Magnetic excitations from the singlet ground state in the S=1/2 quasi-one-dimensional system Sr_14-xY_xCu₂₄O₄₁

Inelastic 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. Strong magnetic inelastic peaks, which originate from the simple chains, have been observed at 9–14 meV in Sr₁₄Cu₂₄O₄₁. These spin gaps have been confirmed to originate from a dimerized state of the chain. The dimers are formed between spins which are separated by 2 and 4 times the distance between the nearest-neighbor copper ions in the simple chain. In addition, a weaker magnetic inelastic peak around 11 meV, which originates from a dimerized state in the ladder, has also been observed. These dimers are formed between the nearest-neighbor copper ions which are connected by the interladder coupling. We have also studied the effect of Y³⁺ substitution for Sr²⁺ site on the dimerized states. It was found that the yttrium substitution suppresses the gap energies drastically. Possible origins of the dimerized ground state and the excitations are discussed. © 1996 The American Physical Society.