YBaCuO Thin Films Research Articles

Temperature dependence of the resistance switching effect studied on the metal/YBa2Cu3O6+x planar junctions

The resistive switching (RS) effect observed in capacitorlike metal/insulator/metal junctions belongs to the most promising candidates for next generation of memory cell technology. It is based on a sudden change of the junction resistance caused by an electric field applied to the metal electrodes. The aim of this work was to study this effect on the structure metal/YBCO6/YBCO7, where YBCO7 is a metallic phase and YBCO6 is an insulator phase that arises spontaneously by outdiffusion of oxygen from a few nanometers wide YBCO surface layer. Oriented YBa2Cu3O7 thin films were prepared by the method of magnetron sputtering and consequently planar structures with metal-YBCO junction were made by the means of the optical lithography, ion etching, and vacuum evaporation. On these junctions, the authors have studied the temperature dependence of the RS effect with I-V and dI/dV-V transport measurements down to liquid He temperature. They have determined the temperature dependence of the RS effect threshold voltage in the temperature range 100–300 K and showed that this dependency is compatible with common idea of oxygen ion migration under electric field within the YBCO surface layer.

ChemInform Abstract: Electrodeposition of YBaCuO and ErBaCuO Superconductor Precursor Films.

Abstract Precursor thin films of YBaCuO and ErBaCuO superconductor were produced by pulse plating from the dissolved nitrate salts of yttrium, barium, copper and erbium, barium, copper in dimethylformamide. The films were produced on zirconium as well as silvercoated SrTiO3 and CaTiO3. The electrodeposited films were heat treated in flowing O2. Film composition was determined by energy-dispersive X-ray analysis. Critical temperature transitions were determined by magnetic as well as resistivity measurements. Onset critical temperatures were measured up to 93 and 80 K for YBaCuO and ErBaCuO respectively. In resistivity measurements of YBaCuO on CaTiOa, a small step drop at approximately 240 K was observed. This drop in resistivity disappeared after one thermal cycle. Similar drops in resistivity above 200 K have been observed by other authors. Compared with potentiostatic and galvanostatic techniques, the use of pulse plating enhances nucleation and growth and results in uniform and homogeneous films.

YBCO and LSMO nano-films and sandwiches prepared by magnetron sputtering

DC and RF magnetron sputtering techniques were used for growing nano-films (t<100 nm) of high temperature superconducting (HTS) YBa2Cu3O7 (YBCO) and ferromagnetic (FM) manganite La0.7Sr0.3Mn03 (LSMO) materials on LaAlO3 (LAO) and Al2O3 (ALO) substrates as well as for preparing of single-, double- and three-layer structures in different areas of the same substrates. The procedure allowed growing of structures on LAO substrates where the critical temperature of YBCO thin film components was more than 84 K. The LSMO films grown ALO substrates were ferromagnetic while the YBCO films grown on LSMO/ALO did not demonstrate superconductivity.

Design and fabrication of bandwidth tunable HTS transmit filter using π-shaped waveguides

We have developed a method for tuning the bandwidth of a high-temperature superconducting (HTS) microstrip filter. Several π-shaped waveguides are placed between the resonators, and the bandwidth is tuned in discrete steps by changing the switch states of the waveguides, which changes the coupling coefficient between the resonators. The filter contains 3-pole half-wavelength straight-line resonators and two π-shaped waveguides for bandwidth tuning. It also has several electrical pads distributed around the feed lines for trimming after tuning. The filter was fabricated by depositing YBa2Cu3O7 thin film on an MgO substrate and has a measured center frequency of 5.17GHz and bandwidth of 220MHz. Use of the π-shaped waveguides to adjust the coupling coefficients and the electrical pads to adjust the external quality factors resulted in 80-MHz bandwidth tuning without increased insertion loss.

