13,781 research outputs found
Electron Fabry-Perot interferometer with two entangled magnetic impurities
We consider a one-dimensional (1D) wire along which single conduction
electrons can propagate in the presence of two spin-1/2 magnetic impurities.
The electron may be scattered by each impurity via a contact-exchange
interaction and thus a spin-flip generally occurs at each scattering event.
Adopting a quantum waveguide theory approach, we derive the stationary states
of the system at all orders in the electron-impurity exchange coupling
constant. This allows us to investigate electron transmission for arbitrary
initial states of the two impurity spins. We show that for suitable electron
wave vectors, the triplet and singlet maximally entangled spin states of the
impurities can respectively largely inhibit the electron transport or make the
wire completely transparent for any electron spin state. In the latter case, a
resonance condition can always be found, representing an anomalous behaviour
compared to typical decoherence induced by magnetic impurities. We provide an
explanation for these phenomena in terms of the Hamiltonian symmetries.
Finally, a scheme to generate maximally entangled spin states of the two
impurities via electron scattering is proposed.Comment: 19 page
Effect of Static Disorder in an Electron Fabry-Perot Interferometer with Two Quantum Scattering Centers
In a recent paper -- F. Ciccarello \emph{et al.}, New J. Phys. \textbf{8},
214 (2006) -- we have demonstrated that the electron transmission properties of
a one-dimensional (1D) wire with two identical embedded spin-1/2 impurities can
be significantly affected by entanglement between the spins of the scattering
centers. Such effect is of particular interest in the control of transmission
of quantum information in nanostructures and can be used as a detection scheme
of maximally entangled states of two localized spins. In this letter, we relax
the constraint that the two magnetic impurities are equal and investigate how
the main results presented in the above paper are affected by a static disorder
in the exchange coupling constants of the impurities. Good robustness against
deviation from impurity symmetry is found for both the entanglement dependent
transmission and the maximally entangled states generation scheme.Comment: 4 pages, 5 figure
Comparison of the Spherical Averaged Pseudopotential Model with the Stabilized Jellium Model
We compare Kohn-Sham results (density, cohesive energy, size and effect of
charging) of the Spherical Averaged Pseudopotential Model with the Stabilized
Jellium Model for clusters of sodium and aluminum with less than 20 atoms. We
find that the Stabilized Jellium Model, although conceptually and practically
more simple, gives better results for the cohesive energy and the elastic
stiffness. We use the Local Density Approximation as well as the Generalized
Gradient Approximation to the exchange and correlation energies.Comment: 13 pages, latex, 8 figures, compressed postscript version available
at http://www.fis.uc.pt/~vieir
Self-Similarity of Friction Laws
The change of the friction law from a mesoscopic level to a macroscopic level
is studied in the spring-block models introduced by Burridge-Knopoff. We find
that the Coulomb law is always scale invariant. Other proposed scaling laws are
only invariant under certain conditions.}Comment: Plain TEX. Figures not include
Intrinsic kinetics of biofilms formed under turbulent flow and low substrate concentrations
Reactor operating conditions strongly affect the
behaviour of biofilm systems, namely their stability and
the substrate removal. In this paper, the penetration of
substrate and the activity of biofilms formed by Pseudomonas
fluorescens under turbulent flow and low substrate
concentrations, are studied. A first order diffusion-reaction
model was applied to results of biofilm accumulation
in steady and non-steady-state. The substrate consumption
rate of the biofilm was calculated based on the on-line
determination of the biofilm accumulated on the surface.
This approach is important when the residence time or the
substrate concentration on the reactor is very low. Also,
the mass transfer of substrate inside the biofilm was
measured for every case under study and introduced in the
model.
The fraction of biofilm penetrated by the substrate depends
on the velocity of the fluid that contacts the biological
matrix: contrary to biofilms formed at higher
velocities, lower velocities give raise to non completely
penetrated biofilms. This fact seems to be associated to the
biofilm internal structure in terms of biomass density and
compactness of the matrix. They remove more substrate
per reactor volume, but are less resistant from an hydrodynamic
point of view. In conclusion, biofilms formed at
higher velocities in turbulent flow allow a more stable
reactor operation.Fundação para a Ciência e a Tecnologia (FCT) - PRAXIS XXI, 2.1/BIO/37/94.
União Europeia - Fundo de Desenvolvimento Regional Europeu (FDRE) - INTERREG, 01/REG II/6/96
Physical stability and biological activity of biofilms under turbulent flow and low substrate concentration
The paper focuses on biofilms subject to turbulent
flow and high liquid velocity (of the order of
1 m s ˉ¹) which can be found in heat exchangers, water
distribution systems and in some wastewater reactors. An
overall model describing biofilm development is presented,
which includes the effects of biomass detachment due
to the hydrodynamic forces. A methodology for estimating
substrate consumption from data obtained through continuous
monitoring of biofilm growth is presented. Results
show that the physical stability of the biofilm increases
with the liquid velocity, while the rate of substrate consumption
decreases.Fundação para a Ciência e a Tecnologia - PRAXIS XXI, 2.1/BIO/37/94 .
Uniãoo Europeia - Fundo de Desenvolvimento Regional Europeu - Programme INTERREG - 01/REG II/6/9
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