arXiv subjects

Li Zhang

Publications and source records attributed to Li Zhang.

At least 919 records · Page 51Linked to original sources

arXiv2015

A self-consistent interpretation of the GeV-TeV emission from a distant blazar PKS 1424+240

We propose a scenario for self-consistent interpretation of GeV - TeV spectrum of a distant blazar PKS 1424+240. In this scenario, ultra-high energy (UHE) protons are assumed to exist in the blazar jet and produce gamma rays through synchrotron emission emitted by relativistic protons and pair cascades (resulted from $pγ$ interaction); meanwhile, some of these UHE protons may escape from the jet and are injected into intergalactic space. Therefore, we assume that UHE cosmic rays (CR) originate from relativistic protons in the jet and use energy-independent escape timescale to obtain UHECR injection spectrum. Both contributions of gamma rays injected by the source and secondary gamma rays produced in interactions of UHECRs emitted by the blazar with photon background during their propagation through intergalactic space are calculated. Our results show that this scenario is able to reproduce the GeV-TeV spectrum of PKS 1424+240 self-consistently in a broad range of redshifts $0.6<z<1.3$. The required relativistic jet power $L_p\simeq3\times10^{46}\ \rm erg\ s^{-1}$ only moderately depends on the assumed source redshift, while the proton escape timescale (and the injected UHECR luminosity) strongly depends on $z$. We compare the integral TeV fluxes predicted in our scenario with the sensitivity of the planned Cherenkov Telescope Array (CTA), and discuss the implications to future observations.

astro-ph.HE↗

arXiv2015

Coexistence of superconductivity and ferromagnetism in Sr0.5Ce0.5FBiS2

Through the combination of X-ray diffraction, electrical transport, magnetic susceptibility, and the heat capacity measurements, we studied the effect of Ce doping in the newly discovered SrFBiS${2}$ system. It is found that Sr${0.5}$Ce${0.5}$FBiS${2}$ undergoes a second-order transition below $7.5 K, followed by a superconducting transition with the critical temperature Tc=2.8 K. Our transport, specific heat and DC-magnetization results suggest the presence of bulk ferromagneticcorrelation of Ce ion below 7.5 K that coexist with superconductivity when the temperature is further lowered below 2.8 K.

cond-mat.supr-con↗

arXiv2015

On focal submanifolds of isoparametric hypersurfaces and Simons formula

The focal submanifolds of isoparametric hypersurfaces in spheres are all minimal Willmore submanifolds, mostly being $\mathcal{A}$-manifolds in the sense of A.Gray but rarely Ricci-parallel (\cite{QTY},\cite{LY},\cite{TY3}). In this paper we study the geometry of the focal submanifolds via Simons formula. We show that all the focal submanifolds with $g\geq3$ are not normally flat by estimating the normal scalar curvatures. Moreover, we give a complete classification of the semiparallel submanifolds among the focal submanifolds.

math.DG↗

arXiv2014

Laser-sub-cycle two-dimensional electron momentum mapping using orthogonal two-color fields

The two-dimensional sub-cycle-time to electron momentum mapping provided by orthogonal two-color laser fields is applied to photoelectron spectroscopy. Using neon as the example we gain experimental access to the dynamics of emitted electron wave packets in electron momenta spectra measured by coincidence momentum imaging. We demonstrate the opportunities provided by this time-to-momentum mapping by investigating the influence of the parent ion on the emitted electrons on laser-sub-cycle times. It is found that depending on their sub-cycle birth time the trajectories of photoelectrons are affected differently by the ion's Coulomb field.

physics.atom-ph↗

arXiv2014

Compact Model of Nanowire Tunneling FETs Including Phonon-Assisted Tunneling and Quantum Capacitance

