Abstract
Elementary excitations of an S=1/2 antiferromagnetic Heisenberg chain KCuGaF 6 were investigated through specific heat and electron spin resonance (ESR) measurements. In this compound, a staggered field is induced perpendicular to the external field because of the alternating g tensor and the Dzyaloshinsky–Moriya interaction with an alternating D vector. Such a spin system can be mapped onto the quantum sine-Gordon (SG) model, when subjected to the external magnetic field. Specific heat shows clear evidence of the field-induced gap, which is related to the elementary excitations, solitons and breathers, characteristic of the quantum SG model. q=0 excitations originated from solitons and breathers were directly observed by high-frequency high-field ESR. These experimental results are well described by the quantum SG field theory.
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