YBa2Cu3Oy Research Articles

Microwave surface resistance of YBCO superconducting thin films under high DC magnetic field

We investigated the dc magnetic field and temperature dependences of the microwave surface resistance (Rs) of YBa2Cu3Oy (YBCO) superconducting thin films with various thicknesses. We used YBCO thin films with 300, 500, and 700nm deposited on a MgO (100) substrate by the thermal co-evaporation method. The Rs was measured using the dielectric resonator method at 21.8GHz, and a high dc magnetic field of up to 12T was applied parallel to the c-axis of the YBCO thin films during Rs measurements. The Rs value of YBCO thin films increased when dc magnetic field was applied. These relationships could be explained by using the two-fluid model for high frequency and low magnetic field limits. We defined the viscous drag coefficient η under the low magnetic limit and the high magnetic field limit as the ηlow (B, T, d) and ηhigh (B, T, d), respectively. The ηlow values were approximately two orders of magnitude greater than ηhigh values. Also, all ηlow and ηhigh value decreased or saturated as the temperature and film thickness increased. These results show that η under the low and high magnetic field limit depended on the temperature and film thickness.

Fabrication process of YBCO thin film starting from amorphous film for microstrip line device

Abstract We investigated the process for fabricating YBa2Cu3Oy (YBCO) thin film microstrip lines from amorphous thin films. First, we made amorphous YBCO thin films and then post-annealing was carried out. The amorphous 300-nm-thick YBCO thin films were deposited on a CeO2 buffered r-Al2O3 substrate by pulsed laser deposition. The 50-nm-thick CeO2 buffer layer was made by a facing target sputtering system. We examined the optimal conditions of the post-annealing process by using a two-step crystallization process. The heating rate of the first step was set to 360 K/min until 1023 K, and then the heating rate of the second step was set to 20 K/min until 1073 K. The oxygen pressure was kept to 3000 Pa at 1073 K for 210 min. From the XRD measurement, we observed that the YBCO thin films grew c-axis orientation on the CeO2/r-Al2O3 substrate. Both YBCO and CeO2 φ-scan peaks showed four-fold symmetry and rotated 45 degrees between YBCO (102) grain axis and CeO2 (220) grain axis. The surface roughness, Δφ, and Δφ of YBCO thin films were 35.12 nm, 0.89 degree, and 1.51 degree, respectively. We found these to be the optimal post-annealing conditions in which to make a single phase of YBCO films with Tc of 87 K from amorphous YBCO thin films. Therefore, our process is useful to make high-quality YBCO microstrip lines.

High-rate fabrication of YBCO coated conductors using TFA-MOD method

Abstract The YBa2Cu3O7-y (YBCO) coated conductors derived from metal-organic deposition (MOD) method using the metal salts of trifluoroacetic acid (TFA) have been developed with high critical current property. The long-length YBCO coated conductors have been fabricated by multi-turn reel-to-reel system. Increasing the thickness per single coating in the multi-turn reel-to-reel system is a cost-effective technique for fabrication of the precursor films in the calcination process. In this work, we have developed a new starting solution consisting of non-fluorine salts of yttrium 4-oxopentanoate and copper 2-ethylhexanoate with focusing on increasing the thickness per single coating for a high-rate fabrication of the YBCO coated conductors by the TFA-MOD method. The critical thickness per single coating of the film fabricated from the new starting solution was 0.54 μm/coat. High critical current of 377 A/cm-width with high critical current density (3.0 MA/cm2) was obtained in the YBCO film fabricated on CeO2 buffered LaMnO3/MgO/Gd2Zr2O-7/HastelloyTM substrates using the new starting solution at the thickness per single coating of 0.42 μm/coat.

Variation of applied field angular dependence of critical current density in YBCO thin films against deposition temperature and composition

Abstract For the magnetic flux pinning in YBa2Cu3Oy (YBCO) thin films, artificial pinning centers (APC) like BaZrO3 and BaSnO3 nanorods act effectively when magnetic fields are applied parallel to the c-axis of the YBCO thin films. However, it is necessary that APC exist into a three dimentional shape and random distribution in order to enhance Jc against all angle range of applied magnetic fields. In this study, we reported YBCO thin films with low anisotropy of Jc against the magnetic field applied angle. As a result, using off-stoichiometric target composition of Y: Ba: Cu = 1: 2: 3.4 and high substrate temperatures, the YBCO thin films which were prepared by pulsed laser deposition method at more than 890 °C showed low anisotropic Jc, since the films included pinning centers acting against wide angle range of applied field.

