Magnetic excitations and structural change in the S=1/2 quasi-one-dimensional magnet Sr_14-xY_xCu₂₄O₄₁ (0<~x<~1)

Neutron-scattering measurements have been performed on the S=1/2 quasi-one-dimensional system Sr_14-xY_xCu₂₄O₄₁, which has both simple chains and two-leg ladders of copper ions. We observed that when a small amount of yttrium is substituted for strontium, which is expected to reduce the number of holes, the dimerized state and the structure in the chain are changed drastically. The inelastic peaks originating from the dimerized state of the chain become broader in energy but not in momentum space. This implies that the dimerized state becomes unstable but the spin correlations are unchanged with yttrium substitution. Furthermore, it was observed that nuclear-Bragg-peak intensities originating from the chain show strong temperature and x dependence, which suggests that the chains slide along the c axis as temperature and x are varied.