1,272 research outputs found
Phonon-assisted and magnetic field induced Kondo tunneling in single molecular devices
We consider the Kondo tunneling induced by multiphonon emission/absorption
processes in magnetic molecular complexes with low-energy singlet-triplet spin
gap and show that the number of assisting phonons may be changed by varying the
Zeeman splitting of excited triplet state. As a result, the structure of
multiphonon Kondo resonances may be scanned by means of magnetic field tuning.Comment: 7 pages, 6 figures. Shortened version of this paper will be published
in the Proceedings of the International Conference "Phonons2007" (J. Phys:
Conf. Series
Racial and Ethnic Differences in Falls Among Older Adults: a Systematic Review and Meta-analysis
The aim of this systematic review and meta-analysis was to determine whether differences in reported fall rates exist between different ethnic groups. Searches were carried out on four databases: Medline, Cumulative Index to Nursing and Allied Health Literature (CINAHL), Scopus, and Web of Science. Only English language studies with community-dwelling participants aged 60 + years were included. Studies also needed to compare fall prevalence for at least two or more ethnic groups. Two reviewers independently screened all articles and evaluated study quality. Twenty-three articles were included for systematic review, and meta-analyses were carried out on the 16 retrospective studies that reported falls in the previous 12 months. The Asian group demonstrated significantly lower fall prevalence than all other ethnic groups at 13.89% (10.87, 16.91). The Hispanic group had a fall prevalence of 18.54% (12.95, 24.13), closely followed by the Black group at 18.60% (13.27, 23.93). The White group had the highest prevalence at 23.77% (18.66, 28.88). Some studies provided adjusted estimates of effect statistics for the odds/risk of falls, which showed that differences still existed between some ethnic groups even after adjusting for other risk factors. Overall, differences in fall prevalence do appear to exist between different ethnic groups, although the reasons for these differences currently remain undetermined and require further investigation. These findings highlight the need to provide more ethnically tailored responses to public health challenges, which could potentially increase the adherence to prevention interventions, and allow for a more targeted use of resources
Kondo effect of a Co atom on Cu(111) in contact with an Fe tip
Single Co atoms, which exhibit a Kondo effect on Cu(111), are contacted with
Cu and Fe tips in a low-temperature scanning tunneling microscope. With Fe
tips, the Kondo effect persists with the Abrikosov-Suhl resonance significantly
broadened. In contrast, for Cu-covered W tips, the resonance width remains
almost constant throughout the tunneling and contact ranges. The distinct
changes of the line width are interpreted in terms of modifications of the Co d
state occupation owing to hybridization with the tip apex atoms.Comment: 4 pages, 3 figure
Non-Kondo mechanism for resistivity minimum in spin ice conduction systems
We present a mechanism of resistivity minimum in conduction electron systems
coupled with localized moments, which is distinguished from the Kondo effect.
Instead of the spin-flip process in the Kondo effect, electrons are elastically
scattered by local spin correlations which evolve in a particular way under
geometrical frustration as decreasing temperature. This is demonstrated by the
cellular dynamical mean-field theory for a spin-ice type Kondo lattice model on
a pyrochlore lattice. Peculiar temperature dependences of the resistivity,
specific heat, and magnetic susceptibility in the non-Kondo mechanism are
compared with the experimental data in metallic Ir pyrochlore oxides.Comment: 5 pages, 3 figures, accepted for publication in Physical Review
Letter
Tunable "Doniach Phase Diagram" for strongly-correlated nanoclusters
Exact diagonalization calculations reveal that the energy spacing in
the conduction band tunes the interplay between the {\it local} Kondo and {\it
non local} RKKY interactions, giving rise to a "Doniach phase diagram" for a
nanocluster with regions of prevailing Kondo or RKKY correlations. The parity
of the total number of electrons alters the competition between the Kondo and
RKKY correlations. This interplay may be relevant to experimental realizations
of small rings or quantum dots with tunable magnetic properties. Below a
critical value V of the hybridization the susceptibility exhibits a low-T
exponential activation behavior determined by the interplay of the spin gap and
.Comment: 4 pages, 5 figure
Topological Kondo effect with Majorana fermions
The Kondo effect is a striking consequence of the coupling of itinerant
