NUMERICAL STUDY ON THE GROUND-STATE PHASE-DIAGRAM OF THE S=1 2 XXZ LADDER MODEL/

The ground-state phase diagram of the S=1/2 XXZ ladder model is invest igated by using the density-matrix renormalization-group method. This model is composed of two S=1/2 XXZ chains with ferromagnetic rung coup lings (lambda) and intra-chain anisotropy (Delta), and continuously ch anges from the two independent S=1/2 XXZ chains at lambda=0 to the S=1 XXZ chain in the limit of lambda-->infinity. it is shown that there e xist edge states at Delta=1 and lambda=1 in relation to the four-fold degeneracy of the ground state and that an excitation gap opens nearly at lambda=0 for 0.8 less than or equal to Delta less than or equal to 1. In addition, it is clarified that the transition between the Halda ne phase and the antiferromagnetic phase occurs at lambda=1.15+/-0.01 for Delta=1. A possible phase diagram in the lambda-Delta plane is pro posed, which is consistent with the scaling analysis based on the boso nization method.