Long-range order in a simple model of interacting fermions

We consider a model of spinless fermions on a lattice, interacting through a nearest neighbor repulsion. In the half-filled band case and for dimensionsd 2, we rigorously prove that there is long-range order in some domain of the parameters=(k _B T)^–1 andt/U, wheret is the hopping amplitude of the particles,U the strength of their repulsion, and the inverse temperature. Our technique is based on the usual Peierls argument of classical statistical mechanics but fails for the groundstate. We discuss the specific difficulties introduced by the Fermi statistics.Work supported in part by U.S. NSF grant PHY 90-19433-A02.     

The effect of oxidation on physical properties of porous silicon layers for optical applications

In order to understand the optical loss mechanisms in porous silicon based waveguides, structural and optical studies have been performed. Scanning and transmission electron microscopic observations of porous silicon layers are obtained before and after an oxidation process at high temperature in wet O₂. Pore size and shape of heavily p-type doped Si wafers are estimated and correlated to the optical properties of the material before and after oxidation. The refractive index was measured and compared to that determined by the Bruggeman model.     

Cell refractive index tomography by digital holographic microscopy

For what we believe to be the first time, digital holographic microscopy is applied to perform optical diffraction tomography of a pollen grain. Transmission phase images with nanometric axial accuracy are numerically reconstructed from holograms acquired for different orientations of the rotating sample; then the three-dimensional refractive index spatial distribution is computed by inverse radon transform. A precision of 0.01 for the refractive index estimation and a spatial resolution in the micrometer range are demonstrated.     

Mössbauer study of nanodimensional nickel ferrite – mechanochemical synthesis and catalytic properties

Iron–nickel spinel oxide NiFe₂O₄ nanoparticles have been prepared by the combination of chemical precipitation and subsequent mechanical milling. For comparison, their analogue obtained by thermal synthesis is also studied. Phase composition and structural properties of iron–nickel oxides are investigated by X-ray diffraction and Mössbauer spectroscopy. Their catalytic behavior in methanol decomposition to CO and methane is tested. An influence of the preparation method on the reduction and catalytic properties of iron–nickel samples is established.     

Growth and characterization of A-plane ZnO and ZnCoO based heterostructures

Non polar ZnO and (Zn, Co)O layers were successfully grown on (11̄02) sapphire (R-plane sapphire). The growth process was shown to directly influence the surface morphology as well as the strain state in (112̄0) ZnO (A-plane ZnO). The dominant defect lines seen in photoluminescence were due to basal stacking faults as demonstrated by means of selective photoluminescence and transmission electron microscopy. We present a novel method for growing high quality A-plane ZnO by inserting a (Zn, Co)O thin buffer layer, which strongly reduced the surface roughness. Finally (Zn, Mg)O/ZnO quantum well structures were grown on such a buffer layer. These quantum wells exhibited no intrinsic quantum confined Stark effect. PACS 81.05.Dz; 81.15.Hi; 78.67.Hc; 68.65.Fg