ESR Experiments on Quantum Spin Systems

Our recent work is reviewed with focus on unique features of the application of Electron Spin Resonance(ESR) technique for quantum spin systems, together with a brief survey of historical backgrounds and recent progress in ESR and model systems. A example is presented in order to illustrate ESR powerfulness in view of high sensitivity, high energy resolution, and feasibility of high field measurements by using pulsed magnetic fields. We apply the ESR technique to observe directly, for the first time, quantum sine-Gordon excitations which have paramount importance in understanding unique quantum magnetism of the S=1/2 Heisenberg antiferromagnetic chain system, Cu-benzoate.