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  • 1
    Digitale Medien
    Digitale Medien
    Annealing of Cd-implanted GaAs: Defect removal, lattice site occupation, and electrical activation (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 4248-4256 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A systematic investigation of the behavior of Cd-implanted GaAs after rapid thermal annealing is presented. The use of various experimental techniques gives a detailed picture regarding the annealing process in the low-dose regime (1012 and 1013 cm−2) on a microscopic as well as on a macroscopic scale. Perturbed angular correlation experiments, using the radioactive probe 111mCd, yield information on the immediate environment of the Cd implant on an atomic scale. Rutherford backscattering channeling and photoluminescence spectroscopy give complementary information concerning the overall damage level in the implanted layer, Hall measurements are used to determine the degree of electrical activation of the implanted Cd acceptors. The outdiffusion of the implanted radioactive Cd atoms is also investigated. The removal of defects in the next-nearest neighborhood of the Cd atoms takes place after annealing at 700 K and is accompanied by a general recovering of the crystal lattice. Between 600 and 900 K more distant defects are removed. The observed outdiffusion of about one-third of the dopant atoms after annealing above 600 K is discussed in context with their partial incorporation in extended defects. Although already at 700 K, 80% of the implanted Cd atoms are on substitutional lattice sites with no defects in their immediate environment, an annealing temperature in excess of 1000 K is necessary to obtain electrical activation of the implants. It is concluded that compensating defects, present in ion-implanted GaAs, are the reason for the significantly higher temperature required for electrical activation as compared to the incorporation of the dopants on defect-free, substitutional lattice sites.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.354039
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  • 2
    Digitale Medien
    Digitale Medien
    Thermal effects in pulsed laser assisted scanning tunneling microscopy (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 2574-2580 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The thermal response of a tunneling tip after illumination of the apex with an ultrashort laser pulse of 1 ps duration is investigated. The finite element method is applied to calculate the resulting time-dependent temperature distribution and the thermal expansion taking into account the elastic properties of the tip material. The calculation reveals the three-dimensional movement of the tip apex. The expansion of the tip occurs within a few nanoseconds and after 10 μs the tip has almost reached its original length again. The bending of the tip due to the asymmetric illumination of the tip occurs on the same time scale and is of the same order of magnitude as the axial expansion. Under tunneling conditions the absolute magnitude of the expansion can lead to the formation of nanocontacts. This accounts for the laser induced nanostructuring of surfaces that has been reported in literature. The application of the thermal expansion as a fast switch for the tunneling current is proposed. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.372221
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  • 3
    Digitale Medien
    Digitale Medien
    Femtosecond laser assisted scanning tunneling microscopy (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 4851-4859 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The excitation of the tunneling junction of a scanning tunneling microscope using ultrashort laser pulses combined with detection of a tunneling current component which depends nonlinearly on the laser intensity allows, in principle, to simultaneously obtain ultimate spatial and temporal resolution. To achieve this goal, a laser system that produces ultrashort laser pulses is combined with an ultrahigh vacuum scanning tunneling microscope. The basic technical considerations are discussed and it is shown that atomic resolution can be achieved under pulsed laser excitation of the tunneling junction. The pulsed illumination gives rise to several contributions to the measured total current. Experimental evidence for signal contributions due to thermal expansion, transient surface potentials and multiphoton photoemission are presented. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290706
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  • 4
    Digitale Medien
    Digitale Medien
    Cd-H pairs in GaAs: Identification and stability (1991)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 58 (1991), S. 1751-1753 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The Cd-H complex in 111mCd-doped GaAs implanted with low-energy (150–400 eV) hydrogen atoms is identified and studied by perturbed angular correlation spectroscopy using radioactive 111mCd as a probe. By measuring the fraction of Cd-H pairs in an isochronal annealing experiment, the stability of the pairs is deduced yielding a dissociation energy of ED=1.35(10) eV. After 111mCd implantation but preceding the H loading, the GaAs samples have to be annealed at temperatures exceeding 900 K in order to form Cd-H pairs. These temperatures are in agreement with the temperature range required for electrical activation of Cd implants, suggesting that a Coulombic interaction is responsible for the formation of Cd-H pairs in GaAs.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.105079
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  • 5
    Digitale Medien
    Digitale Medien
    Hot electron tunneling in femtosecond laser-assisted scanning tunneling microscopy (2002)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 81 (2002), S. 286-288 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The combination of scanning tunneling microscopy with femtosecond laser spectroscopy yields simultaneously ultimate spatial and temporal resolution. One possibility to realize this combination is the direct excitation of the tunnel junction in a pump–probe configuration and the detection of a tunnel current component that depends nonlinearly on the laser intensity. The laser-induced signal is expected to be very small, therefore a suitable sample material and a modulation technique is required. In measurements on a GaP(100) surface evidence for tunneling of hot electrons is obtained giving the possibility for local time-resolved tunneling spectroscopy. © 2002 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1491280
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  • 6
    Digitale Medien
    Digitale Medien
