Research: Graphene

 <<  <  1 2 3  >   >> 

Local density of states in momentum space for a hollowlike impurity in graphene
http://prb.aps.org/node/3000
Optical conductivity of strained graphene, as a function of frequency and electric field orientation
Strain-induced Electronic Topological Transitions in graphene
Plasmon dispersion relation for strained graphene, including Local Field Effects

Graphene is an atomic thick single layer of sp2-carbon atoms in a honeycomb lattice. The quite recent realization of sufficiently large graphene flakes in the laboratory has stimulated an enormous outburst of both experimental and theoretical investigation, due to its remarkable mechanical and electronic properties that make graphene an ideal candidate for applications in nanoelectronics. The electronic bands are characterized by two inequivalent Dirac points, which endow the density of states (DOS) with a linear energy dependence at the Fermi level, thus making graphene a zero-gap semiconductor. Graphene is also characterized by outstanding mechanical and elastic properties, which make it one of the hardest materials known. It is also expected that correlations should play a prominent role in establishing ordered phases in the low-energy, massless elementary excitations of such a low-dimensional electronic liquid. In particular, structural, magnetic, and superconducting (SC) instabilities may compete at low temperature.

In this context, isolated impurities may serve as probes of the local electronic ordering of graphene. Motivated by quantum chemistry calculations, showing that molecular adsorption takes place on preferential sites of the honeycomb lattice, we have evaluated the local density of states (LDOS) as a function of energy on an isolated, nonmagnetic impurity and on its neighbouring sites.

Our findings, both in real and in reciprocal space, may help interpreting the results of Fourier-transformed scanning-tunnelling spectroscopy. We also recover a sublinear dependence of the conductivity on the carrier concentration as a generic impurity effect. As in the layered cuprates, the energy dependence of the LDOS around an isolated impurity may provide a direct signature of the possibly unconventional symmetry of an SC order parameter. As a matter of fact, Cooper pairs have been induced in graphene layers by effect of proximity to a SC junction, while intrinsic SC in graphene is presently a subject of intense theoretical investigation. We have classified the possible symmetries of a SC gap compatible with an underlying honeycomb lattice, and characterized the SC phase diagram as a function of doping and coupling constants. We have therefore evaluated the effect of isolated impurities on the LDOS in such an unconventional SC phase, showing that the occurrence of strong resonances within the gapped region is indeed a generic consequence of a d-wave order parameter. While local modifications do affect local electronic properties, global distortions are expected to modify transport or other overall properties.

We have therefore studied the effect of uniaxial strain on the optical conductivity and reflectivity of graphene. While the Dirac-cone approximation is robust for sufficiently small strain, band dispersion linearity breaks down along a given direction, corresponding to the development of anisotropic massive low-energy excitations. At low strain, we recover a linear behaviour of the low-energy DOS, while a band gap opens for sufficiently intense strain, for generic strain directions. This may be interpreted in terms of an electronic topological transition, corresponding to a change of topology of the Fermi line. These features may be observed in the frequency dependence of the longitudinal-optical conductivity as well as of the reflectivity of graphene in the visible range.

(and references therein)