Sample records for a-ga010 studiedby spa-leed

  1. A combined STM and SPA-LEED study of the "explosive" nucleation and collective diffusion in Pb/Si(111) (United States)

    Hattab, H.; Hupalo, M.; Hershberger, M. T.; Horn von Hoegen, M.; Tringides, M. C.


    A novel type of very fast nucleation was recently found in Pb/Si(111) with 4- to 7-layer high islands becoming crystalline in an "explosive" way, when the Pb deposited amount in the wetting layer is compressed to θc 1.22 ML, well above the metallic Pb(111) density. This "explosive" nucleation is very different from classical nucleation when island growth is more gradual and islands grow in size by single adatom aggregation [8]. In order to identify the key parameters that control the nucleation we used scanning tunneling microscopy (STM) and spot profile analysis low energy electron diffraction (SPA-LEED). It was found that the number and duration of steps in iterative deposition used to approach θc and the flux rate have dramatic effects on the crystallization process. Larger depositions over shorter times induce greater spatial coverage fluctuations, so local areas can reach the critical coverage θc easier. This can trigger the collective motion of the wetting layer from far away to build the Pb islands "explosively". The SPA-LEED experiments show that even low flux experiments in iterative deposition experiments can trigger transfer of material to the superstable 7-layer islands, as seen from the stronger satellite rings close to the (00) spot.

  2. Growth of NaCl on thin epitaxial KCl films on Ag(100) studied by SPA-LEED (United States)

    Marquardt, Christian; Paulheim, Alexander; Sokolowski, Moritz


    We investigated the growth of NaCl on thin (100)-oriented films of KCl by spot profile analysis of low energy electron diffraction (SPA-LEED). The underlying question of this investigation was how the system accommodates to the misfit of - 10% between the NaCl and KCl lattices. The KCl films (3 atomic layers thick) were epitaxially grown on a Ag(100) single crystal. We studied the heteroepitaxial growth of NaCl on KCl at 300 K and at 500 K, respectively. At 300 K, the first NaCl monolayer (ML) grows pseudomorphically on the KCl film. From the second layer onward, the NaCl lattice relaxes. The NaCl multilayers roughen, and a small rotational disorder (± 4°) of the NaCl domains is observed. The roughening results from the formation of multilayer islands of limited lateral size due to the misfit to the pseudomorphic first NaCl layer. At a growth temperature of 500 K, no pseudomorphic NaCl layer forms, instead relaxed multilayer island growth of NaCl is observed from the first layer onward. Similarly to the growth at 300 K, we find NaCl multilayer islands of limited lateral size. For both temperatures, we explain this growth behavior by the misfit that makes the adsorption sites at the island edges of the first relaxed NaCl layer less favorable for larger islands, promoting nucleation of multilayer islands.

  3. Reactivity of Au nanoparticles supported over SiO2 and TiO2 studiedby ambient pressure photoelectron spectroscopy

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    Herranz, Tirma; Deng, Xingyi; Cabot, Andreu; Alivisatos, Paul; Liu, Zhi; Soler-Illia, Galo; Salmeron, Miquel


    The influence of the metal cluster size and the identity of the support on the reactivity of gold based catalysts have been studied in the CO oxidation reaction. To overcome the structural complexity of the supported catalysts, gold nanoparticles synthesized from colloidal chemistry with precisely controlled size have been used. Those particles were supported over SiO{sub 2} and TiO{sub 2} and their catalytic activity was measured in a flow reactor. The reaction rate was dependent on the particle size and the support, suggesting two reaction pathways in the CO oxidation reaction. In parallel, ambient pressure photoelectron spectroscopy (APPS) has been performed under reaction conditions using bidimensional model catalysts prepared upon supporting the Au nanoparticles over planar polycrystalline SiO{sub 2} and TiO{sub 2} thin films by means of the Langmuir-Blodgett (LB) technique to mimic the characteristic of the powder samples. In this way, the catalytically active surface was characterized under true reaction conditions, revealing that during CO oxidation gold remains in the metallic state.

