Superconducting YBCO Thin Films Research Articles

Compact high‐selectivity high‐temperature superconducting wideband bandpass filter using triple‐mode stub‐loaded loop resonator

A triangle triple-mode stub-loaded loop resonator (SLLR) is proposed in this letter and used for wideband filter design. Also, this SLLR can produce multiple transmission zeros for high-selectivity bandpass filter (BPF) design without adding extra circuit unit. To verify the unique characteristics of the proposed SLLR, a compact wideband BPF is designed and fabricated using high-temperature superconducting YBCO thin films on a MgO substrate. Furthermore, a novel meander structure is introduced to realize compact size and strong input/output coupling design. The measured results show that the 3-dB fractional bandwidth is of 50.6% (2.55-4.28 GHz), and the selectivity factor of the passband is 0.87 with a maximum insertion loss of 0.07 dB, indicating good agreement with the theoretical expectation.

Low-energy ion beam modified surface property and mechanism of high temperature superconductor YBa2Cu3O7- thin film

The interaction between ion beam and solid target is widely used in material modification. For the high temperature superconducting thin film modification, however, earlier experiments show that the samples are accompanied by the degradation in superconducting properties due to the structural damage of materials. In order to improve surface morphologies and superconducting properties of YBa2Cu3O7- (YBCO) thin films, we introduce a new ion beam structure modification (ISM) method. Although the ion bombardment time parameter effect is not clear, the related mechanism should be clarified. In this paper, the bombardment processes with duration times of 8 min, 10 min and 12 min are investigated in a vacuum chamber with an Ar+ Kaufman ion source, and the direction between the incident ion beam and the normal of sample is fixed at a certain angle. Surface morphologies and the microstructures of YBCO samples are characterized by scanning electron micrographs and X-ray diffraction patterns, respectively. In the respect of superconducting properties, the critical current density Jc is measured by Jc-scanning test. The results indicate that the needle-like a-axis grains and pores disappear gradually with the increase of the ion bombardment time. In order to characterize the effects of ion beam bombardment time on the internal strain in YBCO thin films, the relationship between the full width at half maximum and the Bragg diffraction angle of YBCO (00l) peak is studied by the William-Hall equation. The results show that the internal strain in YBCO thin film increases with increasing the ion beam bombardment time. At the same time, the critical current density Jc value of the sample after ISM processing increases, which is more than 2.2 times higher than that of the initial sample. The main reason for the increases of critical current density Jc in YBCO thin film is due to the drastic shrink of CuO bond caused by the increasing internal strain. Based on the bond contraction pair theory, the shrink of CuO bond improves the energy to break Cooper-pairs, and then increases the current carrying capacity of high temperature superconducting YBCO thin film, especially in copper-oxygen (CuO2) plane. The ISM process might be a useful method of markedly improving the surface morphology, meanwhile, the critical current density Jc value also increases in high temperature superconducting YBCO thin film.

Sign change of the vortex Hall effect in superconducting YBCO thin films with a square pattern of ion-irradiated defect columns

The Hall effect in the mixed state of thin films of the high-temperature superconductor YBa2Cu3O7−δ (YBCO) that were patterned with a square array of regions with suppressed superconducting order parameter is investigated. Cylindrical defect columns penetrating the entire thin YBCO film along its crystallographic c−axis have been created by irradiation with He+ ions through a silicon stencil mask. Distinct peaks of the critical current at commensurate arrangements of magnetic flux quanta with the artificial defect lattice confirm enhanced vortex pinning. Vortex motion not only leads to a dissipative voltage along the current direction but also to a transverse voltage, termed vortex Hall effect. We report on the observation of a novel commensurability effect in the transverse Hall signal. A sign change and a positive peak of the Hall coefficient appear in a narrow magnetic field range around the matching field. The feature appears in the temperature range below the critical temperature, where the Hall effect usually is negative in underdoped and optimally-doped cuprate superconductors. The results indicate that the Hall matching effect originates from enhanced pinning of the vortices along the regular defect columns.

