Microwave Surface Resistance Research Articles

Effects of Proton Irradiation on the Microwave Surface Resistance of ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-\delta}$ Films

Proton irradiations on superconducting bulk materials and thin films have enabled to enhance the critical current density and the upper critical field in the mixed state due to increased vortex pinning. We investigated the microwave surface resistance of proton-irradiated (YBCO) films grown on (LAO) substrates. The intrinsic was measured at 8.5 GHz at temperatures of 7-90 K with the effects of the finite film thickness taken into account. Significant reduction in the of YBCO films were observed at temperatures of 40 K to 70 K when the films were irradiated with 20 MeV proton for 10 minutes to the fluence of with the value of 100 of the proton-irradiated one at 60 K appearing significantly lower than the corresponding one of 140 before irradiation. However, at temperatures below 20 K, the of the irradiated YBCO films was higher than that of the YBCO film before irradiation with the residual of the former appearing higher than that of the latter. No significant changes in the of YBCO films were observed when the films were irradiated with 20 MeV proton for 30-120 minutes to the fluences of to . The reduction in the of proton-irradiated YBCO films at 40-70 K was attributed to enhanced impurity scattering rate of compared to a value of for unirradiated YBCO.

Fabrication of double-sided Tl 2Ba 2CaCu 2O 8 thin films on 2-in. LaAlO 3 substrates by pulsed laser deposition and post-annealing

Double-sided Tl 2Ba 2CaCu 2O 8 (Tl-2212) thin films have been prepared on 2-in. LaAlO 3 substrates by a two-step method in which Tl-free amorphous Ba 2CaCu 2O x precursor films were first deposited on the substrates by pulsed laser deposition and the subsequent thallination, crystallization and oriented growth of the superconducting Tl-2212 films were obtained in a long-duration annealing process in argon atmosphere at temperature around 720–740 °C. The films were characterized by means of X-ray diffraction, scanning electron microscopy and transport measurements. The results show that the as-prepared films are predominately single phase with c-axis perpendicular to the substrate surface, except for the presence of some submicron particles on the film surface. For the both sides of the films, the zero-resistance transition temperature T C0 is about 105 K and the critical current density J C is in excess of 10 6 A/cm 2. The microwave surface resistance R S is below 0.5 mΩ.

Search for non-local quasiparticle effects in optimally doped single crystal YBCO

When a magnetic field is applied perpendicular to the CuO 2 planes of cuprate high temperature superconductors, the normal fluid response is expected to include contributions from quasiparticles with relevant mean free paths greater than the penetration depth. As a result, a non-local electrodynamical response is expected. Optimally doped single crystals of YBCO have been prepared such that a microwave field can be applied parallel to the c-axis with negligible demagnetization effects. We report on efforts to identify non-local quasiparticle effects in these crystals using cavity perturbation measurements of the microwave surface resistance.

The importance of CeO 2 growth temperature and its post-annealing for the improvement of the microwave surface resistance of DyBa 2Cu 3O z films

