arXiv subjects

Y. Zhang

Publications and source records attributed to Y. Zhang.

At least 577 records · Page 32Linked to original sources

arXiv2010

QCD sum rules and thermal properties of Charmonium in the vector channel

The thermal evolution of the hadronic parameters of charmonium in the vector channel, i.e. the J/psi resonance mass, coupling (leptonic decay constant), total width, and continuum threshold is analyzed in the framework of thermal Hilbert moment QCD sum rules. The continuum threshold $s_0$, as in other hadronic channels, decreases with increasing temperature until the PQCD threshold s_0 = 4, m_Q^2 is reached at T \simeq 1.22T_c (m_Q is the charm quark mass) and the J/psi mass is essentially constant in a wide range of temperatures. The other hadronic parameters behave in a very different way from those of light-light and heavy-light quark systems. The total width grows with temperature up to T \simeq 1.04T_c beyond which it decreases sharply with increasing T. The resonance coupling is also initially constant beginning to increase monotonically around T \simeq T_c. This behavior strongly suggests that the J/psi resonance might survive beyond the critical temperature for deconfinement, in agreement with lattice QCD results.

hep-ph↗

arXiv2010

Simultaneous time-optimal control of the inversion of two spin 1/2 particles

We analyze the simultaneous time-optimal control of two-spin systems. The two non coupled spins which differ in the value of their chemical offsets are controlled by the same magnetic fields. Using an appropriate rotating frame, we restrict the study to the case of opposite shifts. We then show that the optimal solution of the inversion problem in a rotating frame is composed of a pulse sequence of maximum intensity and is similar to the optimal solution for inverting only one spin by using a non-resonant control field in the laboratory frame. An example is implemented experimentally using techniques of Nuclear Magnetic Resonance.

quant-ph↗

arXiv2010

Geometry and optics calibration of WFCTA prototype telescopes using star light

The Large High Altitude Air Shower Observatory project is proposed to study high energy gamma ray astronomy ( 40 GeV-1 PeV ) and cosmic ray physics ( 20 TeV-1 EeV ). The wide field of view Cherenkov telescope array, as a component of the LHAASO project, will be used to study energy spectrum and compositions of cosmic ray by measuring the total Cherenkov light generated by air showers and shower maximum depth. Two prototype telescopes have been in operation since 2008. The pointing accuracy of each telescope is crucial to the direction reconstruction of the primary particles. On the other hand the primary energy reconstruction relies on the shape of the Cherenkov image on the camera and the unrecorded photons due to the imperfect connections between photomultiplier tubes. UV bright stars are used as point-like objects to calibrate the pointing and to study the optical properties of the camera, the spot size and the fractions of unrecorded photons in the insensitive areas of the camera.

astro-ph.IM↗

arXiv2010

Inclusion of sdBs in evolutionary population synthesis for binary stellar populations and the application: the determinations of photo-z and galaxy morphology

Subdwarf B stars (sdBs) can significantly change the ultraviolet spectra of populations at age t~1Gyr, and have been even included in the volutionary population synthesis (EPS) models by Han et al. (2007). In this study we present the spectral energy distributions (SEDs) of binary stellar populations (BSPs) by combining the EPS models of Han et al. (2007) and those of the Yunnan group (Zhang et al. 2004, 005), which have included various binary interactions (except sdBs) in EPS models. This set of SEDs is vailable upon request from the authors. Using this set of SEDs of BSPs we build the spectra of Burst, E, S0-Sd and Irr types of galaxies by using the package of Bruzual & Charlot (2003, BC03). Combined with the photometric data (filters and magnitudes), we obtain the photometric redshifts and morphologies of 1502 galaxies by using the Hyperz code of Bolzonella et al. (2000). This sample of galaxies is obtained by removing those objects, mismatched with the SDSS/DR7 and GALEX/DR4, from the catalogue of Fukugita et al. (2007). By comparison the results with the SDSS spectroscopic redshifts and the morphological index of Fukugita et al. (2007), we find that the photo-zs fluctuate with the SDSS spectroscopic redshifts, while the Sa-Sc galaxies in the catalogue of Fukugita et al. (2007) are classified earlier as Burst-E galaxies.

astro-ph.SR↗

arXiv2010

The effects of UV photometry and binary interactions on photometric redshift and galaxy morphology

