Possible localized modes in the uniform quantum Heisenberg chains of Sr 2 CuO 3

A model of mobile bond defects is tentatively proposed to analyze the ``anomalies'' observed on the NMR spectrum of the quantum Heisenberg chains of ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}.$ A bond defect is a local change in the exchange coupling. It results in a local alternating magnetization (LAM), which, when the defect moves, creates 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 form a periodic structure, which extends over almost all the chains. In that regime, the density of bond defects decreases linearly with T.