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Shoji Yamamoto

Publications and source records attributed to Shoji Yamamoto.

71 records · Page 4Linked to original sources
arXiv2000

Characterization of ferrimagnetic Heisenberg chains according to the constituent spins

The low-energy structure and the thermodynamic properties of ferrimagnetic Heisenberg chains of alternating spins $S$ and $s$ are investigated by the use of numerical tools as well as the spin-wave theory. The elementary excitations are calculated through an efficient quantum Monte Carlo technique featuring imaginary-time correlation functions and are characterized in terms of interacting spin waves. The thermal behavior is analyzed with particular emphasis on its ferromagnetic and antiferromagnetic dual aspect. The extensive numerical and analytic calculations lead to the classification of the one-dimensional ferrimagnetic behavior according to the constituent spins: the ferromagnetic ($S>2s$), antiferromagnetic ($S<2s$), and balanced ($S=2s$) ferrimagnetism.

cond-mat.stat-mech↗
arXiv1999

Nuclear spin relaxation in ordered bimetallic chain compounds

A theoretical interpretation is given to recent proton spin relaxation-time (T_1) measurements on NiCu(C_7H_6N_2O_6)(H_2O)_3$\cdot$2H_2O, which is an ideal one-dimensional ferrimagnetic Heisenberg model system of alternating spins 1 and 1/2. The relaxation rate T_1^{-1} is formulated in temrs of the spin-wave theory and is evaluated by the use of a quantum Monte Carlo method. Calculations of the temperature and applied-field (H) dependences of T_1^{-1} are in total agreement with the experimental findings. T_1 behaves as $T_1^{-1}\propto H^{-1/2}$, which turns out an indirect observation of the quadratic dispersion relations dominating the low-energy physics of quantum ferrimagnets.

cond-mat.str-el↗
arXiv1999

Simulated nuclear spin-lattice relaxation in Heisenberg ferrimagnets: Indirect observation of quadratic dispersion relations

In response to recent proton spin relaxation-time measurements on NiCu(pba)(H$_2$O)$_3$$\cdot$2H$_2$O with ${pba}=1,3{-propylenebis(oxamato)}$, which is an excellent one-dimensional ferrimagnetic Heisenberg model system of spin-$(1,{1/2})$, we study the Raman relaxation process in spin-$(S,s)$ quantum ferrimagnets on the assumption of predominantly dipolar hyperfine interactions between protons and magnetic ions. The relaxation time $T_1$ is formulated within the spin-wave theory and is estimated as a function of temperature and an applied field $H$ by a quantum Monte Carlo method. The low-temperature behavior of the relaxation rate $T_1^{-1}$ qualitatively varies with $(S,s)$, while $T_1^{-1}$ is almost proportional to $H^{-1/2}$ due to the characteristic dispersion relations.

cond-mat.str-el↗
arXiv1999

Competing Ground States of the New Class of Halogen-Bridged Metal Complexes

Based on a symmetry argument, we study the ground-state properties of halogen-bridged binuclear metal chain complexes. We systematically derive commensurate density-wave solutions from a relevant two-band Peierls-Hubbard model and numerically draw the the ground-state phase diagram as a function of electron-electron correlations, electron-phonon interactions, and doping concentration within the Hartree-Fock approximation. The competition between two types of charge-density-wave states, which has recently been reported experimentally, is indeed demonstrated.

cond-mat.str-el↗
arXiv1999

Broken-Symmetry Ground States of Halogen-Bridged Binuclear Metal Complexes

Based on a symmetry argument, we study ground states of what we call MMX-chain compounds, which are the new class of halogen-bridged metal complexes. Commensurate density-wave solutions of a relevant multi-band Peierls-Hubbard model are systematically revealed within the Hartree-Fock approximation. We numerically draw ground-state phase diagrams, where various novel density-wave states appear.

cond-mat.str-el↗
arXiv1999

Breakdown of a Magnetization Plateau due to Anisotropy in Heisenberg Mixed-Spin Chains