500 V/200 A fault current limiter modules made of large-area MOD-YBa2Cu3O7 thin films with high-resistivity Au–Ag alloy shunt layers

We developed 500 Vrms/ 200 Arms superconducting thin-film fault current limiter (FCL) modules that can withstand high electric fields (E>30 Vrms cm−1) by usinglarge-area YBa2Cu3O7 (YBCO) thin films with high-resistivity Au–Ag alloy shunt layers.Au–Ag alloy films about 60 nm thick were sputter-deposited onYBCO/CeO2/sapphire films(2.7 cm × 20 cm) prepared using a fluorine-free MOD method. Each 20 cm longAu–Ag/YBCO film was then divided into three segments (each∼5.7 cm long) by four Ag electrodes deposited on the Au–Ag layer, resulting in an effectivelength of 17 cm. The 500 V/200 A FCL modules were then fabricated by first connecting twoof the segmented films in parallel using Ag-sheathed Bi-2223 superconducting tapes andthen connecting in parallel an external resistor and a capacitor for each segment toprotect the Au–Ag/YBCO film from hot spots. Switching tests using a short-circuitgenerator revealed that all the modules carried a superconducting ac current of≥237 Arms and that modules prepared with YBCO films having a relatively homogeneous critical currentIc distributionsuccessfully withstood ≥515 Vrms for five cycles without any damage. These results demonstrate that (a) theFCL modules fabricated here successfully achieved the rated current of200 Arms and ratedvoltage of 500 Vrms and (b) total area of the YBCO films on sapphire substrates required for the 500 V/200 A(100 kV A) module was less than one-third that for conventional thin-film FCLmodules that use gold shunt layers, leading to the significantly reduced cost ofthin-film FCLs. Film damage due to hot spots depended on the difference inIc between the two parallel-connected films and on the inhomogeneity of theIc distribution in the film, and is most probably due to nonlinear current flows at the momentof quenching that cause local overheating.

Terahertz Conductivity of Overdoped Y1−x Ca x B2Cu3O7−δ

In the current work we have investigated the terahertz response of Ca overdoped YBaCuO thin films using both time and frequency domain spectroscopy methods. For both methods a basic data analysis was performed using the two fluid and a variable dielectric function (VDF) models. The imaginary part of the conductivity was proportional to 1/ω, known from the delta-function response. The real part of conductivity showed a well known frequency and temperature dependence, where it increases below Tc and obtains maxima at about 50 K. However, a sharp decrease of the real part of the conductivity was observed at about 10 cm−1. This decrease occurs below Tc and becomes dominant as temperature decreases. It was observed on the 5% and 10% Ca doped samples but it was more dominant on the 10% case. Moreover, this sharp decrease in σ2(ω)at 10 cm−1 was not observed in optimally doped YBCO samples. We would like to stress at this point that these values are much smaller than those obtained by Microwave and Tunneling measurements, arguing for the existence of a complex order parameter in the overdoped regime with an imaginary component of about 1.8 meV.

On the Power Handling Properties of High Temperature Superconducting Microwave Contiguous Duplexer

The power handling properties of a high temperature superconducting (HTS) microwave contiguous duplexer are measured in this letter. The HTS contiguous duplexer was fabricated using YBa2Cu3O7 (YBCO) thin films on a 0.5-mm-thick LaA1O3 substrate, operation frequencies of which are centered at 3.25 GHz and 3.75 GHz with 500-MHz-wide channels. The tested frequency response of power handling property of the HTS contiguous duplexer shows that there is a decreased power handling property on the crossover region; however, this microwave nonlinearity does not affect the engineering application of HTS contiguous duplexers and multiplexers.

