743 research outputs found
Gallium concentration dependence of room-temperature near-bandedge luminescence in n-type ZnO:Ga
We investigated the optical properties of epitaxial \textit{n}-type ZnO films
grown on lattice-matched ScAlMgO substrates. As the Ga doping concentration
increased up to cm, the absorption edge showed a
systematic blueshift, consistent with the Burstein-Moss effect. A bright
near-bandedge photoluminescence (PL) could be observed even at room
temperature, the intensity of which increased monotonically as the doping
concentration was increased except for the highest doping level. It indicates
that nonradiative transitions dominate at a low doping density. Both a Stokes
shift and broadening in the PL band are monotonically increasing functions of
donor concentration, which was explained in terms of potential fluctuations
caused by the random distribution of donor impurities.Comment: accepted for publication for Applied Physics Letters 4 figure
Monte-Carlo simulation of localization dynamics of excitons in ZnO and CdZnO quantum well structures
Localization dynamics of excitons was studied for ZnO/MgZnO and CdZnO/MgZnO
quantum wells (QW). The experimental photoluminescence (PL) and absorption data
were compared with the results of Monte Carlo simulation in which the excitonic
hopping was modeled. The temperature-dependent PL linewidth and Stokes shift
were found to be in a qualitatively reasonable agreement with the hopping
model, with accounting for an additional inhomogeneous broadening for the case
of linewidth. The density of localized states used in the simulation for the
CdZnO QW was consistent with the absorption spectrum taken at 5 K.Comment: 4 figures, to appear in J. Appl. Phy
Optical Properties of LiNbO thin films
The complex dielectric functions of LiNbO were determined using optical
transmittance and reflectance spectroscopies at room temperature. The measured
dielectric function spectra reveal distinct structures at several bandgap
energies. The bandgaps (exciton resonances) in the spectrum were observed at
ca. 2.3, 3.2, 3.9, and 5.1 eV, respectively. These experimental data have been
fit using a model dielectric function based on the electronic energy-band
structure near critical points plus excitonic effects. The features of measured
dielectric functions are, to some extent, reproduced quantitatively by an
ab-initio calculation including the interaction effects between electrons and
holes.Comment: 8 pages, 5 figures, 1 tabl
Analysis on reflection spectra in strained ZnO thin films
Thin films of laser molecular-beam epitaxy grown ZnO films were studied with
respect to their optical properties. 4-K reflectivity was used to analyze
various samples grown at different biaxial in-plane strain. The spectra show
two structures at 3.37 eV corresponding to the A-free exciton transition and at
3.38 eV corresponding to the B-free exciton transition. Theoretical
reflectivity spectra were calculated using the spatial dispersion model. Thus,
the transverse energies, the longitudinal transversal splitting (ELT,), the
oscillator strengths, and the damping parameters were determined for both the
A- and B-free excitons of ZnO. As a rough trend, the strain dependence of the
energy E_LT for the A-excitons is characterized by a negatively-peaking
behavior with a minimum around the zero strain, while ELT for the B-excitons is
an increasing function of the strain field values.Comment: 4 pages, 2 figures, 1 table, conference: ICMAT2005 (Singapore), to
appear in an issue of J. Cryst. Growt
Polarization properties of laser-diode-pumped micro-grained Nd:YAG ceramic lasers
Detailed polarization properties have been examined in laser-diode-pumped
(LD-pumped) micro-grained ceramic Nd:YAG lasers in different microchip cavity
configurations. Stable linearly-polarized single-frequency oscillations, whose
polarization direction coincide with that of an LD pump light, were observed in
an external cavity scheme. While, in the case of a thin-slice laser scheme with
coated reflective ends, elliptically-polarized single-frequency operations took
place in the low pump-power regime and dynamic instabilities appeared,
featuring self-induced antiphase modulations among counter-rotating
circularly-polarized components having slightly different lasing frequencies,
with increasing the pump power.Comment: 9 pages, 5 figure
