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Xuan Li

Publications and source records attributed to Xuan Li.

160 records · Page 9Linked to original sources
arXiv2017

Probing autoionizing states of molecular oxygen with XUV transient absorption: Electronic symmetry dependent lineshapes and laser induced modification

The dynamics of autoionizing Rydberg states of oxygen are studied using attosecond transient absorption technique, where extreme ultraviolet (XUV) initiates molecular polarization and near infrared (NIR) pulse perturbs its evolution. Transient absorption spectra show positive optical density (OD) change in the case of $nsσ_g$ and $ndπ_g$ autoionizing states of oxygen and negative OD change for $ndσ_g$ states. Multiconfiguration time-dependent Hartree-Fock (MCTDHF) calculation are used to simulate the transient absorption spectra and their results agree with experimental observations. The time evolution of superexcited states is probed in electronically and vibrationally resolved fashion and we observe the dependence of decay lifetimes on effective quantum number of the Rydberg series. We model the effect of near-infrared (NIR) perturbation on molecular polarization and find that the laser induced phase shift model agrees with the experimental and MCTDHF results, while the laser induced attenuation model does not. We relate the electron state symmetry dependent sign of the OD change to the Fano parameters of the static absorption lineshapes.

physics.atom-ph↗
arXiv2016

Recent Heavy Flavor Results Utilizing the FVTX in PHENIX at RHIC

Heavy flavor and quarkona production are important hard probes to test the Quantum Chromodynamics (QCD) and measure the properties of the Quark Gluon Plasma (QGP) created in high energy heavy ion collisions. The new PHENIX mid- and forward-rapidity silicon vertex detectors provide precise Distance of Closest Approach (DCA) determination, allowing charm and bottom yields identification, and improve di-muon mass resolution for charmonia studies. The relative $ψ(2S)$ over $J/ψ$(1S) yields measured in $p$+$p$, $p$+Al and $p$+Au data at $\sqrt{s}=$200 GeV help us to understand final state effects on the charmonia production. Direct access to B-meson production has been enabled via measurements of non-prompt $J/ψ\to μ^{+} + μ^{-}$ in $p$+$p$ collisions at $\sqrt{s} = 510$ GeV and Cu+Au collisions at $\sqrt{s} = 200$ GeV. The fraction of B-meson decays in J/$ψ$ yields in the $p$+$p$ measurement agrees with higher energy results from Tevatron and LHC. This fraction is enhanced in Cu+Au collisions suggesting the B-meson yield is not modified by the medium whereas J/$ψ$ is strongly suppressed.

nucl-ex↗
arXiv2016

Novel multilevel preconditioners for the systems arising from plane wave discretization of Helmholtz equations with large wave numbers

In this paper we are concerned with fast algorithms for the systems arising from the plane wave discretizations for two-dimensional Helmholtz equations with large wave numbers. We consider the plane wave weighted least squares (PWLS) method and the plane wave discontinuous Galerkin (PWDG) method. The main goal of this paper is to construct multilevel parallel preconditioners for solving the resulting Helmholtz systems. To this end, we first build a multilevel overlapping space decomposition for the plane wave discretization space based on a multilevel overlapping domain decomposition method. Then, corresponding to the space decomposition, we construct an additive multilevel preconditioner for the underlying Helmholtz systems. Further, we design both additive and multiplicative multilevel preconditioners with smoothers, which are different from the standard multigrid preconditioners. We apply the proposed multilevel preconditioners with a {\it constant} coarsest mesh size to solve two dimensional Helmholtz systems generated by PWLS method or PWDG method, and we find that the new preconditioners possess nearly stable convergence, i.e., the iteration counts of the preconditioned iterative methods (PCG or PGMRES) with the preconditioners increase very slowly when the wave number increases (and the fine mesh size decreases).

