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We investigate the optical properties of an ultrathin film of a topological insulator in the presence of an in-plane magnetic field. We show that due to the combination of the overlap between the surface states of the two layers and the magnetic field, the optical conductivity can show strong anisotropy. This leads to the effective optical activity of the ultrathin film by influencing the circularly polarized incident light. Intriguingly, for a range of magnetic fields, the reflected and transmitted lights exhibit elliptic character. Even for certain values almost linear polarizations are obtained, indicating that the thin film can act as a polaroid in reflection. All these features are discussed in the context of the time reversal symmetry breaking as one of key ingredients for the optical activity., Comment: 7 pages, 6 figures

We investigate the spin-dependent thermoelectric effects in magnetic graphene in both diffusive and ballistic regimes. Employing the Boltzmann and Landauer formalisms we calculate the spin and charge Seebeck coefficients (thermopower) in magnetic graphene varying the spin splitting, temperature, and doping of the junction. It is found that while in normal graphene the temperature gradient drive a charge current, in the case of magnetic graphene a significant spin current is also established. In particular we show that in the undoped magnetic graphene in which different spin carriers belong to conduction and valence bands, a pure spin current is driven by the temperature gradient. In addition it is revealed that profound thermoelectric effects can be achieved at intermediate easily accessible temperatures when the thermal energy is comparable with Fermi energy $k_BT\lesssim \mu$. By further investigation of the spin-dependent Seebeck effect and a significantly large figure of merit for spin thermopower $\mathcal{Z}_{\rm sp}T$, we suggest magnetic graphene as a promising material for spin-caloritronics studies and applications., Comment: 9 pages, 8 figures

We investigate pure spin pumping in graphene by imposing a ferromagnet (F) with rotating magnetization on top of it. Using the generalized scattering approach for adiabatic spin pumping, we obtain the spin current pumped through magnetic graphene to a neighboring normal (N) region. The spin current can be easily controlled by gate voltages and under certain conditions, becomes sufficiently large to be measurable in current experimental setups. In fact it reaches a maximum value when one of the spins are completely filtered due to the vanishing density of states of the corresponding spin species in the ferromagnetic part. Considering an N|F|N structure with a finite ferromagnetic region, it is found that in contrast to the metallic ferromagnetic materials the transverse spin coherence length can be comparable to the length of F denoted by $L$. Subsequently, due to the quantum interferences, the spin current becomes oscillatory function of $JL/\hbar v_F$ in which $J$ is the spin splitting inside F. Finally we show, originated from the controllability of pumped spin into two different normal sides, a profound spin battery effect can be seen in the hybrid N|F|N device., Comment: 7 pages, 6 figures

Motivated by recent experiments revealing superconductivity in MoS$_2$, we investigate the Josephson effect in the monolayer MoS$_2$ at the presence of an exchange splitting. We show that the supercurrent reversal known as $0-\pi$ transition can occur by varying the doping via gate voltages. This is in contrast to common superconductor/ferromagnet/superconductor junctions in which successive $0-\pi$ transition take place with the variation of junction length or temperature. In fact for the case of MoS$_2$ we find that both the amplitude and the period of oscillations show a dependence on the doping which explains the predicted doping induced supercurrent reversal. These effects comes from the dependence of density and Fermi velocity on the doping strength beside the intrinsic spin splitting in the valence band which originates from spin-orbit interaction., Comment: 5 pages, 3 figures

NGC 7789 is an intermediate-age open cluster with an age that is similar to the mean age of contact binary stars. V12 is a bright UMa-type binary star with an orbital period of 0.397 day. The first complete light curves of V12 in V, R, and I bands are presented and analyzed with the W-D (Wilson and Devinney) method. The results shows that V12 is an intermediate-contact binary (f = 43%) with a mass ratio of 3.848, and it is a W-type contact binary where the less massive component is slightly hotter than the more massive one. The asymmetries of the light curves are explained by the presence of a dark spot on the more massive component. The derived orbital inclination (i = 83.6o) indicates that it is a total eclipsing binary, which suggests that the determined parameters are reliable. The orbital period may show a long-term increase at a rate of P = +2.48 x 10-6 days/year that reveals a rapid mass transfer from the less massive component to the more massive one. However, more observations are needed to confirm this conclusion. The presence of an intermediate-contact binary in an intermediate-age open cluster may suggest that some contact binaries have a very short pre-contact timescale. The presence of a third body or/and stellar collision may help to shorten the pre-contact evolution., Comment: 17 Pages

The CCD observations for the W UMa type binary systems GSC 2362-2866 and GSC 107-596 have been analyzed using the Wilson-Devinney Code to determine their photometric orbital and physical parameters. The results show that both systems may be classified as A-subtype of W-UMa eclipsing binary. The mass ratio of GSC 2362-2866 is found to be q = 0.73, with an over-contact degree of f = 0.122%. While the mass ratio of the system GSC 107-596 is found to be q = 0.70 with an over-contact degree of f = 0.166% in V_band and q = 0.69 and f = 0.111% in R band, Comment: 13 Pages

