Dirac Fermions in Graphene and Graphite—a view from angle ...

Figure 7.8. (a) ARPES intensity map taken on HOPG sample at 26 eV photon energy (k z ≈ 0.13
c ∗ ). (b) ARPES intensity map measured at the same conditions as panel a except on a different
spot inside the sample. The arrow points to the additional intensity near E F . (c) ARPES intensity
map measured on single crystal graphite with 50 eV photon energy. The dotted line is extracted
dispersion from EDCs. (d) ARPES intensity maps measured at the same conditions as panel c
except on a different spot. The dotted line is the dispersion extracted from panel c. The open circles
are the dispersions extracted for the additional feature at low energy.
7.6 Conclusions and implication of this study
We report clear evidence that sharp quasiparticle dispersions, in agreement with band structure calculation
along the high symmetry directions, can coexist with a circular Fermi energy intensity map, a definitive
signature of azimuthal disorder 124 . The measured dispersions from HOPG are compared with those measured
from single crystal graphite. The good agreement between data taken on these two different kinds of
samples suggests that more ARPES opportunities can be made available for layered materials even when
high quality single crystals of large size are difficult to obtain. A typical example of such samples is the
recently discovered superconducting C 6 Ca 97 , where intercalation is more favorable in HOPG than in single
crystalline graphite, thus the resulting superconducting C 6 Ca is azimuthally disordered. This study suggests
that even with such an azimuthally disordered sample, it is still promising to study with ARPES to obtain
useful information.
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