Showing papers by "Michael Schmid published in 1995"

Sensors based on piezoelectric resonators

Abstract: A review of sensors based on piezoelectric crystal resonators is presented. The survey focuses on the fundamental resonator modes rather than on the variety of surrounding support configurations in special sensor applications. First, the general properties of vibrating crystal sensors and their inherent superiority are described. The sensor concepts utilizing either homogeneous resonators with temperature and pressure (stress) as primary measurants or composite resonators with areal mass density and viscoelastic properties of the 'foreign' layer as primary measurands are discriminated. A comparison between bulk acoustic wave (BAW) and surface acoustic wave (SAW) resonators with respect to their primary sensitivity functions and principal capabilities for sensor applications is given and the importance of recent investigations on Lamb wave and horizontal polarized shear wave (HPLW) interdigital transducer (IDT) resonators is acknowledged. The importance of mode purity for high dynamic range sensors based on resonators and some aspects of the demand on specialized electronics are emphasized. The present state of established sensors based on primary sensitivities, e.g., quartz-crystal thermometers, pressure transducers, thin-film thickness and deposition-rate monitors, viscoelastic layer analysers (crystal/liquid composite resonators) is reviewed. A selection of the most promising recently investigated vibrating crystal sensors utilizing indirect sensitivities is described, including the wide field of analyte-selective coatings and resonator-based immunosensors or immunoassays. Finally, the potential of alternative piezoelectric materials for future sensor developments is briefly discussed.

Incorporation of subgenomic amounts of DNA as compensation for mutational load in a gynogenetic fish

Abstract: A CENTRAL paradigm in evolutionary biology is that sexual reproduction is advantageous over asexuality1–5. One of the long-term disadvantages asexual forms have to face is Muller's ratchet6. In the absence of recombination, theoretically no genotype can ever produce offspring with fewer mutations than its own load. The accumulation of deleterious mutations and gene combinations that cannot be purged should lead to extinction of parthenogenetic forms within 104–105 generations7,8. Evidence is accumulating, however, that some of these might have survived for such periods or even longer9–14. In the Amazon Molly fish Poecilia Formosa we have detected a process that appears to compensate for disadvantages of asexuality, namely incorporation of subgenomic amounts of DNA from a bisexual host species by microchromosomes.

Loss of telomeric sites in the chromosomes of Mus musculus domesticus (Rodentia:Muridae) during Robertsonian rearrangements.

Abstract: Mouse chromosomes possessing multiple Robertsonian rearrangements (Rb chromosomes) have been examined using fluorescencein situ hybridization with the telomeric consensus sequence (TTAGGG)n. No hybridization signals were detected at the primary constriction of Rb chromosomes. This observation leads us to conclude that the formation of Rb chromosomes in the mouse is invariably associated with the loss of telomeric regions. More significantly, a further alteration in regions flanking the primary constrictions was observed after hybridizing with a minor satellite DNA probe to Rb chromosomes. It seems likely that the breakpoints required for a Robertsonian process do not include telomeric sites exclusively but extend to the adjacent pericentromeric regions of the original acrocentric chromosomes. In contrast to previous reports, these observations demonstrate the elimination of substantial amounts of chromosomal DNA during the formation of mouse Rb chromosomes.

Potential sputtering of lithium fluoride by slow multicharged ions

Abstract: Thin polycrystalline LiF films have been bombarded by slow $(\ensuremath{\le}1$ keV) multicharged ${\mathrm{Ar}}^{q+}$ ions $(q\ensuremath{\le}9)$, in order to study the resulting total sputter yields by means of a quartz crystal microbalance. More than 99% of sputtered particles are neutral and show yields, at given impact energy, in proportion to the potential energy of projectile ions. The respective ``potential sputtering'' process already takes place far below 100 eV impact energy. It can be related to defect production in LiF following electron capture by the multicharged ions, and removes about one LiF molecule per 100 eV of projectile potential energy.

