High Temperature Superconductor Thin Films Research Articles

Novel dual mode disk-shaped resonator filter with HTS thin film

We propose a novel dual mode disk-shaped resonator filter with high temperature superconductor (HTS) thin films. The 5 GHz-band YBa2Cu3O7-x (YBCO) microstrip dual mode disk resonator filter on an MgO (100) substrate keeps a perfect circle shape and a waveguide line of about a half-wavelength that is capacitively-coupled with the disk resonator to generate a dual mode. The fabricated filter had an equivalent frequency response of a two-pole filter with two attenuation poles. The coupled coefficient of two orthogonal modes can be controlled by the length of coupled waveguide line and the gap space between feeder and disk resonator. The fabricated filter showed very low third-order intermodulation distortion (IMD3) of -73 dBc with an output power of 10 W at a temperature of 65 K. In addition, the proposed structure can be fabricated using a side lithography and etching processes. This is an advantage for multi-stage filter applications. We believe this filter is a promising candidate structure for RF transmit system applications.

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.

High-frequency characterization and modeling of paraelectric∕superconductor resonators for high power-handling microwave passive devices

The theoretical conditions required to observe reduced nonlinearities in passive microwave devices made of a paraelectric (PE) thin layer and a high-temperature superconductor (HTS) thin film are presented. In a transmission line (TL) combining both materials, the nonlinear permittivity εr(Vrf) of the PE generates a nonlinear capacitive contribution Cd(Vrf), whereas the nonlinear penetration depth λ(Irf) of the HTS produces a nonlinear inductive contribution Ld(Irf). These contributions act in opposite ways. Under determined conditions that will be defined, they may fully compensate or at least lead to the reduction of the spurious signal level. The HTS nonlinearities may be lowered by employing very thin layers of PE. In this case, the high dielectric loss tangent (tanδ) of PE should not be such an issue. Based on experimental results obtained with superconducting YBa2Cu3O7−δ resonators integrating different thicknesses of paraelectric SrTiO3 (STO) deposited on LaAlO3 substrates, the loss level could then be considered in the simulation of the proposed compensation effect. The results show that the reduction of the unloaded quality factor (Q0) as the thickness of STO increases also impacts the level of spurious signals. Then, an analytic expression for the Q0 of a resonant TL that integrates a PE and a HTS thin layer is proposed. It specifies the conditions for reducing the total nonlinear response. These results could then be applied to realize high power handling HTS passive microwave devices.

Accurate Calculation of Shielding Current Density in HTS Thin Film

A numerical method has been developed for calculating the shielding current density in a high temperature superconducting (HTS) thin film. When the initial-boundary-value problem of the shielding current density is discretized with respect to time and space, improper integrals appear as coefficients of the nonlinear system. In order to evaluate the coefficients accurately, both the double exponential formula and the Gauss-Legendre formula are applied to the improper integrals. As a result, the shielding current density can be accurately determined even in the thin film. As an application of the method, the contactless method for measuring the critical current density has been investigated numerically. The results of computations show a qualitative agreement with the experimental results.

Numerical Simulation of Permanent Magnet Method: Relation Between Critical Current Density and Maximum Repulsive Force

The permanent magnet method for measuring the critical current density of the high-temperature superconducting (HTS) thin film is investigated numerically. The numerical code has been developed for analyzing the time evolution of the shielding current density in the HTS thin film. In the code, the initial-boundary-value problem of the shielding current density is solved numerically. After discretized by using the finite element method (FEM) and the backward Euler method, it is reduced to the problem in which the simultaneous nonlinear equations are solved at each time step. By using the numerical code, the maximum repulsive force is calculated as a function of the critical current density. The results of computations show an excellent proportional relationship between them. This is mainly because the spatial distribution of the shielding current density varies in proportion to the critical current density except for the neighborhood of the film edge.

Universal temperature scaling of flux line pinning in high-temperature superconductingthin films

Dissipation-free current transport in high-temperature superconductors is one of the mostcrucial properties of this class of materials which is directly related to the effective inhibition offlux line movement by defect structures. In this respect epitaxially grown thin films ofYBa2Cu3O7−δ (YBCO) are proving to be the strongest candidates for many widescale applications thatare close to realization. We show that the relation between different defect structures andflux line pinning in these films exhibits universal features which are clearly displayed in adetailed analysis of the temperature-dependent behaviour of local critical currents. Thisallows us to identify different pinning mechanisms at different temperatures to beresponsible for the critical currents found. Additionally, the presence of grainboundaries with very low misorientation angles affects the temperature stability ofthe critical currents, which has important consequences for future applications.

