Thermodynamic properties and shear viscosity over entropy density ratio of nuclear fireball in a quantum-molecular dynamics model

Thermodynamic and transport properties of nuclear fireball created in the central region of heavy-ion collisions below 400 MeV/nucleon are investigated within the isospin-dependent quantum molecular dynamic (IQMD) model. These properties including the density, temperature, chemical potential, entropy density ($s$) and shear viscosity ($\eta$), are calculated by a generalized hot Thomas Fermi formulism and a parameterized function, which was developed by Danielewicz. As the collision goes on, a transient minimal $\eta/s=5/4\pi-10/4\pi$ occurs in the largest compression stage. Besides, the relationship of $\eta/s$ to temperature ($T$) in the freeze-out stage displays a local minimum which is about 9-20 times $1/4\pi$ around $T$ = 8-12 MeV, which can be argued as indicative of a liquid gas phase transition. In addition, the influences of nucleon-nucleon (NN) cross section ($\sigma_{NN}$) and symmetry energy coefficient ($C_{s}$) are also discussed, and it is found that the results are sensitive to $\sigma_{NN}$ but not to $C_{s}$.Comment: 10 pages, 13 figures; Phys. Rev. C (in press) (x-axis of Fig.1 is corrected

Statistical Analysis of Filament Features Based on the H{\alpha} Solar Images from 1988 to 2013 by Computer Automated Detection Method

We improve our filament automated detection method which was proposed in our previous works. It is then applied to process the full disk H$\alpha$ data mainly obtained by Big Bear Solar Observatory (BBSO) from 1988 to 2013, spanning nearly 3 solar cycles. The butterfly diagrams of the filaments, showing the information of the filament area, spine length, tilt angle, and the barb number, are obtained. The variations of these features with the calendar year and the latitude band are analyzed. The drift velocities of the filaments in different latitude bands are calculated and studied. We also investigate the north-south (N-S) asymmetries of the filament numbers in total and in each subclass classified according to the filament area, spine length, and tilt angle. The latitudinal distribution of the filament number is found to be bimodal. About 80% of all the filaments have tilt angles within [0{\deg}, 60{\deg}]. For the filaments within latitudes lower (higher) than 50{\deg} the northeast (northwest) direction is dominant in the northern hemisphere and the southeast (southwest) direction is dominant in the southern hemisphere. The latitudinal migrations of the filaments experience three stages with declining drift velocities in each of solar cycles 22 and 23, and it seems that the drift velocity is faster in shorter solar cycles. Most filaments in latitudes lower (higher) than 50{\deg} migrate toward the equator (polar region). The N-S asymmetry indices indicate that the southern hemisphere is the dominant hemisphere in solar cycle 22 and the northern hemisphere is the dominant one in solar cycle 23.Comment: 51 pages, 12 tables, 25 figures, accepted for publication in ApJ

Electronic Structure of Weakly Correlated Antiferromagnetic Metal SrCrO3: First-principles calculations

By systematic first-principles calculations, we study the electronic structure and magnetic property of SrCrO$_3$. Our results suggest that SrCrO$_3$ is a weakly correlated antiferromagnetic (AF) metal, a very rare situation in transition-metal oxides. Among various possible AF states, the C-type spin ordering with small amount of orbital polarization (dxy orbital is more occupied than the d_{yz/zx} orbital) is favored. The detailed mechanism to stabilize the C-type AF state is analyzed based on the competition between the itinerant Stoner instability and superexchange, and our results suggest that the magnetic instability rather than the orbital or lattice instabilities plays an important role in this system. The experimentally observed compressive tetragonal distortion can be naturally explained from the C-type AF state. By applying the LDA+$U$ method to study this system, we show that wrong ground state will be obtained if $U$ is large.Comment: 17 pages 15 figure

