Bond-alternating Spin Chains Research Articles

Spin dynamics of [formula omitted]=1 bond-alternating spin chains in transverse fields

We calculate dynamical structure factors of the S=1 bond-alternating spin chain in transverse magnetic fields. The numerical results successfully explain the field dependence of the peaks observed in recent inelastic neutron-scattering experiments on NTENP.

Spin dynamics of the half-magnetization plateau in S = 1 bond-alternating spin chains

We investigate dynamical properties of the half-magnetization plateau in S = 1 bond-alternating spin chains. Dynamical structure factors calculated by the exact diagonalization method are compared with the dispersion curves of low-lying excitations obtained by a cluster expansion technique. We find that in the half-magnetization plateau the quintet excitation forms the lowest-lying excitation. The field dependence of the critical exponents of the spin correlation functions are further investigated. On the basis of the results, characteristics of the half-magnetization plateau are discussed.

Dynamical properties of [formula omitted] bond-alternating Heisenberg chains at finite temperatures

Dynamical structure factors of the S = 1 bond-alternating spin chains in the dimer phase are calculated at finite temperature, using the pair dynamical correlated-effective-field approximation. At T = 0 , the delta-function-type peak of the one-magnon mode appears. When temperature is increased, such a sharp peak is broadened and the additional weak peak caused by the excitation from the triplet state to the quintet state emerges in the higher energy region. The results are discussed in comparison with those obtained by the exact diagonalization method.

Dynamical Structure Factors ofS= 1 Bond-Alternating Heisenberg Chains at Finite Temperatures

Dynamical structure factors of the S = 1 bond-alternating spin chains in the dimer state are calculated at finite temperature using the pair dynamical correlated-effective-field approximation. At T = 0, the delta-function-type peak corresponding to the one-magnon mode appears. When temperature increases, such a sharp peak is broadened and the additional weak peak caused by the excitation from the triplet state to the quintet state emerges in the higher energy region. The results are discussed in comparison with those obtained by the exact diagonalization method.

Incommensurate state in a quasi-one-dimensionalS=1∕2bond-alternating antiferromagnet with frustration in magnetic fields

We investigate the critical properties of the $S=1/2$ bond-alternating spin chain with a next-nearest-neighbor interaction in magnetic fields. By the numerical calculation and the exact solution based on the effective Hamiltonian, we show that there is a parameter region where the longitudinal incommensurate spin correlation becomes dominant around the half-magnetization of the saturation. Possible interpretations of our results are discussed. We next investigate the effects of the interchain interaction ($J^{\prime}$). The staggered susceptibility and the uniform magnetization are calculated by combining the density-matrix renormalization group method with the interchain mean-field theory. For the parameters where the dominant longitudinal incommensurate spin correlation appears in the case $J^{\prime}=0$, the staggered long-range order does not emerge up to a certain critical value of $J^{\prime}$ around the half-magnetization of the saturation. We calculate the static structure factor in such a parameter region. The size dependence of the static structure factor at $k=2k_{\rm F}$ implies that the system has a tendency to form an incommensurate long-range order around the half-magnetization of the saturation. We discuss the recent experimental results for the NMR relaxation rate in magnetic fields performed for pentafluorophenyl nitronyl nitroxide.

NMR relaxation rate of F 5PNN in magnetic fields

Motivated by recent experiments for F 5PNN, we investigate NMR relaxation rate 1/ T 1 of S= 1 2 bond-alternating spin chains with a next-nearest-neighbor interaction in magnetic fields. We show that 1/ T 1 shows a power-law behavior with respect to temperature and at certain magnetic field 1/ T 1 becomes independent of temperature. The results qualitatively agree with the experimental findings.

DYNAMICAL PROPERTIES OF FIELD-INDUCED ORDERED-STATES IN S = 1/2 ONE-DIMENSIONAL QUANTUM SPIN SYSTEMS

We calculate the dynamical structure factors of the magnetization-plateau state in the S = 1/2 bond-alternating spin chain with a next-nearest-neighbor interaction. The results show characteristic behaviors depending on the next-nearest-neighbor interaction and the bond-alternation. We discuss the lower excited states in comparison with the exact excitation spectrums of an effective Hamiltonian.

Dynamics and Incommensurate Spin Correlation of the Magnetization Plateau State in theS=1/2 Bond-Alternating Spin Chain with a Next-Nearest-Neighbor Interaction

We investigate dynamical properties of the magnetization plateau state in the S =1 /2 bond-alternating spin chain with a next-nearest-neighbor interaction. The dynamical struc- ture factor shows characteristic behavior depending on the next-nearest-neighbor interaction α andthe bondalternation δ. The static structure factor takes the largest value at q = π for small δ. With increasing δ, the wave number of the largest value shifts towards q = π/2, taking the incommensurate value. where δ denotes the bond alternation, α denotes the NNN interaction, N is the total number of the site, and H is magnetic field. We set J =1 ,gµB = 1 and ¯ = 1. In magnetic field along the z axis, rotational symmetry around the x and y axes is broken, while that around the z axis remains. Therefore, the Hamiltonian can be classified into the subspace according to the magnetization m = M/N with M = N S z . In the following, we fix M = N/4. In the half-magnetization- plateau state with δ ∼ 1 and α ∼ 0, this Hamiltonian can be mapped onto the one-dimensional (1D) S =1 /2Heisenberg-Ising model in zero field. 2),4) The DSF can be calculated numerically, using a continued fraction based on the Lanczos algorithm. 5) We calculated the transverse DSF S x (q, ω), turning our atten- tion to the behavior of the lowest excitation band. Typical results for the DSF are shown in Figs. 1 (a) and (b). When δ becomes large for given α, the largest intensity in given wave number lies in the lowest excited state. The solid lines represent the exact bounds of the elementary excitation for the effective Hamiltonian obtained by

