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Jian Wu

Publications and source records attributed to Jian Wu.

At least 433 records · Page 24Linked to original sources
arXiv2010

THz generation by dual-color pulse in prealigned molecules

We demonstrate that the THz generation in air from a dual-color pulse, composed of the fundamental and second-harmonic waves, can be coherently controlled by field-free molecular alignment. By tuning its time delay to properly match various molecular alignment revivals, the THz generation from the dual-color pulse can be promoted or decreased due to the spatial cross-(de)focusing effect and the alignment-dependent ionization probabilities of the prealigned diatomic molecules of air. For the dual-color pulse of orthogonally polarized fundamental and second-harmonic waves, the polarization of the generated THz radiation can be controlled by the field-free molecular alignment, which functions as a transient dynamic wave-plate for the dual-color pulse with different phase velocities for the orthogonally polarized field components. The plasma effect on the THz generation of the dual-color pulse is also observed, leading to additional intensity and polarization control of the THz radiation.

physics.optics↗
arXiv2010

Spontaneous edge-defect formation and defect-induced conductance suppression in graphene nanoribbons

We present a first-principles study of the migration and recombination of edge defects (carbon adatom and/or vacancy) and their influence on electrical conductance in zigzag graphene nanoribbons (ZGNRs). It is found that at room temperature, the adatom is quite mobile while the vacancy is almost immobile along the edge of ZGNRs. The recombination of an adatom-vacancy pair leads to a pentagon-heptagon ring defect structure having a lower energy than the perfect edge, implying that such an edge-defect can be formed spontaneously. This edge defect can suppresses the conductance of ZGNRs drastically, which provides some useful hints for understanding the observed semiconducting behavior of the fabricated narrow GNRs.

cond-mat.mes-hall↗
arXiv2010

The Physical Conditions of the Intrinsic N V Narrow Absorption Line Systems of Three Quasars

We employ detailed photoionization models to infer the physical conditions of intrinsic narrow absorption line systems found in high resolution spectra of three quasars at z=2.6-3.0. We focus on a family of intrinsic absorbers characterized by N V lines that are strong relative to the Ly-alpha lines. The inferred physical conditions are similar for the three intrinsic N V absorbers, with metallicities greater than 10 times the solar value (assuming a solar abundance pattern), and with high ionization parameters (log U ~ 0). Thus, we conclude that the unusual strength of the N V lines results from a combination of partial coverage, a high ionization state, and high metallicity. We consider whether dilution of the absorption lines by flux from the broad-emission line region can lead us to overestimate the metallicities and we find that this is an unlikely possibility. The high abundances that we infer are not surprising in the context of scenarios in which metal enrichment takes place very early on in massive galaxies. We estimate that the mass outflow rate in the absorbing gas (which is likely to have a filamentary structure) is less than a few solar masses per year under the most optimistic assumptions, although it may be embedded in a much hotter, more massive outflow.

astro-ph.CO↗
arXiv2010

Spin glassiness and power law scaling in a quasi-triangular spin-1/2 compound

We present data on the magnetic properties of two classes of layered spin S=1/2 antiferromagnetic quasi-triangular lattice materials: $Cu_{2(1-x)}Zn_{2x}(OH)_3NO_3$ ($0 < x < 0.65$) and its long chain organic derivatives $Cu_{2(1-x)}Zn_{2x}(OH)_3(C_7H_{15}COO)\cdot mH_2O$ ($0 < x < 0.29$), where non-magnetic Zn substitutes Cu isostructurally. It is found that the long-chain compounds, even in a clean system in the absence of dilution, $x\!=\!0$, show spin-glass behavior, as evidenced by DC and AC susceptibility, and by time dependent magnetization measurements. A striking feature is the observation of a sharp crossover between two successive power law regimes in the DC susceptibility above the freezing temperature. Specific heat data are consistent with a conventional phase transition in the unintercalated compounds, and glassy behavior in the long chain compunds.

