YBaCuO Thin Films Research Articles

High-temperature superconductor quantum flux parametron for energy efficient logic

We report the fabrication and measurement of quantum flux parametron logic from high-transition-temperature Josephson junctions operating at 25 K, above the temperature of liquid helium. The circuits are written directly into the plane of a single-layer thin film of YBa2Cu3O7 using a focused helium ion beam. A single-cell quantum flux parametron was constructed and correct logic operation was verified by using an on-chip superconducting quantum interference device for readout. A three-cell shift register was also fabricated and tested using a three-phase clock cycle. We estimate the average bit energy to be about 1×10−20 J at 1 GHz.

Feasible Parameters of Ohmic Areas of YBaCuO Thin Films Switched via Moving Unstable Border between Superconducting and Normal States

A system of two equations based on one of the classical electricity laws was used to determine the sizes and temperatures of ohmic areas formed under action of overcritical nanosecond electrical pulses. Calculations were performed at five points for three experimentally obtained voltage–current (V-I) dependences for samples with the same geometry but different critical current density values. The system included two additional conditions to satisfy the known descriptive model of transition from superconducting (SC) to a normal (N) state—S-N switching—and to obtain physically acceptable solutions over the entire current range of V-I dependence. The solution for each point takes the form of a function, since the initial temperature increase of the primary channel across the film is entered as a parameter. Two modes of concentrated energy release in the channel were disclosed. Their random appearance leads to an unexpected degradation of the sample. As such, the obtained results correspond to the situations occurring during the experiments. The validity of applying additional conditions to the system is discussed. In the discussion, it is also explained at which moments the moving S-N border acquires the velocity of the order of ~106 m/s, comparable to the Fermi velocity. Consideration to describe the moving unstable S-N border as being constantly in a state of Richtmyer–Meshkov instability is presented.

Fabrication of C-Band High-Temperature Superconducting Microstrip Triplexer with High Accuracy

This article presents a miniaturized C-band triplexer fabricated with high-temperature superconducting (HTS) microstrip lines and to be applied to quantum computers. The triplexer consists of three bandpass filters and branch lines. In the design of the bandpass filters, folded step impedance resonators (SIRs) are firstly used to achieve good stopband performances by harmonic suppression. Secondly, cross-coupling structures are also utilized to further improve the passband edge steepness of the bandpass filters. Finally, the parasitic transmission zeros originating from the assembling box sizes are employed to achieve a deeper out-of-band rejection. Three bandpass filters are designed and connected to the common input port of the triplexer. After that, the triplexer is coarsely optimized by advanced design system (ADS) for time-saving and is finely optimized by Sonnet for better accuracy. The optimized layout of the triplexer is fabricated on a 2-inch diameter and 0.5 mm thickness MgO wafer; here, the MgO wafer is double-sided coated with HTS YBa2Cu3O7 (YBCO) thin films. The measured passband bandwidths (return loss greater than 12 dB) of the triplexer are 6.420-6.680 GHz, 6.780-7.030 GHz, and 7.110-7.380 GHz, respectively, which satisfy the required design specifications.

Detailed look into the unstable S-N border during propagation of the N-zone into the thin YBaCuO film

The focus is on the initial processes which change the shape of the bending S-N border during pulsed electrical S-N switching of thin YBaCuO films. These processes were noted on the SEM images of damaged films. The YBaCuO strip samples of high critical current densities were damaged by overcritical current pulses of nanosecond duration. The processes obtain different properties depending on the effective penetration length that the films have. During the N-zone propagation by the channel, led by the top unstable S-N border, an iterative transformation of the S-N border shape takes place. Two sets of the processes at the single transformation step, demonstrating weak and strong coherent jets action, are visualized and described. The origin of coherent jet formation, which is the driving force of the processes, is discussed. A peculiar interpretation of the moving top of the N-zone as a band in the state of Richtmayer-Meshkov instability is also proposed. It appears that devices with elements of the micro-scale dimensions should manifest the ultra-high speed of the S-N switching.

