Richard Zallen

Brookite-rich titania films made by pulsed laser deposition

Abstract Pulsed laser deposition has been used to prepare thin films of TiO2 on silicon and other substrates. X-ray diffraction and Raman scattering results show that brookite is the main phase present in the films. Brookite has not previously been reported for pulsed laser deposited titania thin films.

Inter-polytype conversion and layer-layer coupling in PbI2

Abstract The occurrence of a thermally-induced 2H → 4H inter-polytype conversion in PbI2 has been observed via the appearance, in Raman scattering, of a shear-motion rigid-layer line at 14 cm-1 and a Davydov splitting of an intra-layer line. Analysis of these data has yielded a comparison of the layer-layer coupling strength in PbI2 with that in other layer crystals, as well as an estimate of the stiffness of the iodine-iodine interlayer bond.

Cell-pattern disordering during convergent extension in Drosophila

Convergent extension is the cell-rearrangement process by which a developing embryo elongates to establish the head-to-tail body axis. In the early Drosophila embryo, this process occurs within a one-cell-thick epithelial layer. Using confocal microscopy, images were collected of the two-dimensional cell pattern at four stages during convergent extension in wild-type embryos and at one stage in two classes of mutant embryos. The cellular topology was analysed in terms of the statistical distribution p(n), the frequency of occurrence of n-sided cells. For wild-type embryos, the results demonstrate progressive cell-pattern disordering during convergent extension. The second moment (variance) of p(n) triples to 1.1 while the peak at p(6) drops from 0.65 to 0.38. The fraction of fourfold vertices (four edges meeting) increases from 2% to 8%. Quantitative analysis of interface orientations reveals that the initial degree of hexatic edge-orientational order essentially disappears during the course of convergent extension. The degree of cell-pattern disordering in the two mutants resembles distinct stages in the wild type and correlates with the extent of axis elongation.

Interband spectra of As2S3 and As2Se3 crystals and glasses

Abstract The spectral dependence of α( h ν), ϵ 2 ( h ν), and n eff ( h ν) for crystalline and amorphous As 2 S 3 and As 2 Se 3 have been obtained by Kramers-Kronig analysis of interband reflectivity data. Two prominent edges, at 2–3 eV and at 7–8 eV, are observed in all four solids and are interpreted as the thresholds for transitions from nonbonding and from bonding filled states, respectively.

Raman‐scattering studies of silicon‐implanted gallium arsenide: The role of amorphicity

A series of Raman‐scattering experiments were carried out on GaAs implanted with Si+ and with SiF+3 ions, both before and after annealing, for samples subjected to fluences spanning a wide range. The implantation‐induced amorphization of the damage layer was cleanly observed via the evolution, with increasing fluence, of the broad three‐band continuum of amorphous GaAs which extends from near zero up to 300 cm−1. Annealing recovers the simple line spectrum of crystalline GaAs, but with a changed longitudinal‐optical/transverse‐optical intensity ratio which indicates a departure from epitaxial regrowth. Three lines observed near 400 cm−1 in heavily implanted samples were identified with silicon vibrational local modes. The effect of annealing on these local‐mode lines is not an intensity increase but is instead a line narrowing which reveals an annealing‐induced sharpening of the distribution of local settings sampled by the substitutional silicons. In particular, a line (at 381 cm−1) assigned to the Si‐at...

The effect of temperature on libron frequencies in molecular crystals: Implications for TTF-TCNQ

Abstract Results for the effect of pressure and oemperature on lattice-phonon frequencies in six crystals (As 4 S 4 , S 8 , S 4 N 4 , C 10 H 8 , C 14 H 10 , C 16 H 10 ) have been analyzed to separate the phonon-excitation-driven (“explicit”) and the thermal-expansion-driven (“implicit”) contributions to d v /d T . The data establish that the volume-driven implicit effect dominates the temperature dependence of external-mode frequencies in molecular crystals . This finding has a bearing upon recent proposals for the role of librons in the electron transport of TTF-TCNQ.

Raman spectra of As4S4 polymorphs: Structural implications for amorphous As2S3 films

Abstract Raman-scattering spectra of α-As 4 S 4 and β-As 4 S 4 have been determined at 300 and 10 K. Although similar in overall aspect, the spectral signatures of the two polymorphs are clearly distinct. We have made a careful comparison of these first-order crystal line spectra to the sharp features reported in the Raman spectra of freshly-deposited films of amorphous As 2 S 3 . Prior proposals for the presence of As 4 S 4 molecules in the unannealed films are supported by these comparisons, but recent contentions that actual microcrystals of β-As 4 S 4 are present in the as-deposited material are clearly contradicted by the absence of any of the lattice-phonon lines which are prominent in the crystals at frequencies below 70 cm -1 .

The lattice polarizability of PbI2

Abstract We have measured the far-i.r. reflectivity spectrum of the layer crystal PbI2 for the in-plane polarization. A strong reststrahlen band is observed, with TO and LO phonon frequencies located at 52 and 108 cm−1. The large lattice contribution accounts for three-quarters of the low-frequency dielectric constant of 26. The deduced infrared effective charge and electronic polarizability of PbI2 indicates that the intralayer bonding is closely related to the chemical bonding in the IV–VI rocksalt-structure crystals.

Intervalenceband and plasmon optical absorption in heavily doped GaAs:C

By using direct numerical-solution techniques for the reflectance (R) and transmittance (T) equations of a multilayer structure, we have analyzed infrared R and T measurements on heavily doped p-type GaAs:C films grown by molecular beam epitaxy. The optical properties, for films with hole concentrations up to 1.4×1020 cm−3, were determined for photon energies from 0.07 to 0.6 eV, in which region plasmon (intraband) and intervalenceband contributions are in competition. Our results for the optical absorption coefficient resolve two separate peaks located (at high doping) at about 0.1 and 0.2 eV. By carrying out calculations of the intervalenceband (IVB) absorption processes for our dopings, we identify the peak near 0.2 eV with light-hole to heavy-hole IVB transitions, and we attribute the lower-energy peak to hole-plasmon excitations. Our experimental absorption spectra are very well described by a model combining the IVB contribution to the dielectric function with a plasmon contribution. The hole-plasmo...

Surface charging effects on valence band spectra in X-ray photoemission: Crystalline and Amorphous As2S3

Abstract In X-ray photoemission (XPS) studies on insulators, strong electric fields associated with surface charging can perturb the observed spectra. We find that the standard technique of flooding with thermal electrons to neutralize the net charge does not eliminate this effect for valence band spectra, but that the use of thin or (for photoconductors) illuminated samples does solve the problem. These conclusions are demonstrated by experiments revealing new structure in, and real differences between, the XPS valence band spectra of crystalline and amorphous As 2 S 3 .