374 research outputs found
Resonant Tunneling through Multi-Level and Double Quantum Dots
We study resonant tunneling through quantum-dot systems in the presence of
strong Coulomb repulsion and coupling to the metallic leads. Motivated by
recent experiments we concentrate on (i) a single dot with two energy levels
and (ii) a double dot with one level in each dot. Each level is twofold
spin-degenerate. Depending on the level spacing these systems are physical
realizations of different Kondo-type models. Using a real-time diagrammatic
formulation we evaluate the spectral density and the non-linear conductance.
The latter shows a novel triple-peak resonant structure.Comment: 4 pages, ReVTeX, 4 Postscript figure
Fano Effect through Parallel-coupled Double Coulomb Islands
By means of the non-equilibrium Green function and equation of motion method,
the electronic transport is theoretically studied through a parallel-coupled
double quantum dots(DQD) in the presence of the on-dot Coulomb correlation,
with an emphasis put on the quantum interference. It has been found that in the
Coulomb blockage regime, the quantum interference between the bonding and
antiboding DQD states or that between their Coulomb blockade counterparts may
result in the Fano resonance in the conductance spectra, and the Fano peak
doublet may be observed under certain non-equilibrium condition. The
possibility of manipulating the Fano lineshape is predicted by tuning the
dot-lead coupling and magnetic flux threading the ring connecting the dots and
leads. Similar to the case without Coulomb interaction, the direction of the
asymmetric tail of Fano lineshape can be flipped by the external field. Most
importantly, by tuning the magnetic flux, the function of four relevant states
can be interchanged, giving rise to the swap effect, which might play a key
role as a qubit in the quantum computation.Comment: 7 pages, 5 figure
Retention in an antiretroviral therapy programme during an era of decreasing drug cost in Limbe, Cameroon
<p>Abstract</p> <p>Background</p> <p>In 2002, Cameroon initiated scale up of antiretroviral therapy (ART); on 1 October 2004, a substantial reduction in ART cost occurred. We assessed the impact of this event and other factors on enrolment and retention in care among HIV-infected patients initiating ART from February 2002 to December 2005 at the single ART clinic serving the Southwest Region in Limbe, Cameroon.</p> <p>Methods</p> <p>We retrospectively analyzed clinical and pharmacy payment records of HIV-infected patients initiating ART according to national guidelines. We compared two cohorts of patients, enrolled before and after 1 October 2004, to determine if price reduction was associated with enhanced enrolment. We assessed factors associated with retention and survival by Cox proportional hazards models. Retention in care implied patients who had contact with the healthcare system as of 31 December 2005 (including those who were transferred to continue care in other ART centres), although these patients may have interrupted therapy at some time. A patient who was not retained in care may have dropped out (lost to follow up) or died.</p> <p>Results</p> <p>Mean enrolment rates for 2920 patients who initiated ART before and after the price reduction were 46.5 and 95.5 persons/month, respectively (p < 0.001). The probabilities of remaining alive and in care were 0.66 (95% CI 0.64-0.68) at six months, 0.58 (95% CI 0.56-0.60) at one year, 0.47 (95% CI 0.45-0.49) at two years and 0.35 (95% CI 0.32-0.38) at three years; they were not significantly different between the two cohorts of patients enrolled before and after the price reduction over the first 15 months of comparable follow up (hazard ratio 1.1; 95% CI 0.9-1.2, p = 0.27). In multivariable analysis using multiple imputations to compensate for missing values, factors associated with dropping out of care or dying were male gender (HR 1.33 [1.18-1.50], p = 0.003), treatment paid by self, family or partly by other (HR 3.05 [1.99-4.67], p < 0.001), and, compared with residents of Limbe, living more than 150 km from Limbe (HR 1.41 [1.18-1.69], p < 0.001), or being residents of Douala (HR 1.51 [1.16-1.98], p < 0.001).</p> <p>Conclusions</p> <p>Reducing the cost of ART increased enrolment of clients in the programme, but did not change retention in care. In a system where most clients pay for ART, an accessible clinic location may be more important than the cost of medication for retention in care. Decentralizing ART clinics might improve retention and survival among patients on ART.</p
Higher-Order Results for the Relation between Channel Conductance and the Coulomb Blockade for Two Tunnel-Coupled Quantum Dots
We extend earlier results on the relation between the dimensionless tunneling
channel conductance and the fractional Coulomb blockade peak splitting
for two electrostatically equivalent dots connected by an arbitrary number
of tunneling channels with bandwidths much larger than the
two-dot differential charging energy . By calculating through second
order in in the limit of weak coupling (), we illuminate
the difference in behavior of the large- and
small- regimes and make more plausible extrapolation to the
strong-coupling () limit. For the special case of
and strong coupling, we eliminate an apparent ultraviolet
divergence and obtain the next leading term of an expansion in . We show
that the results we calculate are independent of such band structure details as
the fraction of occupied fermionic single-particle states in the weak-coupling
theory and the nature of the cut-off in the bosonized strong-coupling theory.