  4. Controlling atomistic processes on Pb films via quantum size effects and lattice rotation

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    Binz, Steven [Iowa State Univ., Ames, IA (United States)


    The two main techniques used to record the data in this dissertation were Spot Profile Analysis - Low Energy Electron Diffraction (SPA-LEED) and Scanning Tunneling Microscopy (STM). A specific data analysis technique for LEED data called G(S) curves is described in depth. G(S) curves can provide a great deal of structural information about the surface; including step heights, island size, and island separation. The effects of quantum size effects (QSE) on the diffusion and critical island sizes of Pb and In on Pb films are reported. Pb depositions on the 2D In phases {radical}3 and {radical}31 to see how the phases affect the Pb growth and its strong QSE are reported.

  5. Adsorption of phthalocyanines on noble metal surfaces; Adsorption von Phthalocyaninen auf Edelmetalloberflaechen

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    Kroeger, Ingo


    In this thesis the adsorbate systems CuPc/Ag(111), CuPc/Au(111), CuPc/Cu(111), H2Pc and TiOPc/Ag(111) were investigated and characterized in great detail using complementary methods. The focus of the experiments was the determination of lateral geometric structures with spot-profile-analysis low energy electron diffraction (SPA-LEED) and scanning tunneling microscopy (STM), as well as the measurement of adsorption heights using the method of normal incidence X-ray standing waves (NIXSW). High resolution electron energy loss spectroscopy (HREELS) was used to characterize the vibronic properties of the molecule and the interface dynamical charge transfer (IDCT). The electronic structure and the charge transfer into the molecule were investigated with ultraviolet photoelectron spectroscopy (UPS). The most important results of this work are related with the interplay between adsorbate-substrate and adsorbate-adsorbate interaction of Phthalocyanines in the submonolayer regime. (orig.)

  6. Surface stress and large-scale self-organization at organic-metal interfaces

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    Pollinger, Florian


    The role of elastic interactions, particularly for the self-organized formation of periodically faceted interfaces, was investigated in this thesis for archetype organic-metal interfaces. The cantilever bending technique was applied to study the change of surface stress upon formation of the interface between 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) and Ag(111). The main focus of this work was on the investigation of the formation of the long-range ordered, self-organized faceted PTCDA/Ag(10 8 7) interface. Reciprocal space maps of this interface were recorded both by spot profile analysis low energy electron diffraction (SPA-LEED) and low energy electron microscopy (LEEM) in selected area LEED mode. Complementary to the reciprocal data, also microscopic real-space LEEM data were used to characterize the morphology of this interface. Six different facet faces ((111), (532), (743), (954), (13 9 5), and (542)) were observed for the preparation path of molecular adsorption on the substrate kept at 550 K. Facet-sensitive dark-field LEEM localized these facets to grow in homogeneous areas of microscopic extensions. The temperature-dependence of the interface formation was studied in a range between 418 K and 612 K in order to learn more about the kinetics of the process. Additional steeper facets of 27 inclination with respect to the (111) surface were observed in the low temperature regime. Furthermore, using facet-sensitive dark-field LEEM, spatial and size distributions of specific facets were studied for the different temperatures. Moreover, the facet dimensions were statistically analyzed. The total island size of the facets follows an exponential distribution, indicating a random growth mode in absence of any mutual facet interactions. While the length distribution of the facets also follows an exponential distribution, the width distribution is peaked, reflecting the high degree of lateral order. This anisotropy is temperature-dependent and occurs

  7. Morphological transformation of the silicon(112) surface through metal adsorption. Faceting vs. stabilization; Morphologische Transformation der Silizium(112) Oberflaeche durch Metalladsorption. Facettierung vs. Stabilisierung

