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ESR study on the excited state energy spectrum of SrCu$_2$(BO$_3$)$_2$ -a central role of multiple-triplet bound states

H. Nojiri·H. Kageyama·Y. Ueda·M. Motokawa
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Abstract

The excited state energy spectrum of a two-dimensional dimer system SrCu$_2$(BO$_3$)$_2$ has been investigated by high field ESR. Various types of multiple-triplet bound states-excited singlet, triplet and quintet states are identified, besides two non-degenerated one-triplet excitations. Extremely strong intensity is found for these higher order bound states, which is an important consequence of the strong suppression of the one-triplet process due to the orthogonality of dimers. A new bound singlet state is found slightly below the energy gap of the one-triplet state, indicating the proximity to a quantum critical point. A band of two-triplet bound state splits into at least eight sharp and discrete levels, which indicates the contribution of distant neighbor interactions. A bound quintet state is observed, which is the direct evidence of two-triplet object made up of four $S=1/2$ spins. The energy of the lowest quintet becomes lower than that of the one-triplet excitation before the closing of the lowest spin gap. It indicates that the two-triplet bound state touches to the ground singlet state before the one-triplet state does. The present investigation exhibits an essential role of various multiple-triplet bound states in the magnetic excitation spectrum of SrCu$_2$(BO$_3$)$_2$.

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H. Nojiri, H. Kageyama, Y. Ueda, M. Motokawa. 2003-04-27. ESR study on the excited state energy spectrum of SrCu$_2$(BO$_3$)$_2$ -a central role of multiple-triplet bound states. https://doi.org/10.1143/jpsj.72.3243

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