Dynamical structure factors of the magnetization plateau state in theS=12bond-alternating spin chain with a next-nearest-neighbor interaction

We calculate the dynamical structure factors of the magnetization plateau state in the $S=\frac{1}{2}$ bond-alternating spin chain with a next-nearest-neighbor interaction. The results show characteristic behaviors depending on the next-nearest-neighbor interaction $\ensuremath{\alpha}$ and the bond alternation \ensuremath{\delta}. We discuss the lower excited states in comparison with the exact excitation spectrums of an effective Hamiltonian. From the finite size effects, characteristics of the lowest excited states are investigated. The dispersionless mode of the lowest excitation appears in adequate sets of $\ensuremath{\alpha}$ and \ensuremath{\delta}, indicating that the lowest excitation is localized spatially and forms an isolated mode below the excitation continuum. We further calculate the static structure factors. The largest intensity is located at $q=\ensuremath{\pi}$ for small $\ensuremath{\delta}$ in fixed \ensuremath{\alpha}. With increasing \ensuremath{\delta}, the wave number of the largest intensity shifts towards $q=\ensuremath{\pi}/2,$ taking the incommensurate value.

Dynamical Structure Factors of theS=1/2Bond-Alternating Spin Chain with a Next-Nearest-Neighbor Interaction in Magnetic Fields

The dynamical structure factor of the S=1/2 bond-alternating spin chain with a next-nearest-neighbor interaction in magnetic field is investigated using the continued fraction method based on the Lanczos algorithm. When the plateau exists on the magnetization curve, the longitudinal dynamical structure factor shows a large intensity with a periodic dispersion relation, while the transverse one shows a large intensity with an almost dispersionless mode. The periodicity and the amplitude of the dispersion relation in the longitudinal dynamical structure factor are sensitive to the coupling constants. The dynamical structure factor of the S=1/2 two-leg ladder in magnetic field is also calculated in the strong interchain-coupling regime. The dynamical structure factor shows gapless or gapful behavior depending on the wave vector along the rung.

Non-classical magnetization process of quantum spin chains

The magnetization process of bond-alternating spin chains is investigated using the bosonization technique. The magnetization plateau recently observed in both numerical and experimental studies is naturally understood in terms of an effective Hamiltonian. Striking similarity to the metal-insulator transition can be seen if we identify the spin stiffness and the susceptibility with the Drude weight and the compressibility, respectively. Spin correlation functions are also calculated.

Phase Transitions ofS=3/2andS=2XXZ Spin Chains with Bond Alternation

We study the phase diagram of S=3/2 and S=2 bond-alternating spin chains numerically. In previous papers, the phase diagram of S=1 XXZ spin chain with bond-alternation was shown to reflect the hidden $Z_{2}\times Z_{2}$ symmetry. But for the higher S Heisenberg spin chain, the successive dimerization transition occurs, and for anisotropic spin chains the phase structure will be more colorful than the S=1 case. Using recently developed methods, we show directly that the phase structure of the anisotropic spin chains relates to the $Z_{2}\times Z_{2}$ symmetry.

An Efficient Monte Carlo Approach to Low-Lying Excitations of Quantum Spin Chains

We give a full description of a recently developed efficient Monte Carlo Approach to low-lying excitations of one-dimensional quantum spin systems. The idea is in a word expressed as extracting the lower edge of the excitation spectrum from imaginary-time quantum Monte Carlo data at a sufficiently low temperature. First, the method is applied to the antiferromagnetic Heisenberg chains of S=1/2, 1, 3/2, and 2. In the cases of S=1/2 and S=1, comparing the present results with the previous findings, we discuss the reliability of the method. The spectra for S=3/2 and S=2 turn out to be massless and massive, respectively. In order to demonstrate that our method is very good at treating long chains, we calculate the S=2 chain with length up to 512 spins and give a precise estimate of the Haldane gap. Second, we show its fruitful use in studying quantum critical phenomena of bond-alternating spin chains. Using the conformal invariance of the system as well, we calculate the central charge of the critical S=1 chain, which results in the Gaussian universality class. Third, we study an alternating-spin system composed of two kinds of spins S=1 and 1/2, which shows the ferrimagnetic behavior. We find a quadratic dispersion relation in the small-momentum region. The numerical findings are qualitatively explained well in terms of the spin-wave theory. Finally, we argue a possibility of applying the method to the higher excitations, where we again deal with the S=1 Heisenberg antiferromagnet and inquire further into its unique low-energy structure. All the applications demonstrate the wide applicability of the method and its own advantages.

Magnetization processes in bond-alternating quantum spin chains

The magnetization processes of bond-alternating spin chains are investigated using the bosonization technique. The magnetization plateau recently observed in both numerical and experimental studies is naturally understood in terms of an effective Hamiltonian. Spin correlation functions are also calculated.

Continuum-field description of one-dimensional dimerized spin-1/2 Heisenberg antiferromagnets.

Effective nonlinear field theory for a semiclassical long-wavelength description of one-dimensional S=1/2 Heisenberg antiferromagnets with dimerized ground states is derived within an approach using two-site coherent states. The method is applied to bond-alternating anisotropic spin chains in presence of external magnetic field and next-nearest-neighbor interactions. Linear and nonlinear excitations in singlet and magnetic phases are studied. Soliton solutions are found and their properties under semiclassical quantization are discussed. \textcopyright{} 1996 The American Physical Society.