arXiv subjects

Feng Li

Publications and source records attributed to Feng Li.

At least 361 records · Page 20Linked to original sources

arXiv2015

Mean-field potential effects on particle and antiparticle elliptic flows in the beam-energy scan program at RHIC

The elliptic flow splitting between particles and their antiparticles has recently been observed by the STAR Collaboration in the beam-energy scan program at the Relativistic Heavy Ion Collider. In studies based on transport models, we have found that this splitting can be explained by the different mean-field potentials acting on particles and their antiparticles in the produced baryon-rich matter. In particular, we have shown that the experimentally measured relative elliptic flow difference can help constrain the vector coupling constant in the Nambu-Jona-Lasinio model used in describing the partonic stage of heavy-ion collisions. This information is useful for locating the critical point in the QCD phase diagram and thus understanding the phase structure of QCD.

nucl-th↗

arXiv2015

Ambipolar Solution-processed Hybrid Perovskite Phototransistors

Organolead halide perovskites have attracted substantial attention due to their excellent physical properties, which enable them to serve as the active material in emerging hybrid solid-state solar cells. Here we investigate the phototransistors based on hybrid perovskite films, and provide direct evidence for their superior carrier transport property with ambipolar characteristics. The field-effect mobilities for triiodide perovskites at room temperature are measured as 0.18 (0.17) cm2 V-1 s-1 for holes (electrons), which increase to 1.24 (1.01) cm2 V-1 s-1 for mixed halide perovskites. The photoresponsivity of our hybrid perovskite devices reaches 320 A W-1, which is among the largest values reported for phototransistors. Importantly, the phototransistors exhibit an ultrafast photoresponse speed of less than 10 μs. The solution-based process and excellent device performance strongly underscore hybrid perovskites as promising material candidates for photoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2015

Star formation properties of galaxy cluster A1767

Abell 1767 is a dynamically relaxed, cD cluster of galaxies with a redshift of 0.0703. Among 250 spectroscopically confirmed member galaxies within a projected radius of 2.5r_{200}, 243 galaxies (~ 97%) are spectroscopically covered by the Sloan Digital Sky Survey (SDSS). Based on this homogeneous spectral sample, the stellar evolutionary synthesis code, STARLIGHT, is applied to investigate the stellar populations and star formation histories (SFHs) of cluster galaxies. The star formation properties of galaxies, such as mean stellar ages, metallicities, stellar masses, and star formation rates (SFRs), are presented as the functions of local galaxy density. Strong environmental effect is found in the manner that massive galaxies in the high-density core region of cluster tend to have higher metallicities, longer mean stellar ages, and lower specific star formation rates (SSFRs), and their recent star formation activities have been remarkably suppressed. In addition, the correlations of the metallicity and SSFR with stellar mass are confirmed.

astro-ph.GA↗

arXiv2014

Deep sub-threshold $Ξ$ and $Λ$ production in nuclear collisions with the UrQMD transport model

We present results on deep sub threshold hyperon production in nuclear collisions, with the UrQMD transport model. Introducing anti-kaon+baryon and hyperon+hyperon strangeness exchange reactions we obtain a good description of experimental data on single strange hadron production in Ar+KCl reactions at $E_{lab}=1.76$ A GeV. We find that the hyperon strangeness exchange is the dominant process contributing to the $Ξ^-$ yield, however remains short of explaining the $Ξ^-/Λ$ ratio measured with the HADES experiment. We also discuss possible reasons for the discrepancy with previous studies and the experimental results, finding that many details of the transport simulation may have significant effects on the final $Ξ^-$ yield.

