4,085 research outputs found
Phase Behavior of Melts of Diblock-Copolymers with One Charged Block
In this work we investigated the phase behavior of melts of block-copolymers
with one charged block by means of dissipative particle dynamics with explicit
electrostatic interactions. We assumed that all the Flory-Huggins \c{hi}
parameters were equal to 0 and showed that the charge correlation attraction
solely can cause microphase separation with long-range order; a phase diagram
was constructed by varying the volume fraction of the uncharged block and the
electrostatic interaction parameter {\lambda}. The obtained phase diagram was
compared to the phase diagram of corresponding neutral diblock-copolymers.
Surprisingly, the differences between these phase diagrams are rather subtle;
the same phases are observed, and the positions of the ODT points are similar
if the {\lambda}-parameter is considered as an "effective" \c{hi}-parameter.
Next, we studied the position of the ODT for lamellar structure depending on
the chain length N. It turned out that while for the uncharged
diblock-copolymer the product \c{hi}crN was almost independent of N, for the
diblock-copolymers with one charged block we observed a significant increase in
{\lambda}crN upon increasing N. It can be attributed to the fact that the
counterion entropy prevents the formation of ordered structures. This was
supported by studying the ODT in diblock-copolymers with charged blocks and
counterions cross-linked to the charged monomer units. The ODT for such systems
was observed at significantly lower values of {\lambda} with the difference
being more pronounced at longer chain lengths N. The diffusion of counterions
in the obtained ordered structures was studied and compared to the case of a
system with the same number of charged groups but homogeneous structure; the
diffusion coefficient in a direction in the lamellar plane was found to be
higher than in any direction in homogeneous structure
Observation of time quasicrystal and its transition to superfluid time crystal
We report experimental realization of a quantum time quasicrystal, and its
transformation to a quantum time crystal. We study Bose-Einstein condensation
of magnons, associated with coherent spin precession, created in a flexible
trap in superfluid He-B. Under a periodic drive with an oscillating
magnetic field, the coherent spin precession is stabilized at a frequency
smaller than that of the drive, demonstrating spontaneous breaking of discrete
time translation symmetry. The induced precession frequency is incommensurate
with the drive, and hence the obtained state is a time quasicrystal. When the
drive is turned off, the self-sustained coherent precession lives a
macroscopically-long time, now representing a time crystal with broken symmetry
with respect to continuous time translations. Additionally, the magnon
condensate manifests spin superfluidity, justifying calling the obtained state
a time supersolid or a time super-crystal
Formation of TiC-Cu nanocomposites by a reaction between Ti25Cu75 melt-spun alloy and carbon
In this work, Ti25Cu75 melt-spun partially amorphous alloy was used as a source of Ti and Cu to synthesize in-situ TiC-Cu nanocomposites. The reaction between the alloy and carbon started during ball milling and continued during Spark Plasma Sintering. At the same time, during ball milling, the alloy experienced phase transformations: crystallization of the amorphous phase was followed by decomposition of TiCu3. Copper crystallites formed during the alloy transformations were the reason for the presence of copper regions 0.5â1âŻÂľm in size free from TiC nanoparticles in the sintered composites. The Ti-Cu intermetallics transformed into non-agglomerated TiC 10â20âŻnm in size distributed in the copper matrix. The hardness of the synthesized TiC-Cu nanocomposites exceeded that of composites obtained by conventional sintering of ball-milled Ti-C-Cu powders
Einstein and Brans-Dicke frames in multidimensional cosmology
Inhomogeneous multidimensional cosmological models with a higher dimensional
space-time manifold M= M_0 x M_1 ...x M_n are investigated under dimensional
reduction to a D_0-dimensional effective non-minimally coupled sigma-model
which generalizes the familiar Brans-Dicke model.
