241 research outputs found
Mesoscopic Fluctuations in Quantum Dots in the Kondo Regime
Properties of the Kondo effect in quantum dots depend sensitively on the
coupling parameters and so on the realization of the quantum dot -- the Kondo
temperature itself becomes a mesoscopic quantity. Assuming chaotic dynamics in
the dot, we use random matrix theory to calculate the distribution of both the
Kondo temperature and the conductance in the Coulomb blockade regime. We study
two experimentally relevant cases: leads with single channels and leads with
many channels. In the single-channel case, the distribution of the conductance
is very wide as fluctuates on a logarithmic scale. As the number of
channels increases, there is a slow crossover to a self-averaging regime.Comment: 4 pages, 3 figure
Magnetotransport through a strongly interacting quantum dot
We study the effect of a magnetic field on the conductance through a strongly
interacting quantum dot by using the finite temperature extension of Wilson's
numerical renormalization group method to dynamical quantities. The quantum dot
has one active level for transport and is modelled by an Anderson impurity
attached to left and right electron reservoirs. Detailed predictions are made
for the linear conductance and the spin-resolved conductance as a function of
gate voltage, temperature and magnetic field strength. A strongly coupled
quantum dot in a magnetic field acts as a spin filter which can be tuned by
varying the gate voltage. The largest spin-filtering effect is found in the
range of gate voltages corresponding to the mixed valence regime of the
Anderson impurity model.Comment: Revised version, to appear in PRB, 4 pages, 4 figure
Fano effect of a strongly interacting quantum dot in contact with superconductor
The physics of a system consisting of an Aharonov Bohm (AB) interferometer
containing a single level interacting quantum dot (QD) on one of its arms, and
attached to normal (N) and superconducting (S) leads is studied and elucidated.
Here the focus is directed mainly on N-AB-S junctions but the theory is capable
of studying S-AB-S junctions as well. The interesting physics comes into play
under the conditions that both the Kondo effect in the QD and the the Fano
effect are equally important.It is found the conductance of the junction is
suppressed as the Fano effect becomes more dominant.Comment: 4 pages, Talk to be given at the NATO Conference MQO, Bled, Slovenia
7-10 September 200
Kondo effect in multielectron quantum dots at high magnetic fields
We present a general description of low temperature transport through a
quantum dot with any number of electrons at filling factor . We
provide a general description of a novel Kondo effect which is turned on by
application of an appropriate magnetic field. The spin-flip scattering of
carriers by the quantum dot only involves two states of the scatterer which may
have a large spin. This process is described by spin-flip Hubbard operators,
which change the angular momentum, leading to a Kondo Hamiltonian. We obtain
antiferromagnetic exchange couplings depending on tunneling amplitudes and
correlation effects. Since Kondo temperature has an exponential dependence on
exchange couplings, quantitative variations of the parameters in different
regimes have important experimental consequences. In particular, we discuss the
{\it chess board} aspect of the experimental conductance when represented in a
grey scale as a function of both the magnetic field and the gate potential
affecting the quantum dot
Interference and interaction effects in multi-level quantum dots
Using renormalization group techniques, we study spectral and transport
properties of a spinless interacting quantum dot consisting of two levels
coupled to metallic reservoirs. For strong Coulomb repulsion and an applied
Aharonov-Bohm phase , we find a large direct tunnel splitting
between the levels of
the order of the level broadening . As a consequence we discover a
many-body resonance in the spectral density that can be measured via the
absorption power. Furthermore, for , we show that the system can be
tuned into an effective Anderson model with spin-dependent tunneling.Comment: 5 pages, 4 figures included, typos correcte
Non-equilibrium Kondo effect in asymmetrically coupled quantum dot
The quantum dot asymmetrically coupled to the external leads has been
analysed theoretically by means of the equation of motion (EOM) technique and
the non-crossing approximation (NCA). The system has been described by the
single impurity Anderson model. To calculate the conductance across the device
the non-equilibrium Green's function technique has been used. The obtained
results show the importance of the asymmetry of the coupling for the appearance
of the Kondo peak at nonzero voltages and qualitatively explain recent
experiments.Comment: 7 pages, 6 figures, Physical Review B (accepted for publication
Resonance Kondo Tunneling through a Double Quantum Dot at Finite Bias
It is shown that the resonance Kondo tunneling through a double quantum dot
(DQD) with even occupation and singlet ground state may arise at a strong bias,
which compensates the energy of singlet/triplet excitation. Using the
renormalization group technique we derive scaling equations and calculate the
differential conductance as a function of an auxiliary dc-bias for parallel DQD
described by SO(4) symmetry. We analyze the decoherence effects associated with
the triplet/singlet relaxation in DQD and discuss the shape of differential
conductance line as a function of dc-bias and temperature.Comment: 11 pages, 6 eps figures include
Photon-Phonon-assisted tunneling through a single-molecular quantum dot
Based on exactly mapping of a many-body electron-phonon interaction problem
onto a one-body problem, we apply the well-established nonequilibrium Green
function technique to solve the time-dependent phonon-assisted tunneling at low
temperature through a single-molecular quantum dot connected to two leads,
which is subject to a microwave irradiation field. It is found that in the
presence of the electron-phonon interaction and the microwave irradiation
field, the time-average transmission and the nonlinear differential conductance
display additional peaks due to pure photon absorption or emission processes
and photon-absorption-assisted phonon emission processes. The variation of the
time-average current with frequency of the microwave irradiation field is also
studied.Comment: 9 pages, 6 figures, submitted to Phys. Rev. B. accepted by Phys. Rev.
Spin Fluctuation and Persistent Current in a Mesoscopic Ring Coupled to a Quantum Dot
We investigate the persistent current influenced by the spin fluctuations in
a mesoscopic ring weakly coupled to a quantum dot. It is shown that the Kondo
effect gives rise to some unusual features of the persistent current in the
limit where the charge transfer between two subsystems is suppressed. Various
aspects of the crossover from a delocalized to a localized dot limit are
discussed in relation with the effect of the coherent response of the Kondo
cloud to the Aharonov-Bohm flux.Comment: 4 pages, 2 figure
Anti-Kondo resonance in transport through a quantum wire with a side-coupled quantum dot
An interacting quantum dot side-coupled to a perfect quantum wire is studied.
Transport through the quantum wire is investigated by using an exact sum rule
and the slave-boson mean field treatment. It is shown that the Kondo effect
provides a suppression of the transmission due to the destructive interference
of the ballistic channel and the Kondo channel. At finite temperatures,
anti-resonance behavior is found as a function of the quantum dot level
position, which is interpreted as a crossover from the high temperature Kondo
phase to the low temperature charge fluctuation phase.Comment: 4 pages Revtex, 3 eps figure
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