We investigate the dynamical structure factors of the S = 1/2 spin ladder systems with a diagonal interaction in the magnetization-plateau state. The results show characteristic behaviors depending on the chain interaction α, the ladder interaction 1 + δ and the diagonal interaction 1 - δ. The large weights are located at q = π, when δ is small. With increasing δ, the wavenumber of the large weights shifts towards q = π/2 taking the incommensurate value. The almost dispersionless behavior comes out at several combinations of α and δ, indicating that the elementary excitation is localized spatially. We discuss our results in comparison with the exact excitation spectrums of the S = 1/2 XXZ model which is an effective Hamiltonian of the present system in α ~ 0 and δ ~ 1. Dynamical structure factors of some S = 1/2 one-dimensional quantum spin systems in the magnetic fields were measured by inelastic neutron-scattering experiments1,2 and it has been shown the theoretical results from the numerical diagonalization of finite systems can explain them.3–5
Spin-chirality separation andS3symmetry breaking in the magnetization plateau of the quantum spin tube