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  • 1
    Publication Date: 2014-12-13
    Description: Detailed investigations of the quadratic nonlinear response of a series of new polyimides with covalently attached chromophore DR13 are performed by the Maker fringes method in the range of fundamental wavelength from 850 to 1450 nm. Polymer films with thickness of 100–400 nm were spin-coated on glass substrates and corona poled. For these materials, the maximum values of the second harmonic generation coefficients d 33 are 80–120 pm/V. A red shift of the nonlinear response dispersion with respect to the linear absorption spectrum was observed for the DR13 chromophore. The temperature dependences of linear absorption and nonlinear coefficients d 33 for studied structures are observed. It was found that the temperature changes of the absorption spectra lead to appreciable contribution to the value of the nonlinear coefficient d 33 . The demonstrated high temperature stability (up to 120 °C) of chromophore-containing polyimide thin films makes it possible to eliminate the degradation of their nonlinear optical properties in the future applications of such structures.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 2
    Publication Date: 2014-10-08
    Description: Patterns of plastic deformation of nanocrystalline (NC) technical grade VT1-0 titanium, are studied in quasi-static tensile experiments, with the average size of the grain d , ranging from 35 nm to 2 μ m, at the temperature interval 4.2 K 〈 T 〈 395 K. The wide range of grain size, and grain size distribution, was made possible by cryomechanical grain fragmentation, which involves rolling at liquid nitrogen temperature, and subsequent annealing. At temperatures of T  ≲ 30 K, smooth deformation curves become wavy, and as the temperature is continually lowered to T jump ≈ 22 K, they become jagged. A correlation is found between the relative amplitude of the stress jump Δσ/σ and the rate of strain hardening θ = (∂σ/∂ e ) ė . A significant increase in plasticity is observed, especially noticeable at temperatures T ≲140 K, if a small fraction (≈15%) of submicron-sized grains is present. This is explained by a combination of dynamic grain growth under the influence of tensile stress, and nanotwinning activated in submicron grains. At cryogenic temperatures, abnormal grain growth favors nanotwinning during deformation. In nanometer-sized grains ( d ≲ 50 nm), twinning is not observed.
    Print ISSN: 1063-777X
    Electronic ISSN: 1090-6517
    Topics: Physics
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  • 3
    Publication Date: 2014-10-10
    Description: Patterns of plastic deformation of nanocrystalline (NC) technical grade VT1-0 titanium, are studied in quasi-static tensile experiments, with the average size of the grain d , ranging from 35 nm to 2 μ m, at the temperature interval 4.2 K 〈 T 〈 395 K. The wide range of grain size, and grain size distribution, was made possible by cryomechanical grain fragmentation, which involves rolling at liquid nitrogen temperature, and subsequent annealing. At temperatures of T  ≲ 30 K, smooth deformation curves become wavy, and as the temperature is continually lowered to T jump ≈ 22 K, they become jagged. A correlation is found between the relative amplitude of the stress jump Δσ/σ and the rate of strain hardening θ = (∂σ/∂ e ) ė . A significant increase in plasticity is observed, especially noticeable at temperatures T ≲140 K, if a small fraction (≈15%) of submicron-sized grains is present. This is explained by a combination of dynamic grain growth under the influence of tensile stress, and nanotwinning activated in submicron grains. At cryogenic temperatures, abnormal grain growth favors nanotwinning during deformation. In nanometer-sized grains ( d ≲ 50 nm), twinning is not observed.
