Interlayer interaction and electronic screening in multilayer graphene

The unusual transport properties of graphene are the direct consequence of a peculiar bandstructure near the Dirac point. We determine the shape of the pi bands and their characteristic splitting, and the transition from a pure 2D to quasi-2D behavior for 1 to 4 layers of graphene by angle-resolved photoemission. By exploiting the sensitivity of the pi bands to the electronic potential, we derive the layer-dependent carrier concentration, screening length and strength of interlayer interaction by comparison with tight binding calculations, yielding a comprehensive description of multilayer graphene's electronic structure

Massive enhancement of electron-phonon coupling in doped graphene by an electronic singularity

The nature of the coupling leading to superconductivity in layered materials such as high-Tc superconductors and graphite intercalation compounds (GICs) is still unresolved. In both systems, interactions of electrons with either phonons or other electrons or both have been proposed to explain superconductivity. In the high-Tc cuprates, the presence of a Van Hove singularity (VHS) in the density of states near the Fermi level was long ago proposed to enhance the many-body couplings and therefore may play a role in superconductivity. Such a singularity can cause an anisotropic variation in the coupling strength, which may partially explain the so-called nodal-antinodal dichotomy in the cuprates. Here we show that the topology of the graphene band structure at dopings comparable to the GICs is quite similar to that of the cuprates and that the quasiparticle dynamics in graphene have a similar dichotomy. Namely, the electron-phonon coupling is highly anisotropic, diverging near a saddle point in the graphene electronic band structure. These results support the important role of the VHS in layered materials and the possible optimization of Tc by tuning the VHS with respect to the Fermi level.Comment: 8 page

Quasiparticle Transformation During a Metal-Insulator Transition in Graphene

Here we show, with simultaneous transport and photoemission measurements, that the graphene terminated SiC(0001) surface undergoes a metal-insulator transition (MIT) upon dosingwith small amounts of atomic hydrogen. We find the room temperature resistance increases by about 4 orders of magnitude, a transition accompanied by anomalies in the momentum-resolved spectral function including a non-Fermi Liquid behaviour and a breakdown of the quasiparticle picture. These effects are discussed in terms of a possible transition to a strongly (Anderson) localized ground state.Comment: 11 pages, 4 figure

Fuel-Supply-Limited Stellar Relaxation Oscillations: Application to Multiple Rings around AGB Stars and Planetary Nebulae

We describe a new mechanism for pulsations in evolved stars: relaxation oscillations driven by a coupling between the luminosity-dependent mass-loss rate and the H fuel abundance in a nuclear-burning shell. When mass loss is included, the outward flow of matter can modulate the flow of fuel into the shell when the stellar luminosity is close to the Eddington luminosity $L_{\rm Edd}$. When the luminosity drops below $L_{\rm Edd}$, the mass outflow declines and the shell is re-supplied with fuel. This process can be repetitive. We demonstrate the existence of such oscillations and discuss the dependence of the results on the stellar parameters. In particular, we show that the oscillation period scales specifically with the mass of the H-burning relaxation shell (HBRS), defined as the part of the H-burning shell above the minimum radius at which the luminosity from below first exceeds the Eddington threshold at the onset of the mass loss phase. For a stellar mass M_*\sim 0.7\Msun, luminosity L_*\sim 10^4\Lsun, and mass loss rate |\dot M|\sim 10^{-5}\Msun yr$^{-1}$, the oscillations have a recurrence time $\sim 1400$ years $\sim 57\tau_{\rm fsm}$, where $\tau_{\rm fsm}$ is the timescale for modulation of the fuel supply in the HBRS by the varying mass-loss rate. This period agrees with the $\sim$ 1400-year period inferred for the spacings between the shells surrounding some planetary nebulae, and the the predictied shell thickness, of order 0.4 times the spacing, also agrees reasonably well.Comment: 15 pages TeX, 1 ps figure submitted to Ap

Small scale rotational disorder observed in epitaxial graphene on SiC(0001)

Interest in the use of graphene in electronic devices has motivated an explosion in the study of this remarkable material. The simple, linear Dirac cone band structure offers a unique possibility to investigate its finer details by angle-resolved photoelectron spectroscopy (ARPES). Indeed, ARPES has been performed on graphene grown on metal substrates but electronic applications require an insulating substrate. Epitaxial graphene grown by the thermal decomposition of silicon carbide (SiC) is an ideal candidate for this due to the large scale, uniform graphene layers produced. The experimental spectral function of epitaxial graphene on SiC has been extensively studied. However, until now the cause of an anisotropy in the spectral width of the Fermi surface has not been determined. In the current work we show, by comparison of the spectral function to a semi-empirical model, that the anisotropy is due to small scale rotational disorder ($\sim\pm$ 0.15$^{\circ}$) of graphene domains in graphene grown on SiC(0001) samples. In addition to the direct benefit in the understanding of graphene's electronic structure this work suggests a mechanism to explain similar variations in related ARPES data.Comment: 5 pages, 4 figure

Highly p-doped graphene obtained by fluorine intercalation

We present a method for decoupling epitaxial graphene grown on SiC(0001) by intercalation of a layer of fluorine at the interface. The fluorine atoms do not enter into a covalent bond with graphene, but rather saturate the substrate Si bonds. This configuration of the fluorine atoms induces a remarkably large hole density of p \approx 4.5 \times 1013 cm-2, equivalent to the location of the Fermi level at 0.79 eV above the Dirac point ED .Comment: 4 pages, 2 figures, in print AP

Morphology of graphene thin film growth on SiC(0001)

Epitaxial films of graphene on SiC(0001) are interesting from a basic physics as well as applications-oriented point of view. Here we study the emerging morphology of in-vacuo prepared graphene films using low energy electron microscopy (LEEM) and angle-resolved photoemission (ARPES). We obtain an identification of single and bilayer of graphene film by comparing the characteristic features in electron reflectivity spectra in LEEM to the PI-band structure as revealed by ARPES. We demonstrate that LEEM serves as a tool to accurately determine the local extent of graphene layers as well as the layer thickness

Unoccupied electronic states of icosahedral Al-Pd-Mn quasicrystals: Evidence of image potential resonance and pseudogap

We study the unoccupied region of the electronic structure of the fivefold symmetric surface of an icosahedral (i) Al-Pd-Mn quasicrystal. A feature that exhibits parabolic dispersion with an effective mass of (1.15±0.1)me and tracks the change in the work function is assigned to an image potential resonance because our density functional calculation shows an absence of band gap in the respective energy region. We show that Sn grows pseudomorphically on i−Al−Pd−Mn as predicted by density functional theory calculations, and the energy of the image potential resonance tracks the change in the work function with Sn coverage. The image potential resonance appears much weaker in the spectrum from the related crystalline Al-Pd-Mn surface, demonstrating that its strength is related to the compatibility of the quasiperiodic wave functions in i−Al−Pd−Mn with the free-electron-like image potential states. Our investigation of the energy region immediately above EF provides unambiguous evidence for the presence of a pseudogap, in agreement with our density functional theory calculations