Search

First-principles calculations were performed jointly with muon-spin (μSR) spectroscopy experiments in order to examine the electrical activity of hydrogen in mixed-cation chalcopyrite Cu(In1−x,Gax)...

Sapphire $(\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3})$ has been investigated by the muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$ method in several experiments in the past. The main $\ensuremath{\mu}\mathrm{SR}$ component is a diamagnetic-like signal with a fast relaxation. Because of this diamagnetic-like behavior, the signal was assigned to either positively charged muonium $({\mathrm{Mu}}^{+})$ or negatively charged muonium $({\mathrm{Mu}}^{\ensuremath{-}})$, but neither of the two assignments was satisfactory (the so-called ``sapphire puzzle''). We have proposed that the signal is due to a weakly paramagnetic muonium configuration (transition state) which is formed during the reaction of muonium with the host lattice. In the present paper, we report new experimental data on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ and discuss these and earlier data in the ${\mathrm{Mu}}^{\ensuremath{-}}$ and in the transition state model. Calculations based on density functional theory were also performed with detailed findings on the energetics of the different muonium configurations and their migration energies. We conclude that the transition state model is more plausible than the ${\mathrm{Mu}}^{\ensuremath{-}}$ model, but the ${\mathrm{Mu}}^{\ensuremath{-}}$ interpretation cannot be excluded completely. In addition, the evidence is presented that the bare muon performs local motion but no long-range diffusion below room temperature in the microsecond time range.

Lattice vacancies are a major concern for the use of the Y3Al5O12 garnet (YAG) in optical applications. They are known to trap charge carriers preventing them from reaching luminescence centers. This reduces useful photon emission and deteriorates performance. Recent efforts to characterize such defects include experimental works by positron-annihilation spectroscopy (PAS) where extensive positron trapping was reported and attributed to defects made up of both cation and oxygen vacancies. The present study reports first-principles calculations for monovacancy and divacancy defects in YAG by means of conventional and two-component density-functional theory. The defect formation energies and corresponding charge-transition levels in the gap were initially determined. The ability of the lower-energy defects to act as positron-trapping centers was then examined. Corresponding positron lifetimes and binding energies to defects were calculated and compared to the experimental PAS data. The lifetimes of aluminum monovacancies agreed well with experiment if gradient corrections are included in the electron-positron correlation. Association of oxygen with aluminum vacancies was found to lead to stable negatively-charged divacancy complexes which also trap positrons. These defects are characterized by longer positron lifetimes, in agreement with the experimental observations.

Recent studies by positron-annihilation spectroscopy (PAS) in single-crystal and nanostructured yttria-stabilized zirconia (YSZ) revealed extensive positron trapping at vacancy-type point defects and other structural imperfections. The present work reports first principles calculations of formation energies and positron lifetimes of Zr vacancies in 10.3 mol% cubic YSZ. The lifetime calculations are based on two-component density-functional theory within the local-density approximation for the electron-positron correlation energy. Gradient-correction effects were also examined. Zr monovacancies were found to be potent trapping sites for positrons with large binding energies and lifetimes in excess of 220 ps. A strong dependency of the lifetimes on the Zr site and local coordination number was observed, a consequence of the structural disorder of the stabilized lattice. In contrast, oxygen vacancies did not act as attractive centers for positrons, either as single defects or as nearest neighbors to seven-fold coordinated Zr vacancies. Defect association of the Zr vacancies with hydrogen led to stable ([Formula: see text]-H) complexes with hydrogen bound to oxygen ions which are nearest neighbors to the vacancies. The negatively-charged ([Formula: see text]-H) defects were also found to trap positrons with corresponding lifetimes strongly dependent upon the hydrogen content within the complex.

The formation and migration energies of interstitial hydrogen in rutile TiO2 are obtained from first principles calculations. The computational approach was based on density functional theory with a semilocal generalised-gradient approximation functional, supplemented with an on-site Hubbard term to account for correlation among the Ti 3d electrons. Charge-transition levels are calculated and compared to previous theoretical studies. The donor character of hydrogen is examined in depth, focusing in particular on the tendency to form polaron-like configurations with the unpaired electron trapped at nearby titanium ions. Distinct minimum-energy paths of hydrogen migration and associated energy barriers were determined by the nudged elastic-band method. The present findings show clearly the strong anisotropy in the energy barriers for migration within the open c channels as opposed to migration crossing adjacent channels of the rutile lattice. For the rate-limiting step which leads to macroscopic diffusion along the c axis the corresponding rate and diffusion coefficient were also determined from transition-state theory. The results are discussed in connection to existing measurements of hydrogen diffusion and recent findings from electron paramagnetic resonance, electron-nuclear double resonance and muonium spectroscopies that probed the spatial localization of the electron spin.

The formation and migration energies of interstitial hydrogen in rutile TiO

Early transition metals (TMs) of the 3d and 4d rows are undesired contaminants in solar- and electronic-grade Si. From the theoretical standpoint, understanding the properties of these TMs in silicon still remains a challenging problem owing to the strong correlations among the TM d-electrons. The present study proposes a first-principles Hubbard-corrected DFT+U approach, with on-site parameters accounting separately for electron Coulomb (U) and exchange (J) effects. We use this approach together with conventional DFT to determine electrical levels and migration barriers of early 3d (Ti, V and Cr) and 4d (Zr, Nb and Mo) TMs in Si. Comparisons with experimental data allowed us to uniquely assign the deep levels in the gap appraising also the effect of on-site correlation. Our results also resolve existing controversies in the literature concerning the type and origin of the donor levels of Cr and Mo. For all the metals, with the exception of Cr, high barriers of interstitial diffusion are obtained, thus confirming that most of these TMs are slow diffusers in silicon.

