38,682 research outputs found
LDA+Gutzwiller Method for Correlated Electron Systems
Combining the density functional theory (DFT) and the Gutzwiller variational
approach, a LDA+Gutzwiller method is developed to treat the correlated electron
systems from {\it ab-initio}. All variational parameters are self-consistently
determined from total energy minimization. The method is computationally
cheaper, yet the quasi-particle spectrum is well described through kinetic
energy renormalization. It can be applied equally to the systems from weakly
correlated metals to strongly correlated insulators. The calculated results for
SrVO, Fe, Ni and NiO, show dramatic improvement over LDA and LDA+U.Comment: 4 pages, 3 figures, 1 tabl
Bifurcation Boundary Conditions for Switching DC-DC Converters Under Constant On-Time Control
Sampled-data analysis and harmonic balance analysis are applied to analyze
switching DC-DC converters under constant on-time control. Design-oriented
boundary conditions for the period-doubling bifurcation and the saddle-node
bifurcation are derived. The required ramp slope to avoid the bifurcations and
the assigned pole locations associated with the ramp are also derived. The
derived boundary conditions are more general and accurate than those recently
obtained. Those recently obtained boundary conditions become special cases
under the general modeling approach presented in this paper. Different analyses
give different perspectives on the system dynamics and complement each other.
Under the sampled-data analysis, the boundary conditions are expressed in terms
of signal slopes and the ramp slope. Under the harmonic balance analysis, the
boundary conditions are expressed in terms of signal harmonics. The derived
boundary conditions are useful for a designer to design a converter to avoid
the occurrence of the period-doubling bifurcation and the saddle-node
bifurcation.Comment: Submitted to International Journal of Circuit Theory and Applications
on August 10, 2011; Manuscript ID: CTA-11-016
A relativistic calculation of super-Hubble suppression of inflation with thermal dissipation
We investigated the evolution of the primordial density perturbations
produced by inflation with thermal dissipation. A full relativistic analysis on
the evolution of initial perturbations from the warm inflation era to a
radiation-dominated universe has been developed. The emphasis is on tracking
the ratio between the adiabatic and the isocurvature mode of the initial
perturbations. This result is employed to calculate a testable factor: the
super-Hubble suppression of the power spectrum of the primordial perturbations.
We show that based on the warm inflation scenario, the super-Hubble suppression
factor, , for an inflation with thermal dissipation is at least 0.5. This
prediction does not depend on the details of the model parameters. If is
larger than 0.5, it implies that the friction parameter is larger than
the Hubble expansion parameter during the inflation era.Comment: 22 pages, 3 figures, use RevTex, accepted by Class. Quant. Gra
Effect of high-K dielectrics on charge transport in graphene
The effect of various dielectrics on charge mobility in single layer graphene
is investigated. By calculating the remote optical phonon scattering arising
from the polar substrates, and combining it with their effect on Coulombic
impurity scattering, a comprehensive picture of the effect of dielectrics on
charge transport in graphene emerges. It is found that though high-
dielectrics can strongly reduce Coulombic scattering by dielectric screening,
scattering from surface phonon modes arising from them wash out this advantage.
By comparing the room-temperature transport properties with narrow-bandgap
III-V semiconductors, strategies to improve the mobility in single layer
graphene are outlined.Comment: 6 pages, 4 Figure
Transverse self-modulation of ultra-relativistic lepton beams in the plasma wakefield accelerator
The transverse self-modulation of ultra-relativistic, long lepton bunches in
high-density plasmas is explored through full-scale particle-in-cell
simulations. We demonstrate that long SLAC-type electron and positron bunches
can become strongly self-modulated over centimeter distances, leading to wake
excitation in the blowout regime with accelerating fields in excess of 20 GV/m.
We show that particles energy variations exceeding 10 GeV can occur in
meter-long plasmas. We find that the self-modulation of positively and
negatively charged bunches differ when the blowout is reached. Seeding the
self-modulation instability suppresses the competing hosing instability. This
work reveals that a proof-of-principle experiment to test the physics of bunch
self-modulation can be performed with available lepton bunches and with
existing experimental apparatus and diagnostics.Comment: 8 pages, 8 figures, accepted for publication in Physics of Plasma
Approximation methods for control of acoustic/structure models with piezoceramic actuators
The active control of acoustic pressure in a 2-D cavity with a flexible boundary (a beam) is considered. Specifically, this control is implemented via piezoceramic patches on the beam which produces pure bending moments. The incorporation of the feedback control in this manner leads to a system with an unbounded input term. Approximation methods in this manner leads to a system with an unbounded input term. Approximation methods in the context of linear quadratic regulator (LQR) state space control formulation are discussed and numerical results demonstrating the effectiveness of this approach in computing feedback controls for noise reduction are presented
The origin of scale-scale correlations of the density perturbations during inflation
We show that scale-scale correlations are a generic feature of slow-roll
inflation theories. These correlations result from the long-time tails
characteristic of the time dependent correlations because the long wavelength
density perturbation modes are diffusion-like. A relationship between the
scale-scale correlations and time-correlations is established providing a way
to reveal the time correlations of the perturbations during inflation. This
mechanism provides for a testable prediction that the scale-scale correlations
at two different spatial points will vanish.Comment: Accepted for publication, International Journal of Modern Physics,
vol. 8 No.6 (Dec 1999
Spin waves in the block checkerboard antiferromagnetic phase
Motivated by the discovery of new family 122 iron-based superconductors, we
present the theoretical results on the ground state phase diagram, spin wave
and dynamic structure factor of the extended Heisenberg model. In
the reasonable physical parameter region of , we fi{}nd the
block checkerboard antiferromagnetic order phase is stable. There are two
acoustic branches and six optical branches spin wave in the block checkerboard
antiferromagnetic phase, which has analytic expression in the high symmetry
points. To compare the further neutron scattering experiments, we discuss the
saddlepoint structure in the magnetic excitation spectrum and calculate the
predicted inelastic neutron scattering pattern based on linear spin wave
theory
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