1,075 research outputs found
Kondo effect in a quantum dot coupled to ferromagnetic leads and side-coupled to a nonmagnetic reservoir
Equilibrium transport properties of a single-level quantum dot tunnel-coupled
to ferromagnetic leads and exchange-coupled to a side nonmagnetic reservoir are
analyzed theoretically in the Kondo regime. The equilibrium spectral functions
and conductance through the dot are calculated using the numerical
renormalization group (NRG) method. It is shown that in the antiparallel
magnetic configuration, the system undergoes a quantum phase transition with
increasing exchange coupling , where the conductance drops from its maximum
value to zero. In the parallel configuration, on the other hand, the
conductance is generally suppressed due to an effective spin splitting of the
dot level caused by the presence of ferromagnetic leads, irrespective of the
strength of exchange constant. However, for ranging from J=0 up to the
corresponding critical value, the Kondo effect and quantum critical behavior
can be restored by applying properly tuned compensating magnetic field.Comment: (8 pages, 8 figs) to appear in PR
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
Fano-Kondo effect in side-coupled double quantum dots at finite temperatures and the importance of the two-stage Kondo screening
We study the zero-bias conductance through the system of two quantum dots,
one of which is embedded directly between the source and drain electrodes,
while the second dot is side-coupled to the first one through a tunneling
junction. Modeling the system using the two-impurity Anderson model, we compute
the temperature-dependence of the conductance in various parameter regimes
using the numerical renormalization group. We consider the non-interacting
case, where we study the extent of the departure from the conventional Fano
resonance line shape at finite temperatures, and the case where the embedded
and/or the side-coupled quantum dot is interacting, where we study the
consequences of the coexistence of the Kondo and Fano effects. If the
side-coupled dot is very weakly interacting, the occupancy changes by two when
the on-site energy crosses the Fermi level and a Fano-resonance-like shape is
observed. If the interaction on the side-coupled dot is sizeable, the occupancy
changes only by one and a very different line-shape results, which is strongly
and characteristically temperature dependent. These results suggest an
intriguing alternative interpretation of the recent experimental results study
of the transport properties of the side-coupled double quantum dot [Sasaki et
al., Phys. Rev. Lett. 103, 266806 (2009)]: the observed Fano-like conductance
anti-resonance may, in fact, result from the two-stage Kondo effect in the
regime where the experimental temperature is between the higher and the lower
Kondo temperature.Comment: 9 pages, 11 figures. In V2: updated references, 3 new figures,
additional discussio
Quantum size effects in Pb layers with absorbed Kondo adatoms: Determination of the exchange coupling constant
We consider the magnetic interaction of manganese phtalocyanine (MnPc)
absorbed on Pb layers which were grown on a Si substrate. We perform an
ab-initio calculation of the density of states and Kondo temperature as
function of the number of Pb monolayers. Comparison to experimental data [Phys.
Rev. Lett. 99, 256601 (2007)] then allows us to determine the exchange coupling
constant J between the spins of the adsorbed molecules and those of the Pb
host. This approach gives rise to a general and reliable method for obtaining
J, by combining experimental and numerical results.Comment: Accepted for publication in Phys. Rev. B as Brief Repor
Valence-band satellite in the ferromagnetic nickel: LDA+DMFT study with exact diagonalization
The valence-band spectrum of the ferromagnetic nickel is calculated using the
LDA+DMFT method. The auxiliary impurity model emerging in the course of the
calculations is discretized and solved with the exact diagonalization, or, more
precisely, with the Lanczos method. Particular emphasis is given to spin
dependence of the valence-band satellite that is observed around 6 eV below the
Fermi level. The calculated satellite is strongly spin polarized in accord with
experimental findings.Comment: REVTeX 4, 8 pages, 5 figure
Mixed-state aspects of an out-of-equilibrium Kondo problem in a quantum dot
We reexamine basic aspects of a nonequilibrium steady state in the Kondo
problem for a quantum dot under a bias voltage using a reduced density matrix,
which is obtained in the Fock space by integrating out one of the two
conduction channels. The integration has been carried out by discretizing the
conduction channels preserving the two-fold degeneracy due to the left-going
and right-going scattering states. The remaining subspace is described by a
single-channel Anderson model, and the statistical weight is determined by the
reduced density matrix. In the noninteracting case, it can be constructed as
the mixed states that show a close similarity to the high-temperature
distribution in equilibrium. Specifically, if the system has an inversion
symmetry, the one-particle states in an energy window between the two chemical
potentials \mu_R and \mu_L are occupied, or unoccupied, completely at random
with an equal weight. The Coulomb interaction preserves these aspects, and the
correlation functions can be expressed in a Lehmann-representation form using
the mixed-state statistical weight.Comment: 8 pages, 3 figure
Suppression of Kondo-assisted co-tunneling in a spin-1 quantum dot with Spin-Orbit interaction
Kondo-type zero-bias anomalies have been frequently observed in quantum dots
occupied by two electrons and attributed to a spin-triplet configuration that
may become stable under particular circumstances. Conversely, zero-bias
anomalies have been so far quite elusive when quantum dots are occupied by an
even number of electrons greater than two, even though a spin-triplet
configuration is more likely to be stabilized there than for two electrons. We
propose as an origin of this phenomenon the spin-orbit interaction, and we show
how it profoundly alters the conventional Kondo screening scenario in the
simple case of a laterally confined quantum dot with four electrons.Comment: 5 pages, 3 figures, submitted 05May201
Non-equilibrium Transport in the Anderson model of a biased Quantum Dot: Scattering Bethe Ansatz Phenomenology
We derive the transport properties of a quantum dot subject to a source-drain
bias voltage at zero temperature and magnetic field. Using the Scattering Bethe
Anstaz, a generalization of the traditional Thermodynamic Bethe Ansatz to open
systems out of equilibrium, we derive exact results for the quantum dot
occupation out of equilibrium and, by introducing phenomenological spin- and
charge-fluctuation distribution functions in the computation of the current,
obtain the differential conductance for large U/\Gamma. The Hamiltonian to
describe the quantum dot system is the Anderson impurity Hamiltonian and the
current and dot occupation as a function of voltage are obtained numerically.
We also vary the gate voltage and study the transition from the mixed valence
to the Kondo regime in the presence of a non-equilibrium current. We conclude
with the difficulty we encounter in this model and possible way to solve them
without resorting to a phenomenological method.Comment: 20 pages, 20 figures, published versio
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