A physics-based compact model for silicon gate-all-around (GAA) nanowire tunneling FETs (NW-tFETs) with good accuracy has been developed by considering Phonon-Assisted Tunneling (PAT) and transition from Quantum Capacitance Limit (QCL) to Classical Limit (CL) during the device-size scaling. The impact of PAT results in the broadening of a single electron-energy level to an energy band with density-of-states (DOS) distribution of Lorentzian shape. As a consequence, the tunneling probability at the edge of tunneling window no longer changes abruptly from zero to having a finite value. By adjusting the parameters in the Lorentzian function, an accurate fitting to the measured transfer characteristics in the subthreshold region is made possible. Besides, with an analytical formula to calculate the channel potential, the model is able to cover naturally the transition from QCL to CL regime when the device size is scaled. Furthermore, on-voltage is defined to facilitate the modeling and fitting processes. Comparisons with the experimental data demonstrate the model accuracy across all device operation regions and the flexibility in model parameter extraction is also shown.

cond-mat.mes-hall↗

arXiv2014

Understanding the TeV emission from a distant blazar PKS 1424+240 in a lepto-hadronic jet model

We investigate the formation of TeV spectrum of a distant blazar PKS 1424+240 residing at a redshift $z\geq0.6$ in two scenarios in the frame of a lepto-hadronic jet model taking both the uncertainties of the extragalactic background light (EBL) and its redshift into account. In the first scenario, TeV emission is attributed to the synchrotron emission of pair cascades resulting from $pγ$ interaction; in the second scenario, TeV emission is attributed to the proton-synchrotron emission, and an internal absorption due to interaction with the photons around the jet is included. The results show that in the first scenario the 68\% upper limit of its redshift within which this scenario can explain the VERITAS TeV spectrum in 2009 well is $\sim0.75$; in the second scenario, this upper limit of the redshift becomes $\sim1.03$. However, the second scenario can be excluded because it requires an unreasonable photon field around the jet with a luminosity of $\sim10^{43}\rm \ erg\ s^{-1}$. In conclusion, the jet model can explain its TeV spectrum with a low EBL density if $0.6<z<0.75$.

astro-ph.HE↗

arXiv2014

Detecting Differential Expression from RNA-seq Data with Expression Measurement Uncertainty

High-throughput RNA sequencing (RNA-seq) has emerged as a revolutionary and powerful technology for expression profiling. Most proposed methods for detecting differentially expressed (DE) genes from RNA-seq are based on statistics that compare normalized read counts between conditions. However, there are few methods considering the expression measurement uncertainty into DE detection. Moreover, most methods are only capable of detecting DE genes, and few methods are available for detecting DE isoforms. In this paper, a Bayesian framework (BDSeq) is proposed to detect DE genes and isoforms with consideration of expression measurement uncertainty. This expression measurement uncertainty provides useful information which can help to improve the performance of DE detection. Three real RAN-seq data sets are used to evaluate the performance of BDSeq and results show that the inclusion of expression measurement uncertainty improves accuracy in detection of DE genes and isoforms. Finally, we develop a GamSeq-BDSeq RNA-seq analysis pipeline to facilitate users, which is freely available at the website http://parnec.nuaa.edu.cn/liux/GSBD/GamSeq-BDSeq.html.

q-bio.GN↗

arXiv2014

Ramanujan-type Congruences for Overpartitions Modulo 16

Let $\overline{p}(n)$ denote the number of overpartitions of $n$. Recently, Fortin-Jacob-Mathieu and Hirschhorn-Sellers independently obtained 2-, 3- and 4-dissections of the generating function for $\overline{p}(n)$ and derived a number of congruences for $\overline{p}(n)$ modulo $4$, $8$ and $64$ including $\overline{p}(5n+2)\equiv 0 \pmod{4}$, $\overline{p}(4n+3)\equiv 0 \pmod{8}$ and $\overline{p}(8n+7)\equiv 0 \pmod{64}$. By employing dissection techniques, Yao and Xia obtained congruences for $\overline{p}(n)$ modulo $8, 16$ and $32$, such as $\overline{p}(48n+26) \equiv 0 \pmod{8}$, $\overline{p}(24n+17)\equiv 0 \pmod{16}$ and $\overline{p}(72n+69)\equiv 0 \pmod{32}$. In this paper, we give a 16-dissection of the generating function for $\overline{p}(n)$ modulo 16 and we show that $\overline{p}(16n+14)\equiv0\pmod{16}$ for $n\ge 0$. Moreover, by using the $2$-adic expansion of the generating function of $\overline{p}(n)$ due to Mahlburg, we obtain that $\overline{p}(\ell^2n+r\ell)\equiv0\pmod{16}$, where $n\ge 0$, $\ell \equiv -1\pmod{8}$ is an odd prime and $r$ is a positive integer with $\ell \nmid r$. In particular, for $\ell=7$, we get $\overline{p}(49n+7)\equiv0\pmod{16}$ and $\overline{p}(49n+14)\equiv0\pmod{16}$ for $n\geq 0$. We also find four congruence relations: $\overline{p}(4n)\equiv(-1)^n\overline{p}(n) \pmod{16}$ for $n\ge 0$, $\overline{p}(4n)\equiv(-1)^n\overline{p}(n)\pmod{32}$ for $n$ being not a square of an odd positive integer, $\overline{p}(4n)\equiv(-1)^n\overline{p}(n)\pmod{64}$ for $n\not\equiv 1,2,5\pmod{8}$ and $\overline{p}(4n)\equiv(-1)^n\overline{p}(n)\pmod{128}$ for $n\equiv 0\pmod{4}$.

