1,518,356 research outputs found
Self consistent theory of unipolar charge-carrier injection in metal/insulator/metal systems
A consistent device model to describe current-voltage characteristics of
metal/insulator/metal systems is developed. In this model the insulator and the
metal electrodes are described within the same theoretical framework by using
density of states distributions. This approach leads to differential equations
for the electric field which have to be solved in a self consistent manner by
considering the continuity of the electric displacement and the electrochemical
potential in the complete system. The model is capable of describing the
current-voltage characteristics of the metal/insulator/metal system in forward
and reverse bias for arbitrary values of the metal/ insulator injection
barriers. In the case of high injection barriers, approximations are provided
offering a tool for comparison with experiments. Numerical calculations are
performed exemplary using a simplified model of an organic semiconductor.Comment: 21 pages, 8 figure
Quantum superconductor-metal transition
We consider a system of superconducting grains embedded in a normal metal. At
zero temperature this system exhibits a quantum superconductor-normal metal
phase transition. This transition can take place at arbitrarily large
conductance of the normal metal.Comment: 13 pages, 1 figure include
Measurements of Antenna Surface for a Millimeter-Wave Space Radio Telescope II; Metal Mesh Surface for Large Deployable Reflector
Large deployable antennas with a mesh surface woven by fine metal wires are
an important technology for communications satellites and space radio
telescopes. However, it is difficult to make metal mesh surfaces with
sufficient radio-frequency (RF) performance for frequencies higher than
millimeter waves. In this paper, we present the RF performance of metal mesh
surfaces at 43 GHz. For this purpose, we developed an apparatus to measure the
reflection coefficient, transmission coefficient, and radiative coefficient of
the mesh surface. The reflection coefficient increases as a function of metal
mesh surface tension, whereas the radiative coefficient decreases. The
anisotropic aspects of the reflection coefficient and the radiative coefficient
are also clearly seen. They depend on the front and back sides of the metal
mesh surface and the rotation angle. The transmission coefficient was measured
to be almost constant. The measured radiative coefficients and transmission
coefficients would cause significant degradation of the system noise
temperature. In addition, we carried out an astronomical observation of a
well-known SiO maser source, R Cas, by using a metal mesh mirror on the NRO
45-m radio telescope Coude system. The metal mesh mirror considerably increases
the system noise temperature and slightly decreases the peak antenna
temperature. These results are consistent with laboratory measurements.Comment: 13 pages, 5 figures, accepted for publication in PAS
Damped Lyman-alpha Absorption Associated with an Early-Type Galaxy at Redshift z = 0.16377
We report new HST and ground-based observations of a damped Lyman-alpha
absorption system toward the QSO 0850+4400. The redshift of the absorption
system is z = 0.163770 and the neutral hydrogen column density of the
absorption system is log N = 19.81 cm**-2. The absorption system is by far the
lowest redshift confirmed damped Lyman-alpha absorption system yet identified,
which provides an unprecedented opportunity to examine the nature, impact
geometry, and kinematics of the absorbing galaxy in great detail. The
observations indicate that the absorption system is remarkable in three
respects: First, the absorption system is characterized by weak metal
absorption lines and a low metal abundance, possibly less than 4% of the solar
metal abundance. This cannot be explained as a consequence of dust, because the
neutral hydrogen column density of the absorption system is far too low for
obscuration by dust to introduce any significant selection effects. Second, the
absorption system is associated with a moderate-luminosity early-type S0
galaxy, although the absorption may actually arise in one of several very faint
galaxies detected very close to the QSO line of sight. Third, the absorbing
material moves counter to the rotating galaxy disk, which rules out the
possibility that the absorption arises in a thin or thick co-rotating gaseous
disk. These results run contrary to the expectation that low-redshift damped
Lyman-alpha absorption systems generally arise in the gas- and metal-rich inner
parts of late-type spiral galaxies. We suggest instead that mounting evidence
indicates that low-redshift galaxies of a variety of morphological types may
contain significant quantities of low metal abundance gas at large
galactocentric distances.Comment: 15 pages, LaTex, 4 figures, to be published in The Astronomical
Journa
Corrosion resistant thermal barrier coating
A thermal barrier coating system for protecting metal surfaces at high temperature in normally corrosive environments is described. The thermal barrier coating system includes a metal alloy bond coating, the alloy containing nickel, cobalt, iron, or a combination of these metals. The system further includes a corrosion resistant thermal barrier oxide coating containing at least one alkaline earth silicate. The preferred oxides are calcium silicate, barium silicate, magnesium silicate, or combinations of these silicates
Methods For Growing A Non-phase Separated Group-iii Nitride Semiconductor Alloy
Systems and methods for MBE growing of group-III Nitride alloys, comprising establishing an average reaction temperature range from about 250 C to about 850 C; introducing a nitrogen flux at a nitrogen flow rate; introducing a first metal flux at a first metal flow rate; and periodically stopping and restarting the first metal flux according to a first flow duty cycle. According to another embodiment, the system comprises a nitrogen source that provides nitrogen at a nitrogen flow rate, and, a first metal source comprising a first metal effusion cell that provides a first metal at a first metal flow rate, and a first metal shutter that periodically opens and closes according to a first flow duty cycle to abate and recommence the flow of the first metal from the first metal source. Produced alloys include AlN, InN, GaN, InGaN, and AlInGaN.Georgia Tech Research Corporatio
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