Self-vacancies in Gallium Arsenide: an ab initio calculation

We report here a reexamination of the static properties of vacancies in GaAs by means of first-principles density-functional calculations using localized basis sets. Our calculated formation energies yields results that are in good agreement with recent experimental and {\it ab-initio} calculation and provide a complete description of the relaxation geometry and energetic for various charge state of vacancies from both sublattices. Gallium vacancies are stable in the 0, -, -2, -3 charge state, but V_Ga^-3 remains the dominant charge state for intrinsic and n-type GaAs, confirming results from positron annihilation. Interestingly, Arsenic vacancies show two successive negative-U transitions making only +1, -1 and -3 charge states stable, while the intermediate defects are metastable. The second transition (-/-3) brings a resonant bond relaxation for V_As^-3 similar to the one identified for silicon and GaAs divacancies.Comment: 14 page

Managing the supercell approximation for charged defects in semiconductors: finite size scaling, charge correction factors, the bandgap problem and the ab initio dielectric constant

The errors arising in ab initio density functional theory studies of semiconductor point defects using the supercell approximation are analyzed. It is demonstrated that a) the leading finite size errors are inverse linear and inverse cubic in the supercell size, and b) finite size scaling over a series of supercells gives reliable isolated charged defect formation energies to around +-0.05 eV. The scaled results are used to test three correction methods. The Makov-Payne method is insufficient, but combined with the scaling parameters yields an ab initio dielectric constant of 11.6+-4.1 for InP. Gamma point corrections for defect level dispersion are completely incorrect, even for shallow levels, but re-aligning the total potential in real-space between defect and bulk cells actually corrects the electrostatic defect-defect interaction errors as well. Isolated defect energies to +-0.1 eV are then obtained using a 64 atom supercell, though this does not improve for larger cells. Finally, finite size scaling of known dopant levels shows how to treat the band gap problem: in less than about 200 atom supercells with no corrections, continuing to consider levels into the theoretical conduction band (extended gap) comes closest to experiment. However, for larger cells or when supercell approximation errors are removed, a scissors scheme stretching the theoretical band gap onto the experimental one is in fact correct.Comment: 11 pages, 3 figures (6 figure files). Accepted for Phys Rev

Biodiversity patterns of Arctic diatom assemblages in lakes and streams: Current reference conditions and historical context for biomonitoring

Comprehensive assessments of contemporary diatom distributions across the Arctic remain scarce. Furthermore, studies tracking species compositional differences across space and time, as well as diatom responses to climate warming, are mainly limited to paleolimnological studies due to a lack of routine monitoring in lakes and streams across vast areas of the Arctic. The study aims to provide a spatial assessment of contemporary species distributions across the circum-Arctic, establish contemporary biodiversity patterns of diatom assemblages to use as reference conditions for future biomonitoring assessments, and determine pre-industrial baseline conditions to provide historical context for modern diatom distributions. Diatom assemblages were assessed using information from ongoing regulatory monitoring programmes, individual research projects, and from surface sediment layers obtained from lake cores. Pre-industrial baseline conditions as well as the nature, direction and magnitude of changes in diatom assemblages over the pastc.200 years were determined by comparing surface sediment samples (i.e. containing modern assemblages) with a sediment interval deposited prior to the onset of significant anthropogenic activities (i.e. containing pre-1850 assemblages), together with an examination of diatoms preserved in contiguous samples from dated sediment cores. We identified several biotypes with distinct diatom assemblages using contemporary diatom data from both lakes and streams, including a biotype typical for High Arctic regions. Differences in diatom assemblage composition across circum-Arctic regions were gradual rather than abrupt. Species richness was lowest in High Arctic regions compared to Low Arctic and sub-Arctic regions, and higher in lakes than in streams. Dominant diatom taxa were not endemic to the Arctic. Species richness in both lakes and streams reached maximum values between 60 degrees N and 75 degrees N but was highly variable, probably reflecting differences in local and regional environmental factors and possibly sampling effort. We found clear taxon-specific differences between contemporary and pre-industrial samples that were often specific to both ecozone and lake depth. Regional patterns of species turnover (beta-diversity) in the pastc.200 years revealed that regions of the Canadian High Arctic and the Hudson Bay Lowlands to the south showed most compositional change, whereas the easternmost regions of the Canadian Arctic changed least. As shown in previous Arctic diatom studies, global warming has already affected these remote high latitude ecosystems. Our results provide reference conditions for future environmental monitoring programmes in the Arctic. Furthermore, diatom taxa identification and harmonisation require improvement, starting with circum-Arctic intercalibrations. Despite the challenges posed by the remoteness of the Arctic, our study shows the need for routine monitoring programmes that have a wide geographical coverage for both streams and lakes

Oral rehabilitation of maxilla: Technical consideration

Oral rehabilitation of edentulous maxilla is particularly difficult because of the lack of bone in correspondence of maxillary sinuses, therefore, the surgeon is forced to place implants in sites where bone is more prevalent. In addition, patients require more frequent oral immediate rehabilitation in order to reduce the discomfort related to wearing a total denture. A viable solution to provide stability and retention of the prosthesis in a short time is represented by the technique of intraoral welding supporting total denture or fixed prosthesis. This goal may be achieved thanks to the technique of welding titanium bars onto implant abutments. In fact, the procedure can be performed directly in the mouth, eliminating the possibility of errors or distortions due to prosthetic procedures. This paper describes a case report and the most recent data regarding long-term success and high predictability of intraorally-welded titanium bar in immediate loading implants.Sin financiación1.711 JCR (2020) Q4, 132/146 Endocrinology & Metabolism0.284 SJR (2020) Q3, 167/232 Endocrinology, Diabetes and MetabolismNo data IDR 2020UE

Oral rehabilitation of edentulous jaws with one-piece implants: A case series

The popularity of one piece implants has increased considerably between patients and dentists. The advantages of one-piece immediate loading for rehabilitation of edentulous mandibles is to reduce the number of interventions and timing of prosthetic. These parameters can be better controlled with a one-piece implant. Twenty-one patients with one-piece implants inserted in totally edentulous mandibles were considered for this retrospective study. Inclusion criteria were: Good oral hygiene, absence of lesions of the oral mucosa, no smoking or smoking less than 20 cigarettes a day, drinking less than 2 glasses of wine a day, good general health no pregnancy. Twenty-one (12 females 9 males) patients were enrolled in this retrospective study. The mean follow-up was 1 years. A total 84 one-piece implants (Biohorizon, Italy) were inserted in edentulous mandible. Implants diameter was 3.0 mm in all fixtures. Implants length was equal and longer than 12 mm in 44 and 40 fixtures respectively. 48 were inserted in females 36 in males (range 33-67; mean age 58.3). One-piece immediate loading implants has no difference in survival rate respect to two-piece implant and delayed loading for rehabilitation of totally edentulous mandibles. In conclusion one-piece immediate loading implant is a reliable device for mandible rehabilitation.Sin financiación1.711 JCR (2020) Q4, 132/146 Endocrinology & Metabolism0.284 SJR (2020) Q3, 167/232 Endocrinology, Diabetes and MetabolismNo data IDR 2020UE