A.V. Narlikar

High field performance of nanodiamond doped MgB2 superconductor

The results from magnetotransport and magnetization of nanodiamond doped MgB2-nDx are reported. Superconducting transition temperature (Tc) is not affected significantly by x up to x=0.05 and latter decreases slightly for higher x>0.05. R(T) vs H measurements show higher Tc values under same applied magnetic fields for the nanodiamond added samples, resulting in higher estimated Hc2 values. From the magnetization measurements, it was found that irreversibility field value Hirr for the pristine sample is 7.5T at 4K and the same is increased to 13.5T for 3wt% nD added sample at the same temperature. The Jc(H) plots at all temperatures show that Jc value is lowest at all applied fields for pristine MgB2 and the sample doped with 3wt% nD gives the best Jc values at all fields. These results are discussed in terms of extrinsic pinning due to dispersed n-diamond in the host MgB2 matrix along with the intrinsic pinning due to possible substitution of C at boron site and increased interband scattering for highly ...

Effect of substrate temperature and heat treatment on the microstructure of diamondlike carbon films

Nonhydrogenated diamondlike carbon films prepared at a substrate temperature (ST) of 100, 300, and 500 °C by the laser ablation of graphite on a single‐crystal silicon substrate have been characterized by scanning tunneling microscopy for the surface structure and Raman spectroscopy for the microstructure. Distorted pentagonal and hexagonal rings are observed on the surface of the film grown at 100 °C while only hexagonal rings are observable for the one grown at 500 °C. The rise in ST is found to increase the surface roughness. To assign the various coexisting carbonaceous species formed at different growth temperatures and to check their thermal stability, heat treatment was performed at up to 1300 °C in vacuum and 600 °C in air. The changes occurring on heat treatment in vacuum in these films around 600 °C have been correlated with the release of defects from the threefold network. Likewise, 950 °C temperature has been associated with the conversion of disordered tetrahedral bonding to a distorted trig...

Superconductivity above 90 K in low Tc phase Bi2Ca1Sr2Cu2Ox

Abstract Superconductivity in the low T c Bi-based 2-1-2-2 cuprate has been studied as a function of processing parameters. T c ( R =0) above 91 K has been realized through the optimization of process conditions. XRD of the samples indicate subtle changes in respect of intensity and splitting of certain peaks. The results are compared with those on Bi-based 2-2-2-3 and Y-based 1-2-3 cuprates. The enhanced T c and the reduced δT c of the 2-1-2-2 compound have been suggested to be the consequence of improved atomic ordering and orthorhombicity.

High resolution STM/STS studies of the role of oxygen in superconductivity of Bi-2122 and optimization of Tc

The presence of excess oxygen intercalated in Bi-O redox layers has been found to be detrimental to the Tc of the Bi-2122 phase. High resolution STM has been effectively used to locate intercalated oxygen and STS studies have revealed Bi-O layers becoming more insulating with intercalation. By suitable nitrogenation when the excess oxygen is depleted, more conducting redox layers give rise to an improved coupling between Cu-O planes of the neighbouring demi-unit cells in the c-direction, and it is this improved coupling that is responsible for enhanced Tc. In this way it has been possible to optimize Tc(R=0) for the single phase Bi-2122 cuprate at 96 K with a transition width of 3 K, which is the best value reported to date for the bulk ceramic sample of this material.

Superconductivity and resistivity studies in Bi2Sr2Ca1−xMxCu2O8+y (M=Eu, Dy, Tm and 0⩽x⩽0.6)

Abstract Electrical resistivity measurements have been performed on the Bi 2 Sr 2 Ca 1−x M x Cu 2 O 8+y (M=Eu, Dy, Tm) system with various heat treatments. A gradual change in normal state resistivity ( ϱ n ) and T c has been observed with increasing dopant concentration. The results provide experimental evidence for a transition from a metallic behaviour to a variable range hopping regime at low temperatures. In the insulator regime (highly semiconducting-like) the resistivity behaviour is found to turn from 2D to 3D variable range hopping for x ⩾0.6. The results are discussed in terms of a theoretical model which determines the intrinsic competition between superconductivity and localization in an essentially localized system. T c data are compared with the earlier reported findings.