Using the Hyperz code and a template spectral library which consists of 4 observed galaxy spectra from Coleman, Wu & Weedman (CWW, 1980) and 8 spectral families built with evolutionary population synthesis models, we present photometric redshift estimates (photo-z) for a spectroscopic sample of 6,531 galaxies, and morphologies for a morphological sample of 1,502 bright galaxies. All galaxies are matched with the SDSS DR7 and GALEX DR4. The inclusion of Fuv or Nuv or both photometry decreases the number of catastrophic identifications (CIs, |z_phot -z_spec| > 1.0). If CIs are removed, the inclusion of both Fuv and Nuv photometry mainly increases the number of non-CIs in the low redshift, g-r < 0.8 and fainter r-magnitude regions. The inclusion of binary interactions (BIs) mainly increases the number of non-CIs and decreases the deviations in the 0.3 < g-r < 0.8 region in the case of only using optical photometry. The inclusion of UV photometry would decrease and increase the probability that early types are classified as Burst and E types, respectively, and increase that late types are classified as CWW-Sbc and CWW-Scd types. If CIs are excluded, the inclusion of UV data mainly raises the identifications of late types in all redshift, bluer g-r and r > 14 regions. Moreover, BIs mainly affect the determinations of E and S0 types. Nuv -u = 1.94 and 5.77-1.47(u-r) = Fuv discriminators can be used as morphology selection indicators. These two criteria have comparable reliability and completeness for selecting early- and late-type galaxies to C=2.6 criterion and higher completeness for early-type selection than u-r=2.22 criterion.

astro-ph.SR↗

arXiv2010

Out-of-plane momentum and symmetry dependent superconducting gap in Ba0.6K0.4Fe2As2

The three-dimensional band structure and superconducting gap of Ba0.6K0.4Fe2As2 are studied with high-resolution angle-resolved photoemission spectroscopy. In contrast to previous results, we have identified three hole-like Fermi surfaces near the zone center with sizable out-of-plane or kz dispersion. The superconducting gap on certain Fermi surface shows significant kz-dependence. Moreover, we found that the superconducting gap sizes are different at the same Fermi momentum for two bands with different spatial symmetries (one odd, one even). Our results provide further information on the rich superconducting gap structure in iron pnictides, and a distinct test for theories.

cond-mat.supr-con↗

arXiv2010

Tuning electronic structure of graphene via tailoring structure: theoretical study

Electronic structures of graphene sheet with different defective patterns are investigated, based on the first principles calculations. We find that defective patterns can tune the electronic structures of the graphene significantly. Triangle patterns give rise to strongly localized states near the Fermi level, and hexagonal patterns open up band gaps in the systems. In addition, rectangular patterns, which feature networks of graphene nanoribbons with either zigzag or armchair edges, exhibit semiconducting behaviors, where the band gap has an evident dependence on the width of the nanoribbons. For the networks of the graphene nanoribbons, some special channels for electronic transport are predicted.

cond-mat.mes-hall↗

arXiv2010

Surface and bulk electronic structures of LaOFeAs studied by angle resolved photoemission spectroscopy

The electronic structure of LaOFeAs, a parent compound of iron-arsenic superconductors, is studied by angleresolved photoemission spectroscopy. By examining its dependence on photon energy, polarization, sodium dosing and the counting of Fermi surface volume, both the bulk and the surface contributions are identified. We find that a bulk band moves toward high binding energies below structural transition, and shifts smoothly across the spin density wave transition by about 25 meV. Our data suggest the band reconstruction may play a crucial role in the spin density wave transition, and the structural transition is driven by the short range magnetic order. For the surface states, both the LaO-terminated and FeAs-terminated components are revealed. Certain small band shifts are verified for the FeAs-terminated surface states in the spin density wave state, which is a reflection of the bulk electronic structure reconstruction. Moreover, sharp quasiparticle peaks quickly rise at low temperatures, indicating of drastic reduction of the scattering rate. A kink structure in one of the surface band is shown to be possibly related to the electron-phonon interactions.

