  Magnetization plateaux emerging in quantum spin systems due to spontaneously
breaking of translational symmetry have been reported both theoretically and
experimentally. The broken symmetry can induce reconstruction of elementary
excitations such as Goldstone and Higgs modes, whereas its microscopic
mechanism and reconstructed quasi-particle in magnetization-plateau phases have
remained unclear so far. Here we theoretically study magnetic excitations in
the magnetization-plateau phases of a frustrated spin ladder by using dynamical
density-matrix renormalization-group method. Additionally, analytical
approaches with perturbation theory are performed to obtain intuitive view of
magnetic excitations. Comparison between numerical and analytical results
indicates the presence of a reconstructed quasi-particle originating from
spontaneously broken translational symmetry, which is realized as a collective
mode of spin trimer called trimeron.
