High Temperature Superconductor Thin Films Research Articles

The growth mechanism and properties of Bi2212 thin films on miscut substrates by sol-gel method

A sol-gel methodology is employed to prepare Bi2212 thin films on single crystal LaAlO3 miscut substrates with nominal tilt angles of 0°, 5°, 10°, and 15°. The study focuses on examining the phase purity, crystal structure, surface morphology, microstructure, and properties of these thin films. Experimental results indicate that the growth of Bi2212 thin films on miscut substrates proceeds through the step-flow mode using the sol-gel method. Higher tilt angles lead to an increase in the conventional resistivity of Bi2212 thin films. Tilted Bi2212 thin films exhibit a typical transverse Seebeck effect when comparing the resistivity in different directions. Notably, the Seebeck coefficient difference reaches 12 V/K when the substrate tilt angle is set at 15°. Meanwhile, tilted Bi2212 thin films consistently exhibit outstanding superconducting transition temperature (Tc) while demonstrating lower critical current density (Jc) values. These findings offer valuable insights for the development of Bi2212 high temperature superconducting thin films (HTSFs) in thermoelectric and superconductive devices, serving as a reference point for achieving optimal film characteristics.

Levitation force measurement and characteristics analysis of double sided high temperature superconducting thin film device

In this paper, the levitation characteristics of double sided high temperature superconducting (HTS) thin film device is proposed and measured. Based on the unique advantages such as, passive self-stable levitation, higher payload, lower energy consumption, less maintenance cost, etc., HTS-magnetic levitation (maglev) technology has been applied in the traditional fields (rail transit and energy storage fields) and novel fields (astrophysics, space station microgravity, vibration isolation fields). As the key core components, HTS device should meet the requirements of volume, weight and cost, to improve the system operation efficiency. Thus, considering the low profile, miniaturization and lightweight characteristics of the double sided YBa2Cu3O7-x (YBCO) thin film, we measured its levitation and relaxation characteristics. Meanwhile, we also designed the YBCO thin film with meander-lines, to explore the levitation characteristic evolution mechanism. It can be found from the experimental results that, the single piece of YBCO thin film with/without meander-lines can offer the levitation force more than 400 mN and 800 mN, respectively. The signature of this work is that the double sided YBCO thin film has the potential to be employed as the core device in the special application fields (such as astrophysics and space station), to supply the sufficient micro levitation force in limited volume and weight.

A Broad Stopband and High Rejection HTS Lowpass Filter for Quantum Computers

This paper presents a novel approach for extending the upper stopband of lowpass filters. The proposed technique involves dividing the hairpin unit cells' coupled lines into multiple smaller components, which shifts the harmonics to higher frequencies while maintaining the cut-off frequency. Additionally, the stopband is further extended by modifying the tapping position to suppress the second harmonic. For demonstration, an improved dual-hairpin unit cell is devised, which is able to extend the stopband up to 11.3 times the cut-off frequency. Furthermore, a cascaded lowpass filter utilizing multiple improved dual-hairpin unit cells is also designed, which is divided into two sub-filters to suppress enclosure resonance. The cascaded filter is fabricated on an HTS (high-temperature superconducting) thin film deposited on a MgO substrate, demonstrating a cut-off frequency of 483 MHz, extending the stopband to 25.1 GHz (51.9 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_c</i> ), affording a rejection level exceeding 60 dB and insertion loss below 0.04 dB. Notably, the roll-off rate exceeds 569 dB/GHz, and the size measures only 0.144 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ_g</i> × 0.030 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ_g</i> .