It is argued that the Einstein frame should be considered as the physical
one. The general prescription for the Einstein frame reformulation of known
solutions in the Brans-Dicke frame is given. As an example, the reformulation
is demonstrated explicitly for the generalized Kasner solutions where it is
shown that in the Einstein frame there are no solutions with inflation of the
external space.Comment: 27 pages, Revte
Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering: Determined by the OLYMPUS Experiment
The OLYMPUS collaboration reports on a precision measurement of the
positron-proton to electron-proton elastic cross section ratio, ,
a direct measure of the contribution of hard two-photon exchange to the elastic
cross section. In the OLYMPUS measurement, 2.01~GeV electron and positron beams
were directed through a hydrogen gas target internal to the DORIS storage ring
at DESY. A toroidal magnetic spectrometer instrumented with drift chambers and
time-of-flight scintillators detected elastically scattered leptons in
coincidence with recoiling protons over a scattering angle range of to . The relative luminosity between the two beam species
was monitored using tracking telescopes of interleaved GEM and MWPC detectors
at , as well as symmetric M{\o}ller/Bhabha calorimeters at
. A total integrated luminosity of 4.5~fb was collected. In
the extraction of , radiative effects were taken into account
using a Monte Carlo generator to simulate the convolutions of internal
bremsstrahlung with experiment-specific conditions such as detector acceptance
and reconstruction efficiency. The resulting values of , presented
here for a wide range of virtual photon polarization ,
are smaller than some hadronic two-photon exchange calculations predict, but
are in reasonable agreement with a subtracted dispersion model and a
phenomenological fit to the form factor data.Comment: 5 pages, 3 figures, 2 table
Measurement of the Top Quark Pair Production Cross Section in the All-jets Decay Channel
We present a measurement of tbar-t production using multijet final states in
pbar-p collisions at a center-of-mass energy of 1.8 TeV, with an integrated
luminosity of 110.3 pb(-1). The analysis has been optimized using neural
networks to achieve the smallest expected fractional uncertainty on the tbar-t
production cross section, and yields a cross section of 7.1 +/- 2.8(stat.) +/-
1.5(syst.) pb, assuming a top quark mass of 172.1 GeV/c^(2). Combining this
result with previous D0 measurements, where one or both of the W bosons decay
leptonically, gives a tbar-t production cross section of 5.9 +/- 1.2(stat) +/-
1.1(syst) pb.Comment: 6 pages, 3 figures; no substative change in revisio
Measurement of the t t-bar production cross section in the dilepton channel in pp collisions at sqrt(s) = 7 TeV
The t t-bar production cross section (sigma[t t-bar]) is measured in
proton-proton collisions at sqrt(s) = 7 TeV in data collected by the CMS
experiment, corresponding to an integrated luminosity of 2.3 inverse
femtobarns. The measurement is performed in events with two leptons (electrons
or muons) in the final state, at least two jets identified as jets originating
from b quarks, and the presence of an imbalance in transverse momentum. The
measured value of sigma[t t-bar] for a top-quark mass of 172.5 GeV is 161.9 +/-
2.5 (stat.) +5.1/-5.0 (syst.) +/- 3.6(lumi.) pb, consistent with the prediction
of the standard model.Comment: Replaced with published version. Included journal reference and DO
Combined search for the quarks of a sequential fourth generation
Results are presented from a search for a fourth generation of quarks
produced singly or in pairs in a data set corresponding to an integrated
luminosity of 5 inverse femtobarns recorded by the CMS experiment at the LHC in
2011. A novel strategy has been developed for a combined search for quarks of
the up and down type in decay channels with at least one isolated muon or
electron. Limits on the mass of the fourth-generation quarks and the relevant
Cabibbo-Kobayashi-Maskawa matrix elements are derived in the context of a
simple extension of the standard model with a sequential fourth generation of
fermions. The existence of mass-degenerate fourth-generation quarks with masses
below 685 GeV is excluded at 95% confidence level for minimal off-diagonal
mixing between the third- and the fourth-generation quarks. With a mass
difference of 25 GeV between the quark masses, the obtained limit on the masses
of the fourth-generation quarks shifts by about +/- 20 GeV. These results
significantly reduce the allowed parameter space for a fourth generation of
fermions.Comment: Replaced with published version. Added journal reference and DO
Measurement of the Top Quark Pair Production Cross Section in pbar-p Collisions Using Multijet Final States
We have studied tbar-t production using multijet final states in pbar-p
collisions at a center-of-mass energy of 1.8 TeV, with an integrated luminosity
of 110.3 pb(-1). Each of the top quarks with these final states decays
exclusively to a bottom quark and a W boson, with the W bosons decaying into
quark-antiquark pairs. The analysis has been optimized using neural networks to
achieve the smallest expected fractional uncertainty on the tbar-t production
cross section, and yields a cross section of 7.1 +/- 2.8(stat.) +/- 1.5(syst.)
pb, assuming a top quark mass of 172.1 GeV/c^(2). Combining this result with
previous D0 measurements, where one or both of the W bosons decay leptonically,
gives a tbar t production cross section of 5.9 +/- 1.2(stat) +/- 1.1(syst) pb.Comment: 30 pages, 32 figures; no substative change in revisio
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