Fabrication of Very Thin High-Temperature-Superconducting Films Required for High-sensitivity Optics/Superconductivity Converters

We studied a method to obtain superconductivity even in very thin films of YBa2Cu3Oy (YBCO). These thinner YBCO films will be able to be applied to optics/superconductivity (opt/super) converters with high optical sensitivity. Opt/super converters composed of YBCO nano-bridges or nano-wires require a low critical current that is determined by the film thickness. According to X-ray diffraction, critical temperature (Tc) and residual resistivity ratio for the substrates of MgO (100), SrTiO3 (100) and NdGaO3 (110), superconducting features are influenced by the lattice mismatching layer in YBCO closing to a hetero-epitaxy interface between YBCO and single crystal substrate. YBCO films with different thicknesses varying from 80 to 10nm were prepared on SrTiO3, using a suitable temperature. The values of Tc slightly decreased when reducing YBCO thickness from 80 to 20 nm, and then declined sharply at less than 20nm. It is considered that a layer in the beginning of deposition less than 20nm causes stress in the crystal lattice. The Tc of 20 nm YBCO film on SrTiO3 was 80.8 K. It can work above liquid N2 temperature.

Fabrication and Microwave Properties of Asymmetric Dual-band High-Tc Superconducting Filter

Abstract An asymmetric dual-band band-pass filter (DBPF) is proposed for the applications of IEEE 802.11b/g (2.4 GHz ∼ 2.48 GHz) on the multimode wireless local area networks (WLAN). The high temperature superconducing (HTS) filter was fabricated by pattering YBa2Cu3Oy (YBCO) films double-sided deposited on 20 × 20 mm 2 LaAlO3 substrates with an RF sputtering technique and by putting them in copper housings. The simulation results show the asymmetric dual-band feature of two passbands at 2.45 and 2.48 GHz, each with a minimum in-band insertion loss of about 0.3 dB and bandwidths of 20 and 23 MHz, respectively. The realized HTS DBPF shows two passbands at 2.47 and 2.49 GHz with maximum insertion losses of 1.75 and 3.17 dB at 77 K, respectively. The measured results show a good HTS DBPF performance. Moreover, the temperature-dependent frequency responses can be well described by the modified two-fluid model based formulas, indicating that the frequency shift in HTS DBPFs is dominated by the temperature dependence of the magnetic penetration depth.

Intrinsic Surface Resistance of YBCO Thin Films Under DC Magnetic Field

We investigated the dc magnetic field and temperature dependences of the microwave intrinsic surface resistance (Rsint) of YBa2Cu3Oy (YBCO) superconducting thin films with various thicknesses. We used YBCO films from 100, 200, and 300 nm thick YBCO films deposited on CeO2/Al2O3 substrates. The Rsint is means the surface resistance of YBCO film without the dielectric loss of the substrates. The Rsint was obtained from measured surface resistance (Rs) by using a phenomenological equation. The Rs was measured using the dielectric resonator method at 21.8 GHz. A dc magnetic field of up to 5.0 T was applied parallel to the c-axis of the superconducting thin films during the Rs measurements. The Rsint of the YBCO thin films increased when the dc magnetic field was applied. These relations could be explained with the two-fluid model for high-frequency and high-magnetic field limits. The Rsint ratio (defined as Rsint (5T)/Rsint (0T)) slightly decreased with the film thickness. These results indicate that the Rsint of HTS films depends on the film thickness in the dc magnetic field and the Rsint in a dc magnetic field may be related to the pinning center in the HTS films.

