36 research outputs found
Role of surface anisotropy for magnetic impurities in electron dephasing and energy relaxation and their size effect
Recently the electron dephasing and energy relaxation due to different
magnetic impurities have been extensively investigated experimentally in thin
wires and in this Letter these quantities are theoretically studied. It was
shown earlier that a magnetic impurity in a metallic host with strong
spin-orbit interaction experiences a surface anisotropy of the form which causes size effects for impurities with integer spin.
Here we show that the dephasing and the energy relaxation are influenced by the
surface anisotropy in very different ways for integer spin having a singlet
ground state. That must result also in strong size effects and may resolve the
puzzle between the concentrations estimated from the two kind of experiments.Comment: 4 pages, 5 eps figure
Can the Spin-Orbit Interaction Break the Channel Degeneracy of the Two-Channel Orbital Kondo Problem?
Two-level systems (TLS) interacting with conduction electrons are possibly
described by the two-channel Kondo Hamiltonian. In this case the channel
degeneracy is due to the real spin of the electrons. The possibility of
breaking that degeneracy has interest on his own. In fact, we show that the
interaction of the conduction electrons with a spin-orbit scatterer nearby the
TLS leads to the breaking of the channel degeneracy only in the case of
electron-hole symmetry breaking. The generated channel symmetry breaking
TLS-electron couplings are, however, too weak to result in any observable
effects. Our analysis is also relevant for heavy fermion systems.Comment: 6 pages, LaTeX using epl style, 3 eps figure
Visualizing the Formation of the Kondo Lattice and the Hidden Order in URu2Si2
Heavy electronic states originating from the f atomic orbitals underlie a
rich variety of quantum phases of matter. We use atomic scale imaging and
spectroscopy with the scanning tunneling microscope (STM) to examine the novel
electronic states that emerge from the uranium f states in URu2Si2. We find
that as the temperature is lowered, partial screening of the f electrons' spins
gives rise to a spatially modulated Kondo-Fano resonance that is maximal
between the surface U atoms. At T=17.5 K, URu2Si2 is known to undergo a 2nd
order phase transition from the Kondo lattice state into a phase with a hidden
order parameter. From tunneling spectroscopy, we identify a spatially
modulated, bias-asymmetric energy gap with a mean-field temperature dependence
that develops in the hidden order state. Spectroscopic imaging further reveals
a spatial correlation between the hidden order gap and the Kondo resonance,
suggesting that the two phenomena involve the same electronic states
Quantum dots as scatterers in electronic transport : interference and correlations
Conductance through a system consisting of a wire with side-attached quantum
dots is calculated. Such geometry of the device allows to study the coexistence
of quantum interference, electron correlations and their influence on
conductance. We underline the differences between "classical" Fano resonance in
which the resonant channel is of single-particle nature and "many-body" Fano
resonance with the resonant channel formed by Kondo effect. The influence of
electron-electron interactions on the Fano resonance shape is also analyzed.Comment: 13 pages, 3 figures, to appear in Solid State Communications, elsart
styl
Theory of Fano-Kondo effect of transport properties through quantum dots
The Fano-Kondo effect in zero-bias conductance is investigated based on a
theoretical model for the T-shaped quantum dot. The conductance as a function
of the gate voltage is generally characterized by a Fano asymmetric parameter
q. With varying temperature the conductance shows a crossover between the high
and low temperature regions compared with the Kondo temperature T_K: two Fano
asymmetric peaks at high temperatures and the Fano-Kondo plateau inside a Fano
peak at low temperatures. Temperature dependence of conductance is calculated
numerically by the Finite temperature density matrix renormalization group
method (FT-DMRG).Comment: 8 pages, 7 figure
Superconducting proximity effect in the presence of strong spin scattering
We report measurements of the four terminal temperature dependent resistance
of narrow Au wires implanted with ~100 ppm Fe impurities in proximity to
superconducting Al films. The wires show an initial decrease in resistance as
the temperature is lowered through the superconducting transition of the Al
films, but then show an increase in resistance as the temperature is lowered
further. In contrast to the case of pure Au wires in contact with a
superconducting film, the resistance at the lowest temperatures rises above the
normal state resistance. Analysis of the data shows that, in addition to
contributions from magnetic scattering and electron-electron interactions, the
temperature dependent resistivity shows a substantial contribution from the
superconducting proximity effect, which exists even in the presence of strong
spin scattering.Comment: 4 pages, 3 figure
Size Dependence In The Disordered Kondo Problem
We study here the role randomly-placed non-magnetic scatterers play on the
Kondo effect. We show that spin relaxation effects (with time )in the
vertex corrections to the Kondo self-energy lead to an exact cancellation of
the singular temperature dependence arising from the diffusion poles. For a
thin film of thickness and a mean-free path , disorder provides a
correction to the Kondo resistivity of the form
that explains both the disorder and sample-size depression of the Kondo effect
observed by Blachly and Giordano (PRB {\bf 51}, 12537 (1995)).Comment: 11 pages, LaTeX, 2 Postscript 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
Anomalous behavior of spin wave resonances in Ga_{1-x}Mn_{x}As thin films
We report ferromagnetic and spin wave resonance absorption measurements on
high quality epitaxially grown Ga_{1-x}Mn_{x}As thin films. We find that these
films exhibit robust ferromagnetic long-range order, based on the fact that up
to seven resonances are detected at low temperatures, and the resonance
structure survives to temperatures close to the ferromagnetic transition. On
the other hand, we observe a spin wave dispersion which is linear in mode
number, in qualitative contrast with the quadratic dispersion expected for
homogeneous samples. We perform a detailed numerical analysis of the
experimental data and provide analytical calculations to demonstrate that such
a linear dispersion is incompatible with uniform magnetic parameters. Our
theoretical analysis of the ferromagnetic resonance data, combined with the
knowledge that strain-induced anisotropy is definitely present in these films,
suggests that a spatially dependent magnetic anisotropy is the most likely
reason behind the anomalous behavior observed.Comment: 9 pages, including 6 figure
Dephasing of Electrons in Mesoscopic Metal Wires
We have extracted the phase coherence time of electronic
quasiparticles from the low field magnetoresistance of weakly disordered wires
made of silver, copper and gold. In samples fabricated using our purest silver
and gold sources, increases as when the temperature
is reduced, as predicted by the theory of electron-electron interactions in
diffusive wires. In contrast, samples made of a silver source material of
lesser purity or of copper exhibit an apparent saturation of
starting between 0.1 and 1 K down to our base temperature of 40 mK. By
implanting manganese impurities in silver wires, we show that even a minute
concentration of magnetic impurities having a small Kondo temperature can lead
to a quasi saturation of over a broad temperature range, while
the resistance increase expected from the Kondo effect remains hidden by a
large background. We also measured the conductance of Aharonov-Bohm rings
fabricated using a very pure copper source and found that the amplitude of the
conductance oscillations increases strongly with magnetic field. This set
of experiments suggests that the frequently observed ``saturation'' of
in weakly disordered metallic thin films can be attributed to
spin-flip scattering from extremely dilute magnetic impurities, at a level
undetectable by other means.Comment: 16 pages, 11 figures, to be published in Physical Review