1,262 research outputs found
Effect of concentration on the performance of quantum dot intermediate-band solar cells
Implementation of a high-efficiency quantum dot intermediate-band solar cell (QD-IBSC) must accompany a sufficient photocurrent generation via IB states. The demonstration of a QD-IBSC is presently undergoing two stages. The first is to develop a technology to fabricate high-density QD stacks or a superlattice of low defect density placed within the active region of a p-i-n SC, and the second is to realize half-filled IB states to maximize the photocurrent generation by two-step absorption of sub-bandgap photons. For this, we have investigated the effect of light concentration on the characteristics of QDSCs comprised of multi-layer stacks of self-organized InAs/GaNAs QDs grown with and without impurity doping in molecular beam epitaxy
Symmetry Breaking and False Vacuum Decay after Hybrid Inflation
We discuss the onset of symmetry breaking from the false vacuum in generic
scenarios in which the mass squared of the symmetry breaking (Higgs) field
depends linearly with time, as it occurs, via the evolution of the inflaton, in
models of hybrid inflation. We show that the Higgs fluctuations evolve from
quantum to classical during the initial stages. This justifies the subsequent
use of real-time lattice simulations to describe the fully non-perturbative and
non-linear process of symmetry breaking. The early distribution of the Higgs
field is that of a smooth classical gaussian random field, and consists of
lumps whose shape and distribution is well understood analytically. The lumps
grow with time and develop into ``bubbles'' which eventually collide among
themselves, thus populating the high momentum modes, in their way towards
thermalization at the true vacuum. With the help of some approximations we are
able to provide a quasi-analytic understanding of this process.Comment: 33 pages, 16 figures, LaTeX, uses revtex. Version to be published in
Phys. Rev. with minor change
Hemodynamic performance of different stent strategies for coronary bifurcations. Evaluation with a mathematical model
Purpose: Best percutaneous treatment strategy for lesions in coronary bifurcations is an ongoing subject of debate. There is limited data that analyses the effect of the different bifurcation strategies on coronary flow. Our aim is to evaluate the influence of different bifurcation stenting strategies on hemodynamic parameters, both in the main vessel (MV) and side branch (SB)
Are short-term variations in solar oscillation frequencies the signature of a second solar dynamo?
In addition to the well-known 11-year solar cycle, the Sun's magnetic
activity also shows significant variation on shorter time scales, e.g. between
one and two years. We observe a quasi-biennial (2-year) signal in the solar
p-mode oscillation frequencies, which are sensitive probes of the solar
interior. The signal is visible in Sun-as-a-star data observed by different
instruments and here we describe the results obtained using BiSON, GOLF, and
VIRGO data. Our results imply that the 2-year signal is susceptible to the
influence of the main 11-year solar cycle. However, the source of the signal
appears to be separate from that of the 11-year cycle. We speculate as to
whether it might be the signature of a second dynamo, located in the region of
near-surface rotational shear.Comment: 6 pages, 2 figures, proceedings for SOHO-24/GONG 2010 conference, to
be published in JPC
Shape-resonant superconductivity in nanofilms: from weak to strong coupling
Ultrathin superconductors of different materials are becoming a powerful
platform to find mechanisms for enhancement of superconductivity, exploiting
shape resonances in different superconducting properties. Here we evaluate the
superconducting gap and its spatial profile, the multiple gap components, and
the chemical potential, of generic superconducting nanofilms, considering the
pairing attraction and its energy scale as tunable parameters, from weak to
strong coupling, at fixed electron density. Superconducting properties are
evaluated at mean field level as a function of the thickness of the nanofilm,
in order to characterize the shape resonances in the superconducting gap. We
find that the most pronounced shape resonances are generated for weakly coupled
superconductors, while approaching the strong coupling regime the shape
resonances are rounded by a mixing of the subbands due to the large energy gaps
extending over large energy scales. Finally, we find that the spatial profile,
transverse to the nanofilm, of the superconducting gap acquires a flat behavior
in the shape resonance region, indicating that a robust and uniform multigap
superconducting state can arise at resonance.Comment: 7 pages, 4 figures. Submitted to the Proceedings of the Superstripes
2016 conferenc
Observation of shell effects in superconducting nanoparticles of Sn
In a zero-dimensional superconductor, quantum size effects(QSE) not only set
the limit to superconductivity, but are also at the heart of new phenomena such
as shell effects, which have been predicted to result in large enhancements of
the superconducting energy gap. Here, we experimentally demonstrate these QSE
through measurements on single, isolated Pb and Sn nanoparticles. In both
systems superconductivity is ultimately quenched at sizes governed by the
dominance of the quantum fluctuations of the order parameter. However, before
the destruction of superconductivity, in Sn nanoparticles we observe giant
oscillations in the superconducting energy gap with particle size leading to
enhancements as large as 60%. These oscillations are the first experimental
proof of coherent shell effects in nanoscale superconductors. Contrarily, we
observe no such oscillations in the gap for Pb nanoparticles, which is ascribed
to the suppression of shell effects for shorter coherence lengths. Our study
paves the way to exploit QSE in boosting superconductivity in low-dimensional
systems
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