18,008 research outputs found
The Gluon Spin in the Chiral Bag Model
We study the gluon polarization contribution at the quark model
renormalization scale to the proton spin, , in the chiral bag model. It
is evaluated by taking the expectation value of the forward matrix element of a
local gluon operator in the axial gauge . It is shown that the confining
boundary condition for the color electric field plays an important role. When a
solution satisfying the boundary condition for the color electric field, which
is not the conventionally used but which we favor, is used, the has a
positive value for {\it all} bag radii and its magnitude is comparable to the
quark spin polarization. This results in a significant reduction in the
relative fraction of the proton spin carried by the quark spin, which is
consistent with the small flavor singlet axial current measured in the EMC
experiments.Comment: Corrections to figure
Partial spin freezing in the quasi-two-dimensional La2(Cu,Li)O4
In conventional spin glasses, the magnetic interaction is not strongly
anisotropic and the entire spin system freezes at low temperature. In
La2(Cu,Li)O4, for which the in-plane exchange interaction dominates the
interplane one, only a fraction of spins with antiferromagnetic correlations
extending to neighboring planes become spin-glass. The remaining spins with
only in-plane antiferromagnetic correlations remain spin-liquid at low
temperature. Such a novel partial spin freezing out of a spin-liquid observed
in this cold neutron scattering study is likely due to a delicate balance
between disorder and quantum fluctuations in the quasi-two dimensional S=1/2
Heisenberg system.Comment: 4 pages, 4 figure
Disks around massive young stellar objects: are they common?
We present K-band polarimetric images of several massive young stellar
objects at resolutions 0.1-0.5 arcsec. The polarization vectors around
these sources are nearly centro-symmetric, indicating they are dominating the
illumination of each field. Three out of the four sources show elongated
low-polarization structures passing through the centers, suggesting the
presence of polarization disks. These structures and their surrounding
reflection nebulae make up bipolar outflow/disk systems, supporting the
collapse/accretion scenario as their low-mass siblings. In particular, S140
IRS1 show well defined outflow cavity walls and a polarization disk which
matches the direction of previously observed equatorial disk wind, thus
confirming the polarization disk is actually the circumstellar disk. To date, a
dozen massive protostellar objects show evidence for the existence of disks;
our work add additional samples around MYSOs equivalent to early B-type stars.Comment: 9 pages, including 2 figures, 1 table, to appear on ApJ
Electronic structure of YbB: Is it a Topological Insulator or not?
To resolve the controversial issue of the topological nature of the
electronic structure of YbB, we have made a combined study using density
functional theory (DFT) and angle resolved photoemission spectroscopy (ARPES).
Accurate determination of the low energy band topology in DFT requires the use
of modified Becke-Johnson exchange potential incorporating the spin-orbit
coupling and the on-site Coulomb interaction of Yb electrons as large
as 7 eV. We have double-checked the DFT result with the more precise GW band
calculation. ARPES is done with the non-polar (110) surface termination to
avoid band bending and quantum well confinement that have confused ARPES
spectra taken on the polar (001) surface termination. Thereby we show
definitively that YbB has a topologically trivial B 2-Yb 5
semiconductor band gap, and hence is a non-Kondo non-topological insulator
(TI). In agreement with theory, ARPES shows pure divalency for Yb and a -
band gap of 0.3 eV, which clearly rules out both of the previous scenarios of
- band inversion Kondo TI and - band inversion non-Kondo TI. We
have also examined the pressure-dependent electronic structure of YbB,
and found that the high pressure phase is not a Kondo TI but a
\emph{p}-\emph{d} overlap semimetal.Comment: The main text is 6 pages with 4 figures, and the supplementary
information contains 6 figures. 11 pages, 10 figures in total To be appeared
in Phys. Rev. Lett. (Online publication is around March 16 if no delays.
Void-mediated formation of Sn quantum dots in a Si matrix
Atomic scale analysis of Sn quantum dots (QDs) formed during the molecular beam-epitaxy (MBE) growth of Sn_xSi_(1−x) (0.05 ⩽ x ⩽ 0.1) multilayers in a Si matrix revealed a void-mediated formation mechanism. Voids below the Si surface are induced by the lattice mismatch strain between Sn_xSi_(1−x) layers and Si, taking on their equilibrium tetrakaidecahedron shape. The diffusion of Sn atoms into these voids leads to an initial rapid coarsening of quantum dots during annealing. Since this formation process is not restricted to Sn, a method to grow QDs may be developed by controlling the formation of voids and the diffusion of materials into these voids during MBE growth
Optical identification of ISO far-infrared sources in the Lockman Hole using a deep VLA 1.4 GHz continuum survey
By exploiting the far-infrared(FIR) and radio correlation, we have performed
the Likelihood-Ratio analysis to identify optical counterparts to the
far-infrared sources in the Lockman Hole. Using the likelihood ratio analysis
and the associated reliability, 44 FIR sources have been identified with radio
sources. Redshifts have been obtained for 29 out of 44 identified sources. One
hyper-luminous infrared galaxy (HyLIRG) with and four ultraluminous infrared
galaxies (ULIRGs) are identified in our sample. The space density of the FIR
sources at z = 0.3-0.6 is 4.6\times 10^{-5}Mpc^{-3}, implying a rapid evolution
of the ULIRG population. Most of \ISO FIR sources have their FIR-radio ratios
similar to star-forming galaxies ARP 220 and M82. At least seven of our FIR
sources show evidence for the presence of an active galactic nucleus (AGN) in
optical emission lines, radio continuum excess, or X-ray activity. Three out of
five (60%) of the ULIRG/HyLIRGs are AGN galaxies. Five of the seven AGN
galaxies are within the ROSAT X-ray survey field, and two are within the
XMM-Newton survey fields. X-ray emission has been detected in only one source,
1EX030, which is optically classified as a quasar. The non-detection in the
XMM-Newton 2-10 keV band suggests a very thick absorption obscuring the central
source of the two AGN galaxies. Several sources have an extreme FIR luminosity
relative to the optical R-band, L(90\mu\mathrm{m})/L(R) > 500, which is rare
even among the local ULIRG population. While source confusion or blending might
offer an explanation in some cases, they may represent a new population of
galaxies with an extreme activity of star formation in an undeveloped stellar
system -- i.e., formation of bulges or young ellipticals.Comment: 55 pages, 16 figures. To appear in A
Micromechanics Fatigue Damage Analysis Modeling for Fabric Reinforced Ceramic Matrix Composites
A micromechanics analysis modeling method was developed to analyze the damage progression and fatigue failure of fabric reinforced composite structures, especially for the brittle ceramic matrix material composites. A repeating unit cell concept of fabric reinforced composites was used to represent the global composite structure. The thermal and mechanical properties of the repeating unit cell were considered as the same as those of the global composite structure. The three-phase micromechanics, the shear-lag, and the continuum fracture mechanics models were integrated with a statistical model in the repeating unit cell to predict the progressive damages and fatigue life of the composite structures. The global structure failure was defined as the loss of loading capability of the repeating unit cell, which depends on the stiffness reduction due to material slice failures and nonlinear material properties in the repeating unit cell. The present methodology is demonstrated with the analysis results evaluated through the experimental test performed with carbon fiber reinforced silicon carbide matrix plain weave composite specimens
- …