Magnetization Plateaus and Magnetic Excitations in the S = 1/2 Spin Ladder Antiferromagnet NH4CuCl3

Field-induced magnetic phases are investigated in one-dimensional S = 1/2 generalized spin ladder systems having the lattice modulation along the leg with the wavevector π/2a, a being the lattice constant, using numerical calculations. We point out that there exist cases where the 1/4- as well as the 3/4-magnetization plateaus appear but the 1/2-plateau does not, for an appropriate set of the parameters. Triplet, excitations in the plateau regions are discussed in connection with the experimental result, which provides possible implications for the important role of the interaction between triplets.