electrons to a quantum spin with degenerate energy levels. While degeneracies
are commonly thought to arise from symmetries or fine-tuning of parameters, the
recent emergence of Majorana fermions has brought to the fore an entirely
different possibility: a "topological degeneracy" which arises from the
nonlocal character of Majorana fermions. Here we show that nonlocal quantum
spins formed from these degrees of freedom give rise to a novel "topological
Kondo effect". This leads to a robust non-Fermi liquid behavior, known to be
difficult to achieve in the conventional Kondo context. Focusing on mesoscopic
superconductor devices, we predict several unique transport signatures of this
Kondo effect, which would demonstrate the non-local quantum dynamics of
Majorana fermions, and validate their potential for topological quantum
computation
The Strong Coupling Fixed-Point Revisited
In recent work we have shown that the Fermi liquid aspects of the strong
coupling fixed point of the s-d and Anderson models can brought out more
clearly by interpreting the fixed point as a renormalized Anderson model,
characterized by a renormalized level , resonance width,
, and interaction , and a simple prescription for their
calculation was given using the numerical renormalization group (NRG). These
three parameters completely specify a renormalized perturbation theory (RPT)
which leads to exact expressions for the low temperature behaviour. Using a
combination of the two techniques, NRG to determine ,
, and , and then substituting these in the RPT
expressions gives a very efficient and accurate way of calculating the low
temperature behaviour of the impurity as it avoids the necessity of subtracting
out the conduction electron component. Here we extend this approach to an
Anderson model in a magnetic field, so that , ,
and become dependent on the magnetic field. The de-renormalization
of the renormalized quasiparticles can then be followed as the magnetic field
strength is increased. Using these running coupling constants in a RPT
calculation we derive an expression for the low temperature conductivity for
arbitrary magnetic field strength.Comment: Contribution to JPSJ volume commemorating the 40th anniversary of the
publication of Kondo's original pape
Slave-boson approach to the infinite-U Anderson-Holstein impurity model
The infinite- Anderson-Holstein impurity model is studied with a focus on
the interplay between the strong electron correlation and the weak
electron-phonon interaction. The slave boson method has been employed in
combination with the large degeneracy expansion (1/N) technique. The charge and
spin susceptibilities and the phonon propagator are obtained in the
approximation scheme where the saddle point configuration and the Gaussian 1/N
fluctuations are taken into account. The spin susceptibility is found not to be
renormalized by electron-phonon interaction, while the charge susceptibility is
renormalized.
From the renormalized charge susceptibility the Kondo temperature is found to
increase by the electron-phonon interaction. It turns out that the bosonic 1/N
Gaussian fluctuations play a very crucial role, in particular, for the phonon
propagator.Comment: 12pages, 3 figures. Published in Physical Review
Theory of Fano-Kondo effect in quantum dot systems: temperature dependence of the Fano line shapes
The Fano-Kondo effect in zero-bias conductance is studied based on a
theoretical model for the T-shaped quantum dot by the finite temperature
density matrix renormalization group method. The modification of the two Fano
line shapes at much higher temperatures than the Kondo temperature is also
investigated by the effective Fano parameter estimated as a fitting parameter.Comment: 2 pages, 2 figures, the proceeding of SCES'0
Magnetic phases in the correlated Kondo-lattice model
We study magnetic ordering of an extended Kondo-lattice model including an
additional on-site Coulomb interaction between the itinerant states. The model
is solved in the dynamical mean-field theory using Wilson's numerical
renormalization group approach as impurity solver. For a bipartite lattice we
find at half filling the expected antiferromagnetic phase. Upon doping this
phase is gradually suppressed and hints towards phase separation are observed.
For large doping the model exhibits ferromagnetism, the appearance of which can
at first sight be explained by Rudermann-Kittel-Kasuya-Yosida interaction.
However, for large values of the Kondo coupling significant differences to
a simple Rudermann-Kittel-Kasuya-Yosida picture can be found. We furthermore
observe signs of quantum critical points for antiferromagnetic Kondo coupling
between the local spins and band states
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