4PO9-30 Third-order local nonlinear microwave response of YBa₂Cu₃0₇

Evgeny E. Pestov, Vladislav V. Kurin, Yury N. Nozdrin, and Michael G. Rzhavin, Institute for Physics of Microstructures, Russian Academy of Sciences, 603600, Nizhny Novgorod, GSP-105, Russia

Presenting Author: E.E. Pestov

HTS thin film continue to be of interest for passive device application at microwave frequencies, but nonlinear effects may limit the performance. We report a set of experimental data on local third-harmonic generation at microwave frequencies (0.5 GHz) in YBa₂Cu₃0₇ from 77 K to T_c. For the local investigation of the nonlinear response the probe with inductive coupling was elaborated. The probe consists of 50 m m wire connecting the inner and outer conductor of the coaxial cable. It is plotted the map of nonlinear microwave response of HTS film YBa₂Cu₃0₇ below T_c with high resolution. The third-harmonic power is measured as a function of temperature, input power and dc magnetic field at some places of the film. The temperature dependence demonstrates a peak of nonlinearity below T_c. The behavior of the third-harmonic power can be described the power-low with exponent n~1.5-2.5. Also it is shown relation between critical current density J_c and nonlinear microwave response.

4PO9-31 Nonlinear Surface Resistance of YBCO Thin Films:Defects Dependent Power Handling Capability Defects Dependent Power Handling Capability

Jaroslaw Wosik, Lei-Ming Xie, and Stuart A. Long, Texas Center for Superconductivity at University of Houston and Electrical and Computer Engineering Department, University of Houston, Houston, TX 77204-5932, USA

Presenting Author: J. Wosik

We present an overview of high power microwave losses in high quality YBCO epitaxial superconducting thin films. The YBCO films measured in this experiment were deposited onto MgO, LaAlO₃, NdGaO₃ and/or R-cut sapphire substrates by the thermal reactive co-evaporation (Theva) or sputtering (DuPont Superconductivity Center) methods. Microwave power-handling capabilities have been investigated using a 14 GHz shielded sapphire cavity, up to 150 W of the input power. Both frequency and time domain methods of surface impedance measurements were used.

To simulate an electromagnetic cavity response a nonlinear R(P)-L(P)-C equivalent circuit model and a Fabry-Perot resonator model, including a cavity heat transfer features, were used for frequency domain and time domain methods, respectively. The analysis of experimental data showed, that three types of surface resistance (R_s) on rf power dependencies can be distinguished: (1) R_s is linear up to very high B_rf values (50 mT); (2) R_s is linear up to medium B_rf values, above which R_s increase is proportional to B_2rf and (3) when R_s proportionality to B_2rf begins at low rf fields and in addition, no rf breakdown can be observed. The correlation between presence of certain type of weak links in films and R_s(B_rf) dependencies will be discussed. Also nonlinearity related to global and local heating on defects will be shown.

4PO9-32 Microwave losses induced by vortex propagation in highly epitaxial YBaCuO films

R. Monaco 1, M. Boffa 2, C. Beneduce 2, A.M. Cucolo 2, and M.C. Cucolo 2. 1 Istituto di Cibernetica del CNR, Italy. 2 Dipartimento di Fisica, Universita' di Salerno, Italy.

Presenting Author: R.L. Monaco

The rf and microwave losses of devices based on YBaCuO superconducting films are dominated by extrinsic mechanisms that can be controlled by a careful device fabrication and design. Often the granular nature of the films shows up in the measurements of the residual surface resistance. We have measured the surface impedance of a large number of highly epitaxial, dc sputtered YBaCuO films patterned by argon ion milling as meanderline microstrip resonator (T_c>90K and D T_c<1K). The plot of R_s vs. H_rf shows that the excess surface resistance is determined by Josephson vortices propagating in the grain boundaries, in good agreement with the theoretical prediction of Halbritter.

Further, the temperature and frequency dependence of Z_s is consistent with this hypotesis. We also correlate the measurements of Z_s with the measurements of two tone, third order intermodulation products.

4PO9-33 Microwave intermodulation study of superconducting films in the presence of an external dc field

R. Monaco 1, A. Andreone 2, A. Cassinese 2, F. Palomba 2, and G. Pica. 1 Istituto di Cibernetica del CNR, Arco Felice, Italy. 2 I.N.F.M. and Dipartimento di Scienze Fisiche, Universita' di Napoli Federico II, Piazzale Tecchio 80, I-80125, Naples, Italy.

