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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 6238-6246 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Inspired by the recent experimental demonstration of ultrafast x-ray absorption spectroscopy, we present a framework for the calculation of extended x-ray absorption fine structure (EXAFS) spectra on the ultrafast (femtosecond to picosecond) time scale. Model calculations for gas phase I2, evolving under the influence of laser pumping, demonstrate that ultrafast EXAFS has the potential to serve as a direct probe of nuclear dynamics, including time-dependent interatomic separations and relative orientations. The feasibility of ultrafast EXAFS as a viable and useful experimental technique is discussed. © 1999 American Institute of Physics.
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 7434-7450 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theoretical framework for determining the lineshapes of single molecules in low temperature glasses is presented. Our methods, in contrast to previous efforts, include coupling between two level systems (TLSs). This framework is applied to the physical system consisting of the chromophore terrylene embedded in the amorphous host polystyrene. We analyze the effect of TLS-TLS coupling on both individual lineshapes and linewidth histograms. Our results indicate that, although TLS-TLS coupling is certainly capable of producing noticeable changes in individual spectral lines relative to the uncoupled results, linewidth histograms are relatively unaffected by said coupling. An interpretation of this result is suggested. © 1998 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 77 (1995), S. 5739-5746 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The linear absorption coefficient of InAs/InxGa1−xSb superlattices is optimized with respect to layer widths, indium content, substrate type and substrate orientation, interface type, and choice of buffer layers based on a model envelope-function approach (EFA) involving the solution of a 6×6 EFA Hamiltonian (heavy, light, and conduction bands) for wave functions and subband energies. Free-standing superlattices as well as superlattices matched to a number of substrates are considered. In general, increasing the indium mole content from 0 to 0.4 doubles the magnitude of absorption. Changing the substrate orientation from [001] to [111] significantly increases absorption in all cases studied due to the increased heavy-hole mass and the larger InAs-conduction-band–InGaSb-valence-band offset in the [111] direction. The use of an In0.4Ga0.6Sb substrate leads to higher absorption because all the beneficial effects of strain are placed in the InAs layer, which is more sensitive to strain than is the InGaSb layer. The larger valence–conduction-band offset for InSb than for GaAs interfaces also leads to higher absorption. The model results agree best with available data when a 100 meV InAs-conduction-band–GaSb-valence-band offset is used. Specific superlattice parameters that optimize absorption for free-standing superlattices on GaSb at three cutoff wavelengths are proposed. © 1995 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 6876-6883 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We consider the line shapes of single molecules in low temperature glasses due to dipole–dipole interaction between the molecules and other particles of the system. Motivated by computer simulations, we employ a simplified formula for the absorption lines and derive an analytic expression for the distribution of the variance of such absorption lines. The simplest version of this distribution—derived for pointlike particles without cutoff of the interaction at small distances—already accounts for the qualitative features of width histograms measured experimentally or gained by computer simulation. We further analyze the effect of the minimal approach distance between the chromophore and the perturbing particles and employ an approximate relation between the variance and the full width at half maximum of the absorption lines. We find that the main characteristics of the histogram of widths stem from the (homogeneous) distribution of interactions in space and do not reflect the distribution of the internal parameters of the underlying model. © 1998 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 8995-9004 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using a recently developed cell theory of liquids and glasses, a simple model calculation of the equilibrium and quenched properties of two-dimensional systems is presented. The particular model chosen is essentially a Bernal liquid, so that the distribution of molecular coordination numbers plays a central role. The basic model of Bernal is simplified, however, in that the Bernal polygons available to the system are restricted to a few basic types and is further modified to allow for small-amplitude vibrations of the molecules about their lattice sites. Quenching is then effected by freezing the coordination-number distribution (which is determined by a single ordering parameter, in the present, simplified treatment) at its "glass-transition'' value. The equilibrium and quenched thermodynamic functions are calculated analytically, and substantial qualitative agreement with well-known characteristics of real liquids and glasses is obtained. © 1995 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6216-6221 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new kind of cell theory—one which formally accounts for the possibility of amorphous phases—is presented. The equilibrium partition function is transformed exactly by first classifying all particle configurations according to multidimensional potential-energy minima and then assigning to each minimum a system-spanning set of cells such that, for configurations near enough the minimum, each cell contains one particle. The theory is then extended to general amorphous systems, and approximate descriptions of supercooled liquids and glass transitions are given.