2,479 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
Magnetic Field Effects on Quasiparticles in Strongly Correlated Local Systems
We show that quasiparticles in a magnetic field of arbitrary strength can
be described by field dependent parameters. We illustrate this approach in the
case of an Anderson impurity model and use the numerical renormalization group
(NRG) to calculate the renormalized parameters for the levels with spin
, , resonance width
and the effective local quasiparticle interaction . In the Kondo or strong correlation limit of the model the progressive
de-renormalization of the quasiparticles can be followed as the magnetic field
is increased. The low temperature behaviour, including the conductivity, in
arbitrary magnetic field can be calculated in terms of the field dependent
parameters using the renormalized perturbation expansion. Using the NRG the
field dependence of the spectral density on higher scales is also calculated.Comment: 15 pages, 17 figure
Tunneling exponents sensitive to impurity scattering in quantum wires
We show that the scaling exponent for tunneling into a quantum wire in the
"Coulomb Tonks gas" regime of impenetrable, but otherwise free, electrons is
affected by impurity scattering in the wire. The exponent for tunneling into
such a wire thus depends on the conductance through the wire. This striking
effect originates from a many-body scattering resonance reminiscent of the
Kondo effect. The predicted anomalous scaling is stable against weak
perturbations of the ideal Tonks gas limit at sufficiently high energies,
similar to the phenomenology of a quantum critical point.Comment: 5 pages, 2 figures; slightly extended version of the published
articl
The Crooks relation in optical spectra - universality in work distributions for weak local quenches
We show that work distributions and non-equilibrium work fluctuation theorems
can be measured in optical spectra for a wide class of quantum systems. We
consider systems where the absorption or emission of a photon corresponds to
the sudden switch on or off of a local perturbation. For the particular case of
a weak local perturbation, the Crooks relation establishes a universal relation
in absorption as well as in emission spectra. Due to a direct relation between
the spectra and work distribution functions this is equivalent to universal
relations in work distributions for weak local quenches. As two concrete
examples we treat the X-ray edge problem and the Kondo exciton.Comment: 4+ pages, 1 figure; version as publishe
Transport properties of a multichannel Kondo dot in a magnetic field
We study the nonequilibrium transport through a multichannel Kondo quantum
dot in the presence of a magnetic field. We use the exact solution of the
two-loop renormalization group equation to derive analytical results for the g
factor, the spin relaxation rates, the magnetization, and the differential
conductance. We show that the finite magnetization leads to a coupling between
the conduction channels which manifests itself in additional features in the
differential conductance.Comment: 4 pages, 4 figure
Photoluminiscence of a quantum dot hybridized with a continuum
We calculate the intensity of photon emission from a trion in a single
quantum dot, as a function of energy and gate voltage, using the impurity
Anderson model and variational wave functions. Assuming a flat density of
conduction states and constant hybridization energy, the results agree with the
main features observed in recent experiments: non-monotonic dependence of the
energy on gate voltage, non-Lorentzian line shapes, and a line width that
increases near the regions of instability of the single electron final state to
occupations zero or two.Comment: 4 pages, 3 figures, Journal-ref adde
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
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.
The impact of active workstations on workplace productivity and performance: a systematic review
Active workstations have been recommended for reducing sedentary behavior in the workplace. It is important to understand if the use of these workstations has an impact on worker productivity. The aim of this systematic review was to examine the effect of active workstations on workplace productivity and performance. A total of 3303 articles were initially identified by a systematic search and seven articles met eligibility criteria for inclusion. A quality appraisal was conducted to assess risk of bias, confounding, internal and external validity, and reporting. Most of the studies reported cognitive performance as opposed to productivity. Five studies assessed cognitive performance during use of an active workstation, usually in a single session. Sit-stand desks had no detrimental effect on performance, however, some studies with treadmill and cycling workstations identified potential decreases in performance. Many of the studies lacked the power required to achieve statistical significance. Three studies assessed workplace productivity after prolonged use of an active workstation for between 12 and 52 weeks. These studies reported no significant effect on productivity. Active workstations do not appear to decrease workplace performance
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