Design of High-Order Resonator HTS Diplexer with Very Different FBW

Adopting the deformed stepped impedance micro-strip line structure resonators (SIRs), the external weak coupling hairpin SIR (HSIR) and external strong coupling opening SIR (OSIR) resonators are designed, respectively. These two types of resonators are convenient for creating an ultra-narrow band and broadband filters, respectively, and can broaden the second harmonic passband. A high isolation diplexer without an impedance matching structure is realized by cascading 12-order and 14-order HTS filters composed of OSIR and HSIR resonators with a high isolation T-shaped structure. The diplexer is fabricated on a thin YBCO/MgO/YBCO (a MgO wafer with YBa2Cu3O7 thin film deposited on both sides) film with a dimension of 31.85 mm × 17.28 mm, a thickness of 0.5 mm, and a dielectric constant of 9.8. At 77 K, the measured central frequency of the diplexer is 2395 MHz and 3300 MHz, the fractional bandwidths (FBW) are 1.25% and 24.24%, the out-of-band rejections are greater than 60 dB/MHz, the 2f0 are located at 5.1 GHz and 6.6 GHz, the insertion loss is less than 0.20 dB, and the return losses are better than 16 dB and 15 dB. The diplexer has the advantages of a simple design method, compact structure, low insertion loss, high sideband rejection, and high isolation.

Synthesis and design of asymmetric cul-de-sac structure HTS filter with transmission zero introduced in Pairs

An asymmetric prototype of a cul-de-sac topology filter to introduce finite frequency transmission zeros (FTZs) in pairs with the least cross-coupling is synthesized and designed in this paper, which is suitable for the design of the high temperature superconducting (HTS) narrowband filter with high selectivity sideband suppression and symmetrical frequency response about the central frequency, especially. The resonator in the cul-de-sac structure filter can adopt the same frequency resonator without self-coupling so that the cul-de-sac structure filter can have fewer coupling coefficients and fewer resonator types. These characteristics of cul-de-sac structure filters are convenient for the design of high-order HTS filters with temperature shifts. A typical 7-order HTS filter with a central frequency of 4000 MHz, 5% relative bandwidth, five coupling coefficients, one cross-coupling, and one pair of FTZs is designed and fabricated on a 0.5 mm-thick MgO wafer with double-sided YBa2Cu3O7 (YBCO) thin films employing this cul-de-sac filter theory. The analyzed frequency response characteristic obtained with full-wave tools of this filter agrees well with the synthesis results and measured results at 77 K (Kelvin temperature).

Investigation of diethanolamine (DEA) as a chelating agent in the fabrication of fluorine-free propionate route YBa2Cu3O7 (YBCO) thin films

The role of diethanolamine (DEA) as a chelating agent was investigated in a fluorine-free precursor solution for the growth of superconducting YBa2Cu3O7 (YBCO) thin films via chemical solution deposition. Infrared spectroscopy and thermal analyses were employed to elucidate the interactions between the chelating agent and propionate-based coating solution. The physical properties of the as-obtained YBCO films were investigated to assess the effect of DEA addition on film growth. Special emphasis was placed on the determination of superconducting transport properties. These have been investigated in wide magnetic field (0–18 T) and temperature (4.5–77 K) ranges to account for various possible application scenarios. The 100 nm thick YBCO film deposited on a (001) SrTiO3 single-crystal substrate exhibited a critical current density of 4 MA cm−2 at 77 K in self-field.

Descriptive model of the transition from superconducting to normal state in thin high quality YBaCuO films by nanosecond electrical pulses