We report here the influence of growth temperature of CeO 2 buffer layer T ( CeO 2 ) as well as the post-annealing of the buffer layer on the crystallinity and the microwave surface resistance R s of DyBa 2Cu 3O z (DBCO) films grown by pulsed laser deposition (PLD). It is found that (i) an increase in the T ( CeO 2 ) facilitates the epitaxial growth of the CeO 2 films, which is a prerequisite to obtain the high quality superconducting films and (ii) the post-annealing of buffer layer at 1050 °C in flowing O 2 for 1 h leads to a profound improvement in the morphology and in the crystallinity of CeO 2 films. Apparently, there exists a critical growth temperature ( T ( CeO 2 ) = 800 – 820 ∘ C , as found in this and previous study [J.C. Nie, H. Yamasaki, Y. Nakagawa, K.D. Bagarinao, M. Murugesan, H. Obara, Y. Mawatari, J. Crystal Growth 284 (2005) 417]) for CeO 2, below which the crystalline quality of CeO 2 films might not be improved merely by the post-annealing. It is explained that for T ( CeO 2 ) < 800 ∘ C , the as-grown CeO 2 grains are longitudinal in shape, and it forms corrugated structure upon annealing. This poor morphology yields a deteriorated crystallinity (i.e., a large value of Δ ω and Δ φ, and the formation of secondary phase) for the CeO 2 as well as the overlying DBCO films, and hence a poor microwave performance of DBCO films for T ( CeO 2 ) < 800 ∘ C . We also observed that the R s in DBCO films monotonously decreased with increase in the growth temperature of CeO 2. Further, the post-annealing of the CeO 2 buffer layer prior to DBCO deposition greatly helps to reduce the R s at the liquid N 2 temperature region, which is immensely required for the use of superconducting films in the passive microwave device components. Thus, the 800–820 °C of T ( CeO 2 ) and the post-annealing of CeO 2 at 1050 °C in flowing O 2 for 1 h may be readily exploited to grow RBCO (R=Y or rare-earth elements) films for microwave applications on the technologically viable r-Al 2O 3 substrates.

Effect of oxygen vacancies on nonlinear dielectric properties of SrTiO3 thin films

Ferroelectrics (FE) are a class of nonlinear dielectrics which exhibit an electric field dependent dielectric constant. High temperature superconductors (HTSC) offer lower microwave surface resistance than conventional materials. There have been significant interest and effort in combining the low microwave losses of HTSC with the voltage control of FE dielectric constant er to implement high quality voltage tunable microwave devices, such as phase shifters, filters, delay lines, and tunable oscillators [1–4]. In such devices, it is desirable to have a large capacitance change ratio [tunability = (Cmax–Cmin)/Cmax] under a certain electric field range accompanied by a small dielectric loss. SrTiO3 (STO) thin films which posses a transition peak located at 60–70 K is one of the most interesting FE materials due to its high nonlinearity and reasonably low dielectric loss. A main focus on research for effective utilization of STO thin films is the development of materials with simultaneously optimized permittivity (absolute magnitude as well as its electric field dependence.) and low dielectric loss (tand). However, the tunability and dielectric loss in STO thin films have not been comparable to that of bulk materials. Comparing with STO single crystal, most STO thin films showed low dielectric constant, low tunability and large dielectric loss. Many reasons have been suggested to the degradation in dielectric properties of STO thin films. The tetragonal distortion (ratio of in-plane and surface normal lattice parameters, D = a/c), the dead layer near interface, and the local polar regions near charged defects like oxygen vacancies are expected to degrade the dielectric properties of STO thin films [5–7]. Several researchers have investigated the effect of oxygen vacancies on dielectric properties of heteroepitaxially grown STO thin films [8, 9]. It is worthwhile to note that the study of oxygen vacancies on dielectric properties of hetero-epitaxial STO thin films is complicated because of the additional effect of film stress due to film-substrate crystal lattice mismatch and difference in thermal expansion coefficients. There have been no systematic studies on the influence of oxygen vacancies on dielectric properties of STO thin films. In this study, STO thin films were homoepitaxially grown on Nb doped STO (Nb:STO) substrate. No additional stress is caused by lattice constant mismatch and thermal expansion coefficient differences between films and substrate in the homo-epitaxial STO thin films. The effect of oxygen vacancies on dielectric properties of STO thin films are presented. STO thin films were deposited on Nb:STO substrates by pulsed laser deposition (PLD). KrF excimer laser radiation (Lamda Physik Compex 201) with repetition rate of 5 Hz was focused on a rotating target set in the chamber through a quartz window. The incident angle of laser beam to the target surface was fixed to be 45 . The output laser pulse energy was kept as 150 mJ which yielded a growth rate of 0.02 nm per pulse. The laser energy density was about 2 J/cm. X. Z. Liu B. W. Tao (&) Y. R. Li The National Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science & Technology of China, Jianshe Road, Chengdu 610054, China e-mail: bwtao@uestc.edu.cn