nucl-th↗

arXiv2014

Partonic mean-field effects on matter and antimatter elliptic flows

Using a partonic transport model based on the Nambu-Jona-Lasinio model, we study the effect of scalar and vector mean fields on the elliptic flows of quarks and antiquarks in relativistic heavy ion collisions in Au+Au collisions at $\sqrt{s_{\rm NN}}=7.7~$GeV and impact parameter $b=8~{\rm fm}$ that leads to the production of a baryon-rich matter. Although the scalar mean field, which is attractive for both quarks and antiquarks, reduces both their elliptic flows, the vector mean field, which is repulsive for quarks and attractive for antiquarks, leads to a splitting of their elliptic flows, and this effect increases with the strength of the vector coupling in the baryon-rich quark matter. Converting quarks and antiquarks at hadronization to hadrons via the quark coalescence model, we further study the dependence of the transverse momentum integrated relative elliptic flow differences between protons and antiprotons, lambda and anti-lambdas, and positively and negatively charged kaons on the strength of the quark vector coupling. These results are then compared with the experimental data measured by the STAR Collaboration in the Beam Energy Scan program at the Relativistic Heavy Ion Collider.

nucl-th↗

arXiv2014

Heavy quark correlations and the effective volume for quarkonia production

Using the Boltzmann transport approach, we study the effective volume of a correlated heavy quark pair in a partonic medium based on their collision rate. We find that the effective volume is finite and depends sensitively on the momentum of the heavy quark and the temperature of the medium. Generally, it increases linearly with time $t$ at the very beginning and the increase then becomes slower due to multiple scattering, and finally it increases linearly with respect to $t^{3/2}$. We further find that the colliding heavy quark pair has an effective temperature similar to that of the medium even though their initial transverse momentum spectra are far from thermal equilibrium.

nucl-th↗

arXiv2014

Organic light-emitting diodes using open-shell molecule as emitter: the emission from doublet

We fabricate OLEDs using a stable neutral π radical, BDPA, as the emitter. There is only one electron in the singly occupied molecular orbital (SOMO) of this open-shell molecule. This feature makes the excited state of open-shell molecules be neither singlet nor triplet, but doublet. The key issue of how to harvest the triplet energy in an OLED is thus bypassed, due to the radiative decay of doublet is totally spin allowed. In the BDPA-based OLED, the emission was confirmed to be from the electronic transition from LUMO to SOMO, via the frontier molecular orbital analysis combined with the spectroscopy measurements. The maximum luminance of the OLEDs is 4879 cd/m2 which is comparable to the first reported Fluorescence-, Phosphorecence- and TADF-based OLEDs.

physics.chem-ph↗

arXiv2014

Patterned silicon substrates: a common platform for room temperature GaN and ZnO polariton lasers

A new platform for fabricating polariton lasers operating at room temperature is introduced: nitride-based distributed Bragg reflectors epitaxially grown on patterned silicon substrates. The patterning allows for an enhanced strain relaxation thereby enabling to stack a large number of crack-free AlN/AlGaN pairs and achieve cavity quality factors of several thousands with a large spatial homogeneity. GaN and ZnO active regions are epitaxially grown thereon and the cavities are completed with top dielectric Bragg reflectors. The two structures display strong-coupling and polariton lasing at room temperature and constitute an intermediate step in the way towards integrated polariton devices.

cond-mat.mes-hall↗

arXiv2013

Asymptotic Composite Estimation

Composition methodologies in the current literature are mainly to promote estimation efficiency via direct composition, either, of initial estimators or of objective functions. In this paper, composite estimation is investigated for both estimation efficiency and bias reduction. To this end, a novel method is proposed by utilizing a regression relationship between initial estimators and values of model-independent parameter in an asymptotic sense. The resulting estimators could have smaller limiting variances than those of initial estimators, and for nonparametric regression estimation, could also have faster convergence rate than the classical optimal rate that the corresponding initial estimators can achieve. The simulations are carried out to examine its performance in finite sample situations.

stat.ME↗

arXiv2013

Elliptic flow splitting as a probe of the QCD phase structure at finite baryon chemical potential

