Electric-Dipole Active Two-Magnon Excitation in <i>a</i> <i>b</i> Spiral Spin Phase of a Ferroelectric Magnet Gd <sub/>0.7</sub> Tb <sub/>0.3</sub> MnO <sub/>3</sub>Kida, Noriaki; Yamasaki, Yuichi; Shimano, Ryo; Arima, Taka-hisa; Tokura, Yoshinori · Journal of the Physical Society of Japan · 2008