Properties Optimization for Perovskite Oxide Thin Films by Formation of Desired Microstructure

Perovskite oxide materials are very important for the electronics industry, because they exhibit promising properties. With an interest in the obvious applications, significant effort has been invested in the growth of highly crystalline epitaxial perovskite oxide thin films in our laboratory. And the desired structure of films was formed to achieve excellent properties. Y₁Ba₂Cu₃O 7-x (YBCO) superconducting thin films were simultaneously deposited on both sides of 3 inch wafer by inverted cylindrical sputtering. Values of microwave surface resistance R s (75 K, 145 ㎓, 0 T) smaller than 100 mΩ were reached over the whole area of YBCO thin films by pre-seeded a self-template layer. For implementation of voltage tunable high-quality varactor, A tri-layer structured SrTiO₃ (STO) thin films with different tetragonal distortion degree was prepared in order to simultaneously achieve a large relative capacitance change and a small dielectric loss. Highly a-axis textured Ba 0.65 Sr 0.35 TiO₃ (BST65/35) thin films was grown on Pt/Ti/SiO₂/Si substrate for monolithic bolometers by introducing Ba 0.65 Sr 0.35 RuO₃ (BSR65/35) thin films as buffer layer. With the buffer layer, the leakage current density of BST65/35 thin films were greatly reduced, and the pyroelectric coefficient of 7.6×10?? C ㎝?² K?¹ was achieved at 6 V/㎛ bias and room temperature.

Magnetic hysteresis in the microwave surface resistance of Nb samples in the critical state

We discuss the hysteretic behavior of the field-induced variations of the microwave surface resistance in superconductors in the critical state. Measurements have been performed in a bulk sample of Nb and a powdered one at different values of the temperature. We discuss a model, based on the Coffey and Clem theory, in which we take into account the flux distribution inside the sample, due to the critical state. The experimental results are justified quantitatively in the framework of our model. We show that by fitting the experimental data it is possible to determine the value of the critical current density and its field dependence.

Microwave and Terahertz Surface Resistance of MgB2 Thin Films

The knowledge of the surface resistance Rs of superconducting thin film at microwave and terahertz (THz) regions is significant to design, make and assess superconducting microwave and THz electronic devices. In this paper we reported the Rs of MgB2 films at microwave and THz measured with sapphire resonator technique and the time-domain THz spectroscopy, respectively. Some interesting results are revealed in the following: (1) A clear correlation is found between Rs and normal-state resistivity right above Tc, ρ0, i.e., Rs decreases almost linearly with the decrease of ρ0. (2) A low residual Rs ,l ess than 50µ� at 18 GHz is achieved by different deposition techniques. In addition, between 10 and 14 K, MgB2 has the lowest Rs compared with two other superconductors Nb3Sn and the high-temperature superconductor YBa2Cu3O7−δ(YBCO). (3) From THz measurement it is found that the Rs of MgB2 up to around 1 THz is lower than that of copper and YBCO at the temperature below 25 K. (4) The frequency dependence of Rs follows ω n ,w hereω is angular frequency, and n is power index. However, n changes from 1.9 at microwave to 1.5 at THz. The above results clearly give the evidences that MgB2 thin film, compared with other superconductors, is of advantage to make superconducting circuits working in the microwave and THz regions.

Microwave surface resistance measurement of hts films using dielectric resonators

An open-ended dielectric resonator method and experimental apparatus for measuring microwave surface resistance of the high Tc superconducting thin films are reported. Making use of two sapphire rods the quality factor related to the losses of the resonator except its superconducting part can be evaluated. The Rs value of an individual superconducting thin film can be determined without any assistance of other superconducting films or calibrators. This measurement provides a convenient and absolute approach for Rs-measurement of high Tc superconducting thin films. Also the error in the Rs-measurement with the method was discussed.

