arXiv subjects

Qiang Li

Publications and source records attributed to Qiang Li.

At least 559 records · Page 31Linked to original sources

arXiv2015

Substitution of Ni for Fe in superconducting Fe$_{0.98}$Te$_{0.5}$Se$_{0.5}$ depresses the normal-state conductivity but not the magnetic spectral weight

We have performed systematic resistivity and inelastic neutron scattering measurements on Fe$_{0.98-z}$Ni$_z$Te$_{0.5}$Se$_{0.5}$ samples to study the impact of Ni substitution on the transport properties and the low-energy ($\le$ 12 meV) magnetic excitations. It is found that, with increasing Ni doping, both the conductivity and superconductivity are gradually suppressed; in contrast, the low-energy magnetic spectral weight changes little. Comparing with the impact of Co and Cu substitution, we find that the effects on conductivity and superconductivity for the same degree of substitution grow systematically as the atomic number of the substituent deviates from that of Fe. The impact of the substituents as scattering centers appears to be greater than any contribution to carrier concentration. The fact that low-energy magnetic spectral weight is not reduced by increased electron scattering indicates that the existence of antiferromagnetic correlations does not depend on electronic states close to the Fermi energy.

cond-mat.supr-con↗

arXiv2014

Observation of the chiral magnetic effect in ZrTe5

The chiral magnetic effect is the generation of electric current induced by chirality imbalance in the presence of magnetic field. It is a macroscopic manifestation of the quantum anomaly in relativistic field theory of chiral fermions (massless spin $1/2$ particles with a definite projection of spin on momentum) -- a dramatic phenomenon arising from a collective motion of particles and antiparticles in the Dirac sea. The recent discovery of Dirac semimetals with chiral quasi-particles opens a fascinating possibility to study this phenomenon in condensed matter experiments. Here we report on the first observation of chiral magnetic effect through the measurement of magneto-transport in zirconium pentatelluride, ZrTe_5. Our angle-resolved photoemission spectroscopy experiments show that this material's electronic structure is consistent with a 3D Dirac semimetal. We observe a large negative magnetoresistance when magnetic field is parallel with the current. The measured quadratic field dependence of the magnetoconductance is a clear indication of the chiral magnetic effect. The observed phenomenon stems from the effective transmutation of Dirac semimetal into a Weyl semimetal induced by the parallel electric and magnetic fields that represent a topologically nontrivial gauge field background.

cond-mat.str-el↗

arXiv2014

Energy Efficiency Optimization for MIMO-OFDM Mobile Multimedia Communication Systems with QoS Constraints

It is widely recognized that besides the quality of service (QoS), the energy efficiency is also a key parameter in designing and evaluating mobile multimedia communication systems, which has catalyzed great interest in recent literature. In this paper, an energy efficiency model is first proposed for multiple-input multiple-output orthogonal-frequency-division-multiplexing (MIMO-OFDM) mobile multimedia communication systems with statistical QoS constraints. Employing the channel matrix singular-value-decomposition (SVD) method, all subchannels are classified by their channel characteristics. Furthermore, the multi-channel joint optimization problem in conventional MIMO-OFDM communication systems is transformed into a multi-target single channel optimization problem by grouping all subchannels. Therefore, a closed-form solution of the energy efficiency optimization is derived for MIMO-OFDM mobile mlutimedia communication systems. As a consequence, an energy-efficiency optimized power allocation (EEOPA) algorithm is proposed to improve the energy efficiency of MIMO-OFDM mobile multimedia communication systems. Simulation comparisons validate that the proposed EEOPA algorithm can guarantee the required QoS with high energy efficiency in MIMO-OFDM mobile multimedia communication systems.

cs.IT↗

arXiv2014

Magneto-optical characterizations of FeTe$_{0.5}$Se$_{0.5}$ thin films with critical current density over 1 MA/cm$^2$

We performed magneto-optical (MO) measurements on FeTe$_{0.5}$Se$_{0.5}$ thin films grown on LaAlO$_3$ (LAO) and Yttria-stabilized zirconia (YSZ) single-crystalline substrates. These thin films show superconducting transition temperature, ${T_c}$ $\sim$ 19 K, 4 K higher than the bulk sample. Typical roof-top patterns can be observed in the MO images of thin films grown on LAO and YSZ, from which a large and homogeneous critical current density, ${J_c}$, over 1 $\times$ 10$^6$ A/cm$^2$ at 5 K was obtained. Magnetic flux penetration measurement reveals that the current is almost isotropically distributed in the two thin films. Compared with bulk crystals, FeTe$_{0.5}$Se$_{0.5}$ thin film demonstrates not only higher ${T_c}$, but also much larger ${J_c}$, which is attractive for applications.

