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Shijun Mao

Publications and source records attributed to Shijun Mao.

35 records · Page 2Linked to original sources
arXiv2020

Magnetic field effect on pion superfluid

Magnetic field effect on pion superfluid phase transition is investigated in frame of a Pauli-Villars regularized NJL model. Instead of directly dealing with charged pion condensate, we apply the Goldstone's theorem (massless Goldstone boson $π^+$) to determine the onset of pion superfluid phase, and obtain the phase diagram in magnetic field, temperature, isospin and baryon chemical potential space. At weak magnetic field, it is analytically proved that the critical isospin chemical potential of pion superfluid phase transition is equal to the mass of $π^+$ meson in magnetic field. The pion superfluid phase is retarded to higher isospin chemical potential, and can survive at higher temperature and higher baryon chemical potential under external magnetic field.

nucl-th↗
arXiv2019

Chiral crossover characterized by Mott transition at finite temperature

We discuss the proper definition for the chiral crossover at finite temperature, based on the Goldstone's theorem. Different from the usually used maximum change of chiral condensate, we propose to define the crossover temperature by the Mott transition of pseudo-Goldstone bosons, which, by definition, guarantees the Goldstone's theorem. We analytically and numerically demonstrate this property in frame of a Pauli-Villars regularized NJL model. In external magnetic field, we find that the Mott transition temperature shows an inverse magnetic catalysis effect.

nucl-th↗
arXiv2018

Dynamically generated magnetic moment in the Wigner-function formalism

We study how the mass and magnetic moment of the quarks are dynamically generated in nonequilibrium quark matter. We derive the equal-time transport and constraint equations for the quark Wigner function in a magnetized quark model and solve them in the semi-classical expansion. The quark mass and magnetic moment are self-consistently coupled to the Wigner function and controlled by the kinetic equations. While the quark mass is dynamically generated at the classical level, the quark magnetic moment is a pure quantum effect, induced by the quark spin interaction with the external magnetic field.

hep-th↗
arXiv2018

Pions in magnetic field at finite temperature

Pions in external magnetic field are investigated in the frame of a Pauli-Villars regularized Nambu--Jona-Lasinio model. The meson propagators in terms of quark bubbles in Ritus and Schwinger schemes are analytically derived, and pion masses are numerically calculated in the Ritus scheme. For neutral and charged pions at finite temperature, there exist respectively one and three mass jumps at the corresponding Mott transition points, due to the discrete energy levels of the two constituent quarks in magnetic field.

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arXiv2017

Chiral Symmetry Restoration and Quark Deconfinement beyond Mean Field in a Magnetized PNJL Model

We study chiral symmetry restoration and quark deconfinement beyond mean field approximation in a magnetized PNJL model. The feedback from mesons to quarks modifies the quark coupling constant and Polyakov potential. As a result, the separate critical temperatures for the two phase transitions at mean field level coincide and the magnetic catalysis becomes inverse magnetic catalysis, when the meson contribution is included.

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arXiv2017

Effect of Quark Dimension Reduction on Goldstone Mode in Magnetic Field

The meson static properties are investigated in Pauli-Villars regularized Nambu--Jona-Lasinio model in strong magnetic field. The quark dimension reduction leads to not only the magnetic catalysis effect on chiral symmetry restoration but also a sudden jump of the mass of the Goldstone mode at the Mott transition temperature.

hep-ph↗
arXiv2016

From Inverse to Delayed Magnetic Catalysis in Strong Magnetic Field

We study magnetic field effect on chiral phase transition in a Nambu--Jona-Lasinio model. In comparison with mean field approximation containing quarks only, including mesons as quantum fluctuations in the model leads to a transition from inverse to delayed magnetic catalysis at finite temperature and delays the transition at finite baryon chemical potential. The location of the critical end point depends on the the magnetic field non-monotonously.

hep-th↗
arXiv2016

Inverse Magnetic Catalysis in Nambu--Jona-Lasinio Model beyond Mean Field

We study inverse magnetic catalysis in the Nambu--Jona-Lasinio model beyond mean field approximation. The feed-down from mesons to quarks is embedded in an effective coupling constant at finite temperature and magnetic field. While the magnetic catalysis is still the dominant effect at low temperature, the meson dressed quark mass drops down with increasing magnetic field at high temperature due to the dimension reduction of the Goldstone mode in the Pauli-Villars regularization scheme.

hep-ph↗
arXiv2015

Deconfinement Phase Transition with External Magnetic Field in Friedberg-Lee Model

The deconfinement phase transition with external magnetic field is investigated in the Friedberg-Lee model. In the frame of functional renormalization group, we extend the often used potential expansion method for continuous phase transitions to the first-order phase transition in the model. By solving the flow equations we find that, the magnetic field displays a catalysis effect and it becomes more difficult to break through the confinement in hot and dense medium.

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arXiv2014

Structure of Compact Stars in Pion Superfluid Phase

The gross structure of compact stars composed of pion superfluid quark matter is investigated in the frame of Nambu-Jona-Lasinio model. Under the Pauli-Villars regularization scheme, the uncertainty of the thermodynamic functions for inhomogeneous states is cured, and the LOFF state appeared in the hard cutoff scheme is removed from the phase diagram of pion superfluid. Different from the unpaired quark matter and color superconductor, the strongly coupled pion superfluid in the state close to the BCS limit is a possible candidate of compact stars with mass $M\simeq 3M_{\odot }$ and radius $R\simeq 14$ km.

