arXiv subjects
Publications and source records attributed to Wei-Min Sun.
We address the problem of angular momentum decomposition of a free photon. We propose a natural and physical scheme for separating the total angular momentum operator for a free photon into an orbital part and a spin part with the hope that it solves this long-standing problem and it could provide a natural basis for further study of the problem of angular momentum decomposition of a gauge field system.
In this paper we study the chiral phase transition of QCD at finite temperature and density by using the rank-2 confining separable gluon propagator model in the framework of Dyson-Schwinger Equations. The critical end point is located at (T_CEP, μ_CEP) = (69 MeV, 270.3 MeV). It is also found that the first order phase transition might not end at one point, but experiences a two-phase coexisting meta-stable state. A comparison with the results in the previous literature is given.
In this paper, we generalize the improved quasi-particle model proposed in J. Cao et al., [ Phys. Lett. B {\bf711}, 65 (2012)] from finite temperature and zero chemical potential to the case of finite chemical potential and zero temperature, and calculate the equation of state (EOS) for (2+1) flavor Quantum Chromodynamics (QCD) at zero temperature and high density. We first calculate the partition function at finite temperature and chemical potential, then go to the limit $T=0$ and obtain the equation of state (EOS) for cold and dense QCD, which is important for the study of neutron stars. Furthermore, we use this EOS to calculate the quark-number density, the energy density, the quark-number susceptibility and the speed of sound at zero temperature and finite chemical potential and compare our results with the corresponding ones in the existing literature.
Based on the linear response of the fermion propagator with respect to an external field, we first derive a model-independent expression for the staggered spin susceptibility in which the influence of the full pseudoscalar vertex function is included. This expression for the staggered spin susceptibility is quite different from that given in the previous literature. The numerical values of the staggered spin susceptibility are calculated within the framework of the Dyson-Schwinger approach. Our numerical result shows that the nonperturbative dressing effects on the fermion propagator is very important when one studies the staggered spin susceptibility which corresponding to antiferromagnetic correlation in both Nambu phase and Winger phase.
The observation of $1.97\pm0.04$ solar-mass neutron-like star gives constraint on the equation of state (EOS) of cold, condensed matter. In this paper, the EOS for both pure quark star and hybrid star with a quark core described by quasi-particle model are considered. The parameters of quasi-particle model which affect the mass of both quark star and hybrid star can be constrained by the observation.
In this paper we study the Aharonov-Bohm (A-B) effect and Anderson-Higgs mechanism in Ginzburg-Landau model of superconductors from the perspective of the decomposition of U(1) gauge potential. By the Helmholtz theorem, we derive exactly the expression of the transverse gauge potential $\vec{A}_\perp$ in A-B experiment, which is gauge-invariant and physical. For the case of a bulk superconductor, we find that the gradient of the total phase field $θ$ provides the longitudinal component ${\vec A}_{\parallel}$, which reflects the Anderson-Higgs mechanism. For the case of a superconductor ring, the gradient of the longitudinal phase field $θ_1$ provides the longitudinal component ${\vec A}_{\parallel}$, while the transverse phase field $θ_2$ produces new physical effects such as the flux quantization inside a superconducting ring.
We study dynamical chiral symmetry breaking (DCSB) in an effective QED$_{3}$ theory of d-wave high temperature cuprate superconductors under a uniform magnetic field. At zero temperature, the external magnetic field induces a mixed state by generating vortices in the condensate of charged holons. The growing magnetic field suppresses the superfluid density and thus reduces the gauge field mass which is opened via the Anderson-Higgs mechanism. By numerically solving the Dyson-Schwinger gap equation, we show that the massless fermions acquires a dynamical gap through DCSB mechanism when the magnetic field strength $H$ is above a critical value $H_{c}$ and the fermion flavors $N$ is below a critical value $N_{c}$. Further, it is found that both $N_{c}$ and the dynamical fermion gap increase as the magnetic field $H$ grows. It is expected that our result can be tested in phenomena in high temperature cuprate superconductors.
In this letter, based on Kubo's formula and the QCD low energy theorem, we propose a direct formula for calculating the bulk viscosity of QCD at finite chemical potential $μ$ and zero temperature. According to this formula, the bulk viscosity at finite $μ$ is totally determined by the dressed quark propagator at finite $μ$. We then use a dynamical, confining Dyson-Schwinger equation model of QCD to calculate the bulk viscosity at finite $μ$. It is found that no sharp peak behavior of the bulk viscosity at finite $μ$ is observed, which is quite different from that of the bulk viscosity at finite temperature.
We analyze the problem of spin decomposition for an interacting system from a natural perspective of constructing angular momentum eigenstates. We split, from the total angular momentum operator, a proper part which can be separately conserved for a stationary state. This part commutes with the total Hamiltonian and thus specifies the quantum angular momentum. We first show how this can be done in a gauge-dependent way, by seeking a specific gauge in which part of the total angular momentum operator vanishes identically. We then construct a gauge-invariant operator with the desired property. Our analysis clarifies what is the most pertinent choice among the various proposals for decomposing the nucleon spin. A similar analysis is performed for extracting a proper part from the total Hamiltonian to construct energy eigenstates.
