arXiv subjects

Y. H. Zhang

Publications and source records attributed to Y. H. Zhang.

At least 109 records · Page 6Linked to original sources

arXiv2010

Weak field magnetoresistance of narrow-gap semiconductor InSb

The weak antilocalization effect of InSb film in perpendicular as well as tilted magnetic field is investigated. It is found that the InSb film has quasi-two-dimensional feature and the Nyquist mechanism dominates decoherence. The two dimensionality is also verified further and the influence of roughness effect and Zeeman effect on weak antilocalization effect is studied by systematically investigating the anisotropy of weak field magnetoresistance with respect to magnetic field. It is also found that the existence of in-plane field can effectively suppress the weak antilocalization effect of InSb film and the roughness effect plays an important role in the anisotropy.

cond-mat.mtrl-sci↗

arXiv2010

Confirmation of room-temperature long range magnetic order in GaN:Mn

We propose a method for nano-scale characterization of long range magnetic order in diluted magnetic systems to clarify the origins of the room temperature ferromagnetism. The GaN:Mn thin films are grown by metal-organic chemical vapor deposition with the concentration of Ga-substitutional Mn up to 3.8%. Atomic force microscope (AFM) and magnetic force microscope (MFM) characterizations are performed on etched artificial microstructures and natural dislocation pits. Numerical simulations and theoretical analysis on the AFM and MFM data have confirmed the formation of long range magnetic order and ruled out the possibility that nano-clusters contributed to the ferromagnetism. We suggest that delocalized electrons might play a role in the establishment of this long range magnetic order.

cond-mat.mtrl-sci↗

arXiv2010

Vortex Phase Diagram of Layered Superconductor Cu0.03TaS2 for H || c

The magnetization and anisotropic electrical transport properties have been measured in high quality Cu0.03TaS2 single crystal. A pronounced peak effect has been observed, indicating that the high quality and homogeneity are vital to peak effect. A kink has been observed in the magnetic field H dependence of the in-plane resistivity ρab for H || c, which corresponds to a transition from activated to diffusive behavior of vortex liquid phase. In the diffusive regime of the vortex liquid phase, the in-plane resistivity ρab shows ρab $\propto$ H0.3 relation, which does not follow the Bardeen-Stephen law for free flux flow. Finally, a simplified vortex phase diagram of Cu0.03TaS2 for H || c is given.

cond-mat.supr-con↗

arXiv2010

Investigation of excited states in $^{18}$Ne via resonant elastic scattering of $^{17}$F+p and its astrophysical implication in the stellar reaction of $^{14}$O($α$,$p$)$^{17}$F

Properties of proton resonances in $^{18}$Ne have been investigated efficiently by utilizing a technique of proton resonant elastic scattering with a $^{17}$F radioactive ion (RI) beam and a thick proton target. A 4.22~MeV/nucleon $^{17}$F RI beam was produced via a projectile-fragmentation reaction, and subsequently separated by a Radioactive Ion Beam Line in Lanzhou ({\tt RIBLL}). Energy spectra of the recoiled protons were measured by two sets of $Δ$E-E silicon telescope at center-of-mass scattering angles of $θ_{c.m.}$$\approx$175${^\circ}$$\pm$5${^\circ}$, $θ_{c.m.}$$\approx$152${^\circ}$$\pm$8${^\circ}$, respectively. Several proton resonances in $^{18}$Ne were observed, and their resonant parameters have been determined by an $R$-matrix analysis of the differential cross sections in combination with the previous results. The resonant parameters are related to the reaction-rate calculation of the stellar $^{14}$O($α$,$p$)$^{17}$F reaction, which was thought to be the breakout reaction from the hot CNO cycles into the $rp$-process in x-ray bursters. Here, $J^π$=(3$^-$, 2$^-$) are tentatively assigned to the 6.15-MeV state which was thought the key 1$^-$ state previously. In addition, a doublet structure at 7.05 MeV are tentatively identified, and its contribution to the resonant reaction rate of $^{14}$O($α$,$p$)$^{17}$F could be enhanced by at least factors of about 4$\sim$6 in comparison with the previous estimation involving only a singlet. The present calculated resonant rates are much larger than those previous values, and it may imply that this breakout reaction could play a crucial role under x-ray bursters conditions.

nucl-ex↗

arXiv2008

Pressure effect on superconductivity and magnetism in FeSex

In this paper, pressure effect on superconductivity and magnetism has been investigated in FeSex (x = 0.80, 0.88). The magnetization curves display anomaly at Ts1 106 K and Ts2 78 K except for the superconducting diamagnetic transition around Tc 8 K. The magnetic anomaly at Ts1 and Ts2 can be related to a ferromagnetic and an antiferromagnetic phase transition, respectively, as revealed by specific heat measurements. The application of pressure not only raises Tc, but also increases both Ts1 and Ts2. This system shows clear evidence that superconductivity arises in a phase with strong magnetic character and the superconductivity coexists with magnetism. In addition, the specific heat anomaly associated with the superconducting transition seems to be absent.