math.NA↗
arXiv2015

Jet measurements in polarized p+p collisions at STAR at RHIC

Jet production in polarized $p+p$ collisions at $\sqrt{s} = 200$ GeV and $\sqrt{s} = 500$ GeV provides a powerful probe to study gluons inside the proton. The Solenoidal Tracker at RHIC (STAR) has the capability, with nearly full azimuthal ($2π$) coverage, to reconstruct jets at mid-rapidity ($|η| < 1$). The latest STAR inclusive jet longitudinal double-spin asymmetry $A_{LL}$ measured in 200 GeV $p+p$ collisions provides better constraints on the polarized gluon distribution $Δg(x)$ for $0.05<x<0.2$ than previous measurements. A recent global QCD fit (DSSV 2014) which includes the 2009 RHIC results provides the first evidence of non-zero gluon contribution to the proton spin. A new inclusive jet cross section using the anti-$k_{T}$ algorithm provides potential insights into the unpolarized gluon distribution function, and the new inclusive jet $A_{LL}$ measurement in 510 GeV $p+p$ collisions shows consistent $x_{T}$ scaling with the 200 GeV result. Future measurements with continuing high energy polarized proton-proton running at $\sqrt{s} = 500$ GeV at RHIC and detector upgrades in the forward direction will explore the gluonic contribution to the proton spin at low $x$ region.

hep-ex↗
arXiv2014

Experimental demonstration of longitudinal beam phase space linearizer in a free-electron laser facility by corrugated structures

Removal of residual linear energy chirp and intrinsic nonlinear energy curvature in the relativistic electron beam from radiofrequency linear accelerator is of paramount importance for efficient lasing of a high-gain free-electron laser. Recently, it was theoretically and experimentally demonstrated that the longitudinal wakefield excited by the electrons itself in the corrugated structure allows for precise control of the electron beam phase space. In this Letter, we report the first utilization of a corrugated structure as beam linearizer in the operation of a seeded free-electron laser driven by a 140 MeV linear accelerator, where a gain of ~10,000 over spontaneous emission was achieved at the second harmonic of the 1047 nm seed laser, and a free-electron laser bandwidth narrowing by about 50% was observed, in good agreement with the theoretical expectations.

physics.acc-ph↗
arXiv2014

Complete quantum state selectivity in cold molecular beams using deflection-resistant dark states in a STIRAP configuration

One of the main goals of chemical dynamics is the creation of molecular beams composed of a single (vibrational, rotational, and magnetic) quantum state of choice. In this paper we show that it is possible to achieve {\it complete} quantum state selectivity by producing resistance to electromagnetically induced deflection (EID) and that the state to be selected can be "dialed in" at will. We illustrate the method by showing in detail how to purify thermal beams of the LiRb and IF molecules to yield molecular beams composed of a variety of pre-chosen single internal quantum states and/or superpositions of such states. We expect that this method will be implemented in all subsequent explorations of the fundamentals of chemical reactions and their control, and the use of cold molecules as a vehicle for studying some of the most profound issues of quantum dynamics.

quant-ph↗
arXiv2014

Extracting molecular potentials from insufficient spectroscopic information

We extend our recently developed inversion method to extract excited state potentials from fluorescence line positions and line strengths. We consider a previous limitation of the method arising due to insufficient input data in cases where the relatively weaker emission data are not experimentally available. We develop a solution to this problem by "regenerating" these weak transition lines via applying a model potential, e.g. a Morse potential. The result of this procedure, illustrated for the Q-branch emission from the lowest three vibrational levels of the B($^1 Π)$ state of LiRb, is shown to have an error of $0.29$ cm$^{-1}$ in the classically allowed region and a global error of $5.67$ cm$^{-1}$ for $V\le E(ν'=10)$. The robustness of this procedure is also demonstrated by considering the statistical error in the measured line intensities.

quant-ph↗
arXiv2014

Recent results of gluon and sea quark polarization measurements in polarized proton-proton collisions at STAR