Magnetic shielding performance of superconducting YBCO thin film in a multilayer device structure

Magnetic shielding performance of superconducting YBaCu2O7−x (YBCO) thin film on an YBCO microbridge was analyzed in a multilayer structure. A sandwich type multilayer structure was fabricated onto a single crystal (100) SrTiO3 (STO) substrate in the form of YBCO/STO/YBCO by depositing a thin STO interlayer in between two YBCO layers. The top YBCO was patterned as 20μm width meander-type microbridges and the bottom layer YBCO was used as magnetic shield. YBCO and STO thin films were deposited by dc and rf magnetron sputtering respectively, and the patterning was performed by using standard photolithography and wet etching. In order to enhance long-term stability of the final device, an additional STO thin film was deposited onto the device as an encapsulation layer. Electrical and magnetic characterizations of the YBCO thin film layers were carried out by means of ac magnetic susceptibility (χ–T) and resistance vs. temperature (R–T) measurements. The current–voltage (I–V) measurements were performed on the microbridges at 77K by observing the shielding performance of the bottom YBCO layer under various applied magnetic fields. The results were compared with that of a same-type single layer YBCO device without a shielding layer. The zero field critical current value of the single layer 20μm wide YBCO device was measured as 30mA and decreased down to 20mA as the field increased up to 100mT. The same measurements on the multilayer device showed that the critical current values remained almost constant around 27mA as the applied field increased.

Fabrication of high-Tc superconducting multilayer structure with YBa2Cu3O7 − x thin films separated by SrTiO3 interlayers

Multilayer thin film structures are attractive for superconducting microelectronics, but they are difficult to achieve on high Tc superconductors such as YBa2Cu3O7−x (YBCO) thin films. The conventional process has been based on fabricating the YBCO thin films on multiple buffer layers resulting in a single superconducting upper layer on a multilayer-type structure; here we present a full fabrication process for the growth of a multilayer superconducting thin film structure that contains multiple superconducting YBCO thin film layers separated by SrTiO3 (STO) interlayers in sandwich-type configuration. The deposition process was optimized for dc and rf magnetron sputtering techniques for the deposition of YBCO and STO thin films respectively. An YBCO/STO/YBCO/STO/YBCO type configuration was fabricated on a commercial single crystal (100) STO substrate by precisely tuning the deposition process parameters. Superconducting properties of YBCO layers were tested by ac magnetic susceptibility and resistances versus temperature measurements after the deposition of each additional layer. In order to demonstrate potential device application, a microbridge pattern was fabricated onto the top YBCO layer and current–voltage analysis was performed.

The coherent gradient sensor for film curvature measurements at cryogenic temperature

Coherent Gradient Sensor (CGS) system is presented for measurement of curvatures and nonuniform curvatures changes in film-substrate systems at cryogenic temperature. The influences of the interface of refrigerator and itself on the interferograms which are accounting for the temperature effect are successfully eliminated. Based on the measurement technique, the thermal stresses (including the radial stress, circumferential stress and shear stress) of superconducting YBCO thin-film are obtained by the extended Stoney's formula during the heating process from 30K to 150K. Take the superconducting YBCO thin film as an example, the thermal stresses of which are gained successfully.

Study of Matching Fields in Kagomé Patterned YBCO Thin Films

Superconducting YBCO thin films have been prepared by the pulsed laser deposition (PLD) technique. YBCO films have been patterned by kagome antidot lattice. The optical lithography technique has been used to get the required pattern. We have studied the variation of the critical current with magnetic field in kagome patterned YBCO films. The longitudinal critical current exhibits jumps which are found to be asymmetrically positioned in magnetic field. These steps are found mainly at nonmatching fields. Around the zero field limit the critical current remains independent of magnetic field. The impact of the DC current and ac-superposition in controlling the vortex motion in patterned film has been discussed.

Dependence of the structural, electrical and magnetic properties of the superconductive YBCO thin films on the deposition rate

In this study, YBCO thin films on single crystal LaAlO3 (100) substrates have been grown using DC inverted cylindrical magnetron sputtering technique and the effect of the deposition rate on these films is investigated. Three different deposition rates are used to produce superconducting YBCO thin films with 150 nm of thickness on (100) LaAlO3 single crystal substrate at 780 0C. The samples are analyzed in detail by means of XRD, R-T, χ-T, M-H and AFM characterizations and also the critical current densities (Jc) are derived from the magnetic hysteresis curves using the modified Bean formula [1]. The critical current density at 50 K was found to be in the range of 3.107 A/m2 to 8. 107 A/m2 with a deposition rate between 2nm/min and 1.2nm/min. A correlation has been obtained so that as the film deposition rate increases, the surface smoothness and crystalline quality of the films significantly deteriorate, resulting in a significant decrease in Jc.