High resolution STM/STS studies of Bi-O redox layers and superconductivity in pure and substituted Bi-2122 high-Tc cuprate

Abstract Effects arising from the intercalation of excess oxygen in Bi-O redox layers of the Bi-2122 compound have been studied with a STM/STS technique which has been successfully used for obtaining atomically resolved images. The excess oxygen is incorporated by substituting Dy 3+ at the Ca 2+ -site as well as by oxygenating the pure sample. It has been possible to detect and directly observe the excess oxygen atoms in the Bi-O layer which is in agreement with the information inferred from neutron diffraction and HRTEM studies of other workers. The STS studies show that there is a gradual increase in its room temperature energy gap and a decrease in the LDOS, indicating the Bi-O layer to become insulating with intercalation of excess oxygen. Depletion of excess oxygen through nitrogenation or Pb substitution at the Bi-site produces the reverse effect. The importance of improving the metallicity of the redox layer is elaborated and it is demonstrated that the T c of the bulk Bi-2122 phase can be significantly sharpened and raised to 96 K by depleting the excess oxygen from the layer. The redox reaction and the influence of the resistive state of the Bi-O layers in relation to the observed superconductivity results are discussed.

Effect of Zn substitution on para- to ferromagnetic transition temperature in La0.67Ca0.33Mn1−xZnxO3 colossal magnetoresistance materials

Structural, magnetic, and thermal measurements are carried out on the La0.67Ca0.33Mn1−xZnxO3 system with x=0.0–0.50. The structural characterization of the samples is done by Rietveld analysis of their neutron diffraction patterns. Zn substitutes at Mn site isostructurally until x=0.30. Oxygen content remains nearly invariant with x. Magnetic and thermal measurements as well as the electrical resistance show a para- to-ferromagnetic transition at Tp, which decreases with an increase of x. For low Zn concentration (until x=0.075) the decrease dTp/dx is smaller than for the larger concentrations of Zn. Relative decrease dTp/dx at higher concentrations (x>0.10) is similar to that observed earlier for the La0.67Ca0.33Mn1−xFexO3 system. For the transition at Tp, the related change of magnetic entropy (ΔStrs) are calculated from the heat capacity data and indicate that for x=0 the expected value ΔStrs=12.8 J/mol K is recovered.

Study of columnar amorphization and structural symmetry changes produced by swift heavy ion irradiation in YBa2Cu3O7−y thin films using STM

Abstract Columnar amorphization and structural symmetry changes produced by 250 MeV Ag+17 ion irradiation in epitaxial thin films of YBa2Cu3O7−y were studied using scanning tunneling microscopy (STM). The columnar amorphized regions were found to have sizes between 2–3 nm and structural distortion up to 4–5 nm size around the ion tracks. The change in spacing between the nearest columnar defect as a function of ion fluence was studied. The observed columnar amorphization is discussed in the framework of electronic energy loss of swift heavy ions which excites the electronic system of the materials leading to the deterioration of structural symmetry due to energy transfer to lattice via electron-phonon interactions.

Neutron diffraction on Er1−xCaxBa2Cu3O7−δ (0.0≤x≤0.3) system: possible oxygen vacancies in CuO2 planes

Abstract In the system Er 1− x Ca x Ba 2 Cu 3 O 7− δ with x =0.0 to 0.3, the superconducting transition temperature ( T c ), as measured from AC susceptibility, decreases with increasing Ca substitution, x . Detailed analysis of the powder neutron diffraction patterns of these samples shows a decrease in oxygen content with increasing x . The orthorhombic distortion decreases slightly and the c -parameter increases slightly with increasing x . Both, the buckling angle [(Cu(2)O(2)Cu(2) angle)] and the planer Cu(2)O(2) distance in CuO 2 planes, increase with increasing x . Increased Cu(2)O(2) bond distance indicates a slight decrease in p-type hole carriers in CuO 2 planes. On comparison with reported results on T c vs. p-type carriers in high T c superconducting compounds, it is inferred that decrease in p-type carriers alone cannot account for sharp T c depression with increasing x in Er 1− x Ca x Ba 2 Cu 3 O 7− δ system. With increasing x , oxygen vacancies are created in CuO 2 planes, which along with increased buckling angle might be partly responsible for T c suppression in this system.

Superconductivity and possible carrier localization in YBa2Cu3−xZinxO7−y

Resistivity and Hall data on single phase YBa2Cu3−xZnxO7−y ceramic samples are reported. Though Tc decreases at a rate of 13 K/at.% as x increases, the carrier density seems to be little affected at 300 K. However, the Hall mobility decreases and varies hardly with temperature as x increases. The normal state resistivity behaviour changes from metallic to semiconducting like which is compared with other cuprate superconductors. The resulting decrease in Tc is discussed in terms of the possible carrier localization. Some appropriate models are briefly reviewed.