Translational Symmetry Broken Magnetization Plateau of the S=1/2 Spin Ladder and the Bond-Alternating Chain with the anisotropy

The magnetization plateau is one of interesing phenomena in the field of the condensed matter physics. It was proposed as the spin gap induced by the external magnetic field[1]. According to the rigorous theorem derived from the Lieb-Schultz-Mattis one, the necessary condition of the magnetization plateau is the relation Q(S-m)=integer, where S and m are the total spin and the magnetization per unit cell, respectively, and Q is the periodicity of the wave function. The numerical diagonalization of finite-size clusters and the size scaling analysis[2,3] indicated that the spin-3/2 antiferromagnetic chain with the single-ion anisotropy exhibits the 1/3 magnetization plateau with Q=1. Recent numerical diagonalization and the level spectroscopy analyes indicated that the translational symmetry broken magnetization plateau with Q=2 would appear with two competing anisotropies in several one-dimentional systems; the S=1, S=3/2 and S=2 antiferromagnetic chains[4-6], the S=1/2 ferromagnetic and antiferromagnetic bond-alternating chain[7] and the spin ladder[8]. In the present study using the same analyses we found that even a single anisotropy can induce the Q=2 magnetization plateau in the S=1/2 spin ladder and the bond-alternating chain. The phase diagrams at half the saturation magnetization and the zero-filed ground state are presented.

[1]M. Oshikawa, M. Yamanaka and I. Affleck, Phys. Rev. Lett. 78, 1984 (1997).