The effect of susceptor inclination angle on the quality of superconducting YBCO thin films prepared by a photo-assisted MOCVD system

Superconducting YBa2Cu3O7−x thin films were deposited on LaAlO3 (100) substrates by a photo-assisted metal-organic chemical vapor deposition (MOCVD) technique. In our experiment, growth temperature, chamber pressure and growth time were all fixed; the only process variable was the susceptor inclination angle (Φ) between the axis of gas inlet and the susceptor surface. It was found that the growth rate of YBCO films increases monotonically as Φ increases, whereas the superconducting critical current density (Jc) has a maximum (1.2×106A/cm2) near Φ=22.5°. The experimental results were found to be well correlated with the three-dimensional simulations of gas flow inside the MOCVD reactor, with Φ being the only process variable.

Preparation of YBa2Cu3O7–x superconducting solutions and films from alkoxide-based precursors using sol–gel method and investigation of their chemical reaction mechanisms

In the development of coated superconductors sol–gel technique has been widely used as an effective processing method, especially in making long-length wires and tapes. However, one drawback associated with the deposition of YBa2Cu3O7–x and Gd2O3 buffer layer on Ni tape is the adhesion characteristic at interfaces YBCO–Gd2O3 and Gd2O3–NiO and NiO–Ni substrate. In this paper, two strategies for adhesion enhancement of multilayered ceramic oxide coatings on Ni substrate were proposed: (1) formation of chemical bonds through surface condensation reactions, and development of ceramic networks through diffusion of alkoxide precursors. The current research has focused on the fabrication and adhesion of YBCO coatings on buffered Ni substrate using nine solutions prepared from Y, Ba and Cu alkoxides, solvent and chelating agent. Only two of YBCO solutions were successfully obtained while the rest of them were unsuccessful. Among these solutions we scrutinized chemical reaction mechanisms of a successful and an unsuccessful solution for comparison. How the chemical bonds and solubility were affected by the acids, base and solvents used in the solutions was demonstrated. It was shown that pH of the solution, homogeneity of the solution and gelation, steric affect of the chemicals in sol–gel solutions are significant issues to obtain high quality superconducting YBCO thin films. In addition, X-ray diffraction (XRD) was used to analyze phase structure of YBCO and compare results of chemical reactions obtained by a chemdraw programme. Scanning electron microscope (SEM) studies was carried out to examine microstructures of YBCO films produced from alkoxide precursors, solvent, chelating agent and modifying liquid chemical materials such as triethanolamine or ammonium hydroxide. It was found to be YBCO, Gd2O3, NiO and Ni phases in YBCO/Gd2O3/Ni sample from XRD analysis. That the solution prepared by using triethanolamine provided the best film quality and adhesion was determined by chemical reactions and gelation behaviour in the solutions and SEM observations.

Water vapour pressure influence on the kinetics of the superconducting YBCO thin films epitaxic growth by the TFA–MOD method

