2,030 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
Kondo Resonance Decoherence by an External Potential
The Kondo problem, for a quantum dot (QD), subjected to an external bias, is
analyzed in the limit of infinite Coulomb repulsion by using a consistent
equations of motion method based on a slave-boson Hamiltonian. Utilizing a
strict perturbative solution in the leads-dot coupling, T, to T^4 and T^6
orders, we calculate the QD spectral density and conductance, as well as the
decoherent rate that drive the systemm from the strong to the weak coupling
regime. Our results indicate thet the weak coupling regime is reached for
voltages larger than a few units of the Kondo temperature.Comment: 5 figure
Magnetic impurities in the honeycomb Kitaev model
We study the effect of coupling magnetic impurities to the honeycomb lattice
spin-1/2 Kitaev model in its spin liquid phase. We show that a spin-S impurity
coupled to the Kitaev model is associated with an unusual Kondo effect with an
intermediate coupling unstable fixed point K_c J/S separating topologically
distinct sectors of the Kitaev model. We also show that the massless spinons in
the spin liquid mediate an interaction of the form
S_{i\alpha}^{2}S_{j\beta}^{2}/R_{ij}^{3} between distant impurities unlike the
usual dipolar RKKY interaction S_{i\alpha}S_{j\alpha}/R_{ij}^{3} noted in
various 2D impurity problems with a pseudogapped density of states of the spin
bath. Furthermore, this long-range interaction is possible only if the
impurities (a) couple to more than one neighboring spin on the host lattice and
(b) the impurity spin is not a spin-1/2.$Comment: 4 pages, 3 figures, Published versio
Magnetoconductance through a vibrating molecule in the Kondo regime
The effect of a magnetic field on the equilibrium spectral and transport
properties of a single-molecule junction is studied using the numerical
renormalization group method. The molecule is described by the
Anderson-Holstein model in which a single vibrational mode is coupled to the
electron density. The effect of an applied magnetic field on the conductance in
the Kondo regime is qualitatively different in the weak and strong
electron-phonon coupling regimes. In the former case, the Kondo resonance is
split and the conductance is strongly suppressed by a magnetic field , with the Kondo temperature. In the strong
electron-phonon coupling regime a charge analog of the Kondo effect develops.
In this case the Kondo resonance is not split by the field and the conductance
in the Kondo regime is enhanced in a broad range of values of .Comment: 6 pages, 4 figure
Gap formation and soft phonon mode in the Holstein model
We investigate electron-phonon coupling in many-electron systems using
dynamical mean-field theory in combination with the numerical renormalization
group. This non-perturbative method reveals significant precursor effects to
the gap formation at intermediate coupling strengths. The emergence of a soft
phonon mode and very strong lattice fluctuations can be understood in terms of
Kondo-like physics due to the development of a double-well structure in the
effective potential for the ions
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
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
Spectral properties of locally correlated electrons in a BCS superconductor
We present a detailed study of the spectral properties of a locally
correlated site embedded in a BCS superconducting medium. To this end the
Anderson impurity model with superconducting bath is analysed by numerical
renormalisation group (NRG) calculations. We calculate one and two-particle
dynamic response function to elucidate the spectral excitation and the nature
of the ground state for different parameter regimes with and without
particle-hole symmetry. The position and weight of the Andreev bound states is
given for all relevant parameters. We also present phase diagrams for the
different ground state parameter regimes. This work is also relevant for
dynamical mean field theory extensions with superconducting symmetry breaking.Comment: 22 pages, 12 figure
Spectral Densities of Response Functions for the O(3) Symmetric Anderson and Two Channel Kondo Models
The O(3) symmetric Anderson model is an example of a system which has a
stable low energy marginal Fermi liquid fixed point for a certain choice of
parameters. It is also exactly equivalent, in the large U limit, to a localized
model which describes the spin degrees of freedom of the linear dispersion two
channel Kondo model. We first use an argument based on conformal field theory
to establish this precise equivalence with the two channel model. We then use
the numerical renormalization group (NRG) approach to calculate both
one-electron and two-electron response functions for a range of values of the
interaction strength U. We compare the behaviours about the marginal Fermi
liquid and Fermi liquid fixed points and interpret the results in terms of a
renormalized Majorana fermion picture of the elementary excitations. In the
marginal Fermi liquid case the spectral densities of all the Majorana fermion
modes display a |omega| dependence on the lowest energy scale, and in addition
the zero Majorana mode has a delta function contribution. The weight of this
delta function is studied as a function of the interaction U and is found to
decrease exponentially with U for large U. Using the equivalence with the two
channel Kondo model in the large U limit, we deduce the dynamical spin
susceptibility of the two channel Kondo model over the full frequency range. We
use renormalized perturbation theory to interpret the results and to calculate
the coefficient of the ln omega divergence found in the low frequency behaviour
of the T=0 dynamic susceptibility.Comment: 26 pages, 18 figures, to be published in Eur. Phys. J.
- …