601 research outputs found
The Jahn-Teller instability with accidental degeneracy
The Jahn-Teller theorem predicts that orbitally degenerate complexes in solids can lower their energy by a distortion which lowers their symmetry. Exact degeneracy is not necessary for this instability; usually it suffices if the actual separation of the energy levels involved is less than the energy reduction which would result if they were exactly degenerate. We discuss various cases of accidental degeneracy in cubic and tetrahedral systems. With accidental degeneracy it is possible to get mixed distortions which involve both trigonal and tetragonal distortions. These have probably been observed for the negative vacancy in silicon, and possibly for the neutral vacancy in diamond. The theory is applied to both these cases, and it is compared with the earlier qualitative arguments of Watkins
The optical absorption of the neutral vacancy in diamond
Optical absorption by the neutral vacancy in diamond has been predicted to occur between the ground 1E state and the 1T2 excited state. Both these states are orbitally degenerate and should show Jahn Teller distortions. We have calculated the Huang-Rhys Factor for the transition (the fraction of the intensity in the zero phonon line) and the first and second moments of the absorption band. Also we discuss the response of the zero phonon line to externally applied stresses. Our model assumes that the vacancy interacts with just six modes, corresponding to the normal modes of the neighbours of the vacancy. The coupling of the electronic and nuclear motion is estimated from an LCAO model.
The results are compared with observations of the GR1 band, which has been attributed to the vacancy. The predicted Huang-Rhys factor S ~ 3.7 and the first and second moments of the band support this identification. Further our model predicts that luminescence from this band should be unpolarised, as observed. However the predicted stress splitting of the zero phonon line differs from that observed. Possible explanations of this are discussed
Nanoelectronics
In this chapter we intend to discuss the major trends in the evolution of
microelectronics and its eventual transition to nanoelectronics. As it is well
known, there is a continuous exponential tendency of microelectronics towards
miniaturization summarized in G. Moore's empirical law. There is consensus that
the corresponding decrease in size must end in 10 to 15 years due to physical
as well as economical limits. It is thus necessary to prepare new solutions if
one wants to pursue this trend further. One approach is to start from the
ultimate limit, i.e. the atomic level, and design new materials and components
which will replace the present day MOS (metal-oxide-semi- conductor) based
technology. This is exactly the essence of nanotechnology, i.e. the ability to
work at the molecular level, atom by atom or molecule by molecule, to create
larger structures with fundamentally new molecular orga- nization. This should
lead to novel materials with improved physical, chemi- cal and biological
properties. These properties can be exploited in new devices. Such a goal would
have been thought out of reach 15 years ago but the advent of new tools and new
fabrication methods have boosted the field. We want to give here an overview of
two different subfields of nano- electronics. The first part is centered on
inorganic materials and describes two aspects: i) the physical and economical
limits of the tendency to miniaturiza- tion; ii) some attempts which have
already been made to realize devices with nanometric size. The second part
deals with molecular electronics, where the basic quantities are now molecules,
which might offer new and quite interest- ing possibilities for the future of
nanoelectronicsComment: HAL : hal-00710039, version 2. This version corrects some aspect
concerning the metal-insulator-metal without dot
AMPHIBIAN AND REPTILE COLONIZATION OF RECLAIMED COAL SPOIL GRASSLANDS
While habitat loss is a major driver of amphibian and reptile declines globally, a subset of post-industrial landscapes, reclaimed and restored, are creating habitat for these animals. In a previous work, we showed that amphibians and reptiles use reclaimed and restored grasslands. In the present work we quantify captures at drift-fence/pitfall trap arrays over two consecutive years and show that several species of amphibians are not only successfully reproducing but that juveniles are being recruited into the population. In particular, 15,844 amphibians and 334 reptiles representing 25 species (14 amphibians, 11 reptiles) were captured at drift fences in 2009 and 2010. Nine additional reptile species were found opportunistically while conducting other research activities at the study site. Out of a total of 8,064 metamorphosing juveniles we detected 126 malformations, a 1.6% rate. The major malformation types were limbs missing (amelia) or foreshortened (ectromely), eye discolorations, and digits foreshortened (ectrodactyly) or small (brachydactyly). Our data show that reclaimed, restored, and properly managed landscapes can support reproducing populations of amphibians and reptiles with low malformation rates, including species in decline across other portions of their range
Protection strategies for next generation passive optical networks -2
Next Generation Passive Optical Networks-2 (NGPON2) are being considered to upgrade the current PON technology to meet the ever increasing bandwidth requirements of the end users while optimizing the network operators' investment. Reliability performance of NG-PON2 is very important due to the extended reach and, consequently, large number of served customers per PON segment. On the other hand, the use of more complex and hence more failure prone components than in the current PON systems may degrade reliability performance of the network. Thus designing reliable NG-PON2 architectures is of a paramount importance. Moreover, for appropriately evaluating network reliability performance, new models are required. For example, the commonly used reliability parameter, i.e., connection availability, defined as the percentage of time for which a connection remains operable, doesn't reflect the network wide reliability performance. The network operators are often more concerned about a single failure affecting a large number of customers than many uncorrelated failures disconnecting fewer customers while leading to the same average failure time. With this view, we introduce a new parameter for reliability performance evaluation, referred to as the failure impact. In this paper, we propose several reliable architectures for two important NGPON2 candidates: wavelength division multiplexed (WDM) PON and time and wavelength division multiplexed (TWDM) PON. Furthermore, we evaluate protection coverage, availability, failure impact and cost of the proposed schemes in order to identify the most efficient protection architecture