Reduction of strain in high temperature superconductor thin film devices

Multilayer buffer layers containing BaZrO 3 thin films on the bottom and SrTiO 3 thin films on the top were successfully applied to improve the epitaxial growth of the YBa 2Cu 3O 7− x (YBCO) films on the MgO substrates, which have the best correspondence of the thermal expansion coefficient with one of YBCO. The HRTEM and X-ray diffraction studies demonstrated pure c-axis orientation and absence of the in-plane misoriented grains in the YBCO films grown on the buffered MgO substrates. High superconducting parameters and no cracks were observed in such YBCO films even at their thicknesses ≫1 μm. The multilayer flux transformers having total thickness ∼2-μm were prepared on the buffered MgO substrates and demonstrated an improved insulation between the superconducting layers and an increased dynamic range compared to flux transformers on SrTiO 3 substrates.

Numerical Simulation of Shielding Current Density in High-Temperature Superconducting Thin Film

The numerical code has been developed for simulating the time evolution of the shielding current density in the high-temperature superconducting (HTS) thin film. When the governing equation of the shielding current density is discretized by the use of the finite element method (FEM) and the completely implicit method, improper integrals appear as coefficients of the nonlinear system. In order to evaluate the coefficients accurately, the double exponential formula is applied to the integrals. As a result, the shielding current density can be calculated with a high accuracy. By using the code, the noncontact measurement method of the critical current density has been investigated numerically

Noise properties of high-temperature superconducting dc-SQUID gradiometers

We have developed different types of superconducting magnetic field sensors based on high temperature superconducting (HTS) thin films. Here, we describe the fabrication of single layer dc-SQUID gradiometers with bicrystal Josephson junctions for operation in a flip-chip configuration to improve sensor performance. High-quality thin films are known to be essential in achieving suitable electrical properties in these superconducting devices. The influence of sample processing on sensor performance is discussed. The most important parameter for practical applications is the field gradient resolution of the investigated dc-SQUID sensors. To determine this parameter for different gradiometer layouts we measured their noise properties in unshielded as well as magnetically or electrically shielded environments.

無声放電式活性酸素発生機と酸化薄膜形成に於ける酸化力

The authors have studied the low-pressure silent-discharge-type active oxygen generator in terms of its application to the formation of oxide thin films. In this paper the oxidation power of active oxygen in the formation of oxide thin films is compared with that of oxygen and ozone by forming silicon oxide thin films. The authors confirmed that the oxidation power is active oxygen>ozone>oxygen based on the experimental result of the number of x in SiOx thin films. Furthermore, the authors applied active oxygen to the formation of the thin-film high-temperature superconductor and active oxygen was found to be effective for the formation of the thin film with high performance. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 163(1): 1–7, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20671

The influence of external factors on the corrosion resistance of high temperature superconductor thin films against moisture

High temperature superconductor (HTS) thin films have been systematically investigated for their corrosion resistance against moisture by studying the role of external factors such as temperature (T), relative humidity (RH), and the type of substrates in the corrosion. In general, (i) the corrosion is progressed monotonously with increasing T as well as RH, (ii) a threshold level of water vapor is needed to cause degradation, and (iii) between T and RH, the influence of T is more dominant. HTS films on SrTiO3 and CeO2 buffered sapphire (cbs) substrates showed better corrosion stability and a low rate of degradation in the critical current density as compared to that of the film grown on MgO substrate. Between DyBa2Cu3Oz (DBCO) and YBa2Cu3Oz, the former is reproducibly found to have many fold higher corrosion resistance against moisture. This observed enhancement in the corrosion resistance in DBCO could be explained by the improved microstructure in the films and the better lattice matching with the substrate. Thus, the dual advantage of DBCO/cbs films, i.e., the enhanced corrosion stability of DBCO and the appropriate dielectric properties of sapphire, can be readily exploited for the use of DBCO/cbs films in the microwave and power devices.

