38,440 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
Synchronization of the ERDA-NASA 100 LkW wind turbine generator with large utility networks
The synchronizing of a wind turbine generator against an infinite bus under random conditions is studied. With a digital computer, complete solutions for rotor speed, generator power angle, electromagnetic torque, wind turbine torque, wind turbine blade pitch angle, and armature current are obtained and presented by graphs
Transient analysis of unbalanced short circuits of the ERDA-NASA 100 kW wind turbine alternator
Unbalanced short-circuit faults on the alternator of the ERDA-NASA Mod-O100-kW experimental wind turbine are studied. For each case, complete solutions for armature, field, and damper-circuit currents; short-circuit torque; and open-phase voltage are derived directly by a mathematical analysis. Formulated results are tabulated. For the Mod-O wind turbine alternator, numerical calculations are given, and results are presented by graphs. Comparisons for significant points among the more important cases are summarized. For these cases the transients are found to be potentially severe. The effect of the alternator neutral-to-ground impedance is evaluated
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