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Anomalous angular dependence of the dynamic structure factor near Bragg reflections: graphite.
R Hambach1, C Giorgetti, N Hiraoka
1Laboratoire des Solides Irradiés, Ecole Polytechnique, CEA/DSM, CNRS, 91128 Palaiseau, France.
Physical Review Letters
|May 14, 2009
Summary
Electron energy-loss spectra in graphite change dramatically near Bragg reflections due to momentum transfer variations. This effect stems from crystal local field effects and graphite stacking, revealing strong coupling between excitations.
Area of Science:
- Condensed matter physics
- Materials science
- Solid-state physics
Background:
- The electron energy-loss function (EELF) is crucial for understanding electronic excitations in materials.
- Studying EELF at large momentum transfers provides insights into electron correlations and material structure.
- Graphite's unique layered structure and electronic properties make it an ideal system for investigating momentum-dependent phenomena.
Purpose of the Study:
- To investigate the electron energy-loss function of graphite for momentum transfers beyond the first Brillouin zone.
- To elucidate the physical mechanisms causing drastic spectral changes near Bragg reflections.
- To explore the role of crystal local field effects and stacking in graphite's electronic excitations.
Main Methods:
- First-principle calculations employing the random phase approximation.
- Inelastic X-ray scattering (IXS) measurements.
- Analysis of the dynamic structure factor S(q, ω).
Main Results:
- Observed drastic changes in electron energy-loss spectra for minute variations in momentum transfer (q) near Bragg reflections.
- Demonstrated that these spectral changes are governed by crystal local field effects.
- Confirmed the influence of graphite's stacking on the observed phenomena.
- Traced the effect to a strong coupling between electronic excitations at small and large momentum transfers.
Conclusions:
- Crystal local field effects and graphite stacking significantly influence electron energy-loss spectra at large momentum transfers.
- The study reveals a strong coupling between electronic excitations across different momentum regimes in graphite.
- This work provides a deeper understanding of momentum-dependent electronic excitations in layered materials.
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