4,039 research outputs found
Relativistic Compact Objects in Isotropic Coordinates
We present a matrix method for obtaining new classes of exact solutions for
Einstein's equations representing static perfect fluid spheres. By means of a
matrix transformation, we reduce Einstein's equations to two independent
Riccati type differential equations for which three classes of solutions are
obtained. One class of the solutions corresponding to the linear barotropic
type fluid with an equation of state is discussed in detail.Comment: 9 pages, no figures, accepted for publication in Pramana-Journal of
Physic
Rotational Perturbations of Friedmann-Robertson-Walker Type Brane-World Cosmological Models
First order rotational perturbations of the Friedmann-Robertson-Walker metric
are considered in the framework of the brane-world cosmological models. A
rotation equation, relating the perturbations of the metric tensor to the
angular velocity of the matter on the brane is derived under the assumption of
slow rotation. The mathematical structure of the rotation equation imposes
strong restrictions on the temporal and spatial dependence of the brane matter
angular velocity. The study of the integrable cases of the rotation equation
leads to three distinct models, which are considered in detail. As a general
result we find that, similarly to the general relativistic case, the rotational
perturbations decay due to the expansion of the matter on the brane. One of the
obtained consistency conditions leads to a particular, purely inflationary
brane-world cosmological model, with the cosmological fluid obeying a
non-linear barotropic equation of state.Comment: 14 pages, 5 figures, REVTEX
Distinguishing impurity concentrations in GaAs and AlGaAs, using very shallow undoped heterostructures
We demonstrate a method of making a very shallow, gateable, undoped
2-dimensional electron gas. We have developed a method of making very low
resistivity contacts to these structures and systematically studied the
evolution of the mobility as a function of the depth of the 2DEG (from 300nm to
30nm). We demonstrate a way of extracting quantitative information about the
background impurity concentration in GaAs and AlGaAs, the interface roughness
and the charge in the surface states from the data. This information is very
useful from the perspective of molecular beam epitaxy (MBE) growth. It is
difficult to fabricate such shallow high-mobility 2DEGs using modulation doping
due to the need to have a large enough spacer layer to reduce scattering and
switching noise from remote ionsied dopants.Comment: 4 pages, 5 eps figure
Multilevel blocking approach to the fermion sign problem in path-integral Monte Carlo simulations
A general algorithm toward the solution of the fermion sign problem in
finite-temperature quantum Monte Carlo simulations has been formulated for
discretized fermion path integrals with nearest-neighbor interactions in the
Trotter direction. This multilevel approach systematically implements a simple
blocking strategy in a recursive manner to synthesize the sign cancellations
among different fermionic paths throughout the whole configuration space. The
practical usefulness of the method is demonstrated for interacting electrons in
a quantum dot.Comment: 4 pages RevTeX, incl. two figure
Dynamical simulation of transport in one-dimensional quantum wires
Transport of single-channel spinless interacting fermions (Luttinger liquid)
through a barrier has been studied by numerically exact quantum Monte Carlo
methods. A novel stochastic integration over the real-time paths allows for
direct computation of nonequilibrium conductance and noise properties. We have
examined the low-temperature scaling of the conductance in the crossover region
between a very weak and an almost insulating barrier.Comment: REVTex, 4 pages, 2 uuencoded figures (submitted to Phys. Rev. Lett.
Effect of next-nearest neighbor coupling on the optical spectra in bilayer graphene
We investigate the dependence of the optical conductivity of bilayer graphene
(BLG) on the intra- and inter-layer interactions using the most complete model
to date. We show that the next nearest-neighbor intralayer coupling introduces
new features in the low-energy spectrum that are highly sensitive to sample
doping, changing significantly the ``universal'' conductance. Further, its
interplay with interlayer couplings leads to an anisotropy in conductance in
the ultraviolet range. We propose that experimental measurement of the optical
conductivity of intrinsic and doped BLG will provide a good benchmark for the
relative importance of intra- and inter-layer couplings at different doping
levels.Comment: 5 pages, 5 figure
CVaR minimization by the SRA algorithm
Using the risk measure CV aR in �nancial analysis has become
more and more popular recently. In this paper we apply CV aR for portfolio optimization. The problem is formulated as a two-stage stochastic programming model, and the SRA algorithm, a recently developed heuristic algorithm, is applied for minimizing CV aR
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