NMR Response in quasi one-dimensional Spin-1/2 Antiferromagnets

Non-magnetic impurities break a quantum spin chain into finite segments and induce Friedel-like oscillations in the local susceptibility near the edges. The signature of these oscillations has been observed in Knight shift experiments on the high-temperature superconductor YBa$_2$Cu$_3$O$_{6.5}$ and on the spin-chain compound Sr$_2$CuO$_3$. Here we analytically calculate NMR spectra, compare with the available experimental data for Sr$_2$CuO$_3$, and show that the interchain coupling is responsible for the complicated and so far unexplained lineshape. Our results are based on a parameter-free formula for the local susceptibility of a finite spin chain obtained by bosonization which is checked by comparing with quantum Monte Carlo and density-matrix renormalization group calculations.