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    Wiethoff, Christian


    Semiconductor nanowires lately attracted immense attention in worldwide research. Lots of possible applications were discussed and partially realized. Gold is the most commonly used metal in catalyzed nanowire growth, but exhibits poor electronic properties for building integrated circuits. Recently, aluminium was achieved to work as a catalyst and the nanowires grown this way showed extremely smooth surfaces. This is in contrast to surfaces grown with other metals. In this work the modification of the silicon (112) surface by metal adsorption is studied. Since nanowires with [111] growth direction have [112]-like sidewalls, this way a virtually endless nanowire surface is used to study the interaction of gold, silver and aluminium with the silicon surface. Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) is used to analyze the surfaces. This high resolution diffraction method allows for detailed characterization of the surface structures and facets. Gold adsorption leads to eight different surface phases, the most stable one exhibits 111- and 113-facets and is shown to occur on nanowire surfaces if they were grown with gold as a catalyst. Silver adsorption pushes the surface to form 111- and 115-facets. The facet size distribution is quite narrow, so the finite size effect creates intensity minima in the vicinity of the bragg points in diffraction patterns. Aluminium adsorption finally leads to a stabilization of the silicon (112) surface, no facets are formed. This atomic flat surface was seen on nanowire sidewalls, grown with aluminium as catalyst. (orig.)

  8. Coadsorption of potassium and nitrogen on the Ni(100) surface

    CERN Document Server

    Scantlebury, M J


    R-factor analysis carried out over a wide area of parameter space has yielded structural parameters as deep as the third Ni layer. Simulations of data obtained for Ni(100)-(K+N) were not attempted due to the similarity of the PD curves with those of the Ni/N system. From the point of view of PD, there are no structural differences between the pure N and the K coadsorbed systems, in contrast to the SPA-LEED and ARUPS results. A scanning tunnelling microscopy (STM) study has been performed on the Ni(100)-(K+N) system. By gradually dosing the Ni/N surface with K, it was possible to observe the K atoms initially adsorbing preferentially along [001] step edges, before forming protrusions approx 10 A in diameter on the terraces at higher coverages. The disturbance of the substrate underlying these protrusions has been imaged and suggests a local interaction of K with the surface. As the K coverage increased to 0.5 ML, the protrusions appear to coalesce. The K atoms have also been shown to be immobile on the surface...

  9. Defect and structural imperfection effects on the electronic properties of BiTeI surfaces (United States)

    Fiedler, Sebastian; El-Kareh, Lydia; Eremeev, Sergey V.; Tereshchenko, Oleg E.; Seibel, Christoph; Lutz, Peter; Kokh, Konstantin A.; Chulkov, Evgueni V.; Kuznetsova, Tatyana V.; Grebennikov, Vladimir I.; Bentmann, Hendrik; Bode, Matthias; Reinert, Friedrich


    The surface electronic structure of the narrow-gap seminconductor BiTeI exhibits a large Rashba-splitting which strongly depends on the surface termination. Here we report on a detailed investigation of the surface morphology and electronic properties of cleaved BiTeI single crystals by scanning tunneling microscopy, photoelectron spectroscopy (ARPES, XPS), electron diffraction (SPA-LEED) and density functional theory calculations. Our measurements confirm a previously reported coexistence of Te- and I-terminated surface areas originating from bulk stacking faults and find a characteristic length scale of ˜100 nm for these areas. We show that the two terminations exhibit distinct types of atomic defects in the surface and subsurface layers. For electronic states resided on the I terminations we observe an energy shift depending on the time after cleavage. This aging effect is successfully mimicked by depositon of Cs adatoms found to accumulate on top of the I terminations. As shown theoretically on a microscopic scale, this preferential adsorbing behaviour results from considerably different energetics and surface diffusion lengths at the two terminations. Our investigations provide insight into the importance of structural imperfections as well as intrinsic and extrinsic defects on the electronic properties of BiTeI surfaces and their temporal stability.

  10. Catalytic Activity and Photophysical Properties of Biomolecules Immobilized on Mesoporous Silica

    DEFF Research Database (Denmark)

    Ikemoto, Hideki

    hybrid materials used for further study. One metalloenzyme, horseradish peroxidase(HRP), was immobilized on rod-shaped SBA-15 by physical adsorption. The catalytic activity of free and immobilized enzyme was first compared at room temperature. Details of the enzyme kinetics including the apparent...... 2 (LH2) from purple photosynthetic bacteria was immobilized on SBA-15 with hexagonally ordered cylindrical pores or on MCF-type mesoporous silica with disordered cage-like mesopores. To identify the location of LH2, spherical particles with small or large pores were tested for the adsorption of LH2....... LH2 was adsorbed to the particles with large pores, while little LH2 was adsorbed to the particles with small pores. Subsequent observation with fluorescence microscopy confirmed that LH2 is adsorbed in the pores of mesoporous silica. The conjugates of LH2 and mesoporous silica were studiedby steady...