Development of microstructure in Tl-2212 thin films and possible influence on microwave surface resistance values

The microstructures of Tl2Ba2Ca1Cu2O8 (Tl-2212) films are very strongly influenced by the processing parameters used to synthesize the superconducting phase and also control the microwave surface resistance values that are of key importance in the application of these materials in high-frequency devices. We report here on detailed studies of how the mesotexture of Tl-2212 films develops during synthesis at 820 and 855 °C. Our key observation is that the microstructure, and hence the superconducting properties, are controlled by the mechanism by which stress is relieved in the films and that apparently perfectly epitaxial films do not have the best microwave performance because in these samples the stress is relieved by macroscopic defects rather than local, low-angle grain misorientations.

Normalized IMD Spectra of High-T C and Low-T C Superconducting Resonators

Analyzing the results of the two-tone intermodulation distortion (IMD) measurements for the 2.3 GHz YBa2Cu3O7− x and GdBa2Cu3O7− x microstrip resonators we described the IMD spectra by the normalized values of P IMDn/P 0, where P IMDn is the output power of the n-th order IMD product and P 0 is that of the fundamental tone. The circulating power in the resonator was normalized by the power of the third-order intercept: P circ/P TOI. It was found, that the examined resonators exhibited the same dependence of P IMDn/P 0 versus P circ/P TOI and the same dependence of P IMDn/P 0 versus the change of the microwave surface resistance (ΔR S) at any measurement temperature. We reproduced these measurements for the indium microstrip resonators of the same geometry and obtained the same results. We conclude, that the nonlinear response of the superconducting microstrip resonators in the high power range is strongly affected by a geometry-dominated hf loss mechanism (likely the hf hysteretic losses due to magnetic flux penetration), which overshadows other nonlinear mechanisms related to the intrinsic material properties. This conclusion is in agreement with the results of the indium disk resonator measurements.

Fabrication of Double-Sided YBa2Cu3O7 Films on CeO2-Buffered Sapphire Substrates by MOD Process

Double-sided YBa 2 Cu 3 O 7 (YBCO) films were successfully prepared on 50-mm-diameter CeO 2 -buffered sapphire substrates by metalorganic deposition (MOD) process using an acetylacetonate coating solution. Mapping analysis of superconducting current densities (J c ) at 77.3 K revealed that J c values of the double-sided films indicated in excess of 2 MA/cm 2 in the center parts with a small decrease of J c at the outer side of the specimens. The J c values of one side (A) are higher than those of the other side (B). Microwave surface resistance (R s ) of sides A and B of the film exhibited 0.57 and 0.60 mΩ, respectively, at 70 K (12 GHz). The difference in the R s values should be attributed to the slight difference in the J c values, which arose from the surface morphology of the CeO 2 buffer layer and heat treatment conditions during the firing process in MOD.

Microwave Power Dependence Measurement of Surface Resistance of Superconducting Films Using a Dielectric Resonator Method with Circle Fit and Two-Mode Techniques

A system was developed to measure the microwave power dependence of the surface resistance superconductor films. The system uses a dielectric resonator method combined with a circle fit technique and a two-mode technique to measure the microwave surface resistance of superconductor films. For validation, this system was used to measure such surface resistance for superconductor films with different surface morphologies. Significant difference in microwave power dependence of surface resistance was observed. This measurement system proved suitable for evaluating superconducting films for passive microwave devices, including high power devices such as transmitting niters.

Microwave Properties of Sapphire Resonators with a Gap and Their Applicability for Measurements of the Intrinsic Surface Impedance of Thin Superconductor Films

A dielectric resonator with a gap between the top plate and the rest has been useful for measuring the penetration depth (A) of superconductor films, a parameter essential for obtaining the intrinsic microwave surface resistance (R,) of thin superconductor films. We investigated effects of a gap on the microwave properties of TE 0ml -mode sapphire resonators with a gap between the top plate and the rest of the resonator. Regardless of a 10 μm-gap in TE 0ml -mode sapphire resonators, variations of the TE 0ml -mode resonant frequency on temperature (Δf 0 ) as well as TE 0ml -mode unloaded Q remained almost the same due to lack of axial currents inside the resonator and negligible radiation effects. The A of YBa 2 Cu 3 O 7-δ (YBCO) films obtained from a fit to the temperature-dependent Δf 0 appeared to be 195 nm at OK and 19.3 GHz, which was well compared with the corresponding value of 193 nm at 10 kHz measured by the mutual inductance method. The intrinsic R s of YBCO films on the order of 1 mΩ, and the tan 6 of sapphire on the order of 10 -8 at 15 K and 40 GHz could be measured simultaneously using sapphire resonators with a 10 μm-gap.

