1/3 magnetization plateau in SrCu2(BO3)2 -: Stripe order of excited triplets

High-field magnetization measurements up to 57 T have been performed at 0.08 K in a single crystal of a two-dimensional spin-gap material SrCu2(BO3)(2). We successfully observed the predicted plateau at 1/3 of the total magnetization around 50 T, in which the magnetic super structure is characterized by a novel stripe order of triplets. The 1/3 plateau is much wider than the previously observed 1/4 and 1/8 plateaux.