A novel method to improve the morphology of Bi2212 film by PVA-assisted Pechini sol-gel method

Bi2212 high-temperature superconducting thin films (HTSFs) have emerged as the most promising material for terahertz (THz) devices due to their intrinsic Josephson effect. Enhancing the surface smoothness of these films is crucial for their practical application. In this study, metal nitrates were utilized as the raw material to prepare Bi2212 HTSFs on SrTiO3 (100) substrates using the Pechini sol-gel method. The chelating agent ethylenediamine tetraacetic acid (EDTA) and the surfactant polyvinyl alcohol (PVA) were employed to improve the dissolution characteristics of Bi2212 solution. The influence of various parameters such as sintering temperatures, surfactant concentrations, and metal ion ratios was systematically investigated to assess the phase purity, crystallinity, surface morphology, and electrical properties of the Bi2212 superconducting thin films. The results demonstrated that a higher concentration of PVA led to reduced solubility of the Bi2212 solution. Notably, when the PVA concentration was set at 1 %, the fabricated Bi2212 thin film exhibited exceptional epitaxial properties, enhanced Bi2212 phase purity, reduced surface roughness, and higher Tc. These outcomes provide valuable insights for the development of Bi2212 HTSFs in superconducting devices, as they serve as a reference point for achieving optimal film characteristics.

Superconducting Notch Filter for RFI Mitigation in Ground-Based Radio Telescope

This article presents a high-rejection, thin-film high-temperature superconductor, microstrip bandstop filter to prevent a local, and high-power radio-frequency interference (RFI) source from interfering with NASA Goddard Geophysical Astronomical Observatory (GGAO)’s very long baseline interferometry (VLBI) global observing system (VGOS) cryogenic receiver. This filter has an excellent 2.7%, 50-dB-fractional-bandwidth, and a center stopband frequency of 9.41 GHz. It does not contain any narrow or interdigital features as found in some designs, which reduces the fringing electric fields and improves its power handling capability. The YBCO films were grown on 435- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m-thick R-plane sapphire substrate and the anisotropic behavior was modeled and simulated with a high degree of accuracy. The device was tested while cooled to 77 K and the measurements agree well with simulation.

Optimized BaZrO3 nanorod density in YBa2Cu3O matrix for high field applications

To maximize the flux pinning in high-temperature superconductor (HTS) thin film applications, we have experimentally studied the effect of BaZrO3 (BZO) nanorod density within the YBa2Cu3O (YBCO) lattice. Even though the BZO decreases the self-field critical current density and the absolute at high fields is observed being the highest for 4% BZO doped YBCO, the maximized pinning property is observed at the level of 10% of BZO, when the distance between the outer edge of the nanorods is in the order of the diameter of the nanorod. In general, as also theoretically calculated, the flux pinning is increased even above 10% of BZO, but the improvement is limited by disturbance of the nanorod growth, weakening the flux pinning and decreasing the absolute drastically. The results evidently show that by maximizing the flux pinning using higher BZO doping concentration than earlier expected and taking care of the maximum self-field , which is strongly dependent on the electron mean free path, would offer the keys to resolve the challenges in the future HTS power applications.

Functional substrates – a novel approach to tailor transport properties and flux-line pinning in YBa2Cu3O7 – x thin films

Abstract A concept is introduced, using substrates for high-temperature superconductor (HTS) thin-film deposition beyond their usual application as chemical inert, lattice-matched support for the films. The substrates are applied as functional elements in order to controllably modify the growth mode of YBa2Cu3O7– x thin films in the case of vicinal cut SrTiO3 single crystals and, thus, generate regular arrays of pinning centers. Furthermore, the use of biaxial epitaxial strain, caused by lattice misfit of film and substrate, is briefly reviewed for HTS thin films and the application of this concept to tailor the properties of perovskite thin films in general is shown.

Temperature Effect on Selectivity of HTSC Josephson Junction Detector

The most sensitive THz detection is obtained with superconducting Josephson Junction (JJ) detectors, cooled by liquid helium. These detectors require complex cooling systems. JJ detectors implemented in High Temperature Superconductors (HTSC) are considered a viable and more applicable alternative. The frequency of the detected radiation is directly proportional to the voltage of the Shapiro steps. To investigate temperature dependence of the frequency selectivity, i.e., the accuracy of the measured voltage, we implemented JJs detectors in YBa <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">7</sub> HTSC thin films, patterned on MgO bicrystal substrate. We improved the detector sensitivity and reduced losses by placing the JJ between the ends of two strips integrated with the antenna, reducing the high mismatch between the impedance of the JJ and that of the Au bow-tie planar antenna. The detector parameters were determined by extensive simulations. High correlation was obtained between the simulations and the experimental results. Error in the measured <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${{\boldsymbol{V}}_{{\boldsymbol{DC}}}}$</tex-math></inline-formula> , hence in the radiation frequency, was 3.5-7% at 60-70 K, 3-4 orders of magnitude larger than that measured at 40-50 K, less than 0.01%, showing the very strong influence of temperature on the frequency selectivity of high temperature JJs detectors.

