arXiv subjects

Gang Li

Publications and source records attributed to Gang Li.

At least 649 records · Page 36Linked to original sources

arXiv2016

Seed population in large Solar Energetic Particle events and the twin-CME scenario

It has been recently suggested that large solar energetic particle (SEP) events are often caused by twin CMEs. In the twin-CME scenario, the preceding CME is to provide both an enhanced turbulence level and enhanced seed population at the main CME-driven shock. In this work, we study the effect of the preceding CMEs on the seed population. We examine event-integrated abundance of iron to oxygen ratio (Fe/O) at energies above 25 MeV/nuc for large SEP events in solar cycle 23. We find that the Fe/O ratio (normalized to the reference coronal value of $0.134$) $\leq2.0$ for almost all single-CME events and these events tend to have smaller peak intensities. In comparison, the Fe/O ratio of twin-CME events scatters in a larger range, reaching as high as $8$, suggesting the presence of flare material from perhaps preceding flares. For extremely large SEP events with peak intensity above $1000$ pfu, the Fe/O drop below $2$, indicating that in these extreme events the seed particles are dominated by coronal material than flare material. The Fe/O ratios of Ground level enhancement (GLE) events, all being twin-CME events, scatter in a broad range. For a given Fe/O ratio, GLE events tend to have larger peak intensities than non-GLE events. Using velocity dispersion analysis (VDA), we find that GLE events have lower solar particle release (SPR) heights than non-GLE events, \red{agreeing with earlier results by Reames 2009b.

astro-ph.SR↗

arXiv2016

A Statistical Study of the Average Iron Charge Distributions inside Magnetic Clouds for Solar Cycle 23

Magnetic clouds (MCs) are the interplanetary counterpart of coronal magnetic flux ropes. They can provide valuable information to reveal the flux rope characteristics at their eruption stage in the corona, which are unable to be explored in situ at present. In this paper, we make a comprehensive survey of the average iron charge state ( Fe) distributions inside 96 MCs for solar cycle 23 using ACE (Advanced Composition Explorer) data. As the Fe in the solar wind are typically around 9+ to 11+, the Fe charge state is defined as high when the Fe is larger than 12+, which implies the existence of a considerable amount of Fe ions with high charge states (e.g., \geq 16+). The statistical results show that the Fe distributions of 92 (~ 96%) MCs can be classified into four groups with different characteristics. In group A (11 MCs), the Fe shows a bimodal distribution with both peaks higher than 12+. Group B (4 MCs) presents a unimodal distribution of Fe with its peak higher than 12+. In groups C (29 MCs) and D (48 MCs), the Fe remains higher and lower than 12+ throughout ACE passage through the MC, respectively. Possible explanations to these distributions are discussed.

astro-ph.SR↗

arXiv2016

Probing CP-violating $h\bar{t}t$ coupling in $e^{+}e^{-}\rightarrow h γ$

We investigate the possibility of probing the CP-violating $h\bar{t}t$ coupling in the process $e^{+}e^{-}\rightarrow h γ$ at the future high luminosity $e^{+}e^{-}$ colliders. Our numerical results show that the cross section for this process can be significantly increased for the allowed CP phase $ξ$ and center of mass energy. For example the cross section is about 6 times of that in the standard model (SM) for $\sqrt{s}=350~\mathrm{GeV}$ and $ξ=3π/5$ (see text for $ξ$ definition). The simulation for the signal process $e^{+}e^{-}\rightarrow h γ\rightarrow b \bar{b}γ$ and its backgrounds shows that the signal significance can reach more than $4.8σ$ and $2.3σ$ for $\sqrt{s}=350~\mathrm{GeV},\ 500~\mathrm{GeV}$ respectively, with the integrated luminosity $\mathcal{L}=3~\text{ab}^{-1}$ and $ξ\in[π/2,3π/5]$. For $\mathcal{L}=10~\text{ab}^{-1}$, the signal significance can be greater than 5 for $\sqrt{s}=350~\mathrm{GeV}$ and 3.9 for $\sqrt{s}=500~\mathrm{GeV}$ with the CP phase $ξ\in[π/2,3π/5]$. Besides the cross section enhancement, the CP-violating $h\bar{t}t$ coupling will induce the forward-backward asymmetry $A_{FB}$ which is absent in the SM and is a clear signal of new CP violation. Compared with the $A_{FB}$ in the Higgs decay $h\rightarrow l^{+}l^{-}γ$, the $A_{FB}$ can be greatly enhanced in the production process. For example $A_{FB}$ can reach 0.5 for $\cosξ\simeq 0.7$ and $\sqrt{s}=500~\mathrm{GeV}$. Due to the large backgrounds, the significance of the expected $A_{FB}$ can only be observed at $1.7σ$ with $\mathcal{L}=10~\text{ab}^{-1}$ and $\sqrt{s}=500~\mathrm{GeV}$. It is essential to trigger the single photon in the final state to separate the bottom jets arising from scalar or vector bosons, in order to isolate the signal from the backgrounds more efficiently.

