68 research outputs found
Simple model for scanning tunneling spectroscopy of noble metal surfaces with adsorbed Kondo impurities
A simple model is introduced to describe conductance measurements between a
scanning tunneling microscope (STM) tip and a noble metal surface with adsorbed
transition metal atoms which display the Kondo effect. The model assumes a
realistic parameterization of the potential created by the surface and a
d3z2-r2 orbital for the description of the adsorbate. Fano lineshapes
associated with the Kondo resonance are found to be sensitive to details of the
adsorbate-substrate interaction. For instance, bringing the adsorbate closer to
the surface leads to more asymmetric lineshapes while their dependence on the
tip distance is weak. We find that it is important to use a realistic surface
potential, to properly include the tunnelling matrix elements to the tip and to
use substrate states which are orthogonal to the adsorbate and tip states. An
application of our model to Co adsorbed on Cu explains the difference in the
lineshapes observed between Cu(100) and Cu(111) surfaces.Comment: 11 pages, 8 eps figure
STM conductance of Kondo impurities on open and structured surfaces
We study the scanning tunneling microscopy response for magnetic atoms on
open and structured surfaces using Wilson's renormalization group. We observe
Fano resonances associated with Kondo resonances and interference effects. For
a magnetic atom in a quantum corral coupled to the confined surface states, and
experimentally relevant parameters, we observe a large confinement induced
effect not present in the experiments. These results suggest that the Kondo
screening is dominated by the bulk electrons rather than the surface ones.Comment: 6 pages, 6 figure
Interaction between Kondo impurities in a quantum corral
We calculate the spectral densities for two impurities inside an elliptical
quantum corral using exact diagonalization in the relevant Hilbert subspace and
embedding into the rest of the system. For one impurity, the space and energy
dependence of the change in differential conductance observed
in the quantum mirage experiment is reproduced. In presence of another
impurity, is very sensitive to the hybridization between
impurity and bulk. The impurities are correlated ferromagnetically between
them. A hopping eV between impurities destroy the Kondo
resonance.Comment: 4 pages, 4 figure
Influence of microwave fields on the electron transport through a quantum dot in the presence of a direct tunneling between leads
We consider the time-dependent electron transport through a quantum dot
coupled to two leads in the presence of the additional over-dot (bridge)
tunneling channel. By using the evolution operator method together with the
wide-band limit approximation we derived the analytical formulaes for the
quantum dot charge and current flowing in the system. The influence of the
external microwave field on the time-average quantum dot charge, the current
and the derivatives of the average current with respect to the gate and
source-drain voltages has been investigated for a wide range of parameters.Comment: 28 Pages, 11 Postscript figure
Predicting a Gapless Spin-1 Neutral Collective Mode branch for Graphite
Using the standard tight binding model of 2d graphite with short range
electron repulsion, we find a gapless spin-1, neutral collective mode branch
{\em below the particle-hole continuum} with energy vanishing linearly with
momenta at the and points in the BZ. This spin-1 mode has a wide
energy dispersion, 0 to and is not Landau damped. The `Dirac cone
spectrum' of electrons at the chemical potential of graphite generates our
collective mode; so we call this `spin-1 zero sound' of the `Dirac sea'.
Epithermal neutron scattering experiments, where graphite single crystals are
often used as analyzers (an opportunity for `self-analysis'!), and spin
polarized electron energy loss spectroscopy (SPEELS) can be used to confirm and
study our collective mode.Comment: 4 pages of LaTex file, 3 eps figure file
Braggoriton--Excitation in Photonic Crystal Infiltrated with Polarizable Medium
Light propagation in a photonic crystal infiltrated with polarizable
molecules is considered. We demonstrate that the interplay between the spatial
dispersion caused by Bragg diffraction and polaritonic frequency dispersion
gives rise to novel propagating excitations, or braggoritons, with intragap
frequencies. We derive the braggoriton dispersion relation and show that it is
governed by two parameters, namely, the strength of light-matter interaction
and detuning between the Bragg frequency and that of the infiltrated molecules.
