First-order quantum phase transition in the orthogonal-dimer spin chain

We investigate the low-energy properties of the orthogonal-dimer spin chain characterized by a frustrated dimer-plaquette structure. When the competin g antiferromagnetic couplings are varied, the first-order quantum phase tra nsition occurs between the dimer and the plaquette phases, which is accompa nied by nontrivial features due to frustration. besides the discontinuity i n the lowest excitation gap at the transition point, a sharp level crossing occurs for the spectrum in the plaquette phase. We further reveal that the plateau in the magnetization curve at 1/4 of the full moment dramatically changes its character in the vicinity of the critical point. It is argued t hat the first-order phase transition in this system captures some essential properties found in the two-dimensional orthogonal-dimer model proposed fo r SrCu2(BO3)(2).