823 research outputs found
Synthesis and trans-ureation of N,N '-diphenyl-4, 4 '-methylenediphenylene biscarbamate with diamines: a non-isocyanate route (NIR) to polyureas
A non-isocyanate route (NIR) of making polyureas of high molecular weight has been found through transureation of N,N'-diphenyl-4,4'-methylenediphenylene biscarbamate (4,4'-DP-MDC) with a variety of diamines and mixed diamines. The preparation of 4,4'-DP-MDC was achieved readily by carbonylation of 4,4'-methylenedianiline (4,4'-MDA) with diphenyl carbonate (DPC) using organic acids as catalysts. It was found that the highest yield (99%) of pure 4,4'-DP-MDC can be isolated in a toluene solution under mild conditions co-catalyzed by benzoic acid and tertiary amine. Trans-ureation of 4,4'-DP-MDC with aliphatic amines indicated that the process is a highly solvent dependent process and was found to be extremely facile in dimethyl sulfoxide (DMSO) at 80 C and in tetramethylene sulfone (TMS) at 140 C in absence of any catalyst. Particularly, the most effective polymerization process was developed using tetramethylene sulfone (TMS) as the solvent under reduced pressure for concurrently distilling off phenol from the reaction mixture during the polymerization in a shifting equilibrium towards polyurea. However, this solvent-assisted transureation was found to be in-efficient when N, N'-dimethyl-4,4'- methylenediphenylene biscarbamate (4,4'-DM-MDC) was used in a similar condition for comparison. Thus, an efficient green-chemistry process has been developed based on 4,4'-DP-MDC in making urea prepolymers, urea elastomers and urea plastics all in excellent yields without using reactive methylenediphenylene diisocyanate (MDI) or any catalysts in the trans-ureation polymerizations
Exclusive semileptonic B_s decays to excited D_s mesons: Search of D_{sJ}(2317) and D_{sJ}(2460)
We study the exclusive semileptonic decays B_s->D_{s0}^*\ell\bar\nu and
B_s->D_{s1}^*\ell\bar\nu, where p-wave excited D_{s0}^* and D_{s1}^* states are
identified with the newly observed D_{sJ}(2317) and D_{sJ}(2460) states. Within
the framework of HQET the Isgur-Wise functions up to the subleading order of
the heavy quark expansion are calculated by QCD sum rules. The decay rates and
branching ratios are computed with the inclusion of the order of 1/m_Q
corrections. We point out that the investigation of the B_s semileptonic decays
to excited D_s mesons may provide some information about the nature of the new
D_{sJ}^* mesons.Comment: 15 pages, 6 eps figures, RevTeX 4, accepted for publication in Phys.
Rev.
Lepton Flavor Violation in the SUSY-GUT Models with Lopsided Mass Matrix
The tiny neutrino masses measured in the neutrino oscillation experiments can
be naturally explained by the supersymmetric see-saw mechanism. If the
supersymmetry breaking is mediated by gravity, the see-saw models may predict
observable lepton flavor violating effects. In this work, we investigate the
lepton flavor violating process in the kind of neutrino mass
models based on the idea of the ``lopsided'' form of the charged lepton mass
matrix. The constraints set by the muon anomalous magnetic moment are taken
into account. We find the present models generally predict a much larger
branching ratio of than the experimental limit. Conversely,
this process may give strong constraint on the lepton flavor structure.
