L. Ganapathi

Effect of processing geometry in oxygen incorporation and in situ formation of YBa2Cu3O7 superconducting thin films by pulsed laser evaporation technique

In situ formation of YBa2 Cu3 O7 superconducting thin films by the pulsed laser evaporation technique is critically dependent on the processing conditions (substrate temperature, oxygen partial pressure, subsequent annealing, etc.) and on the deposition setup, including the nozzle geometry required for the incorporation of oxygen. We have studied the recovery of the superconducting properties as a function of location of the oxygen jet relative to a fixed substrate‐target setup at a substrate temperature of 550 °C and an oxygen partial pressure of 200 mTorr. Several experiments were performed with the oxygen jet directed on the target and/or on the substrate. During the deposition process, a dc bias voltage of +300 V was applied on an interposing ring, which has resulted in formation of superconducting thin films at a low temperature of 500 °C. Excellent superconducting properties with the Tc zero of 88 K were obtained on films fabricated at 550 °C with the nozzle directed onto the substrate. In contrast,...

Synthesis of superconducting YBa2Cu3O7−δ thin films on nickel‐based superalloy using in situ pulsed laser deposition

Good quality superconducting YBa2Cu3O7−δ thin films have been grown on nickel‐based superalloy, viz., Hastelloy with yttria‐stabilized zirconia buffer layers employing a single chamber in situ pulsed laser deposition process. YBa2Cu3O7−δ films were found to be textured with c axis perpendicular to the substrate. A plot of normalized resistance against temperature exhibited a smooth metallic behavior followed by an onset superconducting transition at 92 K with the zero resistance temperature (Tc0) of 86 K. Critical current density for these films at 77 K was ∼7000 A cm−2 and critical current (Ic) was observed to be proportional to (1 − T/Tc0)1.4.

Properties of YBa2Cu3AgxO7−δ composite superconductors

Superconducting properties and microstructures of the YBa2Cu3AgxO7−δ composites have been studied as a function of silver content, 0 3, small quantities of 211 phase were detected along the grain boundaries. Alloy formation between Ag and Cu is proposed to take place which makes the residual composition poor in copper, resulting in the formation of nonsuperconducting 211 phase. The silver is observed in the form of precipitates at very low concentrations of silver, indicating that silver does not substitute for copper in YBa2Cu3O7. Lattice parameters of the composites were found to be invariant throughout the above composition range. The addition of Ag2O in the starting composition seems to stabilize the stoichiometric YBa2Cu3O7 by way of decomposition to Ag and O. As a result, the incorporation of oxygen and recovery of s...

In situ processing of textured superconducting thin film of Bi(-Pb)-Ca-Sr-Cu-O by excimer laser ablation

We have prepared highly oriented bismuth cuprate thin films (c‐axis textured) on yttria‐stabilized zirconia (100) YS‐ZrO2 substrates at 650 °C by pulsed excimer laser ablation in a 200 mTorr oxygen ambient. The films were annealed in oxygen and nitrogen atmospheres to study the effect on zero‐resistance transition temperature (Tc0). Thin films deposited at 650 °C exhibited a Tc0 of 68 K with onset at 110 K. Post‐annealing for 1 h at 400 °C in oxygen improved the Tc0 to 82 K, while the onset remained the same. X‐ray diffraction, scanning electron microscopy, and Rutherford backscattering channeling studies were performed on these films for correlations between crystal structure, microstructure, and superconducting properties. X‐ray diffraction patterns indicated a 2212 type phase with a=5.39 A and c=30.76 A; preferential orientation of the c axis perpendicular to the substrate was observed. The lattice parameters and x‐ray diffraction patterns were found to be invariant with annealing treatments that impro...

Single‐chamber, in situ processing of superconducting YBa2Cu3O7−δ thin films on stainless steel with yttria‐stabilized zirconia buffer layer

Single‐chamber in situ laser processing of high‐Tc YBa2Cu3O7−δ film on stainless steel (302) substrates with yttria‐stabilized zirconia (YSZ) buffer layers, has been carried out using a multitarget deposition system. YSZ and YBa2Cu3O7−δ films were deposited sequentially by KrF excimer laser (λ=248 nm) at substrate temperature of 650 °C. The films were characterized by x‐ray diffraction, four point probe ac electrical resistivity, scanning electron microscopy and Auger electron spectroscopy (AES) techniques. Due to the matching of the thermal expansion coefficient of YSZ with stainless steel and its diffusion barrier characteristics, good quality high‐Tc films have been grown on stainless steel substrates with superconducting transition temperature, Tc (onset), of 92 K and Tco (zero resistivity temperature) of 84 K. AES depth profiling indicated little interdiffusion of Fe across the interface of YSZ and stainless steel. A laser evaporation method was used for patterning the YBa2Cu3O7−δ films and critical ...

