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Gang Li

Publications and source records attributed to Gang Li.

At least 667 records · Page 37Linked to original sources
arXiv2015

An observational revisit of band-split solar type-II radio bursts

Band split of solar type II radio bursts, discovered several decades ago, is a fascinating phenomenon with the type-II lanes exhibiting two almost-parallel sub-bands with similar morphology. The underlying split mechanism remains elusive. One popular interpretation is that the splitting bands are emitted from the shock upstream and downstream, respectively, with their frequency ratio (γ) determined by the shock compression ratio. This interpretation has been taken as the physical basis for many published references. Here we report an observational analysis of type II events with nice split selected from the ground-based RSTN data from 2001 to 2014, in the metric-decametric wavelength. We investigate the temporal variation and distribution of γ, and conduct correlation analyses on the deduced spectral values. It is found that γ varies in a very narrow range with >80% of γ (one-minute averaged data) being between 1.15 to 1.25. For some well-observed and long-lasting events, γ does not show a systematic variation trend within observational uncertainties, from the onset to the termination of the splits. In addition, the parameters representing the propagation speed of the radio source (presumably the coronal shock) show a very weak or basically no correlation with γ. We suggest that these results do not favor the upstreamdownstream scenario of band splits.

astro-ph.SR↗
arXiv2015

Interacting weak topological insulators and their transition to Dirac semimetal phases

Topological insulators in the presence of strong Coulomb interaction constitute novel phases of matter. Transitions between these phases can be driven by single-particle or many-body effects. On the basis of {\it ab-initio} calculations, we identify a concrete material, {\it i.e.} Ca$_{2}$PtO$_{4}$, that turns out to be a hole-doped weak topological insulator. Interestingly, the Pt-$d$ orbitals in this material are relevant for the band inversion that gives rise to the topological phase. Therefore, Coulomb interaction should be of importance in Ca$_{2}$PtO$_{4}$. To study the influence of interactions on the weak topological insulating phase, we look at a toy model corresponding to a layer-stacked 3D version of the Bernevig-Hughes-Zhang model with local interactions. For small to intermediate interaction strength, we discover novel interaction-driven topological phase transitions between the weak topological insulator and two Dirac semimetal phases. The latter correspond to gapless topological phases. For strong interactions, the system eventually becomes a Mott insulator.

cond-mat.str-el↗
arXiv2015

Weak measurement amplification in optomechanics via a squeezed coherent state pointer

We present a scheme for achieving amplification of the displacement of the mirror in optomechanical cavity using single-photon postselection where the mirror is initially prepared in squeezed coherent state. The amplification depends on the enhanced fluctuations of the squeezed coherent state, and it is is caused by the noncommutativity of quantum mechanics relying on the squeezed coherent state, which can not be explained by the standard weak measurement [1,25].

quant-ph↗
arXiv2015

Weak measurement with a coherent state pointer and its implementation in optomechanical system

Weak measurement with a coherent state pointer and in combination with an orthogonal postselection can lead to a surprising amplification effect, and we give a fire-new physical mechanism about the weak measurement in order to understand this effect. Moreover, this physical mechanism is a general result and based on it, we present a scheme of optomechanical system to implement weak measurement amplification on an orthogonal postselection.

quant-ph↗
arXiv2015

Amplification effects in optomechanics via weak measurement

We revisit the scheme of single-photon weak-coupling optomechanics using post-selection, proposed by Pepper, Ghobadi, Jeffrey, Simon and Bouwmeester [Phys. Rev. Lett. 109, 023601 (2012)], by analyzing the exact solution of the dynamical evolution. Positive and negative amplification effects of the displacement of the mirror's position can be generated when the Kerr phase is considered. This effect occurs when the post-selected state of the photon is orthogonal to the initial state, which can not be explained by the usual weak measurement results. The amplification effect can be further modulated by a phase shifter, and the maximal displacement state can appear within a short evolution time.

quant-ph↗
arXiv2015

Weak measurement combined with quantum delayed-choice experiment and implementation in optomechanical system

Weak measurement [1,19] combined with quantum delayed-choice experiment that use quantum beam splitter instead of the beam splitter give rise to a surprising amplification effect, i.e., counterintuitive negative amplification effect. We show that this effect is caused by the wave and particle behaviours of the system to be and can't be explained by a semiclassical wave theory, due to the entanglement of the system and the ancilla in quantum beam splitter. The amplification mechanism about wave-particle duality in quantum mechanics lead us to a scheme for implementation of weak measurement in optomechanical system.

quant-ph↗
arXiv2015

Well-posedness and general decay of solution for a transmission problem with viscoelastic term and delay