Shear viscosity of hot nuclear matter by the mean free path method

The shear viscosity of hot nuclear matter is investigated by using the mean free path method within the framework of IQMD model. Finite size nuclear sources at different density and temperature are initialized based on the Fermi-Dirac distribution. The results show that shear viscosity to entropy density ratio decreases with the increase of temperature and tends toward a constant value for $\rho\sim\rho_0$, which is consistent with the previous studies on nuclear matter formed during heavy-ion collisions. At $\rho\sim\frac{1}{2}\rho_0$, a minimum of $\eta/s$ is seen at around $T=10$ MeV and a maximum of the multiplicity of intermediate mass fragment ($M_{\text{IMF}}$) is also observed at the same temperature which is an indication of the liquid-gas phase transition.Comment: 5 figs and 5 pages; accepted by Physical Review

Klein tunneling and supercollimation of pseudospin-1 electromagnetic waves

Pseudospin plays a central role in many novel physical properties of graphene and other artificial systems which have pseudospins of 1/2. Here we show that in certain photonic crystals (PCs) exhibiting conical dispersions at k = 0, the eigenmodes near the "Dirac-like point" can be described by an effective spin-orbit Hamiltonian with a pseudospin of 1, treating wave propagations in the upper cone, the lower cone and a flat band (corresponding to zero refractive index) within a unified framework. The 3-component spinor gives rise to boundary conditions distinct from those of pseudospin-1/2, leading to new wave transport behaviors as manifested in Klein tunneling and supercollimation. For example, collimation can be realized more easily with pseudospin-1 than pseudospin-1/2. The special wave scattering properties of pseudospin-1 photons, coupled with the discovery that the effective photonic "potential" can be varied by a simple change of length scale, may offer new ways to control photon transport.Comment: 34 pages, 6 figure

Transition from circular-ribbon to parallel-ribbon flares associated with a bifurcated magnetic flux rope

Magnetic flux ropes play a key role in triggering solar flares in the solar atmosphere. In this paper, we investigate the evolution of active region NOAA 12268 within 36 hours from 2015 January 29 to 30, during which a flux rope was formed and three M-class and three C-class flares were triggered without coronal mass ejections. During the evolution of the active region, the flare emission seen in the H$\alpha$ and ultraviolet wavebands changed from a circular shape (plus an adjacent conjugated ribbon and a remote ribbon) to three relatively straight and parallel ribbons. Based on a series of reconstructed nonlinear force-free fields, we find sheared or twisted magnetic field lines and a large-scale quasi-separatrix layer (QSL) associated with 3D null points in a quadrupolar magnetic field. These features always existed and constantly evolved during the two days. The twist of the flux rope was gradually accumulated that eventually led to its instability. Around the flux rope, there were some topological structures, including a bald patch, a hyperbolic flux tube and a torus QSL. We discuss how the particular magnetic structure and its evolution produce the flare emission. In particular, the bifurcation of the flux rope can explain the transition of the flares from circular to parallel ribbons. We propose a two-stage evolution of the magnetic structure and its associated flares. In the first stage, sheared arcades under the dome-like large-scale QSL were gradually transformed into a flux rope through magnetic reconnection, which produced the circular ribbon flare. In the second stage, the flux rope bifurcated to form the three relatively straight and parallel flare ribbons.Comment: 31 pages, 9 figures, 2 tables; Accepted for publication in Ap

Can We Determine the Filament Chirality by the Filament Footpoint Location or the Barb-bearing?

We attempt to propose a method for automatically detecting the solar filament chirality and barb bearing. We first introduce the unweighted undirected graph concept and adopt the Dijkstra shortest-path algorithm to recognize the filament spine. Then, we use the polarity inversion line (PIL) shift method for measuring the polarities on both sides of the filament, and employ the connected components labeling method to identify the barbs and calculate the angle between each barb and the spine to determine the bearing of the barbs, i.e., left or right. We test the automatic detection method with H-alpha filtergrams from the Big Bear Solar Observatory (BBSO) H-alpha archive and magnetograms observed with the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Four filaments are automatically detected and illustrated to show the results. The barbs in different parts of a filament may have opposite bearings. The filaments in the southern hemisphere (northern hemisphere) mainly have left-bearing (right-bearing) barbs and positive (negative) magnetic helicity, respectively. The tested results demonstrate that our method is efficient and effective in detecting the bearing of filament barbs. It is demonstrated that the conventionally believed one-to-one correspondence between filament chirality and barb bearing is not valid. The correct detection of the filament axis chirality should be done by combining both imaging morphology and magnetic field observations.Comment: 20 pages, 7 figures, accepted for publication in RA

