System-size convergence of point defect properties: The case of the silicon vacancy

We present a comprehensive study of the vacancy in bulk silicon in all its charge states from 2+ to 2-, using a supercell approach within plane-wave density-functional theory, and systematically quantify the various contributions to the well-known finite size errors associated with calculating formation energies and stable charge state transition levels of isolated defects with periodic boundary conditions. Furthermore, we find that transition levels converge faster with respect to supercell size when only the Gamma-point is sampled in the Brillouin zone, as opposed to a dense k-point sampling. This arises from the fact that defect level at the Gamma-point quickly converges to a fixed value which correctly describes the bonding at the defect centre. Our calculated transition levels with 1000-atom supercells and Gamma-point only sampling are in good agreement with available experimental results. We also demonstrate two simple and accurate approaches for calculating the valence band offsets that are required for computing formation energies of charged defects, one based on a potential averaging scheme and the other using maximally-localized Wannier functions (MLWFs). Finally, we show that MLWFs provide a clear description of the nature of the electronic bonding at the defect centre that verifies the canonical Watkins model.Comment: 10 pages, 6 figure

The utility of surface magnetic field measurements in the MAGSAT program

To take full advantage of the global, vector, survey by the Magsat satellite, and international program of augmented surface measurements was proposed. For secular variation and upper mantle conductivity the proposed measurements are global. The repeat station measurements for secular variation should be occupied at 2-3 year intervals. A special observing period in November and December of 1979 is proposed during which simultaneous, continuous, global measurements for upper mantle conductivity studies are to be gathered. Finally, it is recommended that the networks in operation during the IMS extend their operation through the Fall of 1980 to provide correlative data useful for high latitude disturbance studies and for crustal conductivity studies

Initial geomagnetic field model from MAGSAT

Magsat data from magnetically quiet days were used to derive a thirteenth degree and order spherical harmonic geomagnetic field model, MGST(3/80). The model utilized both scalar and vector data and fit that data with standard deviations of 8, 52, 55 and 97 nT for the scalar magnitude, B sub r, B sub theta and B sub phi respectively. When compared with earlier models, the Earth's dipole moment continues to decrease at a rate of about 26 nT/year. Evaluation of earlier models with Magsat data shows that the scalar field at the Magsat epoch is best predicted by the POGO(2/72) model but that the AWC/75 and IGS/75 are better for predicting vector fields

Effect of electron-nuclear spin interactions on electron-spin qubits localized in self-assembled quantum dots

The effect of electron-nuclear spin interactions on qubit operations is investigated for a qubit represented by the spin of an electron localized in a self-assembled quantum dot. The localized electron wave function is evaluated within the atomistic tight-binding model. The magnetic field generated by the nuclear spins is estimated in the presence of an inhomogeneous environment characterized by a random nuclear spin configuration, by the dot-size distribution, by alloy disorder, and by interface disorder. Due to these inhomogeneities, the magnitude of the nuclear magnetic field varies from one qubit to another by the order of 100 G, 100 G, 10 G, and 0.1 G, respectively. The fluctuation of the magnetic field causes errors in exchange operations due to the inequality of the Zeeman splitting between two qubits. We show that the errors can be made lower than the quantum error threshold if an exchange energy larger than 0.1 meV is used for the operation.Comment: 15 pages, 2 figure

Carotid artery stenting with filter protection in high-risk patients showing severe electroencephalographic alterations during carotid endarterectomy

Purpose: To describe the results and efficacy of stent treatment in patients with carotid stenosis who had aborted carotid endarterectomy procedures due to the appearance of severe electroencephalographic (EEG) alterations. Methods: A retrospective study was conducted of 18 patients (11 men; mean age 72 years, range 62–84) with symptomatic high-grade carotid artery stenoses ( 70%) who experienced severe EEG alterations during carotid endarterectomy, causing the procedure to be aborted. Twelve patients had shown no hemodynamic alterations during preoperative transcranial Doppler evaluation after external compression of the common carotid artery; the remaining 6 could not be evaluated. The patients were referred for carotid artery stenting (CAS); 7 had contralateral internal carotid artery stenosis and 5 had contralateral occlusion. Endovascular intervention was carried out using standard techniques under filter protection. Follow-up was scheduled at 3, 6, and 12 months. Results: All patients were successfully treated without immediate complications. The EEG did not display any significant alterations during the endovascular procedure. Mean followup was 43 months. Magnetic resonance imaging at 6 months showed no signs of cerebral ischemia. Color Doppler ultrasound imaging documented normal stent patency in all patients. Conclusion: Patients with symptomatic severe carotid stenosis ( 70%) who are considered at risk due to the appearance of severe EEG alterations during surgical treatment may benefit from CAS with respect to both major and minor complications. Larger studies are needed to confirm these findings. J Endovasc Ther 2006;13:451–456 Key words: carotid endarterectomy, carotid artery, stenosis, electroencephalography, cerebral ischemia, carotid angioplasty, sten

Non-Commutative Geometry and Measurements of Polarized Two Photon Coincidence Counts

Employing Maxwell's equations as the field theory of the photon, quantum mechanical operators for spin, chirality, helicity, velocity, momentum, energy and position are derived. The photon ``Zitterbewegung'' along helical paths is explored. The resulting non-commutative geometry of photon position and the quantum version of the Pythagorean theorem is discussed. The distance between two photons in a polarized beam of given helicity is shown to have a discrete spectrum. Such a spectrum should become manifest in measurements of two photon coincidence counts. The proposed experiment is briefly described.Comment: Latex, 13 pages, 3 figure

Economic burden of HPV9-related diseases : a real world cost analysis from Italy

The objectives of this study were to estimate the economic burden of HPV in Italy, accounting for total direct medical costs associated with nine major HPV-related diseases, and to provide a measure of the burden attributable to HPV 6, 11, 16, 18, 31, 33, 45, 52, 58 infections

Nanotechnology in machining processes: recent advances

In this brief survey, the use of nanoparticle dispersions in machining processes is discussed and the relevant applicational performances are analysed and related to the structural and chemical composition of the embedded nanophase. The paper is divided in two basic parts. In the former, the metalworking nanofluids are classified with respect to the physico-chemical properties of the nanostructured phase suspended in the base fluid. In the latter, some aspects concerning the production of metalworking nanofluids are analysed and a new green and economically viable technique based on a cementation process for metal nanoparticle synthesis is proposed as an alternative approach to the conventional manufacturing techniques