In this paper, we are going to introduce the Kottamia Astronomical Observatory, KAO, to the astronomical community. The current status of the telescope together with the available instrumentations is described. An upgrade stage including a new optical system and a computer controlling of both the telescope and dome are achieved. The specifications of a set of CCD cameras for direct imaging and spectroscopy are given. A grating spectrograph is recently gifted to KAO from Okayama Astrophysical Observatory, OAO, of the National Astronomical Observatories in Japan. This spectrograph is successfully tested and installed at the F/18 Cassegrain focus of the KAO 74" telescope., Comment: 15 Pages

We propose an effective lattice Hamiltonian for monolayer MoS$_2$ in order to describe the low-energy band structure and investigate the effect of perpendicular electric and magnetic fields on its electronic structure. We derive a tight-binding model based on the hybridization of the $d$ orbitals of molybdenum and $p$ orbitals of sulfur atoms and then introduce a modified two-band continuum model of monolayer MoS$_2$ by exploiting the quasi-degenerate partitioning method. Our theory proves that the low-energy excitations of the system are no longer massive Dirac fermions. It reveals a difference between electron and hole masses and provides trigonal warping effects. Furthermore, we predict a valley degeneracy breaking effect in the Landau levels. Besides, we also show that applying a gate voltage perpendicular to the monolayer modifies the electronic structure including the band gap and effective masses., Comment: 7 pages, 3 figures, To appear in PRB

New BVR light curves of the eclipsing binary system NSV 5904 have been constructed based on CCD observations obtained using 1.88-m telescope of Kottamia observatory during the phase of telescope testing and adjusting its optical quality on May, 2009. New times of minima and epoch have been determined from these light curves. Using the Binary Maker 3.0 (BM3) package, a preliminary determination of the photometric orbital and physical parameters of NSV 5904 are given., Comment: 10 pages, published in NRIAG Journal of Astronomy and Astrophysics, Special Issue

Charge transport through a small topological superconducting island in contact with a normal and a superconducting electrode occurs through a cycle that involves coherent oscillations of Cooper pairs and tunneling in/out the normal electrode through a Majorana bound state, the Josephson-Majorana cycle. We illustrate this mechanism by studying the current-voltage characteristics of a superconductor-topological superconductor-normal metal single-electron transistor. At low bias and temperature the Josephson-Majorana cycle is the dominant mechanism for transport. We discuss a three-terminal configuration where the non-local character of the Majorana bound states is emergent., Comment: 6 pages, 4 figures

We present a theory of non-equilibrium superconducting proximity effect in an interacting quantum dot induced by a time-dependent tunnel coupling between dot and a superconducting lead. The proximity effect, that is established when the driving frequency fulfills a gate-voltage-dependent resonance condition, can be probed through the tunneling current into a weakly-coupled normal lead. Furthermore, we propose to generate and manipulate coherent superpositions of quantum-dot states with electron numbers differing by two, by applying pulsed oscillatory variations to the couplings between dot and superconductors., Comment: 6 pages, 5 figures, paper substantially extended: new section, new figure, new references introduced

We theoretically demonstrate the capability of a ferromagnetic-normal (FN) interface in graphene to focus an electron-wave with a certain spin direction. The essential feature is the negative refraction Klein tunneling, which is spin-resolved when the exchange energy of F graphene exceeds its Fermi energy. Exploiting this property, we propose a graphene NFN electronic spin lens through which an unpolarized electronic beam can be collimated with a finite spin-polarization. Our study reveals that magnetic graphene has the potential to be the electronic counterpart of the recently discovered photonic chiral meta-materials that exhibit a negative refractive index for only one direction of the circular polarization of the photon-wave., Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. Lett

This paper is a quantitative analysis on packet switched network with a view to generalize load balancing and determination of appropriate routing algorithm in multipath environment. Several routing algorithms have been introduced for routing of packets from source to destination. Some of them route packets accurately with increased workload and some of them drastically cut down the workload. A few of them can find out a minimum workload deviation for both UDP and TCP packets. We simulated these approaches in a well defined simulator, analyzed and evaluated their performance. After expanding our analysis with varying weights and number of paths we found that the recently proposed routing algorithm Mixed Weighted Fair Routing (MWFR) outperforms the existing routing algorithms by reducing the routing and network overhead and saving the scarce bandwidth as well as CPU consumption for packet switching networks., Comment: 12 Pages, IJCNC Journal 2010