We discuss the critical behavior of the spin-(1,1/2) Heisenberg ferrimagnetic chain in a magnetic field, whose magnetization curve exhibits a plateau at a third of the full magnetization. A bond alternation stabilizes the massive state, whereas an exchange anisotropy causes the breakdown of the plateau and the onset of a gapless spin-fluid state, where the transition, lying in the XY but ferromagnetic region, is of Kosterlitz-Thouless type. In order to elucidate significant quantum effects, we investigate the model of classical version as well.

cond-mat.stat-mech↗
arXiv1999

Critical Behavior of Anisotropic Heisenberg Mixed-Spin Chains in a Field

We numerically investigate the critical behavior of the spin-(1,1/2) Heisenberg ferrimagnet with anisotropic exchange coupling in a magnetic field. A quantized magnetization plateau as a function of the field, appearing at a third of the saturated magnetization, is stable over whole the antiferromagnetic coupling region. The plateau vanishes in the ferromagnetic coupling region via the Kosterlitz-Thouless transition. Comparing the quantum and classical magnetization curves, we elucidate what are essential quantum effects.

cond-mat.stat-mech↗
arXiv1998

Magnetic Properties of Quantum Ferrimagnetic Spin Chains

Magnetic susceptibilities of spin-$(S,s)$ ferrimagnetic Heisenberg chains are numerically investigated. It is argued how the ferromagnetic and antiferromagnetic features of quantum ferrimagnets are exhibited as functions of $(S,s)$. Spin-$(S,s)$ ferrimagnetic chains behave like combinations of spin-$(S-s)$ ferromagnetic and spin-$(2s)$ antiferromagnetic chains provided $S=2s$.

cond-mat.stat-mech↗
arXiv1998

Low-Energy Structure of Heisenberg Ferrimagnetic Spin Chains

Static and dynamic structure factors of Heisenberg ferrimagnetic spin chains are numerically investigated. There exist two distinct branches of elementary excitations, which exhibit ferromagnetic and antiferromagnetic aspects. The ferromagnetic feature is smeared out with the increase of temperature, whereas the antiferromagnetic one persists up to higher temperatures. The scattering intensity is remarkably large at lower boundaries of the ferromagnetic and antiferromagnetic spectra. All these observations are consistent with the ferromagnetic-to-antiferromagnetic crossover in the thermal behavior which has recently been reported.

cond-mat.stat-mech↗
arXiv1998

Novel Density-Wave States of Two-Band Peierls-Hubbard Chains

Based on a symmetry argument we systematically reveal Hartree-Fock broken-symmetry solutions of the one-dimensional two-band extended Peierls-Hubbard model, which covers various materials of interest such as halogen-bridged metal complexes and mixed-stack charge-transfer salts. We find out all the regular-density-wave solutions with an ordering vector $q=0$ or $q=π$. Changing band filling as well as electron-electron and electron-phonon interactions, we numerically inquire further into the ground-state phase diagram and the physical property of each state. The possibility of novel density-wave states appearing is argued.

cond-mat.str-el↗
arXiv1998

Combination of Ferromagnetic and Antiferromagnetic Features in Heisenberg Ferrimagnets

We investigate the thermodynamic properties of Heisenberg ferrimagnetic mixed-spin chains both numerically and analytically with particular emphasis on the combination of ferromagnetic and antiferromagnetic features. Employing a new density-matrix renormalization-group technique as well as a quantum Monte Carlo method, we reveal the overall thermal behavior: At very low temperatures, the specific heat and the magnetic susceptibility times temperature behave like $T^{1/2}$ and $T^{-1}$, respectively, whereas at intermediate temperatures, they exhibit a Schottky-like peak and a minimum, respectively. Developing the modified spin-wave theory, we complement the numerical findings and give a precise estimate of the low-temperature behavior.

