Figure 1

Inelastic x-ray spectra of graphite along $\Gamma \mathrm{\text{−}}K$; $\mathit{q}$ is in units of the distance between $\Gamma$ and $K$. (a) The LO phonon along $\Gamma \mathrm{\text{−}}K$ is observed if $\mathit{q}$ and $\mathit{Q}$ are not parallel, e.g., if $\mathit{H}=(100)$. (b),(c) Along $\Gamma \mathrm{\text{−}}K$ with $\mathit{H}=(2\overline{1}0)$ ($\mathit{q}\parallel \mathit{Q}$), the TO mode is allowed ($A_1$ symmetry). It is observed only at large $\mathit{q}$, where it has a polarization component parallel to $\mathit{Q}=\mathit{H}+\mathit{q}$.Reuse & Permissions

Figure 2

Inelastic x-ray scattering and ab initio calculations of the phonon dispersion of graphite. The symmetry of the branches is indicated; see Table . Filled dots are the experimental data for the LO, TO, and the LA branch. The TO data are connected by a cubic spline extrapolation (dashed line). The experimental error is smaller than the size of the symbols. Those phonon wave vectors $\mathit{q}$, which were not exactly along the $\Gamma \mathrm{\text{−}}M$ or $\Gamma \mathrm{\text{−}}K\mathrm{\text{−}}M$ direction, were projected onto the closest high-symmetry direction. Open dots are the calculated phonon frequencies; they were scaled overall by 1% to smaller frequencies. The lines are a spline extrapolation to the calculated points. Inset: Brillouin zone of graphite in the basal plane with the high-symmetry points $\Gamma$, $K$, and $M$. The arrows indicate the reciprocal lattice vectors with magnitude $H=|\mathit{H}|=4\pi /(\sqrt{3}{a}_0)$, where $a_0$ is the in-plane lattice constant of graphite.Reuse & Permissions