math.CO↗

arXiv2014

Skewless Network Clock Synchronization Without Discontinuity: Convergence and Performance

This paper examines synchronization of computer clocks connected via a data network and proposes a skewless algorithm to synchronize them. Unlike existing solutions, which either estimate and compensate the frequency difference (skew) among clocks or introduce offset corrections that can generate jitter and possibly even backward jumps, our solution achieves synchronization without these problems. We first analyze the convergence property of the algorithm and provide explicit necessary and sufficient conditions on the parameters to guarantee synchronization. We then study the effect of noisy measurements (jitter) and frequency drift (wander) on the offsets and synchronization frequency, and further optimize the parameter values to minimize their variance. Our study reveals a few insights, for example, we show that our algorithm can converge even in the presence of timing loops and noise, provided that there is a well defined leader. This marks a clear contrast with current standards such as NTP and PTP, where timing loops are specifically avoided. Furthermore, timing loops can even be beneficial in our scheme as it is demonstrated that highly connected subnetworks can collectively outperform individual clients when the time source has large jitter. The results are supported by experiments running on a cluster of IBM BladeCenter servers with Linux.

math.OC↗

arXiv2014

Ramanujan-type Congruences for Overpartitions Modulo 5

Let $\overline{p}(n)$ denote the number of overpartitions of $n$. Hirschhorn and Sellers showed that $\overline{p}(4n+3)\equiv 0 \pmod{8}$ for $n\geq 0$. They also conjectured that $\overline{p}(40n+35)\equiv 0 \pmod{40}$ for $n\geq 0$. Chen and Xia proved this conjecture by using the $(p,k)$-parametrization of theta functions given by Alaca, Alaca and Williams. In this paper, we show that $\overline{p}(5n)\equiv (-1)^{n}\overline{p}(4\cdot 5n) \pmod{5}$ for $n \geq 0$ and $\overline{p}(n)\equiv (-1)^{n}\overline{p}(4n)\pmod{8}$ for $n \geq 0$ by using the relation of the generating function of $\overline{p}(5n)$ modulo $5$ found by Treneer and the $2$-adic expansion of the generating function of $\overline{p}(n)$ due to Mahlburg. As a consequence, we deduce that $\overline{p}(4^k(40n+35))\equiv 0 \pmod{40}$ for $n,k\geq 0$. Furthermore, applying the Hecke operator on $ϕ(q)^3$ and the fact that $ϕ(q)^3$ is a Hecke eigenform, we obtain an infinite family of congrences $\overline{p}(4^k \cdot5\ell^2n)\equiv 0 \pmod{5}$, where $k\ge 0$ and $\ell$ is a prime such that $\ell\equiv3 \pmod{5}$ and $\left(\frac{-n}{\ell}\right)=-1$. Moreover, we show that $\overline{p}(5^{2}n)\equiv \overline{p}(5^{4}n) \pmod{5}$ for $n \ge 0$. So we are led to the congruences $\overline{p}\big(4^k5^{2i+3}(5n\pm1)\big)\equiv 0 \pmod{5}$ for $n, k, i\ge 0$. In this way, we obtain various Ramanujan-type congruences for $\overline{p}(n)$ modulo $5$ such as $\overline{p}(45(3n+1))\equiv 0 \pmod{5}$ and $\overline{p}(125(5n\pm 1))\equiv 0 \pmod{5}$ for $n\geq 0$.

