High-power HTS microstrip filters for wireless communication

We developed a novel nondestructive and contactless method for measurements of critical current density (J/sub c/) in high-temperature superconductor (HTS) thin films by using a permanent magnet (Sm/sub 2/Co/sub 17/). This measurement technique is based on a repulsive force (F/sub r/) and attractive force (F/sub a/) between the magnet and HTS film caused by the shielding currents and pinning forces. The F/sub r/ and F/sub a/ were measured by using a high-resolution load sensor changing the distance (L) between the magnet and HTS film. An effective maximum repulsive force (F/sub m,eff/) could be determined from an extrapolated value of the F/sub r/ vs. L curve for L=0 mm. We investigated the relationship of the J/sub c/ to the F/sub m,eff/ in many HTS (YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// and ErBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//) samples. The standard J/sub c/ of HTS films was measured by using an inductive measurement system produced from THEVA GmbH. We found that the J/sub c/ is almost proportional to the two-thirds power of the F/sub m,eff/ normalized by the film thickness. This result indicates that the simple permanent magnet technique can be easily estimate the J/sub c/ of the HTS films. This system has good reproducibility for the J/sub c/ measurements and is useful to provide a rapid, routine characterization of the superconductivity in the HTS materials.

Superconducting quantum interference devices (SQUIDs), which are the most ultrasensitive magnetic sensors, are generally made by microfabrication of superconducting thin films. The bicrystal junction, which is often used in high-temperature superconducting (HTS) Josephson junctions (JJs), has problems expensive bicrystal substrates, and the fact that JJs can only be arranged in a fixed straight line [1-2]. Because the HTS thin film is a type-II superconductor, magnetic fluxes penetrates the HTS thin film and moves for the Lorentz force, which increases the flux noise at low frequencies.In this study, we investigated a method to fabricate low-noise SQUID magnetic sensors by Ga focused ion beam (FIB) irradiation of high-temperature superconducting thin films. When HTS thin film is irradiated with FIB, point defects are introduced into the HTS crystal structure, which leads to changing into normal conduction. The dependence of the critical current change ratio IC/IC0 on the dose was shown in Fig.1. The dependence of the normal resistance RN are also depicted. The thickness of the YBa2Cu3O7-δ (YBCO) film was 50 nm and it was protected by a thin Au layer. IC/IC0 decreased with irradiation fluence and changed into normal conduction at 2.0×1017 ions/cm2; after this point RN increased steeply.HTS nanobridge JJ, were fabricated by FIB irradiation. Figure 2 shows the V-I characteristics of a nanobridge with an HTS film thickness of 50 nm and a bridge width of 120 nm. The critical current was reduced to 264.5 μA and shows a clear Shapiro-steps under microwave irradiation with frequency of 10 GHz. As a method to reduce the magnetic flux noise, we also investigated the introducing of normal conducting pinning sites (anti-dots) into HTS thin films by FIB irradiation. We have confirmed that anti-dots with diameters of several hundred nm can be formed by point irradiation of HTS thin films with FIB. In the future, we will confirm SQUID operation at a nanobridge JJ with an YBCO film thickness of 50 nm and investigate the improvement of SQUID noise characteristics by introducing anti-dots.  Fig. 1. Dependence of IC/IC0 and RN on FIB irradiation fluence.  Fig. 2. V-I characteristics of the HTS nanobridge with a width of 120 nm.

Higher usage of the mobile phone and internet services has increased the demands on the mobile frequency spectrum resulting in the congestion of the frequency bands used for the wireless communication systems. Consequently the adjacent channel interference and hence the number of dropout calls have increased. High Temperature Superconducting (HTS) microstrip filters enable the decrease in the adjacent channel interference, improvement of the audio quality and increase of the sensitivity of wireless communication receivers. Improvements in the performance of microwave HTS devices directly depend on lowering the surface resistance of the HTS thin films. Therefore it is important to fabricate HTS thin films with the lowest possible surface resistance. In general, microwave losses of HTS films depend on several factors such as the substrate material and deposition technique. In this paper we report results on YBa/sub 2/Cu/sub 3/O/sub 7-d/ (YBCO) thin films deposited on Al/sub 2/O/sub 3/ substrates with CeO/sub 2/ sub layer. We have used a microstrip resonator for reflection parameter measurements for data processing. We have carried out the measurements of microstrip meander resonators quality factor at frequency 1.8 GHz. Resonators are prepared by the wet etching technique, from the YBCO films, with sapphire substrate and CeO/sub 2/ sub layer, which is obtained by the laser ablation method. In our investigation we have used a single-port measurement method (using reflection factor).

