Quantum communication through anisotropic Heisenberg XY spin chains

We study quantum communication through an anisotropic Heisenberg XY chain in a transverse magnetic field. We find that for some time t and anisotropy parameter γ, one can transfer a state with a relatively high fidelity. In the strong-field regime, the anisotropy does not significantly affect the fidelity while in the weak-field regime the affect is quite pronounced. The most interesting case is the intermediate regime where the oscillation of the fidelity with time is low and the high-fidelity peaks are relatively broad. This would, in principle, allow for quantum communication in realistic circumstances. Moreover, we calculate the purity, or tangle, as a measure of the entanglement between one spin and all the other spins in the chain and find that the stronger the anisotropy and exchange interaction, the more entanglement will be generated for a given time.     

Solitons and spin complexes in the anisotropic Heisenberg chain

It is shown that in the case of the spin chain with weak anisotropy the quasiclassical energies of the soliton solutions of the Landau-Lifshitz equation coincide with the magnon bound state energies.     

Nonlinear spin excitations in a classical Heisenberg anisotropic helimagnet

The nonlinear spin excitations in an anisotropic helimagnet in the presence of a constant magnetic field are investigated in the classical continuum limit. The helical character is introduced into the model, in analogy with the twist in a cholesteric liquid crystal. After deriving a class of spin wave solutions for the stereographic representation of the Landau-Lifshitz equation, in-plane stationary spin configurations are obtained and their stability is analysed. When the external magnetic field is along the anisotropic axis, modulational instability is observed in the spin lattice, and when the external magnetic field is normal to the anisotropic axis, the spin configurations are unstable. The perturbed spin components show fluctuations in the tail region, while the velocity and amplitude of the soliton remain unaltered.     

各向异性海森伯自旋链中的超椭圆函数波解

在霍尔斯坦-普里马科夫表象中研究了各向异性海森伯自旋链模型.在半经典近似条件下,考虑高阶非线性项和周期性边界条件,应用相干态求出了用雅可比椭圆函数的反函数的组合表示的超椭圆函数波解,并讨论了解的物理意义.     

Field dependent spin dynamics of antiferromagnetic Heisenberg chains

Low temperature data are given for the spectra of longitudinal and transverse correlation functions of an antiferromagnetic classical heisenberg chain. The one dimensional magnet subjected to large external fields is investigated by means of a computer simulation calculation. The results are tied in with the static properties and compared with theoretical predictions.     

Low-temperature spin dynamics of a classical compressible Heisenberg chain

The dynamic spin correlation function of a classical compressible Heisenberg chain is investigated in the low temperature limit. Anharmonic processes, arising from the nonlinearity of the spin equations of motion and from the coupling of the spins to translational degrees of freedom, are taken into account to lowest order. Two-magnon processes and magnon-phonon processes contribute to the spectra. They can give rise to additional resonances but mostly their weight is very small. The main influence of the spin-phonon coupling lies in a change of the width and position of the spin-wave resonance. Comparison is made with previous work.Research assistant of the Belgian National Fund for Scientific Research (N.F.W.O.)     

The dynamics of a generalized Heisenberg ferromagnetic spin chain

We introduce a generalized Heisenberg ferromagnetic spin chain with a four-dimensional target space, and investigate its continuous limit, the generalized continuous Heisenberg model (GCHM). We reduce the dynamics of the GCHM to a nonlinear evolution of space curves in four dimensions. The space curve evolution is expressed in terms of a system of coupled nonlinear equations for the three curvatures, k(1)(x),k(2)(x),k(3)(x), of a curve in R(4). Applying the Painleve analysis to the stationary equations, we conclude that GCHM, in general, is not integrable, unless k(1) is constant. We obtain explicit solutions of the resulting stationary system under the latter condition. (c) 1995 American Institute of Physics.     

NMR study of spin dynamics in disordered Heisenberg paramagnets

The spin dynamics in the disordered paramagnets KMg_1?xMn_xF₃ and Zn_1?xMn_xF₂ were investigated using the F¹⁹ NMR. Linewidths and relaxation rates of those distinct classes of F¹⁹ nuclei that have different numbers of nearest neighbor magnetic ions were obtained as a function of concentration x. The particular importance of the isolated magnetic ion to the low frequency spectral density was established.     

Intrinsic localized modes of a classical discrete anisotropic Heisenberg ferromagnetic spin chain

We report several exact intrinsic localized mode solutions of the classical spin evolution equation of a one-dimensional anisotropic Heisenberg ferromagnetic spin chain in terms of Jacobian elliptic functions. These include one, two and three spin excitations. All these solutions have smooth anticontinuum limits. Their linear stability and semiclassical quantization are also discussed briefly.     

Spin-ordering in S = 1 anisotropic Heisenberg models with nondiagonal spin exchange

The properties of S = 1 anisotropic Heisenberg models with nondiagonal exchange between axial and planar spin components are investigated using Monte Carlo techniques. The quantum nature is taken into account in a semi-classical approximation. The ordering of the spins when applying an external field with axial and planar components is discussed. It is argued that the quantum nature of the spins and the nondiagonal exchange may explain the peculiar shape of the magnetic specific heat of FeBr₂ as well as the weakly first-order phase transition observed in the same compound when a tilted field is applied. Received 24 January 1999     

Dynamic and static excitations of a classical discrete anisotropic Heisenberg ferromagnetic spin chain

Using Jacobi elliptic function addition formulas and summation identities we obtain several static and moving periodic soliton solutions of a classical anisotropic, discrete Heisenberg spin chain with and without an external magnetic field. We predict the dispersion relations of these nonlinear excitations and contrast them with that of magnons and relate these findings to the materials realized by a discrete spin chain. As limiting cases, we discuss different forms of domain wall structures and their properties.     

