Numerical Study on the Ground-State Phase Diagram of the S=1/2 XXZ Ladder Model

Abstract

The ground-state phase diagram of the S=1/2 XXZ laddermodel is investigated by using the density-matrixrenormalization-group method. This model is composed of two S=1/2 XXZchains with ferromagnetic rung couplings (λ ) and intra-chainanisotropy (Δ), and continuously changes from the twoindependent S=1/2 XXZ chains at λ =0 to the S=1 XXZ chain in thelimit of λ → ∞ . It is shown that there existedge states at Δ=1 and λ =1 in relation to thefour-fold degeneracy of the ground state and that an excitation gapopens nearly at λ =0 for 0.8≤ Δ≤ 1. In addition, it is clarified that the transition between the Haldane phase and theantiferromagnetic phase occurs at λ =1.15 ± 0.01 forΔ=1. A possible phase diagram in the λ -Δplane is proposed, which is consistent with the scaling analysis basedon the bosonization method.