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Register a new userFerromagnetism of two-flavor quark matter in chiral and/or color-superconducting phases at zero and finite temperatures
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We study the phase structure of the unpolarized and polarized two-flavor quark matters at zero and finite temperatures within the Nambu--Jona-Lasinio (NJL) model. We focus on the region, which includes the coexisting phase of quark-antiquark and diquark condensates. Generalizing the NJL model so as to describe the polarized quark matter, we compute the thermodynamic potential as a function of the quark chemical potential ($mu$), the temperature ($T$), and the polarization parameter. The result heavily depends on the ratio $G_D / G_S$, where $G_S$ is the quark-antiquark coupling constant and $G_D$ is the diquark coupling constant. We find that, for small $G_D / G_S$, the ferromagnetic phase is energetically favored over the paramagnetic phase. On the other hand, for large $G_D / G_S$, there appears the window in the ($mu, T$)-plane, in which the paramagnetic phase is favored.
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We exploit analytic continuation to prolongate to the region of real chemical potentials the (pseudo)critical lines of QCD with two degenerate staggered fermions at nonzero temperature and quark or isospin density obtained in the region of imaginary chemical potentials. We determine the curvatures at zero chemical potential and quantify the deviation between the cases of finite quark and of finite isospin chemical potential. In both circumstances deviations from a quadratic dependence of the pseudocritical lines on the chemical potential are clearly seen. We try different extrapolations and, for the nonzero isospin chemical potential, confront them with the results of direct Monte Carlo simulations. We also find that, as for the finite quark chemical potential, an imaginary isospin chemical potential can strengthen the transition till turning it into strong first order.
We study the effect of a large magnetic field on the chiral and diquark condensates in a regime of moderately dense quark matter. Our focus is on the inter-dependence of the two condensates through non-perturbative quark mass and strong coupling effects, which we address in a 2-flavor Nambu-Jona-Lasinio (NJL) model. For magnetic fields $eBlesssim 0.01$ GeV$^2$ (corresponding to $Blesssim 10^{18}$G), our results agree qualitatively with the zero-field study of Huang et al., who found a mixed broken phase region where the chiral and superconducting gap are both non-zero. For $eBgtrsim 0.01$ GeV$^2$ and moderate diquark-to-scalar coupling ratio $G_D/G_S$, we find that the chiral and superconducting transitions become weaker but with little change in either transition density. For large $G_D/G_S$ however, such a large magnetic field disrupts the mixed broken phase region and changes a smooth crossover found in the zero-field case to a first-order transition at neutron star interior densities.
We investigate the effect of turning on temperature for the charge neutral phase of two-flavor color superconducting (2SC) dense quark matter in presence of constant external magnetic field. Within the Nambu-Jona-Lasinio model, by tuning the diquark coupling strength, we study the interdependent evolution of the quark Bardeen-Cooper-Schrieffer gap and dynamical mass as functions of temperature and magnetic field. We find that magnetic field $B gtrsim 0.02$ GeV$^2$ ($10^{18}$ G) leads to anomalous temperature behavior of the gap in the gapless 2SC phase (moderately strong coupling), reminiscent of previous results in the literature found in the limit of weak coupling without magnetic field. The 2SC gap in the strong coupling regime is abruptly quenched at ultrahigh magnetic field due to the mismatched Fermi surfaces of up and down quarks imposed by charge neutrality and oscillation of the gap due to Landau level quantization. The dynamical quark mass also displays strong oscillation and magnetic catalysis at high magnetic field, although the latter effect is tempered by nonzero temperature. We discuss the implications for newly born compact stars with superconducting quark cores.
We study the phase structure and condensates of two-flavor QCD at finite isospin chemical potential in the framework of a confining, Dyson-Schwinger equation model. We find that the pion superfluidity phase is favored at high enough isospin chemical potential. A new gauge invariant mixed quark-gluon condensate induced by isospin chemical potential is proposed based on Operator Product Expansion. We investigate the sign and magnitude of this new condensate and show that its an important condensate in QCD sum rules at finite isospin density.
The thermodynamic potential and thermal dependence of low lying mass spectra of scalars and pseudoscalars are evaluated in a generalized Nambu -- Jona-Lasinio model, which incorporates eight-quark interactions. These are necessary to stabilize the scalar effective potential for the light and strange quark flavors, which would be otherwise unbounded from below. In addition it turns out that they are also crucial to i) lower the temperature of the chiral transition, in conformity with lattice calculations, ii) sharpen the temperature interval in which the crossover occurs, iii) or even allow for first order transitions to occur with realistic quark mass values, from certain critical values of the parameters. These are unprecedented results which cannot be obtained within the NJL approaches restricted to quartic and six-quark interactions.
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