2,870 research outputs found
Submicrometric Films of Surface-Attached Polymer Network with Temperature-Responsive Properties
Temperature-responsive properties of surface-attached
poly(N-isopropylacrylamide) (PNIPAM) network films with well-controlled
chemistry are investigated. The synthesis consists of cross-linking and
grafting preformed ene-reactive polymer chains through thiol--ene click
chemistry. The formation of surface-attached and cross-linked polymer films has
the advantage of being wellcontrolled without any caution of no-oxygen
atmosphere or addition of initiators. PNIPAM hydrogel films with same
cross-link density are synthesized on a wide range of thickness, from
nanometers to micrometers. The swelling-collapse transition with temperature is
studied by using ellipsometry, neutron reflectivity, and atomic force
microscopy as complementary surface-probing techniques. Sharp and high
amplitude temperature-induced phase transition is observed for all
submicrometric PNIPAM hydrogel films. For temperature above LCST,
surface-attached PNIPAM hydrogels collapse similarly but without complete
expulsion of water. For temperature below LCST, the swelling of PNIPAM
hydrogels depends on the film thickness. It is shown that the swelling is
strongly affected by the surface attachment for ultrathin films below 150
nm. For thicker films above 150 nm (to micrometers), surface-attached polymer
networks with the same cross-link density swell equally. The density profile of
the hydrogel films in the direction normal to the substrate is confronted with
in-plane topography of the free surface. It results that the free interface
width is much larger than the roughness of the hydrogel film, suggesting
pendant chains at the free surface.Comment: in Langmuir, American Chemical Society, 2015, LANGMUIR, 31 (42),
pp.11516-1152
A trap-based pulsed positron beam optimised for positronium laser spectroscopy
We describe a pulsed positron beam that is optimised for positronium (Ps) laser-spectroscopy experiments. The system is based on a two-stage Surko-type buffer gas trap that produces 4 ns wide pulses containing up to 5 × 105 positrons at a rate of 0.5-10 Hz. By implanting positrons from the trap into a suitable target material, a dilute positronium gas with an initial density of the order of 107 cm−3 is created in vacuum. This is then probed with pulsed (ns) laser systems, where various Ps-laser interactions have been observed via changes in Ps annihilation rates using a fast gamma ray detector. We demonstrate the capabilities of the apparatus and detection methodology via the observation of Rydberg positronium atoms with principal quantum numbers ranging from 11 to 22 and the Stark broadening of the n = 2 → 11 transition in electric fields
Self-consistent Approach to Off-Shell Transport
The properties of two forms of the gradient expanded Kadanoff--Baym
equations, i.e. the Kadanoff--Baym and Botermans-Malfliet forms, suitable to
describe the transport dynamics of particles and resonances with broad spectral
widths, are discussed in context of conservation laws, the definition of a
kinetic entropy and the possibility of numerical realization. Recent results on
exact conservations of charge and energy-momentum within Kadanoff-Baym form of
quantum kinetics based on local coupling schemes are extended to two cases
relevant in many applications. These concern the interaction via a finite range
potential, and, relevant in nuclear and hadron physics, e.g. for the
pion--nucleon interaction, the case of derivative coupling.Comment: 35 pages, submitted to issue of Phys. Atom. Nucl. dedicated to S.T.