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 66 (1989), S. 3082-3087 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In two previous papers [J. Appl. Phys. 62, 3267 (1987); 64, 6373 (1988)], we used a highly accurate numerical model to solve the problem of equilibrium barrier formation in graded Hg1−xCdxTe heterojunctions. However, we concentrated solely on narrow-gap p on wide-gap n heterojunctions in which the barrier forms in the conduction band. The present calculation is an extension of that work, but with emphasis on wide-gap p on narrow-gap n heterojunctions. We perform a full parametric study, varying doping concentrations (symmetric and asymmetric), cadmium compositions, and cadmium grading widths. All band profiles are calculated with and without a valence-band offset. Clear trends are observed. Without the valence-band offset, for the larger grading widths, there is a tendency for the valence band to bow down on the n side. The band bowing acts as a potential barrier for the minority carriers. When the valence-band offset is included though, the existence of a barrier to minority carriers depends on the grading width. For the most narrowly graded junctions, a potential well for the minority carriers is present. In the present study, we assume a valence-band offset of 300 meV for HgTe:CdTe. The effects of elevated temperatures and interdiffusion of dopants are also studied, but appear to be inconsequential for the present cases.
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 64 (1988), S. 6373-6378 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We use a highly accurate numerical computer model to solve the problem of equilibrium barrier formation in graded Hg1−xCdxTe heterojunctions including valence-band offsets. The present calculation is an extension of our previous work [J. Appl. Phys. 62, 3267 (1987)], which incorporated physical features designed to improve upon previous calculations, but employed the common anion rule. First, we analyze and compare our results to the recent work of Oda [Infrared Phys. 27, 49 (1987)] and find significant differences. Then, we make a comparison with the results of our previous work. We observe clear trends between our results with and without the valence-band offset. Unlike Oda, we do not make general predictions with regard to the conditions needed to support the formation and growth of a barrier in the conduction band, but instead find the band profiles to be a complex function of all the junction design parameters. For the present study involving narrow gap p on wide gap n heterojunctions, and the assumed: HgTe:CdTe 300-meV valence-band offset, the band profiles do not differ significantly from the profiles assuming the common anion rule.
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 62 (1987), S. 2533-2540 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We are presenting an analytic model for the figures of merit of a novel extrinsic infrared quantum detector—the blocked impurity band (BIB) detector. The detector consists of top and bottom contacts, a heavily doped active layer, and a nearly intrinsic layer, called the blocking layer, to stop the motion of hopping carriers in the impurity band. The responsivity, gain, excess noise factor, and detectivity of the BIB detector are calculated as functions of the device dimensions, doping concentrations, and the applied reverse bias, which controls the electric field in the depletion region, devoid of hopping carriers, of the device underneath the blocking layer. Central to our model is the inclusion of impact ionization of carriers in the calculation of the detector response and of the associated noise. For practical detector dimensions and doping concentrations, and at 2-V reverse bias, we calculate the responsivity to be on the order of 2 A/W, and detectivities, with 1012 photons/cm2 s background photon flux, on the order of 1013 cm Hz1/2/W. We provide analytic expressions for the figures of merit which should prove useful to other researchers in the field who want to optimize the detector design.
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 62 (1987), S. 310-311 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is shown that the photoconductive gain of a longitudinal detector is given by the ratio of the average carrier lifetime to the interelectrode transit time regardless of the absorption profile of the detector. Both corpuscular and macroscopic points of view are considered.
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