Ching Yao Fong

Scaling properties of exchange and correlation holes of the valence shell of second-row atoms

We study the exchange and correlation hole of the valence shell of second-row atoms using variational Monte Carlo techniques, especially correlated estimates, and norm-conserving pseudopotentials. The well-known scaling of the valence shell provides a tool to probe the behavior of exchange and correlation as a functional of the density and thus test models of density functional theory. The exchange hole shows an interesting competition between two scaling forms\char22{}one caused by self-interaction and another that is approximately invariant under changes in the particle number, related to the known invariance of exchange under uniform scaling to high density with the particle number held constant. The correlation hole shows a scaling trend that is marked by the finite size of the atom relative to the radius of the hole. Both trends are well captured in the main by the Perdew-Burke-Ernzerhof generalized-gradient approximation model for the exchange-correlation hole and energy.

Enhancement of ferromagnetic coupling in Mn∕GaAs digital ferromagnetic heterostructure by free-hole injection

We have studied the effect of the free-hole injection on the ferromagnetic coupling in the Mn∕GaAs digital ferromagnetic heterostructure (DFH) using ab initio electronic-structure methods. The DFH is modeled by a supercell periodically consisting of a δ-doped layer of MnAs and 15 layers of GaAs. The injection of free holes is simulated by assigning a range of missing electrons in unit cell. The δ-doped layer of Mn atoms in GaAs introduces three spin-polarized hole bands which are the consequence of hybridization between the d states of the Mn atoms and the p states of the nearest neighboring As atoms. These spin-polarized holes are confined to the vicinity of the MnAs layer. After the injection of free holes, the Fermi energy is lowered, consequently the number of spin-polarized holes in the layer of MnAs increases monotonously. Our results show the enhancement of the ferromagnetic coupling by the free-hole injection, which is in agreement with the experimental observation.

Optical reflectance measurements of Co1−xCrx

The optical reflectance of Co1−xCrx for 0.12≤x≤0.22 has been measured with photon energy between 1.5 and 4.1 eV. Light was unpolarized. For atmospheric pressure, two sets of samples have been used. Spectra were obtained under vacuum (10−3 Torr) with one set of samples. Samples are in the film form with thicknesses of the order of 1 μm. Between 1.5≤ℏω≤3.5 eV, the measured alloy spectra are similar to that for pure Co with E∥c. The reflectance decreases monotonically as the photon energy increases. This suggests that the alloying Cr to Co at x≤0.22 has not changed appreciably the electronic properties of Co within 3.0 eV of the Fermi energy EF. At higher energy the present results show a minimum in reflectance at 4.0 eV. Therefore, the alloying of Cr seems to show its effect in higher‐energy interband transitions.

A Quantum Monte Carlo Study of the Exchange-Correlation Hole in Silicon Atom and System-Averaged Correlation Holes of Second Row Atoms

The variational Monte Carlo method using the correlated estimates technique and the correlated (Slater-Jastrow) wave function to study the exchange-correlation hole, nxcin open shell atoms are discussed in detail. The spin decomposed nxc for the silicon atom and the system-averaged correlation holes for the second row atoms are presented. We find important nonlocal contributions to the opposite spin nxc. The system-averaged correlation holes show approximate scaling behavior with respect to the number of electron pairs and a correlation radius.

Electronic properties of Si-doped nipi structures in GaAs

The electronic properties of heavily and orderly Si-doped nipi structures in GaAs are studied theoretically using the abinitio self-consistent pseudopotential method within the local density approximation. Two nipi configurations are considered. Besides investigating the nature of the impurity-related band edge states the xy-planar-averaged local ionic and self-consistent potentials are also analyzed. The screening effect of the host crystal on the doping induced potential is found to be small. The effects of the doping induced electric field and the strain due to dopings are also examined.