Possibility of Nd:YAG-PLD Method for Fabricating REBCO Coated Conductors

A Nd:YAG (neodymium-doped yttrium aluminum garnet) laser is environmentally safe, due to the absence of any poisonous or hazardous gases, and also economical, without the need for expensive gases. We can thus expect a safe and low cost <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm REBa}_{2}{\rm Cu}_{3}{\rm O}_{y}$</tex> </formula> (REBCO)-coated conductor fabrication process by using an Nd:YAG laser. We investigated the fabrication of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{y}$</tex></formula> (YBCO) films using a PLD method with a fourth harmonic Nd:YAG laser. The YBCO films showed good superconducting properties; for example, on <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\rm SrTiO}_{3}$</tex></formula> single crystalline substrates, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$T_{\rm c}$</tex></formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$J_{\rm c}$</tex></formula> were about 90 K and 2.5 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm MA/cm}^{2}$</tex></formula> , respectively, at 77 K. In order to enhance <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$J_{\rm c}$</tex></formula> in magnetic fields, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm BaSnO}_{3}$</tex> </formula> was doped into YBCO films by means of a combinatorial Nd:YAG-PLD method. The combinatorial method enabled the growth and study of a library of samples ranging from pure-YBCO to BSO-doped-YBCO. We demonstrated rapid optimization of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm BaSnO}_{3}$</tex> </formula> content by the combinatorial method and concluded that the optimal BSO content was around 3.6 wt%. In addition, in order to enhance material yield of the Nd:YAG-PLD method, we optimized the oxygen pressure and distance between the target and substrate. On a substrate with dimensions of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$10\times 10\ {\rm mm}^{2}$</tex> </formula> , the material yield of a YBCO film reached about 19.0%. These results suggest that the Nd:YAG-PLD method is promising for forming YBCO-coated conductors.

Characteristics of Ultra-Wideband Bandpass YBCO Filter With Impendence Stub

A compact ultra-wideband (UWB) bandpass filter (BPF) is presented for applications to short-range and high-speed wireless communication. Superconducting YBa2Cu3Oy (YBCO) stepped impedance resonators and coupled-line sections as inverter circuits are designed to form the basic filter structure. In the filter design, connected high-low stepped impedance microstrip lines construct the resonators, and open-stub lines are utilized to add return-loss poles in the pass-band and create transmission zeros in the lower/upper stop-band region. Simulation results show that the passband from 3.0 GHz to 8.6 GHz has a 3-dB fractional bandwidth of 99 percent, computed insertion losses better than 0.03 dB, and return losses greater than 15 dB. Rejection levels in the upper/lower stop-bands are better than 20 dB. For fabrication, high-Tc superconducting (HTS) YBCO films were deposited on double-side-polished 0.5-mm-thick MgO (100) substrates by a radio-frequency sputtering system. The filter was made out of patterned double-sided deposited YBCO films integrated with a gold-coated housing. The realized HTS UWB BPF shows a wide passband within 2.9-8.3 GHz with a maximum insertion loss of 0.88 dB. The measured results show good HTS UWB BPF performance. Moreover, the temperature-dependent frequency responses and the insertion loss can be described by the modified two-fluid-model-based formulas, indicating that the frequency shift and the increase in insertion loss for HTS BPF are both dominated by the temperature dependence of the magnetic penetration depth.

Stable barium compounds in YBa2Cu3Oy superconductors

The stability of various amounts of Ba3Cu3In4O12 (334) or BaTbO3 (BTO) in a sintered YBa2Cu3Oy (YBCO) matrix was examined. Samples with added 334 or BTO exhibited critical temperatures (Tc) above 90K for up to 20vol.% addition and improved critical current densities (Jc) under a magnetic field. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis indicated that 334 and BTO did not react with the YBCO matrix under the sintering conditions used. The normalized Jc under a magnetic field of 1T reached a maximum at 14vol.% of 334 addition and 20vol.% BTO addition. YBCO thin films with added BTO showed a gradual decrease in the Tc with increasing BTO content. YBCO films with added 334 showed a constant Tc of 87K up to a 334 content of 4vol.%.

Effect of ZnO and Zn0,95Mn0,05O nano-particle inclusions on YBCO polycrystalline pinning properties

The effect of nano-size ZnO or Zn0.95Mn0.05O (ZnMnO) addition on the microstructure and pinning properties of polycrystalline YBa2Cu3Oy (YBCO) was analyzed. Samples with nano-particles ZnO and Zn0.95Mn005O addition were synthesized by solid state reaction. Microstructure investigation by transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDS), critical current density dependence on applied magnetic field Jc (B) and electrical resistivity as a function of temperature ρ(T) were carried out to evaluate the relative performance of samples. Using the right amount of ZnO or Zn0.95Mn0.05O addition (0.2 wt.%), the flux pinning ability of YBCO has been enhanced. The best flux pinning was observed for Zn0.95Mn0.05O added sample. The flux pinning enhancement suggests magnetic interaction of pinning centers via vortices.

