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L. Zhang

Publications and source records attributed to L. Zhang.

299 records · Page 17Linked to original sources
arXiv2001

Novel Cascaded Ultra Bright Pulsed Source of Polarization Entangled Photons

A new ultra bright pulsed source of polarization entangled photons has been realized using type-II phase matching in spontaneous parametric down conversion process in two cascaded crystals. The optical axes of the crystals are aligned in such a way that the extraordinarily (ordinarily) polarized cone from one crystal overlaps with the ordinarily (extraordinarily) polarized cone from the second crystal. This spatial overlapping removes the association between the polarization and the output angle of the photons that exist in a single type-II down conversion process. Hence, entanglement of photons originating from any point on the output cones is possible if a suitable optical delay line is used. This delay line is particularly simple and easy to implement.

quant-ph↗
arXiv2001

Growth mechanism of superconducting MgB2 films prepared by various methods

The growth mechanisms of MgB2 films obtained by different methods on various substrates are compared via a detailed cross-sectional scanning electron microscopy (SEM) study. The analyzed films include (a) samples obtained by an ex-situ post-anneal at 900 degree of e-beam evaporated boron in the presence of an Mg vapor (exhibiting bulk-like Tc0 about 38.8 K), (b) samples obtained by the same ex-situ 900 degree anneal of pulsed laser deposition (PLD)-grown Mg+B precursors (exhibiting Tc0 ~ 25 K), and (c) films obtained by a low-temperature (600 - 630 degree) in-situ anneal of PLD-grown Mg+B precursors (with Tc0 about 24 K). A significant oxygen contamination was also present in films obtained from a PLD-grown precursors. On the other hand, it is clearly observed that the films obtained by the high-temperature reaction of e-beam evaporated B with Mg vapor are formed by the nucleation of independent MgB2 grains at the film surface, indicating that this approach may not be suitable to obtain smooth and (possibly) epitaxial films.

cond-mat.supr-con↗
arXiv2001

Superconducting magnesium diboride films on Silicon with Tc0 about 24K grown via vacuum annealing from stoichiometric precursors

Superconducting magnesium diboride films with Tc0 ~ 24 K and sharp transition \~ 1 K were successfully prepared on silicon substrates by pulsed laser deposition from a stoichiometric MgB2 target. Contrary to previous reports, anneals at 630 degree and a background of 2x10^(-4) torr Ar/4%H2 were performed without the requirement of Mg vapor or an Mg cap layer. This integration of superconducting MgB2 films on silicon may thus prove enabling in superconductor-semiconductor device applications. Images of surface morphology and cross-section profiles by scanning electron microscopy (SEM) show that the films have a uniform surface morphology and thickness. Energy dispersive spectroscopy (EDS) reveals these films were contaminated with oxygen, originating either from the growth environment or from sample exposure to air. The oxygen contamination may account for the low Tc for those in-situ annealed films, while the use of Si as the substrate does not result in a decrease in Tc as compared to other substrates.

cond-mat.supr-con↗
arXiv2001

Polarization and Variations of BL Lacertae Objects

BL Lacertae objects are an extreme subclass of AGNs showing rapid and large-amplitude variability, high and variable polarization, and core-dominated radio emissions. If a strong beaming effect is the cause of the extreme observation properties, one would expect that these properties would be correlated with each other. Based on the relativistic beaming model, relationships between the polarization and the magnitude variation in brightness, as well as the core- dominance parameter are derived and used statistically to compare with the observational data of a BL Lacertae object sample. The statistical results are consistent with these correlations, which suggests that the polarization, the variation, and the core-dominance parameter are possible indications of the beaming effect.

astro-ph↗
arXiv2001

The Radio and Gamma-Ray Luminosities of Blazars

Based on the $γ$-ray data of blazars in the third EGRET catalog and radio data at 5 GHz, we studied the correlation between the radio and $γ$-ray luminosities using two statistical methods. The first method was the partial correlation analysis method, which indicates that there exist correlations between the radio and $γ$-ray luminosities in both high and low states as well as in the average case. The second method involved a comparison of expected $γ$-ray luminosity distribution with the observed data using the Kolmogorov-- Smirnov (KS) test. In the second method, we assumed that there is a correlation between the radio and $γ$-ray luminosities and that the $γ$-ray luminosity function is proportional to the radio luminosity function. The KS test indicates that the expected gamma-ray luminosity distributions are consistent with the observed data in a reasonable parameter range. Finally, we used different $γ$-ray luminosity functions to estimate the possible 'observed' $γ$-ray luminosity distributions by GLAST.

