Submillimeter-wave ESR study on quantum spin systems

Various types of quantum spin systems have been studied by submillimeter wa ve ESR technique. In SrCu2(BO3)(2), it has been observed that many kinds of "multiple-triplet" excitations appear and this feature has been explained as the extreme localization of the excited states in the Shastry and Suther land model. These localized excitation change to the spin-wave like mode, w hen the energy gap is collapsed in high magnetic field. For Cubenzoate, a d istinct anomaly was found in the temperature dependence of the line width. Absence of the three dimensional ordering indicates that the anomaly is cau sed by the change of the spin dynamics, and it has been discussed in term o f the "breather" picture recently proposed by Oshikawa and Affleck. The pre sent results highlight the important role of ESR for the investigation of m agnetic excitation in quantum spin systems, especially for high magnetic fi elds.