Magnetic excitations in magnetization plateaus of a frustrated spin ladder

Magnetization plateaus 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 quasiparticle 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 the dynamical density-matrix renormalization-group method. Additionally, analytical approaches with the perturbation theory are performed to obtain an intuitive view of magnetic excitations. A comparison between numerical and analytical results indicates the presence of a reconstructed quasiparticle originating from spontaneously broken translational symmetry, which is realized as a collective mode of the spin trimer called a trimeron.