Comparison of resonant tunneling in AlGaAs/GaAs parabolic and diffusion modified quantum wells

Double barrier resonant tunneling diode using annealing induced diffusion modified quantum well is proposed as a viable alternative to that using parabolic quantum well which requires complex techniques to fabricate it. The transmission coefficients are calculated using the hybrid incremental airy function plane wave approach. The room temperature current-voltage characteristics have been calculated using transmission coefficients. The current-voltage characteristics are found to be similar in both diodes.     

Improved Linear Programming Formulations for the Multi-group Discriminant Problem

This paper sets out to solve the multi (more than two)-group classification problem, and develops a new linear programming model which simultaneously determines the cut-off values for the different classification functions. Instead of decomposing the content in the multi-group problem to facilitate computation of the cut-off values, this new model aggregates information contained in the multi-group problem which, intuitively, should provide better estimates of the group boundaries. Furthermore, this new model, one existing LP model, and a statistical approach will be tested by using real-life data.     

On the temperature dependence of the order parameter of liquid crystals over a wide nematic range

Based on the ideas of Landau-de Gennes theory applied to nematic liquid crystals, several forms for the variation of the order parameter as a function of temperature are investigated over a wide nematic range. These functional forms are used to fit the experimental order parameters, determined through the use of C-13 NMR, for 4-methoxybenzylidene-4'-butylaniline (MBBA) and 4-n-pentyl-4'-cyanobiphenyl (5CB), and the physical significance of the parameters is discussed. A comparison of the results shows that the functional form which fits the experimental data best is similar to the Haller equation, a useful relation which is usually regarded as empirical. In this case, the coefficients resulting from a semi-empirical approach based on the Landau-de Gennes treatment may be thought of as quantifying the importance of the structure and rigidity of the liquid crystal in determining the temperature dependence of the order parameter for that liquid crystal. In the process, we have also examined the pretransitional behaviour in the C-13 NMR chemical shifts of liquid crystals observed within a few tenths of a degree above the nematic to isotropic transition temperature.     

Measurement of carbonaceous aerosols: validation of a thermal gravimetric method and its comparison with a thermal optical transmittance method

A high throughput thermal gravimetric method was developed to measure the carbonaceous aerosols and particulates collected on filters. Representative portions or the whole piece of a loaded or blank filter was sampled and conditioned at 105±5 °C for 4 h to drive away water moisture. The sample was then treated at 550±10 °C for 4 h. After each of the heat treatment steps, the sample was conditioned in a humidity and temperature-controlled cabinet for 12 h till constant weight. The weight of the filter before and after the heated treatment was measured and the weight difference between the treatment at 550 and 105 °C was calculated as the weight of the carbon containing substances (CCS). Reference chemical standards and certified reference materials SRM 1649a and 1650a were used to validate the method. CCS concentrations of real aerosol samples collected in the city of Hong Kong during the summer of 2001 were also measured, compared and correlated with the concentrations of total carbon (TC), elemental carbon (EC) and organic carbon (OC) of the samples determined using a thermal optical transmittance (TOT) method as specified in NIOSH method 5040. The weight of organic compounds in airborne particulate matters is usually estimated by multiplying the weight of TC, which is determined by thermal/thermal optical methods, by a factor of 1.2 or 1.4. To this end, a correction factor of approximately 1.6 was found to relate TC to the measured values of CCS in aerosol samples collected in the urban atmosphere of Hong Kong. The proposed procedure for measurement of CCS is simple, easy to follow and requires simple laboratory instrumentation. Typically, the analysis of more than 100 filter samples can be completed within three working days with minimal attention.