hep-ph↗

arXiv2016

Rotational Symmetry Breaking in a Trigonal superconductor Nb-doped Bi2Se3

The search for unconventional superconductivity has been focused on materials with strong spin-orbit coupling and unique crystal lattices. Doped bismuth selenide (Bi$_2$Se$_3$) is a strong candidate given the topological insulator nature of the parent compound and its triangular lattice. The coupling between the physical properties in the superconducting state and its underlying crystal symmetry is a crucial test for unconventional superconductivity. In this paper, we report direct evidence that the superconducting magnetic response couples strongly to the underlying 3-fold crystal symmetry in the recently discovered superconductor with trigonal crystal structure, niobium (Nb)-doped bismuth selenide (Bi$_2$Se$_3$). More importantly, we observed that the magnetic response is greatly enhanced along one preferred direction spontaneously breaking the rotational symmetry. Instead of a simple 3-fold crystalline symmetry, the superconducting hysteresis loop shows dominating 2-fold and 4-fold symmetry. This observation confirms the breaking of the rotational symmetry and indicates the presence of nematic order in the superconducting ground state of Nb-doped Bi$_2$Se$_3$. Further, heat capacity measurements display an exponential decay in superconducting state and suggest that there is no line node in the superconducting gap. These observations provide strong evidence of odd-parity topological superconductivity.

cond-mat.supr-con↗

arXiv2016

Could the observation of $X(5568)$ be resulted by the near threshold rescattering effects?

We investigate the invariant mass distributions of $B_sπ$ via different rescattering processes. Because the triangle singularity (TS) could be present for a very broad incident energy region, it can be expected that the TS peaks may simulate the resonance-like bump $X(5568)$ observed by the D0 collaboration. The highly process-dependent characteristic of TS mechanism offers a criterion to distinguish it from other dynamic mechanisms.

hep-ph↗

arXiv2016

Electron Acceleration at a Coronal Shock Propagating Through a Large-scale Streamer-like Magnetic Field

With a test-particle simulation, we investigate the effect of large-scale coronal magnetic fields on electron acceleration at an outward-propagating coronal shock with a circular front. The coronal field is approximated by an analytical solution with a streamer-like magnetic field featured by partially open magnetic field and a current sheet at the equator atop the closed region. We show that the large-scale shock-field configuration, especially the relative curvature of the shock and the magnetic field line across which the shock is sweeping, plays an important role in the efficiency of electron acceleration. At low shock altitudes, when the shock curvature is larger than that of magnetic field lines, the electrons are mainly accelerated at the shock flanks; at higher altitudes, when the shock curvature is smaller, the electrons are mainly accelerated at the shock nose around the top of closed field lines. The above process reveals the shift of efficient electron acceleration region along the shock front during its propagation. It is also found that in general the electron acceleration at the shock flank is not so efficient as that at the top of closed field since at the top a collapsing magnetic trap can be formed. In addition, we find that the energy spectra of electrons is power-law like, first hardening then softening with the spectral index varying in a range of -3 to -6. Physical interpretations of the results and implications on the study of solar radio bursts are discussed.