math.CO↗

arXiv2014

Weak Localization Effects as Evidence for Bulk Quantization in Thin Films Bi2Se3

Strong spin-orbit coupling in topological insulators results in the ubiquitously observed weak antilocalization feature in their magnetoresistance. Here we present magnetoresistance measurements in ultra thin films of the topological insulator Bi_2Se_3, and show that in the 2D quantum limit, in which the topological insulator bulk becomes quantized, an additional negative magnetoresistance feature appears. Detailed analysis associates this feature with weak localization of the quantized bulk channels, providing thus evidence for this quantization. Examination of the dephasing fields at different temperatures indicates different scattering mechanism in the bulk vs the surface states.

cond-mat.mes-hall↗

arXiv2014

Pulsed Laser Deposition of High-Quality Thin Films of the Insulating Ferromagnet EuS

High-quality thin films of the ferromagnetic-insulator europium(II) sulfide (EuS) were fabricated by pulsed laser deposition on Al2O3 (0001) and Si (100) substrates. A single orientation was obtained with the [100] planes parallel to the substrates, with atomic-scale smoothness indicates a near-ideal surface topography. The films exhibit uniform ferromagnetism below 15.9 K, with a substantial component of the magnetization perpendicular to the plane of the films. Optimization of the growth condition also yielded truly insulating films with immeasurably large resistance. This combination of magnetic and electric properties open the gate for novel devices that require a true ferromagnetic insulator.

cond-mat.mtrl-sci↗

arXiv2014

Electronic phase diagram in the new BiS2-based Sr1-xLaxFBiS2 system

In this paper, we systematically study the electron doping effect in a new BiS2-based system Sr1-xLaxFBiS2(0 0.3, reaching its maximal superconducting transition temperature Tc=3.5 K at x=0.55. In the normal state of superconducting samples, it is clearly seen that there exists a crossover from metallic to semiconducting state below a temperature Tmin, which shifts to lower temperatures with increasing La content. Based on these measurements, the associated electronic phase diagram of Sr1-xLaxFBiS2 system has thus been established.

cond-mat.supr-con↗

arXiv2014

The physical properties of Fermi BL Lac objects jets

We investigate the physical properties of BL Lac objects (BL Lacs) jets by modeling the quasi-simultaneous spectral energy distributions (SEDs) of 22 {\it Fermi} BL Lac objects in the frame of a simple one-zone synchrotron self-Compton (SSC) model. We obtained the best-fit model parameters and their uncertainties for each BL Lac object through the $χ^2$-minimization procedure and discussed their implications on the physical processes. The modeling results show that the one-zone SSC model can successfully fit the SEDs of high-synchrotron-peaked BL Lacs (HBLs) and intermediate-synchrotron-peaked BL Lacs (IBLs), but fails to explain the SEDs of low-synchrotron-peaked BL Lacs (LBLs). The statistical analysis results for model parameters are summarized as follows. (1) No correlation is found between magnetic field ($B$) and the broken energy of relativistic electrons distribution ($γ^{\prime}_{\rm b}$) for HBLs and IBLs, but there are inverse correlations between $γ^{\prime}_{\rm b}$ and the radius of emitting blob ($R^{\prime}_{\rm b}$) as well as the electrons number $K^{\prime}_{\rm e}$ for HBLs and IBLs. It's therefore concluded that the variation of $γ^{\prime}_{\rm b}$ is mainly caused by that of $R^{\prime}_{\rm b}$ rather than $B$ for HBLs and IBLs. (2) The Poynting flux in jets can not account for the observed radiations since the power in Poynting flux is smaller than the radiative power, and the cold protons could be the primary energy carrier in the jets.

