Ag-doped YBa2Cu3O7 Research Articles

Improvement in the Properties of Ag-Doped YBa2Cu3O7x Grain Boundary Josephson Junctions

Ag-doped YBa2Cu3O7–x (YBCO) thin films using 5 to 20 wt% Ag-doped YBCO targets have been grown by a dc sputtering technique on SrTiO3 bicrystals. Critical currents of 4 to 5 × 106 A/cm2 at 77 K were measured in YBCO films doped with 5 wt% Ag which has been found to be higher than the value of 1 × 106 A/cm2 measured in undoped samples. The normal resistivity decreases by a doping of 5 wt% Ag and increases for higher Ag concentrations. The critical temperature, Tc, of the Ag-YBCO films remained unchanged at 92 K as in the undoped YBCO samples. An IcRn product of 170 μV at 77 K was found in Grain Boundary Josephson junctions (GBJJs) with 5 wt% Ag, compared with the value of 100 μV measured in undoped samples at the same temperature. Current–voltage characteristics were measured in GBJJs, showing Shapiro steps under microwave radiation and Fraunhofer patterns with an external magnetic field. The improvement in the normal and superconducting properties of Ag-doped YBCO films has been interpreted using the De Gennes model to establish that YBCO containing metallic Ag addition shows a superconductor–normalmetal–superconductor (S–N–S) behavior, thereby the Ag-doping enhances the weak link behavior and is, therefore, appropriate for electronic applications.

Micro-PIXE and channeling PIXE analysis of Ag-doped YBa2Cu3O7−δ thin films

Nuclear Microscopy, utilizing a 2 MeV He+ beam for channeling Rutherford Backscattering (RBS) and PIXE analysis, was used to characterise Ag-doped YBa2Cu3O7−δ thin films and measure the lateral distribution of the Ag. The samples were prepared by in situ two-beam pulsed laser deposition in order to investigate the effects of such dopings on critical current densities [1,2]. Films deposited at temperatures above 650°C form needle-like surface structures with a length of up to 100 μm; these tend to align with in-plane a–b axis. Results for a sample prepared at a substrate temperature of 730°C and a maximum Ag concentration of 5 at.% are discussed. The needle-like structures were found to be rich in Ag and Cu, and the YBa2Cu3O7−δ film contained 0.02 at.% Ag. Broad beam PIXE-channeling results indicate that 19% of the Ag is substitutional.

The distribution of Ag in Ag-doped YBa2Cu3O7- thin film prepared by dual-beampulsed-laser deposition

<P>The Ag distribution in Ag-doped YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-<IMG SRC="common/delta.gif" ALT="delta" ALIGN="TOP"/></SUB> (YBCO) thin films fabricated by dual-beam pulsed-laser deposition on SrTiO<SUB>3</SUB> (100) substrates has been studied by Auger electron spectroscopy, microproton-induced x-ray emission, atomic force microscopy and scanning electron microscopy. All the results consistently show that Ag aggregated in the bar-like structures observed in the film. These bars are aligned along the <I>a</I>-<I>b</I>-axis or at 45° to the <I>a</I>-<I>b</I>-axis of the YBCO thin film. The main body of the long bars aligned with the <I>a</I>-<I>b</I>-axes of the film was found to be a combination of metallic Ag with other precipitates of YBCO film that may grow from the substrate surface to the YBCO film surface. There were other precipitates aggregating as well at the surface of these bars, such as oxides of Cu and Ba. The short bars that aligned along 45° to the <I>a</I>-<I>b</I>-axes of the film were found to be deficient in Ag but rich in Cu, Ba and O, which could be oxide precipitates of YBCO. The growth mechanisms of the two types of bars seem quite different.</P>

Directly coupled direct current superconducting quantum interference device magnetometers based on ramp-edge Ag:YBa2Cu3O7−x/PrBa2Cu3O7−x/Ag:YBa2Cu3O7−x junctions

Directly coupled dc superconducting quantum interference device (SQUID) magnetometers on LaAlO3 substrates were fabricated using ramp-edge superconductor/normal-metal/superconductor junctions, where Ag-doped YBa2Cu3O7−x was used for the electrode and PrBa2Cu3O7−x for the normal-metal barrier. A flux noise of 8×10−6 Φ0 Hz−1/2 at 10 kHz measured with a dc bias current was achieved at 75 K, which corresponded to a field sensitivity of 400 fTHz−1/2 for a magnetometer with a pick-up loop area of 8.5 mm×7.5 mm. Most significantly, the noise floor increased at lower frequencies with a frequency dependence slightly less than 1/f. The field noise of the SQUID magnetometers increased by only 25% after cycling the devices from zero field to 500 mG. In a static earth’s magnetic field background, the field noise of the SQUID magnetometers increased by less than a factor of 2.

