Preparation and characterization of alkaline-earth substituted superconducting La2CuO4

Metallic-oxygen deficient compounds in ternary systems MOz.sbnd;La₂O₃CuO with M = Ca, Sr, Ba have been prepared by coprecipitation and subsequent low-temperature solid-state reaction, as well as by high-temperature reaction from the corresponding oxides and carbonates. Substitution of trivalent La by divalent alka-line-earth ions results in sharp drops in resistivity in the 30 – 40 K range, first discovered for the Ba-La-Cu-O system. Room-temperature X-ray powder diffraction shows the presence of a perovskite-like layer-type phase of K₂NiF₄-type to be responsible for the high T_c super-conductivity. For single-phase samples of La₂CuO₄:M composition, the resistivity drop and especially the onset of the diamagnetic susceptibility are enhanced, and occur near the composition where an orthorhombic-to-tetragonal structural phase transition is detected by X-ray diffraction.     

The luminescence of magnesium tungstate dihydrate,MgWO4·2H2O

The luminescence of MgWO₄.2H₂O contains two emission bands, one due to the tungstate tetrahedron, and another one related to the hydrate group. The former is compared with the tungstate luminescence of anhydrous MGWO₄.     

Reactive atmosphere processing of oxides (Part II)

The application of the reactive atmosphere process (RAP) to rare-earth oxides to suppress OH⁻ impurity introduction is extended to the reduction of the higher oxide. A marked difference in the action of H₂ and CO is shown in the reduction of Tb₄O₇ to Tb₂O₃.     

Optical properties of AgGaSe2 thin films

AgGaSe₂ films were grown using the flash evaporation technique on glass substrates at various substrate temperatures. The optical absorption of these films in the energy range 1.4–1.9 eV was studied and the films were found to possess direct band gap material. The photoconductivity of the AgGaSe₂ films was studied as a function of (i) light intensity, (ii) temperature and (iii) response times. The photoconductivity response spectra of the AgGaSe₂ films were utilized to determine the band gap energies as a function of the substrate temperature. The implications are discussed.     

IR transmittance studies of hydrogen-free and hydrogenated silicon nitride and silicon oxynitride films deposited by reactive sputtering

Amorphous silicon nitride and silicon oxynitride films were deposited by reactive r.f. sputtering in various reactive ambients of nitrogen, oxygen and hydrogen of different compositions. The IR transmittance of the films was studied as a function of their hydrogen content and the oxygen:nitrogen ratio. The results were discussed in terms of compensation of gap states by hydrogenation. The microscopic structure of the silicon oxynitride films is believed to depend on the oxygen:nitrogen atomic ratio.     

Plasma-enhanced chemical vapor deposition of molybdenum

The plasma-enhanced chemical vapor deposition of molybdenum films from Ar-Mo(CO)₆ gas mixtures and H₂-Mo(CO)₆ gas mixtures was studied. Films deposited from the former mixture contain large quantities of carbon and oxygen before and after annealing, while their sheet resistance remained beyond the range of the four-point probe used here. As-deposited films from the latter mixture contain carbon, but oxygen is present apparently in the form of a hydroxide. On annealing, the resistivity reaches a minimum of 7.5 μΩ cm and oxygen is no longer present in the film.     

Phase transitions in adsorbed layers VIII: Evaluation of phase diagrams for monolayers of argon,krypton and xenon adsorbed on graphite

The theory of monolayer adsorption of simple atomic gases on crystalline solid is presented and is used to calculate the phase diagrams for noble gases adsorbed on graphite. It is shown that the third-order correction terms to the interaction energy between adsorbed atoms must be included in order to obtain results that are in agreement with experimental data. It is also demonstrated that our theory predicts the formation of a commensurate two-dimensional solid phas e for krypton monolayer films and an incommensurate two-dimensional solid phase for both argon and xenon monolayer films on graphite.     

Optical absorption and photoconductovity in amorphous indium selenide thin films

Optical absorption coefficient and photoconductivity measurements were performed on amorphous indium selenide thin films produced by vacuum evaporation. The dependence of the absorption coefficient α on the photon energy ħω at the edge of the absorption band is well described by the relation $\text{α}\text{h}\text{̵}\text{ω = B(}\text{h}\text{̵}\text{ω - E}{}_{\mathrm{<mtext>opt</mtext>}}\text{)}{}^2$ where B is a quality factor and E_opt is the optical band gap. The steady state photoconductivity as a function of the temperature and light intensity is tentatively interpreted in terms of a simple kinetic model proposed by Weiser et al.