cond-mat.stat-mech↗
arXiv1997

Quantum Fluctuations to Cause the Breakdown of the Spin-1 Haldane Phase

Investigating quantum fluctuations in the ground states of S=1 quantum antiferromagnetic spin chains described by the bilinear-biquadratic Hamiltonian, we study a mechanism of the breakdown of the Haldane phase. Based on the valence-bond-solid structure, but replacing two links of them by triplet bonds (crackions), we construct a trial wave function which is singlet and translationally invariant, where the crackion-crackion distance is regarded as a variational parameter. At $β<1/3$, the minimization of the variational energy results in a bound state of crackions, while at $β>1/3$, crackions come to be set free from their bound state with increase of $β$ and the chain length. We point out that the breakdown of the Haldane phase with $β$ approaching 1 can be attributed to the collapse of the bound state and the growth of a short-range repulsive interaction between crackions.

cond-mat.stat-mech↗
arXiv1997

Elementary Excitations of Heisenberg Ferrimagnetic Spin Chains

We numerically investigate elementary excitations of the Heisenberg alternating-spin chains with two kinds of spins 1 and 1/2 antiferromagnetically coupled to each other. Employing a recently developed efficient Monte Carlo technique as well as an exact diagonalization method, we verify the spin-wave argument that the model exhibits two distinct excitations from the ground state which are gapless and gapped. The gapless branch shows a quadratic dispersion in the small-momentum region, which is of ferromagnetic type. With the intention of elucidating the physical mechanism of both excitations, we make a perturbation approach from the decoupled-dimer limit. The gapless branch is directly related to spin 1's, while the gapped branch originates from cooperation of the two kinds of spins.

cond-mat.stat-mech↗
arXiv1997

Thermodynamic Properties of Heisenberg Ferrimagnetic Spin Chains: Ferromagnetic-Antiferromagnetic Crossover

We study thermodynamic properties of the one-dimensional Heisenberg ferrimagnet with antiferromagnetically exchange-coupled two kinds of spins 1 and 1/2. The specific heat and the magnetic susceptibility are calculated employing a modified spin-wave theory as well as a quantum Monte Carlo method. The specific heat is in proportion to $T^{1/2}$ at low enough temperatures but shows a Schottky-like peak at mid temperatures. The susceptibility diverges as $T^{-2}$. We reveal that at low temperatures the model is regarded as a ferromagnet, while at mid temperatures it behaves like a gapped antiferromagnet.

cond-mat.stat-mech↗
arXiv1997

Low-Lying Excitations in the S=1 Antiferromagnetic Heisenberg Chain

In order to confirm the picture of domain-wall excitations in the hidden antiferromagnetic order of the Haldane phase, the structure of the low-lying excitations in the S=1 antiferromagnetic Heisenberg chain is studied by a quantum Monte Carlo method. It is confirmed that there exists a finite energy gap between the first- and the second-excited states at $k=π$ as well as between the ground state and the first-excited state at $k=π$. In the thermodynamic limit, the second-excited state at $k=π$ is separated from the ground state by the gap which is three times as large as the Haldane gap. From the size dependences of the low-lying-excitation energies, the interactions between the elementary excitations in the excited states are concluded to be repulsive.

cond-mat.stat-mech↗
arXiv1996

Matrix product states approach to the Heisenberg ferrimagnetic spin chains

We propose a new version of the matrix product (MP) states approach to the description of quantum spin chains, which allows one to construct MP states with certain total spin and its z-projection. We show that previously known MP wavefunctions for integer-spin antiferromagnetic chains and ladders correspond to some particular cases of our general ansatz. Our method allows to describe systems with spontaneously broken rotational symmetry, like quantum ferrimagnetic chains whose ground state has nonzero total spin. We apply this approach to describe the ground state properties of the isotropic ferrimagnetic Heisenberg chain with alternating spins 1 and 1/2 and compare our variational results with the high-precision numerical data obtained by means of the quantum Monte Carlo (QMC) method. For both the ground state energy and the correlation functions we obtain very good agreement between the variational results and the QMC data.

cond-mat↗