406 research outputs found
Numerical analyses of the nonequilibrium electron transport through the Kondo impurity beside the Toulouse point
Nonequilibrium electron transport through the Kondo impurity is investigated
numerically for the system with twenty conduction-electron levels. The electron
current under finite voltage drop is calculated in terms of the `conductance
viewed as transmission' picture proposed by Landauer. Here, we take into
account the full transmission processes of both the many-body correlation and
the hybridization amplitude up to infinite order. Our results demonstrate, for
instance, how the exact solution of the differential conductance by Schiller
and Hershfield obtained at the Toulouse point becomes deformed by more
realistic interactions. The differential-conductance-peak height is suppressed
below e^2/h with the width hardly changed through reducing the Kondo coupling
from the Toulouse point, whereas it is kept unchanged by further increase of
the coupling. We calculated the nonequilibrium local Green function as well.
This clarifies the spectral property of the Kondo impurity driven far from
equilibrium
Kondo resonance in an ac driven quantum dot subjected to finite bias
We employ the time-dependent non-crossing approximation to study the time
averaged conductance for a single electron transistor in the Kondo regime when
the dot level is sinusoidally driven from its equilibrium position by means of
a gate voltage in finite bias. We find that the average conductance exhibits
significant deviation from the monotonous reduction when the applied bias is
equal to the driving frequency of the dot level. We investigate the effect of
the temperature and the driving frequency on the observed enhancement. We
attribute this behaviour to the overlap of the satellite Kondo peaks with the
split Kondo resonances formed at each lead's Fermi level. We display the
spectral function to put our interpretation into more rigorous footing.Comment: 5 pages, 4 figure
Kondo time scales for quantum dots - response to pulsed bias potentials
The response of a quantum dot in the Kondo regime to rectangular pulsed bias
potentials of various strengths and durations is studied theoretically. It is
found that the rise time is faster than the fall time, and also faster than
time scales normally associated with the Kondo problem. For larger values of
the pulsed bias, one can induce dramatic oscillations in the induced current
with a frequency approximating the splitting between the Kondo peaks that would
be present in steady state. The effect persists in the total charge transported
per pulse, which should facilitate the experimental observation of the
phenomenon.Comment: 5 pages with 4 encapsulated figures which come in separate postscript
files: latex file: text.tex figures: fig1.eps, fig2.eps, fig3.eps, fig4.ep
Fermi liquid theory for the Anderson model out of equilibrium
We study low-energy properties of the Anderson impurity under a finite bias
voltage using the perturbation theory in of Yamada and Yosida in the
nonequilibrium Keldysh diagrammatic formalism, and obtain the Ward identities
for the derivative of the self-energy with respect to . The self-energy is
calculated exactly up to terms of order , and , and the
coefficients are defined with respect to the equilibrium ground state. From
these results, the nonlinear response of the current through the impurity has
been deduced up to order .Comment: 8 pages, 1 figur
Nonlinear Response of a Kondo system: Direct and Alternating Tunneling Currents
Non - equilibrium tunneling current of an Anderson impurity system subject to
both constant and alternating electric fields is studied. A time - dependent
Schrieffer - Wolff transformation maps the time - dependent Anderson
Hamiltonian onto a Kondo one. Perturbation expansion in powers of the Kondo
coupling strength is carried out up to third order, yielding a remarkably
simple analytical expression for the tunneling current. It is found that the
zero - bias anomaly is suppressed by an ac - field. Both dc and the first
harmonic are equally enhanced by the Kondo effect, while the higher harmonics
are relatively small. These results are shown to be valid also below the Kondo
temperature.Comment: 7 pages, RevTeX, 3 PS figures attached, the article has been
significantly developed: time - dependent Schrieffer - Wolff transformation
is presented in the full form, the results are applied to the change in the
direct current induced by an alternating field (2 figures are new
Dynamical 1/N approach to time-dependent currents through quantum dots
A systematic truncation of the many-body Hilbert space is implemented to
study how electrons in a quantum dot attached to conducting leads respond to
time-dependent biases. The method, which we call the dynamical 1/N approach, is
first tested in the most unfavorable case, the case of spinless fermions (N=1).
We recover the expected behavior, including transient ringing of the current in
response to an abrupt change of bias. We then apply the approach to the
physical case of spinning electrons, N=2, in the Kondo regime for the case of
infinite intradot Coulomb repulsion. In agreement with previous calculations
based on the non-crossing approximation (NCA), we find current oscillations
associated with transitions between Kondo resonances situated at the Fermi
levels of each lead. We show that this behavior persists for a more realistic
model of semiconducting quantum dots in which the Coulomb repulsion is finite.Comment: 18 pages, 7 eps figures, discussion extended for spinless electrons
and typo
Kondo effect in real quantum dots
Exchange interaction within a quantum dot strongly affects the transport
through it in the Kondo regime. In a striking difference with the results of
the conventional model, where this interaction is neglected, here the
temperature and magnetic field dependence of the conductance may become
non-monotonic: its initial increase follows by a drop when temperature and
magnetic field are lowered
Theory of Scanning Tunneling Spectroscopy of a Magnetic Adatom on a Metallic Surface
A comprehensive theory is presented for the voltage, temperature, and spatial
dependence of the tunneling current between a scanning tunneling microscope
(STM) tip and a metallic surface with an individual magnetic adatom. Modeling
the adatom by a nondegenerate Anderson impurity, a general expression is
derived for a weak tunneling current in terms of the dressed impurity Green
function, the impurity-free surface Green function, and the tunneling matrix
elements. This generalizes Fano's analysis to the interacting case. The
differential-conductance lineshapes seen in recent STM experiments with the tip
directly over the magnetic adatom are reproduced within our model, as is the
rapid decay, \sim 10\AA, of the low-bias structure as one moves the tip away
from the adatom. With our simple model for the electronic structure of the
surface, there is no dip in the differential conductance at approximately one
lattice spacing from the magnetic adatom, but rather we see a resonant
enhancement. The formalism for tunneling into small clusters of magnetic
adatoms is developed.Comment: 12 pages, 9 figures; to appear in Phys. Rev.
Broken unitarity and phase measurements in Aharonov-Bohm interferometers
Aharonov-Bohm mesoscopic solid-state interferometers yield a conductance
which contains a term , where relates to the magnetic
flux. Experiments with a quantum dot on one of the interfering paths aim to
relate to the dot's intrinsic Friedel transmission phase, .
For closed systems, which conserve the electron current (unitarity), the
Onsager relation requires that . For open systems, we show that
depends in general on the details of the broken unitarity. Although it
gives information on the resonances of the dot, is generally not equal
to . A direct relation between and requires
specific ways of opening the system, which are discussed.Comment: 4 pages, 3 figures(eps). Phys. Rev. Letters (in press
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