ALBERT

Notes: We propose an alternative approach to the 1/D-type expansions for evaluating properties of generalized Gutzwiller wave functions (GWF's) in low dimensions. Our expansion uses the sum rule and symmetries of the trial wave function explicitly. We apply the scheme to an antiferromagnetic generalization of the GWF for the 1D Hubbard model. We find good agreement with known results from variational Monte Carlo calculations and a significant improvement over previous approximations.

Notes: Superconducting YBa2Cu3O7−x films have been deposited in situ onto III-V superlattice substrates. The substrates were GaAs/AlAs superlattices grown by molecular beam epitaxy onto GaAs substrates. For 5000-A(ring) -thick YBCO films grown at 615 °C substrate temperature, we have obtained Tc of 73 K. For thinner films the Tc's are lower, indicating poor interfaces. However, the onset of the superconducting transition is 90 K in all cases. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy (TEM) show these films to be polycrystalline. TEM shows an interaction layer of about 1000 A(ring) at the interface. Low-temperature cathodoluminescence measurements of the substrate show that atomic interdiffusion has intruded at least 5000 A(ring) below the interface.

Notes: The characteristics of the formation and growth of buried oxide layers, formed by oxygen implantation into silicon at lower energies (50–140-keV 16O+), have been studied using secondary-ion mass spectrometry. Some results have been checked and compared with the results obtained by Rutherford backscattering and cross-sectional transmission electron microscopy. The critical doses, required to form a continuous buried stoichiometric oxide layer during implantation (ΦIc) and after annealing (ΦAc) have been estimated from experimental results. The thicknesses of the silicon overlayer (TASi) and buried silicon dioxide layer (TASiO2) for the annealed wafers have also been estimated. A set of semi-empirical formulas for ΦIc, ΦAc, TASi, and TASiO2 has been introduced. These formulas can be used to quickly calculate the critical doses and the layer thickness values.

Notes: Device grade 〈100〉 single crystal silicon wafers have been implanted with 40 keV oxygen ions (16O+) over the dose range of 1×1017–8×1017/cm2 at a temperature of 550±10 °C. Transmission electron microscopy, ion channeling, and secondary ion mass spectroscopy studies show that during implantation the critical dose required to form a buried oxygen-rich amorphous (SiOx, x〈2) layer is lower than 1×1017 O+/cm2. As the dose increases from 1×1017 to 4×1017/cm2 the thickness of the buried SiOx layer increases and there is a corresponding decrease in the thickness of the single crystal silicon top layer, with the oxygen concentration and residual radiation damage playing important roles in determining its position and thickness. A dose of 5×1017/cm2 results in a continuous surface amorphous layer, with a buried SiO2 sublayer being formed in the region corresponding to the implanted oxygen peak. With further increasing dose, the buried SiO2 sublayer grows primarily towards the surface. The results for the sample implanted at a dose of 1×1017/cm2 show that surface defects can be attributed to agglomeration of interstitial silicon atoms created by the internal oxidation process.

Notes: Superconductivity in an indium-diffused GaAs system has been investigated by magnetic-field-modulated microwave absorption and ac susceptibility measurements. These show existence of a superconducting phase in some of the samples. Indium was used to solder GaAs on Mo holder or deposit on the surface of GaAs, following the different heat treatments. This indium-diffused GaAs from surface or back side is most likely the origin of the superconductivity.

Notes: The effects of substrate temperature on the structural and superconducting properties of YBa2Cu3O7−x films prepared by metalorganic chemical vapor deposition have been studied in the temperature range of 550 to 690 °C. Highly c-axis oriented films with transition temperatures of 90 K and critical current densities at 77.5 K and 0 T larger than 106 A/cm2 could be produced at substrate temperatures above 650 °C. As the temperature was decreased, the amount of a-axis orientation in the film increased. Films grown at a substrate temperature of 600 °C showed a strong a-axis orientation with a transition temperature of 83.5 K and a critical current density of 4.5×105 A/cm2 at 77.5 K and 0 T.