Chromosomal evidence for laboratory synthesis of a triploid hybrid between the gynogenetic teleost Poecilia formosa and its host species

Abstract: Cytogenetic analyses were performed on a triploid hybrid individual produced in the laboratory by mating Poecilia formosa, an all-female gynogenetic diploid species, with a normal male of melanistic ornamental Poecilia sphenops (black molly). Direct chromosome preparations revealed 69 (3n) chromosomes in the somatic complement. This cytogenetic observation in conjunction with the pigmentation phenotype leads us to conclude that occasional failure of sperm exclusion may result in the presence of triploid hybrids even in the natural habitats. ? 1995 The Fisheries Society of the British Isles

Chromosome banding in amphibia. XXI: Inversion polymorphism and multiple nucleolus organizer regions in Agalychnis callidryas (Anura, Hylidae)

Abstract: Cytogenetic analyses were performed on several populations of the Central American tree frog Agalychnis callidryas, using conventional methods and banding techniques. The karyotype of this species is distinguished by an inversion polymorphism in chromosome 9, which is either submetacentric or telocentric. The populations examined are in Hardy-Weinberg equilibrium with respect to the two alternative morphs of chromosome 9. This is the first report of the occurrence of an intrapopulational chromosomal inversion polymorphism in the order Anura. In male meiosis, the two chromosomes 9 form a bivalent exhibiting a ring-like pairing configuration with terminal chiasmata in both arms, regardless of whether the paired homologs are heteromorphic or homomorphic. Furthermore, individual specimens of A. callidryas exhibit one or two unexpected 18S+28S ribosomal RNA gene clusters, in addition to the standard nucleolus organizers. The chromosomal localization of these extra nucleolus organizers is identical in all metaphases from the same specimen and shows a specific intraindividual pattern. The karyotype evolution in the phyllomedusine hylids, the structure of the various classes of heterochromatin, and the occurrence and possible origin of the rare inversion polymorphisms and multiple nucleolus organizers in A. callidryas and a few other amphibian species are discussed

Surface stress, surface elasticity, and the size effect in surface segregation.

Abstract: Surface stress and surface elasticity of low-index fcc surfaces have been studied using effective-medium theory potentials. In addition to total-energy calculations giving stress components and elastic data for the surface as a whole, the use of artificial atoms with modified size allows us to probe the stress and elasticity of individual layers. This method of artificial atoms provides a direct way to study the contribution of atomic size to segregation in alloys as well as the driving force of reconstructions driven by surface stress. As an example, we give a qualitative explanation of the face-dependent segregation of Pt-Ni alloys. We also compare results of these atomic-scale calculations with continuum elasticity.

Direct observation of a new growth mode: Subsurface island growth of Cu on Pb(111).

Abstract: Atomically resolved scanning tunneling microscopy on Cu/Pb(111) reveals a new growth mode, contrary to the Volmer-Weber mode expected from the significantly lower surface energy of Pb. (111)oriented Cu islands with a thickness of 3–11 layers are immersed in the Pb substrate and covered by a single close-packed Pb layer. This subsurface growth mode occurring at room temperature can be explained by simple thermodynamic considerations.

Immunostaining and interphase arrangement of field bean kinetochores

Abstract: More than 100 sera from patients with scleroderma CREST (calcinosis, Raynaud phenomenon, esophageal dismotility, sclerodactyly, telangiectasia) were tested in order to detect antigenic nuclear components of the field beanVicia faba (2n=12). Kinetochores of mitotic chromosomes and prekinetochores of interphase cells from root-tip meristems were specifically labelled via an indirect immunofluorescence procedure by antibodies of one of these sera. In 44% of interphase nuclei in which centromeres could be identified, only half (6) of the number of expected prekinetochores (12) was detected, circumstantially indicating at least transient association of homologous centromeres. Some nuclei showed clustering of centromeres at one pole (Rabl configuration). In metaphase chromosomes, each sister kinetochore contained a fluorescent spot. Western blotting of field bean nuclear proteins revealed four antigenic proteins of 28, 30, 64 and 68 kDa.

p ( n ×1) superstructures of Pb on Cu(110)

