Successful replacement of B by C in the series MgB 2-x C x for values of x upto 0.3 are reported. Resistivity and ac susceptibility measurements have been carried out in the samples. Solubility of carbon, inferred from the observed change in the lattice parameter with carbon content indicates that carbon substitutes upto x=0.30 into the MgB 2 lattice. The superconducting transition temperature, T C measured both by zero resistivity and the onset of the diamagnetic signal shows a systematic decrease with increase in carbon content upto x=0.30, beyond which the volume fraction decreases drastically. The temperature dependence of resistivity in the normal state fits to the Bloch-Gruneisen formula for all the carbon compositions studied. The Debye temperatures, θ D , extracted from the fit is seen to decrease with carbon content from 900K to 525K, whereas the electron-phonon interaction parameter, λ, obtained from the McMillan equation using the measured T C and θ D , is seen to increase monotonically from 0.8 in MgB 2 to 0.9 in the x=0.50 sample. The ratio of the resistivities between 300K and 40K versus T C is seen to follow the Testardi correlation for the C substituted samples. The decrease in T C is argued to mainly arise due to large decrease in θ D with C concentration and a decrease in the hole density of states at N(E F ).

UV Raman scattering studies show longitudinal optical (LO) mode up to fourth order in wurtzite GaN nanowire system. Fröhlich interaction of electron with the long range electrostatic field of ionic bonded GaN gives rise to enhancement in LO phonon modes. Good crystalline quality, as indicated by the crystallographic as well as luminescence studies, is thought to be responsible for this significant observation. Calculated size dependence, incorporating size corrected dielectric constants, of electron-phonon interaction energy agrees well with measured values and also predict stronger interaction energy than that of the bulk for diameter below ∼3nm.

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1-longitudinal optical (LO) phonons in free-standing mixed Cd 1−x Zn x S nanocrystals, synthesized using chemical precipitation, are investigated using Raman spectroscopy. As expected for the nanocrystals, the 1-LO modes are found to appear at slightly lower wavenumbers than those in the bulk mixed crystals and exhibit one-mode behavior. On the other hand, the line broadening is found to be much more than that can be accounted on the basis of phonon confinement. From the detailed line-shape analysis it turns out that the substitutional disorder in the mixed crystals contributes much more to the line broadening than the phonon confinement. The linewidth arising from these mechanisms are also extracted from the analysis.

Among the small cation sized rare earth sesquioxides, the reported transition pressure of cubic Tm2O3 is ambiguous. Pressure induced structural phase transition in cubic Tm2O3 has been reinvestigated using the synchrotron X-ray diffraction, Raman spectroscopy, and ab initio density functional theory (DFT) calculations up to a pressure of 25 GPa. Both the X-ray diffraction and Raman spectroscopic measurements revealed an irreversible polymorphic structural phase transition from type-C cubic to type-B monoclinic at around 12 GPa, whereas the same is predicted to be 8 GPa from the density functional theory. The phase transition observed at 12 GPa is in contrast to the literature and the reasoning has been established by other studies, viz., Raman spectroscopy and DFT. A third order Birch-Murnaghan equation of state fit to the experimental compressibility curve yielded a zero pressure bulk modulus of 149(2) GPa with the pressure derivatives 4.8(5) for the parent cubic phase and 169(2) GPa with the pressure derivative 4 for the high pressure monoclinic phase, respectively. These values are in good agreement with the calculated bulk modulus of 146 and 151 GPa for the cubic and monoclinic phases, respectively. Raman modes for the monoclinic phase of Tm2O3 are measured and reported for the first time. The mode Grüneisen parameter of different Raman modes for both cubic and monoclinic phases of Tm2O3 has also been determined. The experimental results are correlated with changes in the density of states near the Fermi level, which are indicative of structural instabilities in the parent cubic structure.

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The valence state, hybridization and electronic band structure of charge ordered AlV2O4 are investigated by measuring the electron energy loss spectra (EELS) and performing band structure calculations using the WIEN2k code. White line ratio and O K edges of V2O5, VO2, V2O3 and AlV2O4, obtained using electron energy loss spectroscopy, are analysed specifically to probe systematically the VO6 octahedra in all of them. The systematic decrease of the L2 intensity and the O K edge intensity from V(5+) in V2O5 to AlV2O4 indicates a progressive increase in the occupancy of the hybridized states, which is corroborated by the absence of a transition from O 1s to hybridized 2t(2g). Band structure calculations on the parent charge frustrated cubic phase and the charge ordered rhombohedral phase clearly document a band gap in the charge ordered state. From the structural information obtained after convergence and the spectroscopic information from EELS, it appears that partial orbital occupancy may lead to a deviation from an integral valence state on all the vanadium in this exotic charge ordered spinel system.