1,557 research outputs found
Universal Static and Dynamic Properties of the Structural Transition in Pb(Zn1/3Nb2/3)O3
The relaxors Pb(ZnNb)O (PZN) and
Pb(MgNb)O (PMN) have very similar properties based on the
dielectric response around the critical temperature (defined by the
structural transition under the application of an electric field). It has been
widely believed that these materials are quite different below with the
unit cell of PMN remaining cubic while in PZN the low temperature unit cell is
rhombohedral in shape. However, this has been clarified by recent high-energy
x-ray studies which have shown that PZN is rhombohedral only in the skin while
the shape of the unit cell in the bulk is nearly cubic. In this study we have
performed both neutron elastic and inelastic scattering to show that the
temperature dependence of both the diffuse and phonon scattering in PZN and PMN
is very similar. Both compounds show a nearly identical recovery of the soft
optic mode and a broadening of the acoustic mode below . The diffuse
scattering in PZN is suggestive of an onset at the high temperature Burns
temperature similar to that in PMN. In contrast to PMN, we observe a broadening
of the Bragg peaks in both the longitudinal and transverse directions below
. We reconcile this additional broadening, not observed in PMN, in terms
of structural inhomogeneity in PZN. Based on the strong similarities between
PMN and PZN, we suggest that both materials belong to the same universality
class and discuss the relaxor transition in terms of the three-dimensional
Heisenberg model with cubic anisotropy in a random field.Comment: 11 pages, 10 figures. Updated version after helpful referee comment
Theoretical analysis of the focusing of acoustic waves by two-dimensional sonic crystals
Motivated by a recent experiment on acoustic lenses, we perform numerical
calculations based on a multiple scattering technique to investigate the
focusing of acoustic waves with sonic crystals formed by rigid cylinders in
air. The focusing effects for crystals of various shapes are examined. The
dependance of the focusing length on the filling factor is also studied. It is
observed that both the shape and filling factor play a crucial role in
controlling the focusing. Furthermore, the robustness of the focusing against
disorders is studied. The results show that the sensitivity of the focusing
behavior depends on the strength of positional disorders. The theoretical
results compare favorably with the experimental observations, reported by
Cervera, et al. (Phys. Rev. Lett. 88, 023902 (2002)).Comment: 8 figure
First-principles study of the structural energetics of PdTi and PtTi
The structural energetics of PdTi and PtTi have been studied using
first-principles density-functional theory with pseudopotentials and a
plane-wave basis. We predict that in both materials, the experimentally
reported orthorhombic phase will undergo a low-temperature phase
transition to a monoclinic ground state. Within a soft-mode framework,
we relate the structure to the cubic structure, observed at high
temperature, and the structure to via phonon modes strongly
coupled to strain. In contrast to NiTi, the structure is extremely close
to hcp. We draw on the analogy to the bcc-hcp transition to suggest likely
transition mechanisms in the present case.Comment: 8 pages 5 figure
Determinisitic Optical Fock State Generation
We present a scheme for the deterministic generation of N-photon Fock states
from N three-level atoms in a high-finesse optical cavity. The method applies
an external laser pulsethat generates an -photon output state while
adiabatically keeping the atom-cavity system within a subspace of optically
dark states. We present analytical estimates of the error due to amplitude
leakage from these dark states for general N, and compare it with explicit
results of numerical simulations for N \leq 5. The method is shown to provide a
robust source of N-photon states under a variety of experimental conditions and
is suitable for experimental implementation using a cloud of cold atoms
magnetically trapped in a cavity. The resulting N-photon states have potential
applications in fundamental studies of non-classical states and in quantum
information processing.Comment: 25 pages, 9 figure
Hybrid materials based on polyethylene and MCM-41 microparticles functionalized with silanes: catalytic aspects of in situ polymerization, crystalline features and mechanical properties
New nanocomposites based on polyethylene have been prepared by in situ polymerization of ethylene in
presence of mesoporous MCM-41. The polymerization reactions were performed using a zirconocene
catalyst either under homogenous conditions or supported onto mesoporous MCM-41 particles, which
are synthesized and decorated post-synthesis with two silanes before polymerization in order to promote
an enhanced interfacial adhesion. The existence of polyethylene chains able to crystallize within
the mesoporous channels in the resulting nanocomposites is figured out from the small endothermic
process, located at around 80 C, on heating calorimetric experiments, in addition to the main melting
endotherm. These results indicate that polyethylene macrochains can grow up during polymerization
either outside or inside the MCM-41 channels, these keeping their regular hexagonal arrangements.
