Measurement of the surface resistance of YBa2Cu3O7?x thin films using stripline resonators

We review methods of measuring surface resistance (R _s ) of thin films using stripline resonators, and present our measurements of theR _s of YBa₂Cu₃O_7–x films as a function of frequency, temperature, and r.f. magnetic field. The films were deposited on LaAlO₃ substrates by two methods: (1) electron-beam coevaporation of Y, BaF, and Cu followed by annealing in O₂, and (2) single-targetin situ sputtering. The measurements were obtained at frequencies from 0.4 to 10 GHz, temperatures from 4 to 90 K, and an r.f. magnetic field range from 0 to 30 Oe. At low temperature and low r.f. field at 0.4 GHz, theR _s values obtained for the two deposition methods are approximately 7×10^–6 and 4×10^–6 , respectively.     

Precision submillimeter wave laser reflection measurements of high-temperature superconductors

Submillimeter wave laser reflection measurements of surface resistance can provide improved capability in the combination of sensitivity, spatial resolution, and frequency range. We have made reflectivity measurements on metals at 1630 GHz with an uncertainty of less than 0.3%. This sensitivity corresponds to a measurement sensitivity for surface resistance of 0.3 . Assuming anf ² frequency scaling of high-temperature superconductor surface resistance from the microwave to the terahertz frequency range, this sensitivity corresponds to about 1 ×10^–5 at 10 GHz. Capability for 10^–7 sensitivity could eventually be possible. Preliminary submillimeter wave reflection measurements of a YBCO thin film have been made with a sensitivity of 1%. Submillimeter wave reflectometry can make it possible to determine the spatial dependence of surface resistance in a wide range of material sizes and shapes. The spatial resolution could be on the order of 0.3–0.5 mm.     

Temperature and Frequency Dependence of Complex Conductance of Ultrathin YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−x</Subscript> Films: Observation of Vortex–Antivortex Pair Unbinding

No Heading We have studied the temperature dependencies of the complex sheet conductance, (, T), of 1–3 unit cell (UC) thick YBa₂Cu₃O_7–x films sandwiched between semiconducting Pr_0.6Y₀.4Ba₂Cu₃O_7–x layers at high frequencies. Experiments have been carried out in a frequency range between: 2–30 MHz with one-spiral coil technique, in 100 MHz-1 GHz frequency range with a new technique using a single spiral coil cavity, and at 30 GHz by aid of a resonant cavity technique. The real, Re M(T), and imaginary parts of the mutual-inductance M(T, ), between a coil and a film, were measured and converted to complex conductivity by aid of the inversion procedure. We have found quadratic temperature dependence of the kinetic inductance, L^k_–1 (T), at low temperatures independent of frequency, with a break in slope at T^dc_BKT, the maximum of real part of conductance and large shift of the onset temperature and the maximum ₁(T) position to higher temperatures with increasing frequency . We obtain from these data the universal ratio T^dc_BKT/L^–1_k (T^dc_BKT) = 25, 25 and 17, nH K for 1, 2 and 3-UC films, respectively in close relation with theoretical prediction of 12 nH K for vortex–antivortex unbinding transition. The activated temperature dependence of the vortex diffusion constant was observed and discussed in the framework of vortex–antivortex pair pinning.PACS numbers: 74.80.Dm, 74.25.Nf, 74.72.Bk, 74.76.Bz.     

Surface impedance of NbN superconducting thin films by Josephson measurements

The microwave quality factorQ of NbN-Pb Josephson junctions has been determined via measurements of the amplitude of Fiske resonant modes in the junctions at different resonant frequencies in the range 30–300 GHz. It is proved that, for our samples, the main contribution to theQ comes from the surface impedance of the filmsR _s . Data show that, for NbN,R _s v^1.5±0.1 atT _c =4.2 K in the frequency range considered. The implications of our results for the possible applications of NbN films both in the context of Josephson microwave devices and as a coating material for rf cavities in high-energy physics are discussed.     

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.     

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.     

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 comparable with the silver value, while the same measurement before sinterization gave a value of 0.5 . 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.     

Low-fired microwave dielectrics in ZnO-TiO2 ceramics doped with CuO and B2O3

The microwave dielectric properties and the microstructure of low-firing zinc titanate ceramics have been investigated. The microstructure of these ceramics have been studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and it was found that CuO addition has the important effect of stabilizing the Zn₂TiO₄ phase. When the sintering temperatures are in the range of 870–950 °C, dielectric constant values ( _r ) saturate at 21–25, the Q×f value is 20 000 GHz (at 10 GHz). The temperature coefficient of resonant frequency ( _f ) was 10 ppm °C^–1. Therefore, the 0.5 wt %-CuO and 1.0 wt %-B₂O₃ doped ZnO-TiO₂ ceramics can be used in multi-layer microwave devices requiring low sintering temperature.     