cond-mat.str-el↗
arXiv2010

Role of Symmetry in the Transport Properties of Graphene Nanoribbons under Bias

The intrinsic transport properties of zigzag graphene nanoribbons (ZGNRs) are investigated using first principles calculations. It is found that although all ZGNRs have similar metallic band structure, they show distinctly different transport behaviors under bias voltages, depending on whether they are mirror symmetric with respect to the midplane between two edges. Asymmetric ZGNRs behave as conventional conductors with linear current-voltage dependence, while symmetric ZGNRs exhibit unexpected very small currents with the presence of a conductance gap around the Fermi level. This difference is revealed to arise from different coupling between the conducting subbands around the Fermi level, which is dependent on the symmetry of the systems.

cond-mat.mes-hall↗
arXiv2010

On the selection effect of radio quasars in the Sloan Digital Sky Survey

We identified a large sample of radio quasars, including those with complex radio morphology, from the Sloan Digital Sky Survey (SDSS) and the Faint Images of Radio Sky at Twenty-cm (FIRST). Using this sample, we inspect previous radio quasar samples for selection effects resulting from complex radio morphologies and adopting positional coincidence between radio and optical sources alone. We find that 13.0% and 8.1% radio quasars do not show a radio core within 1.2 and 2 arcsecs of their optical position, and thus are missed in such samples. Radio flux is under-estimated by a factor of more than 2 for an additional 8.7% radio quasars. These missing radio extended quasars are more radio loud with a typical radio-to-optical flux ratio namely radio loudness RL >100, and radio power P >10^{25} W/Hz. They account for more than one third of all quasars with RL>100. The color of radio extended quasars tends to be bluer than the radio compact quasars. This suggests that radio extended quasars are more radio powerful sources, e.g., Fanaroff-Riley type 2 (FR-II) sources, rather than the compact ones viewed at larger inclination angles. By comparison with the radio data from the NRAO VLA Sky Survey (NVSS), we find that for sources with total radio flux less than 3 mJy, low surface brightness components tend to be underestimated by FIRST, indicating that lobes in these faint radio sources are still missed.

astro-ph.CO↗
arXiv2010

Quantum Manifestations of Graphene Edge Stress and Edge Instability: A First-Principles Study

We have performed first-principles calculations of graphene edge stresses, which display two interesting quantum manifestations absent from the classical interpretation: the armchair edge stress oscillates with a nanoribbon width, and the zigzag edge stress is noticeably reduced by spin polarization. Such quantum stress effects in turn manifest in mechanical edge twisting and warping instability, showing features not captured by empirical potentials or continuum theory. Edge adsorption of H and Stone-Wales reconstruction are shown to provide alternative mechanisms in relieving the edge compression and hence to stabilize the planar edge structure.

cond-mat.mes-hall↗
arXiv2010

Experimental and Theoretical Evidence for Extended Particle Models

We review the experimental searches on those interactions where the fundamental particles could exhibit a non point-like behavior. In particular we focus on the QED reaction measuring the differential cross sections for the process $ \EEGG $ at energies from sqrt{s} =55 GeV to 207 GeV using the data collected with the VENUS, TOPAZ, ALEPH, DELPHI L3 and OPAL from 1989 to 2003. The global fit to the data is 5 standard deviations away from the standard model expectation for the hypothesis of an excited state of the electron, corresponding to the cut-off scale Lambda =12.5 TeV. Assuming that this cut-off scale restricts the characteristic size of QED interaction to 15.7x10^{-18} cm, we perform an effort to assign in a semi-mechanical way all available properties of fundamental particles to a hypothetical classical object. Such object can be modeled as a classical gyroscope consisted of a non rotating inner massive kernel surrounded by an outer rotating massive layer equipped with charged sorted in a way to match the charge contents for different interactions. The model size of an electron agrees with 1.86x10^{-17} cm with the experiment. The introduction of a particle like structure related to gravity allows to estimate the inner mass kernel of an electron to 1.7x10^{-19} cm and the mass of a scaler to 154 GeV. The extension of the model to electrical charged particle-like structure in nonlinear electrodynamics coupled to General Relativity confirms the model in the global geometrical structure of mass and field distribution.

