Raman spectroscopic investigations of phonons of different symmetries in anatase TiO2 nanocrystals synthesized by the sol−gel method are carried out. Out of six Raman active phonons, the line shapes of two Eg modes and one B1g mode have been analyzed quantitatively to distinguish between the contributions of laser-induced local heating, phonon confinement effects, and defects to the line broadening. The line shape asymmetry arising from confinement of optical phonons in the 397 cm−1 B1g mode is found to be of opposite nature than those in the 144 and 639 cm−1 Eg modes. This arises due to the negative dispersion of the 397 cm−1 B1g phonon dispersion curve. The measured spectra show larger broadening than those predicted by a phonon confinement model. The excess broadening, attributed to intrinsic defects, is found to be least for the 144 cm−1 Eg mode and largest for the 397 cm−1 B1g mode. In addition, finite laser power of the excitation wavelength is found to raise the temperature of the nanocrystals, with heating being maximum for the smallest size nanocrystals.

We report high-frequency scattering parameter measurement of large-area monolayer graphene integrated on low-loss quartz substrates. High-quality graphene was grown by chemical vapour deposition on copper, chemically hole doped, and transferred to quartz. Microwave measurements were performed from 0.01 to 110 GHz. Simple microwave models were used to extract graphene impedance parameters. We find that contact resistance is effectively shunted above 3 GHz. Atomically thin large area graphene behaves as a wideband resistor with negligible kinetic inductance and negligible skin effect.

Abstract:X-ray diffraction, resistivity, ac susceptibility and magnetization studies on La 0.67 Ca 0.33 Mn 1-x Ru x O 3 (0 ≤ x < 0.1) were carried out. A significant increase in the lattice parameters indicated the presence of mixed valance state of Ru: Ru 3+ and Ru 4+. The resistivity of the doped compounds exhibited two features: a broad maximum and a relatively sharp peak. While a para to ferromagnetic transition could be observed for the latter peak, no magnetic signal either in ac susceptibility or in magnetization measurements could be observed for the broad maximum. The magnetic moment decreases non linearly from 3.55 to 3 µ B over the Ru composition from 0 to 8.5 at.%. Based on the results of the present studies and on existing literature on the Mn-site substituted systems, we argue that a magnetic phase separation occurs in the Ru doped system. While the sharp peak in the resistivity corresponds to Ru 4+ enriched region with a ferromagnetic coupling with neighboring Mn ions, the broad peak corresponds to a Ru 3+ rich regions, with an antiferromagnetic coupling with neighboring Mn ions.

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For the first time, the effect of Ta 5+ substitution at the Mn site of La 0.67 Ca 0.33 MnO 3 compound is reported. A significant increase in lattice parameters and unit cell expansion indicate larger structural modification upon substitution. The ferromagnetic-metallic ground state is modified to a cluster glass insulator for x40:03: The observed suppression of the ferromagnetic transition temperature T c of $39 K/at% might be the highest reported in Mn-site substituted La 0.67 Ca 0.33 MnO 3 . The well-known effects of changed carrier density, larger average ionic radius at Mn site and dilution of Mn sublattice appear insufficient to explain this strong suppression of T C . r 2004 Published by Elsevier B.V.

We report the lattice parameter variation and the band gap tuning of CdO with Ca by preparing thermodynamically stable Cd1−xCaxO solid solution in the entire composition range 0≤x≤1. The functional dependence of the lattice parameter on Ca concentration is found to deviate from Vegard’s law. The principal band gap is found to vary nonmonotonically over a wide range, from 1.4 to 3.9 eV for 0≤x≤0.8. First principles density functional theory calculations, using full potential linearized augmented plane wave methods also predict a nonlinear variation for the lattice parameter and the optical band gap with Ca concentration. From these calculations, contributions from volume deformation, electron transfer, and structural relaxation are estimated and the results are compared with experiments.

To investigate the polymerization of fullerenes, solid C 60 and C 70 have been subjected simultaneously to high pressures and temperatures, with pressures up to 7.5 GPa and temperatures up to 800°C. X-raydiffraction measurements indicate that the fcc solid C 60 transforms to an orthorhombic structure consisting of a polymerized linear chain of C 60 molecules. The associated changes in the intramolecular vibrational modes have been probed through Raman and infrared measurements. In contrast to solid C 60 , under similar conditions, polymerization is not observed in the case of solid C 70 , although there is a structural transition from the initial hcp structure to a rhombohedral structure. This lack of polymerization in C 70 is discussed in terms of the structure of solid C 70 under high pressure and temperature conditions, coupled with the topochemical features of the C 70 molecule.

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