By assuming that only gravitation exists between dark matter (DM) and normal matter (NM), we study the effects of fermionic DM on the properties of neutron stars using the two-fluid Tolman-Oppenheimer-Volkoff formalism. It is found that the mass-radius relationship of the DM admixed neutron stars (DANSs) depends sensitively on the mass of DM candidates, the amount of DM, and interactions among DM candidates. The existence of DM in DANSs results in a spread of mass-radius relationships that cannot be interpreted with a unique equilibrium sequence. In some cases, the DM distribution can surpass the NM distribution to form DM halo. In particular, it is favorable to form an explicit DM halo, provided the repulsion of DM exists. It is interesting to find that the difference in particle number density distributions in DANSs and consequently in star radii caused by various density dependencies of nuclear symmetry energy tends to disappear as long as the repulsion of accumulated DM is sufficient. These phenomena indicate that the admixture of DM in neutron stars can significantly affect the astrophysical extraction of nuclear equation of state by virtue of neutron star measurements. In addition, the effect of the DM admixture on the star maximum mass is also investigated.Comment: to be published in Phys. Rev. C (2014

We adopt the Nambu-Jona-Lasinio (NJL) model to study the crust-core transition properties in neutron stars (NSs). For a given momentum cutoff and symmetry energy of saturation density in the NJL model, decreasing the slope of the symmetry energy gives rise to an increase in the crust-core transition density and transition pressure. Given the slope of the symmetry energy at saturation density, the transition density and corresponding transition pressure increase with increasing symmetry energy. The increasing trend between the fraction of the crustal moment of inertia and the slope of symmetry energy at saturation density indicates that a relatively large momentum cutoff of the NJL model is preferred. For a momentum cutoff of 500 MeV, the fraction of the crustal moment of inertia clearly increases with the slope of symmetry energy at saturation density. Thus, at the required fraction (7%) of the crustal moment of inertia, the NJL model with momentum cutoff of 500 MeV and a large slope of the symmetry energy of saturation density can give the upper limit of the mass of the Vela pulsar to be above 1.40 M .

Non-equilibrium molecular dynamics simulations of liquid water in picosecond high-power terahertz pulses are performed by using a non-polarizable potential model. Numerical results show that the energy absorption of water molecules exhibits a pronounced resonance with THz pulses in the frequency range of 14-17 THz. With the THz pulse at resonant frequencies, the maximum temperature is about 562K by heating the water at room temperature. Further investigation indicates that the results are independent of the size of the nanoscale water box. The efficiency of energy transfer by resonant absorption is more than seven times of microwave heating. These studies show promising applications of ultrashort THz pulses.

Using molecular dynamics simulations, we investigate systematically the water permeation properties across the single-walled carbon nanotube (SWCNT) in the presence of the terahertz electric field (TEF). With the TEF normal to the nanotube, the fracture of the hydrogen bonds results in the giant peak of net fluxes across the SWCNT with a three-fold enhancement centered around 14THz. The phenomenon is attributed to the resonant mechanisms, characterized by librational, rotational, and rotation-induced responses of in-tube polar water molecules to the TEF. For the TEF along the symmetry axis of the nanotube, the vortical modes for resonances and consequently the enhancement of net fluxes are greatly suppressed by the alignment of polar water along the symmetry axis, which characterizes the quasi one-dimensional feature of the SWCNT nicely. The resonances of water molecules in the TEF can have potential applications in the high-flux device designs used for various purposes.

Considering the importance of Lorentz invariance and chiral symmetry, we adopt the Nambu-Jona-Lasinio (NJL) model that ensures the nuclear matter saturation properties to study the density dependence of the symmetry energy. The negative symmetry energy at high densities that is usually dubbed the super-soft symmetry energy can be obtained from introducing a chiral isovector-vector interaction in the lagrangian, but should be ruled out by the neutron star (NS) stability in the mean-field approximation. It is found that the isovector-scalar interaction in the NJL model can play an important role in softening of the symmetry energy. We have investigated NS properties. The NS maximum mass obtained with various isovector-scalar couplings and momentum cutoffs is well above the 2M⊙, and the NS radius obtained well meets the limits extracted from recent measurements. In particular, the significant reduction of the canonical NS radius occurs with the moderate decrease of the slope of the symmetry energy, while the effect of the symmetry energy on the NS maximum mass remains insignificant as usual.

We study the trend of the nuclear symmetry energy in relativistic mean-field models with appearance of the hyperon and quark degrees of freedom at high densities. On the pure hadron level, we focus on the role of Λ hyperons in influencing the symmetry energy both at given fractions and at charge and chemical equilibriums. The softening of the nuclear symmetry energy is observed with the inclusion of the Λ hyperons that suppresses the nucleon fraction. In the phase with the admixture of quarks and hadrons, the equation of state is established on the Gibbs conditions. With the increase of the quark phase fraction in denser and denser matter, the apparent nuclear symmetry energy decreases till to disappear. This softening would have associations with the observations which need detailed discriminations in dense matter with the admixture of new degrees of freedom created by heavy-ion collisions.

The heating effect of terahertz pulse with various frequencies and intensities on the heavy water solution is investigated using the molecular dynamics simulation. Resonant absorptions are found for both heavy water and light water, but at a different resonant frequency which is about 16 THz for heavy water and 21 THz for light water. This resonant phenomenon can be explained perfectly by the collective rotational modes that may release water molecules from hydrogen bonding. The findings not only illustrate the heating mechanism of heavy water solution under the terahertz pulse irradiation, but also demonstrate a novel difference between light water and heavy water that could have potential applications.

The sensitive correlations between the low-density halo structure and the high-density properties of the nuclear equation of state (EOS) are constructed in light kaonic nuclei with the relativistic mean-field theory. More specifically, the 1p 1/2 halo spreads out linearly with increasing the pressure and sound velocity square at supra-normal densities and decreasing the incompressibility at saturation density. These results suggest that the novel halo in light kaonic nuclei can serve as a sensitive indicator of the nuclear EOS of symmetric matter at supra-normal densities. The experimental production and detection of the light kaonic nuclei, yet to be available, is discussed in some details at last.