Dynamical Properties of a Haldane Gap Antiferromagnet

We study the dynamic spin correlation function of a spin one antiferromagnetic chain with easy-plane single-ion anisotropy. We use exact diagonalization by the Lancz\H os method for chains of lengths up to N=16 spins. We show that a single-mode approximation is an excellent description of the dynamical properties. A variational calculation allows us to clarify the nature of the excitations. The existence of a two-particle continuum near zero wavevector is clearly seen both in finite-size effects and in the dynamical structure factor. The recent neutron scattering experiments on the quasi-one-dimensional antiferromagnet NENP are fully explained by our results.Comment: 14 pages, SphT/92-135 plain tex with Postscript figures included. Postscipt file available by anonymous ftp at amoco.saclay.cea.fr by get pubs.spht/92-135.ps local_file (290 kb) or get pubs.spht/92-135.ps.Z local_file.Z (compressed - 120 kb

Magnon dispersion and thermodynamics in CsNiF_3

We present an accurate transfer matrix renormalization group calculation of the thermodynamics in a quantum spin-1 planar ferromagnetic chain. We also calculate the field dependence of the magnon gap and confirm the accuracy of the magnon dispersion derived earlier through an 1/n expansion. We are thus able to examine the validity of a number of previous calculations and further analyze a wide range of experiments on CsNiF_3 concerning the magnon dispersion, magnetization, susceptibility, and specific heat. Although it is not possible to account for all data with a single set of parameters, the overall qualitative agreement is good and the remaining discrepancies may reflect departure from ideal quasi-one-dimensional model behavior. Finally, we present some indirect evidence to the effect that the popular interpretation of the excess specific heat in terms of sine-Gordon solitons may not be appropriate.Comment: 9 pages 10 figure

Nonlinear excitations in CsNiF3 in magnetic fields perpendicular to the easy plane

Experimental and numerical studies of the magnetic field dependence of the specific heat and magnetization of single crystals of CsNiF3 have been performed at 2.4 K, 2.9 K, and 4.2 K in magnetic fields up to 9 T oriented perpendicular to the easy plane. The experimental results confirm the presence of the theoretically predicted double peak structure in the specific heat arising from the formation of nonlinear spin modes. The demagnetizing effects are found to be negligible, and the overall agreement between the data and numerical predictions is better than reported for the case when the magnetic field was oriented in the easy plane. Demagnetizing effects might play a role in generating the difference observed between theory and experiment in previous work analyzing the excess specific heat using the sine-Gordon model.Comment: 6 pages, 5 figures, submitted to Phys. Rev.

Static properties of an easy-plane ferromagnetic S=1/2 chain:Comparison of numerical results and experimental data on [C6H11NH3]CuBr3

New results of a variant of the numerically exact transfer matrix method have been compared with the experimentally determined static properties of [C6H11NH3]CuBr3 (CHAB). Above T=3.5 K, the available data on the zero-field heat capacity, the excess heat capacity C=C(B) -C(B=0) for B=1, 2, and 3 T, and the magnetization up to 5 T are described with an accuracy that is comparable to the experimental error. Calculations of the spin-spin correlation functions by this method reveal that the fair description of the experimental correlation length in CHAB by a classical spin model is largely accidental. The zero-field susceptibility deduced from these correlation functions is in satisfactory agreement with the reported data.</p

Static properties of ferromagnetic quantum chains: numerical results and experimental data on two S = 1/2 systems

New results of a variant of the numerically exact quantum transfer matrix method have been compared with experimental data on the static properties of [C6H11NH3]CuBr3(CHAB), a ferromagnetic system with about 5% easy-plane anisotropy. Above T=3.5 K, the available data on the zero-field heat capacity, the excess heat capacity ¿C=C(B)-C(B=0), and the magnetization are described with an accuracy comparable to the experimental error. Calculations of the spin-spin correlation functions reveal that the good description of the experimental correlation length in CHAB by a classical spin model is largely accidental. The zero-field susceptibility, which can be deduced from these correlation functions, is in fair agreement with the reported experimental data between 4 and 100 K. The method also seems to yield accurate results for the chlorine isomorph, CHAC, a system with about 2% uniaxial anisotropy

