Anomalous magnetization process in frustrated spin ladders

We study, at T=0, the anomalies in the magnetization curve of the S=1 two-leg ladder with frustrated interactions. We focus mainly on the existence of the $M=\text{Ms}\mathrm{/}2$ plateau, where $\text{Ms}$ is the saturation magnetization. We use analytical methods (degenerate perturbation theory and non-Abelian bosonization) as well as numerical methods (level spectroscopy and density matrix renormalization group), which lead to the consistent conclusion with each other. We also touch on the $M=\text{Ms}\mathrm{/}4$ and $M=(3\mathrm{/}4)\text{Ms}$ plateaux and cusps.