447 research outputs found
Kondo Temperature in Multilevel Quantum Dots
We develop a general method to evaluate the Kondo temperature in a multilevel
quantum dot that is weakly coupled to conducting leads. Our theory reveals that
the Kondo temperature is strongly enhanced when the intradot energy-level
spacing is comparable to or smaller than the charging energy. We propose an
experiment to test our result, which consists of measuring the size-dependence
of the Kondo temperature.Comment: 4 pages, 1 figure and supplementary material. Revised and improved
version, to appear in Phys. Rev. Let
Analysis of the Kondo effect in ferromagnetic atomic-sized contacts
Atomic contacts made of ferromagnetic metals present zero-bias anomalies in
the differential conductance due to the Kondo effect. These systems provide a
unique opportunity to perform a statistical analysis of the Kondo parameters in
nanostructures since a large number of contacts can be easily fabricated using
break-junction techniques. The details of the atomic structure differ from one
contact to another so a large number of different configurations can be
statistically analyzed. Here we present such a statistical analysis of the
Kondo effect in atomic contacts made from the ferromagnetic transition metals
Ni, Co and Fe. Our analysis shows clear differences between materials that can
be understood by fundamental theoretical considerations. This combination of
experiments and theory allow us to extract information about the origin and
nature of the Kondo effect in these systems and to explore the influence of
geometry and valence in the Kondo screening of atomic-sized nanostructures.Comment: 17 pages, 11 figure
Magnetization reversal and nonexponential relaxation via instabilities of internal spin waves in nanomagnets
A magnetic particle with atomic spins ordered in an unstable direction is an
example of a false vacuum that decays via excitation of internal spin waves.
Coupled evolution of the particle's magnetization (or the vacuum state) and
spin waves, considered in the time-dependent vacuum frame, leads to a peculiar
relaxation that is very fast at the beginning but slows down to a
nonexponential long tail at the end. The two main scenarios are linear and
exponential spin-wave instabilities. For the former, the longitudinal and
transverse relaxation rates have been obtained analytically. Numerical
simulations show that the particle's magnetization strongly decreases in the
middle of reversal and then recovers.Comment: 6 EPL pages, 4 figure
Localization Length in Anderson Insulator with Kondo Impurities
The localization length, , in a 2--dimensional Anderson insulator
depends on the electron spin scattering rate by magnetic impurities,
. For antiferromagnetic sign of the exchange, %constant, the time
is {\em itself a function of }, due to the Kondo correlations. We
demonstrate that the unitary regime of localization is impossible when the
concentration of magnetic impurities, , is smaller than a critical
value, . For , the dependence of on the
dimensionless conductance, , is {\em reentrant}, crossing over to unitary,
and back to orthogonal behavior upon increasing . Sensitivity of Kondo
correlations to a weak {\em parallel} magnetic field results in a giant
parallel magnetoresistance.Comment: 5 pages, 1 figur
Simple mechanism for a positive exchange bias
We argue that the interface coupling, responsible for the positive exchange
bias (HE) observed in ferromagnetic/compensated antiferromagnetic (FM/AF)
bilayers, favors an antiferromagnetic alignment. At low cooling field this
coupling polarizes the AF spins close to the interface, which spin
configuration persists after the sample is cooled below the Neel temperature.
This pins the FM spins as in Bean's model and gives rise to a negative HE. When
the cooling field increases, it eventually dominates and polarizes the AF spins
in an opposite direction to the low field one. This results in a positive HE.
The size of HE and the crossover cooling field are estimated. We explain why HE
is mostly positive for an AF single crystal, and discuss the role of interface
roughness on the magnitude of HE, and the quantum aspect of the interface
coupling.Comment: 10 pages, 2 figures, to be published on May 1 issue of PR
Fourier transform spectroscopy of d-wave quasiparticles in the presence of atomic scale pairing disorder
The local density of states power spectrum of optimally doped
BiSrCaCuO (BSCCO) has been interpreted in terms of
quasiparticle interference peaks corresponding to an "octet'' of scattering
wave vectors connecting k-points where the density of states is maximal. Until
now, theoretical treatments have not been able to reproduce the experimentally
observed weights and widths of these "octet'' peaks; in particular, the
predominance of the dispersing "q'' peak parallel to the Cu-O bond
directions has remained a mystery. In addition, such theories predict
"background'' features which are not observed experimentally. Here, we show
that most of the discrepancies can be resolved when a realistic model for the
out-of-plane disorder in BSCCO is used. Weak extended potential scatterers,
which are assumed to represent cation disorder, suppress large-momentum
features and broaden the low-energy "q''-peaks, whereas scattering at order
parameter variations, possibly caused by a dopant-modulated pair interaction
around interstitial oxygens, strongly enhances the dispersing "q''-peaks.Comment: 7 pages, 3 figure
Bound State and Order Parameter Mixing Effect by Nonmagnetic Impurity Scattering in Two-band Superconductors
We investigate nonmagnetic impurity effects in two-band superconductors,
focusing on the effects of interband scatterings. Within the Born
approximation, it is known that interband scatterings mix order parameters in
the two bands. In particular, only one averaged energy gap appears in the
excitation spectrum in the dirty limit. [G. Gusman: J. Phys. Chem. Solids {\bf
28} (1967) 2327.] In this paper, we take into account the interband scattering
within the -matrix approximation beyond the Born approximation in the
previous work. We show that, although the interband scattering is responsible
for the mixing effect, this effect becomes weak when the interband scattering
becomes very strong. In the strong interband scattering limit, a two-gap
structure corresponding to two order parameters recovers in the superconducting
density of states. We also show that a bound state appears around a nonmagnetic
impurity depending on the phase of interband scattering potential.Comment: 28pages, 10 figure
Superconductivity in two-band systems with variable charge carrier density. The case of MgB2
The theory of thermodynamic properties of two-band superconductor with
reduced density charge carriers is developed on the base of phonon
superconducting mechanism with strong electron-phonon interaction. This theory
is adapted to describe the behavior of critical temperature Tc, energy gaps
Delta1, Delta2, and the relative jump of electron specific heat (Cs - Cn)/Cn in
the point T = Tc along with the variation of charge carrier density in the
compound MgB2 when substitutional impurities with different valence are
introduced into the system. It is shown, that according to the filling
mechanism of energy bands which overlap on Fermi surface, the quantities Tc,
Delta1, Delta2 decrease when this compound is doped with electrons and remain
constant or weakly change when the system is doped with holes. The theory
qualitatively agrees with the experimental data. Also is shown that the
consideration of inter- and intraband scattering of electrons on impurity
potential improves this agreement.Comment: 19 pages, 6 figures, 1 table. to be published in JETP (first number
2007
Low Energy Properties of the (n,n) Carbon Nanotubes
According to band theory, an ideal undoped (n,n) carbon nanotube is metallic.
We show that the electron-electron interaction causes it to become Mott
insulating with a spin gap. More interestingly, upon doping it develops
superconducting fluctuations.Comment: 5pages, 2eps figures, one reference added, final version, accepted to
PR
- …