Geometrical Properties of Coupled Oscillators at Synchronization

We study the synchronization of $N$ nearest neighbors coupled oscillators in a ring. We derive an analytic form for the phase difference among neighboring oscillators which shows the dependency on the periodic boundary conditions. At synchronization, we find two distinct quantities which characterize four of the oscillators, two pairs of nearest neighbors, which are at the border of the clusters before total synchronization occurs. These oscillators are responsible for the saddle node bifurcation, of which only two of them have a phase-lock of phase difference equals $\pm$$\pi$/2. Using these properties we build a technique based on geometric properties and numerical observations to arrive to an exact analytic expression for the coupling strength at full synchronization and determine the two oscillators that have a phase-lock condition of $\pm$$\pi$/2.Comment: accepted for publication in "Communications in Nonlinear Science and Numerical Simulations

Light scattering and atomic force microscopy study of InAs island formation on InP

Some aspects of the morphology of InAs island formation on InP have been studied by atomic force microscopy, photoluminescence, photoluminescence excitation spectroscopy, and Raman scattering. The InAs layer is grown by chemical beam epitaxy on top of InP surfaces with sawtooth-like channels. The deposition of a thin InAs layer results in quantum dots strongly aligned along the InP channels. Subsequent annealing in an arsenic atmosphere produces growth and loss of coherency of the islands. Atomic force microscopy shows the changes in size and alignment of the islands. Optical measurements serve to give quantitative estimates of the strain distribution among the top of the InP buffer layer, the wetting layer and the islands for the differently treated samples. (C) 2000 American Institute of Physics. [S0021-8979(00)02003-X].8731165117

Intervalley scattering by acoustic phonons in two-dimensional MoS2 revealed by double-resonance Raman spectroscopy

Double-resonance Raman scattering is a sensitive probe to study the electron-phonon scattering pathways in crystals. For semiconducting two-dimensional transition-metal dichalcogenides, the double-resonance Raman process involves different valleys and phonons in the Brillouin zone, and it has not yet been fully understood. Here we present a multiple energy excitation Raman study in conjunction with density functional theory calculations that unveil the double-resonance Raman scattering process in monolayer and bulk MoS2. Results show that the frequency of some Raman features shifts when changing the excitation energy, and first-principle simulations confirm that such bands arise from distinct acoustic phonons, connecting different valley states. The double-resonance Raman process is affected by the indirect-to-direct bandgap transition, and a comparison of results in monolayer and bulk allows the assignment of each Raman feature near the M or K points of the Brillouin zone. Our work highlights the underlying physics of intervalley scattering of electrons by acoustic phonons, which is essential for valley depolarization in MoS2

Faster HIV-1 Disease Progression among Brazilian Individuals Recently Infected with CXCR4-Utilizing Strains

Introduction: Primary HIV infection is usually caused by R5 viruses, and there is an association between the emergence of CCXR4-utilizing strains and faster disease progression. We characterized HIV-1 from a cohort of recently infected individuals in Brazil, predicted the virus's co-receptor use based on the env genotype and attempted to correlate virus profiles with disease progression. Methods: A total of 72 recently infected HIV patients were recruited based on the Serologic Testing Algorithm for Recent HIV Seroconversion and were followed every three to four months for up to 78 weeks. The HIV-1 V3 region was characterized by sequencing nine to twelve weeks after enrollment. Disease progression was characterized by CD4+ T-cell count decline to levels consistently below 350 cells/mu L. Results: Twelve out of 72 individuals (17%) were predicted to harbor CXCR4-utilizing strains; a baseline CD4,350 was more frequent among these individuals (p = 0.03). Fifty-seven individuals that were predicted to have CCR5-utilizing viruses and 10 individuals having CXCR4-utilizing strains presented with baseline CD4.350; after 78 weeks, 33 individuals with CCR5 strains and one individual with CXCR4 strains had CD4.350 (p = 0.001). There was no association between CD4 decline and demographic characteristics or HIV-1 subtype. Conclusions: Our findings confirm the presence of strains with higher in vitro pathogenicity during early HIV infection, suggesting that even among recently infected individuals, rapid progression may be a consequence of the early emergence of CXCR4-utilizing strains. Characterizing the HIV-1 V3 region by sequencing may be useful in predicting disease progression and guiding treatment initiation decisions.Brazilian Program for STD and AIDSBrazilian Program for STD and AIDSMinistry of Health [914/BRA/3014-UNESCO/Kallas]Ministry of HealthSao Paulo City Health DepartmentSao Paulo City Health Department [2004-0.168.922-7/Kallas]Fundacao de Amparo a Pesquisa do Estado de Sao PauloFundacao de Amparo a Pesquisa do Estado de Sao Paulo [04/15856-9/Diaz]Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES)Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES)Brazilian Ministry of EducationBrazilian Ministry of Educatio

On the cross-over between 2-dimensional and 3-dimensional growth in Si/Ge-n/Si quantum wells

We report Raman scattering and low temperature photoluminescence measurements performed on a series of Molecular-Beam-Epitaxy-grown Si/Ge-n/Si quantum wells with n varying from 3 to 6. Our results are consistent with a gradual evolution of the Si/Ge interface which starts with Ge segregation and formation of terraces for low Ge coverage, to smooth 2-dimensional Ge layers, bounded by interfacial alloy layers, for coverages superior to 4-monolayers. (C) 1998 Elsevier Science Ltd. All rights reserved.107735936

Advantages of the graded-channel SOIFD MOSFET for application as a quasi-linear resistor

In this paper, we analyze the previously unexpected advantages of asymmetric channel engineering on the MOS resistance behavior in quasi-linear operation, such as used in integrated continuous-time tunable filters. The study of the two major figures of merit in such applications as on-resistance and nonlinear harmonic distortion, is supported by both measurements and simulations of conventional and graded-channel (GC) fully depleted silicon-on-insultor (SOI) MOSFETs. The quasi-linear current-voltage characteristics of GC transistors show a decrease of the on-resistance as the length of the low doped region in the channel is increased, as well as an improvement in the third-order harmonic distortion (HD3), when compared with conventional transistors. A method for full comparison between conventional and GC SOI MOSFETs is presented, considering HD3 evolution with on-resistance tuning under low voltage of operation. Results demonstrate the significant advantages provided by the asymmetrical long channel transistors

A PHOTOMODULATED SPECTROSCOPY STUDY OF INXGA1-XAS/GAAS SUPERLATTICES AND QUANTUM-WELLS

We present here a detailed study of photomodulated transmission and reflectivity at room and liquid-nitrogen temperatures of a series of In(x)Ga(1-x)As/GaAs superlattices and a single quantum well. Our samples span a variety of alloy compositions and quantum-well widths. We compare the results of our measurements with the predictions of an envelope-function calculation, which includes wave-vector dependence of the minibands. This comparison allows identification of several spectral features unmistakably arising from miniband dispersion. Also, accurate determination is made of the band-offset parameter, whose value is discussed in the context of those obtained by other authors.69117836784