cond-mat.supr-con↗

arXiv2008

Remote generation of entanglement for individual atoms via optical fibers

The generation of atomic entanglement is discussed in a system that atoms are trapped in separate cavities which are connected via optical fibers. Two distant atoms can be projected to Bell-state by synchronized turning off the local laser fields and then performing a single quantum measurement by a distant controller. The distinct advantage of this scheme is that it works in a regime that $Δ\approxκ\gg g$, which makes the scheme insensitive to cavity strong leakage. Moreover, the fidelity is not affected by atomic spontaneous emission.

quant-ph↗

arXiv2008

XMM-Newton observations of the TeV BL Lac object PKS 2155--304 in 2006: signature of inverse Compton X-ray emission?

This paper reports the first discovery of possible inverse Compoton (IC) X-ray emission below 10 keV in the typical high-energy peaked BL Lac object (HBL) PKS 2155--304. Two XMM-Newton observations performed in 2006 reveal that the 0.6--10 keV X-ray spectra of the source harden ($ΔΓ\sim$ 0.1--0.3) at break energies of about 4 keV. The concave X-ray spectra of the source could be easily interpretated by a mixture of a steep component (i.e., the high energy tail of the synchrotron emission) and a flat one (i.e., the low energy side of the IC emission). However, the steep spectra ($Γ\sim$ 2.3) in the hard X-rays indicate that the synchrotron emission still domiantes over the IC one, while the latter is effectively present and perceived as flattening the synchrotron spectrum in this energy range. The quasi-simultaneous optical-UV-X-ray spectral energy distributions (SEDs) obtained with XMM-Newton suggest that the concave X-ray spectra of the source could be the result of down-ward shift of the synchrotron peak frequency to the optical band, incuring the IC emisssion to become more important in the hard ($\sim 4$--10 keV) X-ray band with respective to other cases in which the synchrotron emission peaks in the UV-soft-X-ray range. This discovery provides a new clue for a smooth transition between HBLs and the low-energy peaked BL Lac objects (LBLs).

astro-ph↗

arXiv2007

Candidate chiral doublet bands in the odd-odd nucleus $^{126}$Cs

The candidate chiral doublet bands recently observed in $^{126}$Cs have been extended to higher spins, several new linking transitions between the two partner members of the chiral doublet bands are observed, and $γ$$-$intensities related to the chiral doublet bands are presented by analyzing the $γ$$-$$γ$ coincidence data collected earlier at the NORDBALL through the $^{116}$Cd$($$^{14}$N, 4n$)$$^{126}$Cs reaction at a beam energy of 65 MeV. The intraband $B(M1)/B(E2)$ and interband $B(M1)_{in}/B(M1)_{out}$ ratios and the energy staggering parameter, S(I), have been deduced for these doublet bands. The results are found to be consistent with the chiral interpretation for the two structures. Furthermore, the observation of chiral doublet bands in $^{126}$Cs together with those in $^{124}$Cs, $^{128}$Cs, $^{130}$Cs and $^{132}$Cs also indicates that the chiral conditions do not change rapidly with decreasing neutron number in these odd-odd Cesium isotopes.

nucl-ex↗

arXiv2006

Multiwavelength Observations of the BL Lacertae Object PKS 2155-304 with XMM-Newton

The optical-UV and X-ray instruments on-board XMM-Ndewton provide an excellent opportunity to perform simultaneous observations of violently variable objects over a broad wavelength range. The UV and X-ray bright BL Lac object PKS 2155-304 has been repeatedly observed with XMM-Ndewton about twice per year. In this paper, we present a detailed analysis of the simultaneous multiwavelength variability of the source from optical to X-rays, based on the currently available XMM-Ndewton observations. These observations probed the intra-day multiwavelength variability at optical-UV and X-ray wavelengths of the source. The UV variability amplitude is substantially smaller than the X-ray one, and the hardness ratios of the UV to X-rays correlates with the X-ray fluxes: the brighter the source, the flatter the UV-X-ray spectra. On 2000 May 30-31 the UV and X-ray light curves were weakly correlated, while the UV variations followed the X-ray ones with no detectable lags on 2000 November 19-21. On 2001 November 30 the source exhibited a major X-ray flare that was not detected in the optical. The intra-day UV and X-ray variability presented here is not similar to the inter-day UV and X-ray variability obtained from the previous coordinated extensive multiwavelength campaigns on the source, indicating that different ``modes'' of variability might be operating in PKS 2155-304 on different timescales or from epoch to epoch.