astro-ph.HE↗

arXiv2013

Nearly optimal minimax estimator for high-dimensional sparse linear regression

We present estimators for a well studied statistical estimation problem: the estimation for the linear regression model with soft sparsity constraints ($\ell_q$ constraint with $0<q\leq1$) in the high-dimensional setting. We first present a family of estimators, called the projected nearest neighbor estimator and show, by using results from Convex Geometry, that such estimator is within a logarithmic factor of the optimal for any design matrix. Then by utilizing a semi-definite programming relaxation technique developed in [SIAM J. Comput. 36 (2007) 1764-1776], we obtain an approximation algorithm for computing the minimax risk for any such estimation task and also a polynomial time nearly optimal estimator for the important case of $\ell_1$ sparsity constraint. Such results were only known before for special cases, despite decades of studies on this problem. We also extend the method to the adaptive case when the parameter radius is unknown.

math.ST↗

arXiv2013

Emitting Electron Spectra and Acceleration Processes in the Jet of PKS 0447-439

We investigate the electron energy distributions (EEDs) and the corresponding acceleration processes in the jet of PKS 0447$-$439 and estimate its redshift through modeling its observed spectral energy distribution (SED) in the frame of a one-zone synchrotron-self Compton (SSC) model. Three EEDs formed in different acceleration scenarios are assumed: the power-law with exponential cut-off (PLC) EED (shock-acceleration scenario or the case of the EED approaching equilibrium in the stochastic-acceleration scenario), the log-parabolic (LP) EED (stochastic-acceleration scenario and the acceleration dominating) and the broken power law (BPL) EED (no acceleration scenario), and then the corresponding fluxes of both synchrotron and SSC are calculated. The model is applied to PKS 0447-439 and modeling SEDs are compared to the observed SED of this object by using the Markov Chain Monte Carlo (MCMC) method. Calculating results show that PLC model fails to fit the observed SED well, while the LP and BPL models give comparably good fits for the observed SED. The results indicate that it is possible that stochastic acceleration process acts in the emitting region of PKS 0447-439 and the EED is far from equilibrium (acceleration dominating) or no acceleration process works (in the emitting region). The redshift of PKS 0447-439 is also estimated in our fitting, and $z=0.16\pm0.05$ for LP case and $z=0.17\pm0.04$ for BPL case.

astro-ph.HE↗

arXiv2013

Emerging Weak Localization Effects on Topological Insulator-Insulating Ferromagnet (Bi_2Se_3-EuS) Interface

Thin films of topological insulator Bi_2Se_3 were deposited directly on insulating ferromagnetic EuS. Unusual negative magnetoresistance was observed near the zero field below the Curie temperature (T_C), resembling the weak localization effect; whereas the usual positive magnetoresistance was recovered above T_C. Such negative magnetoresistance was only observed for Bi_2Se_3 layers thinner than t~4nm, when its top and bottom surfaces are coupled. These results provide evidence for a proximity effect between a topological insulator and an insulating ferromagnet, laying the foundation for future realization of the half-integer quantized anomalous Hall effect in three-dimensional topological insulators.

cond-mat.mtrl-sci↗

arXiv2013

Skewless Network Clock Synchronization

This paper examines synchronization of computer clocks connected via a data network and proposes a skewless algorithm to synchronize them. Unlike existing solutions, which either estimate and compensate the frequency difference (skew) among clocks or introduce offset corrections that can generate jitter and possibly even backward jumps, our algorithm achieves synchronization without these problems. We first analyze the convergence property of the algorithm and provide necessary and sufficient conditions on the parameters to guarantee synchronization. We then implement our solution on a cluster of IBM BladeCenter servers running Linux and study its performance. In particular, both analytically and experimentally, we show that our algorithm can converge in the presence of timing loops. This marks a clear contrast with current standards such as NTP and PTP, where timing loops are specifically avoided. Furthermore, timing loops can even be beneficial in our scheme. For example, it is demonstrated that highly connected subnetworks can collectively outperform individual clients when the time source has large jitter. It is also experimentally demonstrated that our algorithm outperforms other well-established software-based solutions such as the NTPv4 and IBM Coordinated Cluster Time (IBM CCT).

math.OC↗