376 research outputs found
Coulomb Interactions and Ferromagnetism in Pure and Doped Graphene
We study the presence of ferromagnetism in the phase diagram of the
two-dimensional honeycomb lattice close to half-filling (graphene) as a
function of the strength of the Coulomb interaction and doping. We show that
exchange interactions between Dirac fermions can stabilize a ferromagnetic
phase at low doping when the coupling is sufficiently large. In clean systems,
the zero temperature phase diagram shows both first order and second order
transition lines and two distinct ferromagnetic phases: one phase with only one
type of carriers (either electrons or holes) and another with two types of
carriers (electrons and holes). Using the coherent phase approximation (CPA) we
argue that disorder further stabilizes the ferromagnetic phase.Comment: 10 pages; published versio
Electronic properties of graphene multilayers
We study the effects of disorder in the electronic properties of graphene
multilayers, with special focus on the bilayer and the infinite stack. At low
energies and long wavelengths, the electronic self-energies and density of
states exhibit behavior with divergences near half-filling. As a consequence,
the spectral functions and conductivities do not follow Landau's Fermi liquid
theory. In particular, we show that the quasiparticle decay rate has a minimum
as a function of energy, there is a universal minimum value for the in-plane
conductivity of order e^2/h per plane and, unexpectedly, the c-axis
conductivity is enhanced by disorder at low doping, leading to an enormous
conductivity anisotropy at low temperatures.Comment: 4 pages, 4 figure. Reference to exciting new ARPES results on
graphite added (we thank A. Lanzara for sharing the paper prior to its
publication
Transmission through a biased graphene bilayer barrier
We study the electronic transmission through a graphene bilayer in the
presence of an applied bias between layers. We consider different geometries
involving interfaces between both a monolayer and a bilayer and between two
bilayers. The applied bias opens a sizable gap in the spectrum inside the
bilayer barrier region, thus leading to large changes in the transmission
probability and electronic conductance that are controlled by the applied bias.Comment: 10 pages, 8 figures, extended versio
Probing the Electronic Structure of Bilayer Graphene by Raman Scattering
The electronic structure of bilayer graphene is investigated from a resonant
Raman study using different laser excitation energies. The values of the
parameters of the Slonczewski-Weiss-McClure model for graphite are measured
experimentally and some of them differ significantly from those reported
previously for graphite, specially that associated with the difference of the
effective mass of electrons and holes. The splitting of the two TO phonon
branches in bilayer graphene is also obtained from the experimental data. Our
results have implications for bilayer graphene electronic devices.Comment: 4 pages, 4 figure
Electronic states and Landau levels in graphene stacks
We analyze, within a minimal model that allows analytical calculations, the
electronic structure and Landau levels of graphene multi-layers with different
stacking orders. We find, among other results, that electrostatic effects can
induce a strongly divergent density of states in bi- and tri-layers,
reminiscent of one-dimensional systems. The density of states at the surface of
semi-infinite stacks, on the other hand, may vanish at low energies, or show a
band of surface states, depending on the stacking order
Vortex liquid crystals in anisotropic type II superconductors
In a type II superconductor in a moderate magnetic field, the superconductor
to normal state transition may be described as a phase transition in which the
vortex lattice melts into a liquid. In a biaxial superconductor, or even a
uniaxial superconductor with magnetic field oriented perpendicular to the
symmetry axis, the vortices acquire elongated cross sections and interactions.
Systems of anisotropic, interacting constituents generally exhibit liquid
crystalline phases. We examine the possibility of a two step melting in
homogeneous type II superconductors with anisotropic superfluid stiffness from
a vortex lattice into first a vortex smectic and then a vortex nematic at high
temperature and magnetic field. We find that fluctuations of the ordered phase
favor an instability to an intermediate smectic-A in the absence of intrinsic
pinning
Induced parity violating thermal effective action for (2+1)-dimensional fermions interacting with a non-Abelian background
We study the parity breaking effective action in 2+1 dimensions, generated,
at finite temperature, by massive fermions interacting with a non-Abelian gauge
background. We explicitly calculate, in the static limit, parity violating
amplitudes up to the seven point function, which allows us to determine the
corresponding effective actions. We derive the exact parity violating effective
action when . When , there are families of terms that
can be determined order by order in perturbation theory. We attempt to
generalize our results to non-static backgrounds through the use of time
ordered exponentials and prove gauge invariance, both {\it small} and {\it
large}, of the resulting effective action. We also point out some open
questions that need to be further understood.Comment: 24 pages. Version to be published in Physical Review D with an added
appendix on the consequences of thermal gauge invarianc
Derivative Expansion and the Effective Action for the Abelian Chern-Simons Theory at Higher Orders
We study systematically the higher order corrections to the parity violating
part of the effective action for the Abelian Chern-Simons theory in 2+1
dimensions, using the method of derivative expansion. We explicitly calculate
the parity violating parts of the quadratic, cubic and the quartic terms (in
fields) of the effective action. We show that each of these actions can be
summed, in principle, to all orders in the derivatives. However, such a
structure is complicated and not very useful. On the other hand, at every order
in the powers of the derivatives, we show that the effective action can also be
summed to all orders in the fields. The resulting actions can be expressed in
terms of the leading order effective action in the static limit. We prove gauge
invariance, both large and small of the resulting effective actions. Various
other features of the theory are also brought out.Comment: 36 page
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