MAGNETIC EXCITATIONS AND STRUCTURAL-CHANGE IN THE S=1 2 QUASI-ONE-DIMENSIONAL MAGNET/

Neutron-scattering measurements have been performed on the S=1/2 quasi -one-dimensional system Sr14-xYxCu24O41, which has both simple chains and two-leg ladders of copper ions. We observed that when a small amou nt of yttrium is substituted for strontium, which is expected to reduc e the number of holes, the dimerized state and the structure in the ch ain are changed drastically. The inelastic peaks originating from the dimerized state of the chain become broader in energy but not in momen tum space. This implies that the dimerized state becomes unstable but the spin correlations are unchanged with yttrium substitution. Further more. it was observed that nuclear-Bragg-peak intensities originating from the chain show strong temperature and x dependence, which suggest s that the chains slide along the c axis as temperature and x are vari ed.