astro-ph.SR↗

arXiv2016

Heart and diamond Fermi arcs in Pd and Pt oxide topological Dirac semimetals

Topological Dirac semimetals (DSMs) exhibit nodal points through which energy bands disperse linearly in three-dimensional (3D) momentum space, a 3D analogue of graphene. The first experimentally confirmed DSMs with a pair of Dirac points (DPs), Na$_{3}$Bi and Cd$_{3}$As$_{2}$, show topological surface Fermi arc states and exotic magneto-transport properties, boosting the interest in the search for stable and nontoxic DSM materials. Here, based on the {\it ab-initio} band structure calculations we predict a family of palladium and platinum oxides as robust 3D DSMs. The novel topological properties of these compounds are distinct from all other previously reported DSMs, they are rendered by the multiple number of DPs in the compounds. We show that along three unit axes of the cubic lattice there exist three pairs of DPs, which are linked by Fermi arcs on the surface. The Fermi arcs display a Lifshitz transition from a heart- to a diamond-shape upon varying the chemical potential. Corresponding oxides are already available as high-quality single crystals, which is an excellent precondition for the verification of our prediction by photoemission and magneto-transport experiments. Our findings not only predict a number of much robust 3D DSM candidates but also extend DSMs to transition metal oxides, a versatile family of materials, which opens the door to investigate the interplay between DSMs and electronic correlations that is not possible in the previously discovered DSM materials.

cond-mat.mtrl-sci↗

arXiv2016

Decentralized and Collaborative Subspace Pursuit: A Communication-Efficient Algorithm for Joint Sparsity Pattern Recovery with Sensor Networks

In this paper, we consider the problem of joint sparsity pattern recovery in a distributed sensor network. The sparse multiple measurement vector signals (MMVs) observed by all the nodes are assumed to have a common (but unknown) sparsity pattern. To accurately recover the common sparsity pattern in a decentralized manner with a low communication overhead of the network, we develop an algorithm named decentralized and collaborative subspace pursuit (DCSP). In DCSP, each node is required to perform three kinds of operations per iteration: 1) estimate the local sparsity pattern by finding the subspace that its measurement vector most probably lies in; 2) share its local sparsity pattern estimate with one-hop neighboring nodes; and 3) update the final sparsity pattern estimate by majority vote based fusion of all the local sparsity pattern estimates obtained in its neighborhood. The convergence of DCSP is proved and its communication overhead is quantitatively analyzed. We also propose another decentralized algorithm named generalized DCSP (GDCSP) by allowing more information exchange among neighboring nodes to further improve the accuracy of sparsity pattern recovery at the cost of increased communication overhead. Experimental results show that, 1) compared with existing decentralized algorithms, DCSP provides much better accuracy of sparsity pattern recovery at a comparable communication cost; and 2) the accuracy of GDCSP is very close to that of centralized processing.

cs.IT↗

arXiv2016

Hidden-charm Pentaquark States in Heavy Ion Collisions at the Large Hadron Collider

In the framework of the quark combination, we derive the yield formulas and study the yield ratios of the hidden-charm pentaquark states in ultra-relativistic heavy ion collisions. We propose some interesting yield ratios which clearly exhibit the production relationships between different hidden-charm pentaquark states. We show how to employ a specific quark combination model to evaluate the yields of exotic $P_c^+(4380)$, $P_c^+(4450)$ and their partners on the basis of reproducing the yields of normal identified hadrons, and execute the calculations in central Pb+Pb collisions at $\sqrt{s_{NN}}= 2.76$ TeV as an example.

hep-ph↗

arXiv2016

A Loop-philic Pseudoscalar

We construct a weakly-coupled renormalizable model to explain the $750\mbox{GeV}$ diphoton excess. The $750\mbox{GeV}$ resonance (denoted as $X(750)$) is interpreted as a pseudoscalar coming from a complex singlet. The model also naturally provides a dark matter candidate. One most attractive feature of the model is that decays of $X(750)$ are all loop-induced so the diphoton rate is not diluted by unwanted tree level branching fractions. Relevant Yukawa interactions need not to be tuned to near non-perturbative region to explain the rate. The model is highly predictive, including the pseudoscalar nature of $X(750)$, and two nearly mass-degenerate exotic quarks carrying electric charge $5/3$ and $2/3$, respectively. Rich phenomenology is expected with respect to collider searches, flavor physics and dark matter detection, if $X(750)$ can be pinned down by future LHC experiments.

