206 research outputs found
Possible strong electron-lattice interaction and giant magneto-elastic effects in Fe-pnictides
The possibility for an appreciable many-body contribution to the
electron-phonon interaction (EPI) in Fe-pnictides is discussed in the model
where EPI is due to the electronic polarization of As- ions. The EPI-pol
coupling ismuch larger than the one obtained in the LDA band structure
calculations. It contributes significantly to the intra-band s-wave pairing and
an appreciable positive As-isotope effect in the superconducting critical
temperature is expected. In the Fe-breathing mode the linear (in the
Fe-displacements) EPI-pol coupling vanishes, while the non-linear (quadratic)
one is very strong. The part of the EPI-pol coupling, which is due to the
"potential" energy (the Hubbard U) changes, is responsible for the giant
magneto-elastic effects in MFe_{2}As_{2}, M=Ca, Sr, Ba since it gives much
larger contribution to the magnetic pressure than the band structure effects
do. This mechanism is contrary to the LDA prediction where the magneto-elastic
effects are due to the "kinetic" energy effects, i.e. the changes in the
density of states by the magneto-elastic effects. The proposed $EPI-pol is
expected to be operative (and strong) in other Fe-based superconductors with
electronically polarizable ions such as Se, Te, S etc., and in high-temperature
superconductors due to the polarizability of the O-ions.Comment: 6 pages, 2 figures; new References are added, text improved, typos
correcte
Unconventional superconducting pairing by conventional phonons
The common wisdom that the phonon mechanism of electron pairing in the
weak-coupling Bardeen-Cooper-Schrieffer (BCS) superconductors leads to
conventional s-wave Cooper pairs is revised. An inevitable anisotropy of sound
velocity in crystals makes the phonon-mediated attraction of electrons
non-local in space providing unconventional Cooper pairs with a nonzero orbital
momentum in a wide range of electron densities. As a result of this anisotropy
quasi-two dimensional charge carriers undergo a quantum phase transition from
an unconventional d-wave superconducting state to a conventional s-wave
superconductor with more carriers per unit cell. In the opposite
strong-coupling regime rotational symmetry breaking appears as a result of a
reduced Coulomb repulsion between unconventional bipolarons dismissing thereby
some constraints on unconventional pairing in the Bose-Einstein condensation
(BEC) limit. The conventional phonons, and not superexchange, are shown to be
responsible for the d-wave symmetry of cuprate superconductors, where the
on-site Coulomb repulsion is large.Comment: 4 pages, 4 figures, more references adde
Response time of a normal-superconductor hybrid system under the step-like pulse bias
The response of a quantum dot coupled with one normal lead and a
superconductor lead driven by a step-like pulse bias is studied using the
non-equilibrium Green function method. In the linear pulse bias regime, the
responses of the upwards and downwards bias are symmetric. In this regime the
turn-on time and turn-off time are much slower than that of the normal system
due to the Andreev reflection. On the other hand, for the large pulse bias
, the instantaneous current exhibits oscillatory behaviors with the
frequency . The turn on/off times are in (or shorter than)
the scale of , so they are faster for the larger bias . In
addition, the responses for the upwards and downwards bias are asymmetric at
large . The turn-on time is larger than the turn-off time but the
relaxation time \cite{note1} depends only on the coupling strength and
it is much smaller than the turn-on/off times for the large bias .Comment: 8 pages, 4 figures, accepted for publication in Phys. Rev.
The authenticity and quality of Rhodiola rosea products
BACKGROUND: Rhodiola rosea L. Crassulaceae, root (Golden Root, Arctic Root) is a high-value herbal medicinal product, registered in the UK for the treatment of stress-induced fatigue, exhaustion and anxiety based on traditional use and used throughout Europe as a herbal medicinal product for similar indications. Numerous unregistered supplements are also available. There are several Chinese species used in traditional Chinese medicine (TCM), including Rhodiola crenulata (Hook.f. & Thomoson) that is believed to be a common adulterant in the R. rosea value chain. AIMS: The project is embedded in a larger study aiming to investigate the diverse value chains that lead to the production of R. rosea as an herbal medicinal product or supplement. Here we focus on a comparison of the quality of the finished products and assess any phytochemical variation between products registered under the Traditional Herbal Medicine Products Directive (THMPD) and products obtained from the market without any registration (i.e. generally unlicensed supplements). Our key aim is to establish the extent of the problem in terms of adulteration of consumer products claiming to contain R. rosea (or R. crenulata). METHODS: Approximately 40 commercial products (granulated powders and extracts) were sourced from different suppliers. We analysed these samples using high performance thin layer chromatography (HPTLC), mass spectrometry (MS) and (1)H NMR spectroscopy coupled with multi-variate analysis software following a method previously developed by our group for the analysis of turmeric products. RESULTS: We investigate the phytochemistry of the different species and assess the potential of R. crenulata as an adulterant at the end of the R. rosea value chains. The consistency of the products varies significantly. Approximately one fifth of commercial products that claimed to be R. rosea did not contain rosavin (the key reference markers used to distinguish R. rosea from related species). Moreover some products appeared not to contain salidroside, another marker compound found in other Rhodiola species. Approximately 80% of the remaining commercial products were lower in rosavin content than the registered products and appeared to be adulterated with other Rhodiola species. CONCLUSIONS: The variation in phytochemical constituents present in Rhodiola products available to European buyers via the internet and other sources is a major cause for concern. Adulteration with different species, and other sometimes unknown adulterants, appears to be commonplace. Good quality systems and manufacturing practices, including those required under the THMPD, enable consumers to have confidence that products are authentic and meet a high specification for quality and safety
Quantifying Demonstration Quality for Robot Learning and Generalization
Learning from Demonstration (LfD) seeks to democratize robotics by enabling
diverse end-users to teach robots to perform a task by providing
demonstrations. However, most LfD techniques assume users provide optimal
demonstrations. This is not always the case in real applications where users
are likely to provide demonstrations of varying quality, that may change with
expertise and other factors. Demonstration quality plays a crucial role in
robot learning and generalization. Hence, it is important to quantify the
quality of the provided demonstrations before using them for robot learning. In
this paper, we propose quantifying the quality of the demonstrations based on
how well they perform in the learned task. We hypothesize that task performance
can give an indication of the generalization performance on similar tasks. The
proposed approach is validated in a user study (N = 27). Users with different
robotics expertise levels were recruited to teach a PR2 robot a generic task
(pressing a button) under different task constraints. They taught the robot in
two sessions on two different days to capture their teaching behaviour across
sessions. The task performance was utilized to classify the provided
demonstrations into high-quality and low-quality sets. The results show a
significant Pearson correlation coefficient (R = 0.85, p < 0.0001) between the
task performance and generalization performance across all participants. We
also found that users clustered into two groups: Users who provided
high-quality demonstrations from the first session, assigned to the
fast-adapters group, and users who provided low-quality demonstrations in the
first session and then improved with practice, assigned to the slow-adapters
group. These results highlight the importance of quantifying demonstration
quality, which can be indicative of the adaptation level of the user to the
task
The role of microtubule movement in bidirectional organelle transport
We study the role of microtubule movement in bidirectional organelle
transport in Drosophila S2 cells and show that EGFP-tagged peroxisomes in cells
serve as sensitive probes of motor induced, noisy cytoskeletal motions.
