Magnetization plateau in the spin ladder with alternating rung exchange

Abstract

We have studied the ground-state phase diagram of a spin ladder with alternating rungexchange in a magnetic filed, in the limit where the rung coupling is dominant. In this limit the model is mappedonto an XXZ Heisenberg chain in uniform and staggered longitudinal magnetic fields, where the amplitude of the staggeredfield is ∼δ. We have shown that the magnetization curve of the system exhibits a plateau atmagnetization equal to half of the saturation value. The width of a plateau scales asδν,where ν = 4/5 in the case of a ladder with isotropic antiferromagnetic legs andν = 2 in the case of a ladder with isotropic ferromagnetic legs. We have calculated four critical fields(Hc1 ± and Hc2 ± ) corresponding to transitions between different magnetic phases of the system.We have shown that these transitions belong to the universality class of thecommensurate–incommensurate transition. The possibility for the realization of a new typeof spin-Peierls instability, characterized by the spontaneous appearance of an alternatingrung exchange in a uniform magnetic field and at a system magnetization equal to the halfof its saturation value, is briefly discussed.