High-temperature dynamics of the anisotropic Heisenberg chain studied by moment methods

The dynamics of the anisotropic spin-1/2 nearest-neighbour Heisenberg chain is studied at infinite temperature. Low-order coefficients of the short-time expansions are computed for spin-spin and energy-density-energy-density correlation functions for cyclical as well as for open-ended chains. The commutator algebra necessary to generate these coefficients may be performed by a computer. The series obtained for the spin correlation function (up to ordert ¹⁴ for a bulk spin and up to ordert ¹⁸ for a boundary spin) and for the energy density correlation function are the longest ones available up to now. The coefficients are used to construct rigorous upper and lower bounds to autocorrelation functions and near-neighbour correlation functions.     

The dynamics of a central spin in quantum Heisenberg XY model

In the thermodynamic limit, we present an exact calculation of the time dynamics of a central spin coupling with its environment at finite temperatures. The interactions belong to the Heisenberg XY type. The case of an environment with finite number of spins is also discussed. To get the reduced density matrix, we use a novel operator technique which is mathematically simple and physically clear, and allows us to treat systems and environments that could all be strongly coupled mutually and internally. The expectation value of the central spin and the von Neumann entropy are obtained.     

Dynamical spin structure factor for the anisotropic spin-1/2 Heisenberg chain

The longitudinal spin structure factor for the XXZ-chain at small wave vector q is obtained using Bethe ansatz, field theory methods, and the density matrix renormalization group. It consists of a peak with a peculiar, non-Lorentzian shape and a high-frequency tail. We show that the width of the peak is proportional to q2 for finite magnetic field compared to q3 for a zero field. For the tail we derive an analytic formula without any adjustable parameters and demonstrate that the integrability of the model directly affects the line shape.     

Three-qubit海森堡XYZ各向异性自旋链系统QMA熵不确定度的量子调控

本文研究了three-qubit海森堡XYZ各向异性自旋链系统各项参数和外加磁场对量子存储支撑(Quantum memory assisted,QMA)熵不确定度及其下限的调控行为,以及对被测系统与存储系统之间量子纠缠的影响.结果表明:增大系统的Dzyaloshinski-Moriya(DM)相互作用强度、降低系统温度、增大沿Z正方向磁场强度可以提高被测系统与存储系统之间的量子纠缠,降低系统QMA熵不确定度及其下限,被测系统与存储系统之间的量子纠缠与QMA熵不确定度及其下限呈类反相关.     

Replica symmetry breaking transition of the weakly anisotropic Heisenberg spin glass in magnetic fields

The spin and the chirality orderings of the three-dimensional Heisenberg spin glass with the weak random anisotropy are studied under applied magnetic fields by equilibrium Monte Carlo simulations. A replica symmetry breaking transition occurs in the chiral sector accompanied by the simultaneous spin-glass order. The ordering behavior differs significantly from that of the Ising spin glass, despite the similarity in the global symmetry. Our observation is consistent with the spin-chirality decoupling-recoupling scenario of a spin-glass transition.     

Finite temperature dynamics of vortices in the two dimensional anisotropic Heisenberg model

We study the effects of finite temperature on the dynamics of non-planar vortices in the classical, two-dimensional anisotropic Heisenberg model with XY- or easy-plane symmetry. To this end, we analyze a generalized Landau-Lifshitz equation including additive white noise and Gilbert damping. Using a collective variable theory with no adjustable parameters we derive an equation of motion for the vortices with stochastic forces which are shown to represent white noise with an effective diffusion constant linearly dependent on temperature. We solve these stochastic equations of motion by means of a Green's function formalism and obtain the mean vortex trajectory and its variance. We find a non-standard time dependence for the variance of the components perpendicular to the driving force. We compare the analytical results with Langevin dynamics simulations and find a good agreement up to temperatures of the order of 25% of the Kosterlitz-Thouless transition temperature. Finally, we discuss the reasons why our approach is not appropriate for higher temperatures as well as the discreteness effects observed in the numerical simulations. Received: 27 April 1998 / Revised: 2 September 1998 / Accepted: 10 September 1998     

The quantum correlation of anisotropic Heisenberg spin chain under the control of inhomogeneous magnetic field

The quantum correlation (quantum entanglement and quantum discord)’s dynamical behavior characteristics of Heisenberg XXZ spin chain with Dzyaloshinskii–Moriya (DM) interaction heterogeneous magnetic field that manipulated by sinusoidal wave are investigated in this paper. The results indicate that quantum correlation of anisotropic Heisenberg XXZ spin chain can be regulated effectively by magnetic field intensity B and magnetic field uniformity cos θ   of the external magnetic field. Under the effects of DM interaction in qubits, the quantum correlation's dynamics evolution process appears sudden death and birth. But DM interaction has a critical value D_CZ$D_{CZ}$ which is connected with other quantum correlation versus parameters. Only when Dz≥D_CZ$D_z\ge D_{CZ}$, sudden death and birth can be obviously observed under the rest given parameters.