Symmetry Breaking in Few Layer Graphene Films

Recently, it was demonstrated that the quasiparticle dynamics, the layer-dependent charge and potential, and the c-axis screening coefficient could be extracted from measurements of the spectral function of few layer graphene films grown epitaxially on SiC using angle-resolved photoemission spectroscopy (ARPES). In this article we review these findings, and present detailed methodology for extracting such parameters from ARPES. We also present detailed arguments against the possibility of an energy gap at the Dirac crossing ED.Comment: 23 pages, 13 figures, Conference Proceedings of DPG Meeting Mar 2007 Regensburg Submitted to New Journal of Physic

Enhancement of chemical activity in corrugated graphene

Simulation of chemical activity of corrugated graphene within density functional theory predicts an enhancement of its chemical activity if the ratio of height of the corrugation (ripple) to its radius is larger than 0.07. Further growth of the curvature of the ripples results in appearance of midgap states which leads to an additional strong increase of chemisororption energy. These results open a way for tunable functionalization of graphene, namely, depending of curvature of the ripples one can provide both homogeneous (for small curvatures) and spot-like (for large curvatures) functionalization.Comment: 7 pages 3 figures. One figure added, description of the shape of ripples expanded. Final version, to be published in J. Phys. Chem.

Energy gaps, topological insulator state and zero-field quantum Hall effect in graphene by strain engineering

Among many remarkable qualities of graphene, its electronic properties attract particular interest due to a massless chiral character of charge carriers, which leads to such unusual phenomena as metallic conductivity in the limit of no carriers and the half-integer quantum Hall effect (QHE) observable even at room temperature [1-3]. Because graphene is only one atom thick, it is also amenable to external influences including mechanical deformation. The latter offers a tempting prospect of controlling graphene's properties by strain and, recently, several reports have examined graphene under uniaxial deformation [4-8]. Although the strain can induce additional Raman features [7,8], no significant changes in graphene's band structure have been either observed or expected for realistic strains of approx. 10% [9-11]. Here we show that a designed strain aligned along three main crystallographic directions induces strong gauge fields [12-14] that effectively act as a uniform magnetic field exceeding 10 T. For a finite doping, the quantizing field results in an insulating bulk and a pair of countercirculating edge states, similar to the case of a topological insulator [15-20]. We suggest realistic ways of creating this quantum state and observing the pseudo-magnetic QHE. We also show that strained superlattices can be used to open significant energy gaps in graphene's electronic spectrum

Electronic properties of bilayer and multilayer graphene

We study the effects of site dilution disorder on the electronic properties in graphene multilayers, in particular the bilayer and the infinite stack. The simplicity of the model allows for an easy implementation of the coherent potential approximation and some analytical results. Within the model we compute the self-energies, the density of states and the spectral functions. Moreover, we obtain the frequency and temperature dependence of the conductivity as well as the DC conductivity. The c-axis response is unconventional in the sense that impurities increase the response for low enough doping. We also study the problem of impurities in the biased graphene bilayer.Comment: 36 pages, 42 figures, references adde

Hysteresis of Electronic Transport in Graphene Transistors

Graphene field effect transistors commonly comprise graphene flakes lying on SiO2 surfaces. The gate-voltage dependent conductance shows hysteresis depending on the gate sweeping rate/range. It is shown here that the transistors exhibit two different kinds of hysteresis in their electrical characteristics. Charge transfer causes a positive shift in the gate voltage of the minimum conductance, while capacitive gating can cause the negative shift of conductance with respect to gate voltage. The positive hysteretic phenomena decay with an increase of the number of layers in graphene flakes. Self-heating in helium atmosphere significantly removes adsorbates and reduces positive hysteresis. We also observed negative hysteresis in graphene devices at low temperature. It is also found that an ice layer on/under graphene has much stronger dipole moment than a water layer does. Mobile ions in the electrolyte gate and a polarity switch in the ferroelectric gate could also cause negative hysteresis in graphene transistors. These findings improved our understanding of the electrical response of graphene to its surroundings. The unique sensitivity to environment and related phenomena in graphene deserve further studies on nonvolatile memory, electrostatic detection and chemically driven applications.Comment: 13 pages, 6 Figure

Quantum capacitance measurements of electron-hole asymmetry and next-nearest-neighbor hopping in graphene

Contains fulltext : 119943.pdf (preprint version ) (Open Access

Limits on charge carrier mobility in suspended graphene due to flexural phonons

The temperature dependence of the mobility in suspended graphene samples is investigated. In clean samples, flexural phonons become the leading scattering mechanism at temperature $T \gtrsim 10\,\,$K, and the resistivity increases quadratically with $T$. Flexural phonons limit the intrinsic mobility down to a few $\text{m}^2/\text{Vs}$ at room $T$. Their effect can be eliminated by applying strain or placing graphene on a substrate.Comment: 4 pages, 3 figure

Limits on Charge Carrier Mobility in Suspended Graphene due to Flexural Phonons

The temperature dependence of the mobility in suspended graphene samples is investigated. In clean samples, flexural phonons become the leading scattering mechanism at temperature $T \gtrsim 10\,\,$K, and the resistivity increases quadratically with $T$. Flexural phonons limit the intrinsic mobility down to a few $\text{m}^2/\text{Vs}$ at room $T$. Their effect can be eliminated by applying strain or placing graphene on a substrate.Comment: 4 pages, 3 figure