Low-energy excitations in a one-dimensional orthogonal dimer model with the Dzyaloshinski-Moriya interaction

Effects of the Dzyaloshinski-Moriya (DM) interaction on low-energy excitations in a one-dimensional orthogonal-dimer model are studied by using the perturbation expansions and the numerical diagonalization method. In the absence of the DM interaction, the triplet excitations show two flat spectra with three-fold degeneracy, which are lebeled by magnetization M=0, ±1. These spectra split into two branches with M=0 and with M=±1 by swithing-on of the DM interaction and besides the curvature appears in the triplet excitations with M=±1 more strongly than those of M=0.