Reliability of battery energy storage systems (BESS) used for online applications, such as electric vehicles and smart grid, depends heavily on the accuracy and rapidness of the state of charge (SOC) estimation. Moreover, to achieve a robust SOC estimation, the battery model parameter identification process is of significant importance. This paper examines a combination of the adaptive unscented Kalman filter (AUKF) and the fast upper diagonal recursive least square (FUDRLS) for the parameter identification and SOC estimation processes, respectively. The analysis focuses on on-line applications and the results are compared with previous work. Experimental validation based on various setups and load conditions is conducted, whereas the advantages of the proposed combination are highlighted.

Over the last decade the use of battery energy storage systems (BESS) on different applications, such as smart grid and electric vehicles, has been increasing rapidly. Therefore, the development of an electrical model of a battery, capable to estimate the states and the parameters of a battery during lifetime is of critical importance. To increase the lifetime, safety and energy usage, appropriate algorithms are used to estimate, with the lowest estimation error, the state of charge of the battery, the battery impedance, as well as its remaining capacity. This paper focuses on the development of model-based online condition monitoring algorithms for Li-ion battery cells, which can be extended to battery modules and systems. The condition monitoring algorithms were implemented after considering an optimal trade-off between their accuracy and overall complexity.

This paper describes a model-based evaluation analysis of grid connected Energy Storage Systems (ESS) that provide a set of grid services: energy arbitrage, distribution investment deferral and frequency regulation. The purpose of this evaluation is to determine the most profitable way to operate an ESS among different investment scenarios and dispatch schedules. Depending on the selected operational profile, revenue streams of the stacked services are aggregated and compared with capital and operating costs over the ESS lifetime, providing whether an investment on ESS is viable or not. Parameters that affect the feasibility of an ESS investment are highlighted. Results indicate that modular investments are prevalent as far as distribution investment deferral is considered, especially in combination with lucrative services as frequency regulation and energy arbitrage.

The increasing penetration of Renewable Energy Sources (RES) and generation uncertainties, brought to the fore new challenges and problems regarding efficient Distribution Networks (DNs) operation. Energy Storage Systems (ESS) can play a significant role in more reliable, secure and flexible DN operation since they can deal with difficult-to-predict changes. This study provides a detailed methodology among the corresponding mathematical formulation for the optimal sizing and allocation of ESS considering optimum operation schedule.

The atomistic and electronic structures of isolated hydrogen states in BeO were studied by ab initio calculations and muonium spectroscopy ((Formula presented.) SR). Whereas standard density-functional theory with a semi-local GGA functional led to a detailed probing of all possible minimum-energy configurations of hydrogen further calculations with the hybrid HSE06 functional provided improved properties avoiding band-gap and self-interaction errors. Similarly to earlier findings for the other wide-gap alkaline-earth oxide, MgO, hydrogen in BeO is also predicted to be an amphoteric defect with the pinning level, E((Formula presented.)), positioned in the mid-gap region. Both donor and acceptor levels were found too deep in the gap to allow for hydrogen to act as a source of free carriers. Whereas, hydrogen in its positively-charged state, (Formula presented.), adopts exclusively hydroxide-bond OH configurations, (Formula presented.) and (Formula presented.) instead show a preference to occupy cage-like interstitial sites in the lattice. (Formula presented.) in particular displays a multitude of minimum-energy configurations: its lowest-energy ground state resembles closely a trapped-atom state with a nearly spherical spin-density profile. In contrast to the strongly ionic MgO, (Formula presented.) in BeO was further found to stabilise in additional higher-energy elongated-bond OH configurations whose existence should be traced to a partial covalent character of the Be–O bonding. Calculations of the proton-electron hyperfine coupling for all (Formula presented.) states showed that the ground-state interstitial (Formula presented.) configuration is dominated by an isotropic hyperfine interaction with a magnitude very close to the vacuum value, a finding corroborated by the (Formula presented.) SR-spectroscopy data. © 2017 Informa UK Limited, trading as Taylor & Francis Group.

Cation vacancies in metal oxides have high formation energies and, thus, are not as abundant as their oxygen-related counterparts. Nonetheless, they can be readily created during non-equilibrium processes. Positron-annihilation spectroscopy (PAS) is a well suited technique of probing such defects since negatively-charged lattice vacancies are deep potential wells which can act as positron traps. The present study reports first-principles calculations of positron trapping at Zr monovacancies in monoclinic zirconia. The binding of positrons and associated lifetimes for these defects were obtained within two-component density-functional theory under different approximations for the electron-positron correlation. The role of hydrogen was also explored. Low-energy vacancy-hydrogen defect complexes were determined for one and two hydrogen atoms bound to a single Zr monovacancy. Hydrogen decoration of the vacancy affected the localization of the positron leading to an appreciable decrease of the lifetime. The study was supplemented with PAS measurements on samples of monoclinic zirconia. From the PAS data two distinct, high-intensity components were resolved originating from different defect states. The corresponding lifetimes of 187 ps and 225 ps were very close to the theoretical predictions.