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arXiv2013

BCS-BEC crossover in relativistic Fermi systems

We review the BCS-BEC crossover in relativistic Fermi systems, including the QCD matter at finite density. In the first part we study the BCS-BEC crossover in a relativistic four-fermion interaction model and show how the relativistic effect affects the BCS-BEC crossover. In the second part, we investigate both two-color QCD at finite baryon density and pion superfluid at finite isospin density, by using an effective Nambu--Jona-Lasinio model. We will show how the model describes the weakly interacting diquark and pion condensates at low density and the BEC-BCS crossover at high density.

cond-mat.quant-gas↗
arXiv2012

Meson Scattering in a Pion Superfluid

Instead of the fermion-fermion scattering which identifies the BCS-BEC crossover in cold atom systems, boson-boson scattering is measurable and characterizes the BCS-BEC crossover at quark level. We study $π$-$π$ scattering in a pion superfluid described by the Nambu--Jona-Lasinio model. We found that the scattering amplitude drops down monotonically with decreasing isospin density and finally vanishes at the boundary of the phase transition. This indicates a BCS-BEC crossover in the pion superfluid.

hep-ph↗
arXiv2010

Helical Edge Modes near Transition to Topological Insulator with Indirect Gap

Helical edge modes are characteristic of topological insulators in two dimensions. This paper demonstrates that helical edge modes remain across transitions to ordinary insulators or to semimetals under certain condition. Straight and zigzag edges are considered in a tight-binding model on square lattice. We focus on the case of indirect gap in bulk topological insulators, and obtain the spectrum of edge modes on both sides of transitions. For straight edge, the helical edge mode in topological insulators with strong particle-hole asymmetry has a reentrant region in momentum space. Edge modes show up even in ordinary insulators, but are absent in semimetals. In zigzag edge, the helical edge mode survives in both semimetals and ordinary insulators. However, the edge modes are absent inside the energy gap of ordinary insulators. All results are obtained analytically.

cond-mat.mtrl-sci↗
arXiv2010

Zigzag edge modes in Z2 topological insulator: reentrance and completely flat spectrum

The spectrum and wave function of helical edge modes in Z_2 topological insulator are derived on a square lattice using Bernevig-Hughes-Zhang (BHZ) model. The BHZ model is characterized by a "mass" term M (k) that is parameterized as M (k) = Delta - B k^2. A topological insulator realizes when the parameters Delta and B fall on the regime, either 0 < Delta /B < 4 or 4 < Delta /B < 8. At Delta /B = 4, which separates the cases of positive and negative (quantized) spin Hall conductivities, the edge modes show a corresponding change that depends on the edge geometry. In the (1,0)-edge, the spectrum of edge mode remains the same against change of Delta /B, although the main location of the mode moves from the zone center for Delta /B < 4, to the zone boundary for Delta /B > 4 of the 1D Brillouin zone. In the (1,1)-edge geometry, the group velocity at the zone center changes sign at Delta /B = 4 where the spectrum becomes independent of the momentum, i.e. flat, over the whole 1D Brillouin zone. Furthermore, for Delta/B < 1.354..., the edge mode starting from the zone center vanishes in an intermediate region of the 1D Brillouin zone, but reenters near the zone boundary, where the energy of the edge mode is marginally below the lowest bulk excitations. On the other hand, the behavior of reentrant mode in real space is indistinguishable from an ordinary edge mode.

cond-mat.mes-hall↗
arXiv2010

Analytic Theory of Edge Modes in Topological Insulators

Spectrum and wave function of gapless edge modes are derived analytically for a tight-binding model of topological insulators on square lattice. Particular attention is paid to dependence on edge geometries such as the straight (1,0) and zigzag (1,1) edges in the thermodynamic limit. The key technique is to identify operators that combine to annihilate the edge state in the effective one-dimensional (1D) model with momentum along the edge. In the (1,0) edge, the edge mode is present either around the center of 1D Brillouin zone or its boundary, depending on location of the bulk excitation gap. In the (1,1) edge, the edge mode is always present both at the center and near the boundary. Depending on system parameters, however, the mode is absent in the middle of the Brillouin zone. In this case the binding energy of the edge mode near the boundary is extremely small; about $10^{-3}$ of the overall energy scale. Origin of this minute energy scale is discussed.

cond-mat.mes-hall↗
arXiv2009

BCS-BEC Crossover and Thermodynamics in Asymmetric Nuclear Matter with Pairings in Isospin I=0 and I=1 Channels

The BCS-BEC crossover and phase diagram for asymmetric nuclear superfluid with pairings in isospin I = 0 and I = 1 channels are investigated at mean field level, by using a density dependent nucleon-nucleon potential. Induced by the in-medium nucleon mass and density dependent coupling constants, neutron-proton Cooper pairs could be in BEC state at sufficiently low density, but there is no chance for the BEC formation of neutron-neutron and proton-proton pairs at any density and asymmetry. We calculate the phase diagram in asymmetry-temperature plane for weakly interacting nuclear superfluid, and find that including the I = 1 channel changes significantly the phase structure at low temperature. There appears a new phase with both I = 0 and I = 1 pairings at low temperature and low asymmetry, and the gapless state in any phase with I = 1 pairing is washed out and all excited nucleons are fully gapped.

nucl-th↗