Simultaneous Measurement Of Attenuation Research Articles

Coexistence of two hole phases in high-quality p−GaAs/AlGaAs in the vicinity of Landau-level filling factors ν=1 and ν=(1/3)

We focused on the transverse AC magnetoconductance of a high-mobility $p$-GaAs/AlGaAs quantum well $(p=1.2\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{cm}}^{\ensuremath{-}2})$ in the vicinity of two values of the Landau-level filling factor $\ensuremath{\nu}$: $\ensuremath{\nu}=1$ (integer quantum Hall effect) and $\ensuremath{\nu}=1/3$ (fractional quantum Hall effect). The complex transverse AC conductance ${\ensuremath{\sigma}}_{xx}^{\mathrm{AC}}(\ensuremath{\omega})$ was found from simultaneous measurements of attenuation and velocity of surface acoustic waves propagating along the interface between a piezoelectric crystal and the two-dimensional hole system under investigation. We analyzed both the real and imaginary parts of the hole conductance and compared the similarities and differences between the results for filling factor 1 and filling factor 1/3. Both to the left and to the right of these values maxima of a specific shape, ``wings,'' arose in the $\ensuremath{\sigma}(\ensuremath{\nu})$ dependences at those two $\ensuremath{\nu}$. Analysis of the results of our acoustic measurements at different temperatures and surface acoustic wave frequencies allowed us to attribute these wings to the formation of collective localized states, namely, the domains of a pinned Wigner crystal, i.e., a Wigner solid. While the Wigner solid has been observed in 2D hole systems previously, we were able to detect it at the highest hole density and, therefore, the lowest hole-hole interaction reported.

Infrared tomographic system for monitoring the two-dimensional distribution of atmospheric pollution over limited areas

The authors analyze the feasibility and performance of a particular tomographic system for atmospheric pollution monitoring over limited areas (e.g. urban areas). Such a system exploits attenuation-based infrared measurements of the average concentration of the fundamental molecular species of pollutants along rectilinear paths. First, the paper demonstrates the feasibility of an apparatus based on semiconductor infrared laser diode transmitters and passive retroreflectors, capable of measuring the average concentration of pollutants along rectilinear paths with 2 km maximum length, by exploiting their infrared absorption properties. For each gaseous species of interest, the optimal wavelength is then singled out, with the purpose of applying the derivative method for measuring the corresponding average atmospheric concentration. The optimal wavelengths are determined based on both absorption data of atmospheric components and plausible ranges of variation of their concentration. Finally, the authors present simulations carried out to evaluate the reconstruction of spatial concentration fields of several air pollutants, obtained through a tomographic inversion algorithm exploiting simultaneous attenuation measurements made along different infrared links. Two different network topologies for such measurements are considered.

Instantaneous frequency scaling of rain attenuation at 11.6-17.8 GHz with SIRIO data

The ratio of simultaneous attenuation measurements performed at 11.6 and 17.8 (or 17.0 GHz) with the Italian satellite SIRIO is discussed. The main application of this information is in the field of the "up-path power control" in earth-space telecommunication systems for which the requirement of estimating the uplink attenuation form measurements on the downlink will probably be taken into consideration as a tool to keep constant the total power received on board. The analysis shows that the average ratio between attenuations at 17.8 and 11.6 GHz, conditioned to the downlink attenuation (11.6 GHz) tends to decrease slightly with attenuation; it turns out to be very close to the ratio between equiprobable values of attenuation computed from the concurrent cumulative distributions (long-term frequency scaling ratio); the standard deviation is of the order of 10 percent of the average value and also exhibits some tendency to decrease; both the overall cumulative distributions and the conditional ones are well fitted by log-normal probability models with standard deviation (of the natural logarithm) constant and equal to 0.13-0.14 up to 5.5 dB in the downlink attenuation.

Assessment of principal planes orientation in a terrestrial link at 18 GHz during intense rainfall

Simultaneous measurements of attenuation and depolarisation at 18 GHz collected during a propagation experiment on a 9.5 km terrestrial link have been employed to extract information about the angle formed by the principal planes with respect to the vertical-horizontal orientations (canting angle). Tilted polarisation planes have been adopted in order to increase the system sensitivity and to detect the sign of the angle according to a new technique recently devised.

Determination of rain anisotropy and effective spreading of the orientation of ellipsoidal raindrops during intense rainfall

Depolarization measurements performed in circular polarization at centimeter wavelength have frequently shown depolarization values lower than those theoretically expected by application of the Oguchi theory, which assumes equioriented ellipsoidal raindrops. This discrepancy can be explained by considering that in reality the drop-axes are oriented at a given instant according to a certain statistical distribution the spreading of which causes a decrease in the anisotropy (differential propagation constants along the principal planes), which is the prime source of rain depolarization (both in circular and in linear polarization). The reduction of anisotropy with respect to the equioriented drops model is experimentally assessed and the spreading necessary in a particular model distribution for the same reduction is determined in this communication. The study is based upon simultaneous measurements of attenuation and depolarization in circular polarization gathered in 1974-1975 during the Magnola experiment (Italy) on a low-elevation terrestrial path ( 9\deg ) at 11.6 GHz. The results confirmed that the equiorientation of raindrops is too severe an assumption, being the true anisotropy 20-30 percent (on average) less than the expected value in case of equiorientation. This figure corresponds to an average spreading of about 20-30\deg which is in agreement with the results given by other authors.