We present a systematic study of isolated hydrogen in diverse forms of ${\mathrm{ZrO}}_{2}$ (zirconia), both undoped and stabilized in the cubic phase by additions of transition-metal oxides $({\mathrm{Y}}_{2}{\mathrm{O}}_{3},{\mathrm{Sc}}_{2}{\mathrm{O}}_{3}$, MgO, CaO). Hydrogen is modeled by using muonium as a pseudoisotope in muon-spin spectroscopy experiments. The muon study is also supplemented with first-principles calculations of the hydrogen states in scandia-stabilized zirconia by conventional density-functional theory (DFT) as well as a hybrid-functional approach which admixes a portion of exact exchange to the semilocal DFT exchange. The experimentally observable metastable states accessible by means of the muon implantation allowed us to probe two distinct hydrogen configurations predicted theoretically: an oxygen-bound configuration and a quasiatomic interstitial one with a large isotropic hyperfine constant. The neutral-oxygen-bound configuration is characterized by an electron spreading over the neighboring zirconium cations, forming a polaronic state with a vanishingly small hyperfine interaction at the muon. The atom-like interstitial muonium is observed also in all samples but with different fractions. The hyperfine interaction is isotropic in calcia-doped zirconia $[{A}_{\mathrm{iso}}=3.02(8)$ GHz], but slightly anisotropic in the nanograin yttria-doped zirconia $[{A}_{\mathrm{iso}}=2.1(1)$ GHz, $D=0.13(2)$ GHz] probably due to muons stopping close to the interface regions between the nanograins in the latter case.

The electronic structure of hydrogen impurity in ${\mathrm{Lu}}_{2}{\mathrm{O}}_{3}$ was studied by first-principles calculations and muonium spectroscopy. The computational scheme was based on two methods which are well suited to treat defect calculations in $f$-electron systems: first, a semilocal functional of conventional density-functional theory (DFT) and secondly a $\text{DFT}+U$ approach which accounts for the on-site correlation of the $4f$ electrons via an effective Hubbard-type interaction. Three different types of stable configurations were found for hydrogen depending upon its charge state. In its negatively charged and neutral states, hydrogen favors interstitial configurations residing either at the unoccupied sites of the oxygen sublattice or at the empty cube centers surrounded by the lanthanide ions. In contrast, the positively charged state stabilized only as a bond configuration, where hydrogen binds to oxygen ions. Overall, the results between the two methods agree in the ordering of the formation energies of the different impurity configurations, though within $\text{DFT}+U$ the charge-transition (electrical) levels are found at Fermi-level positions with higher energies. Both methods predict that hydrogen is an amphoteric defect in ${\mathrm{Lu}}_{2}{\mathrm{O}}_{3}$ if the lowest-energy configurations are used to obtain the charge-transition, thermodynamic levels. The calculations of hyperfine constants for the neutral interstitial configurations show a predominantly isotropic hyperfine interaction with two distinct values of 926 MHz and 1061 MHz for the Fermi-contact term originating from the two corresponding interstitial positions of hydrogen in the lattice. These high values are consistent with the muonium spectroscopy measurements which also reveal a strongly isotropic hyperfine signature for the neutral muonium fraction with a magnitude slightly larger (1130 MHz) from the ab initio results (after scaling with the magnetic moments of the respective nuclei).

The development of engineering-level models requires adoption of physical mechanisms that underlie observed phenomena. This paper reviews several cases where parameter-free, atomic-scale, quantum mechanical calculations led to the identification of specific physical mechanisms for phenomena relating to performance, reliability, radiation effects, and aging issues in microelectronics. More specifically, we review recent calculations of electron mobilities that are based on atomic-scale models of the Si–SiO 2 interface and elucidate the origin of strain-induced mobility enhancement. We then review extensive work that highlights the role of hydrogen as the primary agent of reliability phenomena such as negative bias temperature instability (NBTI) and radiation effects, such as enhanced low-dose radiation sensitivity (ELDRS) and dopant deactivation. Finally, we review atomic-scale simulations of recoils induced by energetic ions in Si and SiO 2 . The latter provide a natural explanation for single-event gate rupture (SEGR) in terms of defects with energy levels in the SiO 2 band gap.

The development of engineering-level models requires adoption of physical mechanisms that underlie observed phenomena. This paper reviews several cases where parameter-free, atomic-scale, quantum mechanical calculations led to the identification of specific physical mechanisms for phenomena relating to performance, reliability, radiation effects, and aging issues in microelectronics. More specifically, we review recent calculations of electron mobilities that are based on atomic-scale models of the Si-SiO2 interface and elucidate the origin of strain-induced mobility enhancement. We then review extensive work that highlights the role of hydrogen as the primary agent of reliability phenomena such as Negative Bias Temperature Instability (NBTI) and radiation effects, such as Enhanced Low Dose Radiation Sensitivity (ELDRS) and dopant deactivation. Finally, we review atomic-scale simulations of recoils induced by energetic ions in Si and SiO2. The latter provide a natural explanation for single-event gate rupture (SEGR) in terms of defects with energy levels in the SiO2 band gap.