Presenting Author: R. Monaco

Microwave devices based on High Temperature Superconducting (HTS) films are superior in performance compared to similar non-superconducting devices but at low microwave power only. Their performance degrades drastically at high power, due to the non linear response of superconducting films. Many techniques can be used to quantify this non linearity in practical devices, such as power response, harmonic generation, or two-tone intermodulation (IM). The last one arises when two CW signals at frequencies f1 and f2 mix up together and generate outputs at 2f1-f2 and 2f2-f1. Intermodulation is known to be the most sensitive probe of non linearity in the microwave response of superconducting materials and useful in the investigation of the symmetry of the wave function in HTS too. We report on detailed IM measurements carried out on microstrip resonators made of both high and low T_c superconducting thin films. The effect of an external dc field on intermodulation signals is investigated too. The data show that, the presence of a dc field affects the amplitude of the IM products consistently with changes of the surface impedance. The data are discussed in the context of conventional and unconventional theories.

4PO9-34 Microwave Power-Induced Switching of Josephson Junctions Weak Links in HTS Thin Films

Lei-Ming P. Xie and Jaroslaw Wosik, Texas Center for Superconductivity at University of Houston, and Electrical and Computer Engineering Dept., University of Houston, Houston, TX 77204-5932, USA

Presenting Author: L.-M. Xie

Using a 14 GHz dielectric cavity and time domain technique, the microwave power-handling capabilities of YBa₂Cu₃O_x thin superconducting films have been measured. The YBCO films measured in this experiment were deposited onto LaAlO₃, NdGaO₃ and/or R-cut sapphire substrates by the thermal reactive co-evaporation or dc sputtering methods. For some of the films, measured surface resistance on rf magnetic field (R_s(B_rf)) dependence, showed sudden increase at certain power levels. A microwave dielectric cavity model of the electromagnetic response of the dielectric cavity to a pulsed input signal, combined with a cavity heat transfer model, was used to analyze this effect. It was found that such sudden microwave loss increase is due to the normal state switching of in-grown strong single Josephson junction (JJ). It indicates that even strong single JJ limits power handling capability of YBCO thin films. By analyzing time domain experimental data we have found that junction switching is induced by combination of microwave power and decrease of junction critical current due to junction heating. The same model was used to interpret experimental results for 24 degree bicrystal junction with well known junction parameters. Influence on power handling capability of similar defects, but with distributed junction parameters, will be shown and discussed.

4PO9-35 Effect of Surface Smoothing on Microwave Properties of YBCO Superconducting Thin Films

Rong Liu, Dariush Fathy, Zu-Hua Zhang, Hong-Yin Zhai, Jaroslaw Wosik, and Wei-Kan Chu, Texas Center for Superconductivity at the University of Houston, Houston, TX 77204-5932, USA

Presenting Author: R. Liu

We report on measurements of microwave properties of epitaxial YBa₂Cu₃O₇ (YBCO) films before and after surface smoothing. The YBCO films measured in these experiments were deposited onto MgO and LaAlO₃, substrates by the thermal reactive co-evaporation (Theva). The smoothing was done using argon ion 20 keV cluster beam with dose of 6.6x10¹⁵/cm². After smoothing the films were also annealed in oxygen at temperature of 480°C and at pressure of 600 Torr. Surface roughness of the films was measured before and after smoothing by atomic forced microscope (AFM). The surface resistance measurements were carried out using both frequency-domain and pulse time-domain techniques in 14GHz TE₀₁₁ shielded dielectric cavity.

At low rf power levels, some improvement in R_s versus temperature dependence for both smoothed (four times decreased roughness) and non-smoothed films was observed. However, at higher power levels, for smoothed films onset of nonlinearity of R_s on microwave power depends on film microstructure. For the films on MgO substrate, the nonlinearity onset in the surface resistance occurred at lower rf power than for the virgin film. The observed effects were explained in the frame of presence of weak links damaged by the bombardments. The correlation between microwave performance after smoothing and surface morphology and types of defects will be discussed.