Abstract: The structures of the p(n\ifmmode\times\else\texttimes\fi{}1) superstructures (n=4, 5, and 9) of Pb on Cu(110) in the coverage range between FTHETA=0.75 and 0.8 are revealed by atomically resolved scanning tunneling microscopy. All three superstructures are formed by substitution of every nth row of Cu atoms (n=4, 5, and 9) in the [001] direction by Pb atoms. The Pb atoms in between are lined up in the [1\ifmmode\bar\else\textasciimacron\fi{}10] direction. The p(4\ifmmode\times\else\texttimes\fi{}1) structure appears in two different modifications: one with substitutional rows of Pb atoms and one with a simple overlayer structure without substituted rows of Cu atoms. Alternating succession of these two modifications results in p(12\ifmmode\times\else\texttimes\fi{}1) domains. It is further shown that the p(9\ifmmode\times\else\texttimes\fi{}1) structure is not a succession of p(4\ifmmode\times\else\texttimes\fi{}1) and p(5\ifmmode\times\else\texttimes\fi{}1) but a superstructure on its own. The p(5\ifmmode\times\else\texttimes\fi{}1) structure proposed here agrees with previous x-ray-diffraction data at least as well as the quasihexagonal model proposed earlier. We have, in addition, identified the nature of the phase that has been described incommensurate obtained by desorption of Pb upon annealing above 600 K.

Secondary ion emission from lithium fluoride under impact of slow multicharged ions

Abstract: Secondary ion emission has been investigated for bombardment of polycrystalline lithium fluoride by slow multicharged Ar ions (charge state q ≤ 9, impact energy E k ≤ 500 eV). The F − ions originate from collisional energy transfer, almost independent of the primary ion charge, whereas the F + yield strongly increases with q . The F + ions are produced via interatomic Auger transitions from the F − 2p valence band into projectile states, and their desorption from LiF is controlled by Coulomb interaction of F + with Li + and F − surface ions, and LiF lattice relaxation. At high impact energy, emission of Li + is also mainly due to collisional energy transfer, but toward lower E k the primary ion charge plays an increasingly important role. The present measurements demonstrate that secondary ions account for less than 0.1% of our earlier measured total sputter yields from LiF.

Evidence for an unusual ZW/ZW'/ZZ sex-chromosome system in Scardinius erythrophthalmus (Pisces, Cyprinidae), as detected by cytogenetic and H-Y antigen analyses.

Abstract: Fifty-seven individuals of the European cyprinid fish species Scardinius erythrophthalmus were sexed by gonad histology, and their karyotypes were analyzed by Giemsa staining and C-

Determination of electron-induced total sputter yield of LiF

Abstract: We present measurements of the total sputter yield for LiF at 150°C induced by electrons with an impact energy between 10 and 500 eV. A nearly linear increase with kinetic energy has been measured. The decrease of the yield with increasing electron dose for energies below 100 eV is also shown. For very high electron dose the yield even becomes zero and simultaneously a red coloration of the surface is observed. Low energy ion scattering (LEIS) measurements at such a surface showed a Li enrichment to more than 90%. To reach such a composition with electrons between 40 and 80 eV kinetic energy sputtering of about 20 monolayers LiF is necessary.

Competitive segregation of Si and P on Fe(96.5)Si(3.5) (100) and (110).

Abstract: Annealing an Fe96.5Si3.5 (100)/(110) bicrystal, containing 90 ppm P, leads immediately to a strong segregation of silicon. The Si atoms, however, desegregate subsequently and are displaced by P, whose segregation enthalpy is larger than that of silicon. The corresponding surface structures formed on both faces have been studied using complementary methods: Scanning tunneling microscopy (STM) to obtain atomically resolved geometrical information and Auger electron spectroscopy (AES) for the determination of the surface composition. Si substitutes surface Fe atoms on both faces and forms ordered surface alloys, whereas P occupies hollow sites on the surface. Si and P form c(2 × 2) superstructures on the (100) surface, whereby each segregated phosphorus atom blocks in the average one silicon segregation site. The (110) surface, on the other hand, is characterized by a c(1 × 3) Si superstructure. Due to the anisotropy of this surface the P/Si exchange proceeds by the formation of silicon coverage decreasing domain boundaries within the silicon structure, which are simultaneously occupied by P atoms. Furthermore the comparison of the AES and STM derived phosphorus coverages indicates a P multilayer segregation on the (110) surface.

Deconvolution of STM images using entropy as a regularization functional

Abstract: The Maximum Entropy approach is applied to restore and sharpen scanning tunneling microscopy (STM) images with atomic resolution. Based on the STM theory of Tersoff and Hamann the process of data acquisition can be approximated by the convolution of a localised atomic density of states (i.e. narrow spots in the reconstruction) of the sample and a Gaussian resolution function which limits the resolution. In STM practice a good and robust estimation of the atomic core positions is necessary for different reasons, such as to be able to calculate the characteristics of the atomic lattice or to study non-periodicities.