We show how to generalize the zero-temperature Lanczos method for calculating dynamical correlation functions to finite temperatures. The key is the microcanonical ensemble which allows us to replace the involved canonical ensemble with a single appropriately chosen state; in the thermodynamic limit it provides the same physics as the canonical ensemble but with the evaluation of a single expectation value. We can employ the same system sizes as for zero temperature but, whereas the statistical fluctuations present in small systems are prohibitive, the spectra of the largest system sizes are surprisingly smooth. We investigate, as a test case, the spin conductivity of the spin-1/2 anisotropic Heisenberg model and in particular we present a comparison of spectra obtained by the canonical and microcanonical ensemble methods.

We present a numerical study on the spin and thermal conductivities of the spin-1 Heisenberg chain in the high temperature limit, in particular of the Drude weight contribution and frequency dependence. We use the Exact Diagonalization and the recently developed microcanonical Lanczos method; it allows us a finite size scaling analysis by the study of significantly larger lattices. This work, pointing to a diffusive rather than ballistic behavior is discussed with respect to other recent theoretical and experimental studies. Introduction.-Recently, numerous experiments on quasi-one dimensional (1D) spin-1/2 compounds [1, 2, 3, 4, 5] have confirmed highly anisotropic thermal transport along the direction of the magnetic chains and a large contribution to the thermal conductivity due to the magnetic interactions. This is in agreement with early theoretical proposals [6,7] of ballistic transport in spin-1/2 Heisenberg antiferromagnetic chains (HAFM), that was recently related to the integrability of this system [8,9,10,11]. These developments promoted the theoretical study of several models, as spin-1/2 frustrated chains, ladders and higher spin systems, using numerical methods [12,13,14] or low energy effective theories [15,16,17,18,19].

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 a 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 a 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.

We present a study of the one-dimensional S = 1 antiferromagnetic spin chain with large easy plane anisotropy, with special emphasis on field-induced quantum phase transitions. Temperature and magnetic field dependence of magnetization, specific heat, and thermal conductivity is presented using a combination of numerical methods. In addition, the original S = 1 model is mapped into the low-energy effective S = 1/2 XXZ Heisenberg chain, a model which is exactly solvable using the Bethe ansatz technique. The effectiveness of the mapping is explored, and we show that all considered quantities are in qualitative, and in some cases quantitative, agreement. The thermal conductivity of the considered S = 1 model is found to be strongly influenced by the underlying effective description. Furthermore, we elucidate the low-lying electron spin resonance spectrum, based on a semi-analytical Bethe ansatz calculation of the effective S = 1/2 model.

A special limit of an antiferromagnetic XY Z chain was recently shown to exhibit interesting bulk as well as surface spin-flop transitions at T = 0. Here we provide a complete calculation of the thermodynamics of the bulk transition using a transfer-matrix-renormalization-group (TMRG) method that addresses directly the thermodynamic limit of quantum spin chains. We also shed some light on certain spinwave anomalies at low temperature predicted earlier by Johnson and Bonner. 75.10.Jm, 75.30.Kz