S. Kalavathi

Carbon solubility and superconductivity in MgB2

Abstract Successful replacement of B by C in the series MgB 2− x C x for values of x upto 0.3 is 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 temperature, θ D , extracted from the fit, is seen to decrease with carbon content from 900 to 525 K, 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 300 and 40 K 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 ).

Crystal structure, superconductivity and magnetic properties of the superconducting ferromagnets Gd1.4−xDyxCe0.6Sr2RuCu2O10 (x=0–0.6)

Abstract The structural, electrical and magnetic properties of the superconducting ferromagnets, Gd 1.4− x Dy x Ce 0.6 Sr 2 RuCu 2 O 10 ( x =0–0.6) are systematically investigated as a function of Dy doping and temperature. These compounds are characterized by high temperature superconductivity ( T c ranging from 20 to 40 K depending upon the Dy content) co-existing with weak ferromagnetism with two magnetic transitions ( T M2 ranging from 95 to 106 K and T M1 around 120 K). Doping with Dy gives no significant structural changes except for a minor change in the c / a ratio. However the superconducting transition temperature is significantly suppressed and magnetic ordering temperature enhanced on Dy doping. These effects are described and discussed.

Direct evidence for the decomposition of YBa2Cu3O7−x into orthorhombic and tetragonal phases at 200°C

The variations of room temperature resistivity (ρ300), powder a.c. susceptibility (χ′) and x-ray diffraction (XRD) line profiles of (013), (103) and (110); (003), (010) and (100) reflections in the step scanning mode were investigated on single phase samples of Y1Ba2Cu3O7−x (YBCO), as a function of ageing at 200°C up to 860 h. The average off-stoichiometryx=0.22 remained constant throughout the experiment.ρ300 showed a monotonic increase while the superconducting transition curves showed increasing sharpening with ageing time. XRD of the reflections investigated developed satellite structures or additional intensities at 2θ values different from those of main reflections. The results are consistent with the prediction and earlier observation of the decomposition of the off-stoichiometric YBCO into oxygen ordered orthorhombic and vacancy ordered tetragonal phases.

Structural studies of Nd1.85Ce0.15CuO4 + Ag superconducting system

We have studied for the first time the effect of Ag addition (0–15 wt%) to the superconducting system, Nd1.85Ce0.15CuO4, on its crystal structure and local structural features, using synchrotron X-ray diffraction (SXRD) and Raman spectroscopy, respectively. SXRD and subsequent Rietveld refinement studies on powders of Nd1.85Ce0.15CuO4 + Ag system indicate a small but significant change in lattice parameter upon Ag addition, showing evidence for possible incorporation of Ag to the extent of ∼1 wt%. Raman spectroscopic studies indicate that the parent structure of Nd1.85Ce0.15CuO4 remains unaffected with no major local structural changes on doping with silver. However, all Raman modes show minor phonon hardening upon Ag addition, which is consistent with the unit cell volume reduction as is observed in XRD. A systematic bleaching out of the apical oxygen defect mode was also observed with increased Ag addition. Polarized Raman measurements helped to identify the asymmetric nature of the B1g Raman mode. X-ray diffraction studies on pellets of Nd1.85Ce0.15CuO4 + Ag system further indicate a randomization of preferred orientation upon Ag addition. The superconductivity of the Nd1.85Ce0.15CuO4 + Ag system has been well characterized for all the compositions studied.

Effect of iron doping and oxygen stoichiometry on infrared absorption in Y1Ba2Cu3O7−δ

We report infrared absorption of Y1Ba2Cu3O7−δ as a function of oxygen stoichiometry (0<δ<1) and copper substitution by iron in the spectral range of 450–700 cm−1. The strong bands associated with Cu-O vibrations undergo significant changes in their frequencies and intensities asδ is varied across the orthorhombic to tetragonal phase. These changes coupled with those arising as a result of doping with iron has helped in identifying the nature of the vibrational modes.

Pressure induced structural phase transition in rare earth sesquioxide Tm2O3: Experiment and ab initio calculations

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 Gruneisen 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.

Granularity and annealing effects in superconductivity of the electron doped cuprate system Nd1.85Ce0.15CuO4

We have observed large granular effects in the superconducting behavior of the electron doped cuprate Nd1.85Ce0.15CuO4 synthesized in polycrystalline form by solid state reaction. The resistive transition to the superconducting state reveals a staged behavior due to intra and inter-granular transitions, the former corresponding to superconductivity within the grains and the latter due to long range coherence between the grains. On the other hand the DC magnetization reveals the intrinsic behavior of the grains and a smooth transition from the paramagnetic state to the diamagnetic. Our investigations of electrical current dependence of inter and intra-granular transitions indicatesignificant changes in the former without any change in the latter. The behavior is in line with what is expected in granular superconductors which are related to intra granular and inter granular coupling. Scanning electron microscopy investigations provide morphological evidence of granularity in the material.

Effect of ionic-size change of the rare earth ion on the electrical properties of the hole doped double perovskite Gd0.95Sr0.05BaCo2O5.5

We report the effect of co-doping Y on the electrical properties of the hole doped double perovskite Gd0.95Sr0.05BaCo2O5.5. New compositions Gd0.95−xYxSr0.05BaCo2O5.5 have been successfully synthesized and characterized. A contraction in all three lattice parameters of the orthorhombic unit cell has been observed on Y doping. A reduction in the insulator to metal-like transition temperature (TIM) has been observed upon Y doping from 348K for x=0 to 331K for x=0.2. These observed shifts have been interpreted in terms of the possible mechanisms of the insulator-metal transition. The low temperature electrical resistivity data in the range 300-77K has been fitted to different conduction models. They fit best to a variable range hopping (VRH) model at low temperature below 120K followed by small polaron hopping model (SPH) at intermediate temperature (120-200K). The presence of magnetic polarons in the cobaltite systems has been envisaged recently and this provides further evidence for the same.

Metal - insulator transition upon Ni substitution in Fe1-xNixSb2 Kondo insulating system

The temperature and magnetic field dependent electrical resistivity studies have been performed for Ni substituted Fe1-xNixSb2 Kondo insulating (KI) system in 4.2 to 300 K temperature range up to 12 Tesla field. While KI gap, deduced in 70-330K range, increases monotonically, the variable range hopping parameter extracted in low temperature regime (7-30K) progressively diminishes under Ni substitution. An insulator to metal transition is observed in low temperature regime for x=0.1. Magnetoresistance remains positive for all the samples.

Normal State Resistivity of Aged Nb-65 at% Ti Alloy

In this paper we report the normal state electrical resistance measurements on the superconductor Nb–65at% Ti alloy β quenched to room temperature and aged at 400°C for varying periods. We examine the low temperature data for a power law behaviour and find that the best fit of the data yields an exponent close to 1 irrespective of the aging time.