A descriptive model of normal zone (N-zone) propagation during the pulsed transition from superconducting to normal state (S-N switching) in the YBaCuO thin films is proposed. The model should explain the unexpected film damage. An ideal propagation of the N-zone was simulated using a combination of sinus-like functions, representing an unstable border between superconducting and normal states (S-N border). Plane geometry of the S film causes the concentration of current and expelled magnetic field at the film edges, which allows us to consider the S-N border as a separate object. Three cases, when the S-N border has effective penetration depth (λeff) of 2.45, 1.64, and 1.155 μm widths with accordingly different critical current densities and impact of coherent jets, are considered. To define numerical values of the S-N switching parameters, earlier obtained experimental parameters were taken. It was shown that the ideal propagation is insufficient to explain the damaging process. Based on the scanning electron microscope (SEM) images of damaged films, it is concluded that coherent jets and other magnetic hydrodynamics phenomena exist in the real N-zone propagation. During the propagation, closed S and N areas form and then are contracted, resulting in round dark spots visible in the SEM images. The conservation of the trapped magnetic flux is manifested during this contraction. Numerical calculations of contracting processes show that circular currents at λeff reach the Ginzburg-Landau de-pairing current densities. This explains the unexpected irreversible damage of the high quality thin YBaCuO films after the flash S-N switching.

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.

Measurement of Electrical Properties of Superconducting YBCO Thin Films in the VHF Range.

A new measurement technique of electrical parameters of superconducting thin films at the Very High Frequency (VHF) range is described, based on resonators with microstrip (MS) structures. The design of an optimal resonator was achieved, based on a thorough theoretical analysis, which is required for derivation of the exact configuration of the MS. A theoretical model is presented, from which an expression for the attenuation of a MS line can be derived. Accordingly, simulations were performed, and an optimal resonator for the VHF range was designed and implemented. Production constraints of YBa2Cu3O7 (YBCO) limited the diameter of the sapphire substrate to 3″. Therefore, a meander configuration was formed to fit the long λ/4 MS line on the wafer. By measuring the complex input reflection coefficients of a λ/4 resonator, we extracted the quality factor, which is mainly affected by the dielectric and conductor attenuations. The experimental results are well fitted by the theoretical model. The dielectric attenuation was calculated using the quasi-static analysis of the MS line. An identical copper resonator was produced and measured to compare the properties of the YBCO resonator in reference to the copper one. A quality factor of ~ was calculated for the YBCO resonator, three orders of magnitude larger than that of the copper resonator. The attenuation per unit length of the YBCO layer was smaller by more than five orders of magnitude than that of the copper.

Electric field-induced oxygen vacancies in YBa2Cu3O7.

The microscopic doping mechanism behind the superconductor-to-insulator transition of a thin film of YBa2Cu3O7 was recently identified as due to the migration of O atoms from the CuO chains of the film. Here, we employ density-functional theory calculations to study the evolution of the electronic structure of a slab of YBa2Cu3O7 in the presence of oxygen vacancies under the influence of an external electric field. We find that, under massive electric fields, isolated O atoms are pulled out of the surface consisting of CuO chains. As vacancies accumulate at the surface, a configuration with vacancies located in the chains inside the slab becomes energetically preferred, thus providing a driving force for O migration toward the surface. Regardless of the defect configuration studied, the electric field is always fully screened near the surface, thus negligibly affecting diffusion barriers across the film.

Depression of critical temperature due to residual strain induced by PLD deposition on YBa2Cu3O7-δ thin films

We present the study of a high Tc superconductor under residual strain. We study the growth of a series of thin films of YBa2Cu3O7−δ (YBCO) using the Pulsed Laser Deposition (PLD) technique. The samples exhibit a residual in-plane compressive strain, whose degree depends on the growth conditions and the thickness of the film, and which alters the superconducting behavior. A thorough characterization of the system, correlating structural and superconducting properties is presented. Raman spectroscopy is introduced to characterize the samples and rule out oxygen loss effects. A possible explanation for the nearly linear depression of the critical temperature Tc with c-axis expansion was introduced, in terms of the mutual Coulomb screening of charge fluctuations along the c-axis, between consecutive CuO2 planes in the structure.

