7,956 research outputs found
Tunneling calculations for GaAs-Al(x)Ga(1-x) as graded band-gap sawtooth superlattices
Quantum mechanical tunneling calculations for sawtooth (linearly graded band-gap) and step-barrier AlGaAs superlattices were performed by means of a transfer matrix method, within the effective mass approximation. The transmission coefficient and tunneling current versus applied voltage were computed for several representative structures. Particular consideration was given to effective mass variations. The tunneling properties of step and sawtooth superlattices show some qualitative similarities. Both structures exhibit resonant tunneling, however, because they deform differently under applied fields, the J-V curves differ
Стадії укладення договору лізингу
Узагальнено вітчизняний та зарубіжний досвід укладання лізингових угод. Здійснено структуризацію стадій лізингового процесу.
Ключові слова: лізингова угода, лізингодавець, лізингоодержувач.Обобщен отечественный и зарубежный опыт заключения лизинговых соглашений.
Осуществлена структуризация стадий лизингового процесса.
Ключевые слова: лизинговое соглашение, лизингодатель, лизингополучатель.The generalization of the domestic and international experience in the concluding of leasing agreements is aimed in this article. The consecutive examination of the stages of leasing
process is set up structurally.
Key words: lease contract, leasee, leasor
Atom lithography without laser cooling
Using direct-write atom lithography, Fe nanolines are deposited with a pitch
of 186 nm, a full width at half maximum (FWHM) of 50 nm, and a height of up to
6 nm. These values are achieved by relying on geometrical collimation of the
atomic beam, thus without using laser collimation techniques. This opens the
way for applying direct-write atom lithography to a wide variety of elements.Comment: 7 pages, 11 figure
Homoclinic puzzles and chaos in a nonlinear laser model
We present a case study elaborating on the multiplicity and self-similarity
of homoclinic and heteroclinic bifurcation structures in the 2D and 3D
parameter spaces of a nonlinear laser model with a Lorenz-like chaotic
attractor. In a symbiotic approach combining the traditional parameter
continuation methods using MatCont and a newly developed technique called the
Deterministic Chaos Prospector (DCP) utilizing symbolic dynamics on fast
parallel computing hardware with graphics processing units (GPUs), we exhibit
how specific codimension-two bifurcations originate and pattern regions of
chaotic and simple dynamics in this classical model. We show detailed
computational reconstructions of key bifurcation structures such as Bykov
T-point spirals and inclination flips in 2D parameter space, as well as the
spatial organization and 3D embedding of bifurcation surfaces, parametric
saddles, and isolated closed curves (isolas).Comment: 28 pages, 23 figure
Comparative analysis of radiation dose and low contrast detail detectability using routine paediatric chest radiography protocols
Objectives: To compare low contrast detail (LCD) detectability and radiation dose for routine paediatric chest X-ray (CXR) imaging protocols among various hospitals.
Methods: CDRAD 2.0 phantom and medical grade polymethyl methacrylate (PMMA) slabs were used to simulate the chest region of four different paediatric age groups. Radiographic acquisitions were undertaken on 17 X-ray machines located in eight hospitals using their existing CXR protocols. LCD detectability represented by image quality figure inverse (IQFinv) was measured physically using the CDRAD analyser software. Incident air kerma (IAK) measurements were obtained using a solid-state dosimeter.
Results: The range of IQFinv, between and within the hospitals, was 1.40-4.44 and 1.52-2.18, respectively for neonates; 0.96-4.73 and 2.33-4.73 for a 1-year old; 0.87-1.81 and 0.98-1.46 for a 5-year old and 0.90-2.39 and 1.27-2.39 for a 10-year old.
The range of IAK, between and within the hospitals, was 8.56-52.62 µGy and 21.79-52.62 µGy, respectively for neonates; 5.44-82.82 µGy and 36.78-82.82 µGy for a 1-year old; 10.97-59.22 µGy and 11.75-52.94 µGy for a 5-year old and 13.97-100.77 µGy and 35.72-100.77 µGy for a 10-year old.
Conclusions: Results show considerable variation, between and within hospitals, in the LCD detectability and IAK. Further radiation dose optimisation for the four paediatric age groups, especially in hospitals /X-ray rooms with low LCD detectability and high IAK, are required.
Keywords: Paediatric chest radiography, CDRAD phantom, low contrast detail detectability and radiation dose
Tuning the excited state of photoactive building blocks for metal-templated self-assembly.
The reaction of 2,2':4,4'':4',4'''-quaterpyridyl (qtpy), with d(6) ruthenium(II) (Ru(II) ), and rhenium(I) (Re(I) ) metal centers has been investigated. The pendant pyridyl groups on the products have also been methylated to produce a second series of complexes containing coordinated Meqtpy(2+). The absorption spectra of the complexes are dominated by intraligand and charge-transfer bands. The ruthenium(II) complexes display broad unstructured luminescence consistent with emission from a Ru(d)→diimine(π*) manifold in acetonitrile solutions. In aqueous solutions, their emissions are weaker and the lifetimes are shorter. This effect is particularly acute for complexes incorporating coordinated dipyridylpyrazine, dppz, ligands. Although the emission of the ruthenium(II) complexes containing Meqtpy(2+) is generally shorter than their qtpy analogs, it is notable that solvent-dependent effects are much less intense. The rhenium(I) complexes also display broad unstructured luminescence but, compared with the ruthenium(II) systems, they have a relatively short lifetime in acetonitrile. Electrochemical studies reveal that all of the Ru(II) complexes display chemically reversible metal-based oxidations. Re(I) complexes only display irreversible metal-based oxidations. In most cases, the reduction processes were not fully chemically reversible. The electrochemical and optical studies reveal that the nature of the lowest excited state of these complexes--particularly, the systems incorporating dppz--is highly dependent on the nature of the coordinated ligands. Calculations indicate that, although the excited state of most of the complexes is centered on the qtpy or Meqtpy(2+) ligands, the excited state of the complexes containing dppz ligands is switched away from the dppz by qtpy methylation. A crystallographic study on one of the dicationic ruthenium(II) structures reveals that it forms an inclusion complex with benzene
Slowing heavy, ground-state molecules using an alternating gradient decelerator
Cold supersonic beams of molecules can be slowed down using a switched
sequence of electrostatic field gradients. The energy to be removed is
proportional to the mass of the molecules. Here we report deceleration of YbF,
which is 7 times heavier than any molecule previously decelerated. We use an
alternating gradient structure to decelerate and focus the molecules in their
ground state. We show that the decelerator exhibits the axial and transverse
stability required to bring these molecules to rest. Our work significantly
extends the range of molecules amenable to this powerful method of cooling and
trapping.Comment: 4 pages, 5 figure
Stability of Colloidal Quasicrystals
Freezing of charge-stabilized colloidal suspensions and relative stabilities
of crystals and quasicrystals are studied using thermodynamic perturbation
theory. Macroion interactions are modelled by effective pair potentials
combining electrostatic repulsion with polymer-depletion or van der Waals
attraction. Comparing free energies -- counterion terms included -- for
elementary crystals and rational approximants to icosahedral quasicrystals,
parameters are identified for which one-component quasicrystals are stabilized
by a compromise between packing entropy and cohesive energy.Comment: 6 pages, 4 figure
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