Local spectroscopy and vortex-core imaging on chemically wet-etched surfaces ofYBa2Cu3Oy by scanning tunneling microscopy/spectroscopy

Low-temperature scanning tunneling microscopy/spectroscopy has been performed on chemically wet-etchedsurfaces of YBa2Cu3Oy (YBCO) single crystals. Chemically wet-etched YBCO surfaces are stable in ultrahighvacuum environments, and superconducting gap spectra are observed with highspatial uniformity up to the bulk superconducting transition temperature,Tc. When a magnetic field was applied, local electronic modulations due to vortices were detectedup to 80 K, enabling the real-space imaging of vortices in YBCO at high temperatures (<Tc) and magnetic fields. Furthermore, local spectroscopy revealed spectra with a zero-biasanomaly due to weak impurities and the tendency of vortex pinning by these impurities.

The dc magnetic field, temperature and film thickness dependence of microwave surface resistance of YBCO and DyBCO thin films

We investigated the dc magnetic field and temperature dependences of the microwave surface resistance (Rs) of YBa2Cu3Oy (YBCO) and DyBa2Cu3Oy (DyBCO) superconducting thin films. The YBCO and DyBCO thin films, each with a thickness of 300, 500, or 700nm, were deposited on MgO (100) substrates by the thermal co-evaporation method. The Rs was measured using the dielectric resonator method. A dc magnetic field of up to 5.0T was applied parallel to the c-axis of the superconducting thin films. The results showed that the Rs value had almost the same temperature dependence at various thicknesses in a zero-external field. The Rs of the YBCO and DyBCO thin films increased with the applied dc magnetic field. The DyBCO thin films showed weaker magnetic field dependence of Rs than the YBCO thin films. The Rs ratio (defined as Rs(5T)/Rs(0T)) linearly increased with the film thickness. These results show that pinning strength decreased with an increasing film thickness.

Relationship between intrinsic surface resistance and critical current density of YBCO thin films with various thickness

We investigated the relationship between the intrinsic surface resistance (Rsint) and critical current density (Jc) of YBa2Cu3Oy (YBCO) film thinner than the penetration depth (λL). The measured YBCO films were deposited on CeO2-buffered r-cut Al2O3 substrates by the pulsed laser deposition method. The thicknesses of these films were 300, 200, and 100nm, respectively. The Rsint means the surface resistance of YBCO film removing the loss by the impedance of the substrates. The effective surface resistance (Rseff) including the impedance of the substrate and the Jc of each YBCO film were measured using the dielectric resonator method at 21.8GHz and the inductive method. We calculated Rsint by using phenomenological expressions and the Rseff value. The Rsint values of each YBCO film were almost the same in the measured temperature region. As a result, we found that Rsint was in inverse proportion to the Jc of YBCO film thinner than λL.

Fabrication, Characterization and Study of MOD Multi-Layer YBCO Films

Fabrication of thick YBa2Cu3Oy (YBCO) films with high critical current densities(Jc) has remained one of the major challenges in the coated conductor development process. In this work, we report successful fabrication of several multi-layer YBCO films with thicknesses ranging from 0.1 micrometers to 1.2 micrometers. The films were deposited on a single crystal substrate by using a multi-layer TFA-MOD technique. The effect of variation in the thickness of an individual layer in a multi-layer film on the overall properties of the film was studied by processing multi-layer films with individual layers of 0.1 micrometers, 0.2 micrometers and 0.5 micrometers. The Jc value for the 0.1 micrometers thick single-layer film was as high as 7 MA/cm2, and the Jc for 6-layer1.2 micrometers thick film was 1 MA/cm2 at 77 K and self field. XRD, SEM and TEM were used to study the dependence of Jc on the orientation, texture and microstructure of the film. It was found that the texture, connections between the two successive layers, porosity, and the crack formation affect the Jc of the film. In addition, we report a multi layer fabrication route with a shorter duration of the pyrolysis reaction according to the number of coats.

Initial Stages of Nucleation and Growth in MOD YBCO Films

Key features of the nucleation and growth of metal organic deposited (MOD) Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> (YBCO) films on biaxially-textured nickel substrates with intervening buffer layers are presented. Within this work, we show the formation of an oriented precursor phase, the laminar growth of the YBCO in the presence of a liquid phase, and subsequent formation of relevant defects tied to the growth process. A model is presented for nucleation and growth, second phase development, and porosity formation. Specific microstructural differences with regard to physical vapor deposited films are discussed.

