980 research outputs found
Spin-orbit Scattering and the Kondo Effect
The effects of spin-orbit scattering of conduction electrons in the Kondo
regime are investigated theoretically. It is shown that due to time-reversal
symmetry, spin-orbit scattering does not suppress the Kondo effect, even though
it breaks spin-rotational symmetry, in full agreement with experiment. An
orbital magnetic field, which breaks time-reversal symmetry, leads to an
effective Zeeman splitting, which can be probed in transport measurements. It
is shown that, similar to weak-localization, this effect has anomalous magnetic
field and temperature dependence.Comment: 10 pages, RevTex, one postscript figure available on request from
[email protected]
Time Dependent Current Oscillations Through a Quantum Dot
Time dependent phenomena associated to charge transport along a quantum dot
in the charge quantization regime is studied. Superimposed to the Coulomb
blockade behaviour the current has novel non-linear properties. Together with
static multistabilities in the negative resistance region of the I-V
characteristic curve, strong correlations at the dot give rise to
self-sustained current and charge oscillations. Their properties depend upon
the parameters of the quantum dot and the external applied voltages.Comment: 4 pages, 3 figures; to appear in PR
Kondo resonances and Fano antiresonances in transport through quantum dots
The transmission of electrons through a non-interacting tight-binding chain
with an interacting side quantum dot (QD) is analized. When the Kondo effect
develops at the dot the conductance presents a wide minimum, reaching zero at
the unitary limit. This result is compared to the opposite behaviour found in
an embedded QD. Application of a magnetic field destroys the Kondo effect and
the conductance shows pairs of dips separated by the charging energy U. The
results are discussed in terms of Fano antiresonances and explain qualitatively
recent experimental results.Comment: 4 pages including 4 figure
Suppression of current in transport through parallel double quantum dots
We report our study of the I-V curves in the transport through the quantum
dot when an additional quantum dot lying in the Kondo regime is side-connected
to it. Due to the Kondo scattering off the effective spin on a side-connected
quantum dot the conductance is suppressed at low temperatures and at low
source-drain bias voltages. This zero-bias anomaly is understood as enhanced
Kondo scattering with decreasing temperature.Comment: 14 pages, 8 figure
Many-Body Approch to Spin-Dependent Transport in Quantum Dot Systems
By means of a diagram technique for Hubbard operators we show the existence
of a spin-dependent renormalization of the localized levels in an interacting
region, e.g. quantum dot, modeled by the Anderson Hamiltonian with two
conduction bands. It is shown that the renormalization of the levels with a
given spin direction is due to kinematic interactions with the conduction
sub-bands of the opposite spin. The consequence of this dressing of the
localized levels is a drastically decreased tunneling current for
ferromagnetically ordered leads compared to that of paramagnetically ordered
leads. Furthermore, the studied system shows a spin-dependent resonant
tunneling behaviour for ferromagnetically ordered leads.Comment: 8 pages, 5 figure
Resonant transmission through an open quantum dot
We have measured the low-temperature transport properties of a quantum dot
formed in a one-dimensional channel. In zero magnetic field this device shows
quantized ballistic conductance plateaus with resonant tunneling peaks in each
transition region between plateaus. Studies of this structure as a function of
applied perpendicular magnetic field and source-drain bias indicate that
resonant structure deriving from tightly bound states is split by Coulomb
charging at zero magnetic field.Comment: To be published in Phys. Rev. B (1997). 8 LaTex pages with 5 figure
Gradient descent learning in and out of equilibrium
Relations between the off thermal equilibrium dynamical process of on-line
learning and the thermally equilibrated off-line learning are studied for
potential gradient descent learning. The approach of Opper to study on-line
Bayesian algorithms is extended to potential based or maximum likelihood
learning. We look at the on-line learning algorithm that best approximates the
off-line algorithm in the sense of least Kullback-Leibler information loss. It
works by updating the weights along the gradient of an effective potential
different from the parent off-line potential. The interpretation of this off
equilibrium dynamics holds some similarities to the cavity approach of
Griniasty. We are able to analyze networks with non-smooth transfer functions
and transfer the smoothness requirement to the potential.Comment: 08 pages, submitted to the Journal of Physics
Two-species percolation and Scaling theory of the metal-insulator transition in two dimensions
Recently, a simple non-interacting-electron model, combining local quantum
tunneling via quantum point contacts and global classical percolation, has been
introduced in order to describe the observed ``metal-insulator transition'' in
two dimensions [1]. Here, based upon that model, a two-species-percolation
scaling theory is introduced and compared to the experimental data. The two
species in this model are, on one hand, the ``metallic'' point contacts, whose
critical energy lies below the Fermi energy, and on the other hand, the
insulating quantum point contacts. It is shown that many features of the
experiments, such as the exponential dependence of the resistance on
temperature on the metallic side, the linear dependence of the exponent on
density, the scale of the critical resistance, the quenching of the
metallic phase by a parallel magnetic field and the non-monotonic dependence of
the critical density on a perpendicular magnetic field, can be naturally
explained by the model.
Moreover, details such as the nonmonotonic dependence of the resistance on
temperature or the inflection point of the resistance vs. parallel magnetic are
also a natural consequence of the theory. The calculated parallel field
dependence of the critical density agrees excellently with experiments, and is
used to deduce an experimental value of the confining energy in the vertical
direction. It is also shown that the resistance on the ``metallic'' side can
decrease with decreasing temperature by an arbitrary factor in the degenerate
regime ().Comment: 8 pages, 8 figure
Correlation and symmetry effects in transport through an artificial molecule
Spectral weights and current-voltage characteristics of an artificial
diatomic molecule are calculated, considering cases where the dots connected in
series are in general different. The spectral weights allow us to understand
the effects of correlations, their connection with selection rules for
transport, and the role of excited states in the experimental conductance
spectra of these coupled double dot systems (DDS). An extended Hubbard
Hamiltonian with varying interdot tunneling strength is used as a model,
incorporating quantum confinement in the DDS, interdot tunneling as well as
intra- and interdot Coulomb interactions. We find that interdot tunneling
values determine to a great extent the resulting eigenstates and corresponding
spectral weights. Details of the state correlations strongly suppress most of
the possible conduction channels, giving rise to effective selection rules for
conductance through the molecule. Most states are found to make insignificant
contributions to the total current for finite biases. We find also that the
symmetry of the structure is reflected in the I-V characteristics, and is in
qualitative agreement with experiment.Comment: 25 figure files - REVTEX - submitted to PR
Spin-Orbit-Induced Magnetic Anisotropy for Impurities in Metallic Samples I. Surface Anisotropy
Motivated by the recent measurements of Kondo resistivity in thin films and
wires, where the Kondo amplitude is suppressed for thinner samples, the surface
anisotropy for magnetic impurities is studied. That anisotropy is developed in
those cases where in addition to the exchange interaction with the impurity
there is strong spin-orbit interaction for conduction electrons around the
impurity in the ballistic region. The asymmetry in the neighborhood of the
magnetic impurity exhibits the anisotropy axis which, in the case of a
plane surface, is perpendicular to the surface. The anisotropy energy is
for spin , and the anisotropy constant is
inversionally proportional to distance measured from the surface and
. Thus at low temperature the spin is frozen in a singlet or doublet of
lowest energy. The influence of that anisotropy on the electrical resistivity
is the subject of the following paper (part II).Comment: 28 pages, RevTeX (using epsfig), 8 eps figures included, submitted to
PR
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