Single-layer graphene exhibits intriguing electronic properties stemming from its geometric structure. For example, charge neutral graphene shows strong electronic correlations that are not explained by the theory that describes typical metallic systems. In addition, when the sublattice symmetry is ...
Single-layer graphene exhibits intriguing electronic properties stemming from its geometric structure. For example, charge neutral graphene shows strong electronic correlations that are not explained by the theory that describes typical metallic systems. In addition, when the sublattice symmetry is broken, energy gap, whose absence has been the main reason preventing graphene from industrial applications, is predicted and observed.
Among them, the electron-plasmon interaction, so-called plasmaron, and energy gap are observed at the Dirac energy where the conduction and valence bands of graphene touch at a single point. Although the physical origin of the two phenomena is apparently different, they become non-trivial to differentiate when graphene is placed on an SiC(0001) substrate. More specifically, when the dielectric screening from the substrate is strong compared to that for free-standing graphene, additional band structure induced by the formation of plasmaron is not separated from the quasiparticle band, but leaves its signature at Dirac energy as increased intensity in conjunction with an elongated band structure along energy direction. Meanwhile, the presence of the buffer layer, a carbidic layer with the same geometric structure as graphene but in which the conical dispersion is absent due to the formation of covalent bonds with the substrate, on an SiC substrate can break the sublattice symmetry of single-layer graphene to induce an energy gap at Dirac energy. For this case, the gap region can be filled with spectral intensity originating from the coupling between the localized π states of the buffer layer and the π bands of single-layer graphene.
This controversial issue can be investigated by analyzing electron band structure of buffer layer, single-layer, and double-layer graphene samples using angle-resolved photoemission spectroscopy. Within the plasmaron picture, single- and double-layer graphene grown on the same dielectric substrate may show the similar spectral features at Dirac energy, because both systems are predicted to exhibit the plasmaron. On the other hand, within the in-gap states picture, the spectral intensity at Dirac energy is expected to decrease as the surface is transformed from single-layer to double-layer graphene as their Dirac energy is different in energy with respect to the Fermi energy.
In this study, I have investigated this controversial issue to better understand electronic properties of graphene on an SiC substrate. I found that double-layer graphene does not show the characteristic features of the plasmaron introduced in the single-layer graphene. More specifically, the electronic states from the buffer layer is found to gradually decrease in spectral intensity with the evolution of overlying graphene layers, suggesting that the buffer layer states increasingly couple to the graphene π bands as predicted in the in-gap states picture. In double-layer graphene, the Dirac energy shows relatively weak spectral intensity at Dirac energy compared to that of the valence band maximum and the conduction band minimum, and the extended lines along its valence bands perfectly overlap with the conduction bands. These are in contrast to the maximum spectral intensity at Dirac energy and the mismatching bands of single-layer graphene, and hence not explained by the plasmaron picture. Consequently, these results suggest that the in-gap states picture induced by the buffer layer has a main contribution to the high spectral intensity at Dirac energy of single-layer graphene epitaxially grown on an SiC(0001) substrate.
Single-layer graphene exhibits intriguing electronic properties stemming from its geometric structure. For example, charge neutral graphene shows strong electronic correlations that are not explained by the theory that describes typical metallic systems. In addition, when the sublattice symmetry is broken, energy gap, whose absence has been the main reason preventing graphene from industrial applications, is predicted and observed.
Among them, the electron-plasmon interaction, so-called plasmaron, and energy gap are observed at the Dirac energy where the conduction and valence bands of graphene touch at a single point. Although the physical origin of the two phenomena is apparently different, they become non-trivial to differentiate when graphene is placed on an SiC(0001) substrate. More specifically, when the dielectric screening from the substrate is strong compared to that for free-standing graphene, additional band structure induced by the formation of plasmaron is not separated from the quasiparticle band, but leaves its signature at Dirac energy as increased intensity in conjunction with an elongated band structure along energy direction. Meanwhile, the presence of the buffer layer, a carbidic layer with the same geometric structure as graphene but in which the conical dispersion is absent due to the formation of covalent bonds with the substrate, on an SiC substrate can break the sublattice symmetry of single-layer graphene to induce an energy gap at Dirac energy. For this case, the gap region can be filled with spectral intensity originating from the coupling between the localized π states of the buffer layer and the π bands of single-layer graphene.
This controversial issue can be investigated by analyzing electron band structure of buffer layer, single-layer, and double-layer graphene samples using angle-resolved photoemission spectroscopy. Within the plasmaron picture, single- and double-layer graphene grown on the same dielectric substrate may show the similar spectral features at Dirac energy, because both systems are predicted to exhibit the plasmaron. On the other hand, within the in-gap states picture, the spectral intensity at Dirac energy is expected to decrease as the surface is transformed from single-layer to double-layer graphene as their Dirac energy is different in energy with respect to the Fermi energy.
In this study, I have investigated this controversial issue to better understand electronic properties of graphene on an SiC substrate. I found that double-layer graphene does not show the characteristic features of the plasmaron introduced in the single-layer graphene. More specifically, the electronic states from the buffer layer is found to gradually decrease in spectral intensity with the evolution of overlying graphene layers, suggesting that the buffer layer states increasingly couple to the graphene π bands as predicted in the in-gap states picture. In double-layer graphene, the Dirac energy shows relatively weak spectral intensity at Dirac energy compared to that of the valence band maximum and the conduction band minimum, and the extended lines along its valence bands perfectly overlap with the conduction bands. These are in contrast to the maximum spectral intensity at Dirac energy and the mismatching bands of single-layer graphene, and hence not explained by the plasmaron picture. Consequently, these results suggest that the in-gap states picture induced by the buffer layer has a main contribution to the high spectral intensity at Dirac energy of single-layer graphene epitaxially grown on an SiC(0001) substrate.
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