4,364 research outputs found
Precision Crystal Calorimetry in High Energy Physics
Crystal Calorimetry is widely used in high energy physics because of its
precision. Recent development in crystal technology identified two key issues
to reach and maintain crystal precision: light response uniformity and
calibration in situ. Crystal radiation damage is understood. While the damage
in alkali halides is found to be caused by the oxygen/hydroxyl contamination,
it is the structure defects, such as oxygen vacancies, cause damage in oxides.Comment: 8 pages with 13 eps Figures, RevTe
Electronic and Magnetic Phase Diagram of a Superconductor, SmFeAsO1-xFx
A crystallographic and magnetic phase diagram of SmFeAsO1-xFx is determined
as a function of x in terms of temperature based on electrical transport and
magnetization, synchrotron powder x-ray diffraction, 57Fe Mossbauer spectra
(MS), and 149Sm nuclear resonant forward scattering (NRFS) measurements. MS
revealed that the magnetic moments of Fe were aligned antiferromagnetically at
~144 K (TN(Fe)). The magnetic moment of Fe (MFe) is estimated to be 0.34
myuB/Fe at 4.2 K for undoped SmFeAsO; MFe is quenched in superconducting
F-doped SmFeAsO. 149Sm NRFS spectra revealed that the magnetic moments of Sm
start to order antiferromagnetically at 5.6 K (undoped) and 4.4 K (TN(Sm)) (x =
0.069). Results clearly indicate that the antiferromagnetic Sm sublattice
coexists with the superconducting phase in SmFeAsO1-xFx below TN(Sm), while
antiferromagnetic Fe sublattice does not coexist with the superconducting
phase.Comment: Accepted in New Journal of Physic
Searching for realistic 4d string models with a Pati-Salam symmetry -- Orbifold grand unified theories from heterotic string compactification on a Z6 orbifold
Motivated by orbifold grand unified theories, we construct a class of
three-family Pati-Salam models in a Z6 abelian symmetric orbifold with two
discrete Wilson lines. These models have marked differences from
previously-constructed three-family models in prime-order orbifolds. In the
limit where one of the six compactified dimensions (which lies in a Z2
sub-orbifold) is large compared to the string length scale, our models
reproduce the supersymmetry and gauge symmetry breaking pattern of 5d orbifold
grand unified theories on an S1/Z2 orbicircle. We find a horizontal 2+1
splitting in the chiral matter spectra -- 2 families of matter are localized on
the Z2 orbifold fixed points, and 1 family propagates in the 5d bulk -- and
identify them as the first-two and third families. Remarkably, the first two
families enjoy a non-abelian dihedral D4 family symmetry, due to the geometric
setup of the compactified space. In all our models there are always some color
triplets, i.e. (6,1,1) representations of the Pati-Salam group, survive
orbifold projections. They could be utilized to spontaneously break the
Pati-Salam symmetry to that of the Standard Model. One model, with a 5d E6
symmetry, may give rise to interesting low energy phenomenology. We study gauge
coupling unification, allowed Yukawa couplings and some of their
phenomenological consequences. The E6 model has a renormalizable Yukawa
coupling only for the third family. It predicts a gauge-Yukawa unification
relation at the 5d compactification scale, and is capable of generating
reasonable quark/lepton masses and mixings. Potential problems are also
addressed, they may point to the direction for refining our models.Comment: 58 pages, 5 figures, 4 tables, revtex4 with ams fonts. Version to
appear in NP
Effect of Multiphase Radiation on Coal Combustion in a Pulverized Coal jet Flame
The accurate modeling of coal combustion requires detailed radiative heat transfer models for both gaseous combustion products and solid coal particles. A multiphase Monte Carlo ray tracing (MCRT) radiation solver is developed in this work to simulate a laboratory-scale pulverized coal flame. The MCRT solver considers radiative interactions between coal particles and three major combustion products (CO2, H2O, and CO). A line-by-line spectral database for the gas phase and a size-dependent nongray correlation for the solid phase are employed to account for the nongray effects. The flame structure is significantly altered by considering nongray radiation and the lift-off height of the flame increases by approximately 35%, compared to the simulation without radiation. Radiation is also found to affect the evolution of coal particles considerably as it takes over as the dominant mode of heat transfer for medium-to-large coal particles downstream of the flame. To investigate the respective effects of spectral models for the gas and solid phases, a Planck-mean-based gray gas model and a size-independent gray particle model are applied in a frozen-field analysis of a steady-state snapshot of the flame. The gray gas approximation considerably underestimates the radiative source terms for both the gas phase and the solid phase. The gray coal approximation also leads to under-prediction of the particle emission and absorption. However, the level of under-prediction is not as significant as that resulting from the employment of the gray gas model. Finally, the effect of the spectral property of ash on radiation is also investigated and found to be insignificant for the present target flame
Evidence for orbital ordering in LaCoO3
We present powder and single crystal X-ray diffraction data as evidence for a
monoclinic distortion in the low spin (S=0) and intermediate spin state (S=1)
of LaCoO3. The alternation of short and long bonds in the ab plane indicates
the presence of eg orbital ordering induced by a cooperative Jahn-Teller
distortion. We observe an increase of the Jahn-Teller distortion with
temperature in agreement with a thermally activated behavior of the Co3+ ions
from a low-spin ground state to an intermediate-spin excited state.Comment: Accepted to Phys. Rev.
Multi log-normal density structure in Cygnus-X molecular clouds: A fitting for N-PDF without power-law
We studied the H column density probability distribution function (N-PDF)
based on molecular emission lines using the Nobeyama 45-m Cygnus X CO survey
data. Using the DENDROGRAM and SCIMES algorithms, we identified 124 molecular
clouds in the CO data. From these identified molecular clouds, an N-PDF
was constructed for 11 molecular clouds with an extent of more than 0.4
deg. From the fitting of the N-PDF, we found that the N-PDF could be
well-fitted with one or two log-normal distributions. These fitting results
provided an alternative density structure for molecular clouds from a
conventional picture. We investigated the column density, dense molecular cloud
cores, and radio continuum source distributions in each cloud and found that
the N-PDF shape was less correlated with the star-forming activity over a whole
cloud. Furthermore, we found that the log-normal N-PDF parameters obtained from
the fitting showed two impressive features. First, the log-normal distribution
at the low-density part had the same mean column density ( 10
cm) for almost all the molecular clouds. Second, the width of the
log-normal distribution tended to decrease with an increasing mean density of
the structures. These correlations suggest that the shape of the N-PDF reflects
the relationship between the density and turbulent structure of the whole
molecular cloud but is less affected by star-forming activities.Comment: 14 pages, 7 Figures, Accepted in MNRA
Impurity-induced in-gap state and Tc in sign-reversing s-wave superconductors: analysis of iron oxypnictide superconductors
The sign-reversing fully gapped superconducting state, which is expected to
be realized in oxypnictide superconductors, can be prominently affected by
nonmagnetic impurities due to the interband scattering of Cooper pairs. We
study this problem based on the isotropic two-band BCS model: In oxypnictide
superconductors, the interband impurity scattering is not equal to the
intraband one . In the Born scattering regime, the reduction in Tc is
sizable and the impurity-induced density of states (DOS) is prominent if , due to the interband scattering. Although impurity-induced DOS can yield a
power-law temperature dependence in , a sizable suppression in Tc is
inevitably accompanied. In the unitary scattering regime, in contrast, impurity
effect is very small for both Tc and DOS except at . By comparing theory
and experiments, we expect that the degree of anisotropy in the -wave
gap function strongly depends on compounds.Comment: 16 pages, 5 figures, to be published in New. J. Phy
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