A Novel Plateau in the Magnetization Curve of the Alternating Heisenberg Chain

At zero temperature, the effect of a uniform magnetic field on an alternating antiferromagnetic-ferromagnetic (AF-F) spin-1/2 chain is studied using the numerical Lanczos method. The alternating model is defined as an alternating AF-F Heisenberg chain with the space-modulated dominant antiferromagnetic exchange. A detailed numerical analysis of the low energy excitation spectrum is presented. The energy gap, the magnetization and the string order parameter of the system are calculated for finite chains. It is found that the space modulation on the antiferromagnetic exchange induced two new gaps in the spectrum. In these new gapped regions, the magnetization shows two nonzero plateaus at the values 1/3 and 2/3 of the saturation magnetization.