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Träfflista för sökning "WFRF:(Lu W) srt2:(2000-2004)"

Sökning: WFRF:(Lu W) > (2000-2004)

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1.
  • Lockwood, M, et al. (författare)
  • Coordinated Cluster and ground-based instrument observations of transient changes in the magnetopause boundary layer during an interval of predominantly northward IMF : relation to reconnection pulses and FTE signatures
  • 2001
  • Ingår i: Annales Geophysicae. - 0992-7689. ; 19:10-12, s. 1613-1640
  • Forskningsöversikt (övrigt vetenskapligt)abstract
    • We study a series of transient entries into the low-latitude boundary layer (LLBL) of all four Cluster spacecraft during an outbound pass through the mid-afternoon magnetopause ([X(GSM), Y(GSM), Z(GSM)] approximate to [2, 7, 9] R(E)). The events take place during an interval of northward IMF, as seen in the data from the ACE satellite and lagged by a propagation delay of 75 min that is well-defined by two separate studies: (1) the magnetospheric variations prior to the northward turning (Lockwood et al., 2001, this issue) and (2) the field clock angle seen by Cluster after it had emerged into the magnetosheath (Opgenoorth et al., 2001, this issue). With an additional lag of 16.5 min, the transient LLBL events cor-relate well with swings of the IMF clock angle (in GSM) to near 90degrees. Most of this additional lag is explained by ground-based observations, which reveal signatures of transient reconnection in the pre-noon sector that then take 10-15 min to propagate eastward to 15 MLT, where they are observed by Cluster. The eastward phase speed of these signatures agrees very well with the motion deduced by the cross-correlation of the signatures seen on the four Cluster spacecraft. The evidence that these events are reconnection pulses includes: transient erosion of the noon 630 nm (cusp/cleft) aurora to lower latitudes; transient and travelling enhancements of the flow into the polar cap, imaged by the AMIE technique; and poleward-moving events moving into the polar cap, seen by the EISCAT Svalbard Radar (ESR). A pass of the DMSP-F15 satellite reveals that the open field lines near noon have been opened for some time: the more recently opened field lines were found closer to dusk where the flow transient and the poleward-moving event intersected the satellite pass. The events at Cluster have ion and electron characteristics predicted and observed by Lockwood and Hapgood (1998) for a Flux Transfer Event (FTE), with allowance for magnetospheric ion reflection at Alfvenic disturbances in the magnetopause reconnection layer. Like FTEs, the events are about 1 R(E) in their direction of motion and show a rise in the magnetic field strength, but unlike FTEs, in general, they show no pressure excess in their core and hence, no characteristic bipolar signature in the boundary-normal component. However, most of the events were observed when the magnetic field was southward, i.e. on the edge of the interior magnetic cusp, or when the field was parallel to the magnetic equatorial plane. Only when the satellite begins to emerge from the exterior boundary (when the field was northward), do the events start to show a pressure excess in their core and the consequent bipolar signature. We identify the events as the first observations of FTEs at middle altitudes.
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2.
  • Arnaudov, B., et al. (författare)
  • Free-to-bound radiative recombination in highly conducting InN epitaxial layers
  • 2004
  • Ingår i: Superlattices and Microstructures. - 0749-6036. ; 36:4-6, s. 563-571
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a theoretical simulation of near-band-edge emission spectra of highly conducting n-InN assuming the model of 'free-to-bound' radiative recombination (FBRR) of degenerate electrons from the conduction band with nonequilibrium holes located in the valence band tails. We also study experimental photoluminescence (PL) spectra of highly conducting InN epitaxial layers grown by MBE and MOVPE with electron concentrations in the range (7.7 × 1017-6 × 1018) cm-3 and find that the energy positions and shape of the spectra depend on the impurity concentration. By modeling the experimental PL spectra of the InN layers we show that spectra can be nicely interpreted in the framework of the FBRR model with specific peculiarities for different doping levels. Analyzing simultaneously the shape and energy position of the InN emission spectra we determine the fundamental bandgap energy of InN to vary between Eg = 692 meV for effective mass mn0 = 0.042m0 and Eg =710 meV for mn0 = 0.1m0. © 2004 Elsevier Ltd. All rights reserved.
