Observation of anomalous peaks in the photoelectron spectra of highly oriented pyrolytic graphite: Folding of the band due to the surface charge density wave transition

The angle-resolved photoelectron spectroscopy with low-photon energy (7–16 eV) is used for the investigation of highly oriented pyrolytic graphite at a low temperature. We observed for the first time peaks in the low binding energy region (from the Fermi level to 0.7 eV) of the surface normal photoelectron spectra at 11K, which disappear above ∼30K. Based on the dispersion both along the parallel and normal to the surface, the peaks are ascribed to the emission from the K(H)-point that is backfolded into the Γ(A)-point as a result of the two-dimensional superperiodicity. The surface charge density wave transition is proposed for the driving mechanism of the superperiodicity based on the temperature dependence of the photoelectron intensity.

Figure 1

(a) A series of the photoelectron spectra with various photon energies at 11 K. Electron emission along the surface normal is measured. (b) Photoelectron spectra taken with several photon energies. (c) The intensities as a function of the photon energy for the photoelectrons whose binding energies are 0.02 eV (solid triangles), 0.3 eV (solid squares), and 0.7 eV (solid circles). The lines show the calculated curves with two Lorentzian peaks. The four-times magnified curves (open squares and circles with dotted lines) are also shown for E${}_{\mathrm{B}}$ = 0.3 and 0.7 eV.

Figure 2

(a) The ARPES intensity map in the logarithmic scale at hν = 13 eV taken at 11 K. Open circles show the dispersions of the peaks obtained from the ARPES spectra at hν = 11.5 eV, and the (blue) line shows the dispersion of the π-band near the K-point of the single-crystal graphite according to the previous ARPES result (Ref. 6). (b) The surface-normal photoelectron intensity map in logarithmic scale as a function of the binding energy and the photon energy. The intensities at hν > 13.3 eV are multiplied by 10. Open squares and triangles represent peak positions (see text). (c) The 2D ($k_z$ = 0) BZ for the graphite (solid lines). The dotted lines illustrate the zone folding associated with a $p\left(\sqrt{3}\times \sqrt{3}\right)R{30}^{\circ }$ superperiodicity. (d) The dispersion along $k_z$ of the unoccupied states derived from the open squares and triangles in (b) and the calculated dispersion curves for the K-H (solid black lines) and the Γ-A (dotted red lines) directions (Ref. 2).

Figure 3

Surface normal photoelectron spectra of HOPG at various temperatures of the sample (hν = 11.5 eV). In the inset, solid circles are photoelectron intensities of HOPG at B${}_{\mathrm{E}}$ = 0.016 eV as a function of the reduced temperature (divided by T${}_{\mathrm{C}}$ of 29 K), and other dots are the superlattice reflection intensities of x ray as a result of the CDW transition in various materials (Refs. 17, 18, 19). The solid line is a square of the BCS-type order parameter (Ref. 20).