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