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

Radial oscillations and stability of compact stars in Eddington-inspired Born-Infeld gravity

We study the hydrostatic equilibrium structure of compact stars in the Eddington-inspired Born-Infeld gravity recently proposed by Banados and Ferreira [Phys. Rev. Lett. 105, 011101 (2010)]. We also develop a framework to study the radial perturbations and stability of compact stars in this theory. We find that the standard results of stellar stability still hold in this theory. The frequency square of the fundamental oscillation mode vanishes for the maximum-mass stellar configuration. The dependence of the oscillation mode frequencies on the coupling parameter \kappa of the theory is also investigated. We find that the fundamental mode is insensitive to the value of \kappa, while higher order modes depend more strongly on \kappa.Comment: Minor changes to match the published versio

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

Optical RKKY Interaction between Charged Semiconductor Quantum Dots

We show how a spin interaction between electrons localized in neighboring quantum dots can be induced and controlled optically. The coupling is generated via virtual excitation of delocalized excitons and provides an efficient coherent control of the spins. This quantum manipulation can be realized in the adiabatic limit and is robust against decoherence by spontaneous emission. Applications to the realization of quantum gates, scalable quantum computers, and to the control of magnetization in an array of charged dots are proposed.Comment: 4 pages, 2 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

Optically-controlled single-qubit rotations in self-assembled InAs quantum dots

We present a theory of the optical control of the spin of an electron in an InAs quantum dot. We show how two Raman-detuned laser pulses can be used to obtain arbitrary single-qubit rotations via the excitation of an intermediate trion state. Our theory takes into account a finite in-plane hole $g$-factor and hole-mixing. We show that such rotations can be performed to high fidelities with pulses lasting a few tens of picoseconds.Comment: 6 pages, 4 figures; minor changes, J-ref adde

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)]

Indirect coupling between spins in semiconductor quantum dots

The optically induced indirect exchange interaction between spins in two quantum dots is investigated theoretically. We present a microscopic formulation of the interaction between the localized spin and the itinerant carriers including the effects of correlation, using a set of canonical transformations. Correlation effects are found to be of comparable magnitude as the direct exchange. We give quantitative results for realistic quantum dot geometries and find the largest couplings for one dimensional systems.Comment: 4 pages, 3 figure