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  • 1
    Digitale Medien
    Digitale Medien
    Dislocation generation and crack growth under monotonic loading (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 6249-6264 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The processes of crack growth and dislocation emission induced by the crack tip are investigated. A crystal with cubic lattice of atoms under plane strain conditions is considered. The main principles of the nanofracture mechanics approach employed in this study are outlined. Both ductile and brittle mechanisms of crack growth in the crystal are examined in nano- or interatomic scale. Only the fundamental constants of the classical theory of dislocations are used which include the interatomic spacing, elastic constants, the Schmid friction constant, and the true surface energy of crystal lattice. The efficient solution of the elastic problem for an arbitrary number of dislocations near the crack tip is obtained in terms of complex potential functions. The equilibrium of dislocation pairs near the crack tip during monotonic loading is investigated. It is shown that dislocation generation at the crack tip occurs at certain quantum levels of external load. The magnitude of external load corresponding to crack growth initiation and emission of the first pair of dislocations is calculated. The mathematical problem for an arbitrary N number of dislocation pairs near the crack tip is reduced to a parametric system of N nonlinear equations, where the stress intensity factor of external load KI plays the role of parameter and N the role of discrete time. The minimum value of KI at which the solution of this system of equations exists corresponds to the stress intensity factor at which the Nth pair of dislocations is generated. The numerical method is presented to determine the minimum value of KI. The approximate method of self-consistent field is employed to reduce the order of the system of nonlinear equations. The approximate method is used to calculate the fracture curve KI(lc) relating the value of KI which maintains the crack growth to the crack length increment lc. The exact solution is also studied, and numerical results are given for a crack in an aluminum specimen and involve the quantum levels of external load corresponding to the moments of dislocation generation and the values of the superfine stress intensity factor up to 150 dislocations. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.360572
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  • 2
    Digitale Medien
    Digitale Medien
    Transitions to a new chiral phase in a Langmuir monolayer (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 8216-8220 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Isotherms and x-ray diffraction studies of eicosanoic acid Langmuir monolayers show a phase in which the molecular tilt is intermediate between nearest-neighbor (NN) and next-nearest-neighbor (NNN) directions. The transition from this "I" phase to an NN-tilted structure is first order, with a ∼60° change in the tilt direction, while the transition to an NNN-tilted structure is apparently continuous. These results can be explained using a Landau-type theory for uniaxially distorted lattices, which is a modification of an existing theory for hexagonal lattices [J. V. Selinger and D. R. Nelson, Phys. Rev. Lett. 61, 416 (1988)]. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.473825
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  • 3
    Digitale Medien
    Digitale Medien
    Absolute measurement of spatial and spectral characteristics of bremsstrahlung using the photoexcitation of nuclear isomers (1993)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 2298-2305 
    Details
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: A method of obtaining absolute, direct measurements of the spatial and spectral characteristics of bremsstrahlung is discussed. This technique, called x-ray activation of nuclei (XAN) is based on the use of well-known photonuclear reactions which populate long-lived nuclear isomers. These populations sample incident photon continua at discrete excitation energies and effectively store this information for convenient retrieval following the irradiation of gram-sized targets. Recently a series of experiments has been conducted which has significantly expanded the available data for the photoexcitation of a wide range of isomers at higher energies. Thus it has become feasible to extend the use of XAN to energies approaching 4 MeV. The utility of this technique is demonstrated by the characterization of bremsstrahlung from the newly installed research linac at the University of Texas at Dallas.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1143925
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  • 4
    Digitale Medien
    Digitale Medien
    Vacuum near-field scanning optical microscope for variable cryogenic temperatures (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 3458-3463 
    Details
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: We describe the design of a novel near-field scanning optical microscope for cryogenic temperatures and operation in vacuum. A helium flow cryostat is used for active temperature control of the sample in the range between 8 and 330 K, while all components of the near-field microscope are kept at room temperature. This design greatly simplifies near-field microscopy at variable sample temperatures and permits large piezoelectric scan ranges of up to 100×100×10 μm 3, the implementation of hardware-linearized piezoelectric scan stages, as well as flexible coarse positioning. First experiments with single GaAs quantum wires demonstrate the excellent performance of this new instrument. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1148308