cond-mat.supr-con↗

arXiv2014

A probabilistic approach to quantum Bayesian games of incomplete information

A Bayesian game is a game of incomplete information in which the rules of the game are not fully known to all players. We consider the Bayesian game of Battle of Sexes that has several Bayesian Nash equilibria and investigate its outcome when the underlying probability set is obtained from generalized Einstein-Podolsky-Rosen experiments. We find that this probability set, which may become non-factorizable, results in a unique Bayesian Nash equilibrium of the game.

quant-ph↗

arXiv2014

Remote-excitation and remote-detection of single quantum dot using propagating surface plasmons on silver nanowire

Using propagating surface plasmons (SPs) on silver nanowire (NW), we demonstrate that focused laser light at the end of silver nanowire can excite single quantum dot (QD) microns away from the excitation spot. The QD-NW interaction allows the excited QD convert part of its energy into propagating SPs which then can be detected at the remote sites. Simultaneous multi-QDs remote-excitation and detection are also realized. Furthermore, the tight confinement of propagating SPs around the NW surface enables selective excitation of QDs very close in space, which cannot be realized under conventional excitation condition. This remote excitation and detection approach may find applications in optical imaging and sensing of chemical and biological systems.

physics.optics↗

arXiv2014

Optimization of a Superconducting Magnetic Energy Storage Device via a CPU-Efficient Semi-Analytical Simulation

Recent advances in second generation (YBCO) high temperature superconducting wire could potentially enable the design of super high performance energy storage devices that combine the high energy density of chemical storage with the high power of superconducting magnetic storage. However, the high aspect ratio and considerable filament size of these wires requires the concomitant development of dedicated optimization methods that account for both the critical current density and ac losses in type II superconductors. Here, we report on the novel application and results of a CPU-efficient semi-analytical computer code based on the Radia 3D magnetostatics software package. Our algorithm is used to simulate and optimize the energy density of a superconducting magnetic energy storage device model, based on design constraints, such as overall size and number of coils. The rapid performance of the code is pivoted on analytical calculations of the magnetic field based on an efficient implementation of the Biot-Savart law for a large variety of 3D "base" geometries in the Radia package. The significantly-reduced CPU time and simple data input in conjunction with the consideration of realistic input variables, such as material-specific, temperature and magnetic field-dependent critical current densities have enabled the Radia-based algorithm to outperform finite element approaches by a twenty fold reduction in CPU time at the same accuracy levels. Comparative simulations of MgB2 and YBCO-based devices are performed at 4.2 K, and finally, calculations of the ac losses are computed in order to ascertain the realistic efficiency of the design configurations.

physics.ins-det↗

arXiv2014

Higgs Pair Production: Improved Description by Matrix Element Matching

Higgs pair production is crucial for measuring the Higgs boson self-coupling. The dominant channel at hadron colliders is gluon fusion via heavy-quark loops. We present the results of a fully exclusive simulation of gluon fusion Higgs pair production based on the matrix elements for hh + 0, 1 partons including full heavy-quark loop dependence, matched to a parton shower. We examine and validate this new description by comparing it with (a) Higgs Effective Theory predictions, (b) exact hh + 0-parton sample showered by pythia, and (c) exact hh+1-parton distributions, by looking at the most relevant kinematic distributions, such as PTh, PThh, Mhh spectra, and jet rate as well. We find that matched samples provide an state-of-the-art accurate exclusive description of the final state. The relevant LHE files for Higgs pair productions at the LHC can be accessed via http://hepfarm02.phy.pku.edu.cn/foswiki/CMS/HH, which can be used for relevant experimental analysis.

hep-ph↗

arXiv2013

Combination of Diverse Ranking Models for Personalized Expedia Hotel Searches

The ICDM Challenge 2013 is to apply machine learning to the problem of hotel ranking, aiming to maximize purchases according to given hotel characteristics, location attractiveness of hotels, user's aggregated purchase history and competitive online travel agency information for each potential hotel choice. This paper describes the solution of team "binghsu & MLRush & BrickMover". We conduct simple feature engineering work and train different models by each individual team member. Afterwards, we use listwise ensemble method to combine each model's output. Besides describing effective model and features, we will discuss about the lessons we learned while using deep learning in this competition.

cs.LG↗

arXiv2013

Enhanced low-energy magnetic excitations via suppression of the itinerancy in Fe0.98-zCuzTe0.5Se0.5

