4 research outputs found
Donor states in modulation-doped Si/SiGe heterostructures
We present a unified approach for calculating the properties of shallow
donors inside or outside heterostructure quantum wells. The method allows us to
obtain not only the binding energies of all localized states of any symmetry,
but also the energy width of the resonant states which may appear when a
localized state becomes degenerate with the continuous quantum well subbands.
The approach is non-variational, and we are therefore also able to evaluate the
wave functions. This is used to calculate the optical absorption spectrum,
which is strongly non-isotropic due to the selection rules. The results
obtained from calculations for Si/SiGe quantum wells allow us to
present the general behavior of the impurity states, as the donor position is
varied from the center of the well to deep inside the barrier. The influence on
the donor ground state from both the central-cell effect and the strain arising
from the lattice mismatch is carefully considered.Comment: 17 pages, 10 figure
A matter-dominated cosmological model with variable and . Confrontation of theoretical predictions with observational data
In the framework of renormalization-group improved cosmologies, we analyze
both theoretically and observationally the exact and general solution of the
matter-dominated cosmological equations, using the expression of \Lambda =
\Lambda(G) already determined by the integration method employed in a previous
paper. A rough comparison between such a model and the concordance \LambdaCDM
model as to the magnitude-redshift relationship has been already done, without
showing any appreciable differences. We here perform a more refined study of
how astrophysical data (Union2 set) on type-I supernovae, gamma ray bursts (in
a sample calibrated in a model independent way with the SneIa dataset), and gas
fraction in galaxy clusters (using a sample of Chandra measurements of the
X-ray gas mass fraction) affect the model and constrain its parameters. We also
apply a cosmographic approach to our cosmological model and estimate the
cosmographic parameters by fitting both the supernovae and the gamma ray bursts
datasets. We show that this matter-dominated cosmological model with variable
Newton parameter and variable cosmological term is indeed compatible with the
observations above (on type Ia supernovae, the gamma ray bursts Hubble diagram,
and the gas mass fraction in X-ray luminous galaxy clusters). The cosmographic
approach adopted confirms such conclusions. Finally, it seems possible to
include radiation into the model, since numerical integration of the equations
derived by the presence of both radiation and matter shows that, after
inflation, the total density parameter is initially dominated by the radiation
contribution and later by the matter one.Comment: 31 pages, 9 figure