76 research outputs found
On-wafer, cryogenic characterization of ultra-low noise HEMT devices
Significant advances in the development of high electron-mobility field-effect transistors (HEMT's) have resulted in cryogenic, low-noise amplifiers (LNA's) whose noise temperatures are within an order of magnitude of the quantum noise limit (hf/k). Further advances in HEMT technology at cryogenic temperatures may eventually lead to the replacement of maser and superconducting insulator superconducting front ends in the 1- to 100-GHz frequency band. Key to identification of the best HEMT's and optimization of cryogenic LNA's are accurate and repeatable device measurements at cryogenic temperatures. This article describes the design and operation of a cryogenic coplanar waveguide probe system for the characterization and modeling of advanced semiconductor transistors at cryogenic temperatures. Results on advanced HEMT devices are presented to illustrate the utility of the measurement system
Parameterized optimized effective potential for atoms
The optimized effective potential equations for atoms have been solved by
parameterizing the potential. The expansion is tailored to fulfill the known
asymptotic behavior of the effective potential at both short and long
distances. Both single configuration and multi configuration trial wave
functions are implemented. Applications to several atomic systems are presented
improving previous works. The results here obtained are very close to those
calculated in either the Hartree-Fock and the multi configurational
Hartree-Fock framework.Comment: 8 pages, 3 figure
A Self-Consistent Model for Positronium Formation from Helium Atoms
The differential and total cross sections for electron capture by positrons
from helium atoms are calculated using a first-order distorted wave theory
satisfying the Coulomb boundary conditions. In this formalism a parametric
potential is used to describe the electron screening in a consistent and
realistic manner. The present procedure is self consistent because (i) it
satisfies the correct boundary conditions and post-prior symmetry, and (ii) the
potential and the electron binding energies appearing in the transition
amplitude are consistent with the wave functions describing the collision
system. The results are compared with the other theories and with the available
experimental measurements. At the considered range of collision energies, the
results agree reasonably well with recent experiments and theories.
[Note: This paper will be published on volume 42 of the Brazilian Journal of
Physics
Teaching and Learning of Calculus
This survey focuses on the main trends in the field of calculus education. Despite their variety, the findings reveal a cornerstone issue that is strongly linked to the formalism of calculus concepts and to the difficulties it generates in the learning and teaching process. As a complement to the main text, an extended bibliography with some of the most important references on this topic is included. Since the diversity of the research in the field makes it difficult to produce an exhaustive state-of-the-art summary, the authors discuss recent developments that go beyond this survey and put forward new research questions
The use of diode lasers in non-surgical therapy of periodontitis
Introduction. Periodontitis is a disease of multifactorial etiology and complex clinical manifestation. Removal of subgingival
bacterial biofilm is a fundamental procedure in the treatment of periodontitis. Scaling and root planing (SRP) is currently the
gold standard in the treatment of periodontitis, but in some cases such a procedure does not provide satisfactory results.
The use of lasers in the treatment of periodontitis is one of the alternative adjunctive methods that has recently become
more popular.
Objective. The aim of the study was to evaluate the effectiveness of diode lasers in the treatment of periodontitis, based
on the literature found in the Medline, Pubmed and Scopus databases published from June 2013 – June 2020. Results
were based on a comparison of the effectiveness of SRP treatment in combination with photodynamic therapy or SRP in
combination with laser irradiation of periodontal pockets.
Results. The majority of studies confirmed the effectiveness of periodontal treatment combining SRP with photodynamic
therapy or SRP with laser irradiation, compared to an SRP alone.
Conclusions. Based on the recent literature, it can be concluded that SRP combined with photodynamic therapy or SRP
in combination with a diode laser irradiation are effective methods to improve the outcomes of non-surgical periodontal
treatment
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