Microscopic origin of dimerization in the CuO₂ chains in Sr₁₄Cu₂₄O₄₁

The interplay between structure and magnetic properties of Sr₁₄Cu₂₄O_41+δ with CuO₂ chain and Cu₂O₃ ladder building blocks is studied as a function of oxygen nonstoichiometry. The characteristic decrease in the magnetic susceptibility below 80 K for δ≊0 disappears both with increasing and decreasing δ, and, correspondingly, the periodicity of the superstructure, which arises from the lattice mismatch along the chain direction between the two structure blocks, shows a significant change. The microscopic origin of the singlet ground state is suggested to be the localization of electrons at low temperature in dimers on structurally modulated CuO₂ chains. © 1996 The American Physical Society.