Determining the temperature-dependent London penetration depth in HTS thin films and its effect on SQUID performance

The optimum design of high-sensitivity Superconducting Quantum Interference Devices (SQUIDs) and other devices based on thin high-temperature superconductor (HTS) films requires accurate inductance modeling. This needs the London penetration depth λ to be well defined, not only at 77 K, but also for any operating temperature, given the increasingly widespread use of miniature low-noise single-stage cryocoolers. Temperature significantly affects all inductances in any active superconducting device, and cooling below 77 K can greatly improve device performance; however, accurate data for the temperature dependence of inductance and λ(T) for HTS devices are largely missing in the literature. We report here inductance measurements on a set of 20 different thin-film YBa2Cu3O7−x SQUIDs at 77 K with thickness t = 220 or 113 nm. By combining experimental data and inductance modeling, we find an average penetration depth λ(77)=391 nm at 77 K, which was independent of t. Using the same methods, we derive an empirical expression for λ(T) for a further three SQUIDs measured on a cryocooler from 50 to 79 K. Our measured value of λ(77) and our inductance extraction procedures were then used to estimate the inductances and the effective areas of directly coupled SQUID magnetometers with large washer-style pickup loops. The latter agrees better than 7% with experimentally measured values, validating our measured value of λ(77) and our inductance extraction methods.

A Compact Four-Pole Tunable HTS Bandpass Filter at VHF Band

This article presents a compact, low loss, wide tuning range, high temperature superconducting (HTS) bandpass filter at very high frequency (VHF) band. The filter consists of four zigzag open-loop resonators loaded with gallium arsenide (GaAs) varactors as tuning elements. Each resonator is biased independently. The filter circuit is fabricated on a 3-inch HTS thin film deposited on a LaAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate. The filter demonstrates a frequency tuning range of 73.2-118 MHz, which is a tunability of 48%, an insertion loss of 0.6-2.2 dB, and a 3-dB fractional bandwidth of 3.2-4.3%. Besides, the return loss of the filter is better than 16.6 dB in the whole tuning range, with slight adjustment of bias voltages for different resonators.

Design of a dual-wideband high temperature superconducting filter

A 7-pole dual-wideband high temperature superconducting (HTS) filter based on a quarter-wavelength stub-loaded dual-mode resonator is introduced. The resonance characteristics, coupling patterns, external factor and cross coupling structure are discussed in turn. The filter is fabricated on a double side DyBa2Cu3O7 HTS thin film with a 2 inch-diameter and 0.5-mm-thick MgO substrate. The fabricated filter is measured at 77 K. The measured results show that the filter has an excellent in-band performance with center frequencies of 240 MHz, 541 MHz and passbands of 40 MHz, 140 MHz, respectively. Due to the introduction of transmission zeros, the selectivity of filter is further improved. The measured results agree with the simulated ones substantially.

The Definition of the Paramethers of Superconducting Film for Production of Protection Equipment Against Electromagnetic Environmental Effects

The paper presents the results of evaluating the propagation of a plane electromagnetic wave (EMW) over the surface of a film made of a high-temperature superconductor (HTS) in both superconducting S and normal N states, as well as an analysis of the parameters of a thin HTS film necessary for implementing a device for protection against electromagnetic radiation. Evaluation of the propagation of EMW over the surface of a thin HTS film was performed on the basis of a two-fluid model. As a result of the research, relations were obtained for determining the value of the surface impedance and the depth of penetration of EMW into a superconducting film in S and N states. It is shown that the expression for determining the penetration depth of EMW into a superconducting film in the normal N state is applicable provided that the frequency of the signal field does not exceed the critical value, which is determined by the binding energy of charge carriers at a temperature not exceeding the transition temperature to the superconducting state. Based on the relations for determining the surface impedance of a thin HTS film, relations are obtained for the active surface resistance, which is the real part of the surface impedance, and the surface reactance, which is its imaginary part, in the superconducting and normal states. Using these ratios, the quality parameter of the HTS thin film is introduced. The dependence of the quality factor of the HTS film on its thickness is found. It is shown that the highest value of the quality factor is realized when the film thickness is less than or of the order of the penetration depth. It is noted that this dependence is valid only if the film thickness does not depend on its quality. Keywords — superconducting film, electromagnetic wave, two-fluid model, surface impedance, penetration depth.

