S. Rayaprol

Structural investigations of La-2125 mixed oxide superconducting system

Crystallographic studies on the (La2 − xNdx)CayBa2Cu4 + yOz′ [y = 2x; 0.0 ≤ x ≤ 0.5] samples are carried out using neutron diffraction technique. The analysis of the neutron diffraction patterns for the series of samples with 0.0 ≤ x ≤ 0.5 was done using Rietveld method. The La2Ba2Cu4Oz′ (La-224, x = 0.0) system exhibits tetragonal phase and is a nonsuperconductor, which on addition of a “CaCuO2” rock salt like layer becomes superconducting. It is important to note the significant role played by Ca-doping in improvising the superconducting properties. Analysis of the structural data reveals that, as Ca concentration increases, the unit cell volume decreases while Tc increases with a maximum value of 78 K for y = 1.0. The detailed crystal structure for LaNdCaBCO series of samples has been studied in the light of changes in bond lengths with increase in Ca concentration and its role in “turning on” the superconductivity.

Studies on La2-xPrxCayBa2Cu4+yOz (x = 0.1 - 0.5, y = 2x) type mixed oxide superconductors

The La2-xPrxCayBa2Cu4+yOz (LaPrCaBCO) mixed oxides have been studied for their structural and superconducting properties using X-ray diffraction (XRD), d. c. resistivity, d. c. susceptibility and iodometric double titration. All the LaPrCaBCO samples for x = 0.1 - 0.5, exhibit tetragonal crystalline structure with P 4/mmm space group as determined by Rietveld analysis of the X-ray diffraction data. With increasing x, enhancement in Tc is observed, which is quite interesting for Pr substituted high Tc oxides. Maximum Tc ~ 58 K has been observed for x = 0.5(La-2125 stoichiometry). The results of structural studies and superconducting property measurements are presented in light of increase in Tc in LaPrCaBCO system with increasing Pr concentration.Abstract The La 2− x Pr x Ca y Ba 2 Cu 4+ y O z (LaPrCaBCO) mixed oxides have been studied for their structural and superconducting properties using X-ray diffraction (XRD), d.c. resistivity, d.c. susceptibility and iodometric double titration techniques. The Rietveld analysis of the XRD data of all the LaPrCaBCO samples ( x =0.1–0.5) exhibits that all the compounds crystallize in tetragonal R-123 type structure (space group P 4/ mmm ). With the increase in x , T c increases up to a maximum value of 58 K ( x =0.5, La-2125 stoichiometry), which is interesting for Pr substituted high T c oxides. The results of structural studies and superconducting property measurements are presented in light of increase in T c in LaPrCaBCO system with increasing Pr concentration.

Effect of Sr-substitution on the restitution of superconductivity in Pr-substituted at rare earth and Ba-site in EuBa2Cu3Oz

Abstract We report the effect of Sr-substitution in restoring the superconductivity of Pr-doped (Eu 1− x Pr x )Ba 2 Cu 3 O z [A] and Eu(Ba 2− x Pr x )Cu 3 O z [B] samples. The structural and superconducting properties of these A and B samples have been investigated using X-ray diffraction, a.c. susceptibility, electrical resistivity, d.c. magnetization and iodometric measurements. It is observed that the superconductivity gets suppressed at the rate of 1.6 K per at.% of Pr substitution at Eu-site due to the localization of mobile holes, while the sample with 15% Pr at Ba-site [B(0.3)] becomes non-superconducting mainly due to the hole filling and localization of holes. However, the increasing substitution of Sr at Ba-site in both A(0.4) and B(0.3) samples upto 25% resulted in the restoration of superconductivity (T c ∼33 K ) due to the delocalization of holes. Interestingly, the increasing substitution of Sr at Eu-site in the non-superconducting B(0.4) sample upto 30%, increases T c from 0 to 52 K mainly due to the hole doping mechanism which is much faster and larger restoration of superconductivity than the Ba-site doping of Sr in the same sample.

