Quantum phase transitions of fermionic atoms in an anisotropic triangular optical lattice

The effect of anisotropy caused by a confining potential on the properties of fermionic cold atoms in a triangular optical lattice is systematically investigated by using the dynamical cluster approximation combined with the continuous time quantum Monte–Carlo algorithm.The quantum phase diagrams which reflect the temperature–interaction relation and the competition between the anisotropic parameter and the interaction are presented with full consideration of the anisotropy of the system.Our results show that the system undergoes a transition from Fermi liquid to Mott insulator when the repulsive interaction reaches a critical value.The Kondo effect also can be observed in this system and the pseudogap is suppressed at low temperatures due to the Kondo effect.A feasible experiment protocol to observe these phenomena in an anisotropic triangular optical lattice with cold atoms is proposed,in which the hopping terms are closely related to the lattice confining potential and the atomic interaction can be adjusted via the Feshbach resonance.     

An improvement of the lattice theory of dislocation for a two-dimensional triangular crystal

The structure of dislocation in a two-dimensional triangular crystal has been studied theoretically on the basis of atomic interaction and lattice statics. The theory presented in this paper is an improvement to that published previously.Within a reasonable interaction approximation, a new dislocation equation is obtained, which remedies a fault existing in the lattice theory of dislocation. A better simplification of non-diagonal terms of the kernel is given. The solution of the new dislocation equation asymptotically becomes the same as that obtained in the elastic theory, and agrees with experimental data. It is found that the solution is formally identical with that proposed phenomenologically by Foreman et al, where the parameter can be chosen freely, but cannot uniquely determined from theory. Indeed, if the parameter in the expression of the solution is selected suitably, the expression can be well applied to describe the fine structure of the dislocation.     

Optimization on Synchrotron Radiation Lattice of BEPCII

The Beijing Electron and Positron Collider II （BEPCII） is a double ring electron positron collider, which can also be used as a synchrotron radiation （SR） light source. Since the BEPCII will start commissioning with SR mode in November 2006, it is essential to have a satisfying SR lattice. Here we first perform frequency map analysis on the original BEPCII SR lattice. The result shows that the resonance is really severe in the lattice and the transverse motion of particles is unstable. Then a new SR lattice is designed to obtain a large dynamic aperture and much more stable transverse motion. The dynamic aperture of the new lattice including nonlinear wiggler effect is calculated to be very good. This new SR lattice can be used in the commissioning stage of the BEPCII.     

Spin-valley-dependent transport and giant tunneling magnetoresistance in silicene with periodic electromagnetic modulations

The transport property of electrons tunneling through arrays of magnetic and electric barriers is studied in silicene.In the tunneling transmission spectrum, the spin-valley-dependent filtered states can be achieved in an incident energy range which can be controlled by the electric gate voltage. For the parallel magnetization configuration, the transmission is asymmetric with respect to the incident angle θ, and electrons with a very large negative incident angle can always transmit in propagating modes for one of the spin-valley filtered states under a certain electromagnetic condition. But for the antiparallel configuration, the transmission is symmetric about θ and there is no such transmission channel. The difference of the transmission between the two configurations leads to a giant tunneling magnetoresistance(TMR) effect.The TMR can reach to 100% in a certain Fermi energy interval around the electrostatic potential. This energy interval can be adjusted significantly by the magnetic field and/or electric gate voltage. The results obtained may be useful for future valleytronic and spintronic applications, as well as magnetoresistance device based on silicene.     

Negative refraction and subwavelength imaging of a photonic-crystal slab for the frequencies in the third band

Negative refraction and subwavelength imaging properties of a two-dimensional （2D） photonic crystal （PC） slab are studied by the finite-difference time-domain method. The PC consists of a triangular lattice of air holes immersed in a dielectric. For a certain frequency range in the third photonic band, the directions of the group velocities and the phase velocities can be opposite, so the PC can work as a kind of negative refractive-index material. The light radiated from a point source can form a subwavelength image spot through the PC slab. Negative refraction and an effective refractive index of the PC slab n = -1 can be achieved for the incident wave with its incident angle within a certain range.     

A Two-Dimensional Photonic Crystal Slab Mirror with Silicon on Insulator for Wavelength 1.3μm

A concrete two-dimensional photonic crystal slab with triangular lattice used as a mirror for the light at wavelength 1.3μm with a silicon-on-insulator （SOI） substrate is designed by the three-dimensional plane wave expansion method. For TE-like modes, the bandgap in the Г-K direction is from 1087nm to 1559nm. The central wavelength in the bandgap is about 1.3μm, hence the incident light at wavelength 1.3μm will be strongly reflected. Experimentally, such a photonic crystal slab is fabricated on an SOI substrate by the combination of EBL and ICP etching. The measurement of its transmission characteristics shows the bandgap edge in a longer wavelength is about 1540 nm. The little discrepancy between the experimental data and the theoretical values is mainly due to the size discrepancy of the fabricated air holes.     

Directed transport of coupled systems in symmetric periodic potentials

In this paper, we discuss the damped unidirectional motions of a coupled lattice in a periodic potential. Each particle in the lattice is subject to a time-periodic ac force. Our studies reveal that a directed transport process can be observed when the ac forces acting on the coupled lattice have a phase shift (mismatch). This directed motion is a collaboration of the coupling, the substrate potential, and the periodic force, which are all symmetric. The absence of any one of these three factors will not give rise to a directed current. We discuss the complex relations between the directed current and parameters in the system. Results in this paper can be accomplished in experiments. Moreover,our results can be generalized to the studies of directed transport processes in more complicated spatially extended systems.     

Controlling of the Polarization States of Electromagnetic Waves Using Epsilon-near-Zero Metamaterials

We demonstrate theoretically that the epsilon-near-zero materials can be utilized to control effectively the polarization conversion of an electromagnetic wave through reflection. The significant feature differing from all other means based on whatever natural materials or metamaterials is that for TM incident wave, the reflected phase is a constant, while for TE wave, the reflected phase is a linear function of the incident angle. The phase difference between them covers the range from-180° to 0°, and the polarization conversions from linear states to elliptical or circular states can be obtained by only adjusting the incident angle. Because no complex structures are employed, our proposal promises a simple approach for manipulating polarization conversion at both terahertz and optical frequencies.     

Generation of a hollow laser beam by a multimode fiber

A simple method to generate a hollow laser beam by multimode fiber is reported.A dark hollow laser beam is generated from a multimode fiber and the dependence of the output beam profile on the incident angle of laser beam is analyzed.The results show that this hollow laser beam can be used to trap and guide cold atoms.     

Diverse solid and supersolid phases of bosons in a triangular lattice

We investigate the ground state of bosons with long-range interactions in the large U limit on a triangular lattice. By mapping this system to the spin-1/2 XXZ model in a magnetic field, we can apply the spin wave theory to this study. We demonstrate how to construct the phase diagrams within the spin wave theory. The phase diagrams are given in an extensive parameter region, where, besides the superfluid phase, diverse solid and supersolid phases are shown to exist in this model. Especially, we find that the phase diagram obtained in this method is consistent with the one obtained previously using numerical techniques in the Ising limit. This confirms the effectiveness of our method. We analyze the stability of all the obtained supersolids and show that they will not be ruined by the quantum fluctuations. We observe that the quantum fluctuations in the stripe supersolid phase could be enhanced by the external field. We also discuss the relevance of our result with the experiment that may be realized with ultracold bosonic polar molecules in a triangular optical lattice.