Magnetization plateau in the S=1/2 Heinsenberg spin chain with next-nearest-neighbor and alternating nearest-neighbor interactions

The magnetization process of the S=1/2 Heisenberg chain with next-nearest-neighbor (NNN) and alternating nearest-neighbor interactions is investigated using the bosonization technique. The existence of the NNN interaction is shown to stabilize a "diluted" dimer order at magnetization m(z)=1/4, leading to a plateau, where two degenerate ground states appear. Effective Hamiltonians describing the low-energy physics in the vicinity of the m(z)=1/4 plateau are also presented. The analogy to metal-insulator transitions is discussed briefly.