    Modeling transport in toroidal plasmas: Status and issues (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 2913-2925 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The scope and detail of physics contained in computational models for fluid (density, momentum, energy) transport in toroidal plasmas have steadily increased during the past two decades. There has been considerable success in the development and verification of models for sources and sinks of particles, energy, momentum, and magnetic flux. Transport codes have collectively become very useful tools in interpreting experimental data and in providing guidance for new experiments. However, a more thorough understanding of the fundamental transport processes of magnetically confined plasmas and development of improved computational models are needed to enhance the predictive capabilities of transport codes. It is argued that fluid transport modeling by itself cannot lead to a complete understanding of transport—there must be a very strong collaboration among theory, experiment, and modeling on both the fluid and kinetic levels.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.859360
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  • 7
    Digitale Medien
    Digitale Medien
    Regeneration potential of CIMMYT durum wheat and triticale varieties from immature embryos (2001)
    Oxford, UK : Blackwell Publishing Ltd
    Plant breeding 120 (2001), S. 0 
    Details
    ISSN: 1439-0523
    Quelle: Blackwell Publishing Journal Backfiles 1879-2005
    Thema: Land- und Forstwirtschaft, Gartenbau, Fischereiwirtschaft, Hauswirtschaft
    Notizen: Twenty-five durum wheat elite advanced lines and released varieties, and five triticale varieties were evaluated for their ability to produce embryogenic callus using three different media. For callus initiation and maintenance there were basal Murashige and Skoog (MS) medium containing double strains of macroelements and 2.5 mg/l 2,4D (DW1), basal MS medium containing 2.0 mg/l 2,4D (DW2), or basal MS medium supplemented with 1.0 mg/l 2,4 D and coconut milk (DW3). Plant regeneration was achieved on basal MS medium with indoleacetic acid and 6-benzylaminopurine, and plants rooted on MS with 1-naphthale-neacetic acid. DW3 medium proved better than the other media tested for embryogenic callus initiation and maintenance. Regeneration rates varied widely with both genotype and initiation medium, with values ranging from no regeneration to 100% regeneration; the plantlets produced per embryo ranged from five to 20. Fourteen of the durum wheat genotypes showed 63–100% regeneration from DW3 callus formation medium, four lines from DW1 medium, and two lines from DW2. Four of the triticale varieties had regeneration of 48–100% from DW3 medium. After six subcultures, over a 6-month period, genotypes lost their ability to regenerate plants. Only 10 lines retained some plant regeneration potential but regeneration was at reduced levels. Successful regeneration of durum wheat and triticale varieties will be used as an integral part of the transformation process.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1046/j.1439-0523.2001.00518.x
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  • 8
    Digitale Medien
    Digitale Medien
    Effects of D-genome chromosomes on crossability of hexaploid triticale (X Triticosecale Wittmack) with maize (1997)
    Oxford, UK : Blackwell Publishing Ltd
    Plant breeding 116 (1997), S. 0 
    Details
    ISSN: 1439-0523
    Quelle: Blackwell Publishing Journal Backfiles 1879-2005
    Thema: Land- und Forstwirtschaft, Gartenbau, Fischereiwirtschaft, Hauswirtschaft
    Notizen: With the aim of producing polyhaploids of hexaploid triticale, 20 genotypes from a CIMMYT breeding programme and eight D-genome chromosome substitution lines of ‘Rhino’ were crossed with maize. In crosses between 20 triticale genotypes and maize, 15 lines produced embryos. Frequencies of embryo formation ranged from 0.0 to 5.4%, with an average of 1.1%. From a total of 200 pollinated spikes, 62 plants were regenerated. Most regenerated plants were polyhaploids with 21 chromosomes, and few aneuhaploids with 22 chromosomes were found. In crosses of triticale substitution lines with maize, all the lines produced embryos, while ‘Rhino’ produced no embryos at all. Higher frequencies of embryo formation were obtained in substitution lines with chromosomes 2D and 4D. These results suggest that D-genome chromosomes in a triticale genetic background have the effect of increasing the frequency of polyhaploid production in triticale x maize crosses.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1111/j.1439-0523.1997.tb01017.x
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  • 9
    Digitale Medien
    Digitale Medien
    Silver nanoparticles on graphite studied by femtosecond time-resolved multiphoton photoemission (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 5428-5434 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Time-resolved multiphoton photoelectron spectroscopy is employed to study collective excitations and their decay dynamics in silver nanoparticles on highly oriented pyrolytic graphite. Resonant excitation of the surface plasmon in the silver nanoparticles with 400 nm femtosecond radiation allows to distinguish between photoemission from the nanoparticles and the substrate. This extends the method of time-resolved two-photon photoemission spectroscopy to inhomogeneous surfaces and permits to probe the dynamics of a confined electron gas. The multiphoton photoelectron spectra, the polarization dependence of the photoelectron yield and the time-resolved measurements reveal the double excitation of the surface plasmon and allow the identification of two different decay channels of the collective excitation. The multiply excited plasmon transfers its total excitation energy to a single photoelectron or decays into at least two single-particle excitations which share the total energy. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.481127
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  • 10
    Digitale Medien
    Digitale Medien
    Effects of steroid hormones and other substances on alarm substance cells and mucous cells in the epidermis of the European minnow, Phoxinus phoxinus (L.), and other Ostariophysi (Pisces) (1985)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of fish biology 27 (1985), S. 0 
    Details
    ISSN: 1095-8649
    Quelle: Blackwell Publishing Journal Backfiles 1879-2005
    Thema: Biologie
    Notizen: The alarm substance cells and mucous cells of various species of Ostariophysi do not react uniformly to treatment with androgen and other steroid hormones. This indicates that the mechanism controlling formation of alarm substance is not uniform in the Ostariophysi and three exist at least two such mechanisms, one of which is operated by androgens and perhaps other steroid hormones of the gonads. Steroid hormones from the adrenal cortex proved ineffective. In the European minnow, anti-androgen (androcur®, cyproteroneacetate) had an efrect opposite to that of androgen, while androgen treatment upon this species led to an increase in the size of mucous cells and a reduction in the alarm substance cells. The epidermis of androgen-treated European minnows contained neither alarm substance cells nor alarm substance, and skin extracts of such fish proved ineffective in behaviour experiments upon fish schools. The effect of further hormone treatments are discussed and the reactions of the European minnow are compared to those of other species.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1111/j.1095-8649.1985.tb03201.x
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