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

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

Electron transport via local polarons at interface atoms

Electronic transport is profoundly modified in the presence of strong electron-vibration coupling. We show that in certain situations, the electron flow takes place only when vibrations are excited. By controlling the segregation of boron in semiconducting Si(111)-3√×3√R30° surfaces, we create a type of adatom with a dangling-bond state that is electronically decoupled from any other electronic state. However, probing this state with scanning tunnelling microscopy at 5 K yields high currents. These findings are rationalized by ab-initio calculations that show the formation of a local polaron in the transport process

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