FINITE-TEMPERATURE DYNAMICS OF THE SPIN-HALF HEISENBERG CHAIN

We study the dynamic susceptibility %(q, omega) of the S = 1/2 Heisenb erg chain. Closed form analytic expression for %(q, omega) at low T mu ch less than J (J is exchange constant) is derived, which contains sub leading logarithmic corrections due to umklapp scattering processes be tween left and right moving excitations. These corrections lead to not iceable deviations from quantum-critical scaling, and can be observed in NMR and neutron scattering experiments. At higher temperatures, we extend the recursion method to finite T using high-temperature expansi ons, and also carry out quantum Monte-Carlo simulations. We find a gra dual transfer of spectral weight from diffusive (q similar to 0) modes at high T much greater than J to propagating antiferromagnetic (q sim ilar to pi) excitations at intermediate T similar to J. (C) 1998 Elsev ier Science B.V. All rights reserved.