Owing to their strong interaction with carriers, early $3d\text{-row}$ (Ti, V, and Cr) and $4d\text{-row}$ (Zr, Nb, and Mo) transition metals (TMs) are undesired contaminants in solar- and electronic-grade Si. The increasing stringent control of contamination levels is urging an accurate picture of their electronic structure. In the present work, the electrical levels and migration energies of these TMs are determined by means of standard density-functional theory (DFT) and a rotationally invariant formulation of DFT+$U$. The latter approach improves on the treatment of electronic correlations at the TM sites and relies on on-site Hubbard Coulomb and Hund's exchange parameters $U$ and $J$, respectively. These are calculated self-consistently from linear-response theory without fitting to experimental data. The effect of correlation was found more pronounced for Ti and V, with a strong impact on the location of their electrical levels. In most cases, the agreement with the experimental data is satisfactory allowing the identification of the type and character of the levels. For Cr and Mo in particular, the results resolve longstanding controversies concerning the type and position of the levels. The obtained migration barriers display moderate charge-state and correlation dependency. High barriers were found for all metals studied, with the exception of Cr, confirming them as slow diffusers in silicon among the whole TM family.

Classical Hartree effects contribute substantially to the success of time-dependent density functional theory, especially in finite systems. Moreover, exchange-correlation contributions have an asymptotic Coulomb tail similar to the Hartree term, and turn out to be crucial in describing response properties of solids. In this work, we analyze in detail the role of the long-range part of the Coulomb potential in the dielectric response of finite and infinite systems, and elucidate its importance in distinguishing between optical and electron energy loss spectra (in the long wavelength limit q--0). We illustrate numerically and analytically how the imaginary part of the dielectric function and the loss function coincide for finite systems, and how they start to show differences as the distance between objects in an infinite array is decreased (which simulates the formation of a solid). We discuss calculations for the model case of a set of interacting and noninteracting beryllium atoms, as well as for various realistic systems, ranging from molecules to solids, and for complex systems, such as superlattices, nanotubes, nanowires, and nanoclusters., This work was funded in part by the EU’s RTN NANOPHASE, the EU’s 6th Framework Programme through the NANOQUANTA Network of Excellence (NMP4-CT-2004-500198), the French “Ministère de la recherche” in the framework of the “ACI Nanostructures,” Spanish DGES, Basque Country University, and Iberdrola S.A. The work of L. K. D. was funded by the joint research program “ISMIR” between CEA and CNRS. Calculations have been done using ABINIT (http://www.abinit.org/), PWSCF (http://www.pwscf.org/), LSI-CP, and LSI-DP (http://theory.polytechnique.fr/codes/). Computer time was granted by IDRIS (project 000544) and by CEA/DSM on the cluster of Compaq Alphaserver (project p543) and NEC SX6 (project p93). A. R. acknowledges support from the Salvador Madariaga program of the Ministry of Science and Education of Spain for a sabbatical leave at the Ecole Polytechnique where this work was started.

arXiv:cond-mat/0308126, We present results for the optical absorption spectra of small-diameter single-walled carbon and boron nitride nanotubes obtained by ab initio calculations in the framework of time-dependent density-functional theory. We compare the results with those obtained for the corresponding layered structures, i.e. the graphene and hexagonal boron nitride sheets. In particular, we focus on the role of depolarization effects, anisotropies, and interactions in the excited states. We show that the random phase approximation reproduces well the main features of the spectra when crystal local field effects are correctly included, and discuss to what extent the calculations can be further simplified by extrapolating results obtained for the layered systems to results expected for the tubes. The present results are relevant for the interpretation of data obtained by recent experimental tools for nanotube characterization, such as optical and fluorescence spectroscopies, as well as polarized resonant Raman scattering spectroscopy. We also address electron energy loss spectra in the small-q momentum-transfer limit. In this case, the interlayer and intertube interactions play an enhanced role with respect to optical spectroscopy., This work was supported by the European Community Research Training Networks NANOPHASE (HPRN-CT-2000-00167), COMELCAN (HPRN-CT-2000-00128), and M-DNA (IST-2001-38051), by the French Ministère de la Recherche through the project NABOCO (Programme No.MPH 724 D), by the SpanishMCyT (MAT2001- 0946), and by the University of the Basque Country (9/UPV 00206.215- 13639/2001). The computer time was granted by IDRIS (Project No. 544), DIPC, and CEPBA (Barcelona).

The energetics of substitutional cation impurities in the bulk volume and at the Σ7 ( 1 012 ) rhombohedral twin boundary in alumina (α-Al 2 O 3 ) were studied by means of ab-initio mixed-basis pseudopotential calculations based on the local density functional theory. For fifteen different substitutional cation impurity species, the positions of the impurity energy levels relative to the valence bands of the oxide were determined, and the type of bonding versus the atomic number of the impurity cation was analysed. For the rhombohedral twin boundary, increased propensity for segregation was found for impurity cations of large ionic radii for which an enhanced overlap between impurity and oxygen electronic states was observed. The effect of structural relaxation at the boundary was studied for the isovalent group-IIIB impurities Sc, Y and La. Segregation broke the screw-rotation symmetry of the pure, unsegregated boundary and altered the interface-plane termination.

Transmission electron microscopy (TEM) and local density functional theory (LDFT) are combined to analyze the microscopic structure of the rhombohedral twin interface in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$. LDFT provides interfacial energetics and atomic and electronic structures for three competing models. With high-resolution TEM the atomic structure at the interface is imaged quantitatively along two orthogonal zone axes. Electron energy loss spectroscopy in TEM with nanoscale spatial resolution yields the interfacial electronic structure. Both experiments confirm the theoretically preferred model quantitatively.