Masked ion beam irradiation of high-temperature superconductors: patterning of nano-size regions with high point-defect density

Ion-beam irradiation of high-temperature superconductors creates different types of defects depending on ion mass, energy and dose. Computer simulations reveal the diversity of the ion-target interactions with YBa2Cu3O7 and are compared to previous experimental results from transmission electron microscopy and electrical transport properties. While protons have a very low efficiency to create defects in YBa2Cu3O7, significantly heavier ions produce defect clusters and inhomogeneous damage in the target material. The situation is exemplarily illustrated by a computer simulation study of the defect cascades produced by H+, He+, Ne+, and Pb+ ions of moderate energy. He+ ions with energy of about 75 keV were found useful for a systematic modification of the electrical properties of high-temperature superconductors, since they do not implant into 100-nm thick films of YBa2Cu3O7 but primarily create point defects by displacement of the oxygen atoms. Such defects are very small and distributed homogeneously in YBa2Cu3O7. The small lateral spread of the collision cascades allows for the patterning of nanostructures by directing a low-divergence beam of He+ ions onto a thin film of YBa2Cu3O7 through a mask. Simulations indicate that the resolution can be about 10 nm. An experimental test with a masked ion beam irradiation confirmed that features with about 200 nm size could be produced in a YBa2Cu3O7 thin film and observed by scanning electron microscopy.

Enhancing Superconducting Parameters by Array of External Magnetic Dots

AbstractEnhancing the pinning force in high-Tc superconductors can be achieved by externally introduced periodic magnetic dots. We use a novel nano-technique to deposit periodic sub-micon Ni dots on YBa2Cu3O7 thin films. The current versus voltage characteristics of an YBa2Cu3O7 thin film strip with uniform Ni dots are measured at various temperatures and magnetic fields. They are compared with the current versus voltage characteristics of a bare YBa2Cu3O7 thin film strip without magnetic dots. It is found the critical current value of the strip with Ni dots reduces with a much slower pace as the magnetic field strength increases in comparison with the value of the bare sample.

Analysis of precursors for crystal growth of YBaCuO thin films in magnetron sputtering deposition

We correlated the crystallinity of YBaCuO films prepared by magnetron sputtering deposition using Ar/O 2 mixture gas with the atomic and molecular composition in the gas phase. YBaCuO films were deposited on MgO substrates at 670 °C. Two-dimensional distributions of Y, Ba, Cu, YO, BaO, and CuO densities and one-dimensional distribution of O density were measured by laser-induced fluorescence spectroscopy. The Y and Ba densities decreased significantly with the increase of the O 2 partial pressure, and they were below the detection limit at an O 2 flow ratio of 10% and a total gas pressure of 53 Pa. The decrease in the Y and Ba densities was compensated by an increase in the YO and BaO densities. The decrease in the Cu density with the increase of the O 2 partial pressure was less significant, while the CuO density was below the detection limit at all the discharge conditions. The O density was evaluated to be 10 12–10 13 cm − 3 , which was much higher than the Cu density. On the other hand, YBaCuO films with high crystallinity were obtained at total gas pressures of 53–80 Pa and O 2 flow ratios of 50–70%. Therefore, it is concluded that the precursors for the deposition of YBaCuO films with high crystallinity are Cu, YO, BaO, and O.

Fabrication of YBa2Cu3O7 thin film on cube-textured Cu tape

We achieved a biaxially oriented YBa2Cu3O7 film on a {100} ⟨001⟩ textured Cu tape with Jc of 1.9×106 A cm−2 at 77 K. A Ni overlayer was deposited by electroplating technique on the {100} ⟨001⟩ textured Cu tape to improve oxidation resistance of Cu substrate, and then YBa2Cu3O7 was deposited on the CeO2/YSZ/CeO2 buffered Ni-electroplated Cu tape. We confirmed that the Ni-electroplated Cu tape lost its ferromagnetism after 700 °C at 1 h heat treatment. Our rolling assisted biaxially textured substrate (RABiTS) type coated conductor can be used for ac applications and is thought to be more cost feasible than the standard RABiTS architecture using Ni-alloy tapes.