Static properties of ferromagnetic quantum chains: numerical results and experimental data on two S = 1/2 systems

New results of a variant of the numerically exact quantum transfer matrix method have been compared with experimental data on the static properties of [C6H11NH3]CuBr3(CHAB), a ferromagnetic system with about 5% easy-plane anisotropy. Above T=3.5 K, the available data on the zero-field heat capacity, the excess heat capacity ¿C=C(B)-C(B=0), and the magnetization are described with an accuracy comparable to the experimental error. Calculations of the spin-spin correlation functions reveal that the good description of the experimental correlation length in CHAB by a classical spin model is largely accidental. The zero-field susceptibility, which can be deduced from these correlation functions, is in fair agreement with the reported experimental data between 4 and 100 K. The method also seems to yield accurate results for the chlorine isomorph, CHAC, a system with about 2% uniaxial anisotropy

Thermal properties of chains of antiferromagnetically coupled spins with s = 1. Numerical evidence of the Haldane gap at nonzero temperatures

By the numerical transfer-matrix method we have computed several thermal properties for a Heisenberg chain of 150 antiferromagnetically coupled spins with quantum number s = 1. We dispensed with any extrapolation to the infinite chain, because the results for the magnetization, the susceptibility, the correlation length, and the static structure factor at wave number q = p have been found to be insensitive to a further increase of the chain length in the temperature regime considered. The characteristic features of our results, that occur at sufficiently low temperatures and low applied fields, can convincingly be explained by the existence of a gap in the excitation spectrum of the order of 0.4 coupling constants and, therefore, nicely complement previous numerical results for corresponding ground-state properties. We have also performed computations for a chain with an additional single-site anisotropy. We find, for appropriate parameter values, excellent agreement with experimental data for the quasi-one-dimensional model compound NENP

Static properties of a ferromagnetic S=1/2 chain system with orthorhombic exchange anisotropy:Comparison of numerical results and experimental data on [C6H11NH3]CuCl3

\u3cp\u3eResults of the numerical quantum-transfer-matrix method (QTM) are compared with experimental data on [C6H11NH3]CuCl3, a quasi-one-dimensional ferromagnetic S=1/2 system with orthorhombic exchange anisotropy. The experimentally observed static properties of this system, such as magnetization, susceptibility, and heat capacity, generally can be described with an accuracy comparable to the experimental error. At temperatures extending up to a few kelvin above the three-dimensional-ordering temperature systematic deviations between theory and the data are found; they are attributed to the (very small) coupling between the chains in this compound.\u3c/p\u3

Static properties of CsNiF3:numerical results, experimental data and soliton bearing models

The static properties of the S=1 easy-plane ferromagnetic chain system CsNiF3 have been calculated by the numerical Quantum Transfer Matrix method (QTM). An analysis of the relevant experimental data shows that the generally accepted spin Hamiltonian accurately describes the individual chains for J / kB=25 K, D / kB=7.7 K, g=2.1. Deviations between theory and experiment are found at temperatures extending up to several kelvin above the three-dimensional ordering temperature, that are attributed to the coupling between the chains. The QTM results are compared with various analytical results, yielding support for the description of solitary excitations in CsNiF3 by the classical sine-Gordon model, extended to include spin fluctuations out of the easy plane and quantum effects.</p

Static properties of cesium trifluoronickelate(I): comparison of numerical results, experimental data, and predictions from soliton-bearing models

The numerical quantum transfer-matrix method has been applied to the s=1 easy-plane ferromagnetic chain system CsNiF3. A detailed analysis of experimental data on the static properties reveals that the generally accepted spin Hamiltonian for this compound gives an accurate description of the individual chains for J/kB=25 K and D/kB=7.7 K. At temperatures up to several degrees Kelvin above the three-dimensional ordering temperature, deviations between theory and experiment are found that are attributed to the coupling between the chains. A comparison of the quantum-transfer-matrix-method results with various analytical results supports the description of solitary excitations in CsNiF3 by the classical sine-Gordon model, extended to include spin fluctuations out of the easy plane and quantum effects