A numerical code has been developed for analyzing the shielding current density in a High-Temperature Superconducting (HTS) thin film. By using the code, the inductive method has been successfully reproduced even when the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> -dependence of the critical current density is taken into account. The results of computations show that the outer radius of the coil must be nearly equal to or less than the sample radius to ensure the sufficient accuracy of the inductive method. Furthermore, Mawatari's conjecture on the third-harmonic voltage generation is justified by use of the numerical simulation.

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.

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

Pulsed laser deposition (PLD) has been used to deposit high quality thin films of Ni81Fe19/Au/YBa2Cu3O7- (delta ) onto (100) oriented substrates of MgO and SrTiO3 for the purpose of fabricating a novel high temperature superconducting three terminal device. The ferromagnet-normal metal-superconductor (F-N-S) structure is currently being investigated to determine the effect of the injection of a spin-polarized current on the order parameter of a high temperature superconducting thin film. High quality films with sharp interfaces, free of defects, are required in order to maximize the spin-injection effect. The surface morphology and transport properties of the YBa2Cu3O7-(delta ) films have been investigated using scanning electron microscopy and ac susceptibility measurements, respectively, as a function of increasing laser fluence. Deposition at 2.0 - 2.4 J/cm2, 790 degrees Celsius and 320 m Torr O2 produces films with a sharp superconducting transition and a smooth surface. The growth of Au on YBCO under different PLD conditions has been observed by atomic force microscopy. Surface clustering of Au occurs at elevated temperatures and is attributed to increased surface mobility. The presence or absence of a background gas influences the cluster size. These results are discussed within the framework of the role of excess energy of PLD adatoms with changing laser fluence and background gas.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this paper a macroscopic phenomenological model for the microwave properties of superconductors is presented. The model is based on the idea that there are two kinds of current carriers, and instead of the first London's equation a new equation is derived. This model can be applied to both low- and high-temperature superconductors. Using this model, an expression for the microwave surface resistance is derived and the surface resistance versus frequency is calculated. The results show that the relation between resistance and frequency is not R/sub 3//spl sim//spl omega//sup 2/ as indicated by both BCS theory and London model, but R/sub s//spl sim//spl omega//sup a/, where a is between 1 and 2 (e.g. a=1.35) for thin film high-T/sub c/ superconductors YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//. The temperature dependence of R/sub s/ is simulated using the given model. These relations and the values of the surface resistance agree well with experimental results. A residual resistance may be interpreted from this model.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

We report on the application of Si and Al ion-implantation patterning to the fabrication of low-loss microwave transmission lines in high-temperature superconductor (HTS) thin films. Using this technique, we have fabricated coplanar waveguide (CPW) transmission lines in YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) thin films deposited on LaAlO/sub 3/ substrates. We have used both resonant and broadband measurements in order to characterize the performance of the resulting transmission line structures. For the broadband measurements, on-wafer calibrations were used to obtain accurate S-parameters and transmission line propagation constants up to 25 GHz. The propagation constants of the ion-implanted transmission lines do not differ significantly from those of lines patterned using conventional ion milling over the frequency range studied, with a value for the attenuation constant of approximately 0.03-0.04 dB/cm at 50 K and 10 GHz. The relatively low losses of the ion-implanted devices demonstrate the effectiveness of this method of patterning for HTS microwave device fabrication.

We study the task of transforming a hard function f, with which any small circuit disagrees on (1 - /spl delta/)/2 fraction of the input, into a harder function f', with which any small circuit disagrees on (1 - /spl delta//sup k/)/2 fraction of the input, for /spl delta/ /spl isin/ (0,1) and k /spl isin/ /spl Nopf/. We show that this process cannot be carried out in a black-box way by a circuit of depth d and size 2/sup o(k2/d)/ or by a nondeterministic circuit of size o(k/log k) (and arbitrary depth). In particular, for k = 2/sup /spl Omega/(n)/, such hardness amplification cannot be done in ATIME(O(1), 2/sup o(n)/. Therefore, hardness amplification in general requires a high complexity. Furthermore, we show that even without any restriction on the complexity of the amplification procedure, such a black-box hardness amplification must be inherently non-uniform in the following sense. Given as an oracle any algorithm which agrees with f' on (1 - /spl delta//sup k/)/2 fraction of the input, we still need an additional advice of length /spl Omega/(k log(1//spl delta/)) in order to compute f correctly on (1 - /spl delta/)/2 fraction of the input. Therefore, to guarantee the hardness, even against uniform machines, of the function f', one has to start with a function f which is hard against non-uniform circuits. Finally, we derive similar lower bounds for any black-box construction of pseudorandom generators from hard functions.