Interaction between the Ferromagnetic Dots and Vortices: Numerical Calculation and Experimental Results

Enhancing the pinning force in high-Tc superconductors can be achieved by externally introduced periodic magnetic dots. We numerically calculate the interaction between ferromagnetic dots and vortices in high-Tc superconductors. The London equation is used to generate two-dimensional vortex lattice. In the matching condition, we calculate the attraction force between magnetic dots and vortices. It is found that in an ideal condition, the pinning force of the magnetic dot reaches 2.5×10−11 N that is more than one order magnitude stronger than the intrinsic pinning force in YBa2Cu3O7 thin films. In the experimental side, we use a novel nano-technique to deposit periodic submicron Ni dots on YBa2Cu3O7 thin films. The current versus voltage characteristics of an YBa2Cu3O7 thin film strip with uniform Ni dots are measured at various temperatures and magnetic fields. They are compared with the current versus voltage characteristics of a bare YBa2Cu3O7 thin film strip without magnetic dots. It is found the critical current value of the strip with Ni dots reduces with a much slower pace as the magnetic field strength increases in comparison with the value of the bare sample.

Q factor of megahertz LC circuits based on thin films of YBaCuO high-temperature superconductor

High-frequency properties of resonant structures based on thin films of YBa2Cu3O7–δ high-temperature superconductor are studied experimentally in the frequency range 30–100 MHz. The structures planar induction coils with a self-capacitance fabricated on neodymium gallate and lanthanum aluminate substrates. The unloaded Q factor of the circuits exceeds 2 × 105 at 77 K and 40 MHz. Possible loss mechanisms that determine the Q factor of the superconducting resonant structures in the megahertz range are considered.

Effect of cyclic annealing on the static and high-frequency characteristics of YBCO-film-based resonant structures

We have studied correlations between the static parameters and high-frequency Q values of megahertz-range resonant LC circuits representing planar coils with self-capacitance manufactured on the basis of thin films of YBa2Cu3O7 − δ (YBCO) high-temperature superconductor. It is established that the static characteristics of resonant circuits reversibly vary in the course of cyclic annealing, while the Q value does not exhibit such changes.

Quality of YBCO thin films grown on LAO substrates exposed to the film deposition - film removal processes

The characteristics are investigated of high temperature superconducting YBa2Cu3O7 (YBCO) films grown on LaAlO3 (LAO) substrates being exposed a different number of times to YBCO film deposition and acid-solution-based cleaning procedures. Possible mechanisms of degradation of the substrate surface quality reflecting on the growing YBCO film parameters are discussed and analyzed.

Imaging of vortex flow in microstructured high-Tc films by laser scanning microscope

We studied the YBa2Cu3O7 thin films with arrays of antidots by imaging the resistive photoresponse with a laser scanning microscope. The films were initially covered by a thin Au layer, which we removed in order to improve the spatial resolution and imaging contrast. This procedure did not change the superconducting properties of the samples. The improved spatial resolution allowed for visualizing the individual flow channels of vortices. The reported measurements with the laser scanning microscope feature direct measurements of the local critical currents of the samples. We show that minor variations of the bias current leave the spatial distribution of vortex flow channels unchanged.

Effect of bulk Ca-substitution in YBaCuO thin films for microwave applications

Calcium diffusion into defects in the high-temperature superconductor (HTS) compound YBa2Cu3O7−x (YBCO) has been previously investigated [D. Seron, D.E. Oates, G. Hammerl, J. Mannhart, P.J. Hirst, R.G. Humphreys, A.C. Anderson, M.A. Hein, J. Derov, Phys. Rev. B 72 (2005) 104511] as a possible way to improve the dc, ac and rf transport properties and to clarify nucleation, growth, and doping of YBCO. In this paper, the microwave properties of homogeneously deposited Ca-substituted YBCO, Y1−yCayBa2Cu3O7−x (YCBCO) films are presented and compared with the previously published Ca-free YBCO films with a Y0.7Ca0.3Ba2Cu3O7−x capping layer. By using different substrates and growth process detailed in the paper, particular defects are created in the YBCO or YCBCO films. In every situation, Ca-substitution is probed and its effect is compared with very high quality epitaxial YBCO films close to the intrinsic limit of microwave nonlinearity for YBCO. Information on the percolation, doping effect, and the role of the defects obtained from the dc properties in the normal state are used to help interpret the nonlinear microwave behavior.

