46,978 research outputs found
Conserved mass models with stickiness and chipping
We study a chipping model in one dimensional periodic lattice with continuous
mass, where a fixed fraction of the mass is chipped off from a site and
distributed randomly among the departure site and its neighbours; the remaining
mass sticks to the site. In the asymmetric version, the chipped off mass is
distributed among the site and the right neighbour, whereas in the symmetric
version the redistribution occurs among the two neighbours. The steady state
mass distribution of the model is obtained using a perturbation method for both
parallel and random sequential updates. In most cases, this perturbation theory
provides a steady state distribution with reasonable accuracy.Comment: 17 pages, 4 eps figure
Heat Bath Particle Number Spectrum
We calculate the number spectrum of particles radiated during a scattering
into a heat bath using the thermal largest-time equation and the
Dyson-Schwinger equation. We show how one can systematically calculate
{d}/{d\omega} to any order using modified real time
finite-temperature diagrams. Our approach is demonstrated on a simple model
where two scalar particles scatter, within a photon-electron heat bath, into a
pair of charged particles and it is shown how to calculate the resulting
changes in the number spectra of the photons and electrons.Comment: 29 pages, LaTeX; 14 figure
Design and simulation of InGaAs/AlAsSb quantum-cascade lasers for short wavelength emission
The design and simulation of an In-0.53Ga-0.47As/Al-0.56As-0.44Sb quantum-cascade laser emitting in the near infrared is presented. Designed using a self-consistent rate equation solver coupled with an energy balance rate equation, the proposed laser has a calculated population inversion of ~20% at 77 K and sufficient gain to achieve room-temperature laser emission at λ ~2.8 µm. Threshold currents in the range 4–8 kA/cm2 are estimated as the temperature increases from 77 K to 300 K. The output characteristics of the proposed laser are compared to an existing λ ~3.1 µm In-0.53Ga-0.47As/Al-0.56As-0.44Sb quantum-cascade structure presented in the literature
Thermal modeling of terahertz quantum-cascade lasers: comparison of optical waveguides
We compare a set of experimental lattice temperature profiles measured in a surface-emitting terahertz (THz) quantum-cascade laser (QCL) with the results of a 2-D anisotropic heat diffusion model. We evaluate the temperature dependence of the cross-plane thermal conductivity (kappaperp) of the active region which is known to be strongly anisotropic due to its superlattice-like nature. Knowledge of kappaperp and its temperature dependence is crucial in order to improve the temperature performance of THz QCLs and this has been used to investigate the longitudinal lattice temperature distribution of the active region and to compare the thermal properties of metal-metal and semi-insulating surface-plasmon THz optical waveguides using a 3-D anisotropic heat diffusion model
Limit theorems for random point measures generated by cooperative sequential adsorption
We consider a finite sequence of random points in a finite domain of a
finite-dimensional Euclidean space. The points are sequentially allocated in
the domain according to a model of cooperative sequential adsorption. The main
peculiarity of the model is that the probability distribution of a point
depends on previously allocated points. We assume that the dependence vanishes
as the concentration of points tends to infinity. Under this assumption the law
of large numbers, the central limit theorem and Poisson approximation are
proved for the generated sequence of random point measures.Comment: 17 page
Yang-Lee Theory for a Nonequilibrium Phase Transition
To analyze phase transitions in a nonequilibrium system we study its grand
canonical partition function as a function of complex fugacity. Real and
positive roots of the partition function mark phase transitions. This behavior,
first found by Yang and Lee under general conditions for equilibrium systems,
can also be applied to nonequilibrium phase transitions. We consider a
one-dimensional diffusion model with periodic boundary conditions. Depending on
the diffusion rates, we find real and positive roots and can distinguish two
regions of analyticity, which can identified with two different phases. In a
region of the parameter space both of these phases coexist. The condensation
point can be computed with high accuracy.Comment: 4 pages, accepted for publication in Phys.Rev.Let
Spontaneous Symmetry Breaking in a Non-Conserving Two-Species Driven Model
A two species particle model on an open chain with dynamics which is
non-conserving in the bulk is introduced. The dynamical rules which define the
model obey a symmetry between the two species. The model exhibits a rich
behavior which includes spontaneous symmetry breaking and localized shocks. The
phase diagram in several regions of parameter space is calculated within
mean-field approximation, and compared with Monte-Carlo simulations. In the
limit where fluctuations in the number of particles in the system are taken to
zero, an exact solution is obtained. We present and analyze a physical picture
which serves to explain the different phases of the model
On Protostellar Disks in Herbig Ae/Be Stars
The spectral shape of IR emission from Herbig Ae/Be stars has been invoked as
evidence for accretion disks around high-mass protostars. Instead, we present
here models based on spherical envelopes with dust density profile
that successfully explain the observed spectral shapes. The spectral energy
distributions (SEDs) of eight primary candidates for protostellar disks are
fitted in detail for all wavelengths available, from visual to far IR. The only
envelope property adjusted in individual sources is the overall visual optical
depth, and it ranges from 0.3 to 3. In each case, our models properly reproduce
the data for both IR excess, visual extinction and reddening. The success of
our models shows that accretion disks cannot make a significant contribution to
the radiation observed in these pre-main sequence stars.Comment: 10 pages, 2 Postscript figures (included), uses aaspp4.sty. To appear
in Astrophysical Journal Letter
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