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3.
  • Darakchieva, Vanya, 1971-, et al. (författare)
  • Deformation potentials of the E1 (TO) and E2 modes of InN
  • 2004
  • Ingår i: Applied Physics Letters. - 0003-6951. ; 84:18, s. 3636-3638
  • Tidskriftsartikel (refereegranskat)abstract
    • The determination of deformation potentials of E1(TO) and E 2 modes of InN were discussed. The deformation potentials were evaluated for two sets of stiffness constants using x-ray diffraction, IR spectroscopic ellipsometry (IRSE), Raman scattering, and Grüneisen parameter values. The InN layer were grown on GaN buffer layers on (0001) sapphire by molecular beam epitaxy. It was found that the strain-free values of the InN E1(TO) mode was 477.9 cm-1 and 491.9 cm -1 for the E2 modes.
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4.
  • Darakchieva, Vanya, et al. (författare)
  • Infrared ellipsometry and Raman studies of hexagonal InN films : Correlation between strain and vibrational properties
  • 2004
  • Ingår i: Superlattices and Microstructures. - 0749-6036. ; 36:4-6, s. 573-580
  • Tidskriftsartikel (refereegranskat)abstract
    • The vibrational properties of InN films with different strain have been studied using Infrared ellipsometry and Raman scattering spectroscopy. We have established a correlation between the phonon mode parameters and the strain, which allows the determination of the deformation potentials and the strain-free frequencies of the InN E1(TO) and E2 modes. The LO phonons and their coupling to the free-carrier plasmon excitations are also discussed in relation to the carrier concentration in the films. © 2004 Elsevier Ltd. All rights reserved.
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5.
  • Fu, Y, et al. (författare)
  • Electron mobilities, Hall factors, and scattering processes of n-type GaN epilayers studied by infrared reflection and Hall measurements
  • 2003
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - American Physical Society. - 1098-0121. ; 67:11
  • Tidskriftsartikel (refereegranskat)abstract
    • We have studied the drift and Hall mobilities of electrons in metal-organic chemical-vapor-deposited wurtzite GaN thin films on sapphire substrate by infrared (IR) reflection and Hall measurements. By analyzing the Hall factor (the ratio between the drift mobility obtained from IR reflection spectra and the Hall mobility from the Hall measurements), it has been concluded that the electron mobility in GaN epilayer is determined by the ionized impurity when the electron concentration is low. At a high carrier concentration of 3.2x10(18) cm(-3), electronic states of more than 70 meV become populated taking into account the thermal excitation, so that the optical-phonon-scattering process becomes activated (the optical-phonon energy is 69.43 meV obtained from IR reflection measurements). Thus, in highly doped wurtzite GaN epilayers, ionized-impurity- and optical-phonon-scattering processes jointly determine the carrier transport properties.
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6.
  • Fu, Y, et al. (författare)
  • Photocurrents of 14 mu m quantum-well infrared photodetectors
  • 2003
  • Ingår i: Journal of Applied Physics. - American Institute of Physics. - 0021-8979. ; 93:12, s. 9432-9436
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the factors that determine photogenerated carriers and response wavelengths of photocurrents of long wavelength (similar to14 mum) quantum well (QW) infrared photodetectors (QWIPs). The material structures of QWIPs are first characterized by the photoluminescence measurements (PL). By calculating the density of photogenerated carriers in the continuum above the energy barriers using the PL calibrated QWIP structures, we have demonstrated that due to the sample quality, the photocarriers can be either in miniband states (Bloch states in the multiple quantum wells), or they transport from one quantum well to the next in the form of running waves. By including possible scattering processes at the QWIP working temperature to link the theoretically calculated photocarrier density with the experimentally measured photocurrent, it is shown that the width of the photocurrent peaks of 14 mum GaAs/AlGaAs QWIPs under investigation is determined by the optical phonon emissions of photocarriers. We have further calculated the densities of photocarriers in the QWIPs reported in the literature. It is shown that the Bloch wave boundary conditions are appropriate for QWIPs with narrow QWs, whereas running wave boundary conditions are appropriate for wide QWs.
