297 research outputs found
Fundamental Limitations on Gain of THz QCL
We analyze the main physical processes in quantum cascade lasers with spatial
separation between the region of photon radiation and LO phonon emission
providing depopulation of the lower level of the optical transition. Our
purpose is to find reasons of reduction of the population inversion at low
photon energy and search for ways of its increase. The expression for the
population inversion is obtained from equation for simplified density matrix.
This allows us, on one hand, to take into account coherence of tunneling
between different levels and, on the other hand, to understand its role in
transition probabilities in a simple way. We come to the conclusion that
population inversion in THz lasers can be significantly increased by
optimization of tunneling matrix element between the two regions and LO phonon
emission time. The optimal value of the matrix element is smaller than its
maximal possible value. As well, the optimal LO phonon emission time is larger
than the time reached at LO phonon resonant emission
InAs-GaSb laser: Prospects for efficient THz emission
We suggest to use InAs/GaSb coupled quantum wells for THz lasing. In these
heterostructures THz lasing is based not on intersubband but on interband
transitions. Crucial advantages of this design in comparison with intersubband
lasers are (i) a large value of the interband dipole matrix element and (ii)
easier maintaining of population inversion. These advantages lead to a gain of
two orders of magnitude higher than for intersubband lasing. Even higher gain
can be obtained in special design InAs/GaSb W-structures where a hybridization
gap of 1-3THz is formed and optical density of states is singular.Comment: 14 pages, 2 figures. Accepted for publication in Applied Physics
Letter
Nonadiabatic charged spherical evolution in the postquasistatic approximation
We apply the postquasistatic approximation, an iterative method for the
evolution of self-gravitating spheres of matter, to study the evolution of
dissipative and electrically charged distributions in General Relativity. We
evolve nonadiabatic distributions assuming an equation of state that accounts
for the anisotropy induced by the electric charge. Dissipation is described by
streaming out or diffusion approximations. We match the interior solution, in
noncomoving coordinates, with the Vaidya-Reissner-Nordstr\"om exterior
solution. Two models are considered: i) a Schwarzschild-like shell in the
diffusion limit; ii) a Schwarzschild-like interior in the free streaming limit.
These toy models tell us something about the nature of the dissipative and
electrically charged collapse. Diffusion stabilizes the gravitational collapse
producing a spherical shell whose contraction is halted in a short
characteristic hydrodynamic time. The streaming out radiation provides a more
efficient mechanism for emission of energy, redistributing the electric charge
on the whole sphere, while the distribution collapses indefinitely with a
longer hydrodynamic time scale.Comment: 11 pages, 16 Figures. Accepted for publication in Phys Rev
Ultra-light Axions: Degeneracies with Massive Neutrinos and Forecasts for Future Cosmological Observations
A generic prediction of string theory is the existence of many axion fields.
It has recently been argued that many of these fields should be light and, like
the well known QCD axion, lead to observable cosmological consequences. In this
paper we study in detail the effect of the so-called string axiverse on large
scale structure, focusing on the morphology and evolution of density
perturbations, anisotropies in the cosmic microwave background and weak
gravitational lensing of distant galaxies. We quantify specific effects that
will arise from the presence of the axionic fields and highlight possible
degeneracies that may arise in the presence of massive neutrinos. We take
particular care understanding the different physical effects and scales that
come into play. We then forecast how the string axiverse may be constrained and
show that with a combination of different observations, it should be possible
to detect a fraction of ultralight axions to dark matter of a few percent.Comment: 24 pages, 16 figures, this version: corrected typos, some comments
added, matches published versio
Relativistic Structure, Stability and Gravitational Collapse of Charged Neutron Stars
Charged stars have the potential of becoming charged black holes or even
naked singularities. It is presented a set of numerical solutions of the
Tolman-Oppenheimer-Volkov equations that represents spherical charged compact
stars in hydrostatic equilibrium. The stellar models obtained are evolved
forward in time integrating the Einstein-Maxwell field equations. It is assumed
an equation of state of a neutron gas at zero temperature. The charge
distribution is taken as been proportional to the rest mass density
distribution. The set of solutions present an unstable branch, even with charge
to mass ratios arbitrarily close to the extremum case. It is performed a direct
check of the stability of the solutions under strong perturbations, and for
different values of the charge to mass ratio. The stars that are in the stable
branch oscillates and do not collapse, while models in the unstable branch
collapse directly to form black holes. Stars with a charge greater or equal
than the extreme value explode. When a charged star is suddenly discharged, it
don't necessarily collapse to form a black hole. A non-linear effect that gives
rise to the formation of an external shell of matter (see Ghezzi and Letelier
2005), is negligible in the present simulations. The results are in agreement
with the third law of black hole thermodynamics and with the cosmic censorship
conjecture.Comment: 27 pages, 14 figures, 4 tables, paper accepte
Magnetic Field Limitations on Advection Dominated Flows
Recent papers discussing advection dominated accretion flows (ADAF) as a
solution for astrophysical accretion problems should be treated with some
caution because of their uncertain physical basis. The suggestions underlying
ADAF involve ignoring the magnetic field reconnection in heating of the plasma
flow, assuming electron heating due only to binary Coulomb collisions with
ions. Here, we analyze the physical processes in optically thin accretion flows
at low accretion rates including the influence of an equipartition turbulent
magnetic field. For these conditions there is continuous destruction of
magnetic flux by reconnection.