4PO9-36 Study of Correlation between the Microstructure and Phase Inhomogeneities of Y-Ba-Cu-O Epitaxial Films and their dc and Microwave Properties

A.K. Vorobiev 1, Y.N. Drozdov 1, S.A. Gusev 1, V.L. Mironov 1, N.V. Vostokov 1, E.B. Kluenkov 1, S.V. Gaponov 1, and V.V. Talanov 2. 1 Institute for Physics of Microstructures RAS, 603600, Nizhny Novgorod, GSP-105, Russia. 2 University of Maryland, Center for Superconductivity Research, College Park, MD 20742, USA.

Presenting Author: A.K. Vorobiev

The influence of various kinds of structure and phase inhomogeneities on electrical properties of YBCO thin films prepared by inverted cylindrical magnetron sputtering has been investigated. A simultaneous analysis of the changes in the microstructure and electrical parameters has allowed to define a contribution of each kind of inhomogeneities in the restriction of the films electrical parameters. It has been found out that the transition temperature depends mainly on deviations of the c-axis lattice parameter from its optimum value, the critical current density depends mainly on the out of plain misorientation of the film domains, and the microwave surface resistance depends mainly on the volume of the high-angle misoriented domains. By using an atomic force and scanning tunneling microscopy the regions of YBCO films with modified properties around Cu-rich secondary phase particles were observed. It is suggested that superconducting properties of these regions are worse than the matrix due to Cu-depletion. Usually the relative area of these inhomogeneities is about 30 % and can occupy up to 50 % and therefore, mostly determines the electrical properties of films. It is suggested that the processes of cation disorder deforming the film lattice play an important role in formation of structure and phase inhomogeneities in YBCO thin films.

4PO9-37 The Surface Impedance of GdBa₂Cu₃O_7-d

C.E. Gough, R.J. Ormeno and G. Yang, Superconductivity Research Group, University of Birmingham, Edgbaston, Birmingham, United Kingdom, B15 2TT

Presenting Author: C.E. Gough

The complex surface impedance Z_s=[im _{rm 0v} /s ₁-is ₂)]^1/2 of a number of GdBa₂Cu₃O_7-d single crystals has been measured using a cavity perturbation technique at 10, 15 and 21 GHz in the temperature range between 1.3 K and T_c. Below 15 K a marked increase in surface resistance and reactance is observed associated with the paramagnetic and antiferromagnetic alignment of the Gd spins. The measured temperature and frequency dependences can be described rather accurately by a model which assumes a negligibly small interaction between the Gd spins and the electrons in the superconducting state, with a magnetic susceptibility c (T,v )=C/[(T+T_N)(1+iv t _s)], where the Néel temperature T_N=2.25 K, the Curie constant C=nm _B²p²/3k_B and the Gd spin relaxation time t _s is a strong function of temperature. Above T_N, t _s has a component varying as 1/(T-T_N), while below T_N it increases like T^-5. Present efforts aim at extending the accessible temperature range to values below 1 K and at including data on oxygen deficient crystals (d » 0.5).

4PO9-39 Magnetic Imaging of High-T_c Thin Films and Devices by Scanning SQUID Microscope

K. Tanabe 1, K. Suzuki 1, Yijie Li 1, T. Utagawa 1, N. Inoue 1, T. Sugano 1, Y. Honami 2, U. Kawabe 2, M. Hidaka 3, T. Satoh 3, and S. Tahara 3. 1 Superconductivity Research Laboratory, ISTEC, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan. 2 Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan. 3 Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan.

Presenting Author: K. Tanabe

Magnetic flux trapped in superconducting thin films causes serious problems for electronic devices such as an increase of flux noise in high-T_c SQUIDs and an operation error in digital devices. In the present study, to investigate the behavior of flux trapping in high-T_c thin films, we have directly observed flux quanta trapped in high-T_c thin films and multilayer devices by using a scanning SQUID microscope (SSM) system in which the sample temperature can be varied in a wide range of 4 - 100 K.

NdBa₂Cu₃O_y (NBCO, T_c = 89 - 92 K) and (Hg,Re)Ba₂CaCu₂O_y ((Hg,Re)-1212, T_c = 113 - 116 K) thin film patterns with different size and moats or grain boundaries as well as an SFQ sampler circuit chip were cooled below T_c under various ambient field and their magnetic images were taken by a SSM with Nb-based SQUIDs. Trapped flux quanta (vortices) were clearly observed in a wide temperature range. The number and positions of trapped vortices in NBCO thin films were found to be unchanged even at temperatures near the film T_c, which is consistent with the rather strong pinning in this material. No trapped flux was observed for the field less than the threshold value predicted by Clem.

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications.