Magnetization plateau in the S=1/2 spin ladder with alternating rung exchange

We have studied the ground-state phase diagram of a spin ladder with alternating rung exchange Jnperp=Jperp(1+(-1)nδ) in a magnetic filed, in the limit where the rung coupling is dominant. In this limit the model is mapped onto an XXZ Heisenberg chain in uniform and staggered longitudinal magnetic fields, where the amplitude of the staggered field is ∼δ. We have shown that the magnetization curve of the system exhibits a plateau at magnetization equal to half of the saturation value. The width of a plateau scales as δν, where ν = 4/5 in the case of a ladder with isotropic antiferromagnetic legs and ν = 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 the commensurate-incommensurate transition. The possibility for the realization of a new type of spin-Peierls instability, characterized by the spontaneous appearance of an alternating rung exchange in a uniform magnetic field and at a system magnetization equal to the half of its saturation value, is briefly discussed