First-principles density-functional calculations have been performed for basal-plane stacking faults in sapphire (α-Al 2 O 3 ). The present results confirm predictions of empirical atomistic studies where these faults are found to have high energies. Examination of the cation-layer stacking sequences along [0001] shows that important changes in the Al-Al coordination occur; with respect to the bulk, several cations have additional or no cation nearest neighbours in adjacent basal cation layers. The obtained generalized stacking-fault energies for displacement paths along the close-packed directions of the oxygen sublattice are discussed in connection with existing theories of basal slip and twinning.

The local interfacial structure and bonding at the rhombohedral twin interface in α-Al 2 O 3 (corundum) was studied by means of first-principles electronic-structure calculations based on the local density functional theory. Two sets of geometrical interface models were selected, one with a terminating oxygen layer at the interface, the other with a termination by interstitial vacancies of the corundum structure. Optimized interface configurations were obtained by minimization of the total internal energy with respect to relative translation states of the adjoining grains and relaxations of all atomic positions. One vacancy-terminated configuration was found with a very low interface energy, a well-defined relative translation state with screw-rotation symmetry and a highly ordered atomic structure, which minimizes the mutual repulsion between the neighboring like-charged ions. A second metastable configuration with vacancy termination and a higher interface energy was also obtained as well as a metastable oxygen-terminated structure with an interface energy between those of the two vacancy-terminated configurations. The theoretical results for the interfacial structures and translation states are discussed with respect to recent experimental investigations of the twin interface by high-resolution transmission electron microscopy. Furthermore, the calculated interfacial site-projected densities of electron states (PDOS) display significant differences from the bulk-crystal PDOS in the conduction bands.

Hydrogen is a common impurity in oxides and has been studied extensively by first-principles electronic structure methods. From the calculated charge-transition levels and their position with respect to the conduction-band edge, definitive conclusions can be drawn concerning the electrical activity of hydrogen either as an isolated defect or as part of a defect complex with intrinsic defects of the host lattice. For those oxides such as yttria-stabilized zirconia, which in many cases are used in polycrystalline or nanocrystalline forms, the interaction of hydrogen with grain boundaries needs to be better understood. Using both density-functional theory in the generalized-gradient approximation and a hybrid-functional approach, the present study reports on the types of isolated hydrogen configuration that can be stabilized at the core of the Σ5(310) tilt grain boundary, an interface whose atomistic structure has been determined in good detail by Z-contrast transmission electron microscopy. Initially, the present calculations elucidated the major relaxation modes that lead to low-energy structures for this boundary. Hydrogen exhibited dual behavior by binding to oxygen ions in bond-type OH(-) configurations in its positively charged state, H(+), whereas the negative H(-) species occupied preferably interstitial positions in the available empty space of the grain-boundary core regions. The neutral paramagnetic state, H(0), detected recently in muonium-based spectroscopic studies, was found to be stable in two different configurations: a deep-donor bond-type and a higher-energy quasiatomic interstitial. These configurations are characterized in terms of the trapping character of their excess electron, the spatial localization of the spin density and the resulting hyperfine parameters.

The effects of non-central forces in atomistic studies of grain boundaries in molybdenum and tungsten, the transition metals with half-filled d-band, are investigated. For this purpose we have used two different types of potential which include different number of moments of the local density of electronic states when evaluating the total energy: the central-force Finnis-Sinclair potentials which include the scalar second moment and the potentials constructed by Carlsson which include the fourth and the matrix second moments. The energy terms associated with these two moments represent non-central interactions and assure that the bcc-fcc structural energy difference is reproduced with good accuracy. For the three boundaries studied, the non-central forces have been found to be very important in determining the lowest energy structures. In particular, the energy differences between multiple structures depend on specific orientations and geometries of the atomic clusters at and near the interface. ...

Hydrogen is a common impurity in oxides and is known to exhibit dual behavior: It can act either as a dopant or alternatively as a compensating impurity, depending on whether its transition (pinning) level, $E(+/\ensuremath{-})$, intersects the conduction band or lies deep in the energy gap. In the present work the incorporation of isolated hydrogen in 10.3 mol% yttria-stabilized zirconia was studied by ab initio calculations employing a semilocal exchange-correlation density functional and a hybrid-functional approach. Equilibrium sites and formation energies were determined for the different charged states of hydrogen and the role of intrinsic oxygen vacancies needed to stabilize the cubic phase of the oxide was particularly examined. Hydrogen was found to be an amphoteric impurity with the equilibrium charge-transition levels, $E(q,{q}^{\ensuremath{'}})$, lying deep inside the gap. Whereas, in its positively charged state, H${}^{+}$, hydrogen was found exclusively to form a dative-type bond with the lattice oxygens, the negatively charged and neutral states also adopt interstitial configurations provided by the empty cubes and the intrinsic structural vacancies of the anion sublattice. Two distinct paramagnetic configurations of hydrogen, H${}^{0}$, are predicted and both of them induce deep localized levels in the band gap. The first configuration is higher-energy compact atomlike with the hydrogen at the interstitial sites, whereas the second one is a bond-type deep donor configuration with the unpaired $4d$ electron localized predominantly at an undercoordinated Zr cation in the vicinity of the impurity. Minimum-energy paths and corresponding classical barriers of migration were also determined for H${}^{0}$ with the aid of the nudged elastic-band method providing insight on the feasibility of site interplay of H${}^{0}$ and interconversion among its interstitial and donor configurations. Oxygen vacancies and lattice relaxation were found to have a major effect on the energy profiles of the paths, the position of the transition states, and the magnitude of the migration barriers.

