Relativistic effect of spin and pseudospin symmetries

Dirac Hamiltonian is scaled in the atomic units $\hbar =m=1$, which allows us to take the non-relativistic limit by setting the Compton wavelength $\rightarrow 0$. The evolutions of the spin and pseudospin symmetries towards the non-relativistic limit are investigated by solving the Dirac equation with the parameter $\lambda$. With $\lambda$ transformation from the original Compton wavelength to 0, the spin splittings decrease monotonously in all spin doublets, and the pseudospin splittings increase in several pseudospin doublets, no change, or even reduce in several other pseudospin doublets. The various energy splitting behaviors of both the spin and pseudospin doublets with $\lambda$ are well explained by the perturbation calculations of Dirac Hamiltonian in the present units. It indicates that the origin of spin symmetry is entirely due to the relativistic effect, while the origin of pseudospin symmetry cannot be uniquely attributed to the relativistic effect.Comment: 15 pages, 7 figures, accepted by PR

Electronic Structure and Linear Optical Properties of Sr2_{2}CuO2_{2}Cl2_{2} Studied from the First Principles Calculation

First-principles calculations with the full-potential linearized augmented plane-wave (FP-LAPW) method have been performed to investigate detailed electronic and linear optical properties of Sr$_{2}$CuO$_{2}$Cl$_{2}$, which is a classical low-dimensional antiferromagnet (AFM) charge transfer ({\it CT}) insulator. Within the local-spin-density approximation (LSDA) plus the on-site Coulomb interaction $U$ (LADA+$U$) added on Cu 3d orbitals, our calculated band gap and spin moments are well consistent with the experimental and other theoretical values. The energy dispersion relation agrees well with the angle resolved photoemission measurements. Its linear optical properties are calculated within the electric-dipole approximation. The absorption spectrum is found to agree well with the experimental result.Comment: 5 pages, 5 figure

Thermal stress prediction for direct-chill casting of a high strength aluminum alloy.

Direct chill (D.C.) casting is one of the most important semi-continuous methods for the production of high strength aluminum alloys. The enormous unevenly cooling of ingots during the casting process can cause significant thermally induced stresses, which may result in solidification cracking. The control of the cracking during DC casting is a state-of-art technology, and many finite element models have been applied to simulate the solidification process during ingot casting. So far, most of the simulations can predict the thermal fields of the ingot accurately, but very few works can get satisfactory thermal stress profiles. One of the major difficulties is the lack of valid thermo-mechanical properties for constitutive modeling of as-cast ingots. The mechanical properties of a high strength aerospace aluminum alloy 7050 was studied in the as-cast ingot form. A thermo-elastic-plastic constitutive model was adopted to summarize the ingot strength and deformation behavior over a wide temperature range from the melting point to room temperature. In addition, the dependence of ingot properties on the casting structure as well as the cooling history at different ingot locations were determined. The cooling history of 7050 ingots can be divided into two portions at every location. The solidification rate between liquidus (635{dollar}\\sp\\circ{dollar}C/1175{dollar}\\sp\\circ{dollar}F) and solidus (524{dollar}\\sp\\circ{dollar}C/975{dollar}\\sp\\circ{dollar}F) decides the cast microstructure, which exhibits various coarse grain structures with notable dendrite segregation. After solidification, the cooling rate of solid ingots will influence the formation of the precipitation phases and their morphology. Both portions of the cooling history were considered as the parameters in the constitutive models. A finite element model (FEM) was developed to predict the thermal stress distribution in DC cast aluminum ingots by employing a commercial FEM code ABAQUS. The in-situ measured temperature profiles was input as the thermal conditions through a user subroutine, and the material constitutive model was employed in the modeling. In addition, fracture toughness of as-cast ingots was investigated experimentally through on-cooling K{dollar}\\sb{lcub}\\rm IC{rcub}{dollar} tests for material from the center and surface of Al-7050 ingot

Electron transport through Al-ZnO-Al: an {\it ab initio} calculation

The electron transport properties of ZnO nano-wires coupled by two aluminium electrodes were studied by {\it ab initio} method based on non-equilibrium Green's function approach and density functional theory. A clearly rectifying current-voltage characteristics was observed. It was found that the contact interfaces between Al-O and Al-Zn play important roles in the charge transport at low bias voltage and give very asymmetric I-V characteristics. When the bias voltage increases, the negative differential resistance occurs at negative bias voltage. The charge accumulation was calculated and its behavior was found to be well correlated with the I-V characteristics. We have also calculated the electrochemical capacitance which exhibits three plateaus at different bias voltages which may have potential device application.Comment: 10 pages, 6 figure