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  • American Institute of Physics (AIP)  (5)
  • Society of Geomagnetism and Earth, Planetary and Space Sciences  (3)
  • Institute of Physics
  • 1995-1999  (8)
  • 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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  • 6
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    Observations of the Magnetosheath near the Nominal Tail Axis during the Geomagnetic Storm of January 25, 1993 (1996)
    Society of Geomagnetism and Earth, Planetary and Space Sciences
    Details
    Publication Date: 1996-01-01
    Description: This paper reports plasma and field changes associated with magnetosheath encounters by GEOTAIL near the center of the tail axis at a down tail distance of X=-94 RE during the main phase of a weak geomagnetic storm on January 25, 1993. The magnetosheath entries of the satellite occurred in conjunction with an enhancement in the total pressure, having a temporal scale between 2 and 18 minutes. It is suggested that temporal and/or local structures in the solar wind such as pressure pulses caused these brief encounters. Assuming the flow direction in the magnetosheath to be parallel to the magnetotail boundary, we estimate the tail magnetic flux and the dimension of the tail for the encounter events. It is shown that solar wind variations, which generate geomagnetic storms, alter significantly the orientation as well as the size of the distant tail, such that the magnetosheath can be observed even near the center of the “average” tail. © 1996, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.
    Print ISSN: 0022-1392
    Topics: Geosciences
    Published by Society of Geomagnetism and Earth, Planetary and Space Sciences
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  • 7
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    Large Field Events in the Distant Magnetotail During Magnetic Storms (1996)
    Society of Geomagnetism and Earth, Planetary and Space Sciences
    Details
    Publication Date: 1996-01-01
    Description: Although the magnetic field intensity in the lobe of the magnetotail is usually in the range of 7-10 nT at distances beyond 150 RE as observed by ISEE 3 and GEOTAIL, it becomes at times more than 30 nT. The magnetic field of as large as 53 nT was observed in the distant tail lobe by GEOTAIL at -182 RE on March 9, 1993, when a moderate magnetic storm was in progress. Prom the scan of daily plots of magnetic field data during the two year period of the GEOTAIL observations, intervals were chosen when the lobe field strength exceeding 20 nT was registered. Large field events are found to mostly occur during a growing stage of the ring current. Events observed beyond 100 RE downstream from the earth are associated with intermittent and temporal excursions of the spacecraft into the magnetotail. The characteristic time scale of magnetospheric period is approximately 20 minutes. These entries into the magnetotail are usually preceded by a strong southward component of sheath magnetic field. Since magnetic storms are associated with enhanced solar wind flux and IMF conditions, reconfiguration of the magnetotail due to increases of the total pressure in the sheath and directional changes of the sheath plasma flow should control the probability of spacecraft residence in the magnetotail. It is confirmed that the pressure balance approximately holds at the tail magnetopause under enhanced solar wind conditions by using high resolution plasma data. Observations of large lobe field events suggest that additional magnetic fluxes caused by the merging process near the earth during magnetic storms constrain the magnetotail radius from decreasing the nominal value significantly, although the magnetotail is in a compressed state due to the increase in exterior pressure.
    Print ISSN: 0022-1392
    Topics: Geosciences
    Published by Society of Geomagnetism and Earth, Planetary and Space Sciences
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  • 8
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    Geotail Observations of an Unusual Magnetotail under Very Northward IMF Conditions (1996)
    Society of Geomagnetism and Earth, Planetary and Space Sciences
    Details
    Publication Date: 1996-01-01
    Description: On March 29, 1993, The Geotail spacecraft was 134RE downstream from the earth and within 12 RE of the axis of the magnetotail whose orientation was calculated from the solar wind direction measured by IMP 8. The IMF had a magnitude near 12 nT and remained steady and roughly 50° northward for 9 hours. Geotail moved back and forth between two regions, an interior region with characteristics similar to the tail and an exterior region with characteristics similar to the magnetosheath. The flow in the dense rapidly flowing exterior region was lilted 8° toward the tail axis rather than parallel to a cylindrical tail. Calculated boundary normals were primarily in the solar magnetospheric Y direction but with a small X component consistent with a slightly flaring or cylindrical tail. The unusual flow direction in the exterior region along with the presence of slower, less dense, tailward flowing plasma on taillike field lines inside the boundary suggest that the exterior flow has a component across the boundary and into the tail. These data support accumulating evidence from MHD simulations that suggest that the magnetotail assumes a very different configuration when the IMF remains very northward for periods of at least several hours. In this unusual configuration, field lines which would normally extend into the distant tail instead close on the flanks and nearer to the earth, creating a distant tail with less open flux and a reduced Y dimension. © 1996, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.
    Print ISSN: 0022-1392
    Topics: Geosciences
    Published by Society of Geomagnetism and Earth, Planetary and Space Sciences
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