Following this constraint we then find a new kind of the charged lepton mass
matrix. The feature of the structure is that both the elements between the 2-3
and 1-3 generations are ``lopsided''. This structure produces a very small 1-3
mixing and a large 1-2 mixing in the charged lepton sector, which naturally
leads to small and the LMA solution for the solar neutrino
problem.Comment: 24 pages, 8 figure
Applicability of perturbative QCD to decays
We develop perturbative QCD factorization theorem for the semileptonic heavy
baryon decay , whose form factors are
expressed as the convolutions of hard quark decay amplitudes with universal
and baryon wave functions. Large logarithmic
corrections are organized to all orders by the Sudakov resummation, which
renders perturbative expansions more reliable. It is observed that perturbative
QCD is applicable to decays for velocity transfer
greater than 1.2. Under requirement of heavy quark symmetry, we predict the
branching ratio , and determine
the and baryon wave functions.Comment: 12 pages in Latex file, 3 figures in postscript files, some results
are changed, but the conclusion is the sam
On the effects of the magnetic field and the isotopic substitution upon the infrared absorption of manganites
Employing a variational approach that takes into account electron-phonon and
magnetic interactions in perovskites with , the
effects of the magnetic field and the oxygen isotope substitution on the phase
diagram, the electron-phonon correlation function and the infrared absorption
at are studied. The lattice displacements show a strong correlation
with the conductivity and the magnetic properties of the system. Then the
conductivity spectra are characterized by a marked sensitivity to the external
parameters near the phase boundary.Comment: 10 figure
Average Lattice Symmetry and Nanoscale Structural Correlations in Magnetoresistive Manganites
We report x-ray scattering studies of nanoscale structural correlations in
the paramagnetic phases of the perovskite manganites
La(CaSr)MnO,
LaSrMnO, and NdSrMnO. We find
that these correlations are present in the orthorhombic phase in
La(CaSr)MnO, but they disappear
abruptly at the orthorhombic-to-rhombohedral transition in this compound. The
orthorhombic phase exhibits increased electrical resistivity and reduced
ferromagnetic coupling, in agreement with the association of the nanoscale
correlations with insulating regions. In contrast, the correlations were not
detected in the two other compounds, which exhibit rhombohedral and tetragonal
phases. Based on these results, as well as on previously published work, we
propose that the local structure of the paramagnetic phase correlates strongly
with the average lattice symmetry, and that the nanoscale correlations are an
important factor distinguishing the insulating and the metallic phases in these
compounds.Comment: a note on recent experimental work, and a new reference adde
Evidence for charge localization in the ferromagnetic phase of La_(1-x)Ca_(x)MnO_3 from High real-space-resolution x-ray diffraction
High real-space-resolution atomic pair distribution functions of
La_(1-x)Ca_(x)MnO_3 (x=0.12, 0.25 and 0.33) have been measured using
high-energy x-ray powder diffraction to study the size and shape of the MnO_6
octahedron as a function of temperature and doping. In the paramagnetic
insulating phase we find evidence for three distinct bond-lengths (~ 1.88, 1.95
and 2.15A) which we ascribe to Mn^{4+}-O, Mn^{3+}-O short and Mn^{3+}-O long
bonds respectively. In the ferromagnetic metallic (FM) phase, for x=0.33 and
T=20K, we find a single Mn-O bond-length; however, as the metal-insulator
transition is approached either by increasing T or decreasing x, intensity
progressively appears around r=2.15 and in the region 1.8 - 1.9A suggesting the
appearance of Mn^{3+}-O long bonds and short Mn^{4+}-O bonds. This is strong
evidence that charge localized and delocalized phases coexist close to the
metal-insulator transition in the FM phase.Comment: 8 pages, 8 postscript figures, submitted to Phys. Rev.
The Structure of Nanoscale Polaron Correlations in La1.2Sr1.8Mn2O7
A system of strongly-interacting electron-lattice polarons can exhibit charge
and orbital order at sufficiently high polaron concentrations. In this study,
the structure of short-range polaron correlations in the layered colossal
magnetoresistive perovskite manganite, La1.2Sr1.8Mn2O7, has been determined by
a crystallographic analysis of broad satellite maxima observed in diffuse X-ray
and neutron scattering data. The resulting q=(0.3,0,1) modulation is a
longitudinal octahedral-stretch mode, consistent with an incommensurate
Jahn-Teller-coupled charge-density-wave fluctuations, that implies an unusual
orbital-stripe pattern parallel to the directions.Comment: Reformatted with RevTe
Magnetic Properties of YBa_2Cu_3O_{7-\delta} in a self-consistent approach: Comparison with Quantum-Monte-Carlo Simulations and Experiments
We analyze single-particle electronic and two-particle magnetic properties of
the Hubbard model in the underdoped and optimally-doped regime of \YBCO by
means of a modified version of the fluctuation-exchange approximation, which
only includes particle-hole fluctuations. Comparison of our results with
Quantum-Monte Carlo (QMC) calculations at relatively high temperatures () suggests to introduce a temperature renormalization in order to
improve the agreement between the two methods at intermediate and large values
of the interaction .
We evaluate the temperature dependence of the spin-lattice relaxation time
and of the spin-echo decay time and compare it with the results
of NMR measurements on an underdoped and an optimally doped \YBCO sample. For
it is possible to consistently adjust the parameters of the Hubbard
model in order to have a good {\it semi-quantitative} description of this
temperature dependence for temperatures larger than the spin gap as obtained
from NMR measurements. We also discuss the case , which is more
appropriate to describe magnetic and single-particle properties close to
half-filling. However, for this larger value of the agreement with QMC as
well as with experiments at finite doping is less satisfactory.Comment: Final version, to appear in Phys. Rev. B (sched. Feb. 99
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