42,925 research outputs found
Optical Resonator Analog of a Two-Dimensional Topological Insulator
A lattice of optical ring resonators can exhibit a topological insulator
phase, with the role of spin played by the direction of propagation of light
within each ring. Unlike the system studied by Hafezi et al., topological
protection is achieved without fine-tuning the inter-resonator couplings, which
are given the same periodicity as the underlying lattice. The topological
insulator phase occurs for strong couplings, when the tight-binding method is
inapplicable. Using the transfer matrix method, we derive the bandstructure and
phase diagram, and demonstrate the existence of robust edge states. When gain
and loss are introduced, the system functions as a diode for coupled resonator
modes.Comment: 10 pages, 9 figure
The heavy-element abundances of AGB stars and the angular momentum conservation model of wind accretion for barium stars
Adpoting new s-process nucleosynthesis scenario and branch s-process path, we
calculate the heavy-element abundances and C/O ratio of solar metallicity
3M_sun TP-AGB stars. The evolutionary sequence from M to S to C stars of AGB
stars is explained naturally by the calculated results. Then combining the
angular momentum conservation model of wind accretion with the heavy-element
abundances on the surface of TP-AGB stars, we calculate the heavy-element
overabundances of barium stars via successive pulsed accreting and mixing. Our
results support that the barium stars with longer orbital period, P>1600 days,
form through wind accretion scenario.Comment: 14 pages, LaTex, 17 PS figures included, accepted for publication in
A &
NeIII/OII as an oxygen abundance indicator in the HII regions and HII galaxies
To calibrate the relationship between Ne3O2 (Ne3O2 =
log(\neiii/\oii)) and oxygen abundances, we present a
sample of 3000 \hii galaxies from the Sloan Digital Sky Survey (SDSS)
data release four. They are associated with a sample from the literature
intended to enlarge the oxygen abundance region. We calculated the electron
temperatures () of 210 galaxies in the SDSS sample with the direct method,
and of the other 2960 galaxies in SDSS sample calculated with an
empirical method. Then, we use a linear least-square fitting to calibrate the
Ne3O2 oxygen abundance indicator. It is found that the Ne3O2 estimator follows
a linear relation with \zoh\ that holds for the whole abundance range covered
by the sample, from approximately 7.0 to 9.0. The best linear relationship
between the Ne3O2 and the oxygen abundance is calibrated. The dispersion
between oxygen abundance and Ne3O2 index in the metal rich galaxies may come
partly from the moderate depletion of oxygen onto grains. The method
has the virtue of being single-valued and not affected by internal reddening.
As a result, the method can be a good metallicity indicator in the \hii
regions and \hii galaxies, especially in high-redshift galaxies.Comment: 7 pages, 6 figures. A&A accepte
Polarization Induced Switching Effect in Graphene Nanoribbon Edge-Defect Junction
With nonequilibrium Green's function approach combined with density
functional theory, we perform an ab initio calculation to investigate transport
properties of graphene nanoribbon junctions self-consistently. Tight-binding
approximation is applied to model the zigzag graphene nanoribbon (ZGNR)
electrodes, and its validity is confirmed by comparison with GAUSSIAN03 PBC
calculation of the same system. The origin of abnormal jump points usually
appearing in the transmission spectrum is explained with the detailed
tight-binding ZGNR band structure. Transport property of an edge defect ZGNR
junction is investigated, and the tunable tunneling current can be sensitively
controlled by transverse electric fields.Comment: 18 pages, 8 figure
Electron density diagnostic for hot plasmas in coronal regime by using B-like ions
Line ratio of transition lines in boron-like spectra of Si X, S XII,
Ar XIV and Fe XXII has been investigated. Collisional-radiative model
calculations reveal that the line ratio is sensitive to the electron density in
ranges of cm,
cm,
cm and cm, respectively. This
complements the K-shell diagnostics of helium-like ions. By comparison between
the prediction and the measured values, effective electron densities in the
electron beam ion trap (EBIT) plasmas performed by Lepson and collaborators at
Lawrence Livermore EBIT, are estimated to be cm and
cm for sulphur and argon plasmas. In
case of argon, a good agreement is shown with the actual electron density
derived from N VI K-shell spectrum. We further explore the transition
lines of Si X and S XII in the stellar coronal spectra measured with the Low
Energy Transmission Grating Spectrometer combined with High Resolution Camera
on board the {\it Chandra X-ray Observatory}. The constrained electron
densities show a good agreement with the those determined from C V and O VII
K-shell spectra.Comment: 15 pagess, 5 figures and 4 tables, accepted for publication on A
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