Millimeter wave ESR measurements in the frustrated antiferromagnetic spin chain system LiCu2O2

Millimeter wave ESR measurements were performed in high magnetic field on the frustrated antiferromagnetic spin chain system LiCu₂O₂. This system has been thought to be a dimer liquid state, which collapses into an antiferromagnetic order transition at 23 K. The absorptions in the ESR spectra were observed both in the dimer liquid state and the ordered state. The line width and resonance field of the ESR spectra in the ordered state shows monotonical temperature dependence, which may indicate the absence of the additional ordered state below 9 K proposed by the other group. The frequency dependence at 4 K were performed in the ordered state up to 220 GHz. Unusual features were realized which crosses the resonance field of the g=2. This feature differs from the one in the simple antiferromagnetic resonance.