Frequency dependence of slant path rain attenuations at 15 and 35 GHz

The main objective of this paper is to examine the frequency dependence of slant path rain attenuation at 15 and 35 GHz in order to determine how well the attenuation at one frequency can be predicted from that at another; this has application in the design of earth‐to‐satellite communications. Also, some inferences on the drop size distribution of rain can be drawn from simultaneous attenuation measurements at two frequencies. The theory of rain attenuation is reviewed, and it is noted that the attenuation is a complex function of drop size, shape, orientation, index of refraction, and rain intensity along the path. Attenuation ratios are computed on the basis of both drop size and rain rate for frequencies at which there are measured data. Results obtained by other investigators are reviewed, and it is found that frequently, attenuation ratios that would not have been predicted on the basis of a surface rain model such as a Laws and Parsons model have been reported; indications are that attenuation is often produced from a widely dispersed distribution of large raindrops. During eight rainy days in 1975–1976, more than 10,000 simultaneous measurements of 15‐ and 35‐GHz attenuations were recorded in the Boston area. On the basis of these results it is concluded that it is not possible to represent the drop size distribution of rain along a slant path in the Boston area by a Laws and Parsons model, since the ratios of the 35‐GHz attenuations to the 15‐GHz attenuations are significantly below those that would have been predicted by that model. The ability to predict slant path rain attenuation at one frequency on the basis of that at another is shown to be a function of frequency separation, climatology, and the type of statistics which are desired.

Time Division Circular Polarization: A Simple Technique for Increasing Propagation Experiment Productivity

A simple technique, time division circular polarization (TDCP), is described which permits the simultaneous measurement of attenuation and depolarization for two orthogonal linearly polarized waves and two orthogonal circularly polarized waves in a slowly varying linear propagation medium using only a single frequency beacon switched between two linearly polarized antenna ports.

Effects of thermal-neutron irradiation on the elastic constants of copper

The three independent elastic constants of copper irradiated with thermal neutrons below 4 K have been measured. Simultaneous measurements of attenuation and resistivity were used to monitor the defect density. The elastic-constant changes during irradiation per unit concentration $\ensuremath{\gamma}$ of defects, $\frac{d\mathrm{ln}C}{d\ensuremath{\gamma}}$, are -2, -15.8, and -18.1 for the bulk modulus, ${C}_{44}$, and ${C}^{\ensuremath{'}}=\frac{({C}_{11}\ensuremath{-}{C}_{12})}{2}$, respectively. The percentage elastic-constant changes measured at 3.6 K after pulse annealing at temperatures up to 60 K were different for the different elastic constants in the same annealing range. Also the elastic-constant changes were strongly temperature dependent. The results show that there are five different types of elastic-constant effects, each of which depends upon the type of defect involved. This analysis also helps to account for the wide variation of results found previously for measurements using different types of irradiation at different temperatures.

Simultaneous Measurement of Attenuation and Velocity Change with an ltrasonic Continuous Wave Spectrometer

A somewhat simplified version of the continuous wave ultrasonic spectrometer described by Leisure and Moss [Rev. Sci. Instrum. 40, 946 (1969)] has been used to simultaneously measure variations in both sound velocity and attenuation. The system has reproducibly measured velocity changes as small as a few parts in 108 and amplitude changes of a few parts in 104. Procedures for use of the cw system are discussed and results in two different metal samples are presented.

Ultrasonic attenuation and velocity data on aluminum single crystals as a function of deformation and orientation

Aluminum single crystals oriented for single slip and for “polyslip” were deformed in tension, at room temperature. These tests were limited to a total tensile strain of up to 1 per cent. Simultaneous measurements of attenuation and velocity changes were made continuously during the tensile deformation. Both longitudinal and shear waves were used, at frequencies of 10 Mc/s and 13 Mc/s. The observed changes in attenuation in the polyslip orientations were consistent with an explanation based on the number of equally favored slip systems in each case. The results on crystals oriented for single slip indicate that in the easy glide range of strain hardening, dislocation multiplication is confined to the primary slip system only. The end of easy glide is associated with dislocations multiplying in other slip systems. An increase in attenuation is also observed prior to the onset of the macroscopic yield. This increase is presently attributed to an increase in dislocation loop length, caused by a breakaway mechanism. In the early stages of deformation, for all orientations, an increase of ultrasonic velocity, relative to the unstrained condition, is observed. The changes of attenuation and of velocity reported here, are consistent with theoretical results derived from the treatment of dislocation damping given by Granato and Lücke.