Plaquette Singlet State of the Orthogonal Dimer System —Observation by High Pressure ESR Measurement in THz Region

Abstract

We have performed the high-pressure high-field ESR measurements on the orthogonal dimer spin system SrCu₂(BO₃)₂ in THz region. By observing its low-lying states directly, we succeeded in detecting the quantum phase transition from the dimer singlet state to the plaquette singlet state at P=1.85±0.05 GPa. We also succeeded in determining the pressure dependence of the intradimer and interdimer exchange interactions J and J′ precisely. The corresponding critical point was obtained to be J′/J=0.660±0.003.