Doped Graphene Layers Research Articles

Angle-resolved photoemission study of the graphite intercalation compoundKC8: A key to graphene

Electrons in isolated graphene layers are a two-dimensional gas of massless Dirac Fermions. In realistic devices, however, the electronic properties are modified by elastic deformations, interlayer coupling and substrate interaction. Here, we unravel the electronic structure of noninteracting, doped graphene layers by revisiting the stage one graphite intercalation compound ${\text{KC}}_{8}$. To this end we apply angle-resolved photoemission spectroscopy and ab initio calculations. The full experimental dispersion is in excellent agreement with calculations of doped graphene once electron correlations are included at the $GW$ level (Greens function $G$ of the Coulomb interaction $W$). This highlights that ${\text{KC}}_{8}$ has negligible interlayer coupling allowing us to access the full experimental Dirac cone.

Collective modes of doped graphene and a standard two-dimensional electron gas in a strong magnetic field: Linear magnetoplasmons versus magnetoexcitons

A doped graphene layer in the integer quantum-Hall regime reveals a highly unusual particle-hole excitation spectrum, which is calculated from the dynamical polarizability in the random-phase approximation. We find that the elementary neutral excitations in graphene in a magnetic field are unlike those of a standard two-dimensional electron gas: in addition to the upper-hybrid mode, the particle-hole spectrum is reorganized in linear magnetoplasmons that disperse roughly parallel to $\ensuremath{\omega}={v}_{F}q$, instead of the usual horizontal (almost dispersionless) magnetoexcitons. These modes could be detected in an inelastic light-scattering experiment.

Collective Modes of the Massless Dirac Plasma

We develop a theory for the long-wavelength plasma oscillation of a collection of charged massless Dirac particles in a solid, as occurring, for example, in doped graphene layers, interacting via the long-range Coulomb interaction. We find that the long-wavelength plasmon frequency in such a doped massless Dirac plasma is explicitly nonclassical in all dimensions with the plasma frequency being proportional to 1/sqrt[variant Planck's over 2pi]. We also show that the long-wavelength plasma frequency of the D-dimensional superlattice made from such a plasma does not agree with the corresponding D + 1-dimensional bulk plasmon frequency. We compare and contrast such Dirac plasmons with the well-studied regular palsmons in metals and doped semiconductors which manifest the usual classical long-wavelength plasma oscillation.

Electronic structure and electron-phonon coupling of doped graphene layers inKC8

We propose graphite intercalation compounds (GICs) as a material system with precisely the same electronic properties as doped few layer graphene. Despite the fact that GICs have been around for the last four decades, this fact has gone unnoticed so far. Especially, we focus on the electronic energy bands of ${\text{KC}}_{8}$ which correspond to a doped graphene monolayer. We provide extensive theoretical and experimental evidence for this claim employing a combined angle-resolved photoemission and theory approach using tight-binding, standard density-functional theory and including electron-electron correlation on a $GW$ level. We observe a strong momentum-dependent kink in the quasiparticle dispersion at 166 meV highlighting electron-phonon coupling to an in-plane transversal optical phonon. These results are key for understanding both the unique electronic properties of doped graphene layers and superconductivity in ${\text{KC}}_{8}$.

Optical properties of doped graphene layers

The reflectance of a graphene monolayer, as well as of a system of monolayers, is calculated in the infrared range. A quantum expression for the conductivity in the collisionless regime that depends on the frequency, the temperature, and the concentration of carriers is used in the calculations. Above the threshold of the interband electron absorption, the reflectance decreases with increasing frequency. With decreasing temperature, excitation of plasmons in the system of layers is possible in a narrow range near the threshold, which results in the occurrence of a deep and sharp minimum in the frequency dependence of the reflectance.

Magnetotransport in high mobility epitaxial graphene

AbstractEpitaxial graphene layers grown on single‐crystal SiC have large structural coherence domains and can be easily patterned into submicron structures using standard microelectronics lithography techniques. Patterned structures show two‐dimensional electron gas properties with mobilities exceeding 3 m2/Vs. Magnetotransport measurements (Shubnikov–de Haas oscillations) indicate that the transport properties are dominated by the highly doped graphene layer at the silicon carbide interface. They reveal the Dirac nature of the charge carriers as predicted for a single graphene layer. The properties of Dirac fermions can be conveniently explored in epitaxial graphene with long electronic phase coherence at the micron scale. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)