Power handling capabilities of superconducting YBCO thin films: Thermally induced nonlinearity effects

We have measured the microwave power dependence of the surface impedance Z_s of YBa₂Cu₃O_x thin films up to very high microwave power levels. Films with different crystal qualities, including one with a bicrystal Josephson junction, were investigated. The experiments included both frequency-domain and pulsed time-domain measurements using a 14 GHz TE₀₁₁ dielectric cavity. Our results demonstrate that the dissipation of heat, generated by rf currents in the superconducting film, contributes to the observed nonlinearities in the surface resistance. The relative extent of this contribution is determined primarily by the film quality. A simple Fabry-Perot resonator model, combined with a cavity heat transfer model, was used to analyze the effects of such nonlinearities on the electromagnetic response of the dielectric cavity to a pulsed input signal.     

How Accurately Can the Surface Resistance of Various Superconducting Films Be Measured with the Sapphire Hakki–Coleman Dielectric Resonator Technique?

It has been shown in the past that a random uncertainty in surface resistance measurements of HTS films as low as 1.1% can be achieved. However for comparative studies between differing samples and different laboratories the absolute uncertainty should be used instead. In this paper we discuss random and absolute uncertainties of R _S measurements for three types of superconducting thin films, namely YBa₂Cu₃O_7−δ, Tl(Ba,Sr)₂Ca₂Cu₃O_y, and MgB₂ when using an optimized Hakki–Coleman dielectric resonator.     

Growth mode and dielectric properties in laser MBE grown multilayer of SrTiO3 and YBa2Cu3Oy

We have prepared perovskite SrTiO₃/YBa₂Cu₃O_y/LaAlO₃ multilayer using laser molecular beam epitaxy technique. The evolution of growth mode from YBa₂Cu₃O_y followed by SrTiO₃ was observed using in-situ reflective high-energy electron diffraction. The high-quality crystal structures of the grown films were also evidenced from X-ray diffraction analysis. The Raman scattering measurements were employed to study the lattice dynamic properties of SrTiO₃ layer in the temperature range from 10 to 300 K. Strain and oxygen vacancies possibly contribute to the lowering of symmetry and the breaking of the central symmetry in SrTiO₃. Temperature dependence of the dielectric constant and loss of thin films under different applied frequencies were evaluated.     

Intermodulation Measurements on High Temperature Superconducting Thin Films

A high temperature superconducting GdBa₂Cu₃O_7– thin film was characterized using a parallel plate resonator at 5.5 GHz and a dielectric puck resonator at 10 GHz, to observe the dependence of surface resistance on microwave power at the fundamental frequency, and the production of intermodulation products using a two-tone measurement setup, respectively. The electromagnetic field pattern of the dielectric puck-HTS thin film mode was modeled using the commercial software package MAFIA. The intermodulation product was compared with a simple RSJ model.     

Nonlinear Power Handling of YBa2Cu3O7−x Films and Microwave Devices

Physical mechanisms which limit the power handling of YBa₂Cu₃O_7?x films and devices are discussed in terms of a quantitative classification scheme. The possible limitations are devided into magnetic or thermal, and global or local in nature. Analytical estimations are compared with measurements of YBa₂Cu₃O_7?x films (Ø = 1″–2″) using a niobium-shielded sapphire resonator at 19 GHz, and disk resonators at 2 GHz. Magnetic effects are found to play an essential role in nonoptimized films in terms of weak-links, and in high-quality films if the lower critical field Bcl is reached. The majority of films and disk resonators appear to suffer from microwave heating. Global heating appears predominantly at CW operation. Local heating results mainly from defects in films of medium quality. Defect-induced quenches are observed at moderate field levels, sometimes resulting in an irreversible degradation of the power handling.     

