NMR relaxation rates in spin-1/2 antiferromagnetic chains

We consider the low-temperature behavior of NMR relaxation rates $T_1$ and $T_{2G}$ in the spin-1/2 antiferromagnetic chain. We find that $T_1 \propto const$and$T_{2G} \propto \sqrt{T}$, with logarithmic corrections. We determine both constants and logarithmic terms by matching perturbative RG with exact results, so that the final expressions for the relaxation rates do not contain any free parameters. Our theoretical results are in excellent agreement with NMR experiments in Sr${}_2$CuO${}_3$.