Softening of the Longitudinal Phonon Mode along the [100] Direction in GdB6

K. Iwasa ； R. Igarashi ； K. Saito ； C. Laulhé ； T. Orihara ； S. Kunii ； K. Kuwahara ； H. Nakao ； Y. Murakami ； F. Iga ； M. Sera ； S. Tsutsui ； H. Uchiyama ； A. Q. R. Baron

《Chinese Journal of Physics》 49卷1期 (2011/02) Pp. 231-238

並列摘要

We have investigated the phonon of GdB6, which exhibits antiferromagnetic and structural first-order phase transition by using the inelastic x-ray scattering technique. The dispersion relation curve of the longitudinal acoustic mode propagating along the simple cubic [100] axis shows the maximum energy around the wave vector q=ξ, 0, 0), ξ=0.2-0.3, and it bends down with approaching the Brillouin zone boundary. The lower-energy zone-boundary mode at q1=(1=2, 0, 0) corresponds to the structural modulation in the ordered phase. The energy of this mode is 75% of the maximum value on the branch at 300 K and further decreases by 10% with decreasing temperature down to the transition temperature T(subscript N)=16 K so that this phonon mode softens considerably far above the transition temperature. On the other hand, the reference material YbB6 that is considered to have no localized magnetic moments does not show such softening in the inelastic neutron scattering measurement. The observation indicates a strong electron-phonon coupling in GdB6, which is expected to be magnetoelastic-type interaction between 4f states and displacement of Gd ions.