The incorporation of interstitial hydrogen in yttria was studied by means of ab initio calculations based on density-functional theory (DFT) and muonium spin polarization spectroscopy (μSR). The density-functional calculations, based on a semilocal functional within the GGA-PBE and a hybrid functional, uncovered multiple geometrical configurations for the neutral, H0, and the negatively charged, H , states of hydrogen, thus demonstrating the existence of metastable minimum-energy sites. It was observed that the low-energy configurations for H0 and H are similar: they prefer to relax in deep, interstitial sites, whereas the equilibrium configurations for the positively charged state, H , were bond-type configurations with the hydrogen forming a covalent O-H bond with an O anion. For all neutral and negative configurations, localized defect levels were found inside the gap. Overall, the results for the formation energies obtained by the two different functionals are qualitatively similar; an amphoteric behavior was found for hydrogen after considering the lowest-energy structures for each charge state. The calculated acceptor transition level, obtained by the hybrid functional and seen near midgap, is consistent with μSR data from literature. The results are consistent with the present μSR data, where the observed diamagnetic signal is attributed to a donor-like muonium at the oxygen-bonded configurations and the paramagnetic signal to an acceptor-like deep muonium at the interstitial sites.

We present a systematic study of isolated hydrogen in $\ensuremath{\alpha}$-TeO${}_{2}$ (paratellurite) by means of muon-spin spectroscopy measurements complemented by ab initio calculations based on density-functional theory (DFT). The observable metastable states accessible by means of the muon implantation allowed us to probe both the donor and the acceptor configurations of hydrogen, as well as to follow their dynamics. A shallow donor state with an ionization energy of 6 meV as well as a deep acceptor state are proposed, together with their atomic-level configurations and associated formation energies obtained from the DFT calculations. The latter show a tendency of interstitial hydrogen to bind strongly to bridging oxygen ions but also to coexist at sites deeper in the interior of the Te--O--Te rings with a more atomiclike character and a defect level in the gap. Atomlike interstitial muonium was observed; it has a hyperfine interaction of about 3.5 GHz. Charge and site changes with temperature are discussed.

This paper introduces key ingredients of the dielectric response of $\ensuremath{\alpha}$-alumina that go beyond an independent-particle (IP) treatment of the valence-electron excitations. The optical-response functions were calculated from first- principles both at the Bethe--Salpeter and the random-phase approximation (RPA) levels. Excitonic effects obtained within the Bethe--Salpeter framework were found essential for reproducing the low-energy part of the experimental spectra (below 15 eV) and the bound exciton in particular. For higher energies, local-field effects introduced through the RPA modified considerably the IP results and provided a satisfactory account of the reflectivity spectra and of the position and shape of the dominant bulk plasmon resonance in the electron energy-loss spectra.

While yttrium and impurity segregation at interfaces of yttria-stabilized zirconia (YSZ) has been intensively studied experimentally, the mechanisms governing the propensity for segregation are still not fully understood. The segregation energetics of yttrium and aluminum, another common segregant at interfaces of YSZ, were studied by means of first principles calculations based on density functional theory. Site-dependent formation energies were calculated following the substitutional incorporation of yttrium and aluminum in the near-interface region of the Σ5(310) grain boundary in cubic zirconia, for which recent experimental data revealed strong yttrium enrichment. Aluminum segregation was found to be strongly favored, accompanied by extensive changes in its local environment and coordination. Yttrium displayed a segregation propensity dependent on the site of substitution that correlated with the ability of its surrounding environment to accommodate the misfit strain by a breathing-type relaxation and increase of the nearest-neighbor yttrium-oxygen distances. Formation energies of oxygen vacancies were also determined near the interface, both as isolated defects and also by considering cosegregation with yttrium; the ensuing defect association led to stable yttrium-vacancy complexes and increased the energy gain from yttrium incorporation at the core of the grain boundary.

We report a case of a woman with Gitelman syndrome who presented to our hospital mainly due to hyperemesis. Following her admission, intravenous potassium and magnesium supplementation was commenced to counter the observed hypokalemia and hypomagnesemia. Hyperemesis receded and although serum potassium remained low, she became asymptomatic. Oral potassium and magnesium supplementation was administered throughout pregnancy and biweekly ion level measurements were scheduled. Despite the intensive replacement, ion levels remained constantly low. She delivered at 38 weeks with an elective caesarean section because of a breech presentation, a healthy female baby weighing 3350g. Neonatal electrolyte profile was normal. © 2010 Informa UK, Ltd.

The development of engineering-level models requires adoption of physical mechanisms that underlie observed phenomena. This paper reviews several cases where parameter-free, atomic-scale, quantum mechanical calculations led to the identification of specific physical mechanisms for phenomena relating to performance, reliability, radiation effects, and aging issues in microelectronics. More specifically, we review recent calculations of electron mobilities that are based on atomic-scale models of the Si-Si O2 interface and elucidate the origin of strain-induced mobility enhancement. We then review extensive work that highlights the role of hydrogen as the primary agent of reliability phenomena such as Negative Bias Temperature Instability (NBTI) and radiation effects, such as Enhanced Low Dose Radiation Sensitivity (ELDRS) and dopant deactivation. Finally, we review atomic-scale simulations of recoils induced by energetic ions in Si and Si O2 . The latter provide a natural explanation for single-event gate rupture (SEGR) in terms of defects with energy levels in the Si O2 band gap.

