We have fabricated YBa₂Cu₃O_7-(delta ) (YBCO) superconducting line resonator and tunable line resonator on ferroelectric Sr_0.5Ba_0.5TiO₃ (SBTO) buffered MgO(100) substrate and discussed the frequency shift mechanism of superconductor as a function of temperature and bias voltage, respectively. The resonators were designed using superconducting YBCO epitaxial thin films. Optimized resonator shown the resonant frequency of 10 GHz at 77 K. The YBCO films were grown in situ by pulsed laser deposition technique at 750 degree(s)C and oxygen partial pressure of 200 mTorr. The resonators have linear microstrip line separated by a gap of 5 micrometers and 0.5 mm, respectively. A gap is intentionally introduced to generate mainly a capacitive series reactance. The equivalence circuit of line resonator is a II network consisted of three capacitances. As the series capacitance C₁₂ of SBTO ferroelectric thin films was changed by a bias voltage applied on the strip conductors including the gap, resonance frequency was shifted about 20 MHz from the unbiased center frequency of 10 GHz. The variation of resonance peak could be explained by a serial capacitance model. To find a central frequency mechanism depending on temperature, we fit the raw data using f(T)/f(10 K) and simple power law model. The shifting of the resonant frequencies due to temperature was fit to a two-fluid model, BCS theory and empirical formula. Also the surface impedance of superconducting YBCO films as a function of temperature at 10 GHz has been estimated by a transmission line method.

We have designed the microstrip-type multipole (7-pole and 9-pole) lowpass filters consisting of both transmission lines and open stubs. The filters were fabricated on high temperature superconducting (HTS) YBa₂Cu₃O_7-(delta ) (YBCO) thin films grown on MgO(100) substrates by pulsed laser deposition. For 7-pole lowpass filter, the measured insertion losses were within 0.5 dB, and up to 8 GHz the passband shows very flat with ripples of less than 0.05 dB. For 9-pole filter, we observed the insertion loss of 5.0 dB and the ripples of 0.64 dB. The skirt became steep and the off-band rejection increased large as the number of poles increases.

A new bandpass filter design is presented which uses arbitrary image impedance of the superconducting thin film coupled lines. In order to investigate the performance of different superconducting bandpass filters with different image impedances, we have selected and designed 4-pole Chebyshev 50 (Omega) and 75 (Omega) coupled line superconducting bandpass filters which have the same passband of 11.7 approximately 12.4 GHz. The measured result of the 75 (Omega) coupled line superconducting bandpass filter agrees with the result of the 50 (Omega) coupled line superconducting bandpass filter. The technique used in this paper to change the filter structure is particularly attractive because of its application to improve the power-handling ability of superconducting passive device and its possibility to reduce the size of superconducting thin film microwave device which has the limitation of superconducting thin film size.

The full wave analysis in the spectral domain of the Transverse Transmission Line (TTL) is used in the analysis of the shielded and open two layers microstrip lines considering the superconductor strip and semiconductor substrate. The superconductor effect is included with the boundary condition of the surface impedance that is calculated from an advanced two-fluid model. Considering that the thickness of the strip is more thin than the penetration depth, the surface current and the fields within the strip are approximately uniforms. For a substrate with high permitivity, the density of current will be practically equal to current in the bottom of the strip. Applying the Moment method is obtained the homogeneous matrix system, and the complex root of the characteristic matrix is the complex propagation constant of the structure, including the phase constant and the attenuation constant. Results are presented for the complex propagation constant, of the shielded and open two layers superconducting microstrip lines versus the frequency and the temperature, where the effects of the material superconductor are shown.

We present a brief description of the role played by the motion of magnetic vortices in the power dependence and non-linearity of high T_c superconductors at rf and microwave frequencies. We then review the current understanding of vortex motion at rf and microwave frequencies, and present broadband (45 MHz - 50 GHz) experimental results which shows a striking crossover in the behavior of the vortex dynamics from a low-frequency interaction- dominated regime, to a high frequency essentially single-particle regime. Finally, we discuss the impact these different regimes of vortex motion have on the design and operation of high T_c rf and microwave devices.

This article introduces a theoretical model for analyzing nonlinear electrodynamic effects induced by flux-motion in anisotropic type-II superconductors. The model is phenomenological and is based on the continuum analysis of flux-dynamics of type-II superconductors in the mixed state, which accounts for both the critical-state effect and material anisotropic effect. Besides, a complete set of nonlinear field equations and effective constitutive equations are formulated in combination with generalized London equations and Maxwell's equations, where the incomplete Meissner effect and the normal conduction fluid effect are also taken into account. In contrast to the well-known Bean's critical-state model, the new model may account for the effect of localized vibration of flux-lines in ac fields and the possible effect of inertia of flux-lines in the superconductor, which may be of particular interest in studying microwave applications of some type-II superconductors at extremely high- frequencies. By analyzing quantitatively the vortex dynamic effects, we propose some modifications of Bean's critical-state model in treating electrodynamic problems of type-II superconductors in ac fields of extremely high frequencies.

Two tone tests performed on resonators and filters fabricated from YBCO thick film coated, 3D resonators, indicate the importance of understanding intermodulation distortion. The films in this investigation were produced by reactive texturing and melt processing. The generation of third order intermodulation (IMD) products was observed at elevated rf power levels. Correlation between IMD level and rf field dependence of surface impedance is demonstrated. The contribution of grain boundaries to IMD is discussed in terms of the resistively shunted junction model.