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  • 5
    Digitale Medien
    Digitale Medien
    High intensity electron channeling and perspectives for a bright tunable x-ray source (1990)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 57 (1990), S. 2956-2958 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Using the continuous wave electron beam of the new Darmstadt superconducting electron accelerator and high intensity axial channeling in a diamond crystal, the possibility for the production of an intense tunable photon source in the x-ray region is investigated. Due to low emittance of the machine and its high current it has become possible, for the first time, to produce channeling radiation in the energy region of a few keV with an intensity of up to 2×1010 photons/s in an x-ray energy window of 10%. This is six orders of magnitude more than what has been observed previously.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.103738
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  • 6
    Digitale Medien
    Digitale Medien
    The intermolecular .pi.-amphoteric character of p-tricyanovinyldimethylaniline (1979)
    s.l. : American Chemical Society
    Journal of the American Chemical Society 101 (1979), S. 7079-7080 
    Details
    ISSN: 1520-5126
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/ja00517a057
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  • 7
    Digitale Medien
    Digitale Medien
    Electroluminescence of a cubic GaN/GaAs (001) p–n junction (2000)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 76 (2000), S. 13-15 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A cubic GaN p–n diode has been grown on n-type GaAs (001) substrates by plasma assisted molecular epitaxy. For p- and n-type doping, elemental Mg and Si beams have been used, respectively. The optical properties are characterized by photoluminescence at room temperature and 2 K. Current–voltage and capacitance–voltage measurements of the cubic GaN n+–p junction are performed at room temperature. The electroluminescence at 300 K is measured through a semitransparent Au contact. A peak emission at 3.2 eV with a full width at half maximum as narrow as 150 meV is observed, indicating that near-band edge transitions are the dominating recombination processes in our device. A linear increase of the electroluminescence intensity with increasing current density is measured. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.125640
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  • 8
    Digitale Medien
    Digitale Medien
    Near-field scanning optical microscopy of polarization bistable laser diodes (1996)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 69 (1996), S. 2471-2473 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: TE/TM polarization bistability in a λ=1.3 μm ridge-waveguide InGaAsP/InP bulk laser is studied by near-field scanning optical microscopy with an optical resolution of better than λ/8. The near-field mode profiles of TE and TM emission show different lateral widths and distinctly different mode center positions. This lateral shift is related to a nonuniform strain distribution along the active layer. Based on this strain gradient, we present a model that accounts for the hysteresislike current dependence of the polarization resolved laser output. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.117501
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  • 9
    Digitale Medien
    Digitale Medien
    Exciton transport into a single GaAs quantum wire studied by picosecond near-field optical spectroscopy (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 2176-2178 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We report a time-resolved near-field luminescence study of excitonic real-space transfer into single GaAs quantum wires. Excitons generated by local optical excitation in a 250 nm spot undergo diffusive transport over a length of several microns and are subsequently trapped into the quantum wire by optical phonon emission. Local energy barriers in the vicinity of the quantum wire, originating from the epitaxial growth mechanism of the nanostructure, directly influence the real-space transfer dynamics and trapping efficiency. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.122414
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  • 10
    Digitale Medien
    Digitale Medien
    Optical near-field photocurrent spectroscopy: A new technique for analyzing microscopic aging processes in optoelectronic devices (1996)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 69 (1996), S. 3981-3983 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The potential of optical near-field photocurrent spectroscopy for analyzing microscopic aging processes in optoelectronic devices is demonstrated. The technique combines the subwavelength spatial resolution of near-field optics with tunable laser excitation, allowing for selective investigation of specific parts of the device structure. Experiments on GaAs/(AlGa)As high power laser diodes before and after accelerated aging provide direct visualization of defect growth within the p-i-n junction and information on aging-enhanced recombination processes close to the laser facet. The effect of wave guiding of the exciting light on the image formation is discussed. The nondestructiveness makes this technique a particularly attractive method for in situ analysis in high power laser diodes. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.117844
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