Using a partonic transport model based on the 3-flavor Nambu-Jona-Lasinio model and a relativistic hadronic transport model to describe, respectively, the evolution of the initial partonic and the final hadronic phase of heavy-ion collisions at energies carried out in the Beam-Energy Scan program of the Relativistic Heavy Ion Collider, we have studied the effects of both the partonic and hadronic mean-field potentials on the elliptic flow of particles relative to that of their antiparticles. We find that to reproduce the measured relative elliptic flow differences between nucleons and antinucleons as well as between kaons and antikaons requires a vector coupling constant as large as 0.5 to 1.1 times the scalar coupling constant in the Nambu-Jona-Lasinio model. Implications of our results in understanding the QCD phase structure at finite baryon chemical potential are discussed.

nucl-th↗

arXiv2013

Effect of substrate temperature on the spin transport property in C60-based spin valve devices

We report the effect of the substrate temperature on the magnetoresistance (MR) of the C60-based spin valve (SV) devices with the sandwich configuration of La0.67Sr0.33MnO3 (LSMO)/C60/cobalt (Co). The C60 interlayer deposited at different substrate temperatures resulted in four types of devices. We observed all types of devices showed a monotonic increase in MR ratio with the substrate temperature. And more interesting, an especially large MR (28.5%) were obtained in the device fabricated at higher substrate temperature, while for the other types of devices, the MR magnitudes were just about a few percent. Based on the I-V measurements, as well as SEM and AFM characteristics, we have obtained that the higher substrate temperature can cause many pits and hollows in the organic film, and these pits will increase the tunneling probability of spin-polarized carriers from one ferromagnetic electrode to the other.

cond-mat.mtrl-sci↗

arXiv2012

Efficient Bayesian Multivariate Surface Regression

Methods for choosing a fixed set of knot locations in additive spline models are fairly well established in the statistical literature. While most of these methods are in principle directly extendable to non-additive surface models, they are less likely to be successful in that setting because of the curse of dimensionality, especially when there are more than a couple of covariates. We propose a regression model for a multivariate Gaussian response that combines both additive splines and interactive splines, and a highly efficient MCMC algorithm that updates all the knot locations jointly. We use shrinkage priors to avoid overfitting with different estimated shrinkage factors for the additive and surface part of the model, and also different shrinkage parameters for the different response variables. This makes it possible for the model to adapt to varying degrees of nonlinearity in different parts of the data in a parsimonious way. Simulated data and an application to firm leverage data show that the approach is computationally efficient, and that allowing for freely estimated knot locations can offer a substantial improvement in out-of-sample predictive performance.

stat.CO↗

arXiv2012

Deep Subwavelength Electromagnetic Transparency through Dual Metallic Gratings with Ultranarrow Slits

In this Letter, we study the transmission response of microwaves through two identical metallic plates machined with ultranarrow slit arrays. The measured and calculated transmission spectra consistently display a striking transmission peak at wavelength much larger than any characteristic length of the structure (e.g., about twenty-fold of the lattice period), which can not be directly explained by the existing mechanisms. Both the LC-circuit-based microscopic picture and the effective-medium-based macroscopic model are established to capture the essential physics behind such unexpected resonance at the deep subwavelength scale. Prospective applications of this novel transmission property can be anticipated, considering the merits of compact and excellent immunity to structural imperfections.

physics.optics↗

arXiv2012

ZnO-Based Polariton Laser Operating at Room Temperature: From Excitonic to Photonic Condensate

A laser threshold is determined by the gain condition, which has been progressively reduced by the use of heterostructures and of quantum confinement. The polariton laser is the ultimate step of this evolution: coherent emission is obtained from the spontaneous decay of an exciton-polariton condensate, without the achievement of any gain condition. ZnO, with its unique excitonic properties, is the best choice for a blue/UV-emitting polariton laser device. We report on the fabrication of a new family of fully hybrid microcavities that combine the best-quality ZnO material available (bulk substrate) and two dielectric distributed Bragg reflectors, demonstrating large quality factors (>2500) and Rabi splittings (~200 meV). Low threshold polariton lasing is achieved between 4 and 300 K and for excitonic fractions ranging between 12% and 96 %. A phase diagram highlighting the role of LO phonon-assisted relaxation in this polar semiconductor is established, and a remarkable switching between polariton modes is demonstrated.

