Unoccupied Region Research Articles

Inverse-photoemission measurement of iron oxides on polycrystalline Fe.

We measured the inverse-photoemission spectra (IPES) of iron oxides grown on an Fe substrate and calculated the band structure of paramagnetic FeO to analyze the IPES. The band-structure calculation showed some overlap between Fe 3d states and O 2p states in the region of occupied states, and s,p-like states of Fe in the unoccupied region. Isochromatic IPES showed two structures at \ensuremath{\sim}2.5 and \ensuremath{\sim}7.5 eV above the Fermi energy. We estimated from the IPES data a full band gap of 2.5 eV for FeO.

Comparative inverse photoemission study of high-Tc superconductors

Inverse photoemission spectroscopy has been performed on LaSrCuO, YBaCuO, and BiSrCaCuO to study the electronic structure of unoccupied region. It was found that the spectrum of Bi2Sr2CaCu2O8 has a clear Fermi edge, thereby giving a direct evidence for the existence of a Fermi surface (Fermi-liquid states), while those of YBa2 Cu3O7 and La1·85Sr0·15CuO4 exhibit little intensity at the Fermi level. A remarkable and prompt reduction of the Fermi-edge intensity was observed for the La-system. The reduction and absence of the Fermi-edge structure is ascribed to the surface degradation probably due to desorption of oxygen from the surface.

Fractal structures from an evaporation/condensation model

Several simple evaporation/condensation models in which all occupied surface sites have the same evaporation probability and condensation occurs on contact with the first occupied site have been investigated using Monte Carlo simulations. In two dimensional simulations these models generate a ‘‘cloud’’ of clusters which at long times are characterized by a fractal-like geometry and an exponential cluster size distribution. The nature of the systems at long times depends on the type of trajectories which the particles follow. Brownian trajectories lead to a cloud of clusters with a fractal dimensionality close to 1.0 (in 2D simulations), whereas linear trajectories generate structures with a fractal dimensionality close to 4/3. In similar one dimensional simulations, there is no evidence for the formation of fractal-like structures at any stage in the evolution of the systems and the equilibrium state consists of a dense region and a completely unoccupied region. The results of three dimensional simulations are inconclusive.

Vacancies and the energy spectrum of refractory metal compounds: TiC and TiO

The influence of structural vacancies in metal and non-metal sublattices on the electronic structure of titanium carbide and oxide is studied by the nonempirical Hartree-Fock-Slater method in the cluster approximation. The main valence band changes for non-stoichiometric compounds are connected with the narrowing of bands due to subtraction of a number of electronic states and an increase of density of states near the Fermi level. Vacancy states appear to be localized in the unoccupied region of the energy spectrum; their admixture in the valence band is very small. The valency of metal atoms is shown to vary continuously in accordance with the stoichiometry ratio. The vacancy charge is very much smaller than the formal ionicity of the atom removed. It is shown that in defect compounds there is essentially no bonding charge connected with the vacancy center. The results obtained are compared with the data of previous calculations and physico-chemical properties.

Germinal Basis of Thyroxin and Female Hormone Effect on Barb Origin in Saddle Feathers

The subcutaneous injection of thyroxin or female hormone into the Brown Leghorn male or capon brings about modifications in saddle feathers which, on the basis of c-isochrone relations, signify the origin and growth of definitive barb primordia in a previously undifferentiated ventral field of the germ (Fraps and Juhn, Fraps). With sufficient concentration of thyroxin, a greater or lesser number of barb apices appear to arise practically simultaneously. Female hormone brings about only slight changes of a similar nature in breast feathers, suggesting that the ventral regions of these germs normally carry a full complement of barb primordia. The only direct observation bearing on this indicated difference in germinal organization was made by Lillie and Juhn, who noted that the unoccupied ventral region of saddle feather germs appeared to be considerably wider than the comparable region in breast germs; Lillie and Juhn, however, attributed this difference largely to the lowness of ridges in saddle germs. A comparison of breast and saddle feather germs prepared before and following injection of thyroxin has therefore been made with a view toward establishing the embryonic basis of the ventral differentiations indicated by c-isochrone analyses of regenerated feathers. Figs. 1-5 represent, rather schematically, the ventral regions of regenerating germs split along their dorsal faces and spread into the plane of the paper. Apical levels are cut away (excepting in Fig. 1), as are also the undifferentiated germ bases. The sloping lines indicate barbs and barb primordia visible in the several preparations. Increasing diameters of barb bases with increasing distances of bases from sites of origin are not shown in the figures, nor are differences in slopes of barb primordia in different germs or regions. The essential relations illustrated by the several figures follow.