arXiv subjects

Mei Huang

Publications and source records attributed to Mei Huang.

At least 163 records · Page 9Linked to original sources

arXiv2008

Non-conformality and non-perfectness of fluid near phase transition

We investigate the thermodynamic and transport properties of the real scalar field theory at weak as well as strong couplings in the Hartree approximation of Cornwall-Jackiw-Tomboulis (CJT) formalism. To our surprise, we find that near phase transition, all the thermodynamic and transport properties of the simplest real scalar model at certain strong coupling agree well with the lattice results of the complex QCD system. We also demonstrate that the system near phase transition is non-conformal, and behaves as a non-perfect fluid.

hep-ph↗

arXiv2008

Phase Transitions and the Perfectness of Fluids

We calculate the ratio eta/s, the shear viscosity (eta) to entropy density (s), which characterizes how perfect a fluid is, in weakly coupled real scalar field theories with different types of phase transitions. The mean-field results of the eta/s behaviors agree with the empirical observations in atomic and molecular systems such as water, He, N, and all the matters with data available in the NIST database. These behaviors are expected to be the same in N component scalar theories with an O(N) symmetry. We speculate these eta/s behaviors are general properties of fluid shared by QCD and cold atoms. Finally, we clarify some issues regarding counterexamples of the conjectured universal eta/s bound found in Refs.[16,17].

hep-ph↗

arXiv2008

Toward a more realistic holographic QCD model

By matching the $Dp-Dq$ system in type II superstring theory with the Regge trajectories of vector and axial-vector meson spectra, a possible 5-dimension metric of a $Dp-Dq$ soft-wall model has been determined. The dependence of the metric parameters on Regge trajectory parameters has been demonstrated. It is shown that the models defined in $Dp$ brane background for $p=3$ or 4 can be consistent with experimental data.

hep-ph↗

arXiv2007

Unconventional Color Superconductor

Superfluidity or superconductivity with mismatched Fermi momenta appears in many systems such as charge neutral dense quark matter, asymmetric nuclear matter, and in imbalanced cold atomic gases. The mismatch plays the role of breaking the Cooper pairing, and the pair-breaking state cannot be properly described in the framework of standard BCS theory. I give a brief review on recent theoretical development in understanding unconventional color superconductivity, including gapless color superconductor, the chromomagnetic instabilities and the Higgs instability in the gapless phase. I also introduce a possible new framework for describing unconventional color superconductor.

hep-ph↗

arXiv2006

Inhomogeneity driven by Higgs instability in gapless superconductor

The fluctuations of the Higgs and pseudo Nambu-Goldstone fields in the 2SC phase with mismatched pairing are described in the nonlinear realization framework of the gauged Nambu--Jona-Lasinio model. In the gapless 2SC phase, not only Nambu-Goldstone currents can be spontaneously generated, but the Higgs field also exhibits instablity. The Nambu-Goldstone currents generation indicates the formation of the single plane wave LOFF state and breaks rotation symmetry, while the Higgs instability favors spatial inhomogeneity and breaks translation invariance. In this paper, we focus on the Higgs instability which has not drawn much attention yet. The Higgs instability cannot be removed without a long range force, thus it persists in the gapless superfluidity and induces phase separation. In the case of g2SC state, the Higgs instability can only be partially removed by the electric Coulomb energy. However, it is not excluded that the Higgs instability might be completely removed in the charge neutral gCFL phase by the color Coulomb energy.

hep-ph↗

arXiv2006

Higgs instability in gapless superfluidity/superconductivity

In this letter we explore the Higgs instability in the gapless superfluid/superconducting phase. This is in addition to the (chromo)magnetic instability that is related to the fluctuations of the Nambu-Goldstone bosonic fields. While the latter may induce a single-plane-wave LOFF state, the Higgs instability favors spatial inhomogeneity and cannot be removed without a long range force. In the case of the g2SC state the Higgs instability can only be partially removed by the electric Coulomb energy. But this does not exclude the possibility that it can be completely removed in other exotic states such as the gCFL state.

hep-ph↗

arXiv2006

Mismatch Induced Type Transition of a Superconductor

We use the Ginzburg-Landau theory near the transition temperature in order to examine the behavior of an inhomogeneous superconductor in the presence of a magnetic field. We find that a transition from type I to type II superconductivity occurs. Furthermore we calculate the critical value of the mismatch and estimate the contributions of the fluctuations of the condensate and the higher order terms in the Ginzburg-Landau framework.

cond-mat.supr-con↗

arXiv2006

Spontaneous Nambu-Goldstone Currents Generation Driven by Mismatch

We review recent progress of understanding and resolving instabilities driven by mismatch between the Fermi surfaces of the pairing quarks in 2-flavor color superconductor. With the increase of mismatch, the 2SC phase exhibits chromomagnetic instability as well as color neutral baryon current instability. We describe the 2SC phase in the nonlinear realization framework, and show that each instability indicates the spontaneous generation of the corresponding pseudo Nambu-Golstone current. The Nambu-Goldstone currents generation state covers the gluon phase as well as the one-plane wave LOFF state. We further point out that, when charge neutrality condition is required, there exists a narrow unstable LOFF (Us-LOFF) window, where not only off-diagonal gluons but the diagonal 8-th gluon cannot avoid the magnetic instability. In this Us-LOFF window, the diagonal magnetic instability cannot be cured by off-diagonal gluon condensate in the color superconducting phase.

