Low-Energy Charge-Density Excitations in MgB$_{2}$: Striking Interplay   between Single-Particle and Collective Behavior for Large Momenta
  A sharp feature in the charge-density excitation spectra of single-crystal MgB$_{2}$, displaying a remarkable cosine-like, periodic energy dispersion with momentum transfer ($q$) along the $c^{*}$-axis, has been observed for the first time by high-resolution non-resonant inelastic x-ray scattering (NIXS). Time-dependent density-functional theory calculations show that the physics underlying the NIXS data is strong coupling between single-particle and collective degrees of freedom, mediated by large crystal local-field effects. As a result, the small-$q$ collective mode residing in the single-particle excitation gap of the B $\pi$ bands reappears periodically in higher Brillouin zones. The NIXS data thus embody a novel signature of the layered electronic structure of MgB$_{2}$. 
Y.Q. Cai, P.C. Chow, O.D. Restrepo, Y. Takano, K. Togano, H. Kito, H.   Ishii, C.C. Chen, K.S. Liang, C.T. Chen, S. Tsuda, S...