High-field ESR on quantum spin systems

High-field electron spin resonance (ESR) experiments have been performed on quantum spin systems. A distinct held-induced crossover between the gap-le ss spinon regime and the gapped sine-Gordon regime is observed for the Heis enberg quantum spin chain; CuBenzoate. In a Shastry and Sutherland model co mpound SrCu2(BO3)(2), a set of multiple-triplet excitations has been resolv ed by utilizing high-resolution ESR. The splitting of the two-triplet bound state into a few discrete energy levels is considered as the evidence of t he extreme localization of the triplet excitations in this compound. The pr esent results indicate the unique and powerful features of ESR in the study of magnetic excitations. (C) 2001 Elsevier Science B.V. All rights reserve d.