S-N border instability, magnetic flux trapping and cumulative effect during pulsed S-N switching of high quality YBaCuO thin films

Unexpected irreversible damage appeared in samples while investigating pulsed S-N switching in YBaCuO thin films. Examination of a scanning electron microscope image of a damaged film containing a defect revealed a step-like motion of the N-zone. Restoration of crack movement dynamics points to the formation of coherent jets. This would mean that cumulative convergence of unknown incompressible media took place. The known model of an edge barrier, when the S-N border, characterized by width λeff, bounds a semi elliptical N-zone at the film edges, was applied for clarification of the cumulative convergence. In that model, counteracting forces cause a wave-like unstable form of the barrier S-N border, and further current growth increases the amplitude of wave-like distortions of the border. Then, current reconnection separates bulges from the N-zone and induces the appearance of the circular currents with a trapped magnetic flux. Heat release at the places with trapped flux cause an expansion in these locations and coherent jets originate from touch points due to cumulative convergence. We substantiate that specific properties of the S-N border at λeff, such as high tensile strength, very high flexibility and also incompressibility, are responsible for the appearance of the cumulative effect. Thus, the N-zone propagation into S state begins by motion of the wave-like border. The non-damaging step-like N-zone propagation into the films without defects is clarified. It is concluded that both the critical current and the damage current are determined by the same barrier with the unstable S-N border, and, therefore, the irreversible damage of the films comes quickly after the current reaches the critical value.

Dynamic properties of high-Tc superconducting nano-junctions made with a focused helium ion beam

The Josephson junction (JJ) is the corner stone of superconducting electronics and quantum information processing. While the technology for fabricating low Tc JJ is mature and delivers quantum circuits able to reach the “quantum supremacy”, the fabrication of reproducible and low-noise high-Tc JJ is still a challenge to be taken up. Here we report on noise properties at RF frequencies of recently introduced high-Tc Josephson nano-junctions fabricated by mean of a Helium ion beam focused at sub-nanometer scale on a YBa2Cu3O7 thin film. We show that their current-voltage characteristics follow the standard Resistively-Shunted-Junction (RSJ) circuit model, and that their characteristic frequency fc = (2e/h)IcRn reaches ~300 GHz at low temperature. Using the “detector response” method, we evidence that the Josephson oscillation linewidth is only limited by the thermal noise in the RSJ model for temperature ranging from T ~ 20 K to 75 K. At lower temperature and for the highest He irradiation dose, the shot noise contribution must also be taken into account when approaching the tunneling regime. We conclude that these Josephson nano-junctions present the lowest noise level possible, which makes them very promising for future applications in the microwave and terahertz regimes.

Dominant factors for the pinning enhancement by large artificial partial and complete antidots in superconducting films

Partially (blind) or fully perforated antidots (BADs or ADs, respectively) fabricated by laser lithography and ion beam etching in superconducting YBa2Cu3O7 (YBCO) films possess a cumulative circumference wall surface, which is found to be responsible for pinning vortices and/or magnetic flux, whereby increasing the critical current density (Jc) relative to corresponding plain (unpatterned) films. Using a variety of contrastingly shaped, relatively large 2–5 μm BADs and ADs, the Jc of YBCO thin films has been effectively increased within the relatively narrow band of wall surfaces of antidots, independent whether patterns are AD or BAD type. Independence of antidot types indicates that enhancement is insensitive whether magnetic flux is inside the ADs, or vortices are inside the BADs. Within this AD wall surface band region a clear shape dependence also emerges. This finding may also provide a guide for superconducting devices requiring maximal Jc and reduced associated vortex movement noise.

An HTS power switch using YBCO thin film controlled by AC magnetic field

This report is on the design of and results from a fast-acting, high power density high temperature superconducting switch. The switch uses thin films of YBa2Cu3O7−x on a sapphire substrate switched into the off-state through application of an AC magnetic field to the superconductor, which causes any transport current to experience a dynamic resistance. Results show reliable and repeatable switching of a 10 × 30 mm section of thin film between an on-state with Ic of 100 A and an off-state of 9 mΩ, with off-state voltage exceeding 100 mV. On-state to off-state transition time is less than 1 ms and off-state to on-state transition time is around 15 ms. These results are promising for further development of a high power, fast-acting superconducting power switching device for use in applications such as flux pumps.