Notes: A novel selective-area texturing process using a pulsed laser to produce a new form of texture in only the landing zone is discussed. By rotating the media under the focused pulsed-laser beam, a row of crater shaped dimples are created in the landing zone only. The rest of the disk remains smooth and untextured. Contact start-stop (CSS) test results from a lubricated C/CoCrTa/Cr media on the laser-textured substrate exhibited a constant static friction coefficient of 0.17 even after 12-K CSS cycles, with no apparent friction buildup. This unique behavior is attributed to the rounded and smooth surface topography of the laser-textured media.

Notes: This work investigates the relative role of the pure geometrical confinement and the strength of the surface effect on the dynamics of liquids in porous silica glasses prepared by the sol-gel process. The deuteron NMR spin-lattice relaxation times T1 of several molecular liquids in porous silica glasses are reported as function of pore size in the range from 18 to 143 A(ring) over the temperature range from 260 to 310 K. Molecular liquids studied include strongly interacting polar liquids such as pyridine-d5, aniline-d5, and nitrobenzene-d5, whereas the saturated cyclic hydrocarbon liquids of cyclohexane-d12 and cis-decalin-d18 represent the weakly interacting liquids. In a first approximation, toluene-d1 and dioxane-d18 are chosen as examples of liquids with intermediate interactions with the silica surface. The experimental relaxation data are analyzed by using the two-state, fast-exchange model which is found to be valid for the strongly interacting liquids and liquids with intermediate interactions. In terms of this model, the viscosity of the surface layer for pyridine-d5 is about 30 times higher than that for bulk liquid pyridine.The importance of the two-dimensional approach to describe motional dynamics of liquids confined to pores smaller than 30 A(ring) is illustrated in the case of weakly interacting liquid of cyclohexane-d12. Additional information on the relative role of surface interactions and the pure topological effects on the dynamics of liquids in confined geometries was obtained by using surface-modified glasses in which the surface hydroxyl groups were replaced by OSi(CH3)2OC2H5 groups. Indeed, the effects of surface modifications on the 2H T−11 are most pronounced for strongly interacting liquids whereas they are absent for cyclohexane. In agreement with the concept of two-dimensional behavior of liquids in small pores, one finds that the low-frequency relaxation times, namely, the spin–spin relaxation time T2, and the spin-lattice relaxation time in the rotating coordinate frame, T1ρ, remain unchanged by surface modification. In fact, this is a consequence of logarithmic enhancement of the spectral density at low frequencies so that the effect of pure geometrical confinement on the T−12 and T−11ρ relaxation rates is much larger than any relaxation rate changes arising from surface modification. Several selected NMR T1 experiments on pyridine-d5 confined to anopore and zeolites are also presented.

Notes: The possible connection between the equilibrium structure of a solution and the chemical reaction dynamics that occur in that solution has been discussed by Adelman and co-workers. In this work, we present a computational demonstration of this connection using molecular dynamics simulations and the generalized Langevin equation (GLE). A favorable example of a reaction loosely based on thermally activated Cl+Cl2→Cl2+Cl in argon solvent is used for this demonstration by (1) computing equilibrium solution structural information in terms of the Ar–Ar and Ar–Cl radial distribution functions, both from integral equations and from molecular dynamics; (2) deriving a memory function for Cl in argon solvent from the radial distribution functions and the Ar–Cl potential; and (3) using this memory function in a simple GLE to compute the dynamics of the reaction. Energy flow results both for climbing and descending the barrier are in gratifying agreement with the dynamics of the same reaction as computed by full deterministic molecular dynamics.

Notes: The microwave spectra of three isotopic Ge species of germyl azide, GeH3N3, have been assigned. The splittings due to internal rotation of the germyl group have been analyzed yielding a threefold barrier of 86.6(1) cm−1. An electric dipole moment of 2.58(2) D has been determined from the Stark effect. The following refined structural parameters have been obtained with a fixed and slightly bent azide group: r(GeN)=1.866(12) A(ring), r(GeH)=1.497(8) A(ring), α(GeNN)=115.9(14)°, α(HGeH)=112.2(10)° and a tilt of the symmetric germyl group of 4.0° away from the azide group.