The development of engineering-level models requires adoption of physical mechanisms that underlie observed phenomena. This paper reviews several cases where parameter-free, atomic-scale, quantum mechanical calculations led to the identification of specific physical mechanisms for phenomena relating to performance, reliability, radiation effects, and aging issues in microelectronics. More specifically, we review recent calculations of electron mobilities that are based on atomic-scale models of the Si-Si O2 interface and elucidate the origin of strain-induced mobility enhancement. We then review extensive work that highlights the role of hydrogen as the primary agent of reliability phenomena such as Negative Bias Temperature Instability (NBTI) and radiation effects, such as Enhanced Low Dose Radiation Sensitivity (ELDRS) and dopant deactivation. Finally, we review atomic-scale simulations of recoils induced by energetic ions in Si and Si O2 . The latter provide a natural explanation for single-event gate rupture (SEGR) in terms of defects with energy levels in the Si O2 band gap.

International audience; The electron energy-loss function of graphite is studied for momentum transfers q beyond the first Brillouin zone. We find that near Bragg reflections the spectra can change drastically for very small variations in q. The effect is investigated by means of first principle calculations in the random phase approximation and confirmed by inelastic x-ray scattering measurements of the dynamic structure factor S(q,ω). We demonstrate that this effect is governed by crystal local field effects and the stacking of graphite. It is traced back to a strong coupling between excitations at small and large momentum transfers.

The dielectric-response functions of crystalline ropes of metallic single-walled carbon nanotubes were determined from time-dependent density-functional theory in the random-phase approximation. Interband transitions and plasmonic excitations were studied as a function of momentum transfer. The impact of the tube diameter was shown for the (n,n) armchair-type series (n ranging from 3 to 8) covering a diameter range from 4 to 11 Å. Helicity effects were examined for the thinnest tubes, the armchair (3,3) versus the zigzag (5,0) configurations. Our results give detailed insight into the various kinds of excitations that can be observed in ropes of nanotubes. Recent experimental findings and trends by electron energy loss and Raman spectroscopies are reproduced and explained. © 2008 The American Physical Society., This work was supported by the EC-RTN program NANOPHASE Contract No. HPRN-CT-2000-00167, by the Spanish MEC Contract No. FIS2007-65702-C02-01 and Grupos Consolidados UPV/EHU of the Basque Country Government 2007, and by the European Community through NoE Nanoquanta Contract No. NMP4-CT-2004-500198, SANES Contract No. NMP4-CT-2006-017310, DNA-NANODEVICES Contract No. IST-2006-029192, and NANO-ERA Chemistry projects, UPV/EHU SGIker Arina. A.R. also acknowledges support from the Ecole Polytechnique during a sabbatical leave in 2001 when this work was started. Computer time was granted by IDRIS Project No. 544.

We use first-principles calculations and experimental data to demonstrate that impurity segregation at heterointerfaces is governed by several factors. In particular, Hf impurities avoid the ${\text{Si-SiO}}_{2}$ interface when present in the ${\text{SiO}}_{2}$ side, might segregate if present in the Si side, but do not cross into ${\text{SiO}}_{2}$. Substitutional Hf impurities in ${\text{SiO}}_{2}$, as revealed by a through-focal series of $Z$-contrast images, act as markers for Si sites, suggesting ordering in the first two Si planes of the amorphous ${\text{SiO}}_{2}$. Finally, we show that dopants in Si segregate at the interface by adopting several distinct configurations and also do not cross into ${\text{SiO}}_{2}$.

International audience; We have measured a strictly linear pi-plasmon dispersion along the axis of individualized single wall carbon nanotubes, which is completely different from plasmon dispersions of graphite or bundled single wall carbon nanotubes. Comparative ab initio studies on graphene based systems allow us to reproduce the different dispersions. This suggests that individualized nanotubes provide viable experimental access to collective electronic excitations of graphene, and it validates the use of graphene to understand electronic excitations of carbon nanotubes. In particular, the calculations reveal that local field effects (LFE) cause a mixing of electronic transitions, including the 'Dirac cone', resulting in the observed linear dispersion.

A statement of the status quo of the PV power systems in Greece, as well as their possible contribution towards the enhancement of power distribution reliability, is the scope of the present paper. Siting and installation of PV systems is performed according to a recent Greek law and by considering environmental and geographical constraints. Meteorological data about solar radiation and temperature are computed, formulated and imported to appropriate software in order to simulate the PV units and generate their power output. The above estimated power production is compared to data concerning load shedding. Assuming that a specific proportion of the eventually unsupplied power could be provided by the accessed power generation of the PV units, the reliability of the distribution system is improved. This improvement is quantified using the distribution system reliability indices SAIDI, SAIFI and CAIDI. The resulting improvement is finally expressed in financial terms through the reduction of interruption costs.

The management of the common acute lymphoblastic leukemia in pregnancy has been controversial. We report a case of a 16-year-old primigravida with acute lymphoblastic leukemia, first presented in pregnancy, which was treated with aggressive chemotherapy protocols. Full remission of the disease was achieved. The neonate was born at 32 weeks following a cesarean section. The woman remains in complete remission, continuing maintenance chemotherapy, 18 months following diagnosis. The offspring did not show any abnormality in physical examinations or laboratory tests and keeps growing normally 18 months after birth. Copyright © 2007 S. Karger AG.