The reconnection is expected to significantly heat the electrons which can
efficiently emit magnetobremstrahlung radiation. Because of this electron
emission, the radiative efficiency of the ADAF is not small. We suggest that
the small luminosities of nearby galactic black holes is due to outflows rather
than ADAF accretion.Comment: 7 pages, 3 figures, Submitted to Ap
A variational approach to the stochastic aspects of cellular signal transduction
Cellular signaling networks have evolved to cope with intrinsic fluctuations,
coming from the small numbers of constituents, and the environmental noise.
Stochastic chemical kinetics equations govern the way biochemical networks
process noisy signals. The essential difficulty associated with the master
equation approach to solving the stochastic chemical kinetics problem is the
enormous number of ordinary differential equations involved. In this work, we
show how to achieve tremendous reduction in the dimensionality of specific
reaction cascade dynamics by solving variationally an equivalent quantum field
theoretic formulation of stochastic chemical kinetics. The present formulation
avoids cumbersome commutator computations in the derivation of evolution
equations, making more transparent the physical significance of the variational
method. We propose novel time-dependent basis functions which work well over a
wide range of rate parameters. We apply the new basis functions to describe
stochastic signaling in several enzymatic cascades and compare the results so
obtained with those from alternative solution techniques. The variational
ansatz gives probability distributions that agree well with the exact ones,
even when fluctuations are large and discreteness and nonlinearity are
important. A numerical implementation of our technique is many orders of
magnitude more efficient computationally compared with the traditional Monte
Carlo simulation algorithms or the Langevin simulations.Comment: 15 pages, 11 figure
Effect of Neutrino Heating on Primordial Nucleosynthesis
We have modified the standard code for primordial nucleosynthesis to include
the effect of the slight heating of neutrinos by annihilations. There
is a small, systematic change in the He yield, , which is insensitive to the value of the baryon-to-photon ratio
for 10^{-10}\la \eta \la 10^{-9}. We also find that the
baryon-to-photon ratio decreases by about 0.5\% less than the canonical factor
of 4/11 because some of the entropy in pairs is transferred to
neutrinos. These results are in accord with recent analytical estimates.Comment: 14 pages/4 Figs (upon request
Exchange bias phenomenon in (Nd1-xYx)2/3Ca1/3MnO3 (x = 0, 0.1) perovskites
Exchange bias phenomenon, evident of antiferromagnetic-ferromagnetic phase segregation state, has been observed in (Nd1-xYx)2/3Ca1/3MnO3 (x = 0, 0.1) compounds at low temperatures. A contribution to the total magnetization of the compounds due to the ferromagnetic phase has been evaluated. It has been found that yttrium doping leads to the growth of the ferromagnetic phase fraction. The ferromagnetic phase in the doped compound has a lower coercivity Hc and more rectangular form of the hysteresis loop. The values of the exchange bias field HEB and coercivity are found to be strongly dependent on the cooling magnetic field Hcool. In sufficiently high magnetic fields, Hcool > 5 kOe, HEB in the doped compound is about twice as low as in the parent compound. This difference is attributed to a lower exchange interaction and higher saturation magnetization of the ferromagnetic phase in (Nd0.9Y0.1)2/3Ca1/3MnO3
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