Motivated by the homometallic intertwining double-chain ferrimagnets A_3_Cu_3_(PO_4_)_4_ (A=Ca,Sr,Pb), we investigate the low-energy structure of their model Hamiltonian H=\sum_n_[J_1_(S_{ n :1}_+S_{ n :3}_) +J_2_(S_{ n+1:1}+S_{ n-1:3}_)]\cdotS_{ n:2}_, where S_{ n:l}_ stands for the Cu^{ 2+}^ ion spin labeled l in the nth trimer unit, with particular emphasis on the range of bond alternation 0<J_2/J_1<1. Although the spin-wave theory, whether up to O(S^1^) or up to O(S^0^), claims that there exists a flat band in the excitation spectrum regardless of bond alternation, a perturbational treatment as well as the exact diagonalization of the Hamiltonian reveals its weak but nonvanishing momentum dispersion unless J_2_=J_1_ or J_2_=0. Quantum Monte Carlo calculations of the static structure factor further convince us of the low-lying excitation mechanism, elucidating similarities and differences between the present system and alternating-spin linear-chain ferrimagnets.
Categories: cond-mat.str-el cond-mat.stat-mech