1. Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan
2. Yamato Scientific Co., Ltd., 2-1-6 Nihonbashihoncho, Chuo-ku, Tokyo 103-8432, Japan
3. Toshiba Electron Tubes and Devices Co., Ltd., 1385 Shimoishigami, Ohtawara 324-8550, Japan