53 research outputs found
Spin-Orbit-Induced Kondo Size Effect in Thin Films with 5/2-spin Impurities
Recently, for spin impurities quite different size dependence of the
Kondo contribution to the resistivity was found experimentally than for S=2.
Therefore previous calculation about the effect of the spin-orbit-induced
magnetic anisotropy on the Kondo amplitude of the resistivity is extended to
the case of impurity spin which differs from the integer spin case as
the ground state is degenerated. In this case the Kondo contribution remains
finite when the sample size goes to zero and the thickness dependence in the
Kondo resistivity is much weaker for Cu(Mn). The behavior of the Kondo
coefficient as a function of the thickness depends on the Kondo temperature,
that is somewhat stronger for larger . Comparing our results with a recent
experiment in thin Cu(Mn) films, we find a good agreement.Comment: 8 pages, ReVTeX + 4 figures (Postscript
Shape-induced magnetic anisotropy in dilute magnetic alloys
We extend the theory of the surface-induced magnetic anisotropy to mesoscopic
samples with arbitrary geometry. The shape-induced anisotropy of impurity spins
in small brick-shaped grains of dilute magnetic alloys is studied in detail.
The surface-induced blocking of a magnetic-impurity spin is shown to be very
sensitive to geometric parameters of a grain. This implies that the apparent
discrepancy between the experimental data of different groups on the size
dependence of the Kondo resistivity can result from different microstructure of
the used samples. In order to interpret recent experimental data on the
anomalous Hall effect in thin polycrystalline Fe doped Au films, we analyse the
magnetisation of impurity spins as a function of the impurity position and of
the grain shape.Comment: 10 pages, 6 figures, E-mail addresses: [email protected],
[email protected], [email protected]
Revised theory of the magnetic surface anisotropy of impurities in metallic mesoscopic samples
In several experiments the magnitude of the contribution of magnetic
impurities to the Kondo resistivity shows size dependence in mesoscopic
samples. It was suggested ten years ago that magnetic surface anisotropy can be
responsible for the size dependence in cases where there is strong spin-orbit
interaction in the metallic host. The anisotropy energy has the form where is the vector perpendicular to the
plane surface, is the spin of the magnetic impurity and is
inversely proportional to distance measured from the surface. It has been
realized that in the tedious calculation an unjustified approximation was
applied for the hybridizations of the host atom orbitals with the conduction
electrons which depend on the position of the host atoms. Namely, the momenta
of the electrons were replaced by the Fermi momentum . That is
reinvestigated considering the -dependence which leads to singular energy
integrals and in contrary to the previous result is oscillating like
and the distance dependence goes like in the
asymptotic region. As the anisotropy is oscillating, for integer spin the
ground state is either a singlet or a doublet depending on distance , but in
the case of the doublet there is no direct electron induced transition between
those two states at zero temperature. Furthermore, for half-integer ()
spin it is always a doublet with direct transition only in half of the cases.Comment: 10 pages, 4 figure
Theory of magnetoresistance in films of dilute magnetic alloys
Earlier a magnetic anisotropy for magnetic impurities nearby the surface of
non-magnetic host was proposed in order to explain the size dependence of the
Kondo effect in dilute magnetic alloys. Recently Giordano has measured the
magnetoresistance of dilute Au(Fe) films for different thicknesses well above
the Kondo temperature . In this way he verified the existence of that
anisotropy even for such a case where the Kondo effect is not dominating. For
detailed comparison of that suggestion with experiments, the magnetic field
dependence of the magnetoresistance is calculated in the lowest approximation,
thus in the second order of the exchange coupling. The strength of the
anisotropy is very close to earlier estimates deduced from the size dependence
of the Kondo resistivity amplitude.Comment: (11 pages, 8 figures, essential changes compared to the old version
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
Microscopic theory for quantum mirages in quantum corrals
Scanning tunneling microscopy permits to image the Kondo resonance of a
single magnetic atom adsorbed on a metallic surface. When the magnetic impurity
is placed at the focus of an elliptical quantum corral, a Kondo resonance has
been recently observed both on top of the impurity and on top of the focus
where no magnetic impurity is present. This projection of the Kondo resonance
to a remote point on the surface is referred to as quantum mirage. We present a
quantum mechanical theory for the quantum mirage inside an ideal quantum corral
and predict that the mirage will occur in corrals with shapes other than
elliptical
Energy relaxation due to magnetic impurities in mesoscopic wires: Logarithmic approach
The transport in mesoscopic wires with large applied bias voltage has
recently attracted great interest by measuring the energy distribution of the
electrons at a given point of the wire, in Saclay. In the diffusive limit with
negligible energy relaxation that shows two sharp steps at the Fermi energies
of the two contacts, which are broadened due to the energy relaxation. In some
of the experiments the broadening is reflecting an anomalous energy relaxation
rate proportional to instead of valid for Coulomb
electron-electron interaction, where is the energy transfer. Later it has
been suggested that such relaxation rate can be due to electron-electron
interaction mediated by Kondo impurities. In the present paper the latter is
systematically studied in the logarithmic approximation valid above the Kondo
temperature. In the case of large applied bias voltage Kondo resonances are
formed at the steps of the distribution function and they are narrowed by
increasing the bias. An additional Korringa energy broadening occurs for the
spins which smears the Kondo resonances, and the renormalized coupling can be
replaced by a smooth but essentially enhanced average coupling (factor of
8-10). Thus the experimental data can be described by formulas without
logarithmic Kondo corrections, but with enhanced coupling. In certain regions
of large bias, that averaged coupling depends weakly on the bias. In those
cases the distribution function depends only on the ratio of the electron
energy and the bias, showing scaling behavior. The impurity concentrations
estimated from those experiments and other dephasing experiments can be very
different, and a possible explanation considering the surface spin anisotropy
due to strong spin-orbit interaction is the subject of our earlier paper.Comment: 12 pages, RevTex
Spin-Orbit-Induced Magnetic Anisotropy for Impurities in Metallic Samples II. Finite Size Dependence in the Kondo Resistivity
The electrical resistivity including the Kondo resistivity increase at low
temperature is calculated for thin films of dilute magnetic alloys. Assuming
that in the non-magnetic host the spin-orbit interaction is strong like in Au
and Cu, the magnetic impurities have a surface anisotropy calculated in part I.
That anisotropy hinders the motion of the spin. Including that anisotropy the
effective electron-impurity coupling is calculated by using the second order
renormalization group equations. The amplitude of the Kondo resistivity
contribution is reduced as the position of the impurity approaches the surface
but the increase occurs approximately at the bulk Kondo temperature. Different
proximity effects observed by Giordano are also explained qualitatively where
the films of magnetic alloys are covered by pure second films with different
mean free path. The theory explains the experimental results in those cases
where a considerable amount of impurities is at the surface inside the
ballistic region.Comment: 39 pages, RevTeX (using epsfig), 15 eps figures included, submitted
to PR
Many-body theory of the quantum mirage
In recent scanning tunneling microscopy experiments, confinement in an
elliptical corral has been used to project the Kondo effect from one focus to
the other one. I solve the Anderson model at arbitrary temperatures, for an
impurity hybridized with eigenstates of an elliptical corral, each of which has
a resonant level width delta. This width is crucial. If delta < 20 meV, the
Kondo peak disappears, while if delta > 80 meV, the mirage disappears. For
particular conditions, a stronger mirage with the impurity out of the foci is
predicted.Comment: 5 pages, 5 figures. Some clarifications of the method added, and a
reference included to show that the hybridization of the impurity with bulk
states can be neglecte
- …