A study of ion-core potentials used in low-energy electron diffraction calculations

Various models for the construction of ion-core potentials presently used in LEED intensity spectra calculations are tested for nickel. We find that self-consistent iterations of these ion-core potentials do not produce significant improvement over the simple linear superposition scheme in building LEED potentials. For the exchange part of the potential, the non-local Hartree-Fock scheme works poorly compared to the Slater approximation. The better results of the Slater exchange are attributed to its compensating for the omission of correlation effects from the Hartree-Fock formula.     

XANES: Determination of bond angles and multi-atom correlations in order and disordered systems

In contrast to conventional single-scattering probes, X-ray absorption near edge structure (XANES) is sensitive, through multiple scattering of the excited electron, not only to pairwise atomic correlations but also to higher order correlations, including bond angles. We present a flexible theory, based on a full multiple scattering cluster calculation, which can be used to extract such information in ordered, disordered and other complex systems now being studied using synchrotron radiation.     

Perfect corner reflector

We numerically investigate the electromagnetic field radiated by a line source within a perfect lens [Phys. Rev. Lett. 85, 3966 (2000)] consisting of two orthogonal planes delimiting positive and negative index media. Use of a coordinate transformation [J. Phys. Condens Matter 15, 6345 (2003)] together with a well-adapted transfer-matrix method permits rigorous calculation of the vector field. We find that two negative corners combine to make a cavity that traps light along closed trajectories. Finally, we numerically show that the field presents some spatial oscillations with a period that is proportional to absorption sigma inside the negative materials as 1/ln sigma and that it is associated with an infinite density of states when sigma tends toward 0.     

Optical design of reflectionless complex media by finite embedded coordinate transformations

Transformation optics offers an unconventional approach to the control of electromagnetic fields. The transformation optical structures proposed to date, such as electromagnetic "invisibility" cloaks and concentrators, are inherently reflectionless and leave the transmitted wave undisturbed. Here, we expand the class of transformation optical structures by introducing finite, embedded coordinate transformations, which allow the electromagnetic waves to be steered or focused. We apply the method to the design of several devices, including a parallel beam shifter and a beam splitter, both of which are reflectionless and exhibit unusual electromagnetic behavior as confirmed by 2D full-wave simulations.