantum critical state at the critical field $H_{c}$ =33.0T. Above $H_{c}$ , we observed the string excitation at the wave number =, which is characteristic of the =1/2 spin chain in the quantum critical state in the magnetic fields. We also observed a crossover of the string excitation to the magnon bound state above the saturation fields $H_{s}$ =43.9T upon increasing the magnetic fields. Finally, we discuss how the string excitation and magnon bound state can be excited by the electric dipole transition due to spin quadrupolar resonance.

論文

Low temperature behavior of itinerant ferromagnet realized in extended Nagaoka mechanism

大西弘明; 宮下精二*

JPS Conference Proceedings (Internet), 38, p.011157_1 - 011157_6, 2023/05

https://doi.org/10.7566/JPSCP.38.011157

We have proposed a Hubbard model which exhibits itinerant ferromagnetism by extending the model of Nagaoka ferromagnetism. In the present work, we confirm that the idea of the realization of itinerant ferromagnetism works in two- and three-dimensional lattices as well as a one-dimensional lattice which we studied previously. We also investigate the temperature dependence of the magnetic ordering and present a magnetic phase diagram in the space of the temperature and the chemical potential. In the obtained phase diagram, we find a V-shape structure for the magnetic ordering near a quantum phase transition. Besides, we find an additional peak exhibiting a dome structure. Temperature dependences of related spin correlation functions are studied in detail.

論文

Finite-temperature properties of extended Nagaoka ferromagnetism; Ordering processes and precursor of a quantum phase transition between itinerant ferromagnetic and Mott antiferromagnetic states

大西弘明; 宮下精二*

Physical Review B, 106(13), p.134436_1 - 134436_13, 2022/10

https://doi.org/10.1103/PhysRevB.106.134436

被引用回数：0 パーセンタイル：0(Materials Science, Multidisciplinary)

We study finite-temperature properties of a Hubbard model including sites of a particle bath which was proposed as a microscopic model to show itinerant ferromagnetism at finite electron density. We use direct numerical methods, such as exact diagonalization and random vector methods. The temperature dependence of quantities is surveyed in the full range of the temperature. We find that the specific heat has several peaks, which correspond to ordering processes in different energy scales. In particular, magnetic correlations develop at very low temperature. The system exhibits an itinerant ferromagnetic state or an antiferromagnetic state of the Mott insulator depending on the chemical potential of the particle bath and the Coulomb interaction. From a microscopic viewpoint, the competition between these two types of magnetic states causes a peculiar ordering process of local spin correlations. Some local ferromagnetic correlations are found to be robust, which indicates that the ferromagnetic correlation originates from the motion of itinerant electrons in a short-range cluster.

論文

Excitation and transport of bound magnon clusters in frustrated ferromagnetic chain

大西弘明

Journal of Physics; Conference Series, 2207, p.012045_1 - 012045_6, 2022/03

https://doi.org/10.1088/1742-6596/2207/1/012045

By exploiting density-matrix renormalization group methods, we study magnetic and transport properties of a frustrated quantum spin chain with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchange interactions in an applied magnetic field. In high magnetic fields, there appear a series of spin multipole liquid ground states such as quadrupole, octupole, and hexadecapole, which are two-, three-, and four-magnon bound states, respectively. Since the low-energy excitation is governed by multi-magnon bound states, we envisage that bound magnon clusters would carry spin current. We will report detailed analyses of multipole excitation spectra and current correlation functions, and discuss how the magnon clusters contribute to the transport.

論文

Optical selection rules of the magnetic excitation in the = $frac{1}{2}$ one-dimensional Ising-like antiferromagnet BaCo $_{2}$ V $_{2}$ O $_{8}$

木村尚次郎*; 大西弘明; 奥谷顕*; 赤木暢*; 鳴海康雄*; 萩原政幸*; 奥西巧一*; 金道浩一*; He, Z.*; 谷山智康*; et al.

Physical Review B, 105(1), p.014417_1 - 014417_9, 2022/01

https://doi.org/10.1103/PhysRevB.105.014417

被引用回数：3 パーセンタイル：34.67(Materials Science, Multidisciplinary)

In this study, we clarify the selection rules of optical transitions from the ground state to the magnetic excited states in the quasi-one-dimensional quantum Ising-like antiferromagnet BaCo $_{2}$ V $_{2}$ O $_{8}$ with spin = $frac{1}{2}$ using the high-field electron spin resonance measurement with illuminating polarized electromagnetic waves. We demonstrate that the unconventional magnetic excitation via the pair creation of quasipaticles at wave numbers $q_{z}=pi/2$ and in the field-induced quantum critical state can couple with both oscillating magnetic and electric fields. Our density matrix renormalization group calculations indicate that the observed selection rules can be explained by two- and four-fold periodical spin interactions, originating from the screw chain structure in BaCo $_{2}$ V $_{2}$ O $_{8}$ , in addition to the magnetoelectric coupling between spins and electric dipoles.

