38,822 research outputs found
Carrier transport in 2D graphene layers
Carrier transport in gated 2D graphene monolayers is theoretically considered
in the presence of scattering by random charged impurity centers with density
. Excellent quantitative agreement is obtained (for carrier density ) with existing experimental data (Ref.
\onlinecite{kn:novoselov2004, kn:novoselov2005, kn:zhang2005, kn:kim2006,
kn:fuhrer2006}). The conductivity scales linearly with in the theory,
and shows extremely weak temperature dependence. The experimentally observed
asymmetry between electron and hole conductivities is explained by the
asymmetry in the charged impurity configuration in the presence of the gate
voltage, while the high-density saturation of conductivity for the highest
mobility samples is explained as a crossover between the long-range and the
point scattering dominated regimes. We argue that the experimentally observed
saturation of conductivity at low density arises from the charged impurity
induced inhomogeneity in the graphene carrier density which becomes severe for
.Comment: 5 pages, 4 figures, published in Phys. Rev. Let
Cosmological Vorticity in a Gravity with Quadratic Order Curvature Couplings
We analyse the evolution of the rotational type cosmological perturbation in
a gravity with general quadratic order gravitational coupling terms. The result
is expressed independently of the generalized nature of the gravity theory, and
is simply interpreted as a conservation of the angular momentum.Comment: 5 pages, revtex, no figure
Second-order Perturbations of the Friedmann World Model
We consider instability of the Friedmann world model to the second-order in
perturbations. We present the perturbed set of equations up to the second-order
in the Friedmann background world model with general spatial curvature and the
cosmological constant. We consider systems with the completely general
imperfect fluids, the minimally coupled scalar fields, the electro-magnetic
field, and the generalized gravity theories. We also present the case of null
geodesic equations, and the one based on the relativistic Boltzmann equation.
In due stage a decomposition is made for the scalar-, vector- and tensor-type
perturbations which couple each other to the second-order. Gauge issue is
resolved to each order. The basic equations are presented without imposing any
gauge condition, thus in a gauge-ready form so that we can use the full
advantage of having the gauge freedom in analysing the problems. As an
application we show that to the second-order in perturbation the relativistic
pressureless ideal fluid of the scalar-type reproduces exactly the known
Newtonian result. As another application we rederive the large-scale conserved
quantities (of the pure scalar- and tensor-perturbations) to the second order,
first shown by Salopek and Bond, now from the exact equations. Several other
applications are made as well.Comment: 61 pages; published version in Phys. Rev.
Cosmological perturbations in a generalized gravity including tachyonic condensation
We present unified ways of handling the cosmological perturbations in a class
of gravity theory covered by a general action in eq. (1). This gravity includes
our previous generalized gravity and the gravity theory motivated
by the tachyonic condensation. We present general prescription to derive the
power spectra generated from vacuum quantum fluctuations in the slow-roll
inflation era. An application is made to a slow-roll inflation based on the
tachyonic condensation with an exponential potential.Comment: 5 page
A conserved variable in the perturbed hydrodynamic world model
We introduce a scalar-type perturbation variable which is conserved in
the large-scale limit considering general sign of three-space curvature (),
the cosmological constant (), and time varying equation of state. In a
pressureless medium is {\it exactly conserved} in all scales.Comment: 4 pages, no figure, To appear in Phys. Rev.
Conserved cosmological structures in the one-loop superstring effective action
A generic form of low-energy effective action of superstring theories with
one-loop quantum correction is well known. Based on this action we derive the
complete perturbation equations and general analytic solutions in the
cosmological spacetime. Using the solutions we identify conserved quantities
characterizing the perturbations: the amplitude of gravitational wave and the
perturbed three-space curvature in the uniform-field gauge both in the
large-scale limit, and the angular-momentum of rotational perturbation are
conserved independently of changing gravity sector. Implications for
calculating perturbation spectra generated in the inflation era based on the
string action are presented.Comment: 5 pages, no figure, To appear in Phys. Rev.
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