Possible localized modes in the uniform quantum Heisenberg chains of Sr2CuO3

A model of mobile bond defects is tentatively proposed to analyze the "anom alies" observed on the NMR spectrum of the quantum Heisenberg chains of Sr2 CuO3. A bond defect is a local change in the exchange coupling. It results in a local alternating magnetization (LAM), which, when the defect moves, c reates a flipping process of the local field seen by each nuclear spin. At low temperature, when the overlap of the LAM becomes large, the defects for m a periodic structure, which extends over almost all the chains. In that r egime, the density of bond defects decreases linearly with T.