Unoccupied electronic structure of graphite probed by angle-resolved photoemission spectroscopy

We report the observation of anomalous bands in graphite valence band structure in angle-resolved photoemission spectroscopy (ARPES) experiments. The photon energy dependence of these bands shows a constant kinetic energy nature. Our results are supported by the very low energy electron diffraction data reported on graphite surfaces which essentially map the unoccupied states representing the photoemission final states. This suggests that the ARPES technique is capable of probing the unoccupied electronic states governed by the secondary electron emission process, along with the occupied bands of solids.

Figure 1

(a) ARPES spectra of HOPG measured at room temperature using He ii $\alpha$ resonance line along k${}_{\left|\right|y}$ and (b) k${}_{\left|\right|x}$ symmetry directions. (c) LEED image of HOPG using 70 eV electrons. The black dashed lines are guides to the eye showing the weak anomalous bands.

Figure 2

ARPES spectra of single-crystal graphite using 41 eV photons along (a) $\Gamma$$M$ and (b) $\Gamma$$K$ symmetry direction. (c) LEED pattern of single-crystal graphite at 70 eV with symmetry directions shown. The blue dashed line around 14 eV in [$E-E_F$] energy scale corresponds to SEE feature.

Figure 3

ARPES spectra of graphite single crystal using (a) 47 eV photon energy along $\Gamma$$K$ symmetry direction, (b) 53 eV photon energy along $\Gamma$$K$ direction, (c) 70 eV photon energy along $\Gamma$$K$ direction, (d) 70 eV photon energy along $\Gamma$$M$ direction. The ARPES spectra for 70 eV photon energy along $\Gamma$$K$ direction is shown as waterfall graph in (e). Anomalous bands are shown with black dashed lines. The blue dashed lines show the (${\sigma }^{*}$) features due to the SEE process.

Figure 4

Comparison of the energy position and dispersion of SEE features observed in the ARPES spectra with the VLEED results reported by Strocov et al. (Ref. 21) on graphite single crystals. Results from the target current spectroscopy (TCS) (Ref. 18) and IPS (Ref. 19) experiments and ab initio calculations of Tatar and Rabii (Ref. 2) are also compared. The width of the measured SEE features in ARPES spectra are shown as error bars.