We have performed resistivity and inelastic neutron scattering measurements on three samples of Fe0.98-zCuzTe0.5Se0.5 with z = 0, 0.02, and 0.1. It is found that with increasing Cu doping the sample's resistivity deviates progressively from that of a metal. However, in contrast to expectations that replacing Fe with Cu would suppress the magnetic correlations, the low-energy (no larger than 12 meV) magnetic scattering is enhanced in strength, with greater spectral weight and longer dynamical spin-spin correlation lengths. Such enhancements can be a consequence of either enlarged local moments or a slowing down of the spin fluctuations. In either case, the localization of the conduction states induced by the Cu doping should play a critical role. Our results are not applicable to models that treat 3d transition metal dopants simply as effective electron donors.

cond-mat.supr-con↗

arXiv2013

The CERN LHC Sensitivity on measuring WZGamma Production and Anomalous WWZGamma Coupling

In this paper we present for the first time a detailed Monte Carlo study of measuring WZGamma production with pure leptonic decays and probing anomalous quartic gauge-boson WWZGamma couplings at the sqrt(s) = 14 TeV LHC, with parton shower and detector simulation effects taken into account. We find that with an integrated luminosity of 100 fb-1 and proper selection cuts, the Standard Model WZGamma signal significance can be improved to as much as 3 sigma. After reviewing previous parametrization on anomalous WWZGamma couplings (see e.g.an/Lambda^2 or k2m/Lambda^2 as shown in Ref. [17]), we propose a more general parametrization scheme with 4 free inputs leading only to genuine WWZGamma aQGC couplings. Finally, our numerical results show that one can reach constraints at 95% confidence level of -5.7 * 10^{-5} GeV^{-2} < k2m/Lambda^2 < 5.5 * 10^{-5} GeV^{-2} and -2.2 x 10^{-5} GeV^{-2} < an/Lambda^2 < 2.4 * 10^{-5} GeV^{-2}, which are more stringent than LEP's results by three orders of magnitude.

hep-ph↗

arXiv2013

Photon-pair jet production via gluon fusion at the LHC

Photon-pair direct or jet-associated productions are important for relevant standard model measurement, Higgs and new physics searches at the LHC. The loop-induced gluon-fusion process gg -> γγg, which although formally contributes only at the next-to-next-to-leading order to γγj productions, may get enhanced by the large gluon flux at the LHC. We have checked and confirmed previous results on gg -> γγ, γγg at one loop, using now the traditional Feynman diagram based approach and taking into account the quark mass effects, and further updated them for the 7 and 14 TeV LHC with new inputs and settings. We provide the details and results of the calculations, which involves manipulation of rank-5 pentagon integrals. Our results show that the gluon-fusion process can contribute about 10% of the Born result, especially at small M_{γγ} and PT_{γγ}, and increase further the overall scale uncertainty. Top quark loop effects are examined in detail, which shows importance near or above the threshold M_{γγ} \gsim 2m_t.

hep-ph↗

arXiv2013

Probing W+W-gamma Production and Anomalous Quartic Gauge Boson Couplings at the CERN LHC

Triple gauge boson associated production at the LHC serves as an interesting channel to test the robustness of the Standard Model. Any deviation from its SM prediction may indicate possible existence of relevant new physics, e.g., anomalous quartic gauge boson couplings. In this paper, a Monte-Carlo feasibility study of measuring WWA production with pure leptonic decays and probing anomalous quartic gauge-boson (e.g., WWAA) couplings, is presented in detail for the first time, with parton shower and detector simulation effects taken into account. Our results show that at the sqrt{s} = 14 TeV LHC with an integrated luminosity of 100 (30) fb-1, one can reach a significance of 9 (5) sigma to observe the SM WWA production, and can constrain at the 95% CL the anomalous WWAA coupling parameters, e.g., a_{0,c}^W/Λ^2 (see Ref.[15] for their definitions), at 10^{-5} GeV^{-2}, respectively.

hep-ph↗

arXiv2013

Spatially Selective Artificial-Noise Aided Transmit Optimization for MISO Multi-Eves Secrecy Rate Maximization