The STAR experiment at RHIC is carrying out a comprehensive high-energy spin physics program to understand the internal structure and dynamics of the proton in polarized proton-proton collisions at $\sqrt{s} = 200$ GeV and $\sqrt{s} = 500/510$ GeV. STAR has the capability, with nearly full azimuthal coverage, to reconstruct leptons, hadrons and jets in the mid-rapidity region ($|η|<1$). The results for inclusive jet longitudinal double spin asymmetries taken during the 2009 RHIC run indicate the first non-zero gluon contribution ($Δg(x,Q^{2}) / g(x,Q^{2})$) to the proton spin for $0.05<x<1$ (Bjorken-x: momentum fraction of partons). Recent longitudinal single-spin asymmetry measurements of $W^{+/-}$ bosons at $\sqrt{s} = 500/510$ GeV in polarized proton-proton collisions provide a direct probe of the polarized anti-u and anti-d quark distributions ($Δ\bar{u}(x,Q^{2})$, $Δ\bar{d}(x,Q^{2})$). These results better constrain the polarized gluon and sea quark distributions of the proton in the RHIC sensitive kinematic region. Future measurements with continuing high energy polarized proton-proton run at RHIC and detector upgrade will explore the gluonic contribution to the proton spin in extended range.

hep-ex↗
arXiv2014

Joint Beamforming Design and Time Allocation for Wireless Powered Communication Networks

This paper investigates a multi-input single-output (MISO) wireless powered communication network (WPCN) under the protocol of harvest-then-transmit. The power station (PS) with reliable power supply can replenish the passive user nodes by wireless power transfer (WPT) in the downlink (DL), then each user node transmits independent information to the sink by a time division multiple access (TDMA) scheme in the uplink (UL). We consider the joint time allocation and beamforming design to maximize the system sum-throughput. The semidefinite relaxation (SDR) technique is applied to solve the nonconvex design problem. The tightness of SDR approximation, thus the global optimality, is proved. This implies that only one single energy beamformer is required at the PS. Then a fast semiclosed form solution is proposed by exploiting the inherent structure. Simulation results demonstrate the efficiency of the proposed algorithms from the perspectives of time complexity and information throughput.

cs.IT↗
arXiv2014

A Low-Complexity Algorithm for Throughput Maximization in Wireless Powered Communication Networks

This paper investigates a wireless powered communication network (WPCN) under the protocol of harvest-then-transmit,where a hybrid access point with constant power supply replenishes the passive user nodes by wireless power transfer in the downlink,then each user node transmit independent information to the hybrid AP in a time division multiple access (TDMA) scheme in the uplink.The sum-throughput maximization and min-throughput maximization problems are considered in this paper.The optimal time allocation for the sum-throughput maximization is proposed based on the Jensen's inequality,which provides more insight into the design of WPCNs.A low-complexity fixed-point iteration algorithm for the min-throughput maximization problem,which promises a much better computation complexity than the state-of-the-art algorithm.Simulation results confirm the effectiveness of the proposed algorithm.

cs.IT↗
arXiv2013

Magnetic domain and magnetic resistance phase transition in strongly correlated electronic material of perovskites junction

The junction magnetoresistivity and domain phase transition were studied between ZnO and La0.4Gd0.1Sr0.5CoO3 thin films grown on LaAlO3 (100) substrates epitaxially by pulse laser deposit. The ferromagnetic transformation into phase-separated (two phase) state was displayed below Tc~127 and has observed that the lattice change discontinuously in the doped cobalt perovskites La0.4Gd0.1Sr0.5CoO3. The Ginzburg-Landau phase field is introduced to deduce antiferroelectric domain structure in LGSCO thin film. On the basis of the domain structures, the phase boundary of thin film is strongly dependent on the combination of electric-mechanical coupling. The phase transformation into phase separated state occurs below Tc~127-128K, and have displayed that the lattice constants change discontinuously at the transformation. The positive MR of ZnO/LGSCO heterojunction exhibited the MIT behavior at 0.2 T is 4.86%, at 0.5 T is 6.05% for approximately 140K.