AMPHIBIAN RESPONSE TO A LARGE-SCALE HABITAT RESTORATION IN THE PRAIRIE POTHOLE REGION
Over the next half-century, scientists anticipate that nearly one third of the currently recognized 7,450 amphibian species will become extinct. Many organizations have responded to the challenge of conserving amphibian biodiversity, some indirectly. Under the auspices of the Iowa Great Lakes Management Plan, the United States Fish and Wildlife Service, Department of Natural Resources, and their partners have been implementing habitat restoration efforts designed to protect water quality, provide recreational opportunities, and benefit wildlife at the regional level. With this program, over 130 wetlands have been created in the past 30 years on recently purchased public lands—one of the largest wetland restoration projects conducted in the Prairie Pothole Region of the Great Plains. While amphibians were not the main target of these restorations, we show that in response, 121 new breeding populations of native Northern Leopard Frogs (Lithobates pipiens; n = 80) and Eastern Tiger Salamanders (Ambystoma tigrinum; n = 41) have been established; in addition, we found 19 populations of non-native American Bullfrogs (L. catesbeianus). Using the program PRESENCE, we show that leopard frog occupancy was greatest in newer (<18 years old), intermediate-sized wetlands, and that tiger salamander occupancy was greatest in small wetlands without fish and larval bullfrogs. These data imply that because native amphibians responded positively to these newly established wetlands, habitat availability has likely been a factor in limiting population numbers. Further, these data suggest the presence of fishes and introduced bullfrogs interferes with the ability of tiger salamanders to colonize restored wetlands
Quantum treatment of inelastic interactions for the modeling of nanowire field-effect transistors
During the last decades, the Nonequilibrium Green's function (NEGF) formalism has been proposed to develop nano-scaled device-simulation tools since it is especially convenient to deal with open device systems on a quantum-mechanical base and allows the treatment of inelastic scattering. In particular, it is able to account for inelastic effects on the electronic and thermal current, originating from the interactions of electron-phonon and phonon-phonon, respectively. However, the treatment of inelastic mechanisms within the NEGF framework usually relies on a numerically expensive scheme, implementing the self-consistent Born approximation (SCBA). In this article, we review an alternative approach, the so-called Lowest Order Approximation (LOA), which is realized by a rescaling technique and coupled with Padé approximants, to efficiently model inelastic scattering in nanostructures. Its main advantage is to provide a numerically efficient and physically meaningful quantum treatment of scattering processes. This approach is successfully applied to the three-dimensional (3D) atomistic quantum transport OMEN code to study the impact of electron-phonon and anharmonic phonon-phonon scattering in nanowire field-effect transistors. A reduction of the computational time by about×6 for the electronic current and×2 for the thermal current calculation is obtained. We also review the possibility to apply the first-order Richardson extrapolation to the Padé N/N-1 sequence in order to accelerate the convergence of divergent LOA series. More in general, the reviewed approach shows the potentiality to significantly and systematically lighten the computational burden associated to the atomistic quantum simulations of dissipative transport in realistic 3D systems
Functional methods in the theory of magnetoimpurity states of electrons in quantum wires
Functional methods are used to study magnetoimpurity states of electrons in
nanostructures. The Keldysh formalism is applied to these states. The theory is
illustrated using a quantum wire sample with impurity atoms capable of
localizing electrons in a magnetic field. The characteristics of
magnetoimpurity states of electrons in the wire are calculated using the model
of a Gaussian separable potential.Comment: 15 pages, 1 figur
Composition, structure, and stability of the rutile TiO_2(110) surface: oxygen depletion, hydroxylation, hydrogen migration and water adsorption
A comprehensive phase diagram of lowest-energy structures and compositions of
the rutile TiO_2(110) surface in equilibrium with a surrounding gas phase at
finite temperatures and pressures has been determined using density functional
theory in combination with a thermodynamic formalism. The exchange of oxygen,
hydrogen, and water molecules with the gas phase is considered. Particular
attention is given to the convergence of all calculations with respect to
lateral system size and slab thickness. In addition, the reliability of
semilocal density functionals to describing the energetics of the reduced
surfaces is critically evaluated. For ambient conditions the surface is found
to be fully covered by molecularly adsorbed water. At low coverages, in the
limit of single, isolated water molecules, molecular and dissociative
adsorption become energetically degenerate. Oxygen vacancies form in strongly
reducing, oxygen-poor environments. However, already at slightly more moderate
conditions it is shown that removing full TiO_2 units from the surface is
thermodynamically preferred. In agreement with recent experimental observations
it is furthermore confirmed that even under extremely hydrogen-rich
environments the surface cannot be fully hydroxylated, but only a maximum
coverage with hydrogen of about 0.6-0.7 monolayer can be reached. Finally,
calculations of migration paths strongly suggest that hydrogen prefers to
diffuse into the bulk over desorbing from the surface into the gas phase.Comment: 17 pages, 11 figures, to appear in PR
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