論文

Spin excitations of the =1/2 one-dimensional Ising-like antiferromagnet BaCo $_{2}$ V $_{2}$ O $_{8}$ in transverse magnetic fields

奥谷顕*; 大西弘明; 木村尚次郎*; 竹内徹也*; 木田孝則*; 森道康; 三宅厚志*; 徳永将史*; 金道浩一*; 萩原政幸*

Journal of the Physical Society of Japan, 90(4), p.044704_1 - 044704_9, 2021/04

https://doi.org/10.7566/JPSJ.90.044704

被引用回数：3 パーセンタイル：30.35(Physics, Multidisciplinary)

The high-field multifrequency electron spin resonance (ESR), the magnetization, and the specific heat of single crystals of BaCo $_{2}$ V $_{2}$ O $_{8}$ (a quasi-one-dimensional Ising-like antiferromagnet with ) were measured in two different transverse magnetic fields: and . A softening of the lowest ESR mode was observed near the saturation field of the magnetization around 40 T in , whereas it occurs around 10 T in , far below the saturation field. These observations were reproduced by numerical calculations using the density-matrix renormalization group methods, including the staggered magnetic fields induced by a uniform external magnetic field and the interchain magnetic exchange interaction. We found that the directional dependence of the lowest mode originates mainly from the remarkable angular dependence of the induced magnetic field. The interchain interaction is also quantitatively important for reproducing the experimental results. The directional dependence of the lowest mode is demonstrated by the particle-hole excitation of spinons using the Jordan-Wigner transformation.

論文

Magnetic excitations and transport properties in frustrated ferromagnetic chain

大西弘明

Journal of Magnetism and Magnetic Materials, 479, p.88 - 90, 2019/06

https://doi.org/10.1016/j.jmmm.2019.02.006

被引用回数：0 パーセンタイル：0(Materials Science, Multidisciplinary)

In a spin nematic state of a spin-1/2 frustrated ferromagnetic chain, low-energy excitations are governed by bound magnons, so that bound magnons carry spin current and thermal current. Regarding magnetic excitations, we find gapless longitudinal and gapped transverse spin excitation spectra, in accordance with quasi-long-range longitudinal and short-range transverse spin correlations, respectively, while we observe gapless quadrupole excitations, signaling quasi-long-range quadrupole correlations. In the present work, we investigate spin and thermal transport properties by exploiting numerical methods, such as exact diagonalization and density-matrix renormalization group. We will discuss the effects of bound magnon propagation in spin and thermal transport, based on detailed analyses of current-current correlation functions in the spin nematic ground state, and temperature and field dependencies of the spin Drude weight and the thermal Drude weight.

論文

Dynamical quadrupole structure factor of frustrated ferromagnetic chain

大西弘明

Physica B; Condensed Matter, 536, p.346 - 349, 2018/05

https://doi.org/10.1016/j.physb.2017.10.056

被引用回数：2 パーセンタイル：10.36(Physics, Condensed Matter)

In a spin-1/2 $J_{1}$ - $J_{2}$ Heisenberg chain with ferromagnetic $J_{1}$ and antiferromagnetic $J_{2}$ , the ground state in high magnetic fields is a quadrupole state, the so-called spin nematic state. To understand the properties of the spin nematic state from the viewpoint of excitation dynamics, we have studied dynamical spin structure factors by density-matrix renormalization group methods. In the spin nematic regime, we find gapless longitudinal and gapped transverse spin excitation spectra, in accordance with quasi-long-ranged longitudinal and short-ranged transverse spin correlations, respectively. These features can be examined by inelastic neutron scattering experiments. However, they are indirect evidence for the spin nematic state. Here, we proceed to analyze dynamical quadrupole structure factors to clarify excitation dynamics in the quadrupole channel. We observe gapless excitations at =, signaling quasi-long-ranged antiferro-quadrupole correlations. Information of spectral intensity distribution would be useful to discuss how we can detect the spin nematic state directly by experiments.

論文

Neutron scattering study of yttrium iron garnet

社本真一; 伊藤孝; 大西弘明; 山内宏樹; 稲村泰弘; 松浦直人*; 赤津光洋*; 樹神克明; 中尾朗子*; 茂吉武人*; et al.