Fabrication of HTS Low-Noise Nanobridge Josephson Junction by Gallium FIB

High-temperature superconductor (HTS) Josephson junctions (JJs) are primarily based on artificial grain boundaries. Bi-crystal JJs are typically utilized in HTS superconducting quantum interference devices (SQUIDs). However, the performances of bi-crystal JJs depend significantly on the quality of the interface at the junction. In this study, we investigate a method for forming nanobridge JJs via gallium focused ion beam (FIB) irradiation. A 100-nm-thick YBa <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">7-</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">δ</sub> was deposited on an MgO (100) substrate via pulsed laser deposition. An additional 20-nm-thick Au layer, which serves as a protection layer preventing ion damage during alignment, was deposited in situ. HTS microchannels with a width of 4 μm were defined via photolithography and argon ion milling. The HTS microchannel widths are decreased via FIB line scans to create nanobridges. The JJs are fabricated using two types of HTS thin films, in which one has a higher critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ) that the other. The fabrication process using the gallium FIB was optimized and nanobridges with a junction width of 80 nm were prepared. As a result, a better with higher J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> (5 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) HTS films allowed for making junctions exhibiting up to five Shapiro steps at 77 K.

Control of the nanosized defect network in superconducting thin films by target grain size

A nanograined YBCO target, where a great number of grain boundaries, pores etc. exist, is shown to hold an alternative approach to future pulsed laser deposition based high-temperature superconductor thin film and coated conductor technologies. Although the nanograined material is introduced earlier, in this work, we comprehensively demonstrate the modified ablation process, together with unconventional nucleation and growth mechanisms that produces dramatically enhanced flux pinning properties. The results can be generalized to other complex magnetic oxides, where an increased number of defects are needed for modifying their magnetic and electrical properties, thus improving their usability in the future technological challenges.

Design of sextuple-mode triple-ring HTS UWB filter using two-round interpolation* *Project supported by the National Natural Science Foundation of China (Grant No. 61471094).

A single-stage ring resonator capable of introducing six modes within the ultra-wideband (UWB) passband is presented. The sextuple-mode resonator consists of three rings and three sets of stepped-impedance open stubs. Based on this sextuple-mode resonator, a UWB filter fed by the interdigital-coupling line (ICL) is designed. And we propose a two-round interpolation method to obtain the filter’s initial dimensions. The designed filter is fabricated on a double-sided YBCO/MgO/YBCO high-temperature superconducting (HTS) thin film for demonstration. The experimental results show that this UWB filter produces eight resonances in the passband eventually, which effectively improves the in-band reflection and the band-edge steepness. Moreover, the upper stopband performance is enhanced due to the transmission zeros (TZs) generated by the stepped-impedance open stubs and the ICL structure. The measured good performance verifies the practicability of the two-round interpolation approach, which can also be extended to other odd–even-mode filter designs.

A 12-pole VHF band high selective high temperature superconducting filter

A 12-pole VHF band high temperature superconducting (HTS) filter with a spiral quarter-wavelength resonator is introduced. The coupling characteristics of the proposed resonator, cross structure and external quality factor is analyzed in detail. The HTS filter is fabricated on a 3-inch-diameter double-sided DyBa2Cu3O7 HTS thin film with a LAO substrate and measured with a low-temperature test system at 77 K. The measured filter has a center frequency of 119.93 MHz and a 1-dB passband of 10.04 dB. Due to the introduction of transmission zeros, the rectangular coefficient (BW60dB/BW1dB) of the measured filter is about 1.20, which shows excellent selectivity. The measured results are in good agreement with the simulated ones.