Fabrication of MgB 2 superconducting films by different methods*

Abstract MgB2 superconducting films have been successfully fabricated on single crystal MgO (111) and c-AL2O3 substrates by different methods. The film deposited by pulsed laser deposition is c-axis oriented with zero resistance transistion temperature of 38.4K, while the other two films fabricated by chemical vapor deposition and electrophoresis are c-axis textured with zero resistance transistion temperature of 38 K and 39 K, respectively. Magnetization hysteresis measurements yield critical current density J c of 107 A/ cm2 at 15K in zero field for the thin film and of 105 A/ cm2 for the thick film. For the thin film deposited by chemical vapor deposition, the microwave surface resistance at 10 K is found to be as low as 100µΩ, which is comparable with that of a high-quality high-temperature superconducting thin film of YBCO. * Supported by the Major State Basic Research Development Program of China (Grant No. 1999064604)

The combined effect of thermal annealing of MgO substrate and Ca substitution on the surface resistance of YBa2Cu3Oz thin films

Single-layer Y1−xCaxBa2Cu3Oz (YCBCO) thin films (x=0.00, 0.02, 0.05, and 0.10) grown on annealed as well as unannealed MgO substrates have been systematically investigated for their carrier concentration, critical current density Jc, and microwave surface resistance Rs. For x⩽0.05, the grain growth follows a three-dimensional-spiral growth mechanism, while for x=0.10 we observed a mainly two-dimensional-like growth of grains. The results of Hall data reveal that the x=0.05 film is overdoped while films with x=0.02 and 0.10 are underdoped with respect to the x=0.00 film. However, the Hall mobility μH is highly enhanced for the x=0.02 film. Thermal annealing of MgO substrates prior to film deposition results to an improvement in the overall superconducting properties of the film such as suppression of normal-state resistivity, enhancement of Jc, and minimization of Rs both for pure as well as Ca-substituted films. Annealing of MgO substrates enhances the Jc value to a magnitude (i) nearly doubled for x=0.00 films and (ii) more than an order for x=0.02 films. Also it leads to a minimization of the Rs value to (i) more than half and (ii) nearly an order of magnitude higher, respectively, for x=0.00 and x=0.02 films. Furthermore, for the x=0.02 film, below 60K, we realized an enhanced Jc value in self- as well as in large-applied fields. For other than the Ca-2% substituted films, a suppression of Jc with a strong field dependency has been noticed. Furthermore, the Rs value of the x=0.02 film (0.1mΩ at 20K, 21.9GHz) was three times lower compared to that of the x=0.00 film (0.35mΩ at 20K, 21.9GHz). At 20K and 21.9GHz, the Rs value for the x=0.05 film is comparable to that of the x=0.00 film, whereas for the x=0.10 film it is twice that of x=0.00. The low normal-state resistivity, enhanced mobility, high Jc, and the minimized Rs observed for x=0.02 films firmly support the possible improvement of superconducting order parameters near the grain boundaries. Thus, the combined effect of thermal annealing of MgO prior to film deposition and substituting Y with 2% Ca may be readily applied to grow high-quality (high Jc and low Rs) YBCO films for microwave device applications.