Variation of Tc0 in the 110 K superconductor Bi1.5Pb0.5Ca2Sr2Cu3Ox

We have systematically investigated resistance of a bismuth cuprate Bi{sub 1.5}Pb{sub 0.5}Ca{sub 2}Sr{sub 2}Cu{sub 3}O{sub {ital x}} as a function of different annealing treatments. X-ray diffraction and transmission electron microscopy studies were performed on concomitant specimens for correlations between structure and superconducting properties. A sharp 110 K superconducting transition was observed in the sample which was isostructural with the {ital n}=2 member of the (Bi,Pb){sub 2}(Ca,Sr){sub {ital n}+1}Cu{sub {ital n}}O{sub 2{ital n}+4+{delta}} family. The zero resistance transition temperature {ital T}{sub {ital c}0} was found to be sensitive to annealing at 673 K with a best result of 100 K for 30 min annealing. It is proposed that {ital T}{sub {ital c}0} and transition width in bismuth cuprates are sensitive to oxygen stoichiometry.We have systematically investigated resistance of a bismuth cuprate Bi1.5Pb0.5Ca2Sr2Cu3Ox as a function of different annealing treatments. X‐ray diffraction and transmission electron microscopy studies were performed on concomitant specimens for correlations between structure and superconducting properties. A sharp 110 K superconducting transition was observed in the sample which was isostructural with the n=2 member of the (Bi,Pb)2(Ca,Sr)n+1CunO2n+4+δ family. The zero resistance transition temperature Tc0 was found to be sensitive to annealing at 673 K with a best result of 100 K for 30 min annealing. It is proposed that Tc0 and transition width in bismuth cuprates are sensitive to oxygen stoichiometry.

Characteristics of high-Tc LaxY1−xBa2Cu3O7−δ superconductors

Abstract The LaxY1−xBa2Cu3O7−δ (0 0.5, the system starts behaving like a La123 system with a high sensitivity of superconducting and semiconducting properties to heat treatment and oxygen content. The solid solution oxides are shown to exhibit semiconducting, metallic or superconducting behavior with increasing oxygen content. Two different preparative techniques have been used to obtain superconducting solid solution oxides. Two parameters, the orthorhombic structure and the oxygen content, have been identified to primarily influence the superconductivity of the (LaxY1−x)123 oxide. Based on the observations of orthorhombic and semiconducting (LaxY1−x )123 samples, it has been suggested that these two parameters independently influence the properties of the (LaxY1−x)123 oxide for x>0 and both are essential for the superconductivity of these oxides. Our study supports the idea that the exchange of La with Ba in the ideal perovskite A site is a main contributor to the variation of the superconducting properties. Oxygen incorporation in the O(4) site and the randomization of the O(1) and O(5) sites are believed to result in a more symmetric tetragonal structure leading to disruption of the two dimensional Cu-O network, considered essential for the superconductivity of the 123 system.

In Situ processing of LaBa2Cu3O7-δ thin films by laser ablation method

Abstract Thin films of LaBa 2 Cu 3 O 7-δ on the (100) surfaces of LaAlO 3 , MgO and YSZ (Yttria Stabilized Zirconia) have been deposited by the laser ablation method for the first time at low substrate temperature ⩽650°C. The X-ray measurements showed that these films are textured and have their c -axis perpendicular to the substrate. Thin films deposited from an orthohombic bulk target showed a metallic behavior above the superconducting transition temperature ≈90 K and zero resistance was observed above liquid nitrogen for the films on (100) YSZ, and (100) LaAlO 3 substrates. The zero resistance temperatures for the films on YSZ, LaAlO 3 and MgO were 85, 80 and 70 K, respectively. It was interesting to note that films deposited from a tetragonal bulk target showed semiconducting behavior up to 20 K.