Density Matrix Renormalization Group Study of the Spin 1/2 Heisenberg Ladder with Antiferromagnetic Legs and Ferromagnetic Rungs

摘要

The ground state and low lying excitation of the spin 1/2 Heisenberg ladderwith antiferromagnetic leg ($J$) and ferromagnetic rung ($-\lambda J, \lambda>0$) interaction is studied by means of the density matrix renormalizationgroup method. It is found that the state remains in the Haldane phase even forsmall $\lambda \sim 0.02$ suggesting the continuous transition to the gaplessphase at $\lambda = 0$. The critical behavior for small $\lambda$ is studied bythe finite size scaling analysis. The result is consistent with the recentfield theoretical prediction.