Superconductive REBCO Thin Films and Their Nanocomposites: The Role of Rare-Earth Oxides in Promoting Sustainable Energy

The lossless transmission of direct electrical currents in superconductors is very often regarded as an ‘energy superhighway’ with greatly enhanced efficiency. With the discovery of high temperature superconductors (HTS) in the late eighties, the prospect of using these materials in efficient and advanced technological applications became very prominent. The elevated operating temperatures as compared to low temperature superconductors (LTS), relaxing cooling requirements and the gradual development of facile synthesis processes raised hopes for a broad breakthrough of superconductor technology. The impact of superconductor technology on the economy and energy sectors is predicted to be huge if these are utilized on a large scale. The development of superconducting tapes with high critical current density (Jc) is crucial for their use in transmission cables. Many countries these days are running projects to develop wires from these HTS materials and simultaneously field trials are being conducted to assess the feasibility of this technology. These HTS wires can carry electrical currents more than 100 times larger than their conventional counterparts with minimum loss of energy. The increased efficiency of HTS electric power products may result in greatly reduced carbon emissions compared to resulting from using the conventional alternatives. In order to use the thin films of YBa2Cu3O7- (YBCO) and REBCO [RE (rare-earth) = Sm, Gd, Eu etc.], the members of the HTS family, for future technological applications, enhancement of Jc over wide range of temperatures and applied magnetic fields is highly desired. The enhancement of Jc of YBCO and REBCO films has been successfully demonstrated by employing different techniques which include doping by rare earth atoms, incorporating nanoscale secondary phase inclusions into the REBCO film matrix, decoration of the substrate surface etc. which generate artificial pinning centers (APCs). In this review, the development of the materials engineering aspect that has been conducted over the last two decades to improve the current carrying capability of HTS thin films is presented. The effect of controlled incorporation of APCs through various methods and techniques on the superconducting properties of YBCO and REBCO thin films is presented, heading towards superior performance of such superconducting thin films.

Activation energy of oxygen diffusion: A possible indicator of supercurrents through YBa2Cu3O7 grain boundaries

Oxygen diffusion is the main transportation form during the preparation process of polycrystalline superconductor YBa2Cu3O7 (YBCO) thin films. Especially the diffusion of oxygen ions in grain boundaries (GBs) can lead to the formation of oxygen vacancies, and finally have an impact on the critical current density (Jc) of YBCO conductors. To deeply understand the supercurrent-limiting mechanism of GBs, for an YBCO bicrystal (containing a single GB), in this article we intensively studied the diffusion of oxygen ions through GB region by molecular dynamics simulations. The calculated results show that the oxygen diffusion in the YBCO bicrystals can be enhanced in GBs, and specifically which is closely related to the grain boundary angle (θ). The most interesting finding is that the activation energy of oxygen diffusion shows an exponential fall with the grain boundary angle, which is exactly consistent with the relation of the critical current density versus the grain boundary angle (Jc ~ θ) in GBs. We firmly believe that the activation energy of oxygen diffusion should have an essential relevance with the Jc especially in YBCO GBs, and can be as an indicator for predicting supercurrents. For these results we further gave an in-depth discussion.

Accelerated growth by flash heating of high critical current trifluoroacetate solution derived epitaxial superconducting YBa2Cu3O7 films

Flash heating reduces the precursor nanoparticle coarsening and shortens the growth time of epitaxial YBa2Cu3O7 thin films.

Enhancing the critical current of YBa2Cu3O7 thin films by substrate nanoengineering

The critical current density, Jc, can be increased by introducing defects throughout YBa2Cu3O7 superconducting thin films. We propose a new approach of substrate nanoengineering to produce well-controlled defects. LaAlO3 substrates have been ion-etched with different 34 nm deep patterns prior to the deposition of the films. An annealing step at 1000 °C after substrate etching has been undertaken to negate the Ar-ion damage to the surface. The Jc of the so-prepared samples measured at different temperatures has been compared to thin films deposited on plain substrates at the same time. In general, an increase in Jc is observed across all temperatures and fields as a result of both patterning and annealing. In particular, at zero field and 85 K, the largest enhancement of >40% has been recorded for circle and triangle patterns. This new substrate nanoengineering technique is very promising for flux trapping of superconducting devices, particularly because an increase to critical current (Ic) also occurs. Further optimization of depth, size, and shape of the patterns is expected to produce further improvements to Jc.