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

The ground state and low lying excitation of the spin 1/2 Heisenberg ladder with antiferromagnetic leg (J) and ferromagnetic rung (−λJ, λ> 0) interaction is studied by means of the density matrix renormalization group method. It is found that the state remains in the Haldane phase even for small λ ∼ 0.02 suggesting the continuous transition to the gapless phase at λ = 0. The critical behavior for small λ is studied by the finite size scaling analysis. The result is consistent with the recent field theoretical prediction.