Time-dependent spin-correlation functions of the two-dimensional finite (3×3=9) isotropic Heisenberg model (spin=1/2) have been calculated at infinite temperatures. The numerical results thus obtained are similar to those of the linear Heisenberg chain.
The expansion coefficients in powers of time of the two-time spin-pair correlation function are obtained up to terms of order t8 for the Heisenberg magnet of spin ½ at infinite temperature for the square and cubic lattices and up to terms of order t10 for the linear chain. The result is applicable to the isotropic as well as anisotropic Heisenberg magnet where the exchange integrals in the z direction and in the orthogonal plane are assumed to be different. Analysis of the result is left to future work.
We have studied the dynamics of the classical Heisenberg chain at finite and infinite temperatures by computer simulation of an array of 4000 spins. The evaluation of the spin-spin and energy-energy wave-vector- and time-dependent correlation functions out to time $10{J}^{\ensuremath{-}1}$ made it possible to determine the spin and energy diffusion constants. At infinite temperature we find ${D}_{S}=(1.33\ifmmode\pm\else\textpm\fi{}0.10)JS{a}^{2}$ and ${D}_{E}=(3\ifmmode\pm\else\textpm\fi{}1)JS{a}^{2}$. Values of DS have also been determined, with less precision, for ferromagnetic and antiferromagnetic interactions at temperatures ∣J∣kBT=1.0and0.5. Our results are compared with various theoretical estimates. In addition, we calculate the time-dependent spin and energy self-correlation functions.
Numerical calculations on two-dimensional spin-1/2 XXZ Heisenberg models are presented. The computational technique entails solving the time-dependent Schr\"odinger equation. The time propagation of wave functions is performed employing Trotter-Suzuki product formulas. The algorithms are numerically stable and are used to study models involving \ensuremath{\approxeq}107 states. With this approach both the thermodynamic properties as well as the many-particle dynamics can be obtained. Densities of states and zero-temperature dynamic magnetic structure factors are presented. For the isotropic Heisenberg antiferromagnet thermodynamic properties are computed for lattices up to 26 sites.
Numerical calculations on two-dimensional spin-½ XXZ Heisenberg models are presented. The computational technique entails solving the time-dependent Schrödinger equation. The time propagation of wave functions is performed employing Trotter-Suzuki product formulas. The algorithms are numerically stable and are used to study models involving ≈10^7 states. With this approach both the thermodynamic properties as well as the many-particle dynamics can be obtained. Densities of states and zero-temperature dynamic magnetic structure factors are presented. For the isotropic Heisenberg antiferromagnet thermodynamic properties are computed for lattices up to 26 sites.
Numerical calculations have been performed to obtain the exact infinite-temperature time-dependent spin autocorrelation function G(t) for a linear chain of N spins (S=21) interacting by nearest-neighbor Heisenberg exchange for N=5,7,9and11 by a method different from that of Carboni and Richards. Exact results for the first 20 moments and estimates for M22 to M30 of the frequency autocorrelation function for the infinite chain are provided. Excellent agreement is obtained with some recent results obtained by Morita, who, however, gives terms only up to M10. G(t) does not show a simple monotonic diffusive ($\ensuremath{\sim}{t}^{\ensuremath{-}\frac{1}{2}}$) behavior within $\frac{4\ensuremath{\hbar}}{J}$, the time domain up to which we believe our result for N=11 to be equivalent to that of the infinite chain although it is reduced to about 10% of its value at t=0.