hep-ph↗

arXiv2006

Spontaneous current generation in the gapless 2SC phase

It is found that, except chromomagnetic instability, the gapless 2SC phase also exhibits a paramagnetic response to the perturbation of an external color neutral baryon current. The spontaneously generated baryon current driven by the mismatch is equivalent to the one-plane wave LOFF state. We describe the 2SC phase in the nonlinear realization framework, and show that each instability indicates the spontaneous generation of the corresponding pseudo Nambu-Golstone current. We show this Nambu-Goldstone currents generation state covers the gluon phase as well as the one-plane wave LOFF state. We further point out that, when charge neutrality condition is required, there exists a narrow unstable LOFF (Us-LOFF) window, where not only off-diagonal gluons but the diagonal 8-th gluon cannot avoid the magnetic instability. We discuss that the diagonal magnetic instability in this Us-LOFF window cannot be cured by off-diagonal gluon condensate in color superconducting phase, and it will also show up in some constrained Abelian asymmetric superfluid/superconducting system.

hep-ph↗

arXiv2005

Understanding magnetic instability in gapless superconductors

Magnetic instability in gapless superconductors still remains as a puzzle. In this article, we point out that the instability might be caused by using BCS theory in mean-field approximation, where the phase fluctuation has been neglected. The mean-field BCS theory describes very well the strongly coherent or rigid superconducting state. With the increase of mismatch between the Fermi surfaces of pairing fermions, the phase fluctuation plays more and more important role, and "soften" the superconductor. The strong phase fluctuation will eventually quantum disorder the superconducting state, and turn the system into a phase-decoherent pseudogap state.

hep-ph↗

arXiv2004

Screening masses in neutral two-flavor color superconductor

The Debye and Meissner screening masses of the gluons and the photon in neutral and beta-equilibrated dense two-flavor quark matter are calculated. The results are presented in a general form that can be used in gapped as well as gapless color superconducting phases. The results for the magnetic screening masses indicate that the system develops a chromomagnetic instability. Possible consequences of the instability are discussed.

hep-ph↗

arXiv2004

Color Superconductivity at Moderate Baryon Density

This article focuses on the two-flavor color superconducting phase at moderate baryon density. In order to simultaneously investigate the chiral phase transition and the color superconducting phase transition, the Nambu-Gorkov formalism is extended to treat the quark-antiquark and diquark condensates on an equal footing. The competition between the chiral condensate and the diquark condensate is analyzed. The cold dense charge neutral two-flavor quark system is investigated in detail. Under the local charge neutrality condition, the ground state of two-flavor quark matter is sensitive to the coupling strength in the diquark channel. When the diquark coupling strength is around the value obtained from the Fierz transformation or from fitting the vacuum bayron mass, the ground state of charge neutral two-flavor quark matter is in a thermal stable gapless 2SC (g2SC) phase. The unusual properties at zero as well as nonzero temperatures and the chromomagnetic properties of the g2SC phase are reviewed. Under the global charge neutrality condition, assuming the surface tension is negligible, the mixed phase composed of the regular 2SC phase and normal quark matter is more favorable than the g2SC phase. A hybrid nonstrange neutron star is constructed.

hep-ph↗

arXiv2004

The gapless 2SC phase

Recent results on the gapless 2SC phase are reviewed. These include: the thermal stability under the constraint of the local charge neutrality condition, the properties at zero and nonzero temperatures, and the color screening properties.

hep-ph↗

arXiv2004

Chromomagnetic instability in dense quark matter

The results for the Debye and Meissner screening masses of the gluons and the photon in the case of neutral and beta-equilibrated dense two-flavor quark matter are presented. In the limits of the normal phase and the ideal two-flavor color superconducting phase, the screening masses coincide with the known results. Most interestingly, we find that the Meissner screening masses squared can be negative, indicating a plasma-type chromomagnetic instability in dense quark matter.

hep-ph↗

arXiv2003

Neutral Dense Quark Matter

The ground state of dense up and down quark matter under local and global charge neutrality conditions with $β$-equilibrium has at least four possibilities: normal, regular 2SC, gapless 2SC phases, and mixed phase composed of 2SC phase and normal components. The discussion is focused on the unusual properties of gapless 2SC phase at zero as well as at finite temperature.

hep-ph↗

arXiv2003

Two flavor color superconductivity and compact stars

Baryonic matter at high density and low temperature is a color superconductor. In real world, this state of matter may naturally appear inside compact stars. A construction of a hybrid compact star with two flavor color superconducting quark matter in its interior is presented.

hep-ph↗

arXiv2003

Gapless color superconductivity at zero and at finite temperature

We present a detailed study of the recently proposed gapless color superconductivity in neutral two-flavor quark matter in beta-equilibrium at zero as well as at finite temperature. We clarify the nature of the fermionic quasiparticles, and discuss the physical properties of this gapless phase of quark matter. The appearance of two "secondary" spin-1 condensates in the gapless ground state is pointed out. The gluon screening properties are also discussed. It is found that finite temperature properties of the gapless color superconductor are rather unusual. One of the most striking results is that the ratio of the critical temperature to the gap at zero temperature is not a universal number. Moreover, this ratio could become arbitrarily large at some values of the coupling constant. In addition, the value of the gap parameter is not a monotonic function of temperature, although the phase transition is a second order phase transition. In an extreme case, in fact, a nonzero finite temperature gap can appear even if its zero temperature value is vanishing.

hep-ph↗

arXiv2003

Gapless two-flavor color superconductor

A new, gapless two-flavor color superconducting phase that appears under conditions of local charge neutrality and $β$-equilibrium is revealed. In this phase, the symmetry of the ground state is the same as in the conventional two-flavor color superconductor. In the low-energy spectrum of this phase, however, there are only two gapped fermionic quasiparticles, and the other four quasiparticles are gapless. The origin and the basic properties of the gapless two-flavor color superconductor are discussed. This phase is a natural candidate for quark matter in cores of compact stars.

hep-ph↗