The results agree with calculations for metallic junctions in the
limit and improve the previous good
agreement with recent two-channel experiments.Comment: 27 pages, 1 RevTeX file with 4 embedded Postscript figures. Uses eps
Suppression of level hybridization due to Coulomb interactions
We investigate an ensemble of systems formed by a ring enclosing a magnetic
flux. The ring is coupled to a side stub via a tunneling junction and via
Coulomb interaction. We generalize the notion of level hybridization due to the
hopping, which is naturally defined only for one-particle problems, to the
many-particle case, and we discuss the competition between the level
hybridization and the Coulomb interaction. It is shown that strong enough
Coulomb interactions can isolate the ring from the stub, thereby increasing the
persistent current. Our model describes a strictly canonical system (the number
of carriers is the same for all ensemble members). Nevertheless for small
Coulomb interactions and a long side stub the model exhibits a persistent
current typically associated with a grand canonical ensemble of rings and only
if the Coulomb interactions are sufficiently strong does the model exhibit a
persistent current which one expects from a canonical ensemble.Comment: 19 pages, 6 figures, uses iop style files, version as publishe
Fine structure in the off-resonance conductance of small Coulomb blockade systems
We show how a fine, multiple-peak structure can arise in the off-resonance,
zero-bias conductance of Coulomb blockade systems. In order to understand how
this effect comes about one must abandon the orthodox, mean-field understanding
of the Coulomb blockade phenomenon and consider quantum fluctuations in the
occupation of the single-particle electronic levels. We illustrate such an
effect with a spinless Anderson-like model for multi-level systems and an
equation-of-motion method for calculating Green's functions that combines two
simple decoupling schemes.Comment: 5 pages, 3 figures, postscript file also available at
http://www.pa.uky.edu/~palacios/papers/eom.ps One figure added. Discussion of
results extende
Incoherent tunnelling through two quantum dots with Coulomb interaction
The Ohmic conductance and current through two quantum dots in series is
investigated for the case of incoherent tunnelling. A generalised master
equation is employed to include the discrete nature of the energy levels.
Regions of negative differential conductance can occur in the I-V
characteristics. Transport is dominated by matching energy levels, even when
they do not occur at the charge degeneracy points.Comment: RevTeX + epsf.sty + 13 figure
Level Statistics of XXZ Spin Chains with Discrete Symmetries: Analysis through Finite-size Effects
Level statistics is discussed for XXZ spin chains with discrete symmetries
for some values of the next-nearest-neighbor (NNN) coupling parameter. We show
how the level statistics of the finite-size systems depends on the NNN coupling
and the XXZ anisotropy, which should reflect competition among quantum chaos,
integrability and finite-size effects. Here discrete symmetries play a central
role in our analysis. Evaluating the level-spacing distribution, the spectral
rigidity and the number variance, we confirm the correspondence between
non-integrability and Wigner behavior in the spectrum. We also show that
non-Wigner behavior appears due to mixed symmetries and finite-size effects in
some nonintegrable cases.Comment: 19 pages, 6 figure
Bistability in the Tunnelling Current through a Ring of Coupled Quantum Dots
We study bistability in the electron transport through a ring of N coupled
quantum dots with two orbitals in each dot. One orbital is localized (called b
orbital) and coupling of the b orbitals in any two dots is negligible; the
other is delocalized in the plane of the ring (called d orbital), due to
coupling of the d orbitals in the neighboring dots, as described by a
tight-binding model. The d orbitals thereby form a band with finite width. The
b and d orbitals are connected to the source and drain electrodes with a
voltage bias V, allowing the electron tunnelling. Tunnelling current is
calculated by using a nonequilibrium Green function method recently developed
to treat nanostructures with multiple energy levels. We find a bistable effect
in the tunnelling current as a function of bias V, when the size N>50; this
effect scales with the size N and becomes sizable at N~100. The temperature
effect on bistability is also discussed. In comparison, mean-field treatment
tends to overestimate the bistable effect.Comment: Published in JPSJ; minor typos correcte
Growing Grain for Others or How to Detect surplus Production?
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