39,171 research outputs found
Characterisation of the fused silica surface quality with a β-source
A method to characterise the quality of a fused silica surface using a β-source is presented. Two fused silica bars (5×10×400mm3) were fabricated for the Cherenkov detector for proton Flux Measurement installed at vacuum chamber of the Super Proton Synchrotron at CERN. The resolution of such device is defined by the collection efficiency of the Cherenkov light, which is produced by relativistic charged particles in the fused silica. Thus, the surface quality of the radiator should be as good as possible to avoid light losses. The method is based on the scanning of the radiator surface with a90Sr radioactive source and measurements of the Cherenkov light rate, detected by a PMT attached to the quartz bars. The data have been compared with a Monte-Carlo simulation, providing an estimation of the radiator's probability of the total internal reflection and inefficient area at the edges of the bars
Strong exciton binding in quantum structures through remote dielectric confinement
We propose a new type of hybrid systems formed by conventional semiconductor
nanostructures with the addition of remote insulating layers, where the
electron-hole interaction is enhanced by combining quantum and dielectric
confinement over different length scales. Due to the polarization charges
induced by the dielectric mismatch at the semiconductor/insulator interfaces,
we show that the exciton binding energy can be more than doubled. For
conventional III-V quantum wires such remote dielectric confinement allows
exciton binding at room temperature.Comment: 4 pages, 3 PostScript figures embedded, best printed in color. Uses
RevTex, multicol, and psfig styles. To appear in Phys. Rev. Let
On the convergence of Magnetorotational turbulence in stratified isothermal shearing boxes
We consider the problem of convergence in stratified isothermal shearing
boxes with zero net magnetic flux. We present results with the highest
resolution to-date--up to 200 grid-point per pressure scale height--that show
no clear evidence of convergence. Rather, the Maxwell stresses continue to
decrease with increasing resolution. We propose some possible scenarios to
explain the lack of convergence based on multi-layer dynamo systems.Comment: 10 pages, 4 figures, accepted for publication in ApJ Letter
Fully Convective Magnetorotational Turbulence in Stratified Shearing Boxes
We present a numerical study of turbulence and dynamo action in stratified
shearing boxes with zero magnetic flux. We assume that the fluid obeys the
perfect gas law and has finite (constant) thermal diffusivity. We choose
radiative boundary conditions at the vertical boundaries in which the heat flux
is propor- tional to the fourth power of the temperature. We compare the
results with the corresponding cases in which fixed temperature boundary
conditions are applied. The most notable result is that the formation of a
fully convective state in which the density is nearly constant as a function of
height and the heat is transported to the upper and lower boundaries by
overturning motions is robust and persists even in cases with radiative
boundary conditions. Interestingly, in the convective regime, although the
diffusive transport is negligible the mean stratification does not relax to an
adiabatic state.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter
Magnetic Helicities and Dynamo Action in Magneto-rotationally Driven Turbulence
We examine the relationship between magnetic flux generation, taken as an
indicator of large-scale dynamo action, and magnetic helicity, computed as an
integral over the dynamo volume, in a simple dynamo. We consider dynamo action
driven by Magneto-Rotational Turbulence (MRT) within the shearing-box
approximation. We consider magnetically open boundary conditions that allow a
flux of helicity in or out of the computational domain. We circumvent the
problem of the lack of gauge invariance in open domains by choosing a
particular gauge -- the winding gauge -- that provides a natural interpretation
in terms of average winding number of pairwise field lines. We use this gauge
precisely to define and measure the helicity and helicity flux for several
realizations of dynamo action. We find in these cases, that the system as a
whole does not break reflectional symmetry and the total helicity remains small
even in cases when substantial magnetic flux is generated. We find no
particular connection between the generation of magnetic flux and the helicity
or the helicity flux through the boundaries. We suggest that this result may be
due to the essentially nonlinear nature of the dynamo processes in MRT.Comment: 26 pages, 10 figures, ApJ accepte
Coherent phenomena in semiconductors
A review of coherent phenomena in photoexcited semiconductors is presented.
In particular, two classes of phenomena are considered: On the one hand the
role played by optically-induced phase coherence in the ultrafast spectroscopy
of semiconductors; On the other hand the Coulomb-induced effects on the
coherent optical response of low-dimensional structures.
All the phenomena discussed in the paper are analyzed in terms of a
theoretical framework based on the density-matrix formalism. Due to its
generality, this quantum-kinetic approach allows a realistic description of
coherent as well as incoherent, i.e. phase-breaking, processes, thus providing
quantitative information on the coupled ---coherent vs. incoherent--- carrier
dynamics in photoexcited semiconductors.
The primary goal of the paper is to discuss the concept of quantum-mechanical
phase coherence as well as its relevance and implications on semiconductor
physics and technology. In particular, we will discuss the dominant role played
by optically induced phase coherence on the process of carrier photogeneration
and relaxation in bulk systems. We will then review typical field-induced
coherent phenomena in semiconductor superlattices such as Bloch oscillations
and Wannier-Stark localization. Finally, we will discuss the dominant role
played by Coulomb correlation on the linear and non-linear optical spectra of
realistic quantum-wire structures.Comment: Topical review in Semiconductor Science and Technology (in press)
(Some of the figures are not available in electronic form
Ab initio simulations of Cu binding sites in the N-terminal region of PrP
The prion protein (PrP) binds Cu2+ ions in the octarepeat domain of the
N-terminal tail up to full occupancy at pH=7.4. Recent experiments show that
the HGGG octarepeat subdomain is responsible for holding the metal bound in a
square planar coordination. By using first principle ab initio molecular
dynamics simulations of the Car-Parrinello type, the Cu coordination mode to
the binding sites of the PrP octarepeat region is investigated. Simulations are
carried out for a number of structured binding sites. Results for the complexes
Cu(HGGGW)+(wat), Cu(HGGG) and the 2[Cu(HGGG)] dimer are presented. While the
presence of a Trp residue and a H2O molecule does not seem to affect the nature
of the Cu coordination, high stability of the bond between Cu and the amide
Nitrogens of deprotonated Gly's is confirmed in the case of the Cu(HGGG)
system. For the more interesting 2[Cu(HGGG)] dimer a dynamically entangled
arrangement of the two monomers, with intertwined N-Cu bonds, emerges. This
observation is consistent with the highly packed structure seen in experiments
at full Cu occupancy.Comment: 4 pages, conference proceedin
Immune cells and preterm labour:do invariant NKT cells hold the key?
We have developed our original made-to-measure (M2M) algorithm, PRIMAL, with the aim of modelling the Galactic disc from upcoming Gaia data. From a Milky Way like N-body disc galaxy simulation, we have created mock Gaia data using M0III stars as tracers, taking into account extinction and the expected Gaia errors. In PRIMAL, observables calculated from the N-body model are compared with the target stars, at the position of the target stars. Using PRIMAL, the masses of the N-body model particles are changed to reproduce the target mock data, and the gravitational potential is automatically adjusted by the changing mass of the model particles. We have also adopted a new resampling scheme for the model particles to keep the mass resolution of the N-body model relatively constant. We have applied PRIMAL to this mock Gaia data and we show that PRIMAL can recover the structure and kinematics of a Milky Way like barred spiral disc, along with the apparent bar structure and pattern speed of the bar despite the galactic extinction and the observational errors
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