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Low-Energy Charge-Density Excitations in MgB$_{2}$: Striking Interplay between Single-Particle and Collective Behavior for Large Momenta

  • Cai, Y. Q.
  • Chow, P. C.
  • Restrepo, O. D.
  • Takano, Y.
  • Togano, K.
  • Kito, H.
  • Ishii, H.
  • Chen, C. C.
  • Liang, K. S.
  • Chen, C. T.
  • Tsuda, S.
  • Shin, S.
  • Kao, C. C.
  • Ku, W.
  • Eguiluz, A. G.
Published Article
Publication Date
Jul 13, 2006
Submission Date
May 12, 2006
DOI: 10.1103/PhysRevLett.97.176402
arXiv ID: cond-mat/0605320
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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$_{2}$.


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