Solving the brachistochrone and other variational problems with soap films

We show a method to solve the problem of the brachistochrone as well as other variational problems with the help of the soap films that are formed between two suitable surfaces. We also show the interesting connection between some variational problems of dynamics, statics, optics, and elasticity.Comment: 16 pages, 11 figures. This article, except for a small correction, has been submitted to the American Journal of Physic

Comparison of Monolithic Optical Frequency Comb Generators Based on Passively Mode-Locked Lasers for Continuous Wave mm-Wave and Sub-THz Generation

In this paper, two different Passive Mode-Locked Laser Diodes (PMLLD) structures, a Fabry-Perot cavity and a ring cavity laser are characterized and evaluated as monolithic Optical Frequency Comb Generators (OFCG) for CW sub-THz generation. An extensive characterization of the devices under study is carried out based on an automated measurement system that systematically evaluates the dynamic characteristics of the devices, focusing on the figures of merit that define the optimum performance of a pulsed laser source when considered as an OFCG. Sub-THz signals generated with both devices at 60 GHz and 90 GHz are presented and analyzed in terms of electrical linewidth to compare such components for mm-Wave and sub-THz photonic generation. This work offers a systematic comparison of PMLLD devices for OFCG operation and provides reference information of the performance of two different device topologies that can be used for the implementation of photonic integrated sub-THz CW generation.We acknowledge COBRA Research Institute, Eindhoven University of Technology, the Netherlands for the fabrication of the ring lasers within the joint European platform for InP-based components and circuits, JePPIX.Publicad

Recurrence sequences over residual rings

In this work we are carried out an algebraic study of the congruential lineal generator. The obtained results make possible several combinatorial approaches that improve significantly the period length and their behavior

A method to construct refracting profiles

We propose an original method for determining suitable refracting profiles between two media to solve two related problems: to produce a given wave front from a single point source after refraction at the refracting profile, and to focus a given wave front in a fixed point. These profiles are obtained as envelopes of specific families of Cartesian ovals. We study the singularities of these profiles and give a method to construct them from the data of the associated caustic.Comment: 12 pages, 5 figure

Photoluminescence study of excitons in homoepitaxial GaN

High-resolution photoluminescence spectra have been measured in high-quality homoepitaxial GaN grown on a free-standing GaN substrate with lower residual strain than in previous work. Unusually strong and well-resolved excitonic lines were observed. Based on free- and bound exciton transitions some important GaN parameters are derived. The Arrhenius plot of the free A exciton recombination yields a binding energy of 24.7 meV. Based on this datum, an accurate value for the band-gap energy, EG(4.3 K) = 3.506 eV, can be given. From the donor bound excitons and their “two-electron” satellites, the exciton localization energy and donor ionization energy are deduced. Finally, estimates of the electron and hole masses have been obtained within the effective mass [email protected] ; [email protected]

Purely dynamical signature of the orientational glass transition

The dynamics of the freezing transition of the rotator phase crystal of ethanol into its orientational glass phase is monitored by measurements of molecular rotational components in the quasielastic neutron scattering spectrum. We demonstrate that phenomena observed at pico- and nanosecond scales can be mapped onto those shown by a model of infinitely thin hard needles rotating around body-centered-cubic lattice positions. As the model glass transition is of purely dynamical origin, our findings support the idea that the glass transition is purely dynamical and not associated with any thermodynamic phase transition.Dirección General de Investigación Científica y Técnica PB95-0075-C03-0

Clinical, dermoscopic and histopathological evaluation of the Meyerson nevus: case report

O nevo de Meyerson ocorre quando uma rara erupção eczematosa focal e transitória surge ao redor de lesões melanocíticas. O mesmo fenômeno também foi observado em lesões não melanocíticas. O caso relatado é o de um doente masculino, 25 anos, que há dois meses notara surgimento de eritema e prurido, circundando dois nevos, localizados no abdome. As lesões eram atípicas à dermatoscopia e procedeu-se à excisão cirúrgica dos dois nevos. O exame histopatológico revelou nevos melanocíticos compostos displásicos, envolvidos por espongiose e vesículas intraepidérmicas. O presente relato sugere que o fenômeno de Meyerson não modifica as características dermatoscópicas dos nevos.Meyerson nevi occur whenever a rare focal and transitory eczematous eruption arises around melanocytic lesions. The same phenomenon has also been observed in non-melanocytic lesions as well. Herein we report the case of a 25 year old, male patient, who had noticed, two months before, the arising of a pruriginous and erithematous halo around two nevi localized on his abdomen. The lesions were found to be atypical on dermoscopic examination and he was submitted to surgical excision of both nevi. Histopathological examination revealed displastic compound melanocytic nevi, surrounded by intraepidermical vesicles and spongiosis. Present report suggests that Meyerson phenomenon does not seem to alter dermoscopic features of nevi

Residual strain effects on the two-dimensional electron gas concentration of AlGaN/GaN heterostructures

Ga-face AlGaN/GaN heterostructures with different sheet carrier concentrations have been studied by photoluminescence and Raman spectroscopy. Compared to bulk GaN, an energy shift of the excitonic emission lines towards higher energies was observed, indicating the presence of residual compressive strain in the GaN layer. This strain was confirmed by the shift of the E2 Raman line, from which biaxial compressive stresses ranging between 0.34 and 1.7 GPa were deduced. The spontaneous and piezoelectric polarizations for each layer of the heterostructures have been also calculated. The analysis of these quantities clarified the influence of the residual stress on the sheet electron concentration (ns). Possible causes for the discrepancies between the calculated and experimentally determined sheet carrier densities are briefly [email protected] ; [email protected]