We also study defect-induced states when the photonic band gap is divided into
two subgaps by the braggoritonic branches and find that each defect creates two
intragap localized states inside each subgap.Comment: LaTeX, 8 pages, 5 figure
A Tunable Two-impurity Kondo system in an atomic point contact
Two magnetic atoms, one attached to the tip of a Scanning Tunneling
Microscope (STM) and one adsorbed on a metal surface, each constituting a Kondo
system, have been proposed as one of the simplest conceivable systems
potentially exhibiting quantum critical behaviour. We have succeeded in
implementing this concept experimentally for cobalt dimers clamped between an
STM tip and a gold surface. Control of the tip-sample distance with
sub-picometer resolution allows us to tune the interaction between the two
cobalt atoms with unprecedented precision. Electronic transport measurements on
this two-impurity Kondo system reveal a rich physical scenario which is
governed by a crossover from local Kondo screening to non-local singlet
formation due to antiferromagnetic coupling as a function of separation of the
cobalt atoms.Comment: 22 pages, 5 figure
One- and many-body effects on mirages in quantum corrals
Recent interesting experiments used scanning tunneling microscopy to study
systems involving Kondo impurities in quantum corrals assembled on Cu or noble
metal surfaces. The solution of the two-dimensional one-particle Schrodinger
equation in a hard wall corral without impurity is useful to predict the
conditions under which the Kondo effect can be projected to a remote location
(the quantum mirage). To model a soft circular corral, we solve this equation
under the potential W*delta(r-r0), where r is the distance to the center of the
corral and r0 its radius. We expand the Green's function of electron surface
states Gs0 for r<r0 as a discrete sum of contributions from single poles at
energies epsilon_i-I*delta_i. The imaginary part delta_i is the half-width of
the resonance produced by the soft confining potential, and turns out to be a
simple increasing function of epsilon_i. In presence of an impurity, we solve
the Anderson model at arbitrary temperatures using the resulting expression for
Gs0 and perturbation theory up to second order in the Coulomb repulsion U. We
calculate the resulting change in the differential conductance Delta dI/dV as a
function of voltage and space, in circular and elliptical corrals, for
different conditions, including those corresponding to recent experiments. The
main features are reproduced. The role of the direct hybridization between
impurity and bulk, the confinement potential, the size of the corral and
temperature on the intensity of the mirage are analyzed. We also calculate
spin-spin correlation functions.Comment: 13 pages, 12 figures, accepted for publication in Phys. Rev. B.
Calculations of spin correlations within an additional approximation adde
Kondo Effect on Mesoscopic Scale (Review)
Following the discovery of the Kondo effect the bulk transport and magnetic
behavior of the dilute magnetic alloys have been successfully described. In the
last fifteen years new directions have been developed as the study of the
systems of reduced dimensions and the artificial atoms so called quantum dots.
In this review the first subject is reviewed starting with the scanning
tunneling microscope (STM) study of a single magnetic impurity. The next
subject is the reduction of the amplitude of the Kondo effect in samples of
reduced dimension which was explained by the surface magnetic anisotropy which
blocks the motion of the integer spin nearby the surface. The electron
dephasing and energy relaxation experiments are discussed with the possible
explanation including the surface anisotropy, where the situation in cases of
integer and half-integer spins is very different. Finally, the present
situation of the theory of dynamical structural defects is briefly presented
which may lead to two-channel Kondo behavior.Comment: 8 pages, submitted to the JPSJ Special Issue "Kondo effect -- 40
years after the Discovery
Manipulating Kondo Temperature via Single Molecule Switching
Two conformations of isolated single TBrPP-Co molecules on a Cu(111) surface
are switched by applying +2.2 V voltage pulses from a scanning tunneling
microscope tip at 4.6 K. The TBrPP-Co has a spin-active cobalt atom caged at
its center and the interaction between the spin of this cobalt atom and free
electrons from the Cu(111) substrate can cause a Kondo resonance. Tunneling
spectroscopy data reveal that switching from the saddle to a planar molecular
conformation enhances spin-electron coupling, which increases the associated
Kondo temperature from 130 K to 170 K. This result demonstrates that the Kondo
temperature can be manipulated just by changing molecular conformation without
altering chemical composition of the molecule.Comment: To appear in Nano Lett (2006
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