We have found two different regimes in the kinetics of the YBCO formation depending on the water partial pressure at a constant temperature and total flow rate of the carrier gas. The first regime at low partial water pressure shows continual kinetics curves until the end of YBCO growth and the reaction is controlled chemically. The second regime at high partial water pressure shows irreproducible steps in the kinetics curves during the thin films YBCO growth. In this work we suggest that there is formation of a boundary layer of water (Nernst layer) when the partial water pressure is higher than 20hPa at 795°C for a total gas flow rate lower than 2.4×10−2ms−1. These irreproducible steps dues probably to a water boundary layer formation can be eliminated by increasing the stirring rate of the carrier gas. The reaction order of YBa2Cu3O7−x formation respect to the water pressure is n=0.5 when the water boundary layer is not formed, but the apparent reaction order respect to the partial water pressure is zero or negative when the gas flow rate of the carrier gas is not big enough for the elimination of this water layer. This work also evidences that there is an intermediate step in the kinetics curves before the formation of YBCO. This step, which starts at low temperature during the heating ramp (∼400°C) is attributed to the partial elimination of F from the BaF2 precursor to form oxyfluoride compounds. So, at low total flow gas rates and low partial water pressures, the reaction is controlled by diffusion mechanism due to the formation of a HF boundary layer (Nernst layer), because the apparent order of YBCO formation is one respect to the stirring rate. Nevertheless, at high flow gas rates and low partial water pressures, the YBCO formation is controlled chemically, then the apparent order respect to the stirring rate is zero and the HF Nernst layer is eliminated. The apparent Ea for the oxyfluoride formation at low temperatures is only ∼18Kj/mol indicating that this intermediate reaction is controlled by diffusion mechanism even at high stirring rate and relatively low partial water pressure. The apparent Ea for the YBCO formation at partial water pressures higher than 20hPa for a total flow rate of 2.4×10−2ms−1 is only ∼32Kj/mol, indicating that the reaction control is mainly diffusive or mixed (diffusive and chemist).

Laser Scanning Microscopy Studies on Detwinned<tex>$rm YBa_2rm Cu_3rm O_7-delta$</tex>Thin Films

YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films are heavily microtwinned due to the crystal's orthorhombic unit cell and the confining effects of underlying substrate. Single crystal YBCO can be detwinned by thermally annealing the sample while applying uniaxial pressure. A similar technique has been devised to detwin a portion of a YBCO thin film. Through the use of photolithographic techniques, a suspended YBCO microbridge structure is fabricated, but with both ends of the bridge anchored to the substrate. During a subsequent anneal, the ends of the bridge induce a uniaxial stress along its length that arises from the lattice misfit and the thermal expansion coefficient mismatch between YBCO and the underlying materials. Without the confining effects of the substrate the suspended material can either detwin or twin on a macro length scale; the extent of which is largely determined by the substrate material. The ability to produce superconducting YBCO bridges that range from heavily microtwinned, to macrotwinned, to nearly detwinned provides an ideal experimental system. To study these phenomena, an optical and laser scanning microscopy set-up was employed. The images obtained provide insight into the role that twins play in determining the transport properties of superconducting YBCO thin films.

AFM investigations of the morphology features and local mechanical properties of HTS YBCO thin films

In the paper presented here the application of the atomic force microscope (AFM) is considered for evaluation of hardness and Young's modulus of high T c superconducting YBCO thin films of different thickness (from 0.05 to 1 μm) grown on unbuffered SrTiO 3 (film I) and on sapphire with a buffer layer of CeO 2 (film II). The best film features a transition temperature T c of 90 K, critical current density J c ( H=0) of 3 × 10 7 A/cm 2 at 4.2 K and 2 × 10 6 A/cm 2 at 77 K. The relationship between mechanical properties and microstructure of these films was investigated. It was found that all the films comprised well-defined Cu-rich precipitates of different size and with different density on their surface. For both type of films the hardness was measured to be in the range of 12–18 GPa. The Young's modulus of the films was about 180–200 GPa. The nanoindentation and nanoscratching measurements showed that the mechanical strength of the films studied was determined mainly by mechanical failure and surface defects (secondary phases).

Carbon depth profiling of superconducting YBCO thin films on nanometer scale

Abstract Ion beam analysis techniques have been used to probe for carbon contamination in high temperature superconductor thin films. These techniques provide a powerful tool to detect C with limit of detection close to 0.01 at.% and to measure carbon depth profiles on the nanometer scale with a depth resolution of 10 nm at the surface. In the present study, a series of YBa 2 Cu 3 O 7− δ (YBCO) films have been formed on MgO substrates by a sol–gel method. Different film thicknesses and heat treatments were studied. Typical formation temperatures were 770–850 °C resulting in thicknesses from 50 to 600 nm. It was found that, depending on the sample preparation conditions, carbon was incorporated in the films at a concentration of 0.5–4.6 at.%. Carbon was homogeneously distributed throughout the films.

