Gapped ground state in a spin- 12 frustrated square lattice

H. Yamaguchi, N. Uemoto, T. Okubo, Y. Kono, S. Kittaka, T. Sakakibara, T. Yajima, S. Shimono, Y. Iwasaki, and Y. Hosokoshi Department of Physical Science, Osaka Prefecture University, Osaka 599-8531, Japan Department of Physics, the University of Tokyo, Tokyo 113-0033, Japan Department of Physics, Chuo University, Tokyo 112-8551, Japan Institute for Solid State Physics, the University of Tokyo, Chiba 277-8581, Japan Department of Materials Science and Engineering, National Defense Academy, Kanagawa 239-8686, Japan