RBS and ion channeling studies of Ag-doped YBa2Cu3O7−δ targets and films

The location of Ag in Ag-doped YBa 2 Cu 3 O 7−δ (YBCO) films and other high - T c materials (such as Ag-doped BiSrCaCuO films and Ag-sheathed textured BiSrCaCuO wires) is a very important issue for improving high - T c materials. In this work, laser ablated and DC magnetron sputtered YBCO films on (1 0 0) LaAlO 3 and (1 0 0) SrTiO 3 were prepared from sintered Ag-YBCO composite targets (nominally containing 5 wt% Ag) and studied by Rutherford Backscattering Spectrometry (RBS) and ion channeling techniques using 2.0 MeV 4 He + and 7 Li + beams. We have found that the Ag-YBCO targets contain about 3 wt% Ag and most of the retained Ag atoms form some small size Ag precipitates with a typical size smaller than a few microns. We have demonstrated that in very good single crystalline YBCO films, the percentage of retained Ag in substitutional sites can be estimated by ion channeling technique. For example, we have found that about 1.2 wt% Ag atoms remain in the laser ablated Ag-doped films prepared from the Ag-YBCO target and about two-thirds of the retained Ag atoms occupy substitutional sites. The sputtered films contain less retained Ag atoms since the deposition temperature is higher and deposition time is longer than those for laser ablated films.

Grain boundary junctions with Ag-doped YBa2Cu3O7−x epitaxial thin films

Using a dc sputtering method at high oxygen pressures we have deposited epitaxial Ag-doped YBa2Cu3O7−x thin films on SrTiO3 bicrystals with missorientation angle of 36.8°. A 15 % Ag-doped YBa2Cu3O7−x sintered target was used to sputter the films. Critical current of 4–5 106 A/cm2 at 77 K were measured in doped films. When compared with 1 × 106 A/cm2 for undoped films, indicated an improvement of the superconducting properties by Ag doping. 200–300 nm thick Ag-doped films were patterned across the bicrystal line to form 5–20 μm-wide junctions. Current-voltage characteristics of the grain boundary junctions at temperatures from 10 K to TC showed a resistively shunted junction (RSJ) behavior with ICRN values of 160–170 μV at 77 K which are higher than the measured values for undoped junctions.

Enhanced Jc and improved grain-boundary properties in Ag-doped YBa2Cu3O7−δ films

A large increase (∼8×) in critical current density, Jc, was achieved for in situ laser ablated YBa2Cu3O7−δ–Ag films. High-resolution Auger electron spectroscopic investigation indicates that the Ag-doped films are stoichiometric with a relatively low grain-boundary extension (8 nm) in contrast to undoped YBa2Cu3O7−δ films (32 nm). Further analysis suggests that the doped film contains a much lower silver content (&amp;lt;0.15 wt %) than in the target material (5 wt %). These observations are in excellent agreement with the temperature dependence of Jc, the room-temperature resistivity, and the surface resistance results. Thus, Jc enhancement in Ag-doped films can be attributed to their superior properties, viz., improved microstructure characteristics and the reduced resistive grain boundaries.

Magnetic shielding effects in Ag-doped and pure YBa2Cu3O7 − x thin films probed by non-resonant microwave absorption

Abstract A non-resonant microwave absorption technique is used to study the magnetic shielding effects in Ag-doped and pure YBa2Cu3O7 − x(YBCO) thin films for small modulating fields. While Ag-doped YBCO thin films show good shielding for small modulating fields (≈ 1 Oe) until 60 K, the pure YBCO films show significant shielding effect only up to 20 K. We interpret these results in terms of better grain alignment in Ag-doped YBCO films as compared to pure YBCO films, which can lead to better shielding effects.

Microstructural dependence of penetration depth of Ag-doped YBa2Cu3O7−δ thin films probed by atomic force microscopy

The magnetic penetration depth λ of laser ablated 5 wt % Ag-doped YBa2Cu3O7−δ thin films has been measured in the thickness range 1500–4000 Å and in the temperature range 18–88 K using microstrip resonator technique. A correlation of λ(T) with the film microstructure observed with atomic force microscopy has shown that λ(T) depends critically on the film microstructure.

Geometrical model and experimental evidence for the growth of Ag-doped YBa2Cu3O7?x thin films

The improvement in the properties of laser-ablated Ag-doped YBa2Cu3O7−x (YBCO) thin films in both normal and superconducting states has been interpreted using a growth model. The model is simple and is based on the widely accepted characteristics of Ag such as its flux action at high temperature and nonreactivity with YBCO phase.

