Materials Literature
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Needle crystal growth in the presence of kinetic effects and anisotropic surface energy is studied. Within the boundary layer model it is shown that at the dimensionless undercooling Δ > 1, instead of a parabolic dendrite, an “angular” dendrite, having planar sections far from the tip, grows. At Δ c > 1 needle crystal…
Electrical transport properties of a sintered yttrium aluminum garnet (YAG) doped with Ca or Mg have been investigated as a function of temperature and partial pressure of oxygen. Conductivity and ionic transport measurements show Ca- and Mg-doped YAG to be mixed ionic-electronic p-type conductors exhibiting an activa…
Room-temperature photoluminescence (PL) and electrolyte electroluminescence (EL) spectra of polycrystalline CdS samples, in air and in contact with aqueous electrolytes, have been studied as a function of the atmosphere and temperature of annealing, light excitation intensity and wavelength, applied bias, electrolyte…
Optical absorption (\ensuremath{\sim}1--5 eV) in rf-deposited hydrogenated (20--40 at. %) amorphous ``diamondlike'' carbon films has been studied as a function of heat treatment. For E\ensuremath{\gtrsim}2.5 eV a Tauc behavior is exhibited. Optical band gaps ${E}_{g}$ are \ensuremath{\lesssim}2.4 eV. At lower energies…
Deposition of Cu in ultrahigh vacuum on a clean Pd{001} surface leads to the growth of a new crystalline Cu phase with body-centered-tetragonal (bct) structure well ordered up to about 10 layers. A low-energy electron-diffraction (LEED) analysis of 6-layer films finds the bulk lattice parameters to be a=2.75 A\r{} and…
The bulk band structure of the IV-VI semiconductors PbSe and PbTe was determined experimentally through angle-resolved photoemission from cleaved PbSe(100) and PbTe(100) surfaces, using synchrotron radiation for photoexcitation. By analyzing spectra recorded in electron emission normal to the surface over a wide range…
Iron-bismuthate glassy semiconductors were prepared in a range of compositions and their electrical transport properties were measured in the temperature range of 100–500 K. The electrical data were analyzed in the light of the models of polaronic hopping conduction. The analysis showed that the high-temperature condu…
Accurate and explicitly electron-correlated wave functions have been used to determine the dynamic hyperpolarizability tensor (γ) and the dynamic field-gradient polarizabilities (B and C) for helium. The γ-tensor is related to nonlinear optical processes such as the Kerr effect, second- and third-harmonic generation a…
1,3-Bis(2-chloroethyl)-1-nitrosourea (BCNU) has been found to be an effective chemotherapeutic agent against brain tumors. However, because it has a very short half-life in plasma, the exposure of neoplastic cells to BCNU is very brief. The delivery of BCNU may be enhanced by using controlled release polymers. We meas…
The ultrafast photodynamics of four-coordinate nickel(II) porphyrins in noncoordinating solvents has been studied using femtosecond time resolved optical spectroscopy. Unambiguous evidence has been found for the formation of a metastable metal (d,d) excited state in less than 350 fs following excitation of the macrocy…
Thin films for optical second-harmonic generation (SHG) were prepared from a newly designed and synthesized amorphous polymer that incorporated a high density of active nonlinear optical groups (p-nitroaniline as attached side groups). For alignment of the nonlinear groups a very high electric field was applied by a c…
Observations of K -edges are made for solid targets compressed by laser-generated coalescing shocks. Using a short pulse blacklighter to avoid radiative pre-heating of the compressed solid, a blue-shifted chlorine K -edge is observed. Comparisons with simple theoretical models have indicated the relative magnitude of…
We have performed photoluminescence, photoexcitation, and transmission electron microscopy measurements on boron nitride films grown by chemical vapor deposition and later reacted with potassium. After reaction, the potassium atoms were found to intercalate the BN host and to form a (2×2)R0° in-plane structure which i…
By solving the full elastic equations on a two-dimensional lattice we grow cracks under various breaking conditions for a system that is submitted to external shear. We find that deterministic fracture patterns are in general branched and can be fractal. This effect is due to the competition between the direction of g…
We introduce a Ginzburg-Landau model for solid solutions to examine elastic effects near the phase separation. As a first application the spinodal curve is calculated when the elastic moduli depend on the concentration and an external stress is applied. The instability is triggered by fluctuations with wave vectors in…
Valence-band and core x-ray-photoemission spectroscopy has been used to examine the electronic structure of FeOOH and ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$. The known small difference in the core-level Fe 2p spectra of the two compounds is discussed. X\ensuremath{\alpha} calculations of an ${\mathrm{FeO}}_{6}$${\mathr…
We report a systematic study of the segregation of third-column elements involved in group III-V arsenide structures to their (100) surface. The surface composition is obtained by in situ electron spectroscopies, on special structures built by molecular-beam epitaxy. In ternary alloys, an important surface enrichment…
We have obtained and analyzed Raman spectra of single-crystal, hot-pressed, and chemical-vapor-deposited boron carbide materials over their single-phase region (from \ensuremath{\sim}9 to \ensuremath{\sim}20 at. % carbon). These spectra provide insight into the substitutional disorder that characterizes these structur…
Semiempirical potentials for silicon, germanium, and their alloys are derived with use of the modified-embedded-atom-method formalism. Following Baskes [Phys. Rev. Lett. 59, 2666 (1987)], it is found that the host electron density which is a linear superposition of atomic densities in the embedded-atom method (EAM) mu…
The low-temperature near-band-gap photoluminescence of ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{\mathrm{x}}$ is studied over the whole composition range 0\ensuremath{\le}x\ensuremath{\le}1. We identify free- and bound-exciton processes and determine the properties of momentum-conserving phon…