hep-ph↗

arXiv2015

Variable stars observed with the AST3-1 telescope from dome A of antarctica

Dome A in the Antarctic plateau is likely one of the best astronomical observing sites on Earth. The first one of three Antarctic Survey Telescope (AST3-1), a 50/68 cm Schmidt-like equatorial-mount telescope, is the first trackable telescope of China operating in Antarctica and the biggest telescope located in Antarctic inland. AST3-1 obtained huge amounts of data in 2012 and we processed the time-series parts. Here we present light curves of 29 variable stars identified from ten-day observations in 2012 with AST3-1, including 22 newly discovered variable stars. 23 of them are eclipsing binaries and the others are pulsating stars. We present the properties of the 29 variable stars, including the classifications, periods and magnitude ranges in i band. For the 17 eclipsing binaries, the phased light curves are presented with the orbital period values well determined.

astro-ph.SR↗

arXiv2015

Novel primary photoexcitations in $π$-conjugated donor-acceptor copolymers probed by transient magneto-photoinduced-absorption

The saga of the primary photoexcitations in $π$-conjugated polymers has been a source of extraordinary scientific curiosity that has lasted for more than three decades. From soliton excitations in trans-polyacetylene, to singlet and triplet excitons and polarons in other polymers, to charge transfer excitons in blends of polymers and fullerenes, the field has been rich with a variety of different photoexcitation species. Here we show the photogeneration of a novel primary intrachain photoexcitation species, namely the composite multi-exciton (CME) in $π$-conjugated donor-acceptor (DA)-copolymers used in organic photovoltaic (OPV) solar cells. We utilized the magnetic field response of the transient photoinduced absorption from sub-picosecond to millisecond to show in pristine DA-copolymer early photogeneration of the CME species that is composed of four coupled spin 1/2 particles, having unique optical and magnetic signatures. This species decomposes into two independent triplets in the microsecond time domain. Importantly in copolymer/fullerene blends the CME ionization generates photocarriers by a unique process that may enhance the photocurrent in OPV solar cells.

cond-mat.mes-hall↗

arXiv2015

Classicality and amplification in postselected weak measurement

Nearly thirty years ago the possibility of anomalous weak amplfication (AWA) was revealed by Aharonov, Albert and Vaidman [1]. Recently two papers presents two AWA schemes which are beyond the traditional proposal given by them [14, 15]. At the first glance the two papers seems very different. Ref. [14] discusses the thermal light cross-Kerr effect and finds only postselection can give the amplification effect without interference of the postselected meter states, and Ref. [15] shows that only weak interaction itself can give the amplification effect without postselection. Here the relationships between the two papers are pointed out and a generalized framework for AWA via postselecting a pair of orthogonal final states is shown.

quant-ph↗

arXiv2015

Phase diagram of the Hubbard model on the anisotropic triangular lattice

We investigate the Hubbard model on the anisotropic triangular lattice as a suggested effective description of the Mott phase in various triangular organic compounds. Employing the variational cluster approximation and the ladder dual-fermion approach as complementary methods to adequately treat the zero-temperature and the finite-temperature domains, we obtain a consistent picture of the phase diagram as a function of anisotropy and interaction strength. The metal-insulator transition substantially depends on the anisotropy, and so does the nature of magnetism and the emergence of a nonmagnetic insulating phase. We further find that geometric anisotropy significantly influences the thermodynamics of the system. For increased frustration induced by anisotropy, the entropy of the system increases with interaction strength, opening the possibility of adiabatically cooling a frustrated system by an enhancement of electronic correlations.

cond-mat.str-el↗

arXiv2015

Efficient implementation of the parquet equations -- role of the reducible vertex function and its kernel approximation

