D. Y. Xing

Band structure of MgB 2 with different lattice constants

We report a detailed study of the electronic structure of the MgB 2 with different lattice constants by using the full-potential linearized augmented plane-wave method. It is found that the lattice parameters have great effect on the σ band of boron. Our results indicate that increasing the lattice constant along the c axis will increase the density of states at the Fermi level, shift the σ band upward, and increase the hole number in the (r band. So, the superconducting transition temperature T c will be raised correspondingly. Changing the lattice constant along a axis has the opposite effect to that of the c axis. Our result is in agreement with the experiment. A possible way of searching for higher T c superconductor has been indicated, i.e., making MgB 2 to have longer c axis and shorter a,b axis by doping.

Thermal escape from a metastable state in periodically driven Josephson junctions.

Resonant activation and noise-enhanced stability were observed in an underdamped real physical system, i.e., Josephson tunnel junctions. With a weak sinusoidal driving force applied, the thermal activated escape from a potential well underwent resonancelike behaviors as a function of the driving frequency. The resonances also crucially depended on the initial condition of the system. Numerical simulations showed good agreement with the experimental results.

Switching effect in spin field-effect transistors

We study how the conductance of a spin field-effect transistor (SFET) is manipulated by spin-orbit coupling strength, interfacial barrier height, and spin polarization in source and drain. It is shown that the conductance of the SFET exhibits an excellent switching characteristic for high potential barriers. By tuning the split-gate voltage one can vary the Dresselhaus [Phys. Rev. 100, 580 (1955)] spin-orbit coupling strength so as to switch the SFET on or off. On the other hand, in the SFET with almost Ohmic-contact interfaces there is pronounced conductance modulation mainly due to the Rashba [Sov. Phys. Solid State 2, 1190 (1960)] and Dresselhaus spin precession.

Defect Studies in a One-Dimensional Photonic Band Gap Structure

We calculate defect modes in a finite one-dimensional quarter-wavelength stack by the transfer matrix technique, and find that the defect mode frequency depends on the refractive index as well as on the thickness of the defect layer and the transmission coefficient depends on its position. Then, by using the extended-beam-propagation method, we investigate the short pulse transmission. It is found that the amplitude of the electromagnetic field in the defect layer is much larger than that of the incident light, which can be used to enhance nonlinear effects if the defect layer is replaced by a nonlinear material.

A spin injector

We theoretically put forward a spin injector, which consists of a three-terminal ferromagnetic-metal (FM) nonmagnetic-semiconductor (NS)-superconductor (SC) mesoscopic hybrid system. This device can inject not only the spin-up current but also the pure spin current into the NS lead. The crossed Andreev reflection plays a key role in this device. Such a spin injector may be realized within the reach of the present-day technology.

Dynamical phase transition of a driven vortex lattice with disordered pinning

We have numerically solved the overdamped equation of vortex motion in a two-dimensional driven vortex lattice with disordered pinning, in which the driving Lorentz force, the pinning force due to point defects, the intervortex interacting force, and the thermal fluctuation force are taken into account. It is found that the vortex density and pinning strength are two important factors of affecting the melting transition of a vortex lattice. At low magnetic fields, there exist hysteresis loops of the average vortex velocity and the average pinning force vs. the driving force, from which the feature of a first-order melting transition of the vortex motion can be clearly seen. As the magnetic field is increased beyond a critical value, the hysteresis loops disappear and the melting transition is replaced by a second-order glass transition. We have also studied the influence of intervortex interactions on the vortex melting transition by comparing several forms of repulsive forces between the vortices.

Spin-polarized intergrain tunneling model for low-field magnetoresistance in polycrystalline manganites

Based on the spin-polarized intergrain tunneling, the origin of temperature-dependent low-field magnetoresistance (MR) is presented for the polycrystalline manganites. In consideration of the spin-flip intergrain tunneling arising from the Mn ion impurities in the grain-boundary regions, it is shown that the variation of the electronic spin polarization and the inelastic intergrain tunneling induced by the collective excitations of local spins at the grain boundaries are simultaneously responsible for the rapid decay of the low-field MR ratio with increasing temperature. The theoretical results are in agreement with the experimental data of the polycrystalline manganites.

A new kind of nonlinear optical material: the fullerene tube

The third-order nonlinear optical polarizability of -derived fullerene tubes has been studied theoretically. The results indicate that the carbon atom number, symmetry and cap have great effects on the nonlinear optical properties of fullerene tubes. When a fullerene tube has a high carbon atom number the major dynamical response peaks in its spectrum concentrate on a narrow region with lower energy and larger values. In the case with similar atom numbers, the tube without a cap and an armchair tube have larger values than the tube with a cap and a zigzag tube, respectively. In addition, we point out that the tube may be very important in optical device applications because it may have a larger value than a pure carbon nanotube and its value may be controlled by its composition.

Extended Haldane's model and its simulation with ultracold atoms

Haldanes model is extended to a square lattice related close to the so-called d+id state, in which the on-site energy is staggered and the next-nearest-neighbor hopping is anisotropic. From the phase diagram obtained, two types of phases are found, i.e., the normal insulator with Chern number C=0 and the Hall insulator with C=±1. We propose a way of simulating this model with cold atoms in an optical lattice. By measuring the atomic density profile, one can detect this phase diagram.

Theoretical study on the spin-state transition in doped La2-xSrxCoO4

The spin-state transition is an interesting and unresolved problem in layered perovskite La2-xSrxCoO4 under doping, as it involves competition among different magnetic structures. Within the unrestricted Hartree-Fock approximation and using the real space recursion method we have studied the effect of Sr doping on the magnetic and electronic properties of layered perovskite La2CoO4. All configurations in an enlarged double cell among the low-spin (t2g6eg1) and the high-spin (t2g5eg2) states are considered. It is found that the ground state of a doped system takes the antiferromagnetic high-spin state (t2g5-xeg2) for 0<x<0.39, the ferromagnetic high-spin state (t2g5eg2-x) for 0.39≤x<0.52, followed by the ferromagnetic high-spin-low-spin ordered state (t2g5-xeg2-t2g6eg1-x) for 0.52≤x<1.1. The two spin-state transitions in the doping range (0<x<1.1) we studied are in agreement with experimental observations.