Exploration of a Novel HTS Thin-Film Device Combined With Roles of Transformer and Overcurrent Limiter

A novel high-temperature superconducting (HTS) thin-film device based on double-sided YBa <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">7-</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> (YBCO) thin film, combined with roles of transformer and overcurrent limiter, is proposed and tested in this article. Due to the effective improvement on the power density, operating frequency, repeatability, functional integration, and low profile of the high-frequency converters, thin-film transformer (TFT) has been widely employed in power conversion fields. In contrast to the normal conductive materials, the surface resistance of the HTS materials is much smaller. Thus, the TFT fabricated by HTS thin film has lower Joule losses and higher power conversion efficiency. We designed the double-sided YBCO thin-film-based HTS-TFT and measured its parameters at liquid nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). As a comparison, we also developed the copper (Cu) based TFT and measured the same parameters at LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and room temperature. Besides, we also tested the overcurrent limiting performance of the HTS-TFT, which is based on the state change from superconductivity to normal state. It can be found from the contrastive experiments that the HTS-TFT has better operating parameters than the Cu-TFT; meanwhile, the overcurrent can be limited by the HTS winding effectively.

Calculations of Some Doping Nanostructurations and Patterns Improving the Functionality of High-Temperature Superconductors for Bolometer Device Applications

We calculate the effects of doping nanostructuration and the patterning of thin films of high-temperature superconductors (HTS) with the aim of optimizing their functionality as sensing materials for resistive transition-edge bolometer devices (TES). We focus, in particular, on spatial variations of the carrier doping into the CuO layers due to oxygen off-stoichiometry, (that induce, in turn, critical temperature variations) and explore following two major cases of such structurations: First, the random nanoscale disorder intrinsically associated to doping levels that do not maximize the superconducting critical temperature; our studies suggest that this first simple structuration already improves some of the bolometric operational parameters with respect to the conventional, nonstructured HTS materials used until now. Secondly, we consider the imposition of regular arrangements of zones with different nominal doping levels (patterning); we find that such regular patterns may improve the bolometer performance even further. We find one design that improves, with respect to nonstructured HTS materials, both the saturation power and the operating temperature width by more than one order of magnitude. It also almost doubles the response of the sensor to radiation.

Modelling of electromagnetic loss in HTS coated conductors over a wide frequency band

Electromagnetic loss is a crucial factor to evaluate the performance of high temperature superconducting (HTS) coated conductors (CCs). Although the loss characteristics around power frequency have been well studied, it is still unclear how loss varies towards high frequencies above kHz level, which is important to a wide range of applications, such as wireless power transfer. This paper is to investigate the frequency dependence of both magnetization and transport current loss in HTS CCs and therefore, to provide comprehensive analyses through a detailed multilayer model reflecting their actual structures. In this paper, a T-formulation based multilayer numerical model for HTS CCs, consisting of HTS layer, substrate, silver overlayer, and copper stabilizers, has been developed. Both magnetization loss and transport current loss over a wide frequency range, from 50 Hz to 1 MHz, have been simulated and discussed. The results obtained by the existing thin film model based on T-formulation, multilayer model and homogenization model by H-formulation are also presented and compared. The proposed multilayer model has been validated by experimental measurements, which has proven the widely adopted HTS thin film model to be inapplicable for frequencies above 100 Hz. In addition, most magnetization losses occur in the copper stabilizers above 1.2 kHz due to skin effect. Results can be used to study the performance of HTS devices towards high frequencies at kHz or even MHz level.

Series arrays of planar long Josephson junctions for high dynamic range magnetic flux detection

We investigated series arrays of closely spaced, planar long Josephson junctions for magnetic field transduction in Earth’s field, with a linear response and high dynamic range. The devices were fabricated from thin film high-temperature superconductor YBa2Cu3O7−δ (YBCO) thin films, using focused helium ion beam irradiation to create the Josephson barriers. Four series arrays, each consisting of several hundreds of long junctions, were fabricated and electrically tested. From fits of the current-voltage characteristics, we estimate the standard deviation in critical current to be around 25%. Voltage-magnetic field measurements exhibit a transfer function of 42 mV/mT and a linear response over a range of 303 μT at 71 K, resulting in a dynamic range of 124 dB.