Elementary excitations of the S=1/2 one-dimensional Heisenberg antiferromagnet KCuGaF6 with exchange constant J/kB=103 K were investigated by high-frequency electron spin resonance (ESR) measurements combined with a pulsed high magnetic field. When an external magnetic field H is applied in KCuGaF6, a staggered magnetic field h is induced perpendicular to H owing to the staggered g tensor and the Dzyaloshinsky-Moriya (DM) interaction with an alternating D vector. Consequently, KCuGaF6 in a magnetic field is represented by the quantum sine-Gordon (SG) model. We observed many resonance modes including a soliton resonance, breathers, interbreather transitions, and two-breather excitation. Their resonance conditions are beautifully described by the quantum SG field theory with one adjustable parameter c_s=h/H. To investigate the relationship between the Curie term due to the DM interaction and the proportionality coefficient c_s, magnetic susceptibility measurements were also performed varying the external field direction.
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