  11. Courses Studied by New York Librarianship Students Affect Their Perceived Readiness for Employment. A Review of: Creel, S. L., & Pollicino, E. B. (2012. Practitioners’ & LIS students’ perceptions on preparedness in the New York metropolitan area. Education for Information, 29, 53-69. doi: 10.3233/EFI-2010-0911

    Directory of Open Access Journals (Sweden)

    Kirsty Thomson


    Full Text Available Objective – To investigate whetherlibrarianship students felt ready to enter theworkforce, and whether practitioners feltrecent graduates were suitably prepared.Design – Survey.Setting – A university in New York City, andschool and public librarians working in theNew York metropolitan area.Subjects – 55 MLS students, 167 school librarypractitioners, and 181 public librarypractitioners.Methods – Students surveyed practitionersabout new graduates’ readiness to work aslibrarians. The students also assessed theirown readiness.Main Results – Detailed analysis of differencesbetween the responses of the four subjectgroups – public librarians, school librarians,public library students, and school librarystudents – for each of six survey statements isprovided. Practitioners and students felt thatschool librarianship graduates were moreprepared for work than public librarianshipgraduates. This may have been due todifferences in the practical components of theircourses.Conclusion – Preparedness for libraryemployment is related to the courses studiedby librarianship students.

  12. Nanostructure Control: Nucleation and Diffusion Studies for Predictable Ultra Thin Film Morphologies (United States)

    Hershberger, Matthew Thomas

    This thesis covers PhD research on two systems with unique and interesting physics. The first system is lead (Pb) deposited on the silicon (111) surface with the 7x7 reconstruction. Pb and Si are mutually bulk insolubility resulting in this system being an ideal case for studying metal and semiconductor interactions. Initial Pb deposition causes an amorphous wetting layer to form across to surface. Continued deposition results in Pb(111) island growth. Classic literature has classified this system as the Stranski-Krastanov growth mode although the system is not near equilibrium conditions. Our research shows a growth mode distinctly different than classical expectations and begins a discussion of reclassifying diffusion and nucleation for systems far away from the well-studied equilibrium cases. The second system studied investigates the interactions of the Rare Earth metal Dysprosium (Dy) with a carbon based 2D lattice called graphene. Graphene is a 2D material composed of carbon atoms arranged in hexagons, similar to a honeycomb with carbon atoms at each corner. The graphene we used is grown epitaxially from a substrate of silicon carbide. This creates a multilayered playground to study how metals interact both on the surface of graphene and intercalated in between graphene layers. Many types of atoms have been studied in graphene systems, but the rare earths and in particular Dy have not been well investigated. This thesis contributes to the knowledge base of graphene on SiC structure and metal-graphene interactions. These systems have been investigated in ultra-high vacuum (UHV) environments with base pressures around 5.0x10-11 torr. The Pb/Si(111)-7x7 system was investigated with scanning tunneling microscopy (STM) and the Graphene/SiC system was investigated with both STM and Spot Profile Analyzing Low Energy Electron Diffraction (SPA-LEED).

  13. To tilt or not to tilt: Correction of the distortion caused by inclined sample surfaces in low-energy electron diffraction

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    Sojka, Falko, E-mail: [Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 5, 07743 Jena (Germany); Meissner, Matthias; Zwick, Christian; Forker, Roman [Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 5, 07743 Jena (Germany); Vyshnepolsky, Michael; Klein, Claudius; Horn-von Hoegen, Michael [University of Duisburg-Essen, Department of Physics, Lotharstr. 1, 47057 Duisburg (Germany); Fritz, Torsten, E-mail: [Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 5, 07743 Jena (Germany)


