Elementary excitations of S=1/2 one-dimensional antiferromagnet KCuGaF6 in magnetic field and quantum sine-Gordon model

Elementary excitations of the S=1/2 one-dimensional Heisenberg antiferromagnet KCuGaF${}_6$ with exchange constant $J\mathrm{/}{k}_B$=103 K were investigated by high-frequency ESR measurements combined with a pulsed high magnetic field. When an external magnetic field H is applied in KCuGaF${}_6$, 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, KCuGaF${}_6$ 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 $c_s=h\mathrm{/}H$. To investigate the relationship between the Curie term due to the DM interaction and the proportional coefficient $c_s$, magnetic susceptibility measurements were also performed varying the external field direction.