ABSTRACT
Within the framework of effective mass theory, we investigate the effects of spin-orbit interaction (SOI) and Zeeman splitting on the electronic properties of an electron confined in GaAs single quantum rings. Energies and envelope wavefunctions in the system are determined by solving the Schrödinger equation via the finite element method. First, we consider an inversely quadratic model potential to describe electron confining profiles in a single quantum ring. The study also analyzes the influence of applied electric and magnetic fields. Solutions for eigenstates are then used to evaluate the linear inter-state light absorption coefficient through the corresponding resonant transition energies and electric dipole matrix moment elements, assuming circular polarization for the incident radiation. Results show that both SOI effects and Zeeman splitting reduce the absorption intensity for the considered transitions compared to the case when these interactions are absent. In addition, the magnitude and position of the resonant peaks have non-monotonic behavior with external magnetic fields. Secondly, we investigate the electronic and optical properties of the electron confined in the quantum ring with a topological defect in the structure; the results show that the crossings in the energy curves as a function of the magnetic field are eliminated, and, therefore, an improvement in transition energies occurs. In addition, the dipole matrix moments present a non-oscillatory behavior compared to the case when a topological defect is not considered.
ABSTRACT
A theoretical analysis of optical properties in a ZnS/CdS/ZnS core/shell/shell spherical quantum dot was carried out within the effective mass approximation. The corresponding Schrödinger equation was solved using the finite element method via the 2D axis-symmetric module of COMSOL-Multiphysics software. Calculations included variations of internal dot radius, the application of electric and magnetic fields (both oriented along z-direction), as well as the presence of on-center donor impurity. Reported optical properties are the absorption and relative refractive index change coefficients. These quantities are related to transitions between the ground and first excited states, with linearly polarized incident radiation along the z-axis. It is found that transition energy decreases with the growth of internal radius, thus causing the red-shift of resonant peaks. The same happens when the external magnetic field increases. When the strength of applied electric field is increased, the opposite effect is observed, since there is a blue-shift of resonances. However, dipole matrix moments decrease drastically with the increase of the electric field, leading to a reduction in amplitude of optical responses. At the moment impurity effects are activated, a decrease in the value of the energies is noted, significantly affecting the ground state, which is more evident for small internal radius. This is reflected in an increase in transition energies.
ABSTRACT
Life on earth is the result of the work of proteins, the cellular nanomachines that fold into elaborated 3D structures to perform their functions. The ribosome synthesizes all the proteins of the biosphere, and many of them begin to fold during translation in a process known as cotranslational folding. In this work we discuss current advances of this field and provide computational and experimental data that highlight the role of ribosome in the evolution of protein structures. First, we used the sequence of the Ankyrin domain from the Drosophila Notch receptor to launch a deep sequence-based search. With this strategy, we found a conserved 33-residue motif shared by different protein folds. Then, to see how the vectorial addition of the motif would generate a full structure we measured the folding on the ribosome of the Ankyrin repeat protein. Not only the on-ribosome folding data is in full agreement with classical in vitro biophysical measurements but also it provides experimental evidence on how folded proteins could have evolved by duplication and fusion of smaller fragments in the RNA world. Overall, we discuss how the ribosomal exit tunnel could be conceptualized as an active site that is under evolutionary pressure to influence protein folding.
ABSTRACT
OBJECTIVE: To describe and analyze the characteristics and the early risk factors for mortality of noninvasive ventilation (NIV) in critically ill children. STUDY DESIGN: A multicenter, prospective, observational 2-year study carried out with critically ill patients (1 month - 18 years of age) who needed NIV. Clinical data and NIV parameters during the first 12 h of admission were collected. A multilevel mixed-effects logistic regression was performed to identify mortality risk factors. RESULTS: A total of 781 patients (44.2 ± 57.7 months) were studied (57.8% male). Of them, 53.7% had an underlying condition, and 47.1% needed NIV for lower airway respiratory pathologies. Bi-level NIV was the initial support in 78.2% of the patients. Continuous positive airway pressure (CPAP) was used more in younger patients (33.7%) than in older ones (9.7%; p < .001). About 16.7% had to be intubated and 6.2% died. The risk factors for mortality were immunodeficiency (odds ratio [OR] = 11.79; 95% confidence interval [CI] = 2.95-47.13); cerebral palsy (OR = 5.86; 95% CI = 1.94-17.65); presence of apneas on admission (OR = 5.57; 95% CI = 2.13-14.58); tachypnea 6 h after NIV onset (OR = 2.59; 95% CI = 1.30-6.94); and NIV failure (OR = 6.54; 95% CI = 2.79-15.34). CONCLUSION: NIV is used with great variability in types of support. Younger children receive CPAP more frequently than older children. Immunodeficiency, cerebral palsy, apneas on admission, tachypnea 6 h after NIV onset, and NIV failure are the early factors associated with mortality.
Subject(s)
Noninvasive Ventilation , Child , Child, Preschool , Continuous Positive Airway Pressure , Critical Illness , Female , Humans , Infant , Infant, Newborn , Intubation , Male , Prospective Studies , Respiratory InsufficiencyABSTRACT
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