C. S. Sundar

Thermal decomposition of C60

Abstract The influence of heat-treatment of C60 pellets in vacuum at temperatures between 500°C and 900°C has been investigated by a variety of experimental techniques. X-ray measurements show that the diffraction peaks become diffuse at temperatures beyond 700°C. Uv-visible absorption measurements show a sharp diminution in the C60 content of the pellets heat-treated at temperatures beyond 700°C. Resistivity measurements show that the pellets heat-treated at 700 to 900°C are conducting with a negative temperature coefficient of resistivity. From these studies it is inferred that C60 decomposes into amorphous carbon on heat-treatment at temperatures beyond 700°C. Plausible mechanisms for the thermal decomposition of C60 are discussed.

A study of positron distribution and annihilation characteristics in YBa2Cu3O7-x

The positron distribution and lifetimes have been computed for the model structures of orthorhombic YBa2Cu3O7, tetragonal YBa2Cu3O6.5 and tetragonal YBa2Cu3O6, which consist of different ordered arrangements of oxygen vacancies. The positron potential was obtained as a sum of the full lattice electrostatic potential and the correlation potential in the local density approximation. The positron wavefunction and the eigenvalue of the lowest-lying itinerant positron state was obtained by solving the Schrodinger equation in the finite-difference scheme. The positron wavefunction displays maxima at the oxygen vacancy positions and it is seen that the positron probes predominantly the oxygen vacancies in the basal plane. By computing the overlap of the positron density with the core and valence electron densities, the positron lifetimes for YBa2Cu3O7, YBa2Cu3O6.5 and YBa2Cu3O6 are estimated to be 190 ps, 222 ps and 234 ps, respectively. Calculation of the positron wavefunction and lifetime has also been carried out for a positron localised at an isolated oxygen vacancy along the chain in orthorhombic YBa2Cu3O7. The calculated values of lifetimes for different arrangements of vacancies have been compared with the experimental values of positron lifetimes in YBa2Cu3O7-x measured after various heat treatments.

Reversible Pressure-Induced Amorphization in Solid C70 : Raman and Photoluminescence Study

We have studied single crystals of C70 by Raman scattering and photoluminescence in the pressure range from 0 to 3 1.1 GPa. The Raman spectrum at 3 1.1 GPa shows only a broadband similar to that of the amorphous carbon without any trace of the Raman lines of CT0. After releasing the pressure from 3 1.1 GPa, the Raman and the photoluminescence spectra of the recovered sample are that of the starting C70 crystal. These results indicate that the C70 molecules are stable up to 3 1.1 GPa and the amorphous carbon high pressure phase is reversible, in sharp contrast to the results on solid Cm. A qualitative explanation is suggested in terms of intermolecular versus intramolecular interactions.

Experimental study of the decomposition of Y1Ba2Cu3O7−x into tetragonaland orthorhombic phases

Abstract Infrared absorption spectroscopy and X-ray diffraction measurements have been used to show that a single phase oxygen deficient Y 1 Ba 2 Cu 3 O 7− x (YBCO) when annealed at low temperatures (200°C) is unstable towards decomposition into a mixture 0824 1062 of orthorhombic and tetragonal phases. This is in qualitative agreement with the calculated phase diagram of YBCO. The degradation in the superconducting properties (Meissner signal reduced by a factor of 2) that accompanies this annealing treatment is understood as being a direct consequence of the decomposition.

Superconductivity in Ru substituted polycrystalline BaFe 2-x Ru x As 2

The occurrence of bulk superconductivity at ~22 K is reported in polycrystalline samples of BaFe2-xRuxAs2 for nominal Ru content in the range of x=0.75 to 1.125. A systematic suppression of the spin density wave transition temperature (TSDW) precedes the appearance of superconductivity in the system. A phase diagram is proposed based on the measured TSDW and superconducting transition temperature (TC) variations as a function of Ru composition. Band structure calculations, indicate introduction of electron carriers in the system upon Ru substitutiom. The calculated magnetic moment on Fe shows a minimum at x=1.0, suggesting that the suppression of the magnetic moment is associated with the emergence of superconductivity. Results of low temperature and high field Mossbauer measurements are presented. These indicate weakening of magnetic interaction with Ru substitution

Pressure induced dimerisation of C70

Solid C70 has been subjected simultaneously to high pressures and temperatures (HPHT), with pressures upto 7.5 GPa and temperatures upto 750°C. X-ray diffraction measurements on the recovered samples indicate that the initial h.c.p. solid C70 transforms to a rhombohedral structure which recovers to an f.c.c. structure on annealing. The associated changes in the intra molecular vibrational modes have been probed through infrared (IR) and Raman measurements. The IR measurements on these HPHT samples show splitting of some of the pristine modes and occurrence of several new modes. These sharp IR modes in the HPHT treated samples, which are seen to be different from that reported for photopolymerised C70, have been attributed to the formation of C70 dimers.

Wide band gap tunability of bulk Cd1−xCaxO

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.

Structure and vibrational properties of carbon tubules

The structure of multilayered carbon tubules has been investigated by electron microscopy and X-ray diffraction. The structure of tubules is characterized by disorder in the stacking of cylindrical graphene sheets. Raman scattering measurements have been carried out in tubules and compared with graphite. The observed features in the Raman spectra in tubules can be understood in terms of the influence of disorder. The additional Raman modes predicted for single layer carbon tubules have not been observed.

Efficacy of surface error corrections to density functional theory calculations of vacancy formation energy in transition metals

We calculate properties like equilibrium lattice parameter, bulk modulus and monovacancy formation energy for nickel (Ni), iron (Fe) and chromium (Cr) using Kohn-Sham density functional theory (DFT). We compare the relative performance of local density approximation (LDA) and generalized gradient approximation (GGA) for predicting such physical properties for these metals. We also make a relative study between two different flavors of GGA exchange correlation functional, namely PW91 and PBE. These calculations show that there is a discrepancy between DFT calculations and experimental data. In order to understand this discrepancy in the calculation of vacancy formation energy, we introduce a correction for the surface intrinsic error corresponding to an exchange correlation functional using the scheme implemented by Mattsson et al (2006 Phys. Rev. B 73 195123) and compare the effectiveness of the correction scheme for Al and the 3d transition metals.

Pressure-induced metallization of BaMn2As2

The temperature and pressure dependent electrical resistivity rho(T,P) studies have been performed on BaMn2As2 single crystal in the 4.2 to 300 K range upto of 8.2 GPa to investigate the evolution of its ground state properties. The rho(T) shows a negative co-efficient of resistivity under pressure upto 3.2 GPa. The occurrence of an insulator to metal transition (MIT) in an external P ~4.5 GPa is indicated by a change in the temperature co-efficient in the rho(T) data at ~36 K . However complete metallization in entire temperature range is seen at a P~5.8 GPa. High pressure XRD studies carried out at room temperature also shows an anomaly in the pressure versus volume curve around P ~ 5 GPa, without a change in crystal structure, indicative of an electronic transition. Further, a clear precipitous drop in rho(T) at ~17 K is seen for P ~5.8 GPa which suggests the possibility of the system going over to a superconducting ground state.