Effect of Growth-Induced Linear Defects on High Frequency Properties of Pulse-Laser Deposited YBa2Cu3O7?δ Films

Growth-induced linear defects are shown to strongly affect the microwave surface resistance, R_s, of highly biaxially oriented high temperature superconductor (HTS) YBa₂Cu₃O_7– (YBCO) films. Measured R_s(77 K) turned out to be 4–5 times higher than in single crystals. The films were deposited by modified pulse-laser technique, J_c(77 K) = (3-6) × 10⁶ A/cm², onto LaAlO₃ substrates. R_s(T) was measured at 134 GHz and 20–100 K. TEM/HREM study of YBCO films deposited at T_s = 750°C–780°C revealed a reduction of edge dislocation density with T_s increase (from 2 · 10¹¹ to 10¹⁰ lines/cm²). YBCO films deposited at T_s = 780°C exhibited the smallest R_s(77 K, 134 GHz) < 120 m and the lowest density of dislocations detected by HREM and X-ray analysis. A nature of the dislocation effect is discussed within a model of local anisotropic elastic deformation in a vicinity of dislocation cores, where T_c variation and an enhancement of normal quasiparticle density are significant.     

Microwave properties of highly oriented YBa2Cu3O7-δ superconducting thin films

We have performed millimeter-wave frequency (94 GHz) measurements on high-quality YBa₂Cu₃O₇- superconducting films on yttrium-stabilized (100) ZrO₂ and MgO substrates. The 0.2m thin films fabricated by magnetron sputteringin situ with the YBa₂Cu₃O₇- 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 m, 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.     

Large-Area YBa2Cu3O7 ? x Films on Sapphire with Excellent Microwave Power Handling Capability

Crack-free thick YBa₂Cu₃O_{7 – x} films are prepared on CeO₂ buffered r-cut sapphire (2 inch in diameter) with thickness up to 700 nm, smooth surfaces (peak-to-valley roughness <10 nm), high critical currents (J _C > 2 MA/cm² at 77 K and 0 T), and low microwave surface resistances (R _s(77K) .4m and R _s(4.2K) .110 at 19.15GHz) comparable to the best values reported in the literature for YBCO films on structurally better matched substrates. These thick YBCO films were able to handle high microwave power corresponding to magnetic field amplitudes (B _HF) up to 54, 37, and 17.4 mT at 4.2, 50, and 77 K, respectively, which for the lower temperatures were limited by the available power of the 25-W HF amplifier. The high-power performance, which to our knowledge belongs to the best reported so far for unpatterned YBa₂Cu₃O_{7 – x} films, was achieved without any degradation of the samples despite frequent thermal cycling.     

Preparation and properties of ferroelectric Pb1 − xCaxTiO3 thin films produced by the polymeric precursor method

Pb_{1 – x} Ca _x TiO₃ thin films with x = 0.24 composition were prepared by the polymeric precursor method on Pt/Ti/SiO₂/Si substrates. The surface morphology and crystal structure, and the ferroelectric and dielectric properties of the films were investigated. X-ray diffraction patterns of the films revealed their polycrystalline nature. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses showed the surface of these thin films to be smooth, dense and crack-free with low surface roughness. The multilayer Pb_{1 – x} Ca _x TO₃ thin films were granular in structure with a grain size of approximately 60–70 nm. The dielectric constant and dissipation factor were, respectively, 174 and 0.04 at a 1 kHz frequency. The 600-nm thick film showed a current density leakage in the order of 10^–7 A/cm² in an electric field of about 51 kV/cm. The C-V characteristics of perovskite thin films showed normal ferroelectric behavior. The remanent polarization and coercive field for the deposited films were 15 C/cm² and 150 kV/cm, respectively.     

Power-Dependent Impedance of High Temperature Superconductor Thin Films: Relation to Harmonic Generation

We present measurements of the power-dependent microwave surface impedance of YBa₂Cu₃O_7– thin films, performed using patterned coplanar waveguide (CPW) resonators at 5.87 GHz and 76 K. We compare these resonator measurements with third-harmonic generation measurements performed on CPW transmission lines of the same geometry patterned onto the same thin-film sample at the same frequency and temperature. We find that the power-dependent surface reactance X_s(P_rf) is directly related to the magnitude of the generated third-harmonic signal, indicating a common origin for both of these manifestations of nonlinearity in high temperature superconductor (HTS) devices. These results are consistent with the nonlinear response generated by a current-dependent penetration depth (J), which provides a material limitation on the linearity that can be achieved in any practical HTS microwave device.     

Field Dependent Microwave Resistivity in YBa2Cu3O7−δ

We investigate the effects of directional pinning due to the layered structure and to columnar defects on the microwave response in YBa₂Cu₃O_7– films. We present measurements of the field-induced microwave resistivity at 48 GHz and 21 GHz taken in various relative orientations between the microwave currents, dc magnetic field and (a, b) planes. From measurements taken in the Lorentz-force-free configuration we experimentally show the relevance of the magnetic field induced increase of quasiparticle (qp) density. We identify the vortex motion contribution, and extract the vortex parameters. We estimate a pinning frequency of order 30 GHz when the field is aligned to the (a, b) plane. Secondly, we show that the introduction of columnar defects gives rise to a strong pinning along the columns, detectable even at 48 GHz. The pinning frequency appears to be of the same magnitude than for pinning by the layered structure.     

