Magnetic excitations from the singlet ground state in the S=1/2 quasi-one-dimensional system ${\mathrm{Sr}}_{14\mathrm{-}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Cu}}_{24}$${\mathrm{O}}_{41}$

Inelastic neutron scattering measurements have been performed on the S=1/2 quasi-one-dimensional system ${\mathrm{Sr}}_{14\mathrm{-}\mathit{x}}$ ${\mathrm{Y}}_{\mathit{x}}$ ${\mathrm{Cu}}_{24}$ ${\mathrm{O}}_{41}$, 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 ${\mathrm{Sr}}_{14}$ ${\mathrm{Cu}}_{24}$ ${\mathrm{O}}_{41}$. 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 ${\mathrm{Y}}^{3+}$ substitution for ${\mathrm{Sr}}^{2+}$ 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.