    Low-energy electron diffraction (LEED) is a widely employed technique for the structural characterization of crystalline surfaces and epitaxial adsorbates. For technical reasons the accessible reciprocal space is limited at a given primary electron energy E. This limitation may be overcome by sweeping E to observe higher diffraction orders decisively enhancing the quantitative examination. Yet, in many cases, such as molecular films with rather large unit cells, the adsorbate reflexes become less pronounced at energies high enough to observe substrate reflexes. One possibility to overcome this problem is an intentional inclination of the sample surface during the measurement at the expense of the quantitative interpretability of then severely distorted diffraction patterns. Here, we introduce a correction method for the axially symmetric distortion in LEED images of tilted samples. We provide experimental confirmation for micro-channel plate LEED and spot-profile analysis LEED instruments using the (7×7) reconstructed surface of a Si(111) single crystal as a reference sample. Finally, we demonstrate that the correction of this distortion considerably improves the quantitative analysis of diffraction patterns of adsorbates since substrate and adsorbate reflexes can be evaluated simultaneously. As an illustrative example we have chosen an epitaxial monolayer of 3,4,9,10-perylenetetracarboxylic dianhydride on Ag(111) that is known to form a commensurate superstructure. - Highlights: • We introduce a method to correct distortions in LEED patterns of tilted surfaces. • Higher diffraction orders unobservable at higher beam energies can be evaluated. • Our procedure makes LEED patterns of tilted samples quantitatively analyzable. • Experimental confirmation with SPA-LEED and MCP-LEED is presented. • The method is applied to PTCDA on Ag(111) confirming earlier literature values.

  14. Studies of Dirac and Weyl fermions by angle resolved photoemission spectroscopy

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    Huang, Lunan [Iowa State Univ., Ames, IA (United States)


    This dissertation consists of three parts. First, we study magnetic domains in Nd2Fe14B single crystals using high resolution magnetic force microscopy (MFM). In addition to the elongated, wavy nano-domains reported by a previous MFM study, we found that the micrometer size, star-shaped fractal pattern is constructed of an elongated network of nano-domains about 20 nm in width, with resolution-limited domain walls thinner than 2 nm. Second, we studied extra Dirac cones of multilayer graphene on SiC surface by ARPES and SPA-LEED. We discovered extra Dirac cones on Fermi surface due to SiC 6 x 6 and graphene 6√ 3 6√ 3 coincidence lattice on both single-layer and three-layer graphene sheets. We interpreted the position and intensity of the Dirac cone replicas, based on the scattering vectors from LEED patterns. We found the positions of replica Dirac cones are determined mostly by the 6 6 SiC superlattice even graphene layers grown thicker. Finally, we studied the electronic structure of MoTe2 by ARPES and experimentally con rmed the prediction of type II Weyl state in this material. By combining the result of Density Functional Theory calculations and Berry curvature calculations with out experimental data, we identi ed Fermi arcs, track states and Weyl points, all features predicted to exist in a type II Weyl semimetal. This material is an excellent playground for studies of exotic Fermions.

  15. Study of photosynthesis process in the presence of low concentrations of clomazone herbicide in tobacco (Nicotiana tabacum

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    Darwish, Majd


    Full Text Available The effect of chemical residues of clomazone on photosynthetic processes has been studiedby using several low concentrations of the herbicide (0, 1, 0.1, 0.01, 0.001, 0.0001 and 0.00001 µM and seedlings of two varieties of tobacco (Nicotiana tabacum L. cv. Virginie vk51, Nicotiana tabacum, L. cv. Xanthi. The content of photosynthetic pigments, the parameters of the chlorophyll-a fluorescence and the JIP-test were performed on an adult leaf (AL and a young leaf (YL, that gave a complementary designto know the action's mode of clomazone on the plant physiological processes. Clomazone reducedthe total chlorophyll (a+b, carotenoids pigments (reduction in size antenna pigments judged by an increase in the chlorophyll a/b ratio in young leaves more than adults leaves. The maximal photochemical efficiency (Fv/Fm of photosystem II (PSII decreased significantly in youngleavescompared to adult leaves and in (Virginie variety than (Xanthi variety. Among the parameters calculated of the JIP-test most affected by the treatment, PIabs, 1-VJ, ABS/RC, DI0/RC, TR0/RC, ET0/RC, ET0/ABS, which indicated acomparable effects of clomazone(1μM, 0.1µM, 0.01µM between the two types of leaves and the varieties used. More, the results showed that the concentration ( 1μM was the most effective amongthe other low concentrations used and the (Virginie variety ismore sensitive than the (Xanthivariety. We conclude that clomazone has probably two combined functions (physiological, toxic judged by the different behavior of both types of leaves in the presence of the herbicide.