Properties of the recently proposed gauge-invariant gluon spin $S_g$ are studied and compared to the usually defined "gluon polarization" $Δg$. By explicit 1-loop calculations in a quark state, it is found that $S_g= \frac 59Δg$. Furthermore, $\frac 45$ of $S_g$ can actually be identified as a "static-field" contribution and shown to cancel exactly an analogous static term in the gluon orbital angular momentum $L_g$, leaving $S_g+L_g$ unaltered. These observations suggest that if properly identified, the gluon contribution to the nucleon spin may be drastically smaller than in the conventional wisdom.
The p-wave and $p+ip$-wave holographic superconductors with fixed DC supercurrent are studied by introducing a non-vanishing vector potential. We find that close to the critical temperature $T_c$ of zero current, the numerical results of both the p wave model and the $p+ip$ model are the same as those of Ginzburg-Landau (G-L) theory, for example, the critical current $j_c \sim (T_c-T)^{3/2}$ and the phase transition in the presence of a DC current is a first order transition. Besides the similar results between both models, the $p+ip$ superconductor shows isotropic behavior for the supercurrent, while the p-wave superconductor shows anisotropic behavior for the supercurrent.
Dynamical chiral symmetry breaking (DCSB) in QED$_{3}$ with finite gauge boson mass is studied in the framework of the rainbow approximation of Dyson-Schwinger equations. By adopting a simple gauge boson propagator ansatz at finite temperature, we first numerically solve the Dyson-Schwinger equation for the fermion self-energy to determine the chiral phase diagram of QED$_3$ with finite gauge boson mass at finite chemical potential and finite temperature, then we study the effect of the finite gauge mass on the phase diagram of QED$_3$. It is found that the gauge boson mass $m_{a}$ suppresses the occurrence of DCSB. The area of the region in the chiral phase diagram corresponding to DCSB phase decreases as the gauge boson mass $m_{a}$ increases. In particular, chiral symmetry gets restored when $m_{a}$ is above a certain critical value. In this paper, we use DCSB to describe the antiferromagnetic order and use the gauge boson mass to describe the superconducting order. Our results give qualitatively a physical picture on the competition and coexistence between antiferromagnetic order and superconducting orders in high temperature cuprate superconductors.
For quantum mechanics of a charged particle in a classical external electromagnetic field, there is an apparent puzzle that the matrix element of the canonical momentum and Hamiltonian operators is gauge dependent. A resolution to this puzzle is recently provided by us in [2]. Based on the separation of the electromagnetic potential into pure gauge and gauge invariant parts, we have proposed a new set of momentum and Hamiltonian operators which satisfy both the requirement of gauge invariance and the relevant commutation relations. In this paper we report a check for the case of the hydrogen atom problem: Starting from the Hamiltonian of the coupled electron, proton and electromagnetic field, under the infinite proton mass approximation, we derive the gauge invariant hydrogen atom Hamiltonian and verify explicitly that this Hamiltonian is different from the Dirac Hamiltonian, which is the time translation generator of the system. The gauge invariant Hamiltonian is the energy operator, whose eigenvalue is the energy of the hydrogen atom. It is generally time-dependent. In this case, one can solve the energy eigenvalue equation at any specific instant of time. It is shown that the energy eigenvalues are gauge independent, and by suitably choosing the phase factor of the time-dependent eigenfunction, one can ensure that the time-dependent eigenfunction satisfies the Dirac equation.
We derive a novel model-independent result for the pion susceptibility in QCD via the isovector-pseudoscalar vacuum polarisation. In the neighbourhood of the chiral-limit, the pion susceptibility can be expressed as a sum of two independent terms. The first expresses the pion-pole contribution. The second is identical to the vacuum chiral susceptibility, which describes the response of QCD's ground-state to a fluctuation in the current-quark mass. In this result one finds a straightforward explanation of a mismatch between extant estimates of the pion susceptibility.
We affirm that the proper momentum defined in [PRL 103:062001 (2009)] does respect exact gauge symmetry and is as measurable as the kinetic momentum. The physical part of the gauge field is also as measurable as the electromagnetic field. The Comment of Ji [arXiv:0910.5022] is due to a misunderstanding of our work, and a typical confusion of our gauge-invariant formalism with the specific Coulomb-gauge calculation.
We examine the conventional picture that gluons carry about half of the nucleon momentum in the asymptotic limit. We reveal that this large fraction is due to an unsuitable definition of the gluon momentum in an interacting theory. If defined in a gauge-invariant and consistent way, the asymptotic gluon momentum fraction is computed to be only about one fifth. This result suggests that the asymptotic limit of the nucleon spin structure should also be reexamined. Possible experimental test of our finding is discussed in terms of novel parton distribution functions.
We derive a model-independent integral formula for chiral susceptibility and attempt to present a continuum model study of it within the framework of Dyson-Schwinger Equations. An appropriate regularization is implemented to remove the temperature-independent quadratic divergence inherent in this quantity. While it demonstrates a second-order phase transition characteristic in the chiral limit, the result obtained supports a crossover at physical current quark masses, which is in good agreement with recent lattice studies.
The quark number susceptibility is expressed as an integral in terms of dressed quark propagator and dressed vector vertex. It is then investigated with the latter two- and three-point functions confronted with a Dyson-Schwinger equation model which accommodates both finite temperature and baryon chemical potential. The critical end point in the phase diagram is identified and the behavior of the quark number susceptibility around the critical end point is highlighted. The characteristic features found agree well with recent lattice QCD simulation results.