Microwave surface resistance and power dependence of thallium-based films deposited onto large-area silver substrates

Microwave surface resistance (R _s ) measurements on large-area (11.4 cm²) Tl-based films deposited onto Ag (Consil 995) substrates have been made at a frequency of 18 GHz as a function of temperature. Deposition onto unoriented Ag substrates yields unoriented films characterized byR _s values of 8.2 and 33.6 m at 10.6 and 77 K, respectively. In contrast, similar deposition onto oriented Ag substrates yields oriented films with values of 12.6 and 14.6 m at 11.2 and 77 K, respectively; corresponding Cu values are 9 and 21 m. Additionally, it is found that the dependence ofR _s on microwave surface magnetic fieldH _s is weaker for the oriented films. These results suggest that the effect of orienting thec-axis in Tl films is twofold: (1) the high-frequency superconducting transition is made considerably sharper, resulting in a lowerR _s value at 77 K, and (2) the rate of increase inR _s with appliedH _s is reduced. Both effects have significant ramifications for the potential application of these materials to high-frequency accelerating cavities.     

Transport properties of InSe x flash evaporated thin films

Thin films of InSe _x were obtained by vacuum evaporation of polycrystalline materials onto substrates at moderate temperatures,T _s. Electrical properties of films grown from different stoichiometries of flash source materials are reported in this work. The temperature dependence of the conductivities shows two conduction regimes. The low temperature regime exhibits aT ^–1/4 conductivity dependence which fits well, using the Mott model, with an average localized states density value ofN(E _F ) 8×10¹⁸cm^–3eV^–1. Hall measurements as a function of temperature show that the predominant conduction mechanism is scattering by grain boundaries in polycystalline films.     

Survey of microwave surface impedance data of high-Tc superconductors—Evidence for nonpairing charge carriers

The microwave surface impedance of the high-T _c oxide superconductors has been measured at many laboratories around the world. A survey of their data between 100 MHz and 150 GHz for polycrystalline as well as single crystalline samples is given, focusing on YBa₂Cu₃O_7– . In comparison to the classical superconductors, these results reveal a very similar temperature dependence of the surface impedance close toT _c but an anomalous high residual surface resistance at lower temperatures. Both features can be explained by the assumption that oxide superconductors contain a significant number of nonpairing charge carriers. Within the framework of a properly extended two-fluid model, this is shown by analysis of our best thin-film data. Moreover, the enhanced losses in polycrystalline material especially for superposed magnetostatic fields result, to a large extent, from the deeper penetration depth. The possible origin of the nonpairing charge carriers and their impact on the applicability of the oxide superconductors is briefly discussed.     

Structural and electrical properties of the Y-Ba-Cu-O superconducting films prepared by spray pyrolysis

High-T _c , superconducting YBa₂Cu₃O_7– thin films have been grown on (100) MgO substrates by a chemical spray pyrolysis method. The crystal structure and surface morphology have been studied by X-ray diffraction and scanning electron microscopy, respectively. The assprayed films were amorphous and insulating, but upon annealing the films became superconducting and show a textured surface morphology with an average grain size of the order of 5–15m. The films were highly oriented with thec-axis being perpendicular to the substrate surface. Three different microstructures were recorded: long rod-shaped grains, platelet or rounded-shape grains, and a melting-like growth. Electrical measurements were carried out in a low-temperature cryostat using a standard d.c. four-probe technique. The onset transition temperature was around 83–86 K, and the completion of the transition to zero resistance was in the range 73–78 K. The magnitude of the measured critical current density was in the range 750–3750 A/cm² at 30 K. A correlation between the resistance of the tunnelling junctions and the critical current density was found from the theoretical models.     

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.     

Simultaneous Double-Sided Deposition of Large Size HTSC Thin Films

An apparatus for simultaneous deposition of large size double-sided YBCO thin films by sputtering from single inverted cyclindrical target was constructed. Double-sided YBCO thin films on LaAlO₃ (100) substrate up to 30 mm in diameter were prepared to test the performance of the apparatus. The microwave surface resistance, R _s (77 K, 10 GHz, 0 T), of the YBCO thin films on both sides of the wafer ranged from 500 to 800 . Values of R _s (75 K, 145 GHz, 0 T) below 60 m were reached over the area of 20 mm × 15 mm on the wafer. The majority of the wafer area given in percent has R _s (75 K, 145 GHz, 0 T) values in the range of 21.5– 45.5 m. Lateral homogeneity of R _s values in the whole wafer was good enough to well meet the microwave application.