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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 YBa2Cu3O6.5 and on the spin-chain compound Sr2CuO3. Here we analytically calculate NMR spectra, compare with the experimental data, and give a simple criterion to determine the impurity concentration. 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 calculations. |
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