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  • 1
    Electronic Resource
    Electronic Resource
    Superlattice parameters for optimum absorption in InAs/InxGa1−xSb superlattice infrared detectors (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 77 (1995), S. 5739-5746 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.359218
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  • 2
    Electronic Resource
    Electronic Resource
    Variable-structure cell model of two-dimensional liquids and glasses (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 8995-9004 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468953
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  • 3
    Electronic Resource
    Electronic Resource
    An investigation of the effects of two level system coupling on single molecule lineshapes in low temperature glasses (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 7434-7450 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476164
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  • 4
    Electronic Resource
    Electronic Resource
    Ultrafast extended x-ray absorption fine structure (EXAFS)—theoretical considerations (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 6238-6246 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479928
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  • 5
    Electronic Resource
    Electronic Resource
    Variance and width of absorption lines of single molecules in low temperature glasses (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 6876-6883 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476102
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