Physical Review B, 97(5), p.054429_1 - 054429_9, 2018/02

https://doi.org/10.1103/PhysRevB.97.054429

被引用回数：18 パーセンタイル：65.6(Materials Science, Multidisciplinary)

イットリウム鉄ガーネットの核および磁気構造と全マグノン分散を中性子散乱により調べた。低エネルギーの分散は強磁性マグノンで期待されるように14meVまで2次関数の分散を示した。積分した動的磁化率"()の虚部は低エネルギーで平方根のエネルギー依存性を示した。"()から絶対値でマグノン状態密度を求めた。その値は理論的に予想されるマグノン分散でひとつのカラリティーモードに対応する。

論文

Quantum phase transitions driven by rhombic-type single-ion anisotropy in the =1 Haldane chain

Tzeng, Y.-C.*; 大西弘明; 大久保毅*; Kao, Y.-J.*

Physical Review B, 96(6), p.060404_1 - 060404_7, 2017/08

https://doi.org/10.1103/PhysRevB.96.060404

被引用回数：24 パーセンタイル：72.72(Materials Science, Multidisciplinary)

The spin-1 Haldane chain is an example of the symmetry-protected-topological (SPT) phase in one dimension. Experimental realization of the spin chain materials usually involves both the uniaxial-type, , and the rhombic-type, , single-ion anisotropies. Here, we provide a precise ground-state phase diagram for a spin-1 Haldane chain with these single-ion anisotropies. Using quantum numbers, we find that the symmetry breaking phase can be characterized by double degeneracy in the entanglement spectrum. Topological quantum phase transitions take place on particular paths in the phase diagram, from the Haldane phase to the large-, large-, or large- phases. The topological critical points are determined by the level spectroscopy method with a newly developed parity technique in the density matrix renormalization group, and the Haldane-large- critical point is obtained with an unprecedented precision, =0.9684713(1). Close to this critical point, a small rhombic single-ion anisotropy can destroy the Haldane phase and bring the system into a -Nel phase. We propose that the compound [Ni(HF)(3-Clpy)]BF is a candidate system to search for the -Nel phase.

論文

Magnetic structure and dispersion relation of the =1/2 quasi-one-dimensional Ising-like antiferromagnet BaCo $_{2}$ V $_{2}$ O $_{8}$ in a transverse magnetic field

松田雅昌*; 大西弘明; 奥谷顕*; Ma, J.*; Agrawal, H.*; Hong, T.*; Pajerowski, D. M.*; Copley, J. R. D.*; 奥西巧一*; 森道康; et al.

Physical Review B, 96(2), p.024439_1 - 024439_8, 2017/07

https://doi.org/10.1103/PhysRevB.96.024439

被引用回数：12 パーセンタイル：51.46(Materials Science, Multidisciplinary)

BaCo $_{2}$ V $_{2}$ O $_{8}$ consists of Co chains, in which Co $^{2+}$ ion carries a fictitious spin $frac{1}{2}$ with Ising anisotropy. We performed elastic and inelastic neutron scattering experiments in BaCo $_{2}$ V $_{2}$ O $_{8}$ in magnetic field perpendicular to the axis which is the chain direction. With applying magnetic field along the axis at 3.5 K, the antiferromagnetic order with the easy axis along the axis, observed in zero magnetic field, is completely suppressed at 8 T, while the magnetic field gradually induces an antiferromagnetic order with the spin component along the axis. We also studied magnetic excitations as a function of transverse magnetic field. The lower boundary of the spinon excitations splits gradually with increasing magnetic field. The overall feature of the magnetic excitation spectra in magnetic field is reproduced by the theoretical calculation based on the spin $frac{1}{2}$ antiferromagnetic chain model, which predicts that the dynamic magnetic structure factor of the spin component along the chain direction is enhanced and that along the field direction has clear incommensurate correlations.

論文

Variational wavefunction for the periodic Anderson model with onsite correlation factors

久保勝規; 大西弘明

Journal of the Physical Society of Japan, 86(1), p.013701_1 - 013701_4, 2017/01

https://doi.org/10.7566/JPSJ.86.013701

被引用回数：3 パーセンタイル：21.19(Physics, Multidisciplinary)

We propose a variational wavefunction containing parameters to tune the probabilities of all the possible onsite configurations for the periodic Anderson model. We call it the full onsite-correlation wavefunction (FOWF). This is a simple extension of the Gutzwiller wavefunction (GWF), in which one parameter is included to tune the double occupancy of the electrons at the same site. We compare the energy of the GWF and the FOWF evaluated by the variational Monte Carlo method and that obtained with the density-matrix renormalization group method. We find that the energy is considerably improved in the FOWF. On the other hand, the physical quantities do not change significantly between these two wavefunctions as long as they describe the same phase, such as the paramagnetic phase. From these results, we not only demonstrate the improvement by the FOWF, but we also gain insights on the applicability and limitation of the GWF to the periodic Anderson model.

