16,406 research outputs found
Silicon nitride-aluminum oxide solid solution (SiAION) formation and densification by pressure sintering
Stirred-ball-mill-blended Si3N4 and Al2O3 powders were pressure sintered in order to investigate the mechanism of solid solution formation and densification in the Si3N4-Al2O3 system. Powder blends with Si3N4:Al2O3 mole ratios of 4:1, 3:2, and 2:3 were pressure sintered at 27.6-MN/sq m pressure at temperatures to 17000 C (3090 F). The compaction behavior of the powder blends during pressure sintering was determined by observing the density of the powder compact as a function of temperature and time starting from room temperature. This information, combined with the results of X-ray diffraction and metallographic analyses regarding solutioning and phase transformation phenomena in the Si3N4-Al2O3 system, was used to describe the densification behavior
Interfacial strain in AlxGa1–xAs layers on GaAs
Detailed analysis of x-ray rocking curves was used to determine the depth profile of strain and composition in a 2500-Å-thick layer of AlxGa1–xAs grown by metalorganic chemical vapor deposition on 100 GaAs. The x value and layer thickness were in good agreement with the values expected from growth parameters. The presence of a transition region, 280 Å thick, was detected by the rocking curve. In this region, the Al concentration varies smoothly from 0 to 0.87. Measurement and control of the sharpness of such interfaces has important implications for heterojunction devices
Frozen light in periodic stacks of anisotropic layers
We consider a plane electromagnetic wave incident on a periodic stack of
dielectric layers. One of the alternating layers has an anisotropic refractive
index with an oblique orientation of the principal axis relative to the normal
to the layers. It was shown recently (A. Figotin and I. Vitebskiy, Phys. Rev.
E68, 036609 2003) that an obliquely incident light, upon entering such a
periodic stack, can be converted into an abnormal axially frozen mode with
drastically enhanced amplitude and zero normal component of the group velocity.
The stack reflectivity at this point can be very low, implying nearly total
conversion of the incident light into the frozen mode with huge energy density,
compared to that of the incident light. Supposedly, the frozen mode regime
requires strong birefringence in the anisotropic layers - by an order of
magnitude stronger than that available in common anisotropic dielectric
materials. In this paper we show how to overcome the above problem by
exploiting higher frequency bands of the photonic spectrum. We prove that a
robust frozen mode regime at optical wavelengths can be realized in stacks
composed of common anisotropic materials, such as YVO₄, LiNb,
CaCO₃, and the like.Comment: to be submitted to Phys. Rev.
Frozen light in photonic crystals with degenerate band edge
Consider a plane monochromatic wave incident on a semi-infinite periodic
structure. What happens if the normal component of the transmitted wave group
velocity vanishes? At first sight, zero normal component of the transmitted
wave group velocity simply implies total reflection of the incident wave. But
we demonstrate that total reflection is not the only possible outcome. Instead,
the transmitted wave can appear in the form of a frozen mode with very large
diverging amplitude and either zero, or purely tangential energy flux. The
field amplitude in the transmitted wave can exceed that of the incident wave by
several orders of magnitude. There are two qualitatively different kinds of
frozen mode regime. The first one is associated with a stationary inflection
point of electromagnetic dispersion relation. This phenomenon has been analyzed
in our previous publications. Now, our focus is on the frozen mode regime
related to a degenerate photonic band edge. An advantage of this new phenomenon
is that it can occur in much simpler periodic structures. This spectacular
effect is extremely sensitive to the frequency and direction of propagation of
the incident plane wave. These features can be very attractive in a variety
practical applications, such as higher harmonic generation and wave mixing,
light amplification and lasing, highly efficient superprizms, etc
Resolution enhancement of multichannel microwave imagery from the Nimbus-7 SMMR for maritime rainfall analysis
A restoration of the 37, 21, 18, 10.7, and 6.6 GHz satellite imagery from the scanning multichannel microwave radiometer (SMMR) aboard Nimbus-7 to 22.2 km resolution is attempted using a deconvolution method based upon nonlinear programming. The images are deconvolved with and without the aid of prescribed constraints, which force the processed image to abide by partial a priori knowledge of the high-resolution result. The restored microwave imagery may be utilized to examined the distribution of precipitating liquid water in marine rain systems
Observation of vortices and hidden pseudogap from scanning tunneling spectroscopic studies of electron-doped cuprate superconductor
We present the first demonstration of vortices in an electron-type cuprate
superconductor, the highest (= 43 K) electron-type cuprate
. Our spatially resolved quasiparticle tunneling spectra
reveal a hidden low-energy pseudogap inside the vortex core and unconventional
spectral evolution with temperature and magnetic field. These results cannot be
easily explained by the scenario of pure superconductivity in the ground state
of high- superconductivity.Comment: 6 pages, 4 figures. Two new graphs have been added into Figure 2.
Accepted for publication in Europhysics Letters. Corresponding author:
Nai-Chang Yeh (E-mail: [email protected]
Scanning Tunneling Spectroscopic Studies of the Effects of Dielectrics and Metallic Substrates on the Local Electronic Characteristics of Graphene
Atomically resolved imaging and spectroscopic characteristics of
graphene grown by chemical vapor deposition (CVD) on copper
foils are investigated and compared with those of mechanical
exfoliated graphene on SiO_2. For exfoliated graphene, the local
spectral deviations from ideal behavior may be attributed to strain
induced by the SiO_2 substrate. For CVD grown graphene, the
lattice structure appears strongly distorted by the underlying
copper, with regions in direct contact with copper showing nearly
square lattices whereas suspended regions from thermal relaxation
exhibiting nearly honeycomb or hexagonal lattice structures. The
electronic density of states (DOS) correlates closely with the
atomic arrangements of carbon, showing excess zero-bias
tunneling conductance and nearly energy-independent DOS for
strongly distorted graphene, in contrast to the linearly dispersive
DOS for suspended graphene. These results suggest that graphene
can interact strongly with both metallic and dielectric materials in
close proximity, leading to non-negligible modifications to the
electronic properties
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