Growth and applications of superconducting and nonsuperconducting oxide epitaxial films

Growth and characterization of high-temperature-superconducting YBa2Cu3O7 and several metallic-oxide thin films by pulsed laser deposition is described here. An overview of substrates employed for epitaxial growth of perovskite-related oxides is presented. Ag-doped YBa2Cu3O7 films grown on bare sapphire are shown to giveT c=90 K, critical current >106 A/cm2 at 77 K and surface resistance =450µΩ. Application of epitaxial metallic LaNiO3 thin films as an electrode for ferroelectric oxide and as a normal metal layer barrier in the superconductor-normal metal-superconductor (SNS) Josephson junction is presented. Observation of giant magnetoresistance (GMR) in the metallic La0·6Pb0·4MnO3 thin films up to 50% is highlighted.

Growth and characterization of laser-deposited Ag-doped YBa2Cu3O7−x thin films on bare sapphire

Microstructural and superconducting properties of YBa2Cu3O7−x thin films grownin situ on bare sapphire by pulsed laser deposition using YBa2Cu3O7−x targets doped with 7 and 10 wt% Ag have been studied. Ag-doped films grown at 730°C on sapphire have shown very significant improvement over the undoped YBa2Cu3O7−x films grown under identical condition. A zero resistance temperature of 90 K and a critical current density of 1·2×106 A/cm2 at 77 K have been achieved on bare sapphire for the first time. Improved connectivity among grains and reduced reaction rate between the substrate and the film caused due to Ag in the film are suggested to be responsible for this greatly improved transport properties.

Surface resistance, residual losses, and granularity in Ag-doped YBa2Cu3O7−δ thin films

We have studied microwave surface resistance Rs and residual resistance Rres in undoped and Ag-doped laser ablated thin-film YBa2Cu3O7−δ microstrip resonators on LaAlO3 substrates. While the undoped films showed a frequency dependence Rs∝fp, where p=1.9±1, Ag-doped films showed p=1.6±1 and 1.4±1 for 5 and 10 wt % Ag doping, respectively. Lower p values and higher Rres observed in Ag-doped films indicate a metallic contribution. However, the advantage of Ag-doped films has been shown to be at 77 K at which they not only have a lower Rs but also a low dRs/dT, thus making them superior to undoped films for microwave devices operating at 77 K.

Effect of time aging on the properties of Ag-doped YBaCuO superconductors

Ag-doped YBa2Cu3O7- delta samples aged in air for 47 months have been examined using structure analysis, resistivity, thermal analysis magnetic measurement, SEM, and TEM (STEM). The experimental results demonstrate that some properties of the samples were hardly changed after being kept in air for a prolonged period of time. On the other hand, the superconductive volume fraction of the sample, the intragranular critical current density Jcg, and the pinning force density Fp were increased. The authors consider that Ag-doping YBa2Cu3O7- delta can effectively prevent samples from degradation.

Large critical currents and improved epitaxy of laser ablated Ag-doped YBa2Cu3O7−δ thin films

Microstructure and critical current densities of laser ablated YBa2Cu3O7−δ thin films doped with 2–20 wt. % Ag have been studied. A critical current density as high as 1.4×107 A cm−2 at 77 K has been realized on 〈100〉 SrTiO3 substrates with YBaCuO films doped with 5 wt. % Ag which has been found to be the optimum. Evidence indicates that the improved microstructure and epitaxy which is a consequence of grain enlargement and alignment caused by Ag is responsible for the high values of critical currents observed.

Dc SQUID made of Ag-doped YBa2Cu3O7 a-b plane-edge junctions

Preparation and characterization of dc SQUIDs based on YBa2Cu3O7/Ag/YBa2Cu3O7 edge junctions is reported. The SQUIDs were prepared in situ by a multistep all laser ablation deposition process. Since the Ag on YBa2Cu3O7 was exposed to 730 °C in O2 ambient, the barrier resistance was quite high, ∼0.6 Ω. The SQUIDs showed voltage periodicity versus magnetic field H⊥ up to ∼80 K and had a maximum response of 30 μV/Φ0 at ∼70 K, where IcRN was 30 μV. The SQUID response versus H⊥ was superimposed on a diffractionlike pattern of its junctions, indicating an improved uniformity in the junction cross section. When the SQUID was exposed to microwave radiation, many pronounced Shapiro steps were observed in the I-V curve, which modulated with the microwave power.