606 research outputs found
Kondo screening cloud in a one dimensional wire: Numerical renormalization group study
We study the Kondo model --a magnetic impurity coupled to a one dimensional
wire via exchange coupling-- by using Wilson's numerical renormalization group
(NRG) technique. By applying an approach similar to which was used to compute
the two impurity problem we managed to improve the bad spatial resolution of
the numerical renormalization group method. In this way we have calculated the
impurity spin - conduction electron spin correlation function which is a
measure of the Kondo compensation cloud whose existence has been a long
standing problem in solid state physics. We also present results on the
temperature dependence of the Kondo correlations.Comment: published versio
Absorption and Emission in quantum dots: Fermi surface effects of Anderson excitons
Recent experiments measuring the emission of exciton recombination in a
self-organized single quantum dot (QD) have revealed that novel effects occur
when the wetting layer surrounding the QD becomes filled with electrons,
because the resulting Fermi sea can hybridize with the local electron levels on
the dot. Motivated by these experiments, we study an extended Anderson model,
which describes a local conduction band level coupled to a Fermi sea, but also
includes a local valence band level. We are interested, in particular, on how
many-body correlations resulting from the presence of the Fermi sea affect the
absorption and emission spectra. Using Wilson's numerical renormalization group
method, we calculate the zero-temperature absorption (emission) spectrum of a
QD which starts from (ends up in) a strongly correlated Kondo ground state. We
predict two features: Firstly, we find that the spectrum shows a power law
divergence close to the threshold, with an exponent that can be understood by
analogy to the well-known X-ray edge absorption problem. Secondly, the
threshold energy - below which no photon is absorbed (above which no
photon is emitted) - shows a marked, monotonic shift as a function of the
exciton binding energy Comment: 10 pages, 9 figure
Dynamical conductance in the two-channel Kondo regime of a double dot system
We study finite-frequency transport properties of the double-dot system
recently constructed to observe the two-channel Kondo effect [R. M. Potok et
al., Nature 446, 167 (2007)]. We derive an analytical expression for the
frequency-dependent linear conductance of this device in the Kondo regime. We
show how the features characteristic of the 2-channel Kondo quantum critical
point emerge in this quantity, which we compute using the results of conformal
field theory as well as numerical renormalization group methods. We determine
the universal cross-over functions describing non-Fermi liquid vs. Fermi liquid
cross-overs and also investigate the effects of a finite magnetic field.Comment: 11 pages in PRB forma
Towards a microscopic description of dimer adsorbates on metallic surfaces
Despite the experimental successes of Scanning Tunneling Microscopy (STM) and
the interest in more complex magnetic nanostructures, our present understanding
and theoretical description of STM spectra of magnetic adatoms is mainly
phenomenological and most often ignores many-body effects. Here, we propose a
theory which includes a microscopic description of the wave functions of the
substrate and magnetic adatoms together with quantum many-body effects. To test
our theory, we have computed the STM spectra of magnetic Cobalt monomers and
dimers adsorbed on metallic Copper surfaces and succesfully compared our
results to recent available experimental data.Comment: 4 pages, 2 figures, discussion of calculation of RKKY interaction and
connection to NRG included. Extended discussion on calculations of the
one-electron parameters of Anderson model. Typos correcte
Quantum Magnetic Impurities in Magnetically Ordered Systems
We discuss the problem of a spin 1/2 impurity immersed in a spin S
magnetically ordered background. We show that the problem maps onto a
generalization of the dissipative two level system (DTLS) with two independent
heat baths, associated with the Goldstone modes of the magnet, that couple to
different components of the impurity spin operator. Using analytical
perturbative renormalization group (RG) methods and accurate numerical
renormalization group (NRG) we show that contrary to other dissipative models
there is quantum frustration of decoherence and quasi-scaling even in the
strong coupling regime. We make predictions for the behavior of the impurity
magnetic susceptibility that can be measured in nuclear magnetic resonance
(NMR) experiments. Our results may also have relevance to quantum computation.Comment: 4 pages, 3 figure
Kondo quantum dot coupled to ferromagnetic leads: Numerical renormalization group study
We systematically study the influence of ferromagnetic leads on the Kondo
resonance in a quantum dot tuned to the local moment regime. We employ Wilson's
numerical renormalization group method, extended to handle leads with a spin
asymmetric density of states, to identify the effects of (i) a finite spin
polarization in the leads (at the Fermi-surface), (ii) a Stoner splitting in
the bands (governed by the band edges) and (iii) an arbitrary shape of the
leads density of states. For a generic lead density of states the quantum dot
favors being occupied by a particular spin-species due to exchange interaction
with ferromagnetic leads leading to a suppression and splitting of the Kondo
resonance. The application of a magnetic field can compensate this asymmetry
restoring the Kondo effect. We study both the gate-voltage dependence (for a
fixed band structure in the leads) and the spin polarization dependence (for
fixed gate voltage) of this compensation field for various types of bands.
Interestingly, we find that the full recovery of the Kondo resonance of a
quantum dot in presence of leads with an energy dependent density of states is
not only possible by an appropriately tuned external magnetic field but also
via an appropriately tuned gate voltage. For flat bands simple formulas for the
splitting of the local level as a function of the spin polarization and gate
voltage are given.Comment: 18 pages, 18 figures, accepted for publication in PR
Assisted hopping and interaction effects in impurity models
We study, using Numerical Renormalization Group methods, the generalization
of the Anderson impurity model where the hopping depends on the filling of the
impurity. We show that the model, for sufficiently large values of the assisted
hopping term, shows a regime where local pairing correlations are enhanced.
These correlations involve pairs fluctuating between on site and nearest
neighbor positions
A renormalization-group analysis of the interacting resonant level model at finite bias: Generic analytic study of static properties and quench dynamics
Using a real-time renormalization group method we study the minimal model of
a quantum dot dominated by charge fluctuations, the two-lead interacting
resonant level model, at finite bias voltage. We develop a set of RG equations
to treat the case of weak and strong charge fluctuations, together with the
determination of power-law exponents up to second order in the Coulomb
interaction. We derive analytic expressions for the charge susceptibility, the
steady-state current and the conductance in the situation of arbitrary system
parameters, in particular away from the particle-hole symmetric point and for
asymmetric Coulomb interactions. In the generic asymmetric situation we find
that power laws can be observed for the current only as function of the level
position (gate voltage) but not as function of the voltage. Furthermore, we
study the quench dynamics after a sudden switch-on of the level-lead couplings.
The time evolution of the dot occupation and current is governed by exponential
relaxation accompanied by voltage-dependent oscillations and characteristic
algebraic decay.Comment: 24 pages, 13 figures; revised versio
Influence of gas flow rate on liquid distribution in trickle-beds using perforated plates as liquid distributors
Two wire mesh tomography devices and a liquid collector were used to study
the influence of the gas flow rate on liquid distribution when fluids
distribution on top of the reactor is ensured by a perforated plate. In
opposition to most of the studies realized by other authors, conditions in
which the gas has a negative impact in liquid distribution were evidenced.
Indeed, the obtained results show that the influence of gas flow rate depends
on the quality of the initial distribution, as the gas forces the liquid to
"respect" the distribution imposed at the top of the reactor. Finally, a
comparison between the two measuring techniques shows the limitations of the
liquid collector and the improper conclusions to which its use could lead
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