Large capacitance enhancement and negative compressibility of two-dimensional electronic systems at LaAlO/SrTiO interfaces
Novel electronic systems forming at oxide interfaces comprise a class of new
materials with a wide array of potential applications. A high mobility electron
system forms at the LaAlO/SrTiO interface and, strikingly, both
superconducts and displays indications of hysteretic magnetoresistance. An
essential step for device applications is establishing the ability to vary the
electronic conductivity of the electron system by means of a gate. We have
fabricated metallic top gates above a conductive interface to vary the electron
density at the interface. By monitoring capacitance and electric field
penetration, we are able to tune the charge carrier density and establish that
we can completely deplete the metallic interface with small voltages. Moreover,
at low carrier densities, the capacitance is significantly enhanced beyond the
geometric capacitance for the structure. In the same low density region, the
metallic interface overscreens an external electric field. We attribute these
observations to a negative compressibility of the electronic system at the
interface. Similar phenomena have been observed previously in semiconducting
two-dimensional electronic systems. The observed compressibility result is
consistent with the interface containing a system of mobile electrons in two
dimensions.Comment: 4 figures in main text; 4 figures in the supplemen
The Effect of Pentoxifylline and Propentofylline(HWA-285) on Post-Ischemic Rat Brain
開始ページ、終了ページ: 冊子体のページ付
Photoemission study of TiO2/VO2 interfaces
We have measured photoemission spectra of two kinds of TiO-capped VO
thin films, namely, that with rutile-type TiO (r-TiO/VO) and that
with amorphous TiO (a-TiO/VO) capping layers. Below the
Metal-insulator transition temperature of the VO thin films, K,
metallic states were not observed for the interfaces with TiO, in contrast
with the interfaces between the band insulator SrTiO and the Mott insulator
LaTiO in spite of the fact that both TiO and SrTiO are band
insulators with electronic configurations and both VO and LaTiO
are Mott insulators with electronic configurations. We discuss possible
origins of this difference and suggest the importance of the polarity
discontinuity of the interfaces. Stronger incoherent part was observed in
r-TiO/VO than in a-TiO/VO, suggesting Ti-V atomic diffusion due
to the higher deposition temperature for r-TiO/VO.Comment: 5 pages, 6 figure
Water release and homogenization by dynamic recrystallization of quartz
To evaluate changes in water distribution generated by
dynamic recrystallization of quartz, we performed infrared (IR) spectroscopy
mapping of quartz in deformed granite from the Wariyama uplift zone in NE
Japan. We analyzed three granite samples with different degrees of
deformation: almost undeformed, weakly deformed, and strongly deformed.
Dynamically recrystallized quartz grains with a grain size of
∼10 µm are found in these three samples, but the
percentages of recrystallized grains and the recrystallization processes are
different. Quartz in the almost-undeformed sample shows wavy grain
boundaries, with a few bulged quartz grains. In the weakly deformed sample,
bulging of quartz, which consumed adjacent host quartz grains, forms regions
of a few hundred micrometers. In the strongly deformed sample, almost all
quartz grains are recrystallized by subgrain rotation. IR spectra of quartz
in the three samples commonly show a broad water band owing to H2O
fluid at 2800–3750 cm−1, with no structural OH bands. Water contents in
host quartz grains in the almost-undeformed sample are in the range of
40–1750 wt ppm, with a mean of 500±280 wt ppm H2O. On the
other hand, water contents in regions of recrystallized grains, regardless
of the recrystallization processes involved, are in the range of 100–510 wt ppm, with a mean of 220±70 wt ppm; these values are low and
homogeneous compared with the contents in host quartz grains. These low water
contents in recrystallized regions also contrast with those of up to 1540 wt ppm in adjacent host grains in the weakly deformed sample. Water
contents in regions of subgrains are intermediate between those in host and
recrystallized grains. These results for water distribution in quartz imply
that water was released by dynamic recrystallization.</p
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