  16. Praseodymia on non-passivated and passivated Si(111) surface

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    Gevers, Sebastian


    In the presented thesis thin praseodymia films on non-passivated and passivated Si(111) substrates were investigated. The first part deals with PDA of praseodymia films with fluorite structure under UHV conditions in the temperature region from RT to 600 C. It is shown that the untreated films, which are oxidized in 1 atm oxygen to obtain fluorite structure, do not exhibit pure PrO{sub 2} stoichiometry as it was assumed before. Instead, they decompose into two laterally coexisting species exhibiting a PrO{sub 2} and a Pr{sub 6}O{sub 11}. oxide phase, respectively. These species are laterally pinned to the lattice parameter of bulk Pr{sub 6}O{sub 11}. Homogeneous oxide films with Pr{sub 6}O{sub 11} phase can be observed after annealing at 100 C and 150 C. If higher annealing temperatures are applied, the film decomposes again into two coexisting species. Finally, after annealing at 300 C, a mixed crystalline film with both Pr{sub 2}O{sub 3} and Pr{sub 2}O{sub 3+{delta}} oxide phases is formed. Again the lateral strain increases due to the tendency of praseodymia phases to increase their lattice parameters during oxygen loss combined with the lateral pinning. This is accompanied by a decrease of crystallite sizes. The increase of the amorphous Pr-silicate interface between Si substrate and oxide at the expense of the crystalline oxide can be observed after annealing at higher temperatures. Furthermore, an increased mosaic spread of the crystallites occurs, which reduces the lateral strain caused by the oxygen loss. Nevertheless, the crystalline structure is stable against further annealing up to temperatures of 600 C. The results obtained from praseodymia films annealed in 1 atm nitrogen show that these films are good candidates to form homogeneous oxide films with pure cub-Pr{sub 2}O{sub 3} structure by subsequent annealing in UHV. Here, a single oxide species is already observed after annealing at 300 C by SPA-LEED measurements. Investigations on oxygen plasma

  17. Investigações sobre doenças de Psittacideos

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    Genesio Pacheco


    to be different from that of the virus of psittacosis studiedby him. We are very much obliged to him for the attention he paid to this verification. Virus of psittacosis - Infectiousness: man, monkey, rabbit, mouse, hen, canary. Neurotropic affinity. Inclusions: small, protoplasmic. Exsiccation: the virus has good power of preservation. Symptoms: inactivity, drowsiness, frequent diarrhoea, oculo-nasal discharge and cough, coma. Duration: 4 to 5 days. Bodily lesions: congestion of intestines, splenomegaly. Virus of S. Paulo - Infects only psittacidae, particularly those of the genus Amazona. No localization in the nervous system. Large, nuclear. Is rapidly destroyed. Inactivity, inappetency, adynamia (drooping of the wings, indifference, leaning its beak against the bars of the cage in order not to fall down; profuse diarrhoea, of whitish stools, at times enterorrhagia; prolonged coma. 2 to 8 days. Foci of yellowish necrosis in liver, spleen and lung. At times, congestion of intestines. Characteristic features common to the two viruses.-They act in great dilutions, filter through tight candles though being partly retained, are preserved under glycerine or Bedson's solution, are stable at 55°C. heat and are destroyed by physical and chemical agents. Both virus diseases are very seldom met with in psittacidae: only once, amongst numberless sick parrots, the author met with a disease of the virus differring from that of psittacosis. This disease, greatly transmissible to man, ought to be more frequent, if it were common in parrots. On the contrary, bacteria cause diseases in these animals with great frequency, presenting variable characters, from a severe epizootic form, rapidly mortal, to ambulatory or silent forms, for the most part developing towards a cure or assuming a chronic character. Amongst the bacteria which cause the infection of this group the salmonellae predominate and amongst them the bacterium discovered by Nocard, as well as a species which in the