Microwave Properties and Critical Current Density of YBCO Thin Films Deposited on CeO2-Buffered Sapphire Substrates Using Sputtering Targets of Different Compositions

The effect of the composition of a YBCO film on the microwave properties deposited on sapphire <R> substrates with a CeO2 buffer layer was investigated. CeO2 and YBCO films were deposited by RF magnetron sputtering with targets of different compositions. Y-rich crack-free YBCO films with a thickness of 0.5 µm, deposited onto a 25-nm-thick CeO2 film, showed good crystallinity and a low microwave surface resistance. Microwave surface resistances of 0.09 mΩ at 20 K and 0.7 mΩ at 77 K were obtained at a frequency of 12 GHz, and a critical current density of 0.1 MA/cm2 at 77 K was estimated for the 0.5-µm-thick YBa1.5Cu2.5Oy film.

Temperature-dependent local electromagnetic characterization of electronic materials by scanning microwave near-field technique

We report a scanning microwave near-field microscope that allows temperature-dependent local electromagnetic characterization of electronic materials. We have established a temperature-controlled sample stage cooled by liquid nitrogen, so that the local electromagnetic properties of the sample could be studied through near-field microwave interaction at temperature varying from 80 to 300 K. Using this instrument, we have studied the magnetic phase transition of an Nd 0.7Sr 0.3MnO 3− δ thin film and an ion-pair complex magnetic material, as well as the homogeneity of the microwave surface resistance of a YBa 2Cu 3O 7− δ superconducting thin film. Experiment results have demonstrated the ability of temperature-dependent local microwave characterization of this microwave microscope for different kind of electronic materials.

Microwave characterization of laser ablated Y 1Ba 2Cu 3O 7− x thin films

In the present study we report the measurements of microwave surface resistance ( R s) of YBCO thin films on LaAlO 3 substrate as a function of temperature, thickness and magnetic field by microstrip resonator technique. The T c( R = 0) of the films is 90 K and J c > 10 6 A/cm 2 at 77 K. The microwave surface resistance has been measured for films of various thicknesses. The value of R s has been found to be initially decreased with increasing film thickness due to increase in number of defects. A minimum microwave surface resistance has been obtained for film thickness of about 300 nm. The increase of R s with film thickness above 300 nm is possibly due to degradation of the film microstructure as observed with Atomic Force Microscopy. Temperature dependence of surface resistance has been studied for best quality films. The field induced variations of surface resistance are also investigated by applying dc magnetic field perpendicular to stripline structure and surface of the film. A general linear and square field dependence of R s at low and high value of fields has been observed with critical field value of 0.4 T which confirms the microwave dissipation induced by flux flow in these resonators at 10 GHz frequency. The hysteresis of R s in dc field observed for field value above critical field shows the higher value of surface resistance in decreasing field than in increasing field which is in agreement with one state critical model and is a characteristic of homogeneous superconductors.

Control of Porosity and Composition in Large-Area YBCO Films to Achieve Micrometer Thickness and High&amp;lt;tex&amp;gt;$J_rm c$&amp;lt;/tex&amp;gt;on Sapphire Substrates

Relatively thick YBCO thin films (thickness d>0.5 /spl mu/m) ideal for fault current limiter as well as microwave applications have been successfully prepared by large-area pulsed laser deposition (PLD) on CeO/sub 2/-buffered sapphire substrates. The attainment of an unusually high film thickness (up to 2.0 /spl mu/m) without microcracking is attributed in part to the presence of pores correlated with yttrium-rich composition in the films. The effect of using YBCO targets with varied Y:Ba:Cu ratios on film porosity and surface morphology was investigated in detail. Using the optimum target composition, uniform critical current densities (J/sub c/) ranging from /spl sim/4.5 MA/cm/sup 2/ at d/spl sim/0.23 /spl mu/m to /spl sim/1.75 MA/cm/sup 2/ at d/spl sim/0.6 /spl mu/m have been achieved. Characterization of a film with d/spl sim/0.5 /spl mu/m showed low microwave surface resistance values [R/sub s/(77.3 K)/spl ap/2.09 m/spl Omega/ and R/sub s/(20 K)/spl ap/227 /spl mu//spl Omega/ at 22 GHz] comparable to the best YBCO films reported by other studies.