Quasi-spin correlations in a frustrated quantum spin ladder

Sugimoto, Takanori*; Mori, Michiyasu  ; Toyama, Takami*; Maekawa, Sadamichi

The quasi-spin correlations in a frustrated quantum spin ladder with one-half magnetization are theoretically studied by using the density-matrix renormalization-group method and the quasi- spin transformation. In this model, the frustration induces a gapless-to-gapful phase transition with a strong rung coupling. The gapful state is observed as the one-half magnetization plateau in the magnetization curve. In the magnetization-plateau state, we find that the quasi-spin dimer has a large expectation value with long-ranged correlations. This result does not only comes in useful to clarify the magnetization-plateau state, but gives a crucial information to understand the magnetization curve of the real compound BiCuPO, whose effective spin model corresponds to ours.