Microwave properties of highly oriented YBa2Cu3O7-δ superconducting thin films

We have performed millimeter-wave frequency (94 GHz) measurements on high-quality YBa₂Cu₃O₇-δ superconducting films on yttrium-stabilized (100) ZrO₂ and MgO substrates. The ～0.2μm thin films fabricated by magnetron sputteringin situ with the YBa₂Cu₃O₇-δ powders as target exhibit superconducting transition temperatures up to 88 K. The critical current density of 6×10⁵ A/cm² at 77 K and the X-ray diffraction spectrum as well as scanning electron microscope photographs indicate these thin films are fullyc-axis oriented, extremely high in density, and universally homogeneous. Millimeter-wave surface resistances have been measured on a hemisphere open resonator in the temperature range of 20 K toT _c and beyond. The surface resistance at 94 GHz and 77 K for these films is found to be about 30 mΩ, nearly 1/4 that for copper, and a drop of two orders in the surface resistance within 4 K is observed, which indicates that these films are good materials for applications in the millimeter-wave range, especially for fabricating microwave devices. We observed such low surface resistance in these thin films due to the near absence of grain and phase boundaries coupled with a high degree of crystalline orientation.     

Oxygen vacancies,zinc impurities,and the intrinsic microwave loss of YBa2Cu3O7−δ

The surface resistance of YBa₂(Cu_1?x Zn _x )₃O_7?δ has been measured in single crystals with oxygen vacancies ranging fromδ=0.03 to 0.15 and for Zn impurity concentrations ofx=0, 0.0015 and 0.0031. Oxygen vacancies in the CuO chains have little influence on the microwave loss untilT _c is reduced substantially. However, Zn impurities in the CuO₂ plane sites drasticallydecrease the surface resistance, showing that the CuO₂ plane conductivity dominates any CuO chain contribution to the microwave properties of YBa₂Cu₃O_7?δ. The reduced microwave loss in the Zn-doped samples mimics the loss observed in state-of-the-art films and indicates that defects must be present in order to achieve the low microwave loss observed in some films. This sensitivity to low levels of defects is a consequence of the rapid increase in the scattering time in YBa₂Cu₃O_7?δ belowT _c .     

Irreversible change of surface impedance of high-Tc thin films due to D.C. magnetic field commutation

The magnetic field dependence of the surface impedance (resistance and reactance) of high-T _c thin films is found employing measurements of the quality factor and the frequency of the parallel plate resonator in a dc magnetic field up to 500 Oe at 10 GHz.c-Oriented YBa₂Cu₃O₇ thin films are examined. Enhancement of surface resistance and inductance with increase of magnetic field is observed. The effect of irreversible increase of surface impedance as compared to its initial values after a cycle of magnetic field commutation is found. A qualitative explanation of the observed effects based on the picture of magnetic vortex penetration and accumulation in the film due to strong pinning is presented.     

Preparation and characterization of YBa2Cu3O7−x samples for microwave applications

We prepared several samples of YBa₂Cu₃O_7?x thick films electrophoretically deposited on metallic substrates, thin films sputtered on dielectric substrates, and bulk pellets, and we tested their microwave properties. Various preparation techniques are described in detail and the importance of appropriate thermal treatment during the fabrication process is emphasized. Experimental results show that electrophoretically deposited thick films on silver substrates reached, after heat treatment, a surface resistance at 50 K of 4×10^?2 Ω comparable with the silver value, while the same measurement before sinterization gave a value of 0.5 Ω. Either bulk pellets or thin films gave worse results, though a lack of sensitivity in our experimental apparatus could have influenced our data. The problem of sensitivity in the characterization of microwave properties of small samples is discussed in the appendices.     

Nonlinear Microwave Response of Epitaxial YBaCuO Films of Varying Oxygen Content on MgO Substrates

We have investigated the nonlinear microwave properties of electron-beam coevaporated YBa₂Cu₃O_7–x films on MgO, using stripline resonators at 2.3 GHz and temperatures 1.7–80 K. The oxygen content of the films ranged from strongly underdoped to overdoped. Above 20 K, the nonlinear response of the resonators was dominated by the superconductor. We could establish clear correlations between the nonlinear surface resistance, two-tone intermodulation (IMD), and oxygen content of the films, which indicate that the superconducting order parameter is important for the nonlinearities. A power-law representation of the nonlinear current-voltage relation would not be appropriate to explain our results phenomenologically. Below 20 K, the dielectric loss tangent of MgO dominated the nonlinear response of the resonators. With increasing power, the dissipation losses decreased markedly, accompanied by enhanced IMD. The surface reactance passed through a shallow minimum at about 5 K, independent of power. We attribute these effects to resonant absorption by impurity states in MgO.     

