We present a local density functional calculation of the effect of an external electric field on the work function change of Pd and Ag adsorption on an Al(001) surface. The adsorption of K has also been considered for comparison. We found that the work functions for all the systems increased linearly when the strength of the external electric field was increased. Since the polarized dlectrons at the interstitial regions between the adsorbate and substrate for Pd/Al(001) and Ag/Al(001) react to the external electric field differently, the subtle differences between Pd/Al(001 and Ag/Al(001) bondings has been characterized through the comparison of the slopes of the work function change versus electric field.

The trends of the conduction band minima with the alloy compositions for the pseudomorphic Si1-x-yGexCy alloys grown on (110) Si and Ge substrates are investigated theoretically with the use of the ab initio pseudopotential method and the virtualcrystal approximation. It is found that the minimum energy gaps are all indirect and not monotonically dependent on the compositions in some cases. In the case of the tensile and compressive strains, the minimum energy gap decreases and increases, respectively, as the C fraction increases and the Ge fraction is constant. Our result is in good agreement with other theoretical results for the Si12xGex system.

Using density functional calculations, we found that the ground state configuration of some bcc (001) surfaces are field dependent because different configurations experience different Stark shifts of surface states. When the W (001) surface is positively biased, an external electric field stabilizes a cs2 3 2d array of surface vacancies, so that the ground state is no longer the well-established "Debe-King" model. Similar behavior is found for Mo (001), but not for Nb (001). The field-induced change of ground state provides a natural explanation for the anomalous behavior of W (001) inside field ion microscopes.

We present a theoretical study of the valence-band offsets (VBOs) of the pseudomorphic Si1-x-yGexCy/Si(001) heterojunctions. Our calculation is based on the ab initio pseudopotential method, the virtual-crystal approximation, and the average-bond-energy theory. It is found that the VBO is sensitive to the strain condition. Initial addition of carbon increases the VBO of the relaxed Si12x2yGexCy/Si(001) heterostructures. For constant Ge fraction, the VBO decreases and increases with increasing the C fraction in the case of the compressive and tensile strains, respectively. The trends of our results are in agreement with those of other theoretical and experimental results.

The ab initio pseudopotential method within the local density functional theory and virtualcrystal approximation is used to study the band gap of the Si1−x−yGexCy (y 0.09) alloys on a Ge (001) substrate. The heterojunction discontinuities are also investigated in the framework of the average bond energy theory in conjunction with the deformation potential method. The calculated results show that the energy gap still remains indirect and only a small amount of C could cause the energy gap to be shrunk signi cantly. The top of the valence bands of the strained Si1−x−yGexCy alloys on Ge (001) is signi cantly lifted and even could be greatly higher than that of Ge by the addition of small amounts of carbon. The trends of our results are consistent with other theoretical data.

We demonstrate that the propagation of a light wave in an elliptic gradient-index medium can be regarded as a joint fractional Fourier transform. The distinctive features of the Wigner distributon function and the recurrence of the original function are analyzed qualitatively. q1998 Elsevier Science B.V. All rights reserved.

In this paper the Laguerre-Gaussian (LG) series representation of the twodimensional fractional Fourier transform is derived from conventional ordinary Fourier transform in polar coordinates. The kernel of this series representation is constituted by Laguerre-Gaussian functions, from which the series representation of a fractional Hankel transform can be easily derived. The connection between the gradient-index medium and the LG series representation is illustrated as an example of its applications.

We present a careful study of the energetics of vacancy and substitutional impurities in aluminum in both the bulk and small cluster environments. The calculations are done within the framework of the local-densityfunctional formalism and are based on the pseudopotential method with plane-wave expansion and periodic boundary conditions. Both the ionic and electronic degrees of freedom are fully relaxed. The electronic structure problem is treated with a preconditioned conjugate-gradient method that applies equally well to insulators and metals, and is suitable for parallel computing. We have considered up to 216 atoms in the supercell, and we show that reliable results can be obtained with 108-atom cells with proper k-point sampling. Vacancy-formation energy, heats of solution of the impurities and the relaxations near the defects are in good agreement with available experimental data. The energetics of substitution in small clusters was found to be rather different from bulk.

A technique based on the path-integral effective-potential theory to calculate the thermodynamic properties of a linear chain of atoms is extended to include all-neighbor interactions. The configuration averages are performed using the classical Monte Carlo technique. The theory is applied to a system interacting with a Mie-Lennard-Jones potential to make contact with earlier work. Results for the internal energy and specific heat are presented and compared with those using only nearest-neighbor interactions and the quantum Monte Carlo method. This is a useful step toward the application of the effective-potential theory to realistic systems with long-range interactions.

The detailed electronic structures and geometries of Si(100) reconstructed surfaces are investigated by a self-consistent calculation with a norm-conserving nonlocal pseudopotential. We concentrate on the dimer model and on (2