  Elementary excitations of an S = 1/2 antiferromagnetic Heisenberg chain
KCuGaF6 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.