An AFM study of the morphology and local mechanical properties of superconducting YBCO thin films

The morphology of thin superconducting YBCO films deposited on sapphire and on SrTiO 3 was studied with the help of atomic force and scanning electron microscopies. The intrinsic mechanical properties in the flat, particles-free and chemically homogeneous regions of the films were determined with the aid of nanoindenting atomic force microscope. Also the microscopy studies revealed the difference in topography of the films, the nanohardness and Young’s modulus of two films were very close to each other. For the indents shallower than 0.2 of the film thickness the Young’s modulus and hardness of the films on two different substrates converged to the values of 210 and 8.5 GPa, respectively. The possible deformation mechanisms determining the localized deformation of intrinsically brittle ceramic films are discussed.

Magnetic Relaxation and Flux Creep in Superconducting YBCO Thin Films

In this work, the temperature dependence of the magnetic relaxation in samples of YBa 2 Cu 3 O 7 (YBCO) thin films of two different qualities grown on MgO by metal organic chemical deposition technique (MOCVD) is measured in the range of 4.2–80 K using an initial applied magnetic field of 0.1 T. The relaxation data are analyzed in terms of a single effective activation energy and then more correctly of a distribution of activation energies. The single activation energies extracted are in broad agreement with published results.

Growth by MOMBE of c-axis superconducting YBCO thin films on different substrates: in situ RHEED monitoring of the growth

Superconducting YBCO c-axis oriented thin films have been grown by MOMBE (metalorganic molecular-beam-epitaxy). Growth of the films on different substrates [100] (Y,Nd)AlO/sub 3/, [100] SrTiO/sub 3/ and [100] MgO has been investigated by in situ monitoring the reflection high energy electron diffraction (RHEED). The paper discusses our results suggesting significant differences in films growth on the three presented substrates. The highest quality (high uniformity, epitaxy and low roughness) has been attained for the films prepared on SrTiO/sub 3/. All films, regardless substrates, have shown values between 81 and 84 K for zero resistance critical temperature T/sub c0/.

Study of the Usual and the Unusual Effect of a DC Magnetic Field on the Microwave Properties of Different Superconducting YBCO Thin Films

An experimental observation of the improvement of the microwave properties of high temperature superconductor thin film subjected to a weak DC magnetic field is reported. This characterization was done with two YBCO films deposited onto LaAlO3 substrates and the coplanar resonator technique was used to extract their microwave properties. The results are presented as a comparison between the behaviors of a sample that exhibits an unusual effect with another sample that has a usual behavior. To understand the origins of such a difference, the crystallographic properties and the microstructure were investigated respectively by mean of X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). A major difference in terms of growth axis orientation and of granularity has been observed between the two samples. Based on the correlated variation of the surface resistance and the surface reactance of the considered sample, the presence of anomalous weak links and also of a-axis grains in one of the films could explain its unusual behavior.

마이크로파 소자응용을 위한 YBCO 박막의 두께 및 증착온도에 관한 특성연구

The effect of the superconducting film thickness on the surface resistance has been investigated. Superconducting YBCO thin films have been grown on MgO substrates by pulsed laser deposition. The dependence of the orientation of YBCO film on thickness has been investigated by X-ray diffraction technique. X-ray diffraction indicated that the film orientation was changed by increasing the film thickness and by changing the substrate temperature. The microwave properties of the films with mixed orientations of a-axis and c-axis will be reported for the applications of microwave devices.

Behavior of an inhomogeneous HTS film in the regime of a resistive current limiter

A high-temperature superconducting (HTS) YBCO thin film covered by a gold layer on sapphire substrate is a very attractive element for building resistive current limiters. The quench dynamics in an HTS thin film with a strong inhomogeneity of the critical current was studied in the circuits with AC and DC voltage sources. The temporal dependence of the normal zone length was found to be of a non-monotonic character. After the normal zone propagates at some distance, it reverses the direction of its motion. The successive returns of the film sections into the superconducting state are observed. Under AC condition, the normal zone can be divided into several parts separated by superconducting regions. This phenomenon is explained by the dependence of the thermal parameters of the sapphire substrate on temperature, in particular, the dramatic reduction of the heat conductivity of the sapphire with the increase of temperature.