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7.
  • Fu, Ying, 1964-, et al. (författare)
  • Photoluminescence spectra of doped GaAs films
  • 2004
  • Ingår i: Applied Physics A: Materials Science & Processing. - 0947-8396. ; 79:3, s. 619-623
  • Tidskriftsartikel (refereegranskat)abstract
    • We have studied the dependence of the photoluminescence (PL) spectrum on the doping level and the film thickness of n-GaAs thin films, both experimentally and theoretically. It has been shown theoretically that modification of the PL spectrum of p-type material by p-type doping is very small due to the large valence-band hole effective mass. The PL spectrum of n-type material is affected by two factors: (1) the electron concentration which determines the Fermi level in the material; (2) the thickness of the film due to re-absorption of the PL signal. For the n-type GaAs thin films under current investigation, the doping level as well as the film thickness can be very well calibrated by the PL spectrum when the doping level is less than 2 x 10(18) cm(-3) and the film thickness is in the range of the penetration length of the PL excitation laser.
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8.
  • Fu, Y, et al. (författare)
  • Quantum mechanical model and simulation of GaAs/AlGaAs quantum well infrared photodetector-I optical aspects
  • 2002
  • Ingår i: Hongwai yu haomibo xuebao. - Science Press. - 1001-9014. ; 21:5, s. 321-326
  • Tidskriftsartikel (refereegranskat)abstract
    • A complete quantum mechanical model for GaAs/AlGaAs quantum well infrared photodetectors(QWIPs) is presented here. The model consisted of four parts: (1) Starting with the description of the electromagnetic field of the infrared radiation in the QWIP, effective component of the vector potential [\A(z)\] along the QWIP growth direction (z-axis) due to the optical diffraction grating was calculated. (2) From the wave transmissions and the occupations of the electronic states, it was discussed that the dark current in the QWIP is determined by the drift-diffusion current of carriers thermally excited from the ground sublevel in the quantum well to extended states above the barrier. (3) The photocurrent was investigated by the optical transition (absorption coefficient between the ground state to excited states due to the nonzero-[\A(z)\]). (4) By studying the inter-diffusion of the At atoms across the GaAs/AlGaAs heterointerfaces, the mobility of the drift-diffusion carriers in the excited states was calculated, so the measurement results of the dark current and photocurrent spectra can be explained theoretically. With the complete quantum mechanical descriptions of (1-4), QWIP device design and optimization are possible.
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9.
  • Fu, Ying, 1964-, et al. (författare)
  • Quantum well infrared detectors
  • 2004
  • Ingår i: Encyclopedia of Nanoscience and Nanotechnology, ed. by H.S. Nalwa. - American Scientific Publisher. - 1-588-83065-9 ; 9, s. 179-97
  • Bokkapitel (övrigt vetenskapligt)
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10.
  • Jiang, Jun, et al. (författare)
  • Length dependence of coherent electron transportation in metal-alkanedithiol-metal and metal-alkanemonothiol-metal junctions
  • 2004
  • Ingår i: Chemical Physics Letters. - 0009-2614. ; 400:4-6, s. 336-340
  • Tidskriftsartikel (refereegranskat)abstract
    • We have applied the elastic-scattering Green's function theory to study the coherent electron transportation processes in both metal-alkanedithiol-metal (gold-[S(CH2)(n)S]-gold, n = 8-14) and metal-alkanemonothiol-metal (gold-[H(CH2)(n)S]-gold, n = 814) at the hybrid density functional theory level. It is shown that the current decreases exponentially with the molecular length. At the low temperature limit the electron decay rate, beta, for alkanedithiol junction is found to be around 0.30/CH2 at 1.0 V bias, much smaller than the calculated value of 0.60/CH2 for alkanemonothiol junction. The decay rate for alkanedithiol junction at the room temperature is neither sensitive to the activation of the Au-S stretching vibrational mode nor to the external bias. The calculated current-voltage characteristics and decay rates for both junctions are in excellent agreement with the corresponding experimental results.
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