NMR RELAXATION RATES FOR THE SPIN-1/2 HEISENBERG CHAIN

The spin-lattice relaxation rate 1/T-1 and the spin-echo decay rate 1/T-2G for the spin-1/2 antiferromagnetic Heisenberg chain are calculated using quantum Monte Carlo and maximum-entropy analytic continuation. The results are compared with recent analytical calculations by Sachdev. If the nuclear hyperfine form factor A(q) is strongly peaked around q=pi, the predicted low-temperature behavior [1/T-1 similar to In-1/2(1/T), 1/T(2G)similar to In-1/2(1/T)/root T] extends up to temperatures as high as T/J approximate to 0.5. If A(q) has significant weight for q approximate to 0 there are large contributions from diffusive long-wavelength processes not taken into account in the theory, and very low temperatures are needed in order to observe the asymptotic T-->0 forms.