NMR relaxation rates for the spin-1/2 Heisenberg chain.

The spin-lattice relaxation rate 1/T1 and the spin echo decay rate 1/T2G for the spin2 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 Aq is strongly peaked around q = π the predicted lowtemperature behavior [1/T1 ∼ ln 1/2(1/T ), 1/T2G ∼ ln 1/2(1/T )/ √ T ] extends up to temperatures as high as T/J ≈ 0.5. If Aq has significant weight for q ≈ 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.