cond-mat.supr-con↗

arXiv2010

Constraints upon the spectral indices of relic gravitational waves by LIGO S5

With LIGO having achieved its design sensitivity and the LIGO S5 strain data being available, constraints on the relic gravitational waves (RGWs) becomes realistic. The analytical spectrum of RGWs generated during inflation depends sensitively on the initial condition, which is generically described by the index $β$, the running index $α_t$, and the tensor-to-scalar ratio $r$. By the LIGO S5 data of the cross-correlated two detectors, we obtain constraints on the parameters $(β, α_t,r)$. As a main result, we have computed the theoretical signal-to noise ratio (SNR) of RGWs for various values of $(β, α_t, r)$, using the cross-correlation for the given pair of LIGO detectors. The constraints by the indirect bound on the energy density of RGWs by BBN and CMB have been obtained, which turn out to be still more stringent than LIGO S5.

gr-qc↗

arXiv2010

Metal to insulator transition on the N = 0 Landau level in graphene

The magnetotransport in single layer graphene has been experimentally investigated in magnetic fields up to 18 T as a function of temperature. A pronounced T-dependence is observed for T < 50 K, which is either metallic, or insulating, depending on the filling factor nu. The metal-insulator transition (MIT) occurs at nu_c ~ 0.65 and in the regime of the dissipative transport, where the longitudinal resistance Rxx > R_K/2. The critical resistivity (Rxx per square) is rho_xx(nu_c) ~ R_K/2 and is correlated with the appearance of zero plateau in Hall conductivity sigma_xy(nu) and peaks in sigma_xx(nu). This leads us to construct a universal low-T (n, B) phase diagram of this quantum phase transition.

cond-mat.mes-hall↗

arXiv2010

Singular extremals for the time-optimal control of dissipative spin 1/2 particles

We consider the time-optimal control by magnetic fields of a spin 1/2 particle in a dissipative environment. This system is used as an illustrative example to show the role of singular extremals in the control of quantum systems. We analyze a simple case where the control law is explicitly determined. We experimentally implement the optimal control using techniques of nuclear magnetic resonance. To our knowledge, this is the first experimental demonstration of singular extremals in quantum systems with bounded control amplitudes.

quant-ph↗

arXiv2010

Strong vortex pinning in Co-doped BaFe2As2 single crystal thin films

We report measurements of the field and angular dependences of Jc of truly epitaxial Co-doped BaFe2As2 thin films grown on SrTiO3/(La,Sr)(Al,Ta)O3 with different SrTiO3 template thicknesses. The films show Jc comparable to Jc of single crystals and a maximum pinning force Fp(0.6Tc) > 5 GN/m3 at H/Hirr ~ 0.5 indicative of strong vortex pinning effective up to high fields. Due to the strong correlated c-axis pinning, Jc for field along the c-axis exceeds Jc for H//ab plane, inverting the expectation of the Hc2 anisotropy. HRTEM reveals that the strong vortex pinning is due to a high density of nanosize columnar defects.

cond-mat.supr-con↗

arXiv2010

SiO and H2O Maser Observations of Red Supergiants in Star Clusters Embedded in the Galactic Disk

We present the result of radio observations of red supergiants in the star cluster, Stephenson's #2, and candidates for red supergiants in the star clusters, Mercer et al. (2005)'s #4, #8, and #13, in the SiO and H$_2$O maser lines.The Stephenson's #2 cluster and nearby aggregation at the South-West contain more than 15 red supergiants. We detected one at the center of Stephenson's #2 and three in the south-west aggregation in the SiO maser line, and three of these 4 were also detected in the H2O maser line. The average radial velocity of the 4 detected objects is 96 km s^{-1}, giving a kinematic distance of 5.5 kpc, which locates this cluster near the base of the Scutum-Crux spiral arm. We also detected 6 SiO emitting objects associated with the other star clusters. In addition, mapping observations in the CO J=1--0 line toward these clusters revealed that an appreciable amount of molecular gas still remains around Stephenson's #2 cluster in contrast to the prototypical red-supergiant cluster, Bica et al.'s #122. It indicates that a time scale of gas expulsion differs considerably in individual clusters.

astro-ph.SR↗

arXiv2010

Strong correlation and massive spectral-weight redistribution induced spin density wave in a-Fe1.06Te