We study the Josephson effect in graphene-based ballistic superconductor-ferromagnet-superconductor (SFS) junctions. We find an oscillatory Josephson coupling $I_c R_N$ of F graphene whose amplitude is nonvanishing for a half-metallic graphene, increases for the exchange fields $h$ above the Fermi energy $E_{F}$ and shows only a slow damping at strong exchange fields $h\gg E_{F}$. We interpret this long range Josephson coupling as the result of the exchange mediated Andreev-Klein process at FS interfaces which enhances the induced antiparallel-spin superconducting correlations in F graphene by increasing $h$ above $E_{F}$. We further demonstrate the existence of regular temperature induced transitions between 0 and $\pi$ couplings in the plane of $T$ and $h$ where the phase boundaries have distinct shapes at the two regimes of $h$ below and above $E_{F}$., Comment: 4 pages, 2 figures

We investigate the Josephson effect in the graphene nanoribbons of length $L$ smaller than the superconducting coherence length and an arbitrary width $W$. We find that in contrast to an ordinary superconducting quantum point contact (SQPC) the critical supercurrent $I_c$ is not quantized for the nanoribbons with smooth and armchair edges. For a low concentration of the carriers $I_c$ decreases monotonically with lowering $W/L$ and tends to a constant minimum for a narrow nanoribbon with $W\lesssim L$. The minimum $I_c$ is zero for the smooth edges but $e\Delta_{0}/\hbar$ for the armchair edges. At higher concentrations of the carriers this monotonic variation acquires a series of peaks. Further analysis of the current-phase relation and the Josephson coupling strength $I_cR_N$ in terms of $W/L$ and the concentration of carriers revels significant differences with those of an ordinary SQPC. On the other hand for a zigzag nanoribbon we find that, similar to an ordinary SQPC, $I_c$ is quantized but to the half-integer values $(n+1/2)4e\Delta_{0}/\hbar$., Comment: 8 pages, 5 figures

We show that Andreev reflection in a junction between ferromagnetic (F) and superconducting (S) graphene regions is fundamentally different from the common FS junctions. For a weakly doped F graphene with an exchange field $h$ larger than its Fermi energy $E_{\rm F}$, Andreev reflection of massless Dirac fermions is associated with a Klein tunneling through an exchange field p-n barrier between two spin-split conduction and valence subbands. We find that this Andreev-Klein process results in an enhancement of the subgap conductance of a graphene FS junction by $h$ up to the point at which the conductance at low voltages $eV\ll \Delta$ is greater than its value for the corresponding non-ferromagnetic junction. We also demonstrate that the Andreev reflection can be of retro or specular types in both convergent and divergent ways with the reflection direction aligned, respectively, closer to and farther from the normal to the junction as compared to the incidence direction., Comment: 4 pages, 2 figures

Fermi line of bilayer graphene at zero energy is transformed into four separated points positioned trigonally at the corner of the hexagonal first Brillouin zone. We show that as a result of this trigonal splitting the minimal conductivity of an undoped bilayer graphene strip becomes anisotropic with respect to the orientation $\theta$ of the connected electrodes and finds a dependence on its length $L$ on the characteristic scale $\ell=\pi/\Delta k\simeq 50 nm$ determined by the inverse of k-space distance of two Dirac points. The minimum conductivity increases from a universal isotropic value $\sigma^{min}_{\bot}=(8/\pi)e^2/h$ for a short strip $L\ll \ell$ to a higher anisotropic value for longer strips, which in the limit of $L\gg \ell$ varies from $(7/3)\sigma^{min}_{\bot}$ at $\theta=0$ to $3\sigma^{min}_{\bot}$ over an angle range $\Delta \theta\sim \ell/L$., Comment: 4 pages, 2 figures

We study Josephson effect in a ballistic graphene strip of length $L$ smaller than the superconducting coherence length and arbitrary width $W$. We find that the dependence of the critical supercurrent $I_{c}$ on $W$ is drastically different for different types of the edges. For \textit{smooth} and \textit{armchair} edges at low concentration of the carriers $I_{c}$ decreases monotonically with decreasing $W/L$ and tends to a constant minimum for a narrow strip $W/L\lesssim1$. The minimum supercurrent is zero for smooth edges but has a finite value $e\Delta_{0}/\hbar$ for the armchair edges. At higher concentration of the carriers, in addition to this overall monotonic variation, the critical current undergoes a series of peaks with varying $W$. On the other hand in a strip with \textit{zigzag} edges the supercurrent is half-integer quantized to $(n+1/2)4e\Delta_{0}/\hbar$, showing a step-wise variation with $W$., Comment: 4 pages, 3 figures

We derive an asymptotic solution of the Einstein field equations which describes the propagation of a thin, large amplitude gravitational wave into a curved space-time. The resulting equations have the same form as the colliding plane wave equations without one of the usual constraint equations.