Preparation of n-type YBa2Cu3Oy films by an electrochemical reaction method

We report on the continuous carrier control from holes to the electrons in YBa2Cu3Oy (YBCO) films by an electrochemical reaction method. In this method, the reduction in the hole concentration or the electron doping occurs at room temperature simply by applying a voltage of ∼1 V between the YBCO film and the Pt electrode in the electrolyte solution of AC1O4 /polyethylene oxide, where A = K or Cs. When the initial sample is slightly overdoped, a dome-like increase and decrease of Tc and an increase of resistance R and Hall coefficient RH, which correspond to a reduction in the hole concentration to ∼1021 cm-3, are observed with increasing voltage or application time. These results suggest that this electrochemical method is a straightforward and effective method for electron doping. Starting with a heavily underdoped sample, which is not superconductive, we succeeded in changing the sign of RH from positive to negative and in rapidly reducing R.

Artificial pinning centres in YBa2Cu3O7−δ thin films by Gd2Ba4CuWOy nanophase inclusions

Artificial pinning centres produced byGd2Ba4CuWOy (2411W) nanophaseinclusions in YBa2Cu3Oy (YBCO) thin films grown by pulsed laser deposition (PLD) using a YBCO targetcontaining 1 mol% 2411W have been investigated. For comparison, YBCO thinfilms have been grown under the same conditions using a pure (commercial)YBCO target. The resulting films were characterized by AC susceptibility(superconducting transition) and SQUID magnetometry (critical current density,Jc),and by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX)spectroscopy and atomic force microscopy (AFM) (structure, morphology and chemical composition).Jc of the film grown from the composite target is practically the same as that measured for thereference sample at 5 K over the whole range of applied field up to 4.5 T. For temperatureshigher than 50 K, however, the presence of the nano-inclusions leads to increasedJc. The relativeenhancement in Jc increases with further increase in temperature.Jc of the film containing nanophase inclusions is between two and three times higher than thatof the reference sample at 77.3 K and 1.5 T. This can be explained by considering thedimensions of nanophase inclusions compared with the temperature-dependent coherencelength. Susceptibility measurements reveal that, apart from a large and sharptransition at about 91 K, there is a secondary, smaller and broader transitionbetween 88 and 84 K in the YBCO film containing nanophase inclusions. Thiscan be explained by substitutions of compatible atoms between YBCO and the2411W phase (Gd with Y), which might also create additional pinning centres ofδTc-type.

Effect of Ba/Y ratio in starting solution on microstructure evolution of YBCO films deposited by advanced TFA-MOD process

We have found that high critical current (IC) values of YBa2Cu3O7-y (YBCO) films are achieved by using the starting solution with the Ba/Y ratio less than 2.0 in the advanced TFA-MOD process. TEM observation revealed that there are few large pores in the YBCO final films with the Ba/Y ratio less than 2.0. Furthermore, from results of investigation of quenched films prepared by cooling rapidly during the calcination process, low Ba content in the starting solution was found to have an effect on attaining the precursor film with low porosity and uniform elemental distribution of cations. As for the crystallization process, there was no significant difference in phases and sizes of non-superconducting precipitates generated before YBCO epitaxial growth, irrespective of the Ba/Y ratio. However, homogeneity of lateral growth rate of the YBCO film is improved, when the starting solution with Ba/Y ratio less than 2.0 is used.

Microstructures and critical current densities of YBCO films containing structure-controlledBaZrO3 nanorods

BaZrO3 nanorods are known to be effective pinning centers asc-axis-correlated pinning centers. Furthermore,BaZrO3 nanorodsin REBa2Cu3Oy (RE: rare-earth element) films are formed by self-assembled stacking ofBaZrO3 using a target mixture of a superconductor andBaZrO3 for pulsed-laser deposition, which is a very easy fabrication technique. The density ofBaZrO3 nanorodsin YBa2Cu3Oy (YBCO) films can be controlled by varying theBaZrO3 content in atarget. The BaZrO3 addition has two functions for superconductivity; one is the improvementof pinning forces due to the addition of pinning centers and the other isTc degradation.The optimum BaZrO3 addition for Jc improvement in magnetic fields is found to be around 3 wt% because of atrade-off between the two functions described above. Furthermore, the length ofBaZrO3 nanorods is found to be controlled using two types of target: pure YBCO and a mixture of YBCO andBaZrO3. Varyingthe BaZrO3 nanorod length has an effect on the pinning mechanism. In particular, magnetic field angle dependences ofJc are variedfrom c-axis-correlated pinning to nearly random pinning by changing the nanorod length. Themagnetic field at the crossover of the pinning mechanism seems to be adjusted by theBaZrO3 nanorod length.