論文

Magnetic excitations of spin nematic state in frustrated ferromagnetic chain

大西弘明

Journal of the Physical Society of Japan, 84(8), p.083702_1 - 083702_4, 2015/08

https://doi.org/10.7566/JPSJ.84.083702

被引用回数：25 パーセンタイル：78.62(Physics, Multidisciplinary)

By exploiting density-matrix renormalization group techniques, we investigate the dynamical spin structure factor of a spin-1/2 Heisenberg chain with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchange interactions in an applied magnetic field. In a field-induced spin nematic regime, we find gapless longitudinal and gapped transverse spin excitation spectra, in accordance with quasi-long-ranged longitudinal and short-ranged transverse spin correlations, respectively. The gapless point coincides with the dominant longitudinal spin correlation, whereas the gap position exhibits a characteristic field dependence contradicting the dominant transverse spin correlation.

論文

Effects of magnetic anisotropy on spin dynamics of ferromagnetic frustrated chain

大西弘明

Journal of Physics; Conference Series, 592(1), p.012109_1 - 012109_6, 2015/03

https://doi.org/10.1088/1742-6596/592/1/012109

被引用回数：8 パーセンタイル：91.48(Physics, Atomic, Molecular & Chemical)

By exploiting density-matrix renormalization group techniques, we investigate the spin dynamics of a spin-1/2 one-dimensional $J_{1}$ - $J_{2}$ XXZ model with competing ferromagnetic $J_{1}$ and antiferromagnetic $J_{2}$ under applied magnetic fields. In a high-field phase, the formation of two-magnon bound states leads to a spin quadrupole state, the so-called spin nematic state. The longitudinal spin and quadrupole correlations are quasi-long-ranged, while the transverse spin correlation is short-ranged. Accordingly, the longitudinal component of the spin excitation has a gapless mode, while the transverse component exhibits a gapped mode. The easy-plane anisotropy disfavors the two-magnon bound state, and in contrast the easy-axis anisotropy stabilizes the two-magnon bound state. We will report detailed analyses of the dependence on exchange coupling parameters and magnetic field, and discuss possible relevance to real compounds such as LiCuVO $_{4}$ .

論文

Doping control of realization of an extended Nagaoka ferromagnetic state from the Mott state

大西弘明; 宮下精二*

Physical Review B, 90(22), p.224426_1 - 224426_11, 2014/12

https://doi.org/10.1103/PhysRevB.90.224426

被引用回数：1 パーセンタイル：4.65(Materials Science, Multidisciplinary)

Inspired by the Nagaoka ferromagnetism, we propose an itinerant model to study the transition between the Mott singlet state and a ferromagnetic state by emulating a doping process in finite lattices. In the Nagaoka ferromagnetism, the total spin of the system takes the maximum value when an electron is removed from the half-filled system. To incorporate a procedure of the electron removal, our model contains extra sites as a reservoir of electrons, and the chemical potential of the reservoir controls the distribution of electrons. We demonstrate that, as a function of the chemical potential, the system exhibits ground-state phase transitions among various values of the total spin, including a saturated ferromagnetic state due to the Nagaoka mechanism at finite hole density.

論文

Ground state of =1 zigzag spin-orbital chain

大西弘明

JPS Conference Proceedings (Internet), 3, p.014025_1 - 014025_5, 2014/06

https://doi.org/10.7566/JPSCP.3.014025

We investigate ground-state properties of a $t_{rm 2g}$ -orbital Hubbard model on a zigzag chain, by exploiting numerical techniques. The electron number per site is set to be two, leading to a local =1 state. That is, the system is a Haldane system with active orbital degrees of freedom. We assume that the $t_{rm 2g}$ level is split into the $a_{rm 1g}$ () and $e_{rm g}$ () levels due to the tetragonal crystalline electric field effect. With varying the level splitting, we observe a quantum phase transition that accompanies the change of the orbital state. We will report the detailed ground-state phase diagram and discuss the spin and orbital states. We will also discuss possible relevance of the obtained results to real compounds such as CaV $_{2}$ O $_{4}$ .