astro-ph↗

arXiv2005

XMM-Newton view of PKS 2155-304: hardness ratio and cross-correlation analysis of EPIC pn observations

With currently available XMM-Newton EPIC pn observations spanned over about 3 years, we present a detailed spectral and temporal variability of the 0.2--10 keV X-ray emission from the X-ray bright BL Lac object PKS 2155-304. The spectral variability is examined with a model independent hardness ratio method. We find that the spectral evolution of the source follows the light curves well, indicating that the spectra harden when the fluxes increase. The plots of hardness ratios versus count rates show that the spectral changes are particularly significant during flares. The cross-correlation functions (CCFs) show that the light curves in the different energy bands are well correlated at different time lags. The CCF peaks (i.e., the maximum correlation coefficients) tend to become smaller with larger energy differences, and the variability in the different energy bands are more correlated for the flares than for the other cases. In most cases the higher energy band variations lead the lower energy band, but in two cases we observed the opposite behavior that the lower energy variability possibly leads the higher energy variability. The time lags increase with the energy differences between the two cross-correlated light curves. The maximum lag is found to be up to about one hour, in support with the findings obtained with previous low Earth orbit X-ray missions. We discuss our results in the context of the particle acceleration, cooling and light crossing timescales.

astro-ph↗

arXiv2005

Phenomenological Analogies in TeV Blazars and Black Hole X-ray Binaries in low state

Blazars are well known for the violent variability from the radio to gamma-ray wavelengths. In this paper, we present a brief summary of the X-ray variability recently obtained for TeV blazars. We also point out the probable similarities of the X-ray variability characteristics between blazars and microblazars. Such phenomenological analogies could be interpreted and unified with the synchrotron X-ray emissions from the jets of the two classes of objects that differ in black hole mass by factors of $\ga 10^7$. The phenomenological analogies in blazars and microblazars (both are jet-dominated) might be parallel to those in black hole X-ray binaries in high/soft state and Seyfert galaxies (both are disk-dominated).

astro-ph↗

arXiv2005

XMM-Newton View of PKS 2155-304: Characterizing the X-ray Variability Properties with EPIC-PN

Starting from XMM-Newton EPIC-PN data, we present the X-ray variability characteristics of PKS 2155-304 using a simple analysis of the excess variance, \xs, and of the fractional rms variability amplitude, fvar. The scatter in \xs\ and \fvar, calculated using 500 s long segments of the light curves, is smaller than the scatter expected for red noise variability. This alone does not imply that the underlying process responsible for the variability of the source is stationary, since the real changes of the individual variance estimates are possibly smaller than the large scatters expected for a red noise process. In fact the averaged \xs and \fvar, reducing the fluctuations of the individual variances, chang e with time, indicating non-stationary variability. Moreover, both the averaged \sqxs (absolute rms variability amplitude) and \fvar show linear correlation with source flux but in an opposite sense: \sqxs correlates with flux, but \fvar anti-correlates with flux. These correlations suggest that the variability process of the source is strongly non-stationary as random scatters of variances should not yield any correlation. \fvar spectra were constructed to compare variability amplitudes in different energy bands. We found that the fractional rms variability amplitude of the source, when significant variability is observed, increases logarithmically with the photon energy, indicating significant spectral variability. The point-to-point variability amplitude may also track this trend, suggesting that the slopes of the power spectral density of the source are energy-independent. Using the normalized excess variance the black hole mass of \pks was estimated to be about $1.45 \times 10^8 M_{\bigodot}$. This is compared and contrasted with the estimates derived from measurements of the host galaxies.

astro-ph↗

arXiv2004

The equivariant Brauer group of a group

We consider the Brauer group ${\rm BM}'(k,G)$ of a group $G$ (finite or infinite) over a commutative ring $k$ with identity. A split exact sequence $$1\longrightarrow {\rm Br}'(k)\longrightarrow {\rm BM}'(k,G)\longrightarrow {\rm Gal}(k,G) \longrightarrow 1$$ is obtained. This generalizes the Fröhlich-Wall exact sequence from the case of a field to the case of a commutative ring, and generalizes the Picco-Platzeck exact sequence from the finite case of $G$ to the infinite case of $G$. Here ${\rm Br}'(k)$ is the Brauer-Taylor group of Azumaya algebras (not necessarily with unit). The method developed in this paper might provide a key to computing the equivariant Brauer group of an infinite quantum group.

math.RA↗

arXiv2004

The Effects of Periodically Gapped Time Series on Cross-correlation Lag Determinations

