2,017 research outputs found
Kondo quantum dot coupled to ferromagnetic leads: Numerical renormalization group study
We systematically study the influence of ferromagnetic leads on the Kondo
resonance in a quantum dot tuned to the local moment regime. We employ Wilson's
numerical renormalization group method, extended to handle leads with a spin
asymmetric density of states, to identify the effects of (i) a finite spin
polarization in the leads (at the Fermi-surface), (ii) a Stoner splitting in
the bands (governed by the band edges) and (iii) an arbitrary shape of the
leads density of states. For a generic lead density of states the quantum dot
favors being occupied by a particular spin-species due to exchange interaction
with ferromagnetic leads leading to a suppression and splitting of the Kondo
resonance. The application of a magnetic field can compensate this asymmetry
restoring the Kondo effect. We study both the gate-voltage dependence (for a
fixed band structure in the leads) and the spin polarization dependence (for
fixed gate voltage) of this compensation field for various types of bands.
Interestingly, we find that the full recovery of the Kondo resonance of a
quantum dot in presence of leads with an energy dependent density of states is
not only possible by an appropriately tuned external magnetic field but also
via an appropriately tuned gate voltage. For flat bands simple formulas for the
splitting of the local level as a function of the spin polarization and gate
voltage are given.Comment: 18 pages, 18 figures, accepted for publication in PR
Latent class analysis of diagnostic tests for adenovirus, Bordetella pertussis and influenza virus infections in German adults with longer lasting coughs
Laboratory tests in adult outpatients with longer lasting coughs to identify a potential causal pathogen are rarely performed, and there is no gold standard for these diagnostic tests. While the diagnostic validity of serological tests for pertussis is well established their potential contribution for diagnosing adenovirus and influenza virus A and B infections is unclear. A sentinel study into the population-based incidence of longer lasting coughs in adults was done in Rostock (former East Germany) and Krefeld (former West Germany). A total of 971 outpatients who consulted general practitioners or internists were included. Inclusion criteria were coughing for 51 week and no chronic respiratory diseases. We evaluated the performance of polymerase chain reaction (PCR) as well as IgG and IgA serology, applying a latent class model for diagnosing infections with adenovirus, B. pertussis, and influenza virus A and B. The adult outpatients first sought medical attention when they had been coughing for a median of 3 weeks. In this situation, direct detection of infectious agents by PCR had a low sensitivity. Modelling showed that additional serological tests equally improved sensitivity and specificity for diagnosis for adenovirus, B. pertussis and influenza virus A and B infections. The combination of serology and PCR may improve the overall performance of diagnostic tests for B. pertussis and also for adenovirus, and influenza virus A and B infections
Ashkin-Teller universality in a quantum double model of Ising anyons
We study a quantum double model whose degrees of freedom are Ising anyons.
The terms of the Hamiltonian of this system give rise to a competition between
single and double topologies. By studying the energy spectra of the Hamiltonian
at different values of the coupling constants, we find extended gapless regions
which include a large number of critical points described by conformal field
theories with central charge c=1. These theories are part of the Z_2 orbifold
of the bosonic theory compactified on a circle. We observe that the Hilbert
space of our anyonic model can be associated with extended Dynkin diagrams of
affine Lie algebras which yields exact solutions at some critical points. In
certain special regimes, our model corresponds to the Hamiltonian limit of the
Ashkin-Teller model, and hence integrability over a wide range of coupling
parameters is established.Comment: 11 pages, minor revision
Gate-controlled spin splitting in quantum dots with ferromagnetic leads in the Kondo regime
科研費報告書収録論文(課題番号:16340097/研究代表者:前川禎通/スピン及び軌道による量子伝導の制御理論)47
Residual Kondo effect in quantum dot coupled to half-metallic ferromagnets
We study the Kondo effect in a quantum dot coupled to half-metallic
ferromagnetic electrodes in the regime of strong on-dot correlations. Using the
equation of motion technique for nonequilibrium Green functions in the slave
boson representation we show that the Kondo effect is not completely suppressed
for anti-parallel leads magnetization. In the parallel configuration there is
no Kondo effect but there is an effect associated with elastic cotunneling
which in turn leads to similar behavior of the local (on-dot) density of states
(LDOS) as the usual Kondo effect. Namely, the LDOS shows the temperature
dependent resonance at the Fermi energy which splits with the bias voltage and
the magnetic field. Moreover, unlike for non-magnetic or not fully polarized
ferromagnetic leads the only minority spin electrons can form such resonance in
the density of states. However, this resonance cannot be observed directly in
the transport measurements and we give some clues how to identify the effect in
such systems.Comment: 15 pages, 8 figures, accepted for publication in J. Phys.: Condens.