The insufficient power-handling capability of high-temperature superconducting microwave filters has been a serious barrier to their application. To clarify the key parameters for improving the power-handling capability of rf filters based on high-temperature superconductors with microstrip structures, we synthesized bandpass filters with different layouts using several kinds of thin film high-temperature superconductors, and performed third-order intermodulation measurements on them. The experimental results indicate that increasing the film thickness and utilizing MBE-grown films of NdBa/sub 2/Cu/sub 3/O/sub 7/ films are effective in obtaining microstrip filters with high power-handling capability.

This chapter introduces the use of substrates for high-temperature superconductor (HTSC) thin-film deposition beyond their usual purpose as chemically inert, lattice-matched support for the films. Substrates are used as functional elements in order to controllably modify the growth mode of YBa 2 Cu 3 O 7 thin films in the case of vicinal-cut SrTiO 3 single crystals or to locally modify the surface morphology and film/substrate lattice mismatch in the case of ion implanted SrTiO 3 . Furthermore, the use of biaxial epitaxial strain is briefly reviewed for HTSC thin films, and the application of this concept to tailor the properties of perovskite thin films in general is shown.

A high temperature superconductor (HTS) microstrip line structure with gilded edges is proposed for improving its power handling capability based on a specified limit of harmonic generation. In this microstrip line structure two narrow and thin gold strips, which have a regular constant (linear) resistivity, are added (gilded) on the edges of the top surface of the HTS strip. A general modeling approach is used. The approach includes the moment method, the multi-port network theory, and the harmonic balance technique. Harmonic power generation and conductor loss are presented.

Summary form only given. The feasibility of cavity-controlled spontaneous emission of a motionless two level atom is analyzed quantum mechanically and nonperturbatively. It is proposed to control the emission by putting the excited atom into the combined field of two lossless modes with their frequencies /spl omega//sub 2,1/ detuned symmetrically about the atomic one /spl omega//sub /spl alpha//, i.e. /spl omega//sub 2,1/=/spl omega//sub /spl alpha///spl plusmn//spl Delta/. The controlling /spl omega//sub 2,1/ modes are isolated from the free radiation field by an one-dimensional ideal cavity (or by two crossed ones) at zero temperature. These modes are taken to be linearly polarized in the same plane. Because the wavelength and cavity dimensions are assumed to be of the same order, the retardation effects are neglected. The spontaneous emission is treated as a radiative decay of the macromolecule-like atom+/spl omega//sub 2,1/ modes-subsystem (the H/sub s/-one) under its interaction with the free H/sub k/-field.

We study the space complexity of refuting unsatisfiable random k-CNFs in the resolution proof system. We prove that for any large enough /spl Delta/, with high probability a random k-CNF over n variables and /spl Delta/n clauses requires resolution clause space of /spl Omega/(n/spl middot//spl Delta//sup -1+/spl epsiv//k-2-/spl epsiv//), for any 0 >/spl radic/n. This bound is nearly tight. Specifically, we show that with high probability, a random 3-CNF with /spl Delta/n clauses requires tree-like refutation size of exp(/spl Omega/(n//spl Delta//sup 1+/spl epsiv//1-/spl epsiv//)), for any 0</spl epsiv/<1/2. Our space lower bound is the consequence of three main contributions. 1. We introduce a 2-player matching game on bipartite graphs G to prove that there are no perfect matchings in G. 2. We reduce lower bounds for the clause space of a formula F in resolution to lower bounds for the complexity of the game played on the bipartite graph G(F) associated with F. 3. We prove that the complexity of the game is large whenever G is an expander graph. Finally, a simple probabilistic analysis shows that for a random formula F, with high probability G(F) is an expander. We also extend our result to the case of G-PHP, a generalization of the pigeonhole principle based on bipartite graphs G. We prove that the clause space for G-PHP can be reduced to the game complexity on G.