The three bright TeV blazars Mrk 421, Mrk 501 and PKS 2155-404 are highly variable in synchrotron X-ray emission. In particular, these sources may exhibit variable time lags between flux variations at different X-ray energy bands. However, there are a number of issues that may significantly bias lag determinations. Edelson et al. (2001) recently proposed that the lags on timescales of hours, discovered by ASCA and BeppoSAX, could be an artifact of periodic gaps in the light curves introduced by the Earth occultation every \~1.6 hr. Using Monte Carlo simulations, in this paper we show that the lags over timescales of hours can not be the spurious result of periodic gaps, while periodic gaps indeed introduces uncertainty larger than what present in the evenly sampled data. The results also show that time lag estimates can be substantially improved by using evenly sampled light curves with large lag to bin-size ratio. Furthermore, we consider an XMM-Newton observation without interruptions and re-sample the light curves using the BeppoSAX observing windows, and then repeat the same cross correlation function (CCF) analysis on both the real and fake data. The results also show that periodic gaps in the light curves do not significantly distort the CCF characters, and indeed the CCF peak ranges of the real and fake data overlap. Therefore, the lags discovered by ASCA and BeppoSAX are not due to periodic gaps in the light curves.

astro-ph↗

arXiv2002

A new list of extra-galactic radio jets

A catalogue of extra-galactic jets is very useful both in observational and theoretical studies of active galaxies. With the use of new powerful radio instruments, the detailed structures of very compact or weak radio sources are investigated observationally and many new radio jets are detected. In this paper, we give a list of 661 radio sources with detected radio jets known to us prior to the end of December 2000. All references are collected for the observations of jets in radio, IR, optical, UV and X-ray wave-bands.

astro-ph↗

arXiv2002

Cross-spectral analysis of the X-ray variability of Mrk 421

Using the cross-spectral method, we confirm the existence of the X-ray hard lags discovered with cross-correlation function technique during a large flare of Mrk 421 observed with BeppoSAX . For the 0.1--2 versus 2--10keV light curves, both methods suggest sub-hour hard lags. In the time domain, the degree of hard lag, i.e., the amplitude of the 3.2--10 keV photons lagging the lower energy ones, tends to increase with the decreasing energy. In the Fourier frequency domain, by investigating the cross-spectra of the 0.1--2/2--10 keV and the 2--3.2/3.2--10 keV pairs of light curves, the flare also shows hard lags at the lowest frequencies. However, with the present data, it is impossible to constrain the dependence of the lags on frequencies even though the detailed simulations demonstrate that the hard lags at the lowest frequencies probed by the flare are not an artifact of sparse sampling, Poisson and red noise. As a possible interpretation, the implication of the hard lags is discussed in the context of the interplay between the (diffusive) acceleration and synchrotron cooling of relativistic electrons responsible for the observed X-ray emission. The energy-dependent hard lags are in agreement with the expectation of an energy-dependent acceleration timescale. The inferred magnetic field (B ~ 0.11 Gauss) is consistent with the value inferred from the Spectral Energy Distributions of the source. Future investigations with higher quality data that whether or not the time lags are energy-/frequency-dependent will provide a new constraint on the current models of the TeV blazars.

astro-ph↗

arXiv2002

Four Years Monitoring of Blazar PKS 2155-304 with BeppoSAX: Probing the Dynamics of the Jet

PKS 2155--304 is one of the brightest blazars in the X-ray band. It was repeatedly monitored with BeppoSax during three long campaigns of about 2 days each in November of 1996, 1997 and 1999. The source underwent different states of intensity and was clearly variable with successive flares detected. This paper presents temporal and spectral analysis to study the X-ray variability trends for a blazar. The rich phenomenology is interpreted in the context of a model where relativistic electrons are accelerated through internal shocks taking place in the jets. The most important parameter turns out to be the initial time interval between the two shells ejected from the central engine to produce the flare, which may determine the structure of the shock and in turn the physical quantities of the emitting region to reproduce the observed trends of the X-ray variability (abridged).

astro-ph↗

arXiv2000

X-ray Emission of Mkn 421: New Clues From Its Spectral Evolution. I. Temporal Analysis

Mkn 421 was repeatedly observed with BeppoSAX in 1997-1998. This is the first of two papers where we present the results of a thorough temporal and spectral analysis of all the data available to us, focusing in particular on the flare of April 1998, which was simultaneously observed also at TeV energies. Here we focus on the time analysis, while the spectral analysis and physical interpretation are presented in the companion paper. The detailed study of the flare in different energy bands reveals very important new results: i) hard photons lag the soft ones by 2-3 ks -a behavior opposite to what is normally found in high energy peak BL Lacs X-ray spectra; ii) the flare light curve is symmetric in the softest X-ray band, while it becomes increasingly asymmetric at higher energies, with the decay being progressively slower than the rise; iii) the flux decay of the flare can be intrinsically achromatic if a stationary underlying emission component is present. The temporal and spectral information obtained challenge the simplest models currently adopted for the (synchrotron) emission and most importantly provide clues on the particle acceleration process.

astro-ph↗