Consider an MISO channel overheard by multiple eavesdroppers. Our goal is to design an artificial noise (AN)-aided transmit strategy, such that the achievable secrecy rate is maximized subject to the sum power constraint. AN-aided secure transmission has recently been found to be a promising approach for blocking eavesdropping attempts. In many existing studies, the confidential information transmit covariance and the AN covariance are not simultaneously optimized. In particular, for design convenience, it is common to prefix the AN covariance as a specific kind of spatially isotropic covariance. This paper considers joint optimization of the transmit and AN covariances for secrecy rate maximization (SRM), with a design flexibility that the AN can take any spatial pattern. Hence, the proposed design has potential in jamming the eavesdroppers more effectively, based upon the channel state information (CSI). We derive an optimization approach to the SRM problem through both analysis and convex conic optimization machinery. We show that the SRM problem can be recast as a single-variable optimization problem, and that resultant problem can be efficiently handled by solving a sequence of semidefinite programs. Our framework deals with a general setup of multiple multi-antenna eavesdroppers, and can cater for additional constraints arising from specific application scenarios, such as interference temperature constraints in interference networks. We also generalize the framework to an imperfect CSI case where a worst-case robust SRM formulation is considered. A suboptimal but safe solution to the outage-constrained robust SRM design is also investigated. Simulation results show that the proposed AN-aided SRM design yields significant secrecy rate gains over an optimal no-AN design and the isotropic AN design, especially when there are more eavesdroppers.

cs.IT↗

arXiv2013

Evolution of superconducting correlations within magnetic-field-decoupled CuO(2) layers of La(1.905)Ba(0.095)CuO(4)

We explore the evolution of superconductivity in La(2-x)Ba(x)CuO(4) with x=0.095 in magnetic fields of up to 35 T applied perpendicular to the CuO(2) planes. Previous work on this material has shown that perpendicular fields enhance both charge and spin stripe order within the planes. We present measurements of the resistivity parallel and perpendicular to the planes, as well as the Hall effect. Measurements of magnetic susceptibility for fields of up to 15 T applied both parallel and perpendicular to the planes provide complementary measures of the superconductivity. We show that fields sufficient to destroy pair tunneling between the planes do not disrupt the superconducting correlations within the planes. In fact, we observe an onset of large amplitude but phase disordered superconductivity within the planes at approximately 30 K that is remarkably insensitive to field. With further cooling, we observe a phase-transition-like drop in the in-plane resistivity to an apparent state of superconductivity, despite the lack of phase coherence between the layers. These observations raise interesting questions concerning the identification of the upper critical field, where pairing is destroyed, in underdoped cuprates.

cond-mat.supr-con↗

arXiv2012

ZZ+jet production via gluon fusion at the LHC

Pair production of Z bosons in association with a hard jet is an important background for Higgs particle or new physics searches at the LHC. The loop-induced gluon-fusion process gg > ZZg contributes formally only at the next-to-next-to-leading order. Nevertheless, it can get enhanced by the large gluon flux at the LHC, and thus should be taken into account in relevant experimental searches. We provide the details and results of this calculation, which involves the manipulation of rank-5 pentagon integrals. Our results show that the gluon-fusion process can contribute more than 10% to the next-to-leading order QCD result and increases the overall scale uncertainty. Moreover, interference effects between Higgs and non-Higgs contributions can become large in phase-space regions where the Higgs is far off-shell.

hep-ph↗

arXiv2012

Conductance of single-atom magnetic junctions: A first-principles study

We present a first-principles investigation to show that the contact conductance of a half conductance quantum ($G_0/2$) found previously does not generally hold for single-atom magnetic junctions composed of a tip and an adatom dsorbed on a surface. The contact conductance of the Ni-Co/Co(111) junction is approximately $G_0/2$, while for the Co-Co/Co(111), Ni-Ni/Ni(111), and Ni-Ni/Ni(001) junctions the contact conductances are 0.80$G_0$, 1.55$G_0$, and 1.77$G_0$, respectively. The deviation from $G_0/2$ is mainly caused by the variation of the spin-down conductance largely determined by the minority $d$ orbitals, as the spin-up one changes little for different junctions.

cond-mat.mes-hall↗

arXiv2012

Transport Properties of Stripe-Ordered High Tc Cuprates

Transport measurements provide important characterizations of the nature of stripe order in the cuprates. Initial studies of systems such as La(1.6-x)Nd(0.4)Sr(x)CuO(4) demonstrated the strong anisotropy between in-plane and c-axis resistivities, but also suggested that stripe order results in a tendency towards insulating behavior within the planes at low temperature. More recent work on La(2-x)Ba(x)CuO(4) with x=1/8 has revealed the occurrence of quasi-two-dimensional superconductivity that onsets with spin-stripe order. The suppression of three-dimensional superconductivity indicates a frustration of the interlayer Josephson coupling, motivating a proposal that superconductivity and stripe order are intertwined in a pair-density-wave state. Complementary characterizations of the low-energy states near the Fermi level are provided by measurements of the Hall and Nernst effects, each revealing intriguing signatures of stripe correlations and ordering. We review and discuss this work.

cond-mat.supr-con↗