Multiple peroxisomes move in unison over large time windows and show
correlations with microtubule tip positions, indicating rapid microtubule
fluctuations in the longitudinal direction. We report the first high-resolution
measurement of longitudinal microtubule fluctuations performed by tracing such
pairs of co-moving peroxisomes. The resulting picture shows that
motor-dependent longitudinal microtubule oscillations contribute significantly
to cargo movement along microtubules. Thus, contrary to the conventional view,
organelle transport cannot be described solely in terms of cargo movement along
stationary microtubule tracks, but instead includes a strong contribution from
the movement of the tracks.Comment: 24 pages, 5 figure
Electron-phonon vertex in the two-dimensional one-band Hubbard model
Using quantum Monte Carlo techniques, we study the effects of electronic
correlations on the effective electron-phonon (el-ph) coupling in a
two-dimensional one-band Hubbard model. We consider a momentum-independent bare
ionic el-ph coupling. In the weak- and intermediate-correlation regimes, we
find that the on-site Coulomb interaction acts to effectively suppress the
ionic el-ph coupling at all electron- and phonon- momenta. In this regime, our
numerical simulations are in good agreement with the results of perturbation
theory to order . However, entering the strong-correlation regime, we find
that the forward scattering process stops decreasing and begins to
substantially increase as a function of , leading to an effective el-ph
coupling which is peaked in the forward direction. Whereas at weak and
intermediate Coulomb interactions, screening is the dominant correlation effect
suppressing the el-ph coupling, at larger values irreducible vertex
corrections become more important and give rise to this increase. These vertex
corrections depend crucially on the renormalized electronic structure of the
strongly correlated system.Comment: 5 pages, 4 eps-figures, minor change
Stokesian jellyfish: Viscous locomotion of bilayer vesicles
Motivated by recent advances in vesicle engineering, we consider
theoretically the locomotion of shape-changing bilayer vesicles at low Reynolds
number. By modulating their volume and membrane composition, the vesicles can
be made to change shape quasi-statically in thermal equilibrium. When the
control parameters are tuned appropriately to yield periodic shape changes
which are not time-reversible, the result is a net swimming motion over one
cycle of shape deformation. For two classical vesicle models (spontaneous
curvature and bilayer coupling), we determine numerically the sequence of
vesicle shapes through an enthalpy minimization, as well as the fluid-body
interactions by solving a boundary integral formulation of the Stokes
equations. For both models, net locomotion can be obtained either by
continuously modulating fore-aft asymmetric vesicle shapes, or by crossing a
continuous shape-transition region and alternating between fore-aft asymmetric
and fore-aft symmetric shapes. The obtained hydrodynamic efficiencies are
similar to that of other low Reynolds number biological swimmers, and suggest
that shape-changing vesicles might provide an alternative to flagella-based
synthetic microswimmers
Two-band Eliashberg equations and the experimental Tc of the diboride Mg1-xAlxB2
The variation of the superconducting critical temperature Tc as a function of
x in the diboride Mg1-xAlxB2 has been studied in the framework of the two-bands
Eliashberg theory and traditional phonon coupling mechanism. We have solved the
two-bands Eliashberg equations using first-principle calculations or simple
assumptions for the variation of the relevant physical quantities. We have
found that the experimental Tc curve can be explained only if the Coulomb
pseudopotential changes with x by tuning the Fermi level toward the sigma band
edge. In polycrystal samples the x dependence of the sigma and pi-band gap has
been found and is in agreement with experiments.Comment: 6 pages, 7 figure
Effect of an Electron-phonon Interaction on the One-electron Spectral Weight of a d-wave Superconductor
We analyze the effects of an electron-phonon interaction on the one-electron
spectral weight A(k,omega) of a d_{x^2-y^2} superconductor. We study the case
of an Einstein phonon mode with various momentum-dependent electron-phonon
couplings and compare the structure produced in A(k,omega) with that obtained
from coupling to the magnetic pi-resonant mode. We find that if the strength of
the interactions are adjusted to give the same renormalization at the nodal
point, the differences in A(k,omega) are generally small but possibly
observable near k=(pi,0).Comment: 10 pages, 14 figures (color versions of Figs. 2,4,10,11,12 available
upon request
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