A.C. Anderson

Stripline resonator measurements of Z/sub s/ versus H/sub rf/ in YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films

A report is presented on measurements of the surface impedance, Z/sub S/, of YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films using a stripline resonator. The films were deposited on LaAlO/sub 3/ substrates by off-axis magnetron sputtering. The authors obtained Z/sub S/ as a function of frequency from 1.5 to 20 GHz, as a function of temperature from 4 K to the transition temperature ( approximately 90 K), and as a function of the RF magnetic field from zero to 300 Oe. At low temperatures the surface resistance, R/sub S/, of the films shows a very weak dependence on the magnetic field up to 225 to 250 Oe. At 77 K, R/sub S/ is proportional to the square of the field. The penetration depth shows a much weaker dependence on the field than does R/sub S/. The origins of the magnetic field dependence of Z/sub S/ are also discussed. >

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

We review methods of measuring surface resistance (Rs) of thin films using stripline resonators, and present our measurements of theRs of YBa2Cu3O7−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, theRs values obtained for the two deposition methods are approximately 7×10−6 and 4×10−6 Ω, respectively.

Surface impedance measurements of YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films in stripline resonators

The microwave surface impedance of thin films of YBa/sub 2/Cu/sub 3/O/sub 7-x/ is measured as a function of temperature, frequency, and RF magnetic field, H/sub RF/, using a stripline-resonator technique. The films were deposited in situ by single-target off-axis magnetron sputtering. The frequency range was from 0.4 to 20 GHz, the temperature range from 4.2 K to T/sub c/, and the RF magnetic field range from 0 to 30 Oe. The surface resistance R/sub S/ at 4.2 K and 1.5 GHz is 4*10/sup -6/ Omega . The penetration depth is determined to be 0.167 mu m in the best film, In some films, R/sub S/ shows a linear dependence on H/sub RF/ and rises rapidly at large fields, showing no evidence of saturation. In others, R/sub S/ shows weak dependence on H/sub RF/ before rising rapidly. These behaviors differ from those observed in bulk ceramic YBa/sub 2/Cu/sub 3/O/sub 7-x/ and in unpatterned films measured in microwave cavities. However, the shape of R/sub S/(H/sub RF/) for the YBa/sub 2/Cu/sub 3/O/sub 7-x/ films is similar to that of Nb and NbN films measured in the same stripline geometry. The authors also present measurements of the intermodulation products in the resonators and discuss the implications of the R/sub S/ and intermodulation measurements for microwave device applications.

High-T/sub c/ superconductive microwave filters

Four-pole microstrip bandpass filters have been fabricated using both postannealed and in-situ-grown thin films of Y-Ba-Cu-O. The 4-GHz, 3% bandwidth filters exhibit, at 77 K, a passband insertion loss as low as 0.3 dB, compared with the 2.8-dB loss of similar gold metallic filters at the same temperature. These results demonstrate that complex passive microwave devices can be designed and implemented using high-temperature superconducting (HTS) material. More aggressive filter designs utilizing many more poles will result in much greater performance advantages for an HTS filter technology over a normal metal technology. Ultimately, ultrasharp-skirt filters can be produced, which would allow for more closely spaced communication channels and more efficient use of microwave communication bands than is currently possible with conventional filters.

High-power HTS microstrip filters for wireless communication

The performance of narrowband microstrip filters with low insertion loss and high power-handling capabilities made from YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) high-temperature superconducting (HTS) thin films is presented. Results are shown for two different designs that were chosen to optimize the power-handling capability. Both filters have a 2-GHz center frequency and 5 poles that incorporate coupled resonators with 10-/spl Omega/ internal impedances. They were made on 5-cm-diameter LaAlO/sub 3/ substrates. Both designs use parallel-coupled feed lines to avoid current crowding, The first design includes backward- and forward-coupled filters, has 1% bandwidth, and has handled over 25 watts of input power at 10 K with less than 0.25 dB compression. The second design has 1.2% bandwidth and uses only forward-coupled resonators. The dissipation loss is less than 0.2 dB at 45 K and it has a third-order intercept of 62 dBm. Another similar filter handled 36 watts of power at 45 K with less than 0.15 dB compression across the passband. We have developed a technique to visualize the power dissipation of the filter by observing the bubbles created by the filter when submerged in liquid helium, showing areas with local defects or where the current distribution is at its peak value. We also discuss several planar high-power filter issues, including material selection and fabrication, device configuration trade-offs, filter structure optimization, and design approaches to maximize power-handling capacity.