    Print ISSN: 1063-777X
    Electronic ISSN: 1090-6517
    Topics: Physics
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  • 4
    Publication Date: 2014-10-22
    Description: We report a procedure to determine the frequency-dependent conductance of quantum Hall structures in a broad frequency domain. The procedure is based on the combination of two known probeless methods—acoustic spectroscopy and microwave spectroscopy. By using the acoustic spectroscopy, we study the low-frequency attenuation and phase shift of a surface acoustic wave in a piezoelectric crystal in the vicinity of the electron (hole) layer. The electronic contribution is resolved using its dependence on a transverse magnetic field. At high frequencies, we study the attenuation of an electromagnetic wave in a coplanar waveguide. To quantitatively calibrate these data, we use the fact that in the quantum-Hall-effect regime the conductance at the maxima of its magnetic field dependence is determined by extended states. Therefore, it should be frequency independent in a broad frequency domain. The procedure is verified by studies of a well-characterized p -SiGe/Ge/SiGe heterostructure.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 5
    Publication Date: 2014-10-28
    Description: For submonolayer quantum dot (QD) based photonic devices, size and density of QDs are critical parameters, the probing of which requires indirect methods. We report the determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer QDs, based on spectral analysis of the optical signature of Aharanov-Bohm (AB) excitons, complemented by photoluminescence studies, secondary-ion mass spectroscopy, and numerical calculations. Numerical calculations are employed to determine the AB transition magnetic field as a function of the type-II QD radius. The study of four samples grown with different tellurium fluxes shows that the lateral size of QDs increases by just 50%, even though tellurium concentration increases 25-fold. Detailed spectral analysis of the emission of the AB exciton shows that the QD radii take on only certain values due to vertical correlation and the stacked nature of the QDs.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 6
    Publication Date: 2014-08-30
    Description: The influence of high magnetic fields on coherent transport is investigated. A monolayer graphene quantum ring is fabricated and the Aharonov-Bohm effect is observed. For increased magnitude of the magnetic field, higher harmonics appear. This phenomenon is attributed to an increase of the phase coherence length due to reduction of spin flip scattering.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
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  • 7
    Publication Date: 2014-08-07
    Description: The appearance of the static domains with depth above 200  μ m in the bulk of MgO-doped lithium niobate single crystals as a result of focused electron beam irradiation of Z + -polar surface was demonstrated. The created domain patterns were visualized by high-resolution methods including piezoresponse force microscopy, scanning electron microscopy, and confocal Raman microscopy. The main stages of the domain structure formation were revealed and explained in terms of the original model.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
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  • 8
    Publication Date: 2014-08-19
    Description: Measurements of the flux of fusion products from high temperature plasmas provide valuable insights into the ion energy distribution, as the fusion reaction rate is a very sensitive function of ion energy. In C-2, where field reversed configuration plasmas are formed by the collision of two compact toroids and partially sustained by high power neutral beam injection [M. Binderbauer et al. , Phys. Rev. Lett.105, 045003 (2010); M. Tuszewski et al. , Phys. Rev. Lett.108, 255008 (2012)], measurements of DD fusion neutron flux are used to diagnose ion temperature and study fast ion confinement and dynamics. In this paper, we will describe the development of a new 3 MeV proton detector that will complement existing neutron detectors. The detector is a large area (50 cm 2 ), partially depleted, ion implanted silicon diode operated in a pulse counting regime. While the scintillator-based neutron detectors allow for high time resolution measurements (∼100 kHz), they have no spatial or energy resolution. The proton detector will provide 10 cm spatial resolution, allowing us to determine if the axial distribution of fast ions is consistent with classical fast ion theory or whether anomalous scattering mechanisms are active. We will describe in detail the diagnostic design and present initial data from a neutral beam test chamber.
    Print ISSN: 0034-6748
    Electronic ISSN: 1089-7623
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
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  • 9
    Publication Date: 2014-06-10
    Description: Deep hole traps were studied in bulk free-standing GaN crystals and in thinner (10–20  μ m) GaN films prepared by hydride vapor phase epitaxy (HVPE) on sapphire. Six hole traps in different combinations were detected in these crystals, H1 (activation energy 0.92–0.94 eV), H2 (0.55 eV), H3 (0.65–0.7 eV), H4 (0.85–0.9 eV), H5 (1.1–1.2 eV), and H6 (0.95–1.05 eV). The dominant traps in all samples were the H5 and H6 traps that were attributed, respectively, to gallium vacancy complexes with oxygen (V Ga -O) and substitutional carbon related centers. We associate the H5 hole traps with the red luminescence bands, the H4 hole traps with the green luminescence bands, and the H6 hole traps with the yellow luminescence bands often observed in HVPE GaN. These attributions are based on the low energy thresholds of the deep traps optical excitation spectra and the depth of the respective trap levels.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 10
    Publication Date: 2014-07-30
    Description: The appearance of ferromagnetic order up to 400 K upon KrF (248 nm) laser irradiation is reported in un-doped SrTiO 3 (STO) single crystal. The high resolution x-ray photo emission spectroscopy (XPS) measurements reveal a strong shift of Sr-, Ti-, and O-related peaks. X-ray diffraction of laser annealed STO does not reveal a signature of either secondary magnetic or amorphous phases. 300 K X-band (∼9.543 GHz) angle-dependent electron paramagnetic resonance (EPR) measurements showed no evidence of additional magnetic peaks upon laser irradiation. XPS and EPR data did not provide a strong evidence of Ti 3+ formation upon laser annealing. No differences in the visible 300 K Raman spectra of pristine and laser annealed STO are noticed. Interestingly, the magnetic moment is decreased by almost 10-fold upon oxygen annealing of laser annealed STO, inferring that oxygen vacancies play an important role in establishing the observed ferromagnetism.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
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