TE013 Mode High Temperature Superconductor Millimeter Resonator

This paper describes a TE₀₁₃ mode dielectric resonator at 36.098 GHz. The dielectric resonator was formed by sandwiching a cylindrical piece of polished dielectric (sapphire) rod between two GdBa₂Cu₃O_7?x (GBCO) High Temperature Superconductor (HTS) thin films. The resonator was tested as a two-port device at a temperature of 77 K. A 62700 unload quality factor Q ₀ was obtained. The surface resistance (Rs) of the HTS film had been determined as 9.4 mΩ at 77 K and 36 GHz.     

Analysis of whispering-gallery superconducting dielectric resonator modes

The whispering-gallery (WG) modes of a superconducting dielectric resonator (SDR) based on a sapphire cylindrical dielectric resonator and a YBa₂Cu₃O_{7 –} shielding cylinder have been studied. A method for the determination of the resonant frequencies and the maximum quality factor of such modes is presented. Calculations have shown that most of the mode energy could be confined between the caustic surface of the WG modes provided the dimensions of the SDR are properly selected, and a magnitude of 10⁹ forQ of the SDR could be estimated. A phenomenal explanation is given to account for such outstanding microwave behavior.     

Nonlinear Power Handling of YBa2Cu3O7?x Films and Microwave Devices

Physical mechanisms which limit the power handling of YBa₂Cu₃O_7–x films and devices are discussed in terms of a quantitative classification scheme. The possible limitations are devided into magnetic or thermal, and global or local in nature. Analytical estimations are compared with measurements of YBa₂Cu₃O_7–x films (Ø = 1–2) using a niobium-shielded sapphire resonator at 19 GHz, and disk resonators at 2 GHz. Magnetic effects are found to play an essential role in nonoptimized films in terms of weak-links, and in high-quality films if the lower critical field Bcl is reached. The majority of films and disk resonators appear to suffer from microwave heating. Global heating appears predominantly at CW operation. Local heating results mainly from defects in films of medium quality. Defect-induced quenches are observed at moderate field levels, sometimes resulting in an irreversible degradation of the power handling.     

Weak-link behavior in superconducting thick films

The surface impedance of melt-processed thick films of YBa₂Cu₃O_7–x on yttria-stabilized zirconia substrates has been measured as a function of temperature over the frequency range 9–18 GHz using an endwall replacement technique. The temperature dependence of both the penetration depth and the surface resistance of thisp-type cuprate system can be described within a two-gap weak-link model. The surface resistance scaled to 10 GHz by the observedf ² frequency dependence gives a value of 1.8 m at 77 K, which compares favorably with the lowest reported values for thick films of this material at microwave frequencies. Trends in material processing indicate that increasing the grain size would further decrease the surface resistance at microwave frequencies.     

Origins of conductive losses at microwave frequencies in YBa2Cu3O7−δ/LaAlO3/YBa2Cu3O7−δ trilayers deposited by pulsed laser deposition

Oriented YBa₂Cu₃O_7?δ/LaAlO₃/YBa₂Cu₃O_7?δ trilayers were deposited by pulsed laser deposition (PLD) onto 〈100〉 MgO and LaAlO₃. Film thicknesses varied from 2000–5000 Å/ layer. A comparision of structure and transport data for the bottom and top superconducting layers indicated a slight decrease in film quality for the top superconducting layer. The critical temperature was lower for the top superconducting layer (90.5 vs. ～90 K) and the microwave surface resistance was higher (increasing from ～2 to 18 mω at 36 GHz, 20 K). The resistivity of the dielectric was estimated to be 10⁶ ω cm, and the loss tangent of the dielectric film at microwave frequencies had an upper limit of 0.01. Cross-sectional TEM analysis of the trilayer structure showed a high density of threading dislocations in the dielectric layer that appeared to nucleate at steps in the underlying superconducting layer. The threading dislocations may serve as conduction paths in the LaAlO₃ layer.     