The electronic structure of a-Fe1.06Te is studied with angle-resolved photoemission spectroscopy. We show that there is substantial spectral weight around Gamma and X, and lineshapes are intrinsically incoherent in the paramagnetic state. The magnetic transition is characterized by a massive spectral-weight transfer over an energy range as large as the band width, which even exhibits a hysteresis loop that marks the strong first order transition. Coherent quasiparticles emerge in the magnetically ordered state due to decreased spin fluctuations, which account for the change of transport properties from insulating behavior to metallic behavior. Our observation demonstrates that Fe1.06Te distinguishes itself from other iron-based systems with more local characters and much stronger interactions among different degrees of freedom, and how a spin density wave is formed in the presence of strong correlation.

cond-mat.supr-con↗

arXiv2010

Electronic structure reconstruction: the driving force behind the magnetic and structural transitions in NaFeAs

The electronic structure of NaFeAs is studied with angle resolved photoemission spectroscopy on high quality single crystals. Large portions of the band structure start to shift around the structural transition temperature, and smoothly evolve as the temperature lowers through the spin density wave transition. Moreover, band folding due to magnetic order emerges around structural transition. Our observation provides direct evidence that the structural and magnetic transitions share the same origin, and are both driven by the electronic structure reconstruction in Fe-based superconductors, instead of Fermi surface nesting.

cond-mat.supr-con↗

arXiv2010

On Temporal Variations of the Multi-TeV Cosmic Ray Anisotropy using the Tibet III Air Shower Array

We analyze the large-scale two-dimensional sidereal anisotropy of multi-TeV cosmic rays by Tibet Air Shower Array, with the data taken from 1999 November to 2008 December. To explore temporal variations of the anisotropy, the data set is divided into nine intervals, each in a time span of about one year. The sidereal anisotropy of magnitude about 0.1% appears fairly stable from year to year over the entire observation period of nine years. This indicates that the anisotropy of TeV Galactic cosmic rays remains insensitive to solar activities since the observation period covers more than a half of the 23rd solar cycle.

astro-ph.HE↗

arXiv2009

Charmonium in the vector channel at finite temperature from QCD sum rules

Thermal Hilbert moment QCD sum rules are used to obtain the temperature dependence of the hadronic parameters of charmonium in the vector channel, i.e. the $J$ / $ψ$ resonance mass, coupling (leptonic decay constant), total width, and continuum threshold. The continuum threshold $s_0$, which signals the end of the resonance region and the onset of perturbative QCD (PQCD), behaves as in all other hadronic channels, i.e. it decreases with increasing temperature until it reaches the PQCD threshold $s_0 = 4 m_Q^2$, with $m_Q$ the charm quark mass, at $T\simeq 1.22 T_c$. The rest of the hadronic parameters behave very differently from those of light-light and heavy-light quark systems. The $J$ / $ψ$ mass is essentially constant in a wide range of temperatures, while the total width grows with temperature up to $T \simeq 1.04 T_c$ beyond which it decreases sharply with increasing T. The resonance coupling is also initially constant and then begins to increase monotonically around $T \simeq T_c$. This behaviour of the total width and of the leptonic decay constant provides a strong indication that the $J$ / $ψ$ resonance might survive beyond the critical temperature for deconfinement.

hep-ph↗

arXiv2009

Observation of TeV Gamma Rays from the Fermi Bright Galactic Sources with the Tibet Air Shower Array

Using the Tibet-III air shower array, we search for TeV gamma-rays from 27 potential Galactic sources in the early list of bright sources obtained by the Fermi Large Area Telescope at energies above 100 MeV. Among them, we observe 7 sources instead of the expected 0.61 sources at a significance of 2 sigma or more excess. The chance probability from Poisson statistics would be estimated to be 3.8 x 10^-6. If the excess distribution observed by the Tibet-III array has a density gradient toward the Galactic plane, the expected number of sources may be enhanced in chance association. Then, the chance probability rises slightly, to 1.2 x 10^-5, based on a simple Monte Carlo simulation. These low chance probabilities clearly show that the Fermi bright Galactic sources have statistically significant correlations with TeV gamma-ray excesses. We also find that all 7 sources are associated with pulsars, and 6 of them are coincident with sources detected by the Milagro experiment at a significance of 3 sigma or more at the representative energy of 35 TeV. The significance maps observed by the Tibet-III air shower array around the Fermi sources, which are coincident with the Milagro >=3sigma sources, are consistent with the Milagro observations. This is the first result of the northern sky survey of the Fermi bright Galactic sources in the TeV region.

astro-ph.HE↗