論文

Nonequilibrium dynamics of multiorbital correlated electron system under time-depending electric field

大西弘明

Physica Status Solidi (B), 250(3), p.553 - 556, 2013/03

https://doi.org/10.1002/pssb.201200792

被引用回数：0 パーセンタイル：0(Physics, Condensed Matter)

To clarify a role of orbital degrees of freedom in the response to an external field, we investigate the real-time dynamics under applied electric fields in a one-dimensional orbital degenerate Hubbard model, by exploiting numerical techniques. Let us here consider a chain along the axis with $e_{rm g}$ -orbital degrees of freedom on each site and one electron per site. In the ground state, the itinerant $d_{3z^2-r^2}$ orbital is singly occupied and the localized $d_{x^2-y^2}$ orbital is empty, while the spin state exhibits antiferromagnetic correlations. After an oscillating electric field is turned on, holon-doublon pairs are created. As the time evolves, the ferro-orbital state is deformed due to the pair-hopping process that is characteristic of multi-orbital systems. Regarding the spin state, we observe the suppression of the antiferromagnetic correlations. We will also address the other filling case.

論文

Spin-orbital state induced by strong spin-orbit coupling

大西弘明

Journal of Physics; Conference Series, 391, p.012102_1 - 012102_4, 2012/12

https://doi.org/10.1088/1742-6596/391/1/012102

被引用回数：6 パーセンタイル：88.01(Physics, Multidisciplinary)

To clarify a crucial role of a spin-orbit coupling in the emergence of novel spin-orbital states in a -electron compound Sr $_{2}$ IrO $_{4}$ , we study ground state properties of a $t_{rm 2g}$ -orbital Hubbard model on a square lattice by Lanczos diagonalization. In the absence of the spin-orbit coupling, the ground state is spin singlet when the Hund's rule coupling is small, while the ground state turns to be a fully polarized ferromagnetic state with the increase of the Hund's rule coupling. On the other hand, when we include the strong spin-orbit coupling, a ferromagnetic state appears even if the Hund's rule coupling is small. In the spin-orbit-induced ferromagnetic state, the total spin is reduced from the fully polarized one, since up-spin and down-spin states are mixed. Regarding the orbital state, we find the so-called complex orbital state, in which $t_{rm 2g}$ -orbital states are mixed with complex coefficients.

論文

Multipole correlations of $t_{rm 2g}$ -orbital Hubbard model with spin-orbit coupling

大西弘明

Journal of the Physical Society of Japan, 80(Suppl.A), p.SA141_1 - SA141_3, 2011/07

https://doi.org/10.1143/JPSJS.80SA.SA141

被引用回数：2 パーセンタイル：20.16(Physics, Multidisciplinary)

We investigate multipole properties in the ground state of a $t_{rm 2g}$ -orbital Hubbard model with the spin-orbit coupling, by exploiting numerical techniques. Here let us consider a one-dimensional chain along the direction with five electrons per site. Due to the spatial anisotropy of $t_{rm 2g}$ orbitals, and orbitals are active in the absence of the spin-orbit coupling. When the spin-orbit coupling is weak, spin and orbital are independent degrees of freedom at local level. With the increase of the spin-orbit coupling, the total angular momentum turns to give a good quantum state. The spin state changes from an antiferromagnetic state to a ferromagnetic state. Regarding the orbital state, the spin-orbit coupling would lead to a complex orbital state. In the presentation, we will discuss how the spin-orbital state can be understood from the viewpoint of multipole.

論文

Dynamics of attractively interacting fermi atoms in one-dimensional optical lattices; Non-equilibrium simulations of fermi superfluidity

奥村雅彦; 大西弘明; 山田進; 町田昌彦

Physica C, 470(Suppl.1), p.S949 - S951, 2010/12

https://doi.org/10.1016/j.physc.2009.11.178

被引用回数：4 パーセンタイル：21.96(Physics, Applied)

近年進展著しいフェルミオン光学格子系において、最近、非平衡状態の生成,観測も行われるようになってきた。その中でも、引力相互作用するフェルミ原子気体の重心運動の実験が行われ、注目を集めている。この実験では引力相互作用が強くなるに連れ、重心運動の減衰が早まることがわかっている。われわれは、この重心運動にスピンインバランス効果がどのように効くのかを、時間依存密度行列繰り込み群を用いて、数値誤差の範囲内で厳密に調べた。その結果、アップスピンとダウンスピンの数に差がある場合、重心運動の減衰がさらに早まることを発見した。さらに詳細に調べた結果、この早い減衰は、二重占有状態とその他の成分が逆位相で運動することに起因することがわかった。