We present an efficient implementation of the parquet formalism which respects the asymptotic structure of the vertex functions at both single- and two-particle levels in momentum- and frequency-space. We identify the two-particle reducible vertex as the core function which is essential for the construction of the other vertex functions. This observation stimulates us to consider a two-level parameter-reduction for this function to simplify the solution of the parquet equations. The resulting functions, which depend on fewer arguments, are coined "kernel functions". With the use of the "kernel functions", the open boundary of various vertex functions in the Matsubara-frequency space can be faithfully satisfied. We justify our implementation by accurately reproducing the dynamical mean-field theory results from momentum-independent parquet calculations. The high-frequency asymptotics of the single-particle self-energy and the two-particle vertex are correctly reproduced, which turns out to be essential for the self-consistent determination of the parquet solutions. The current implementation is also feasible for the dynamical vertex approximation.

cond-mat.str-el↗

arXiv2015

Magnetotransport Measurements of the Surface States of Samarium Hexaboride using Corbino Structures

The recent conjecture of a topologically-protected surface state in SmB$_6$ and the verification of robust surface conduction below 4 K have prompted a large effort to understand the surface states. Conventional Hall transport measurements allow current to flow on all surfaces of a topological insulator, so such measurements are influenced by contributions from multiple surfaces of varying transport character. Instead, we study magnetotransport of SmB$_6$ using a Corbino geometry, which can directly measure the conductivity of a single, independent surface. Both (011) and (001) crystal surfaces show a strong negative magnetoresistance at all magnetic field angles measured. The (011) surface has a carrier mobility of $122\text{ cm}^2/\text{V}\cdot\text{sec}$ with a carrier density of $2.5\times10^{13} \text{ cm}^{-2}$, which are significantly smaller than indicated by Hall transport studies. This mobility value can explain a failure so far to observe Shubnikov-de Haas oscillations. Analysis of the angle-dependence of conductivity on the (011) surface suggests a combination of a field-dependent enhancement of the carrier density and a suppression of Kondo scattering from native oxide layer magnetic moments as the likely origin of the negative magnetoresistance. Our results also reveal a hysteretic behavior whose magnitude depends on the magnetic field sweep rate and temperature. Although this feature becomes smaller when the field sweep is slower, does not disappear or saturate during our slowest sweep-rate measurements, which is much slower than a typical magnetotransport trace. These observations cannot be explained by quantum interference corrections such as weak anti-localization, but are more likely due to an extrinsic magnetic effect such as the magnetocaloric effect or glassy ordering.

cond-mat.str-el↗

arXiv2015

A classical postselected weak amplification scheme via thermal light cross-Kerr effect

In common sense, postselected weak amplification must be related to destructive interference effect of the meter system, and a single photon exerts no effect on thermal field via cross-phasemodulation (XPM) interaction. In this Letter we present, for the first time, a thermal light cross-Kerr effect. Through analysis, we reveal two unexpected results: i) postselection and weak amplification can be explained at a classical level without destructive interference, and ii) weak amplification and weak value are not one thing. After postselection a new mixed light can be generated which is nonclassical. This scheme can be realized via electromagnetically-induced transparency.

quant-ph↗

arXiv2015

Nonthermally Dominated Electron Acceleration during Magnetic Reconnection in a Low-beta Plasma

By means of fully kinetic simulations, we investigate electron acceleration during magnetic reconnection in a nonrelativistic proton--electron plasma with conditions similar to solar corona and flares. We demonstrate that reconnection leads to a nonthermally dominated electron acceleration with a power-law energy distribution in the nonrelativistic low-$β$ regime but not in the high-$β$ regime, where $β$ is the ratio of the plasma thermal pressure and the magnetic pressure. The accelerated electrons contain most of the dissipated magnetic energy in the low-$β$ regime. A guiding-center current description is used to reveal the role of electron drift motions during the bulk nonthermal energization. We find that the main acceleration mechanism is a \textit{Fermi}-type acceleration accomplished by the particle curvature drift motion along the electric field induced by the reconnection outflows. Although the acceleration mechanism is similar for different plasma $β$, low-$β$ reconnection drives fast acceleration on Alfvénic timescales and develops power laws out of thermal distribution. The nonthermally dominated acceleration resulting from magnetic reconnection in low-$β$ plasma may have strong implications for the highly efficient electron acceleration in solar flares and other astrophysical systems.

astro-ph.SR↗