In this paper, we consider a transmission problem in a bounded domain with a viscoelastic term and a delay term. Under appropriate hypothesis on the relaxation function and the relationship between the weight of the damping and the weight of the delay, we prove the well-posedness result by using Faedo-Galerkin method. By introducing suitable Lyaponov functionals, we establish a general decay result, from which the exponential and polynomial types of decay are only special cases.

math.AP↗
arXiv2015

Evidence of the Solar EUV hot channel as a magnetic flux rope from remote-sensing and in-situ observations

Hot channels (HCs), high temperature erupting structures in the lower corona of the Sun, have been proposed as a proxy of magnetic flux ropes (MFRs) since their initial discovery. However, it is difficult to make definitive proof given the fact that there is no direct measurement of magnetic field in the corona. An alternative way is to use the magnetic field measurement in the solar wind from in-situ instruments. On 2012 July 12, an HC was observed prior to and during a coronal mass ejection (CME) by the AIA high-temperature images. The HC is invisible in the EUVI low-temperature images, which only show the cooler leading front (LF). However, both the LF and an ejecta can be observed in the coronagraphic images. These are consistent with the high temperature and high density of the HC and support that the ejecta is the erupted HC. In the meanwhile, the associated CME shock was identified ahead of the ejecta and the sheath through the COR2 images, and the corresponding ICME was detected by \textit{ACE}, showing the shock, sheath and magnetic cloud (MC) sequentially, which agrees with the coronagraphic observations. Further, the MC contained a low-ionization-state center and a high-ionization-state shell, consistent with the pre-existing HC observation and its growth through magnetic reconnection. All of these observations support that the MC detected near the Earth is the counterpart of the erupted HC in the corona for this event. Therefore, our study provides strong observational evidence of the HC as an MFR.

astro-ph.SR↗
arXiv2015

Physics cross sections and event generation of $e^+e^-$ annihilations at the CEPC

The cross sections of the Higgs production and the corresponding backgrounds of $e^+e^-$ annihilations at the CEPC (Circular Electron and Positron Collider) are calculated by a Monte-Carlo method, and the beamstruhlung effect at the CEPC is carefully investigated. The potential for new physics beyond the Standard Model at the CEPC is discussed.

hep-ex↗
arXiv2015

Existence, general decay and blow-up of solutions for a viscoelastic Kirchhoff equation with Balakrishnan-Taylor damping and dynamic boundary conditions

Our aim in this article is to study a nonlinear viscoelastic Kirchhoff equation with strong damping, Balakrishnan-Taylor damping, nonlinear source and dynamical boundary condition. Firstly, we prove the local existence of solutions by using the Faedo-Galerkin approximation method combined with a contraction mapping theorem. We then prove that if the initial data enter into the stable set, the solution globally exists, and if the initial data enter into the unstable set, the solution blows up in a finite time. Moreover, we obtain a general decay result of the energy, from which the usual exponential and polynomial decay rates are only special cases.

math.AP↗
arXiv2015

A compactness theorem on Branson's $Q$-curvature equation

Let $(M, g)$ be a closed Riemannian manifold of dimension $5$. Assume that $(M, g)$ is not conformally equivalent to the round sphere. If the scalar curvature $R_g\geq 0$ and the $Q$-curvature $Q_g\geq 0$ on $M$ with $Q_g(p)>0$ for some point $p\in M$, we prove that the set of metrics in the conformal class of $g$ with prescribed constant positive $Q$-curvature is compact in $C^{4, α}$ for any $0 <α< 1$. We also give some estimates for dimension $6$ and $7$.

math.DG↗
arXiv2015

Topological nature and the multiple Dirac cones hidden in Bismuth high-Tc superconductors

Recent theoretical studies employing density-functional theory have predicted BaBiO$_{3}$ (when doped with electrons) and YBiO$_{3}$ to become a topological insulator (TI) with a large topological gap (~ 0.7 eV). This, together with the natural stability against surface oxidation, makes the Bismuth-Oxide family of special interest for possible applications in quantum information and spintronics. The central question, we study here, is whether the hole-doped Bismuth Oxides, i.e. Ba$_{1-x}$K$_{x}$BiO$_{3}$ and BaPb$_{1-x}$Bi$_{x}$O$_{3}$, which are "high-Tc" bulk superconducting near 30 K, additionally display in the further vicinity of their Fermi energy $E_{F}$ a topological gap with a Dirac-type of topological surface state. Our electronic structure calculations predict the K-doped family to emerge as a TI, with a topological gap above $E_{F}$. Thus, these compounds can become superconductors with hole-doping and potential TIs with additional electron doping. Furthermore, we predict the Bismuth-Oxide family to contain an additional Dirac cone below $E_{F}$ for further hole doping, which manifests these systems to be candidates for both electron- and hole-doped topological insulators.