cond-mat.mtrl-sci↗

arXiv2012

Contributions of hyperon-hyperon scattering to subthreshold cascade production in heavy ion collisions

Using a gauged flavor SU(3)-invariant hadronic Lagrangian, we calculate the cross sections for the strangeness-exchange reactions YY to NΞ(Y=Λ, Σ) in the Born approximation. These cross sections are then used in the Relativistic Vlasov-Uehling-Uhlenbeck (RVUU) transport model to study Ξproduction in Ar+KCl collisions at incident energy of 1.76A GeV and impact parameter b=3.5 fm. We find that including the contributions of hyperon-hyperon scattering channels strongly enhances the yield of Ξ, leading to the abundance ratio Ξ^{-}/(Λ+Σ^{0})=3.38E-3, which is essentially consistent with the recently measured value of $(5.6 \pm 1.2_{-1.7}^{+1.8})\times 10^{-3}$ by the HADES collaboration at GSI.

nucl-th↗

arXiv2012

A Co-Prime Blur Scheme for Data Security in Video Surveillance

This paper presents a novel Coprime Blurred Pair (CBP) model for visual data-hiding for security in camera surveillance. While most previous approaches have focused on completely encrypting the video stream, we introduce a spatial encryption scheme by blurring the image/video contents to create a CBP. Our goal is to obscure detail in public video streams by blurring while allowing behavior to be recognized and to quickly deblur the stream so that details are available if behavior is recognized as suspicious. We create a CBP by blurring the same latent image with two unknown kernels. The two kernels are coprime when mapped to bivariate polynomials in the z domain. To deblur the CBP we first use the coprime constraint to approximate the kernels and sample the bivariate CBP polynomials in one dimension on the unit circle. At each sample point, we factor the 1D polynomial pair and compose the results into a 2D kernel matrix. Finally, we compute the inverse Fast Fourier Transform (FFT) of the kernel matrices to recover the coprime kernels and then the latent video stream. It is therefore only possible to deblur the video stream if a user has access to both streams. To improve the practicability of our algorithm, we implement our algorithm using a graphics processing unit (GPU) to decrypt the blurred video streams in real-time, and extensive experimental results demonstrate that our new scheme can effectively protect sensitive identity information in surveillance videos and faithfully reconstruct the unblurred video stream when two blurred sequences are available.

cs.CV↗

arXiv2011

Time-resolved study of the magnetic field effects on electroluminescence in tri-(8-hydroxyquinoline)- aluminum based organic light emitting devices

We investigated the magnetic field effects (MFEs) in organic light-emitting diodes (OLEDs) through the transient electroluminescence (EL) method. The time-resolved MFEs on the emission were obtained for the first time, which would be a useful method to clarify the underlying mechanisms of the MFEs. The fluorescent dye doped tri-(8-hydroxyquinoline)-aluminum (Alq3) based OLEDs were fabricated. Then, the transient EL was measured both with and without a magnetic field. To explore the time-resolved MFEs on the emission of the device, the excitons population dynamics in the device have been analyzed by a kinetic model. Our results suggest that both the intersystem crossing between the singlet and triplet electron-hole pairs and the triplet-triplet annihilation perturbed by the external magnetic field cause the time-resolved MFEs.

cond-mat.mtrl-sci↗

arXiv2011

Driving conditions dependence of magneto-electroluminescence in tri-(8-hydroxyquinoline)-aluminum based organic light emitting diodes

we investigated the magneto-electroluminescence (MEL) in tri-(8-hydroxyquinoline)-aluminum based organic light-emitting diodes (OLEDs) through the steady-state and transient method simultaneously. The MELs show the great different behaviors when we turn the driving condition from a constant voltage to a pulse voltage. For devices driven by the constant voltage, the MELs are similar with the literature data; for devices driven by the pulse voltage, the MELs are quite different, they firstly increase to a maximum then decrease as the magnetic field increases continuously. Negative MELs can be seen when both the magnetic field and driving voltage are high enough.

physics.chem-ph↗