Hole dispersion and enhancement of antiferromagnetic interaction of localized spins in high-Tc superconductors

In the framework of the extended Hubbard model (copper-oxygen and oxygen-oxygen hopping) the energy and effective mass of a mobile hole is expressed in terms of hopping parameters and the correlators between spins of the background S_nS_n+1. Due to interaction of the mobile hole with localized holes, its effective mass increases about four times.     

Performance evaluation of the R-BTMA protocol in a distributedmobile radio network context

A reservation multiple-access protocol for mobile communication in a decentralized environment is proposed and investigated both analytically and by simulation. The reservation-busy-tone multiple access (R-BTMA) is a fully distributed and reservation-based protocol. It is a medium access control (MAC) protocol designed for short-range communications between vehicles in a decentralized context (i.e., without a base station). The specificity of this protocol relies on the utilization of two different characteristic channels. Information is transmitted on the data channel (DC), whereas the busy-tone channel (BTC) is used for signalling traffic. The range of the BTC is typically longer than the DC one. The throughput and access delay performance are studied in the particular context of two cells sharing a common BTC. The traffic between the vehicles is supposed to be regular and is parameter sharing. The comparison of the model analysis results to those obtained by the simulative approach shows a satisfactory matching     

Growth of InP/InGaAs multiple quantum well structures by chemical beam epitaxy

InP/InGaAs multiple quantum well structures with up to 200 periods have been grown by CBE. These structures exhibit exceptional lateral uniformity, measured as ±1 Å in period, ±13 ppm in lattice mismatch and ±0.5 nm in wavelength across a 2 inch wafer. Good surface morphology, sharp interfaces and excellent growth control have all been demonstrated.     

The new two-dimensional electron gas base HBT (2DEG-HBT):two-dimensional numerical simulation

The bipolar/FET characteristics of the 2DEG-HBT are analyzed extensively by a two-dimensional numerical simulator based on a drift-diffusion model. For bipolar operations at high collector current densities, it is confirmed that the cutoff frequency f_T is determined mainly by the collector transit time of holes and by the charging time of the extrinsic base-collector capacitance C _bc^EXT. The charging times of the emitter and base regions and the base transit time are shown to be negligible. A high cutoff frequency F_T (88 GHz) and current gain h_FE (760) are obtained for an emitter size of 1×10 μm², and undoped collector thickness of 150 nm, and a collector current density J_c of 10⁵ A/cm². The FET operation of the same 2DEG-HBT structure shows a threshold voltage V_th of 0.74 V, the transconductance G_m^max of 80 mS/mm, and maximum cutoff frequency F_T^max of 15 GHz. The dependence of the device performance on material parameters is analyzed extensively from a device design point of view     

Energy oscillations in electron transport across a triangularbarrier

Carrier transport across the semiconductor space-charge region of a silicon triangular barrier diode was investigated by a Monte Carlo simulation. Oscillations of the electron mean kinetic energy are observed as a function of position along the uphill slope of the barrier under bias. At a given point on the uphill slope, the energy distribution function shows an oscillatory behavior, with a periodicity corresponding to the optical phonon energy. These oscillations are shown to be due to the nonequilibrium dynamics of the electron interaction with optical phonons in the situation when other inelastic electron scattering processes are negligible. The energy oscillations are superimposed on a smooth cooling of the distribution in the transport toward the top of the barrier, as current flows through the system. A comparison with the thermionic theory quantifies the importance of nonequilibrium effects in short-range electronic transport     

Below-bandgap refractive index of AlAs/GaAs multiple quantum wells

Precision reflection measurements were performed on GaAs/AlAs superlattices of the same composition but different layer spacings. Nonlinear-least-squares fits to the data were performed to a single layer. Measurements were extracted for the superlattice thickness, thickness of a disturbed interface layer between the superlattice and substrate, the uniformity in composition and/or spacing and the composition. It was demonstrated that these nondestructive measurements in the infrared region (3000 to 12 000 cm^–1) in conjunction with a simple single layer model are capable of accurately yielding the above quantities with high precision.     

An adaptive filter for gravitational-wave antennas

Summary We report on the development of an adaptive optimum filter for processing the data of a resonant bar gravitational-wave detector. This filter, based on the matched-filter theory, is adaptive in the sense that the function it realizes is derived from the actual noise spectrum of the data being analysed (instead from an idealized model of the noise). Its implementation is mostly based on frequency domain techniques. We also report on the application of the new filter to the data of the cryogenic antenna Explorer of the Rome group, with particular reference to the comparison between its performance and that of an otpimum filter with fixed values of the parameters.     

Uniformization and the diversity of Whitehead groups

Techniques of uniformization are used to prove that it is not consistent that the Whitehead groups of cardinality ℵ₁ are exactly the strongly ℵ₁-free groups. Some consequences of the assumption that every strongly ℵ₁-free group of cardinality ℵ₁ is Whitehead are derived. Other results about uniformization are also proved. Research partially supported by NSERC grant #9848. Research partially supported by the BSF. The authors thank Rutgers University for its support. Publication #441.     

Room temperature Dy:YLF laser operation at 4.34 μm

A Dy:YLF laser operating on the ⁶H_11/2 to ⁶H_13/2 transition at 4.34 μm and using a laser pumping scheme is reported. This pumping scheme is necessitated by the short upper-laser-level lifetime and the small effective stimulated-emission cross section. A suitable laser for this application is the Er:YLF laser operating at 1.73 μm. A simple model that approximates Dy:YLF laser performance well is presented. Results on laser performance, including a determination of the slope efficiency and threshold as a function of the output mirror reflectivity and a correlation of the pulse length with the laser output energy, are reported. Overall laser efficiency is found to be limited primarily by the ratio of the pump wavelength to laser output wavelength and the terminated four-level laser operation     

Magnetic stimulation of the motor cortex-theoretical considerations

The aim of this paper is to present a first approximation model for the computation of the electric fields produced in the brain tissues by magnetic stimulation. Results are given in terms of induced electric field and current density caused by coils of different radii and locations. Nontraditional coil locations and assemblies are also considered (multicoil arrangements). Model simulations show that a good control of the excitation spread can be achieved by proper positioning of the coil. It is also predicted that one of the major drawbacks of the technique, i.e., the poor ability to concentrate the current spread into a small brain area can be partially overcome by more effective coil positioning and/or assembly. Finally, some comparisons are made among the results obtained from electric and magnetic stimulation. This is thought to be of great help in the design of experiments aimed to understand the relative role of the different brain structures responsible for the motor response.