High quality ISD MgO buffers on large scale, flexible metal substrates for RF applicationsof HTS thin films

High temperature superconductor (HTS) films on biaxially aligned MgO buffers producedby inclined substrate deposition (ISD) on flexible metal substrates show promisingelectrical properties for current transport and RF applications. The quality andperformance of the HTS film crucially depends on the biaxial texture offered bythe buffer layer. We have investigated and optimized the conditions suitable forproducing highly aligned ISD MgO buffer layers on large scale, flexible metalsubstrates. Thoroughly mechanically ground and electro-chemically polished substrates,homogeneous substrate cooling, compensation for variation of spatial deposition angleand high deposition rates account for excellent results (orientation quality ofΔφ = 7°–8° and Δχ = 2.5°–3.0°) and very good homogeneity over the substrate area. The deposited ReBCO films on topof the MgO ISD buffered metal substrates show critical current densities up to2.2 MA cm−2. The surface resistance of such HTS films measured at 10.9 GHz exhibits excellent values of500 µΩ at 77 Kand 200 µΩ at 30 K. Such surface resistances on HTS films on ISD substrates are a factor of1.8 higher compared than those of HTS films on single crystalline substrates.

Effect of ionizing radiation on noise in MgB 2 thin film – a candidate material for detector development for post-Cassini planetary missions

A thin film of MgB 2, grown on a SiN–Si substrate, with a superconducting transition temperature, T c, near 39 K has been irradiated with γ-rays from a 60Co source. The total dose was 100 Krads (Si). Its pre- and post-irradiation noise has been measured and noise spectral density plots made. No change in d R/d T, pre- and post-irradiation was measured. At the mid-point of the transition, T = 38.24 K, a noise spectral density S V = 0.34 nV Hz −1/2 was measured at 10 Hz. The temperature noise, K n, of the MgB 2 film at different frequencies is compared to that of high temperature superconducting (HTS) thin films (YBaCuO and GdBaCuO with T c ∼ 90 K) used currently in HTS transition-edge bolometers. Even with the observed post-irradiation small increase in noise, its lower T c and K n values predict that high performance far IR bolometers can be developed using MgB 2 as a thermistor.

Ion-beam direct-structuring of high-temperature superconductors

A novel method allows for a direct modification of the electric properties of high-temperature superconductors (HTS) and for patterning of devices into thin films of HTS. A low-divergence beam of light ions is directed at a thin film of HTS through a mask placed at some distance from the surface of the material. It converts the illuminated areas of the film from superconducting to semiconducting and even insulating in a single process. Computer simulations of the ion-target interactions and experimental investigations on 100-nm thick films of YBa 2Cu 3O 7 by X-ray diffraction, electrical resistivity and Hall effect measurements after cumulative irradiation are reported. A central result is that 75 keV He + ions can penetrate through thin films of HTS without implantation effects, create point defects without destruction of the main building blocks of the crystal structure and convert a HTS to an insulating material with technically feasible ion doses of a few 10 15 cm −2.

Heteroepitaxial growth of YBCO/STO thin films for voltage tunable varistors

SrTiO 3 (STO) ferroelectric (FE) thin films associated with high-temperature superconductors (HTSC) is a good compromise for tunable microwave components. The great challenge encountered for frequently used coplanar structured devices based on FE/HTSC (A/B means A is on B) thin films is that it is difficulty to simultaneously keep good superconductivity of HTSC thin films and optimized non-linear dielectric properties of ferroelectrics. Stripline structured devices based on HTSC/FE thin films are prepared in this paper. In the HTSC/FE structured devices, STO thin films with smooth surface were heteroepitaxial grown on LaAlO 3 (1 0 0) (LAO) substrate. The STO thin film showed not only good out-of-plane orientation, but also identical in-plane orientation with substrate. The crystallographic orientation relationship between LAO substrate and STO thin film in a– b plane is [1 0 0] STO∥[1 0 0] LAO. Highly out-of-plane textured YBCO thin films with good superconducting properties were grown onto the STO thin films, and interdigital capacitors with relative capacitance change of 20% and Q-factor of 286 were made from the YBCO/STO/LAO thin films. The experiment results showed that the HTSC/FE multilayers is a good candidate for high- Q varistors.

Temperature and dc magnetic field dependence of the surface resistance in HTS films

We investigated the dependence of the surface resistance (Rs) in high-temperature superconductor (HTS) thin films on a dc magnetic field (parallel to the c-axis) and temperature. Measurements were done using the dielectric resonator method and superconducting magnet system. The YBa2Cu3O7−δ (YBCO) and DyBa2Cu3O7−δ (DyBCO) thin films produced by THEVA GmbH were prepared through a thermal co-evaporation method. The Rs of the YBCO and DyBCO thin films increased with magnitude of a dc magnetic field. The Rs(5T)/Rs(0T) of the DyBCO was lower than that of the YBCO thin films for the temperature region below 40K. From the results of the field-cooled (FC) and zero-field-cooled (ZFC) measurements of Rs, it was found that there was plenty of flux pinning in the DyBCO thin films compared with the YBCO thin films. These results indicate that the DyBCO thin films with much pinning centers are better for use in low temperature nuclear magnetic resonance applications.