High-power high-temperature superconducting microstrip filters for cellular base-station applications

We report narrowband microstrip filters with low insertion loss and high power-handling capabilities made from YBa/sub 2/Cu/sub 3/O/sub 7/(-x) high-temperature superconducting films. One 5-pole filter on a LaAlO/sub 3/ substrate, consisting of backward-coupled and forward-coupled resonators, can handle over 27 W input power at 10 K. It has 1% fractional bandwidth and 10-/spl Omega/ internal impedance. The insertion loss increase in passband at 10 K, as the input power changed from a few mW to 27 W, is less than 0.25 dB. We also report a forward-coupled microstrip filter centered at 2 GHz, with a 1.2% fractional bandwidth, 10-/spl Omega/ internal impedance, and parallel-coupled feed lines. We applied over 10 /spl Omega/ to the filter at 45 K without noticeable degradation of the filter performance. The insertion loss at 45 K is less than 0.2 dB. The return loss is better than 12 dB.<<ETX>>

Infrared refractive indices of LaAlO 3 , LaGaO 3 , and NdGaO 3

We discuss the experimental and theoretical studies of the infrared optical constants of three perovskite materials: LaAlO3, LaGaO3, and NdGaO3. These materials are commonly used as substrates for depositing high-temperature superconducting films. The transmittance and the reflectance are measured with a Fourier-transform infrared spectrometer. At frequencies from 1000 to 10 000 cm−1, for which the substrates are transparent, the refractive index is extracted directly from the measured transmittance and reflectance. Kramers–Kronig relations are employed to yield the refractive index of these materials at frequencies from 100 to 1000 cm−1. We also use a Lorentz phonon oscillator model to obtain the refractive index by fitting the measured data to the model function. The advantages and limitations of each method are discussed. The refractive index for the LaAlO3 substrate is also obtained at 10 and 78 K with a cryogenic accessory. The results are compared with those reported by other research groups. This research provides information necessary for the quantitative study of the optical properties of thin superconducting films.

Superconductive tapped delay lines for microwave analog signal processing

Passive superconducting tapped delay lines have been fabricated for use as matched filters for multigigahertz bandwidth analog signal processing. Specifically, linear frequency-modulated dispersive delay lines, also known as chirp filters, having a bandwidth of 2.6 GHz centered at 4 GHz and a dispersion time of 35 ns have been constructed. The stripline structure consists of a 4000-A-thick patterned niobium film sandwiched between 5-cm-diameter, 125-μm-thick sapphire wafers. Two parallel striplines, each 1.6-m long, are wound in a spiral pattern. The taps are backward-wave couplers formed by bringing the two lines into and out of proximity at specified locations. Pulse expansion and compression have been demonstrated with these devices and are in close agreement with a new theoretical model for this class of signal processors.

Superconductive delay-line technology and applications

Microwave analog signal-processing filters have been realized in the form of coupled niobium striplines on silicon dielectric substrates. Device responses with ± 2-dB amplitude accuracy and 9°-rms phase error have been achieved in amplitude-weighted filters with 37.5 ns of dispersion and 2.3-GHz bandwidths. Relative side-lobe levels of -26 dB and less are currently obtained. The achievable dispersion for stripline circuits on a single pair of 5-cm-diameter, 125-μm-thick wafers is limited to about 40 ns by the electro-magnetic coupling between neighboring lines. To achieve greater dispersion two approaches are under development: (1) Stripline circuits are being fabricated on multiple wafer pairs which are physically stacked and electrically concatenated to produce dispersive delay lines with 4-GHz bandwidth and 75-ns dispersion time. Phenolic resin is used as an adhesive to ensure the mechanical integrity of the stacked structure. (2) A technique to fabricate dense stripline circuits on very thin (15-μm) single-crystal silicon superstrates supported by thicker substrates has been demonstrated and preliminary results will be described. A chirp-transform system capable of real-time spectral analysis has been constructed using a pair of the superconductive delay-line filters. A resolution of 43 MHz over an unprecedented 2400-MHz bandwidth with amplitude uniformity of ±1 dB and side-lobe levels of -18 dB was demonstrated.

Relation between electrical properties and microstructure of YBa2Cu3O7−x thin films deposited by single‐target off‐axis sputtering

The relationship between the deposition conditions and the structural and electrical properties of in situ superconducting YBa2Cu3O7−x thin films deposited by off‐axis magnetron sputtering has been investigated. High‐quality films have been produced with a transition temperature TC (R=0) of 92 K, a critical current density JC (zero field) of 3.3×107 A/cm2 at 4.2 K and 4.8×106 A/cm2 at 77 K, and a microwave surface resistance RS of 2.6×10−6 Ω at 1.5 GHz and 4.2 K which rises to 8.3×10−6 Ω at 77 K. Among the deposition conditions explored, substrate temperature was identified as the most influential in producing these high‐quality films. A quantitative relationship was established between substrate temperature and TC, normal‐state resistivity ρ, JC, orientation distribution, x‐ray‐diffraction peak broadening, lattice expansion, RS, and penetration depth λ. Increasing substrate temperature results in an increase in TC, a decrease in ρ, an increase in JC, an increase in grain size, an increase in the ratio of...