57,089 research outputs found
Levinson's theorem for the Schr\"{o}dinger equation in two dimensions
Levinson's theorem for the Schr\"{o}dinger equation with a cylindrically
symmetric potential in two dimensions is re-established by the Sturm-Liouville
theorem. The critical case, where the Schr\"{o}dinger equation has a finite
zero-energy solution, is analyzed in detail. It is shown that, in comparison
with Levinson's theorem in non-critical case, the half bound state for
wave, in which the wave function for the zero-energy solution does not decay
fast enough at infinity to be square integrable, will cause the phase shift of
wave at zero energy to increase an additional .Comment: Latex 11 pages, no figure and accepted by P.R.A (in August); Email:
[email protected], [email protected]
PopIII signatures in the spectra of PopII/I GRBs
We investigate signatures of population III (PopIII) stars in the
metal-enriched environment of GRBs originating from population II-I (PopII/I)
stars by using abundance ratios derived from numerical simulations that follow
stellar evolution and chemical enrichment. We find that at more than
of PopII/I GRBs explode in a medium previously enriched by PopIII stars
(we refer to them as GRBIIIII). Although the formation of
GRBIIIII is more frequent than that of pristine PopIII GRBs
(GRBIIIs), we find that the expected GRBIIIII observed rate is
comparable to that of GRBIIIs, due to the usually larger luminosities of these
latter. GRBIIIII events take place preferentially in small
proto-galaxies with stellar masses , star formation rates
and metallicities . On the other hand,
galaxies with are dominated by metal enrichment
from PopIII stars and should preferentially host GRBIIIII. Hence,
measured GRB metal content below this limit could represent a strong evidence
of enrichment by pristine stellar populations. We discuss how to discriminate
PopIII metal enrichment on the basis of various abundance ratios observable in
the spectra of GRBs' afterglows. By employing such analysis, we conclude that
the currently known candidates at redshift -- i.e. GRB 050904
\cite[][]{2006Natur.440..184K} and GRB 130606A \cite[][]{2013arXiv1312.5631C}
-- are likely not originated in environments pre-enriched by PopIII stars.Comment: 9 pages, 7 figures; MNRAS accepte
Constraining the PopIII IMF with high-z GRBs
We study the possibility to detect and distinguish signatures of enrichment
from PopIII stars in observations of PopII GRBs (GRBIIs) at high redshift by
using numerical N-body/hydrodynamical simulations including atomic and
molecular cooling, star formation and metal spreading from stellar populations
with different initial mass functions (IMFs), yields and lifetimes. PopIII and
PopII star formation regimes are followed simultaneously and both a top-heavy
and a Salpeter-like IMF for pristine PopIII star formation are adopted. We find
that the fraction of GRBIIs hosted in a medium previously enriched by PopIII
stars (PopIII-dominated) is model independent. Typical abundance ratios, such
as [Si/O] vs [C/O] and [Fe/C] vs [Si/C], can help to disentangle enrichment
from massive and intermediate PopIII stars, while low-mass first stars are
degenerate with regular PopII generations. The properties of galaxies hosting
PopIII-dominated GRBIIs are not very sensitive to the particular assumption on
the mass of the first stars.Comment: 9 pages, 4 figure
Influences of magnetic coupling process on the spectrum of a disk covered by the corona
Recently, much attention has been paid to the magnetic coupling (MC) process,
which is supported by very high emissivity indexes observed in Seyfert 1 galaxy
MCG-6-30-15 and GBHC XTE J1650-500. But the rotational energy transferred from
a black hole is simply assumed to be radiated away from the surrounding
accretion disk in black-body spectrum, which is obviously not consistent with
the observed hard power-law X-ray spectra. We intend to introduce corona into
the MC model to make it more compatible with the observations. We describe the
model and the procedure of a simplified Monte Carlo simulation, compare the
output spectra in the cases with and without the MC effects, and discuss the
influences of three parameters involved in the MC process on the output
spectra. It is shown that the MC process augments radiation fluxes in the UV or
X-ray band. The emergent spectrum is affected by the BH spin and magnetic field
strength at the BH horizon, while it is almost unaffected by the radial profile
of the magnetic field at the disk. Introducing corona into the MC model will
improve the fitting of the output spectra from AGNs and GBHCs.Comment: 15 pages, 5 figures, accepted by A&
The Relativistic Levinson Theorem in Two Dimensions
In the light of the generalized Sturm-Liouville theorem, the Levinson theorem
for the Dirac equation in two dimensions is established as a relation between
the total number of the bound states and the sum of the phase shifts
of the scattering states with the angular momentum :
\noindent The critical case, where the Dirac equation has a finite
zero-momentum solution, is analyzed in detail. A zero-momentum solution is
called a half bound state if its wave function is finite but does not decay
fast enough at infinity to be square integrable.Comment: Latex 14 pages, no figure, submitted to Phys.Rev.A; Email:
[email protected], [email protected]
The structural, mechanical, electronic, optical and thermodynamic properties of t-XAs (X Si, Ge and Sn) by first-principles calculations
The structural, mechanical, electronic, optical and thermodynamic properties
of the t-XAs (X Si, Ge and Sn) with
tetragonal structure have been investigated by first principles calculations.
