Crossover Concentrations Research Articles

Cross-Over Concentration in Polydisperse Polymer Solutions

Correlation length and osmotic compressibility of blends of narrow distribution polystyrenes in benzene solutions were measured by dilute and semidilute regimes. Universal curves were obtained for correlation length and osmotic compressibility. These curves were the same as those of monodisperse polystyrenes. The cross-over concentration C* of polydisperse systems can be calculated by the equation, 1/C*=∑i(wi/Ci*), where wi and Ci* are the weight fractions and cross-over concentrations of i-th components, respectively.

Electronic structure of pseudobinary semiconductor alloysAlxGa1−xAs,GaPxAs1−x, andGaxIn1−xP

A method for calculating detailed electronic properties of the pseudobinary III-V compound semiconductor alloys is presented. The technique begins with realistic band structures obtained for the constituent compounds by fitting the band-gap symmetry-point energies and effective masses to experimental data, where they are available, and to more sophisticated theoretical results. Then the coherent-potential approximation is used to calculate the alloy band structures and scattering rates. Detailed comparisons between the theoretical predictions and experimental data for three alloys ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$, $\mathrm{Ga}{\mathrm{P}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$, and ${\mathrm{Ga}}_{x}{\mathrm{In}}_{1\ensuremath{-}x}\mathrm{P}$ demonstrate the quantitative nature of the method. Bowing parameters for the $\ensuremath{\Gamma}$, $X$, and $L$ gaps and the direct-to-indirect band-gap crossover concentrations are all predicted to within the present degree of experimental certainty.

A comparison of the capabilities of juvenile and adult Penaeus setiferus and Penaeus stylirostris to regulate the osmotic, sodium, and chloride concentrations in the hemolymph

1. 1. The capabilities of juvenile and mature adult Penaeus setiferus and P. stylirostris to regulate the osmotic, sodium, and chloride concentrations in the hemolymph are compared. 2. 2. In P. setiferus and P. stylirostris acclimated to salinities of 9.8 and 10.8%. respectively, juvenile shrimp are stronger hyperosmotic and hyperionic regulators than adults. However, the reduced regulatory capabilities of adult shrimp are not sufficient to require migration to offshore waters for survival. 3. 3. At 40.4%. juvenile P. setiferus are more effective hypoosmotic and hypoionic regulators than adults. However, there is no difference between the regulatory capabilities of juvenile and adult P. stylirostris at 36.2%. 4. 4. Differences in hemolymph concentration between juvenile and adult P. setiferus at 23.5%. indicate that the isosmotic and isoionic crossover concentrations are elevated with maturation.

Correlated random-walk simulations in simple and binary lattices

Random-walk simulations were performed on one-, two-, and three-dimensional, simple and binary lattices with several coordination numbers containing about one million sites. The random walk included a correlation parameter $l$ (Gaussian distribution with given standard deviation) representing a partial directional memory. The walks on the random binary lattices were constrained to sites of one component only (concentration $C$) with the sites of the second component acting as reflecting microboundaries. All simulations were restricted to the percolating cluster. The simple lattice simulations are compared with the well-known asymptotic analytical expressions for simple random walk ($l=1$) and with an expression for correlated walks ($l\ensuremath{\gg}1$). The visitation efficiency increases, as expected, with $C$. It also increases with $l$ for simple and high-$C$ lattices. However, for lower-$C$ lattices the visitation efficiency decreases with $l$, thus giving rise to "crossover concentrations." Our results are given in a series of figures of the efficiency or the number of sites visited versus the number of steps, showing the effects of concentration ($C$), and correlation ($l$). Applications to exciton percolation and coherence are mentioned.