The thermal expansion of Nb₃Sn superconducting composites has been investigated with newly developed electro-optical measurement technique. In this method, deformation of the sample is recorded with two CCD cameras equipped with long distance micro objective lenses. Image processing is accomplished with a high resolution frame grabber and an auxiliary high speed processor board. The linear thermal expansion data of two different samples are reported from room temperature down to 73 K. The results are compared with the linear thermal expansion of copper and Nb₃Sn, the main components of the composite itself. The temperature dependence of (Delta) L/L at low temperature has been generated. A simple analytical method was used to evaluate the values of the mean thermal expansion coefficient ((alpha) ). Good agreement has been found between the experimental results and the analytically evaluated values.

Quality factor and surface resistance of high-T_c superconductor thick film microstrip resonators have been studied at various cryogenic temperatures. Superconducting microstrip line resonators made of YBa₂Cu₃O_7-d (YBCO) material were prepared by screen printing on polycrystalline yttrium stabilized zirconium oxide substrates. Equivalent resonators mae of thick film gold material (ESL 8880-H) were prepared on the same substrate material to determine the resonator geometry factor (G) and by means of it the surface resistance (R_S) of the superconducting resonators. Both conductor patterns and ground planes of the superconducting resonators were made of YBCO material. The resonators were measured in the frequency range 2 GHz - 7 GHz and the temperature range 30 K - 90 K (the gold resonators were measured up to 300 K). At their best, the superconducting resonators showed significant improvement in unloaded quality factor Q₀ (over three times higher) and over 6 dB lower insertion loss values compared to identical thick film gold resonators at 40 K - 50 K temperatures. The crossover temperature was frequency dependent: for example, temperatures of 62 K at 2.3 GHz, 71 K at 4.6 GHz and 55 K at 6.8 GHz frequencies were observed. The surface resistance of the YBCO resonators was calculated at the fundamental, the second and the third harmonic resonant frequencies at 40 K and 77 K. To achieve significant improvement in loss characteristics over thick film gold resonators at these frequencies, operation near 40 K temperatures is required.

Undoped YBa₂Cu₃O_7-(delta ) (YBCO) and 5 wt.% Au doped YBCO thin films were deposited by laser ablation technique on < 100 > LaAlO₃. The stability of the films under temperature stress and 70% relative humidity was studied using accelerated aging tests. Results obtained indicate that the Au-doped films show higher stability than undoped films.

The surface resistance, R_s, at microwave frequencies has been an important qualification parameter for high temperature superconductor (HTS) thin films. HTS thin films with low R_s have been realized on many substrates, and many groups have realized R_s values in the range 300 - 400 (mu) (Omega) at 10 GHz at 77 K with YBa₂Cu₃O_7-(delta ) (YBCO) films on <100> LaAlO₃ substrates. Both microstrip resonator and parallel plate resonator techniques are being used to measure R_s values of HTS thin films. It has been observed that the value of R_s at given frequency and temperature critically depends upon the epitaxial quality and granularity of the films. For example, YBCO films grown on <100> MgO have been found to be granular and weak link limited with a significant microwave power dependence of R_s. On the other hand, YBCO films insitu grown on <100> LaAlO₃ have shown better epitaxy with low R_s. This is obviously due to the much better lattice match of YBCO with <100> LaAlO₃ if the targets used for laser deposition are doped with Ag. Extensive work carried out in our laboratory has shown that a Ag-doping level of around 5 wt.% in YBCO is the optimum which results in YBCO films of much improved quality. We have realized Ag-doped YBCO films with J_c values of 6 - 8 X 10⁶ Acm^-2 at 77 K and a low R_s value of 210 (mu) (Omega) at 10 GHz at 77 K on <100> LaAlO₃. Both these values are the best realized on LaAlO₃ to date. What is equally important is the fact that with Ag-doping the reproducibility of the epitaxial quality of the films improves significantly. This has been found to be due to the enhanced oxygenation of films during growth and the surfactant effect of Ag. Experiments have shown that even the optimum temperature for insitu growth in reduced considerably by Ag-doping. It must be mentioned, however, that the only negative aspect of Ag-doping is the higher microwave residual surface resistance, R_res, observed in these films at (very) low temperatures. This is obviously due to the presence of Ag in the grain boundaries which gives rise to a metallic contribution to R_s which appears as R_res. Nevertheless, Ag-doped YBCO films have been found to be superior to undoped films both due to their higher stability and lower R_s at 77 K, a temperature at which HTS devices are expected to operate.

We have characterized thin film YBa₂Cu₃O_7-(delta ) (YBCO) bicrystal grain boundary junctions by a non-contact microwave absorption method which is based on the study of dynamic flux quantization in these junctions using the field-modulated-microwave absorption technique. This method is capable of probing the inhomogeneity of coupling in the Josephson junctions. The first derivative of the microwave absorption (dP/dB) as a function of B_dc was measured in a TE₁₀₂ microwave cavity at 9.5 GHz. Epitaxial films of YBCO were deposited by laser ablation on 24 degree(s) bicrystal substrates of SrTiO(subscript 3$. Lengths of the investigated junctions ranged from 100 micrometers to 2 mm. We observed series of well-resolved absorption lines as a function of externally applied dc magnetic field. The nonuniformity of the grain boundary junction coupling was determined using analysis based on the line profile, the amplitude modulation period and microwave power dependence of the series. A physical model, based on the microwave absorption analysis in a single non-uniform Josephson junction was used for the data interpretation. The results of our measurements agree with a model in which the grain boundary consists of several parallel subjunctions.