ESR study on the excited state energy spectrum of SrCu2(BO3)2: A central role of multiple-triplet bound states

• Published in: Journal of the Physical Society of Japan 2003-12, Vol.72 (12), p.3243-3253
• Main Authors: NOJIRI, Hiroyuki, KAGEYAMA, Hiroshi, UEDA, Yutaka, MOTOKAWA, Mitsuhiro
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Electron paramagnetic resonance and relaxation; Exact sciences and technology; Magnetic properties and materials; Magnetic resonances and relaxations in condensed matter, mössbauer effect; Magnetically ordered materials: other intrinsic properties; Physics; Spin waves
• ISSN: 0031-9015; 1347-4073
• DOI: 10.1143/JPSJ.72.3243

The excited state energy spectrum of a two-dimensional dimer system SrCu2(BO3)2 has been investigated by high field ESR. Besides two non-degenerated one-triplet excitations, various types of multiple-triplet bound states are identified: singlet, triplet, and quintet. The intensity of these bound states is extremely strong because one-triplet process is strongly suppressed in the orthogonal dimer system. The 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 the two-triplet bound state splits due to distant neighbor interactions. The bound quintet state 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, namely, the quintet touches first to the ground singlet state. The present investigation exhibits an essential role of multiple-triplet bound states in the magnetic excitation spectrum.