Mechanical response is observed to be dependent on the content in mesoporous MCM-41 and on the
crystalline features of polyethylene. Accordingly, stiffness increases and deformability decreases in the
nanocomposites as much as MCM-41 content is enlarged and polyethylene amount within channels is
raised. Ultimate mechanical performance improves with MCM-41 incorporation without varying the
final processing temperature
Role of spinon and spinon singlet pair excitations on phase transitions in superconductors
We examine the roles of massless Dirac spinon and spin singlet pair
excitations on the phase transition in superconductors. Although the
massless spinon excitations in the presence of the spin singlet pair
excitations do not alter the nature of the phase transition at , that
is, the XY universality class, they are seen to induce an additional attractive
interaction potential between vortices, further stabilizing vortex-antivortex
pairs at low temperature for lightly doped high samples.Comment: 5 pages, 1 figur
Direct Measurements of Absolute Branching Fractions for D0 and D+ Inclusive Semimuonic Decays
By analyzing about 33 data sample collected at and around 3.773
GeV with the BES-II detector at the BEPC collider, we directly measure the
branching fractions for the neutral and charged inclusive semimuonic decays
to be and , and determine the ratio of the two branching
fractions to be
Macromolecular theory of solvation and structure in mixtures of colloids and polymers
The structural and thermodynamic properties of mixtures of colloidal spheres
and non-adsorbing polymer chains are studied within a novel general
two-component macromolecular liquid state approach applicable for all size
asymmetry ratios. The dilute limits, when one of the components is at infinite
dilution but the other concentrated, are presented and compared to field theory
and models which replace polymer coils with spheres. Whereas the derived
analytical results compare well, qualitatively and quantitatively, with
mean-field scaling laws where available, important differences from ``effective
sphere'' approaches are found for large polymer sizes or semi-dilute
concentrations.Comment: 23 pages, 10 figure
Development of a novel reagentless, screen-printed amperometric biosensor based on glutamate dehydrogenase and NAD+, integrated with multi-walled carbon nanotubes for the determination of glutamate in food and clinical applications
© 2015 Elsevier B.V. Abstract A screen printed carbon electrode (SPCE) containing the electrocatalyst Meldola's Blue (MB) has been investigated as the base transducer for a reagentless glutamate biosensor. The biopolymer chitosan (CHIT) and multiwalled carbon nanotubes (MWCNTs) were used to encapsulate the enzyme glutamate dehydrogenase (GLDH) and the co-factor nicotinamide adenine dinucleotide (NAD+). The biosensor was fabricated by sequentially depositing the components on the surface of the transducer (MB-SPCE) in a layer-by-layer process, details of which are included in the paper. Each layer was optimised to construct the reagentless device. The biosensor was used in conjunction with amperometry in stirred solution using an applied potential of +0.1 V (vs. Ag/AgCl). Optimum conditions for the analysis of glutamate were found to be: temperature, 35°C; phosphate buffer, pH 7 (0.75 mM, containing 0.05 M NaCl). The linear range of the reagentless biosensor was found to be 7.5-105 μM, and limit of detection was found to be 3 μM (based on n = 5, CV: 8.5% based on three times signal to noise) and the sensitivity was 0.39 nA/μM (±0.025, coefficient of variation (CV) of 6.37%, n = 5). The response time of the biosensor was 20-30 s. A food sample was analysed for monosodium glutamate (MSG). The endogenous content of MSG was 90.56 mg/g with a CV of 7.52%. The reagentless biosensor was also used to measure glutamate in serum. The endogenous concentration of glutamate was found to be 1.44 mM (n = 5), CV: 8.54%. The recovery of glutamate in fortified serum was 104% (n = 5), CV of 2.91%
Measurements of the observed cross sections for exclusive light hadrons containing at , 3.650 and 3.6648 GeV
By analyzing the data sets of 17.3, 6.5 and 1.0 pb taken,
respectively, at , 3.650 and 3.6648 GeV with the BES-II
detector at the BEPC collider, we measure the observed cross sections for
, , ,
and at the three energy
points. Based on these cross sections we set the upper limits on the observed
cross sections and the branching fractions for decay into these
final states at 90% C.L..Comment: 7 pages, 2 figure
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