cond-mat.mtrl-sci↗
arXiv2013

High resolution photoassociation spectroscopy of the $^{6}$Li$_2$ $1^{3}Σ_{g}^{+}$ state

We present experimental observations of seven vibrational levels $v'=20-26$ of the $1^{3}Σ_{g}^{+}$ excited state of Li$_2$ molecules by the photoassociation (PA) of a degenerate Fermi gas of $^6$Li atoms. For each vibrational level, we resolve the rotational structure using a Feshbach resonance to enhance the PA rates from p-wave collisions. We also, for the first time, determine the spin-spin and spin-rotation interaction constants for this state. The absolute uncertainty of our measurements is $\pm 0.00002$ cm$^{-1}$ ($\pm 600$ kHz). We use these new data to further refine an analytic potential for this state.

physics.atom-ph↗
arXiv2012

Forward azimuthal correlations in 200 GeV p+p and d+Au collisions at STAR

Forward particle production and correlation measurements at RHIC can probe low-$x$ gluons. The suppression observed in back-to-back forward $π^{0}$+forward $π^{0}$ correlations at STAR in central d+Au collisions at STAR is consistent with a prediction of the Color Glass Condensate (CGC) calculation, indicating the gold nucleus probed at such low-$x$ is in a dense gluon state. The forward $π^{0}$ + near-forward jet-like cluster azimuthal correlations in 200 GeV p+p and d+Au collisions at STAR are studied, which are sensitive to the intermediate x region between forward+mid-rapidity correlations and forward+forward correlations. Together with the other measurements from STAR, which probe different regions of x, forward+near-forward di-hadron correlations can provide information to understand how sharp is the transition from dilute parton gas to dense gluon state.

nucl-ex↗
arXiv2012

Direct Observation of Giant Saturation Magnetization in Fe16N2

Magnetic materials with giant saturation magnetization have been a holy grail for magnetic researchers and condensed matter physicists for decades because of its great scientific and technological impacts. As described by the famous Slater-Pauling curve the material with highest Ms is the Fe65Co35 alloy. This was challenged in 1972 by a report on the compound Fe16N2 with Ms much higher than that of Fe65Co35. Following this claim, there have been enormous efforts to reproduce this result and to understand the magnetism of this compound. However, the reported Ms by different groups cover a broad range, mainly due to the unavailability of directly assessing Ms in Fe16N2. In this article, we report a direct observation of the giant saturation magnetization up to 2500 emu/cm3 using polarized neutron reflectometry (PNR) in epitaxial constrained Fe16N2 thin films prepared using a low-energy and surface-plasma-free sputtering process. The observed giant Ms is corroborated by a previously proposed Cluster + Atom model, the characteristic feature of which, namely, the directional charge transfer is evidenced by polarization-dependent x-ray absorption near edge spectroscopy (XANES).

cond-mat.mtrl-sci↗
arXiv2011

Rapidity dependence of azimuthal correlations for p+p and d+Au at $\sqrt{s}=200$GeV

Forward di-jet production in d+Au collisions at RHIC provides sensitivity to Bjorken-x between 0.001 and 0.02 for the nuclear gluon density . The STAR experiment at RHIC has continuous azimuthal coverage with electromagnetic calorimeters for pseudo-rapidity from -1 to 4. We have measured forward+mid-rapidity $π^{0}-π^{0}$ and $π^{0}-h$ azimuthal correlations and forward+forward rapidity $π^{0}-π^{0}$ azimuthal correlations. Both analyses use the Forward Meson Spectrometer (FMS) triggered data in RHIC 2008 run p+p and d+Au collisions at $\sqrt{s_{NN}}$=200 GeV. The end-cap electromagnetic calorimeter (EEMC) at STAR covers pseudo-rapidity from 1.07 to 2. This provides us sensitivity to the nuclear gluon density at intermediate x values. Preliminary results on the $π^{0}-π^{0}$ and $π^{0}$ + jet-like azimuthal correlations with both the FMS and the EEMC will be discussed.

hep-ex↗