Structural and superconducting properties of La2−xRxBa2CayCu4+yO10+δ (R=Nd, Gd; y=2x) system

A nonsuperconducting La2Ba2Cu4O9+δ system has been made superconducting by the addition of CaCuO2, along with rare earth doping at the La site, with the stoichiometric composition La2−xRxCayBa2Cu4+yO10+δ (R=Nd, Gd, and y=2x). All the samples with 0.0<x<0.5; y=2x were characterized by x-ray diffraction and neutron diffraction measurements for structural properties and by ac susceptibility, dc resistivity, and iodometric analysis for superconducting properties. The x=0.0 sample displays semiconducting behavior. Increasing x increases Tc from less than 15 K for Nd/Gd samples with x=0.1 to ∼78 K for x=0.5 samples. It is interesting to note that the hole concentration and Tc exhibit a strong correlation with increasing dopant concentration. Neutron diffraction studies confirm the occupancy of Ca and Nd ions at the La site with a concomitant displacement of La on to the Ba site.

Effect of hole filling by Co and hole doping by Ca on the superconductivity of GdBa2Cu3O7−δ

Abstract The structural and superconducting properties of Cu–Co and coupled Gd–Ca/Cu–Co substitution in GdBa 2 Cu 3 O 7− δ have been studied by X-ray diffraction for crystal structure determination, resistivity, a.c. susceptibility for T c determination and iodometry for oxygen content measurements. The oxides (Gd 1− y Ca y )Ba 2 (Cu 1− x Co x ) 3 O 7− δ have been synthesised for 0.0≤ x ≤0.15 and 0.0≤ y ≤0.45. The effect of increasing Co-concentration in GdBa 2 (Cu 1− x Co x ) 3 O 7− δ for 0.0≤ x ≤0.15 is to lower the carrier concentration and decrease T c which is attributed to the hole filling by Co. This suppression in T c has been compensated for by appropriate hole doping with Ca, in (Gd 1− y Ca y )Ba 2 (Cu 1− x Co x ) 3 O 7− δ which shows that for a fixed Co-content T c increases dramatically as the Ca-content increases leading for instance to the oxides x =0.1, y =0.3; x =0.12, y =0.36 and x =0.15, y =0.45 with respective T c ’s of 83 K, 68 K and 63 K whereas the nonsubstituted phases x =0.1, 0.12, 0.15, y =0.0 don’t superconduct.

NEUTRON DIFFRACTION STUDIES ON La2-xDyxCa2xBa2Cu4+2xOz SUPERCONDUCTORS

Structural studies on Dy-substituted La-2125 type superconductors have been carried out by neutron diffraction experiments at room temperature using a monochromatic neutron beam of wavelength (λ) = 1.249 Å. A series of samples with La2-xDyxCa2xBa2Cu4+2xOz stoichiometric composition, forx = 0.1–0.5, have been studied for their structural properties. A tetragonal Y-123 unit cell was taken as the starting model for the Rietveld analysis. All the samples fit into the starting model, with no structural transition taking place with increasing dopant concentration. The results of Rietveld analysis and structural properties will be discussed in detail.

Low Temperature Bond Valence Sum Study of La1.7 Dy0.3 Ca0.6 Ba2 Cu4.6 Oz Oxide Superconductors

Bond Valence Sum (BVS) calculations were performed on neutron diffraction (ND) data measured at 12, 35, 55, 75 and 300 K on one sample (x = 0.3), of a series of La2-xDyxCa2xBa2Cu4+2xOz (La-2125) compounds. ND experiments have been carried out to study the effect of temperature on structure and to relate it with the superconducting properties. The sample, La1.7Dy0.3Ca0.6Ba2Cu4.6Oz (LD3) has been reported earlier, and exhibits superconductivity at 68 K. In this paper we discuss BVS calculations carried out on low temperature neutron diffraction data.

Interplay of lattice strain and spin-polarization in ferromagnetic–insulator–ferromagnetic thin films: La0.7Ca0.3MnO3/LaAlO3/La0.7Sr0.3MnO3

Epitaxial thin films of ferromagnet/insulator/ferromagnet layers (La0.7Ca0.3MnO3/LaAlO3/La0.7Sr0.3MnO3) of different thickness were fabricated by pulsed laser deposition. Microstructural analysis shows island growth on the thicker film surface. dc magnetization study demonstrates the strong signature of domain wall freezing for the composite film. An interesting feature is a cusp/upturn in field-cooled curve in the composite film-1, much below TB, the blocking temperature. The low temperature cusp disappears in the composite film-2. We suggest that a quasistable charge ordering via spin localization at low temperature, especially for film-1, is the origin of this feature.