Background. The aim of this study was to investigate pathologic differences of the placenta in pregnancies complicated by gestational diabetes compared to non-diabetic pregnancies. Methods. Forty singleton pregnancies complicated by gestational diabetes were recruited and compared to 40 consecutive normal pregnancies. A pathologist, blinded to all clinical data, reviewed all histological samples of the placentas. The histological assessment was carried out with regard to the following aspects: fetal vessel thrombosis, villous immaturity, chorangiosis, presence of nucleated fetal red blood cells (NFRBCs), ischemia, infarction, presence of hydropic or avascular villi, lymphohistiocytic villitis and villous fibrinoid necrosis. Results. The presence of degenerative lesions such as fibrinoid necrosis and vascular lesions like chorangiosis was apparent, mainly in the diabetes group. Villous immaturity and the presence of NFRBC as an indication of chronic fetal hypoxia were significantly increased in the placentas of women with diabetes compared with the control group. Fetal/placental weight ratio was significantly lower in the diabetic group. Conclusion. Histological abnormalities were observed more frequently in the diabetic placentas compared to the controls. These findings support the hypothesis that impaired placental function is one of the main reasons for the increased frequency of fetal complications in diabetic pregnancies. © 2008 Taylor & Francis.

We performed ab initio calculations of the optical absorption spectrum and the wave-vector-dependent dielectric and energy-loss functions of graphite in the framework of the random-phase approximation. In the absorption spectrum, the most prominent peaks were analyzed in terms of interband transitions from specific regions of the Brillouin zone. The inclusion of the crystal local-field effects (LFE) in the response had an important influence on the absorption spectrum for light polarization parallel to the c axis. The calculated electron energy-loss spectra, even without LFE, were in very good agreement with existing momentum-dependent energy-loss experiments concerning the peak positions of the two valence-electron plasmons. Important aspects of the line shape and anisotropy of the energy-loss function at large momentum transfer q were also well described: the splitting of the total (π+σ) plasmon and the appearance of peaks originating from interband transitions. Finally, the role of the interlayer interaction was studied, in particular with regard to its effect on the absorption spectrum for light polarization parallel to c, and to the position of the higher-frequency π+σ plasmon., This work was supported by the EC-RTN program NANOPHASE (Contract No. HPRN-CT-2000-00167). A.R. acknowledges support from the Ecole Polytechnique during a sabbatical leave in 2001 where this work was started and partial support from Spanish MCyT(MAT2001-0946), University of the Basque Country (9/UPV 00206.215-13639/2001) and COMELCAN (HPRN-CT-2000-00128). Computer time was granted by IDRIS (Project No. 544).

Presentado al XVII International Winterschool Euroconference on Electronic Properties of Novel Materials celebrado en Austria del 8 al 15 de marzo de 2003., We present a theoretical study of the optical absorption spectrum of small boron‐nitride and carbon nanotubes using time‐dependent density‐functional theory and the random phase approximation. Both for C and BN tubes, the absorption of light polarized perpendicular to the tube‐axis is strongly suppressed due to local field effects. Since BN‐tubes are wide band‐gap insulators, they only absorb in the ultra‐violet energy regime, independently of chirality and diameter. In comparison with the spectra of the single C and BN‐sheets, the tubes display additional fine‐structure which stems from the (quasi‐) one‐dimensionality of the tubes and sensitively depends on the chirality and tube diameter. This fine structure can provide additional information for the assignment of tube indices in high resolution optical absorption spectroscopy.

We determined the anisotropic dielectric response of graphite by means of time-dependent density-functional theory and high-resolution valence electron energy-loss spectroscopy. The calculated loss function was in very good agreement with the experiment for a wide range of momentum-transfer orientations with respect to the graphitic basal planes, provided that local-field effects were included in the response. The calculations also showed strong effects of the interlayer Coulomb interaction on the total π+σ plasmon. This finding must be taken into account for the explanation of recent loss spectra of carbon nanotube materials., This work was supported by the EC-RTN program NANOPHASE (Contract No. HPRN-CT-2000-00167). A. R. acknowledges support from the Ecole Polytechnique during a sabbatical leave, Iberdrola, and DGES. T. P. thanks the ÖAW for an APART grant. Computer time was granted by IDRIS (Project No. 544).

This paper is part of an extended study of oxide materials with the μSR technique. As an example, we present here experimental data on yttria-stabilized zirconia (ZrO2 doped with 8% Y2O3). Three different muon states can be distinguished: i) Deep muonium (less than 17(1)% fraction), seen as a fast-relaxing signal or indirectly via decoupling measurements in high longitudinal fields, ii) μ+ in a paramagnetic environment 62(6)% fraction), characterized by a very weak but clearly-visible hyperfine interaction, and iii) diamagnetic muon 21(1)% fraction); the diamagnetic signal is broadened only by the interaction with nuclear moments. The state corresponding to μ+ in a paramagnetic environment and the diamagnetic state are attributed to the same (oxygen-bound) muon configuration, but we assume that they have different electron surroundings (with or without an unpaired electron in the vicinity). The paramagnetic electron is not captured in the Coulomb potential of the positive muon but is self-trapped (polaron formation) at a nearby Zr ion. The distant electron interacts with the muon only via dipolar magnetic fields. This explains the very weak hyperfine interaction felt by the μ+ state in a paramagnetic environment. A further result of the experiment is that the disappearance of this signal with increasing temperature is not due to ionization of an electron shallowly bound to the muon but is caused by rapid spin fluctuations of the electron, averaging the hyperfine interaction to zero.