Dynamical Structure Factors of the S=1/2 Bond-Alternating Spin Chain   with a Next-Nearest-Neighbor Interaction in Magnetic Fields
  The dynamical structure factor of the S=1/2 bond-alternating spin chain with a next-nearest-neighbor interaction in magnetic field is investigated using the continued fraction method based on the Lanczos algorithm. When the plateau exists on the magnetization curve, the longitudinal dynamical structure factor shows a large intensity with a periodic dispersion relation, while the transverse one shows a large intensity with an almost dispersionless mode. The periodicity and the amplitude of the dispersion relation in the longitudinal dynamical structure factor are sensitive to the coupling constants. The dynamical structure factor of the S=1/2 two-leg ladder in magnetic field is also calculated in the strong interchain-coupling regime.   The dynamical structure factor shows gapless or gapful behavior depending on the wave vector along the rung. 
Mamoru Usami and Sei-ichiro Suga (Osaka University)