Growth of stacked heterostructures of SIS-type with S = YBCO orBSCCO and I = (Ba,Ca)CuO2 or (Sr,Ca)CuO2

We investigated MOCVD growth ofc-axis two-layer and three-layer stacked heterostructures on(100)SrTiO3 of SI, IS or SIS type. S denotes high temperature superconductor thin films ofYBa2Cu3O7 (Y-123),Bi2Sr2Ca2Cu3O10 (Bi-2223)or Bi2Sr2CaCu2O8 (Bi-2212), while I stands for insulating thin films of(Ba,Ca)CuO2 or(Sr,Ca)CuO2. X-ray diffraction, atomic force microscopy and measurements of resistivity versustemperature were performed after the deposition of each layer. The best results,suitable for further device fabrication, were obtained for the SIS structure(100)SrTiO3/Bi- 2223/(Sr,Ca)CuO2/Bi- 2212. Similarstructures with I = (Ba,Ca)CuO2 have shown higher impurity phases and lower superconducting quality of the topsuperconducting layer. For two-layer or three-layer structures, Bi-2223 as a top layercould not be obtained and, instead, a phase with Bi-2212 formed. Structures with(100)SrTiO3/Y- 123/(Sr,Ca)CuO2/Y- 123 or(100)SrTiO3/Y- 123/(Ba,Ca)CuO2/Y- 123 were also promising, but a current leak often occurred due to greater roughness. Structureswith Y-123 are less sensitive to growth temperature than those with BSCCO.

A study on the microwave responses of YBCO and TBCCO thin films by coplanar resonator technique

YBa2Cu3O7(YBCO) thin films have been prepared by thermal coevaporation on LaAlO3(LAO) substrates, and Tl2Ba2CaCu2O8(TBCCO) thin films are synthesized by magnetron sputtering method on LAO substrates. The transition temperature Tc is90 K for YBCO/LAO and 104 K for TBCCO/LAO. Microwave responses of the films are studied systematically by coplanar resonator technique. Energy gaps of the films obtained are Δ0 = 1.04kBTc for YBCO films and Δ0 = 0.84kBTc for TBCCO films by analysing the temperature dependence of resonant frequencies of coplanar resonator. Penetration depth at 0 K λ0 = 198 nm for YBCO films and λ0 = 200 nm for TBCCO films could also be obtained by using the weak coupling theory and two fluid theory. Results of penetration depth and energy gap confirm the weak coupling properties of the films. In addition, microwave surface resistances Rs of YBCO/LAO and TBCCO/LAO are also investigated by analysing the quality factor and insert loss of the coplanar resonator. Surface resistance of TBCCO/LAO is less than that of YBCO/LAO, so that TBCCO/LAO films may have more potential applications.

Atomically flat MOD La0.7Sr0.3MnO3 bufferlayers for high critical current YBa2Cu3O7 TFA films

Superconducting YBa2Cu3O7 (YBCO) thin films were grown by the trifluoroacetates route onto metallorganic depositedLa0.7Sr0.3MnO3 (LSMO) buffer layers. The growth conditions of the LSMO films have been studied in detail andfully strained epitaxial layers with atomically flat surfaces have been obtained with local roughness∼1 nm, as determined by atomic force microscopy. We prove that this surface morphology is a keyparameter for achieving YBCO/LSMO multilayers with high critical current densities in self-field,Jcsf (77 K) = 1.8 MA cm−2. Angular pinning studies demonstrate that the defect structure is modifiedwith respect to standard TFA–YBCO films as a pronounced peak inJc is observed for at high temperatures. This indicates the presence of anisotropic defects playing animportant role in vortex pinning even at the irreversibility line stressing the suitability ofthese all-chemical TFA–YBCO/LSMO multilayers as conductive coated conductorarchitectures.