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1. First Direct Observation of Two-Magnon Bound States in Spin-1 Anisotropic Chains in High Magnetic FieldsS.A. Zvyagin, Dresden High Magnetic Field Laboratory (HLD/FZD) in collaboration withJ. Wosnitza, Dresden High Magnetic Field Laboratory (HLD/FZD) C.D. Batista, LANL Los Alamos V.S. Zapf, M. Jaime, NHMFL Los Alamos,J. Krzystek, NHMFL Tallahassee A. Paduan-Filho, Universidade de Sao Paulo, Brazil What happens with the excitation spectrum in an exchange-coupled spin-1 system if an applied magnetic field is so strong that its magnetization gets fully saturated? In addition to ordinary single-magnon states and two-magnon continuum in spin-1 quantum chains with a large single-ion anisotropy, in the fully spin-polarized phase the theory predicts the existence of so-called two-magnon bound states. These states were postulated as long ago as in 1970, and have since attracted a great deal of attention due to its potential relevance to the intrinsic localized spin modes in anisotropic magnetic media. Using the tunable-frequency sub-mm wavelength electron spin resonance (ESR) technique, we have obtained for the first time a direct signature of the two-magnon bound states. The experiments have been performed on the organic S = 1 chain antiferromagnet NiCl2-4SC(NH2)2 (known as DTN). The parameters of the two-magnon bound states were calculated analytically using results of previous thermodynamic and neutron scattering measurements. Excellent agreement between the theory and experiment was obtained. Figure: (top) ESR spectra of DTN taken at T = 1.6 K and various frequencies; (bottom) frequency vs. field dependence of different excitation modes in DTN. The two-magnon bound-state excitations correspond to ESR modes E and F. Published in: S.A. Zvyagin, J. Wosnitza, C. D. Batista, M. Tsukamoto, N. Kawashima, J. Krzystek, V.S. Zapf, M. Jaime, N. F. Oliveira, Jr., A. Paduan-Filho, Phys. Rev. Lett. 98, 047205, 2007.