15,621 research outputs found
Nuclear modification factor in intermediate-energy heavy-ion collisions
The transverse momentum dependent nuclear modification factors (NMF), namely
, is investigated for protons produced in Au + Au at 1 GeV within
the framework of the isospin-dependent quantum molecular dynamics (IQMD) model.
It is found that the radial collective motion during the expansion stage
affects the NMF at low transverse momentum a lot. By fitting the transverse
mass spectra of protons with the distribution function from the Blast-Wave
model, the magnitude of radial flow can be extracted. After removing the
contribution from radial flow, the can be regarded as a thermal one
and is found to keep unitary at transverse momentum lower than 0.6 GeV/c and
enhance at higher transverse momentum, which can be attributed to Cronin
effect.Comment: 8 pages, 5 figures; aceepted by Physics Letters
Arginine52 controls the photoisomerization process in photoactive yellow protein
International audienc
Positive Semidefiniteness and Positive Definiteness of a Linear Parametric Interval Matrix
We consider a symmetric matrix, the entries of which depend linearly on some
parameters. The domains of the parameters are compact real intervals. We
investigate the problem of checking whether for each (or some) setting of the
parameters, the matrix is positive definite (or positive semidefinite). We
state a characterization in the form of equivalent conditions, and also propose
some computationally cheap sufficient\,/\,necessary conditions. Our results
extend the classical results on positive (semi-)definiteness of interval
matrices. They may be useful for checking convexity or non-convexity in global
optimization methods based on branch and bound framework and using interval
techniques
Doping dependent evolution of magnetism and superconductivity in Eu1-xKxFe2As2 (x = 0-1) and temperature dependence of lower critical field Hc1
We have synthesized the polycrystalline samples of Eu1-xKxFe2As2 (x = 0-1)
and carried out systematic characterization using x-ray diffraction, ac & dc
magnetic susceptibility, and electrical resistivity measurements. We have seen
a clear signature of the coexistence of superconducting transition (Tc = 5.5 K)
with SDW ordering in our under doped sample viz. x = 0.15. The spin density
wave transition observed in EuFe2As2 get completely suppressed at x = 0.3 and
superconductivity arises below 20 K. Superconducting transition temperature Tc
increases with increase in K content and a maximum Tc = 33 K is reached for x =
0.5, beyond which it decreases again. The doping dependent T(x) phase diagram
is extracted from the magnetic and electrical transport data. It is found that
magnetic ordering of Eu-moments coexists with superconductivity up to x = 0.6.
The isothermal magnetization data taken at 2 K for the doped samples suggest 2+
valence states of Eu ions. We also present the temperature dependence of the
lower critical field Hc1 of superconducting polycrystalline samples. The value
of Hc1(0) obtained for x = 0.3, 0.5, and 0.7 after taking the demagnetization
factor into account is 248, 385, and 250 Oe, respectively. The London
penetration depth {\lambda}(T) calculated from the lower critical field does
not show exponential behaviour at low temperature, as would be expected for a
fully gapped clean s-wave superconductor. In contrast, it shows a T2 power-law
feature down to T = 0.4 Tc, as observed in Ba1-xKxFe2As2 and BaFe2-xCoxAs2.Comment: 17 pages, 10 figure
Study on insulation effect of hollow riser
According to the air insulation, the use of riser hollow way to reduce the riser heat transfer, improve the riser insulation effect. In the experimental simulation, the corundum brick inner and outer wall and the air gap temperature were measured. The optimal air gap is 25 mm from the heat transfer theory and the outer wall temperature, but the insula- tion effect is not obvious. To further explore, proposed in the inner and outer walls to 25 mm as a unit into the insula- tion board improvement measures. The results show that the insertion of the insulation board can greatly reduce the temperature of the outer wall and improve the insulation effect. When inserting a heat shield, the heat flow can be reduced by about 50 %. With the increase in the number of inserts inserted, the insulation effect is gradually increased
STM and RHEED study of the Si(001)-c(8x8) surface
The Si(001) surface deoxidized by short annealing at T~925C in the ultrahigh
vacuum molecular beam epitaxy chamber has been in situ investigated by high
resolution scanning tunnelling microscopy (STM) and reflected high energy
electron diffraction (RHEED). RHEED patterns corresponding to (2x1) and (4x4)
structures were observed during sample treatment. The (4x4) reconstruction
arose at T<600C after annealing. The reconstruction was observed to be
reversible: the (4x4) structure turned into the (2x1) one at T>600C, the (4x4)
structure appeared again at recurring cooling. The c(8x8) reconstruction was
revealed by STM at room temperature on the same samples. A fraction of the
surface area covered by the c(8x8) structure decreased as the sample cooling
rate was reduced. The (2x1) structure was observed on the surface free of the
c(8x8) one. The c(8x8) structure has been evidenced to manifest itself as the
(4x4) one in the RHEED patterns. A model of the c(8x8) structure formation has
been built on the basis of the STM data. Origin of the high-order structure on
the Si(001) surface and its connection with the epinucleation phenomenon are
discussed.Comment: 26 pages, 12 figure
Structural study on hole-doped superconductors Pr1-xSrxFeAsO
The structural details in Pr1-xSrxFeAsO (1111) superconducting system are
analyzed using data obtained from synchrotron X-ray diffraction and the
structural parameters are carefully studied as the system is moving from
non-superconducting to hole-doped superconducting with the Sr concentration.
Superconductivity emerges when the Sr doping amount reaches 0.221. The linear
increase of the lattice constants proves that Sr is successfully introduced
into the system and its concentration can accurately be determined by the
electron density analyses. The evolution of structural parameters with Sr
concentration in Pr1-xSrxFeAsO and their comparison to other similar structural
parameters of the related Fe-based superconductors suggest that the interlayer
space between the conducting As-Fe-As layer and the insulating Pr-O-Pr layer is
important for improving Tc in the hole-doped (1111) superconductors, which
seems to be different from electron-doped systems.Comment: 17 pages, 7 figures, 1 tabl
Spiral Chain O4 Form of Dense Oxygen
Oxygen is in many ways a unique element: the only known diatomic molecular
magnet and the capability of stabilization of the hitherto unexpected O8
cluster structure in its solid form at high pressure. Molecular dissociations
upon compression as one of the fundamental problems were reported for other
diatomic solids (e.g., H2, I2, Br2, and N2), but it remains elusive for solid
oxygen, making oxygen an intractable system. We here report the theoretical
prediction on the dissociation of molecular oxygen into a polymeric spiral
chain O4 structure (\theta-O4) by using first-principles calypso method on
crystal structure prediction. The \theta-O4 stabilizes above 2 TPa and has been
observed as the third high pressure phase of sulfur (S-III). We find that the
molecular O8 phase remains extremely stable in a large pressure range of 0.008
- 2 TPa, whose breakdown is driven by the pressure-induced instability of a
transverse acoustic phonon mode at zone boundary, leading to the ultimate
formation of \theta-O4. Remarkably, stabilization of \theta-O4 turns oxygen
from a superconductor into an insulator with a wide band gap (approximately 5.9
eV) originating from the sp3-like hybridized orbitals of oxygen and the
localization of valence electrons. (This is a pre-print version of the
following article: Li Zhu et al, Spiral chain O4 form of dense oxygen, Proc.
Natl. Acad. Sci. U.S.A. (2011), doi: 10.1073/pnas.1119375109, which has been
published online at http://www.pnas.org/content/early/2011/12/27/1119375109 .)Comment: 13 apages, 3 figure
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