1,721,103 research outputs found
Bi incorporation in GaN and AlxGa1-xN alloys
No full textBismuth has been recently suggested as a possible surfactant in the growth of GaN samples in place of As and Sb. In this work we use a density-functional first-principles technique to investigate the solubility and electrical activity of Bi impurities in GaN and AlGaN, two key parameters for surfactants that are so far unknown for bismuth. We find that the Bi solubility in GaN and AlGaN is very low. In n-type and moderate p-type conditions Bi behaves like a donor or an isoelectronic impurity substituting Ga at the cation site, while in extreme p-type conditions a multiple-donor Bi-N defect is formed that hinders efficient p-type doping. In AlGaN, Al and Bi substitutional defects show a tendency to pair, giving origin to an Al-Bi complex, whose electronic properties are similar to those of an isolated Bi-Ga impurity. The results of our investigations suggest that the use of Bi as surfactant is incompatible with an efficient p-type doping. Finally we examine the features of the defects extrinsic levels, identifying the current data on Bi-doped GaN luminescence as a conduction-band-to-donor recombination process for the Bi-Ga defect
Sexual Developmental Disorders in Pediatrics
No full textDisorders of sex development (DSD) are a heterogeneous group of pathologies that result in an alteration in sex determination or differentiation. DSD are estimated to affect 1: 4,500 newborns and according to the 2006 Chicago Consensus classification, DSD can be divided into three categories: those with a 46 XX karyotype, those with a 46 XY karyotype and those relating to sex chromosomes. It is crucial to correctly identify the pathology already in the first days of life to direct the patient and his family to the best path of care. For this reason, the role of the pediatrician is fundamental in the correct identification of the clinical picture and in supporting the family during the long process that involves the management of these patients. To make a diagnosis, it is necessary to follow a path led by a multidisciplinary team that includes several steps such as the execution of the genetic analysis, the evaluation with diagnostic imaging methods and laboratory evaluations. The therapeutic management, on the other hand, is still very complex even if in recent years we have moved from an attitude of early gender reassignment to an approach of watchful waiting to let the patient choose when she/he is mature enough to do so, which gender she/he feels to belong. It should not be forgotten that throughout this process the pediatrician must be both supportive and clinically active in the management of the child and his family
Novel electronically driven surface phase predicted in C/Si(111)
No full textWe predict a novel electronically driven phase for the recently created C/Si(111) surface at 1/3 monolayer coverage. Whereas the isoelectronic surface Sn/Ge(111) is a 3x3 distorted metal and Si/SiC(0001) is an undistorted magnetic Mott insulator, the new phase combines both features. Two of three adatoms in C/Si(111) should form a distorted (3x3) honeycomb sublattice, the third an undistorted insulating and magnetic triangular sublattice. The generally conflicting elements, namely, band energy, favoring distortion, and strong electron correlations favoring a Mott state, actually conspire in this case. This kind of state represents the surface analog of the Fazekas-Tosatti state in the charge density wave compound 1T-TaS2
Electronic and dynamical properties of the MgB2 surface: Implications for the superconducting properties
No full textWe report on a detailed first-principles local-density-functional investigation of the structural, electronic and dynamical properties of a MgB2 free surface in both unreconstructed ideal terminations and in the case of Na coverage. Our study shows that in the case of a B-terminated surface the density of states at the Fermi level is significantly larger than in the bulk, a behavior that might lead to an enhancement of superconducting properties. We find the Mg-terminated surface to be the most stable structure in the whole range admitted by the chemical potentials, in agreement with very recent experimental results. The Na-covered surface is just slightly less stable than the Mg-terminated one in B-rich conditions and cannot be ruled out as a potentially relevant metastable structure. We rationalize the thermodynamical trends and the dynamical properties of the surfaces considered in terms of the B-σ state filling; in particular, we find that the larger the filling the more stable the surface and the higher the frequency of the vibrational modes localized at the crystal surface
MgB2 and BeB2: A comparative study of their electronic and superconducting properties
No full textWe report a detailed study of the electronic and dynamical properties of superconducting MaB(2) and BeB2. Carefully comparing their band structure at different volumes and the corresponding phonon frequencies for the E-2g mode, we are able to explain their very different superconducting behavior in terms of different electron-phonon coupling. Our calculations are performed within the local-density approximation to the density-functional theory, using the full-potential linearized augmented plane wave and plane-wave pseudopotential in order to calculate phonon frequencies with frozen-phonon approach and linear-response theory, respectively. Our results highlight differences and similarities between the two compounds and give information regarding the zone center E-2g phonon frequencies, electron-phonon coupling, valence-band top deformation potentials at different volumes, and Gruneisen parameters. Our results compare well with available experimental measurements
Disorder-induced localisation and suppression of superconductivity in YSr2Cu3O6+x
No full textBy means of ab initio calculations within the local density approximation to density functional theory, we investigate the electronic structure of the 60 K superconductor YSr2Cu3O6+x (YSCO). We focus on the effects of the Sr/Ba substitution and on the main structural modifications induced by this substitution experimentally found in the Sr compound, namely the tetragonal symmetry and the oxygen disorder in the basal plane. In the calculations, this disorder is simulated by using a supercell approach. Due to band structure effects, we find a larger stabilisation free energy of the orthorhombic structure in YBa2Cu3O6+x (YBCO). In YSCO, the tetragonal disordered phase is found to be stabilized by oxygen overdoping (x > 1) and by sufficiently large mass-enhancement factors, γ > 5. The analysis of the atomic site projected density of states suggests that oxygen disorder in the CuO basal planes of YSCO induces hole localisation, which accounts for the large 30 K reduction of Tc with respect to YBCO
Theoretical investigation of FeTe magnetic ordering under hydrostatic pressure
No full textWe investigate the pressure phase diagram of FeTe, predicting structural and magnetic properties in the normal state at zero temperature within density functional theory. We carefully examined several possible different crystal structures over a pressure range up to approximate to 30 GPa: simple tetragonal (PbO type), simple monoclinic, orthorhombic (MnP type), hexagonal (NiAs and wurzite type), and cubic (CsCl and NaCl type). We predict pressure to drive the system through different magnetic ordering (notably also some ferromagnetic phases), eventually suppressing magnetism at around 17 GPa. We speculate the ferromagnetic order to be the reason for the absence of a superconducting phase in FeTe at variance with the case of FeSe. DOI: 10.1103/PhysRevB.87.09451
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