Role of short-range order in manipulating light absorption in disordered media

Structural correlations have a significant effect on light propagation in disordered media. We numerically investigate the role of short-range order in light absorption in thin films with disordered nanoholes. Two types of disordered distributions, including stealthy hyperuniform (SHU) and hard disk (HD) patterns with different degrees of short-range order, are studied. We find that Bragg scattering induced by short-range order results in the appearance of a gradually sharper absorption peak with the increasing of degrees of short-range order ($\chi$, $\phi$). A physical model is proposed to calculate the in-plane angularly differential scattering cross section $d \sigma^*/d \theta$ of thin-film nanostructures with consideration of {the} structure factor $S(q)$. Results reveal that higher level of short-range order can enhance in-plane Bragg scattering in certain wavelengths and directions corresponding to rich and sharp peaks in {the} structure factor $S(q)$, which can further modify morphology-dependent-like resonances of an individual scatterer {and leads } to {large} improvement of absorptivity in thin films. Besides, the comparison results show that SHU structures exhibit better integrated absorption ($IA$) enhancement than both HD and periodic structures. And there is a transition of local-order phase between hexagonal lattice{s} and square lattice{s for SHU structures}, leading to an optimal absorption performance when $\chi$ is around 0.5 of interest. The present study paves a way in controlling light absorption and scattering using novel disordered nanostructures.Comment: 10 pages, 12 figure

Dependence of the Length of Solar Filament Threads on the Magnetic Configuration

High-resolution H$\alpha$ observations indicate that filaments consist of an assembly of thin threads. In quiescent filaments, the threads are generally short, whereas in active region filaments, the threads are generally long. In order to explain these observational features, we performed one-dimensional radiative hydrodynamic simulations of filament formation along a dipped magnetic flux tube in the framework of the chromospheric evaporation-coronal condensation model. The geometry of a dipped magnetic flux tube is characterized by three parameters, i.e., the depth ($D$), the half-width ($w$), and the altitude ($h$) of the magnetic dip. The parameter survey in the numerical simulations shows that allowing the filament thread to grow in 5 days, the maximum length ($L_{th}$) of the filament thread increases linearly with $w$, and decreases linearly with $D$ and $h$. The dependence is fitted into a linear function $L_{th}=0.84w-0.88D-2.78h+17.31$ Mm. Such a relation can qualitatively explain why quiescent filaments have shorter threads and active region filaments have longer threads.Comment: 8 pages, 6 figures, accepted for publication in RA

Pygmy and Giant Dipole Resonances by Coulomb Excitation using a Quantum Molecular Dynamics model

Pygmy and Giant Dipole Resonance (PDR and GDR) in Ni isotopes have been investigated by Coulomb excitation in the framework of the Isospin-dependent Quantum Molecular Dynamics model (IQMD). The spectra of $\gamma$ rays are calculated and the peak energy, the strength and Full Width at Half Maximum (FWHM) of GDR and PDR have been extracted. Their sensitivities to nuclear equation of state, especially to its symmetry energy term are also explored. By a comparison with the other mean-field calculations, we obtain the reasonable values for symmetry energy and its slope parameter at saturation, which gives an important constrain for IQMD model. In addition, we also studied the neutron excess dependence of GDR and PDR parameters for Ni isotopes and found that the energy-weighted sum rule (EWSR) $PDR_{m_1}/GDR_{m_1}$ increases linearly with the neutron excess.Comment: 8 pages, 12 figure