Theory

2010

Energy gaps and a zero-field quantum Hall effect in graphene by strain engineering

Nature Physics 6, 30-33 (2010)

F. Guinea, M. I. Katsnelson & A. K. Geim

 

Among many remarkable qualities of graphene, its electronic properties attract particular interest owing to the chiral character of the charge carriers, which leads to such unusual phenomena as metallic conductivity in the limit of no carriers and the half-integer quantum Hall effect observable even at room temperature.

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Curved Graphene Nanoribbons: Structure and Dynamics of Carbon Nanobelts

Nanotechnology 21 (2010) 075710

B.V.C. Martins, D.S. Galvão

 

Carbon nanoribbons (CNRs) are graphene (planar) structures with large aspect ratio. Carbon nanobelts (CNBs) are small graphene nanoribbons rolled up into spiral-like structures, i. e., carbon nanoscrolls (CNSs) with large aspect ratio. In this work we investigated the energetics and dynamical aspects of CNBs formed from rolling up CNRs. We have carried out molecular dynamics simulations using reactive empirical bond-order potentials. Our results show that similarly to CNSs, CNBs formation is dominated by two major energy contribution, the increase in the elastic energy due to the bending of the initial planar configuration (decreasing structural stability) and the energetic gain due to van der Waals interactions of the overlapping surface of the rolled layers (increasing structural stability). Beyond a critical diameter value these scrolled structures can be even more stable (in terms of energy) than their equivalent planar configurations. In contrast to CNSs that require energy assisted processes (sonication, chemical reactions, etc.) to be formed, CNBs can be spontaneously formed from low temperature driven processes. Long CNBs (length of appr. 30.0 nm) tend to exhibit self-folded racket-like conformations with formation dynamics very similar to the one observed for long carbon nanotubes. Shorter CNBs will be more likely to form perfect scrolled structures. Possible synthetic routes to fabricate CNBs from graphene membranes are also addressed.

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Theory of Fano resonances in graphene: The influence of orbital and structural symmetries on STM spectra

Phys. Rev. B 81, 085413 (2010)

T. O. Wehling, H. P. Dahal, A. I. Lichtenstein, M. I. Katsnelson, H. C. Manoharan, and A. V. Balatsky

 

We theoretically investigate Fano factors arising in local spectroscopy of impurity resonances in graphene. It is demonstrated that Fano line shapes can strongly differ from the antiresonances usually found on metal surfaces. Graphene’s highly symmetric Fermi points make this effect particularly sensitive to the detailed atomistic structure and orbital symmetries of the impurity. After a model discussion based on an Anderson impurity coupled to an electron bath with linearly vanishing density of states, we present first-principles calculations of Co adatoms on graphene. For Co above the center of a graphene hexagon, we find that the two-dimensional E1 representation made of dxz,dyz orbitals is likely responsible for the hybridization and ultimately Kondo screening for cobalt on graphene. Anomalously large Fano q factors depending strongly on the orbitals involved are obtained. For a resonant s-wave impurity, a similarly strong adsorption site dependence of the q factor is demonstrated. These anomalies are striking examples of quantum-mechanical interference related to the Berry phase inherent to the graphene band structure.

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Effects of a quantum measurement on the electric conductivity: Application to graphene

Phys. Rev. B 81, 073403 (2010)

J. Z. Bernád, M. Jääskeläinen, and U. Zülicke

 

We generalize the standard linear-response (Kubo) theory to obtain the conductivity of a system that is subject to a quantum measurement of the current. Our approach can be used to specifically elucidate how back-action inherent to quantum measurements affects electronic transport. To illustrate the utility of our general formalism, we calculate the frequency-dependent conductivity of graphene and discuss the effect of measurement-induced decoherence on its value in the dc limit. We are able to resolve an ambiguity related to the parametric dependence of the minimal conductivity.

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Topological Insulator in the Core of the Superconducting Vortex in Graphene

Phys. Rev. Lett. 104, 066404 (2010)

Igor F. Herbut

 

The core of the vortex in a general superconducting order parameter in graphene is argued to be ordered, with the possible local order parameters forming the algebra U(1)×Cl(3). A sufficiently strong Zeeman coupling of the magnetic field of the vortex to the electron spin breaks the degeneracy in the core in favor of the anomalous quantum Hall state. I consider a variety of superconducting condensates on the honeycomb lattice and demonstrate the surprising universality of this result. A way to experimentally determine the outcome of the possible competition between different types of orders in the core is proposed.

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Oblique terahertz plasmons in graphene nanoribbon arrays

Phys. Rev. B 81, 073404 (2010)

V. V. Popov, T. Yu. Bagaeva, T. Otsuji, and V. Ryzhii

 

We show that the plasmon frequencies in a one-dimensional dense array of doped (or gated) graphene nanoribbons can be changed through the entire terahertz range depending on the angle between the plasmon wave vector and the nanoribbon direction. The overdamped regime for oblique plasmons in the graphene nanoribbon array is discussed.

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