Double-sided YBa2Cu3O7−δ superconducting films prepared by laser ablation with a Si radiation heater

Double-sided superconducting YBa₂Cu₃O_7?δ thin films have been prepared by pulsed laser ablation with a Si radiation heater. Measurements show that those samples are of good quality with zero resistance temperatures of ≥90 K, critical current densities of ≥3 x 10⁶ A/cm² at 77 K and zero field, and 10 GHz microwave surface resistances of 370μΩ (second side) and 1.7 mΩ (first side), respectively. The X-ray diffraction patterns indicate that each side of the samples isc-axis oriented epitaxial YBa₂Cu₃O_7?δ film. Scanning electron microscopy (SEM) reveals that the surface of the films is rather smooth with no particle larger than 1 μm.     

Fabrication and characterization of heteroepitaxial bilayers of La-Ca-Mn-O/Y-Ba-Cu-O

We have fabricated La_0.67Ca_0.33MnO₃/YBa₂Cu₃O_y heteroepitaxial bilayers on SrTiO₃ (1 0 0) substrates by rf magnetron sputtering. The La_0.67Ca_0.33MnO₃/YBa₂Cu₃O_y bilayers were characterized by X-ray diffraction, rocking curve measurements, scanning electron microscope and dc four-probe measurements. Both La_0.67Ca_0.33MnO₃ and YBa₂Cu₃O_y layers show excellent crystallinity indicated by their sharp and narrow rocking curves. The La_0.67Ca_0.33MnO₃/YBa₂Cu₃O_y bilayers exhibit a very smooth surface. The relation between the superconducting critical current density of YBa₂Cu₃O_y and temperature was also investigated.     

Precise Microwave Characterisation of YBa2Cu3O7−δ Films on Sapphire and Lanthanum Aluminate Substrates

We have precisely measured the surface resistance (R _S) of high quality YBCO thin films, deposited on Lanthanum Aluminate and Silver doped YBCO thin films on Cesium buffered Sapphire substrates using the Hakki-Coleman dielectric resonator at frequency of 10 GHz as a function of temperature from 25 K to 85 K. We have also studied the microwave power dependence of YBCO films on both substrates and the frequency dependence of films on LAO. Precise microwave characterisation of HTS films is needed for selection of best films for mobile phone base station receivers.     

Influence of substrate on the excess electrical noise in the normal state of YBa2Cu3O7-δ thin films

We present our studies of the low-frequency excess electrical noise in YBa₂Cu₃O_7-δ thin films in the normal state. We have studied films with varying microstructure deposited on different substrates. The frequency dependence and bias current dependence of the noise power spectral density agree with the behavior expected for noise due to conductance fluctuations. Comparison between films on different substrates shows that the presence of defects such as grain boundaries in the film correlate with significantly enhanced noise levels. The noise levels in our good-quality epitaxial films are several orders of magnitude lower than the anomalously large noise magnitudes reported for YBa₂Cu₃O_7-δ in most of the earlier studies.     

Effects of bismuth replacement for copper on the microstructure and superconductivity of YBa2Cu3Oy

The partial replacement of copper with bismuth in YBa₂Cu₃O_y had led to a new phase instead of crystal site substitution. The resulting YBa₂BiO₆ could also be found whenever YBa₂Cu₃O_y reacted with Bi₂O₃. The YBa₂BiO₆ was formed either at the expense of YBa₂Cu₃O_y or by decomposing it, together with the liberation of copper oxide. Electrical measurements revealed the trend toward increased semiconducting normal state resistivity for bismuth-replaced samples. The insulating YBa₂BiO₆ phase was responsible for the deterioration of both superconductivity and mechanical strength.