hep-ph↗
arXiv2009

Probing the Origins of the CIV and Fe Kalpha Baldwin Effect

We use UV/optical and X-ray observations of 272 radio-quiet Type 1 AGNs and quasars to investigate the CIV Baldwin Effect (BEff). The UV/optical spectra are drawn from the Hubble Space Telescope, International Ultraviolet Explorer and Sloan Digital Sky Survey archives. The X-ray spectra are from the Chandra and XMM-Newton archives. We apply correlation and partial-correlation analyses to the equivalent widths, continuum monochromatic luminosities, and alpha_ox, which characterizes the relative X-ray to UV brightness. The equivalent width of the CIV 1549 emission line is correlated with both alpha_ox and luminosity. We find that by regressing l_UV with EW(CIV) and alpha_ox, we can obtain tighter correlations than by regressing l_UV with only EW(CIV). Both correlation and regression analyses imply that l_UV is not the only factor controlling the changes of EW(CIV); alpha_ox (or, equivalently, the soft X-ray emission) plays a fundamental role in the formation and variation of CIV. Variability contributes at least 60% of the scatter of the EW(CIV)-l_UV relation and at least 75% of the scatter of the of the EW(CIV)-alpha_ox relation. In our sample, narrow Fe Kalpha 6.4 keV emission lines are detected in 50 objects. Although narrow Fe Kalpha exhibits a BEff similar to that of CIV, its equivalent width has almost no dependence on either alpha_ox or EW(CIV). This suggests that the majority of narrow Fe Kalpha emission is unlikely to be produced in the broad emission-line region. We do find suggestive correlations between the emission-line luminosities of CIV and Fe Kalpha, which could be potentially used to estimate the detectability of the Fe Kalpha line of quasars from rest-frame UV spectroscopic observations.

astro-ph.CO↗
arXiv2008

Prediction of Half Metallicity along the Edge of Boron Nitride Zigzag Nanoribbons

First-principles calculations reveal half metallicity in zigzag boron nitride (BN) nanoribbons (ZBNNRs). When the B edge, but not the N edge, of the ZBNNR is passivated, despite being a pure $sp$-electron system, the ribbon shows a giant spin splitting. The electrons at the Fermi level are 100% spin polarized with a half-metal gap of 0.38 eV and its conductivity is dominated by metallic single-spin states. The two states across at the Dirac point have different molecular origins, which signals a switch of carrier velocity. The ZBNNR should be a good potential candidate for widegap spintronics.

cond-mat.mtrl-sci↗
arXiv2008

Adsorption of gas molecules on graphene nanoribbons and its implication for nano-scale molecule sensor

We have studied the adsorption of gas molecules (CO, NO, NO2, O2, N2, CO2, and NH3) on graphene nanoribbons (GNRs) using first principles methods. The adsorption geometries, adsorption energies, charge transfer, and electronic band structures are obtained. We find that the electronic and transport properties of the GNR with armchair-shaped edges are sensitive to the adsorption of NH3 and the system exhibits n type semiconducting behavior after NH3 adsorption. Other gas molecules have little effect on modifying the conductance of GNRs. Quantum transport calculations further indicate that NH3 molecules can be detected out of these gas molecules by GNR based sensor.

cond-mat.mtrl-sci↗
arXiv2008

Suppression of spin-polarization in graphene nanoribbon by edge defect and impurity

We investigate the effect of edge defects (vacancies) and impurities (substitutional dopants) on the robustness of spin-polarization in graphene nanoribbons (GNRs) with zigzag edges, using density-functional-theory calculations. We found that the stability of the spin state and its magnetic moments decrease continuously with increasing concentration of defects or impurities. The system generally becomes non-magnetic at the concentration of one edge defect (impurity) per 10 angstrom. The spin suppression is shown to be caused by reduction and removal of edge states at the Fermi energy. Our analysis implies an important criterion on the GNR samples for spintronics applications.