Our calculated results show that these compounds are mechanically and
dynamically stable. By the study of elastic anisotropy, it is found that the
anisotropic of the t-SnAs is stronger than that
of t-SiAs and
t-GeAs. The band structures and density of states
show that the t-XAs (Si, Ge and Sn) are
semiconductors with narrow band gaps. Based on the analyses of electron density
difference, in t-XAs As atoms get electrons, X
atoms lose electrons. The calculated static dielectric constants,
, are 15.5, 20.0 and 15.1 eV for
t-XAs (X Si, Ge and Sn), respectively. The
Dulong-Petit limit of t-XAs is about 10 J
molK. The thermodynamic stability successively
decreases from t-SiAs to
t-GeAs to t-SnAs.Comment: 14 pages, 10 figures, 6 table
Broadband RCS Reduction of Microstrip Patch Antenna Using Bandstop Frequency Selective Surface
In this article, a simple and effective approach is presented to reduce the Radar Cross Section (RCS) of microstrip patch antenna in ultra broad frequency band. This approach substitutes a metallic ground plane of a conventional patch antenna with a hybrid ground consisting of bandstop Frequency Selective Surface (FSS) cells with partial metallic plane. To demonstrate the validity of the proposed approach, the influence of different ground planes on antenna’s performance is investigated. Thus, a patch antenna with miniaturized FSS cells is proposed. The results suggest that this antenna shows 3dB RCS reduction almost in the whole out-of operating band within 1-20GHz for wide incident angles when compared to conventional antenna, while its radiation characteristics are sustained simultaneously. The reasonable agreement between the measured and the simulated results verifies the efficiency of the proposed approach. Moreover, this approach doesn’t alter the lightweight, low-profile, easy conformal and easy manufacturing nature of the original antenna and can be extended to obtain low-RCS antennas with metallic planes in broadband that are quite suitable for the applications which are sensitive to the variation of frequencies
New U(1) Gauge Symmetry of Quarks and Leptons
Instead of anchoring the seesaw mechanism with the conventional heavy
right-handed neutrino singlet, a small Majorana neutrino mass may be obtained
just as well with the addition of a heavy triplet of leptons per family to the
minimal standard model of particle interactions. The resulting model is shown
to have the remarkable property of accommodating a new U(1) symmetry which is
anomaly-free and may thus be gauged. There are many possible phenomenological
consequences of this proposal which may be already relevant in explaining one
or two recent potential experimental discrepancies.Comment: minor word changes, to appear in MPL
Frost increases internal potassium requirements for alleviation of sterility and grain yield of wheat
Low potassium (K) in topsoils and subsoils is common in the grains belt of West Australia (Weaver and Wong 2011). Frost is increasing in frequency and severity during the spring coinciding with the young microspore stage of pollen development, and is manifest as severe grain yield loss from frost-induced sterility. Our aim was to determine whether K increased wheat crop tolerance to frost during early pollen development and whether this was related to internal K concentrations
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