Magnetic phase diagram of a spin S=1/2 antiferromagnetic two-leg ladder with modulated along legs Dzyaloshinskii-Moriya interaction

We study the ground-state magnetic phase diagram of a spin S = 1/2 antiferromagnetic two-leg ladder with period two lattice units modulated Dzyaloshinskii-Moriya (DM) interaction along the legs. We consider the case of collinear DM vectors and strong rung exchange and magnetic field. In this limit we map the initial ladder model onto an effective spin sigma = 1/2 XXZ chain and study the latter using the continuum-limit bosonization approach. We identified four quantum phase transitions and corresponding critical magnetic fields, which mark transitions from the spin gapped regimes into the gapless quantum spin-liquid regimes. In the gapped phases the magnetization curve of the system shows plateaus at magnetisation M = 0 and to its saturation value per rung M = M-sat = 1. We have shown that the very presence of alternating DM interaction leads to opening of a gap in the excitation spectrum at magnetization M = 0.5M(sat). The width of the magnetization plateau at M = 0.5M(sat), is determined by the associated with the dynamical generation of a gap in the spectrum is calculated and is shown that its length scales as (D0D1/J(2))(alpha) where D-0, D-1 are uniform and staggered components of the DM term, J is the intraleg exchange and alpha <= 3/4 and weakly depends on the DM couplings.