169 research outputs found
Conventional Superconductivity in Fe-Based Pnictides: the Relevance of Intra-Band Electron-Boson Scattering
Various recent experimental data and especially the large Fe-isotope effect
point against unconventional pairings, since the large intra-band impurity
scattering is strongly pair-breaking for them. The strength of the inter-band
impurity scattering in some single crystals may be strong and probably beyond
the Born scattering limit. In that case the proposed s(+-) pairing (hole(h)-
and electron(el)-gaps are of opposite signs) is suppressed but possibly not
completely destroyed. The data imply that the intra-band pairing in the h- and
in the el-band, which are inevitably due to some nonmagnetic el-boson
interaction (EBI), must be taken into account. EBI is either due to phonons
(EPI) or possibly due to excitons (EEI), or both are simultaneously operative.
We discuss their interplay briefly. The large Fe-isotope effect favors the EPI
and the s(+) pairing (the h- and el-gaps are in-phase).Comment: 7 pages, no figures, explanations and argumentations improved,
references adde
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
Efficiency at maximum power: An analytically solvable model for stochastic heat engines
We study a class of cyclic Brownian heat engines in the framework of
finite-time thermodynamics. For infinitely long cycle times, the engine works
at the Carnot efficiency limit producing, however, zero power. For the
efficiency at maximum power, we find a universal expression, different from the
endoreversible Curzon-Ahlborn efficiency. Our results are illustrated with a
simple one-dimensional engine working in and with a time-dependent harmonic
potential.Comment: 6 pages, 3 figure
Observation of the spontaneous vortex phase in the weakly ferromagnetic superconductor ErNiBC: A penetration depth study
The coexistence of weak ferromagnetism and superconductivity in ErNiBC suggests the possibility of a spontaneous vortex phase (SVP) in which
vortices appear in the absence of an external field. We report evidence for the
long-sought SVP from the in-plane magnetic penetration depth of high-quality single crystals of ErNiBC. In addition to
expected features at the N\'{e}el temperature = 6.0 K and weak
ferromagnetic onset at K, rises to a maximum
at K before dropping sharply down to 0.1 K. We assign the
0.45 K-maximum to the proliferation and freezing of spontaneous vortices. A
model proposed by Koshelev and Vinokur explains the increasing as a consequence of increasing vortex density, and its subsequent decrease
below as defect pinning suppresses vortex hopping.Comment: 5 pages including figures; added inset to Figure 2; significant
revisions to tex
Measurement Instruments for the Anthropomorphism, Animacy, Likeability, Perceived Intelligence, and Perceived Safety of Robots
This study emphasizes the need for standardized measurement tools for human robot interaction (HRI). If we are to make progress in this field then we must be able to compare the results from different studies. A literature review has been performed on the measurements of five key concepts in HRI: anthropomorphism, animacy, likeability, perceived intelligence, and perceived safety. The results have been distilled into five consistent questionnaires using semantic differential scales. We report reliability and validity indicators based on several empirical studies that used these questionnaires. It is our hope that these questionnaires can be used by robot developers to monitor their progress. Psychologists are invited to further develop the questionnaires by adding new concepts, and to conduct further validations where it appears necessary. © The Author(s) 2008
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
Mechanisms of amyloid-β34 generation indicate a pivotal role for BACE1 in amyloid homeostasis
The beta‑site amyloid precursor protein (APP) cleaving enzyme (BACE1) was discovered due to its “amyloidogenic” activity which contributes to the production of amyloid-beta (Aβ) peptides. However, BACE1 also possesses an “amyloidolytic” activity, whereby it degrades longer Aβ peptides into a non‑toxic Aβ34 intermediate. Here, we examine conditions that shift the equilibrium between BACE1 amyloidogenic and amyloidolytic activities by altering BACE1/APP ratios. In Alzheimer disease brain tissue, we found an association between elevated levels of BACE1 and Aβ34. In mice, the deletion of one BACE1 gene copy reduced BACE1 amyloidolytic activity by ~ 50%. In cells, a stepwise increase of BACE1 but not APP expression promoted amyloidolytic cleavage resulting in dose-dependently increased Aβ34 levels. At the cellular level, a mislocalization of surplus BACE1 caused a reduction in Aβ34 levels. To align the role of γ-secretase in this pathway, we silenced Presenilin (PS) expression and identified PS2-γ-secretase as the main γ-secretase that generates Aβ40 and Aβ42 peptides serving as substrates for BACE1’s amyloidolytic cleavage to generate Aβ34
Mechanisms of amyloid-β34 generation indicate a pivotal role for BACE1 in amyloid homeostasis
The beta‑site amyloid precursor protein (APP) cleaving enzyme (BACE1) was discovered due to its "amyloidogenic" activity which contributes to the production of amyloid-beta (Aβ) peptides. However, BACE1 also possesses an "amyloidolytic" activity, whereby it degrades longer Aβ peptides into a non‑toxic Aβ34 intermediate. Here, we examine conditions that shift the equilibrium between BACE1 amyloidogenic and amyloidolytic activities by altering BACE1/APP ratios. In Alzheimer disease brain tissue, we found an association between elevated levels of BACE1 and Aβ34. In mice, the deletion of one BACE1 gene copy reduced BACE1 amyloidolytic activity by ~ 50%. In cells, a stepwise increase of BACE1 but not APP expression promoted amyloidolytic cleavage resulting in dose-dependently increased Aβ34 levels. At the cellular level, a mislocalization of surplus BACE1 caused a reduction in Aβ34 levels. To align the role of γ-secretase in this pathway, we silenced Presenilin (PS) expression and identified PS2-γ-secretase as the main γ-secretase that generates Aβ40 and Aβ42 peptides serving as substrates for BACE1's amyloidolytic cleavage to generate Aβ34
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
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