Mat
Level-occupation switching of the Quantum Dot, and phase anomalies in mesoscopic interferometry
For a variety of quantum dots, the widths of different single-particle levels
may naturally differ by orders of magnitude. In particular, the width of one
strongly coupled level may be larger than the spacing between other, very
narrow, levels. We found that in this case many consecutive Coulomb blockade
peaks are due to occupation of the same broad level. Between the peaks the
electron jumps from this level to one of the narrow levels and the transmission
through the dot at the next resonance essentially repeats that at the previous
one. This offers a natural explanation of the salient features of the behavior
of the transmission phase in an interferometer with a QD. The theory of this
effect will be reviewed with special emphasis on the role of the interactions.
New results on the dot-charging measurements and the fine structure of
occupation switchings will be presented, accompanied by the unified description
of the whole series of CB peaks caused by a single broad level. We then discuss
the case where the system approaches the Kondo regime.Comment: 30 pages in IOP format, 11 figure
NRG study of the Kondo effect in the presence of itinerant-electron ferromagnetism
The Kondo effect in quantum dots (QDs) - artificial magnetic impurities -
attached to ferromagnetic leads is studied with the numerical renormalization
group (NRG) method. It is shown that the QD level is spin-split due to presence
of ferromagnetic electrodes, leading to a suppression of the Kondo effect. We
find that the Kondo effect can be restored by compensating this splitting with
a magnetic field. Although the resulting Kondo resonance then has an unusual
spin asymmetry with a reduced Kondo temperature, the ground state is still a
locally-screened state, describable by Fermi liquid theory and a generalized
Friedel sum rule, and transport in the unitary limit is not spin dependent.Comment: 4 pages, 4 figure
Spectroscopic properties of cool Ursa Major group members
Until now, most members of the Ursa Major (UMa) group of stars have been
identified by means of kinematic criteria. However, in many cases kinematic
criteria alone are insufficient to ascertain, whether an individual star is
really a member of this group. Since photometric criteria are ineffective in
the case of cool dwarf members, one must use spectroscopic criteria.
Nevertheless, resulting membership criteria are inconclusive. We reanalyse
spectroscopic properties of cool UMa group dwarfs. In particular, we study the
distribution of iron abundance, the strength of the Li I absorption at 6708 A
and the Li abundance, and the infilling of the core of the H alpha line.
Twenty-five cool and northern bona-fide members are carefully selected from the
literature. Homogeneously measured stellar parameters and iron abundances are
given for all Sun-like stars selected, based on spectra of high resolution and
high signal-to-noise ratio. In addition, we measure the Li equivalent width and
abundance as well as the relative intensity of the H alpha core and the
corresponding chromospheric flux. The studied stars infer an average Ursa Major
group iron abundance of -0.03+-0.05 dex, which is higher by about 0.06 dex than
determined elsewhere. The Li abundance derived of Ursa Major group dwarf stars
is higher than in the Hyades at effective temperatures cooler than the Sun, but
lower than in the younger Pleiades, a result which is independent of the exact
value of the effective temperature adopted. The Sun-like and cooler dwarfs also
display chromospheric infilling of the H alpha core. We present spectroscopic
criteria that may be used to exclude non-members.Comment: accepted for publication in A&A, 19 pages, 10 figures, 7 table
- …