cond-mat.mtrl-sci↗
arXiv2015

Interference effects on Higgs mass measurement in $e^+e^-\to H(γγ) Z$ at CEPC

A high luminosity Circular Electron Positron Collider (CEPC) as a Higgs Factory will be helpful to the precision measurement of the Higgs mass. The signal-background interference effect is carefully studied for the Higgs diphoton decay mode in the associated Z boson production at the future $e^+e^-$ colliders at energy $246 {\rm GeV}$. The mass shifts go up from about $20 {\rm MeV}$ to $50 {\rm MeV}$ for the experimental mass resolution ranging from $0.8 {\rm GeV}$ to $2 {\rm GeV}$.

hep-ph↗
arXiv2015

Hidden physics in the dual-fermion approach - a special case of a non-local expansion scheme

In this work, we present a nonlocal expansion scheme to study correlated electron systems aiming at a better description of its spatial fluctuations at all length scales. Taking the nonlocal coupling as a perturbation to the local degrees of freedom, we show that the nonlocality in the self-energy function can be efficiently constructed from the coupling between local fluctuations. It can provide one unified framework to incorporate nonlocality to both ordered and disordered correlated many-body fermion systems. In this application, we prove that the dual-fermion approach can be understood as a special case of this nonlocal expansion scheme. The scheme presented in this work is constructed without introducing any dual variable, in which the interacting nature and the correlated behaviors of the lattice fermions have a clear physics correspondence. Thus, in this special case, the equivalence of the dual-fermion approach to the nonlocal expansion scheme beautifully reveals the physics origin of the dual variables. We show that the noninteracting dual-fermion Green's function corresponds exactly to a nonlocal coupling of the lattice fermion renormalized by the local single-particle charge fluctuations, and the dual-fermion self-energy behaves as the one-particle fully irreducible components of the lattice Green's function. Not only limited to this specific example, the nonlocal expansion scheme presented in this work can also be applied to other problems depending on the choice of the local degrees of freedom.

cond-mat.str-el↗
arXiv2015

Acceleration phases of a solar filament during its eruption

Filament eruptions often lead to coronal mass ejections (CMEs), which can affect critical technological systems in space and on the ground when they interact with the geo-magnetosphere in high speeds. Therefore, it is an important issue to investigate the acceleration mechanisms of CMEs in solar/space physics. Based on observations and simulations, the resistive magnetic reconnection and the ideal instability of magnetic flux rope have been proposed to accelerate CMEs. However, it remains elusive whether both of them play a comparable role during a particular eruption. It has been extremely difficult to separate their contributions as they often work in a close time sequence during one fast acceleration phase. Here we report an intriguing filament eruption event, which shows two apparently separated fast acceleration phases and provides us an excellent opportunity to address the issue. Through analyzing the correlations between velocity (acceleration) and soft (hard) X-ray profiles, we suggest that the instability and magnetic reconnection make a major contribution during the first and second fast acceleration phases, respectively. Further, we find that both processes have a comparable contribution to accelerate the filament in this event.

astro-ph.SR↗
arXiv2015

Line Strengths of Rovibrational and Rotational Transitions in the X$^2Π$ Ground State of OH

A new line list including positions and absolute intensities (in the form of Einstein $A$ values and oscillator strengths) has been produced for the OH ground X\DP\ state rovibrational (Meinel system) and pure rotational transitions. All possible transitions are included with v$\primed$ and v$\Dprimed$ up to 13, and $J$ up to between 9.5 and 59.5, depending on the band. An updated fit to determine molecular constants has been performed, which includes some new rotational data and a simultaneous fitting of all molecular constants. The absolute line intensities are based on a new dipole moment function, which is a combination of two high level ab initio calculations. The calculations show good agreement with an experimental v=1 lifetime, experimental $μ_\mathrm{v}$ values, and $Δ$v=2 line intensity ratios from an observed spectrum. To achieve this good agreement, an alteration in the method of converting matrix elements from Hund's case (b) to (a) was made. Partitions sums have been calculated using the new energy levels, for the temperature range 5-6000 K, which extends the previously available (in HITRAN) 70-3000 K range. The resulting absolute intensities have been used to calculate O abundances in the Sun, Arcturus, and two red giants in the Galactic open and globular clusters M67 and M71. Literature data based mainly on [O I] lines are available for the Sun and Arcturus, and excellent agreement is found.

astro-ph.SR↗