astro-ph↗
arXiv1999

Basic properties of Gamma-ray loud blazars

In this paper, a method is proposed to determine the basic properties of $γ$-ray loud blazars, among them the central black hole mass, M, the Doppler factor, $δ$, the propagation angle of the $γ$-rays with respect to the symmetric axis of a two-temperature accretion disk, $Φ$, and the distance (i.e. the height above the accretion disk), d at which the $γ$-rays are created, for seven $γ$-ray loud blazars with available GeV variability timescales and in which the absorption effect of a $γ$-ray and the beaming effect have been taken into account. Our results indicate that, if we take the intrinsic $γ$-ray luminosity to be $λ$ times the Eddington luminosity, $L_γ^{in} = λL_{Edd.}$, the masses of the blazars are in the range of $(4 \sim 131)\times 10^{7}M_{\odot}$, the Doppler factors ($δ$) lie in the range of 0.57 to 5.33 the angle ($Φ$) is in the range of $13^{\circ}$ to 43$^{\circ}$ and the distance (d) is in the range of 26R_{g} to 411R_{g}. Our model results are independent of $γ$-ray emission mechanisms but they do depend on the X-ray emission mechanism of the accretion disk.

astro-ph↗
arXiv1999

Eccentric Behavior of Disk Galaxies

A theory is developed for the dynamics of eccentric perturbations $[\propto \exp(\pm iϕ)]$ of a disk galaxy residing in a spherical dark matter halo and including a spherical bulge component. The disk is represented as a large number $N$ of rings with shifted centers and with perturbed azimuthal matter distributions. Account is taken of the dynamics of the shift of the matter at the galaxy's center which may include a massive black hole. The gravitational interactions between the rings and between the rings and the center is fully accounted for, but the halo and bulge components are treated as passive gravitational field sources. Equations of motion and a Lagrangian are derived for the ring + center system, and these lead to total energy and total angular momentum constants of the motion. We study the eccentric dynamics of a disk with an exponential surface density distribution represented by a large number of rings. The inner part of the disk is found to be strongly unstable. Angular momentum of the rings is transferred outward and to the central mass if present, and a trailing one-armed spiral wave is formed in the disk. We also analyze a disk with a modified exponential density distribution where the density of the inner part of the disk is reduced. In this case we find much slower, linear growth of the eccentric motion. A trailing one-armed spiral wave forms in the disk and becomes more tightly wrapped as time increases. The motion of the central mass if present is small compared with that of the disk.

astro-ph↗
arXiv1999

The Goldberger-Treiman Discrepancy in SU(3)

The Goldberger-Treiman discrepancy in SU(3) is analyzed in the framework of heavy baryon chiral perturbation theory (HBChPT). It is shown that the discrepancy at leading order is entirely given by counterterms from the order p^3 Lagrangian, and that the first subleading corrections are suppressed by two powers in the HBChPT expansion. These subleading corrections include meson-loop contributions as well as counterterms from the order p^5 Lagrangian. Some one-loop contributions are calculated and found to be small. Using the three discrepancies (pi-N-N, K-N-Lambda and K-N-Sigma) which can be extracted from existing experimental data, we find that the HBChPT calculation favors the smaller g(pi-N-N) values obtained in recent partial wave analyses.

hep-ph↗
arXiv1998

Improved upper limit on Muonium to Antimuonium Conversion

A new experiment has been set up at the Paul Scherrer Institut to search for muonium to antimuonium conversion. No event was found to fulfil the requested signature which consists of the coincident detection of both constituents of the antiatom in its decay. Assuming an effective (V-A)$\times$(V-A) type interaction an improved upper limit is established for the conversion probability of ${\rm P_{M\bar{M}}} \leq 8 \cdot 10^{-9}$ (90%C.L.), which is almost two orders of magnitude lower compared to previous results and provides a sensitive test for theoretical extensions of the standard model.

nucl-ex↗
arXiv1993

A Gaussian Theory of Superfluid--Bose-Glass Phase Transition

We show that gaussian quantum fluctuations, even if infinitesimal, are sufficient to destroy the superfluidity of a disordered boson system in 1D and 2D. The critical disorder is thus finite no matter how small the repulsion is between particles. Within the gaussian approximation, we study the nature of the elementary excitations, including their density of states and mobility edge transition. We give the gaussian exponent $η$ at criticality in 1D and show that its ratio to $η$ of the pure system is universal.

cond-mat↗