cond-mat.mtrl-sci↗
arXiv2007

Making a field effect transistor on a single graphene nanoribbon by selective doping

Using first-principle electronic structure calculations, we show a metal- semiconductor transition of a metallic graphene nanoribbon with zigzag edges induced by substitutional doping of Nitrogen or Boron atoms at the edges. A field effect transistor consisting of a metal-semiconductor-metal junction can then be constructed by selective doping of the ribbon edges. The current-voltage characteristics of such a prototype device is determined by the first-principle quantum transport calculations.

cond-mat.mtrl-sci↗
arXiv2007

The Sloan Digital Sky Survey Quasar Catalog IV. Fifth Data Release

We present the fourth edition of the Sloan Digital Sky Survey (SDSS) Quasar Catalog. The catalog contains 77,429 objects; this is an increase of over 30,000 entries since the previous edition. The catalog consists of the objects in the SDSS Fifth Data Release that have luminosities larger than M_i = -22.0 (in a cosmology with H_0 = 70 km/s/Mpc, Omega_M = 0.3, and Omega_Lambda = 0.7) have at least one emission line with FWHM larger than 1000 km/s, or have interesting/complex absorption features, are fainter than i=15.0, and have highly reliable redshifts. The area covered by the catalog is 5740 sq. deg. The quasar redshifts range from 0.08 to 5.41, with a median value of 1.48; the catalog includes 891 quasars at redshifts greater than four, of which 36 are at redshifts greater than five. Approximately half of the catalog quasars have i < 19; nearly all have i < 21. For each object the catalog presents positions accurate to better than 0.2 arcsec. rms per coordinate, five-band (ugriz) CCD-based photometry with typical accuracy of 0.03 mag, and information on the morphology and selection method. The catalog also contains basic radio, near-infrared, and X-ray emission properties of the quasars, when available, from other large-area surveys. The calibrated digital spectra cover the wavelength region 3800--9200A at a spectral resolution of ~2000. The spectra can be retrieved from the public database using the information provided in the catalog. The average SDSS colors of quasars as a function of redshift, derived from the catalog entries, are presented in tabular form. Approximately 96% of the objects in the catalog were discovered by the SDSS.

astro-ph↗
arXiv2006

Phase space and quark mass effects in neutrino emissions in a color superconductor

We study the phase space for neutrino emissions with massive quarks in direct Urca processes in normal and color superconducting quark matter. We derive in QCD and the NJL model the Fermi momentum reduction resulting from Fermi liquid properties which opens up the phase space for neutrino emissions. The relation between the Fermi momentum and chemical potential is found to be $p_{F}\approxμ(1-κ)$ with $κ$ depending on coupling constants. We find in the weak coupling regime that $κ$ is a monotonously increasing function of the chemical potential. This implies quenched phase space for neutrino emissions at low baryon densities. We calculate neutrino emissivities with massive quarks in a spin-one color superconductor. The quark mass corrections are found to be of the same order as the contributions in the massless case, which will bring sizable effects on the cooling behavior of compact stars.

hep-ph↗
arXiv2005

Modulating transmission properties of nanoscale transistors by dipole effects near contacts

We theoretically demonstrate that a dipole layer on the electrode can modulate the transmission properties of nanoscale devices by influencing the contact properties, through first principles simulations on carbon nanotube based field effect transistors. The dipole layer is realized by potassium adsorption on Au electrodes, which shifts the electrostatic potential at the near contact region significantly. The dipoles parallel to the direction of the bias voltage may act as a supplement to the effect of gate voltages, while the perpendicular dipoles may modify the interface barrier of the contacts.

cond-mat.mtrl-sci↗