Rapid and simple measurement of critical current density in HTS thin films using a permanent magnet method

We present the results of non-destructive and contactless measurement of the critical current density (Jc) of high-temperature superconductor (HTS) thin films using a permanent magnet method. A cylindrical permanent magnet (Sm2Co17) with a height of 5.0 mm and diameter d of 5.0mm was used. To investigate the margin of the sample setting for the vertical direction, load vs. distance curves for a minimum distance Lmin which varied from 0.5 to 2.0mm were measured. Here, L is the distance from the permanent magnet to a surface of the HTS film. The effective maximum repulsive force Fm,eff was almost constant below Lmin=1.2mm. This result indicates that the measurement system has a wide margin with respect to sample setting in the vertical direction. We investigated the dependence of Fm,eff on the sample area from diameter l=20–4mm and found that the relationship of the critical current density Jc=1.7×10−7 (Fm,eff/t) can be applied for l/d>2 and the required sample size of the HTS films is approximately l=2d for this system. Here, t is the thickness of the HTS thin film. These results indicate that the permanent magnet method can be used to easily estimate the Jc of HTS films at 77K. Additionally, this system has good reproducibility for Jc measurements. It is thus promising as a replacement for conventional inductive measurements and for measurements for Jc mapping of HTS films, tapes, and wires using a small magnet.

A high temperature superconductor tape RF receiver coil for a low field magneticresonance imaging system

High temperature superconductor (HTS) thin films have been applied in making alow loss RF receiver coil for improving magnetic resonance imaging imagequality. However, the application of these coils is severely limited by theirlimited field of view (FOV). Stringent fabrication environment requirements andhigh cost are further limitations. In this paper, we propose a simpler methodfor designing and fabricating HTS coils. Using industrial silver alloy sheathedBi(2−x)PbxSr2Ca2Cu3O10 (Bi-2223) HTS tapes, a five-inch single-turn HTS solenoid coil has been developed, andhuman wrist images have been acquired with this coil. The HTS tape coil hasdemonstrated an enhanced FOV over a six-inch YBCO thin film surface coil at 77 K withcomparable signal-to-noise ratio.

Switches Based on High-Temperature Superconducting Thin Films

Switches based on high-temperature superconducting (HTS) thin films have been developed for the application in fault current limiters and various controlling devices. HTS thin film structures needed for this purpose have usually been fabricated as long multi-layered elements, in which HTS thin film is deposited on a dielectric substrate with high heat conductivity and covered by a thin layer with high electric conductivity. The main challenge has been to provide the fast and simultaneous transition of the whole of switch from the superconducting to the normal state. We experimentally investigated, under DC and AC conditions, the superconducting-normal state (S-N) transition in switches fabricated from parallel and series-connected YBCO thin films. The films were deposited on a sapphire substrate of 0.5 mm in thickness and covered by 100 nm Au layer. The appearance and propagation of normal zones in the films were determined by measuring the voltage drop across their different sections. The results were compared with ones obtained for a single film. The conditions providing a simultaneous S-N transition of the whole of the switch are discussed. It is shown that shunting the film by a resistor improves both the film protection from overheating and the homogeneity of S-N transition in the switch.

Dependence of Surface Resistance in HTS Thin Films on a DC Magnetic Field

We investigated the behavior of the surface resistance (R/sub s/) of high-temperature superconductor (HTS) thin films exposed to a dc magnetic field (parallel to the c-axis). Measurements were done using the dielectric resonator method and superconducting magnet system. The DyBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (DyBCO) and YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) thin films (produced by THEVA GmbH) were prepared through a thermal co-evaporation method. The resonant frequency of the TE/sub 011/ mode in the resonator was approximately 22 GHz. A dc magnetic field of up to 5 T was applied vertically to two parallel superconducting thin films placed into a closed-type Cu cavity. R/sub s/ of the DyBCO and YBCO thin films increased as the applied dc magnetic field increased. The R/sub s/ ratio (defined as R/sub s/(5 T)/R/sub s/(0 T)) of the DyBCO and YBCO thin films at 4.5 K was approximately 5 and 50, respectively. These results indicate that the DyBCO thin film may be better for use in low-temperature nuclear magnetic resonance applications.