Absolute sense of piezoelectric coefficients in the chalcopyrites ZnGeP2 and AgGaS2

The absolute orientation of single crystals of ZnGeP₂ and AgGaS₂ on which the relative sense of the d₁₄ and d₃₆ piezoelectric coefficients had been measured, was determined by the X-ray absorption edge method. Both coefficients were thereby found to be positive, corresponding in the point charge model to positive charge associated with the metal and negative charge with the nonmetal atoms, in contrast with the negative d_33′ reported in GaP.     

Obituary

By the use of the quantum regression theorem the two-time dipole correlation function is calculated for a system characterized by the well-known phenomenological two-level density matrix. Calculation of the scattered light spectrum in the weak signal limit yields a lineshape containing a sharp component following the exciting light frequency and also a broad component characterized by the dephasing width. When no dephasing is present the result is the same as the isolated molecule result. The calculations indicate that broad fluorescence type emission is due to the presence of ensemble correlations which are important oly if the exciting field is present and if real dephasing processes occur. The relationship between these results and the semiclassical results is also discussed.     

Effects of nuclear deformation on neutron total cross sections

Neutron total cross section differences have been determined for ^150,152,154Sm from 0.8 to 15 MeV by measuring transmission ratios. Significant differences were observed for ^152,150Sm and for ^154,150Sm. Evidence is presented that these results can be attributed to nuclear deformation.     

The structure of hexacyclopropylbenzene

The title compound crystallizes in the orthorhombic system, space group Pbnm, a = 9.933 (2), b = 11.068 (2), c = 17.134 (3) Å, z = 4. The structure was solved by direct methods and refined via block-diagonal least squares to R = 0.061. The arrangement of the cyclopropyl rings with respect to the benzene ring approximates point symmetry D_3d. The crystallographic site symmetry is m with the mirror plane perpendicular to the benzene ring and passing through two of its vertices.     

Resonant Raman detection of macular pigment levels in the living human retina

We have used resonant Raman scattering as a novel, noninvasive in vivo optical technique to measure the concentration of macular carotenoid pigments in the living human retina. Using a backscattering geometry and resonant molecular excitation in the visible, we measure the Raman peaks that originate from the single- and double-bond stretch vibrations of the p -conjugated molecule's carbon backbone. The Raman signals scale linearly with carotenoid content, whereas the required laser excitation is well under safety limits for macular exposure. The Raman technique is objective and quantitative and may lead to a new method for rapid screening of carotenoid pigment levels in large human populations that are at risk for vision loss from age-related macular degeneration, the leading cause of blindness of the elderly in the United States.     

Central-upwind scheme for shallow water equations with discontinuous bottom topography

Finite-volume central-upwind schemes for shallow water equations were proposed in [A. Kurganov and G. Petrova, Commun. Math. Sci., 5 (2007), 133–160]. These schemes are capable of maintaining “lake-at-rest” steady states and preserving the positivity of the computed water depth. The well-balanced and positivity preserving features of the central-upwind schemes are achieved, in particular, by using continuous piecewise linear interpolation of the bottom topography function. However, when the bottom function is discontinuous or a model with a moving bottom topography is studied, the continuous piecewise linear approximationmay not be sufficiently accurate and robust.