Belyaev on the occasion of his 80th birthday. Few references are adde
Renormalization in Self-Consistent Approximations schemes at Finite Temperature I: Theory
Within finite temperature field theory, we show that truncated
non-perturbative self-consistent Dyson resummation schemes can be renormalized
with local counter-terms defined at the vacuum level. The requirements are that
the underlying theory is renormalizable and that the self-consistent scheme
follows Baym''s -derivable concept. The scheme generates both, the
renormalized self-consistent equations of motion and the closed equations for
the infinite set of counter terms. At the same time the corresponding
2PI-generating functional and the thermodynamical potential can be
renormalized, in consistency with the equations of motion. This guarantees the
standard -derivable properties like thermodynamic consistency and exact
conservation laws also for the renormalized approximation schemes to hold. The
proof uses the techniques of BPHZ-renormalization to cope with the explicit and
the hidden overlapping vacuum divergences.Comment: 22 Pages 1 figure, uses RevTeX4. The Revision concerns the correction
of some minor typos, a clarification concerning the real-time contour
structure of renormalization parts and some comments concerning symmetries in
the conclusions and outloo
Target mass number dependence of subthreshold antiproton production in proton-, deuteron- and alpha-particle-induced reactions
Data from KEK on subthreshold \bar{\mrm{p}} as well as on and
\mrm{K}^\pm production in proton-, deuteron- and -induced reactions
at energies between 2.0 and 12.0 A GeV for C, Cu and Pb targets are described
within a unified approach. We use a model which considers a nuclear reaction as
an incoherent sum over collisions of varying numbers of projectile and target
nucleons. It samples complete events and thus allows for the simultaneous
consideration of all final particles including the decay products of the
nuclear residues. The enormous enhancement of the \bar{\mrm{p}} cross
section, as well as the moderate increase of meson production in deuteron and
induced compared to proton-induced reactions, is well reproduced for
all target nuclei. In our approach, the observed enhancement near the
production threshold is mainly due to the contributions from the interactions
of few-nucleon clusters by simultaneously considering fragmentation processes
of the nuclear residues. The ability of the model to reproduce the target mass
dependence may be considered as a further proof of the validity of the cluster
concept.Comment: 9 pages, 4 figure
New Measurement of the Relative Scintillation Efficiency of Xenon Nuclear Recoils Below 10 keV
Liquid xenon is an important detection medium in direct dark matter
experiments, which search for low-energy nuclear recoils produced by the
elastic scattering of WIMPs with quarks. The two existing measurements of the
relative scintillation efficiency of nuclear recoils below 20 keV lead to
inconsistent extrapolations at lower energies. This results in a different
energy scale and thus sensitivity reach of liquid xenon dark matter detectors.
We report a new measurement of the relative scintillation efficiency below 10
keV performed with a liquid xenon scintillation detector, optimized for maximum
light collection. Greater than 95% of the interior surface of this detector was
instrumented with photomultiplier tubes, giving a scintillation yield of 19.6
photoelectrons/keV electron equivalent for 122 keV gamma rays. We find that the
relative scintillation efficiency for nuclear recoils of 5 keV is 0.14, staying
constant around this value up to 10 keV. For higher energy recoils we measure a
value around 20%, consistent with previously reported data. In light of this
new measurement, the XENON10 experiment's results on spin-independent
WIMP-nucleon cross section, which were calculated assuming a constant 0.19
relative scintillation efficiency, change from cm to
cm for WIMPs of mass 100 GeV/c, and from
cm to cm for WIMPs of mass 30
GeV/c.Comment: 8 pages, 8 figure
Characterization of the first true coaxial 18-fold segmented n-type prototype detector for the GERDA project
The first true coaxial 18-fold segmented n-type HPGe prototype detector
produced by Canberra-France for the GERDA neutrinoless double beta-decay
project was tested both at Canberra-France and at the Max-Planck-Institut fuer
Physik in Munich. The main characteristics of the detector are given and
measurements concerning detector properties are described. A novel method to
establish contacts between the crystal and a Kapton cable is presented.Comment: 21 pages, 16 Figures, to be submitted to NIM
Direct measurement of piston friction of internal-combustion engines using the floating-liner principle
Piston–cylinder interactions account for a significant portion of frictional losses in an internal-combustion engine. This is mainly as the result of significant changes in the operating conditions (the load, the speed and the temperature) as well as the contact geometry and the encountered topography during a typical engine cycle. These changes alter the regime of lubrication which underlies the mechanisms of friction generation. The multi-variate interactive nature of the problem requires quite complex analyses which do not fully replicate the actual in-situ conditions. Therefore, there is a need for direct measurement of cyclic friction under controlled conditions. The paper describes the use of a novel floating-liner arrangement which is capable of direct measurement of friction, its transitory mechanisms, as well as determination of the regime of lubrication
Raman Response in Doped Antiferromagnets
The resonant part of the electronic Raman scattering response is
calculated within the model on a planar lattice as a function of
temperature and hole doping, using a finite-temperature diagonalization method
for small systems. Results, directly applicable to experiments on cuprates,
reveal on doping a very pronounced increase of the width of the two-magnon
Raman peak, accompanied by a decrease of the total intensity. At the same time
the peak position does not shift substantially in the underdoped regime.Comment: 11 pages revtex, 3 postscript figures. Minor corrections and changes
from previous version, to be published in Phys. Rev.
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