1,686 research outputs found
Variational approach to the excitonic phase transition in graphene
We analyze the Coulomb interacting problem in undoped graphene layers by
using an excitonic variational ansatz. By minimizing the energy, we derive a
gap equation which reproduces and extends known results. We show that a full
treatment of the exchange term, which includes the renormalization of the Fermi
velocity, tends to suppress the phase transition by increasing the critical
coupling at which the excitonic instability takes place.Comment: 4 page
Electron-induced rippling in graphene
We show that the interaction between flexural phonons, when corrected by the
exchange of electron-hole excitations, may place the graphene sheet very close
to a quantum critical point characterized by the strong suppression of the
bending rigidity of the membrane. Ripples arise then due to spontaneous
symmetry breaking, following a mechanism similar to that responsible for the
condensation of the Higgs field in relativistic field theories. In the presence
of membrane tensions, ripple condensation may be reinforced or suppressed
depending on the sign of the tension, following a zero-temperature buckling
transition in which the order parameter is given essentially by the square of
the gradient of the flexural phonon field.Comment: 4 pages, 3 figure
Renormalization group analysis of electrons near a Van Hove singularity.
A model of interacting two dimensional electrons near a Van Hove singularity
is studied, using renormalization group techniques. In hole doped systems, the
chemical potential is found to be pinned near the singularity, when the
electron-electron interactions are repulsive. The RG treatment of the leading
divergences appearing in perturbation theory give rise to marginal behavior and
anisotropic superconductivity.Comment: 4 Latex pages + 5 postcript figure
Electrostatic screening in fullerene molecules
The screening properties of fullerene molecules are described by means of a
continuum model which uses the electronic wavefunctions of planar graphite as a
starting point. The long distance behavior of the system gives rise to a
renormalizable theory, which flows towards a non trivial fixed point. Its
existence implies an anomalous dielectric constant. The screening properties
are neither metallic nor insulating. Alternatively, the intramolecular
screening is obtained from a simple approximation to the electronic
wavefunctions. Intermolecular effects are also calculated. As a consistency
check, it is shown that the observed polarizability of C is well
eproduced.Comment: 7 pages. Revte
Charge distribution and screening in layered graphene systems
The charge distribution induced by external fields in finite stacks of
graphene planes, or in semiinfinite graphite is considered. The interlayer
electronic hybridization is described by a nearest neighbor hopping term, and
the charge induced by the self consistent electrostatic potential is calculated
within linear response (RPA). The screening properties are determined by
contributions from inter- and intraband electronic transitions. In neutral
systems, only interband transitions contribute to the charge polarizability,
leading to insulating-like screening properties, and to oscillations in the
induced charge, with a period equal to the interlayer spacing. In doped
systems, we find a screening length equivalent to 2-3 graphene layers,
superimposed to significant charge oscillations.Comment: 8 page
- …