No abstract

No abstract

Experimental data for the magnetic susceptibility and magnetostriction of CuGeO 3 are analyzed within a one-dimensional antiferromagnetic model with nearest (J 1 ) and next-nearest neighbour interactions (J 2 ). We show that the ratio of the exchange constants in the antiferromagnetic chains of CuGeO 3 amounts to J 2 /J 1 = 0.354(0.01), i.e. it is significantly larger than the critical value for the formation of a spontaneous gap in the magnetic excitation spectrum without lattice dimerization. The susceptibility data are reproduced by our numerical results over the temperature range from 20K to 950K to a high degree of accuracy for J 1 = 80.2(3.0) and J 2 = 28.4(1.8). The pressure dependence of the exchange constants is estimated from magnetostriction data. Furthermore, the specific heat data are checked on consistency against the calculated entropy of the above model.

All eigenstates and eigenvalues are determined for the spin-1 2 XXZ chain Hϭ2J͚ i (S i x S iϩ1for rings with up to Nϭ16 spins, for anisotropies ⌬ϭ0, cos(0.3), and 1. The dynamic spin pair correlations ͗S lϩn (t)S l ͘, (ϭx,z), the dynamic structure factors S (q,), and the intermediate structure factors I (q,t) are calculated for arbitrary temperature T. It is found that for all T, S z (q,) is mainly concentrated on the region ͉͉Ͻ 2 (q), where 2 (q) is the upper boundary of the two-spinon continuum, although excited states corresponding to a much broader frequency spectrum contribute. This is also true for the Haldane-Shastry model and the frustrated Heisenberg model. The intermediate structure factors I (q,t) for ⌬ 0 show exponential decay for high T and large q. Within the accessible time range, the time-dependent spin-correlation functions do not display the long-time signatures of spin diffusion.Approximate analytic expressions for the Tϭ0 dynamic structure factors were conjectured, which take into account known sum rules as well as exact results for the case ⌬ϭ0 and asymptotic results ͑2.5͒ for small q and .

No abstract

We consider the anisotropic Heisenberg spin-1/2 chain in a transverse magnetic field at zero temperature. We first determine all components of the dynamical structure factor by combining exact results with a mean-field approximation recently proposed by Dmitriev et al. ͓JETP 95, 538 ͑2002͔͒. We then turn to the small anisotropy limit, in which we use field theory methods to obtain exact results. We discuss the relevance of our results to neutron scattering experiments on the one-dimensional Heisenberg chain compound Cs 2 CoCl 4 .

No abstract

No abstract