High multipole transitions in NIXS: valence and hybridization in 4f systems

Momentum-transfer (q) dependent non-resonant inelastic x-ray scattering measurements were made at the N4,5 edges for several rare earth compounds. With increasing q, giant dipole resonances diminish, to be replaced by strong multiplet lines at lower energy transfer. These multiplets result from two different orders of multipole scattering and are distinct for systems with simple 4f^0 and 4f^1 initial states. A many-body theoretical treatment of the multiplets agrees well with the experimental data on ionic La and Ce phosphate reference compounds. Comparing measurements on CeO2 and CeRh3 to the theory and the phosphates indicates sensitivity to hybridization as observed by a broadening of 4f^0-related multiplet features. We expect such strong, nondipole features to be generic for NIXS from f-electron systems

High temperature thermal conductivity of 2-leg spin-1/2 ladders

Based on numerical simulations, a study of the high temperature, finite frequency, thermal conductivity $\kappa(\omega)$ of spin-1/2 ladders is presented. The exact diagonalization and a novel Lanczos technique are employed.The conductivity spectra, analyzed as a function of rung coupling, point to a non-diverging $dc-$limit but to an unconventional low frequency behavior. The results are discussed with perspective recent experiments indicating a significant magnetic contribution to the energy transport in quasi-one dimensional compounds.Comment: 4 pages, 4 figure

Photoluminescence and spectral switching of single CdSe/ZnS colloidal nanocrystals in poly(methyl methacrylate)

Emission from single CdSe nanocrystals in PMMA was investigated. A fraction of the nanocrystals exhibiting switching between two energy states, which have similar total intensities, but distinctly different spectra were observed. We found that the spectral shift characteristic frequency increases with the pump power. By using the dynamic shift in the spectral position of emission peaks, we were able to correlate peaks from the same nanocrystal. The measured correlation is consistent with assignment of low energy lines to phonon replicas.Comment: 5 pages, 4 figure

Chainlike silicon nanowires: Morphology, electronic structure and luminescence studies

The chainlike siliconnanowiresSiNWs have been synthesized by fluctuating the pressure of the carrier gas in the growth process. The chainlike SiNWs comprise crystalline Sinanoparticles interconnected by amorphous silicon oxidewires. In addition to the sphere, other interesting shapes such as rectangular and triangular Sinanoparticles in chainlike SiNWs were also observed. X-rayabsorption fine structure shows that the Sinanoparticles in the chainlike SiNWs are crystalline silicon and that the wire has a significantly larger Si oxide to crystalline Si ratio and disorder compared to the normal SiNW. X-ray excited optical luminescence and photoluminescence spectroscopy show that the relatively strong luminescence from the chainlike SiNWs compared to normal SiNWs arises mainly from silicon oxide in the chainlike SiNWs.Research at the University of Western Ontario was supported by the Natural Science and Engineering Research Council (NSERC) of Canada. CSRF was supported by NSERC through a MFA grant and the National Research Council (NRC) of Canada. SRC was supported by the U.S. National Science Foundation under Grant No. DMR-00- 84402. N. B. W. acknowledges the support of a grant from the Research Grants Council of Hong Kong SAR [SiNWs RGC Grant 9040879 (CityU 1024/03)]

A Fixed-Point Iteration Method With Quadratic Convergence

The fixed-point iteration algorithm is turned into a quadratically convergent scheme for a system of nonlinear equations. Most of the usual methods for obtaining the roots of a system of nonlinear equations rely on expanding the equation system about the roots in a Taylor series, and neglecting the higher order terms. Rearrangement of the resulting truncated system then results in the usual Newton-Raphson and Halley type approximations. In this paper the introduction of unit root functions avoids the direct expansion of the nonlinear system about the root, and relies, instead, on approximations which enable the unit root functions to considerably widen the radius of convergence of the iteration method. Methods for obtaining higher order rates of convergence and larger radii of convergence are discussed

Linear and nonlinear optical characteristics of the Falicov-Kimball model

We calculate the linear and nonlinear optical properties of the Falicov-Kimball model for a mixed-valent system within the self-consistent mean-field approximation. Second-harmonic generation can only occur if the mixed-valent state has a built-in coherence between the itinerant d-electrons and the localized f-holes. By contrast, second-harmonic generation cannot occur for solutions of the model with f-site occupation as a good quantum number. As an experimental test of coherence in mixed-valent compounds we propose a measurement of the dynamic second-order susceptibility.Comment: 4 pages, 2 PostScript figures, to appear in Physical Review Letter