Dynamical structure factors ofS=1bond-alternating Heisenberg chains

Abstract

We calculate the dynamical structural factor of the S=1 bond-alternating Heisenberg chain. In the Haldane phase, the lowest excited states form the lower edge of the multimagnon continuum in $0 \leq q \leq q_c$ and the one-magnon mode in $q_c \leq q \leq \pi$. As the system approaches the gapless point, $q_c$ shifts towards $q=\pi$ and the largest integrated intensity of the one-magnon mode is decreased. In the singlet-dimer phase, the one-magnon mode appears in $0 \leq q \leq q_c$. As the bond-alternation becomes strong, $q_c$ shifts towards $q=\pi$. In the antiferromagnetic-ferromagnetic bond-alternation region with a strong ferromagnetic coupling, the lowest excited states form the lower edge of the multimagnon continuum in $0 \leq q \leq 0.2\pi$ and $0.8\pi \leq q \leq \pi$, and the one-magnon mode appears in $0.2\pi<q<0.8\pi$. The largest integrated intensity of the one-magnon mode is 93%, which is slightly smaller than that in the S=1 Haldane-gap system. We further discuss the dynamical structural factor in connection with the inelastic neutron-scattering experiments.