D. Kalokitis

Microwave properties of highly oriented YBa2Cu3O7−x thin films

We have performed intra‐ and extra‐cavity microwave frequency (1–100 GHz) measurements on high quality Y1Ba2Cu3O7−x superconducting thin films on (100) LaAlO3 substrates. The ∼0.3 μm thin films fabricated by the pulsed laser deposition technique exhibit superconducting transition temperatures >90 K, as determined by resistivity and ac susceptibility measurements, and critical current densities of 5×106 A/cm2 at 77 K. Moreover, ion beam channeling minimum yields of ∼3% were measured, indicating the extremely high crystalline quality of films grown on the LaAlO3 substrate. Microwave surface resistance values at 77 K for these films are found to be more than one to two orders of magnitude lower than for copper at 77 K for almost the entire frequency range explored. We postulate that the reason we observe such low surface resistances in these films is the virtual absence of grain and phase boundaries coupled with the high degree of crystallinity. Furthermore, we believe that the residual resistance measured b...

Low-loss substrate for microwave application of high-temperature superconductor films

We have shown that strontium lanthanum aluminate (SrLaAlO4) has considerable potential as a substrate material for high‐temperature superconductor (high Tc) microwave applications. Excellent lattice match is obtainable on a 14° tilt from the c axis. The lattice dimensions encourage ab plane epitaxial growth. The thermal expansion was found to be 7.4×10−6 °C−1. Patterned sputtered niobium resonators on SrLaAlO4 exhibit Q values at 5 K comparable to similar niobium resonators on polycrystalline alumina, (Q≳10 000 from 2 to 15 GHz). Deposited YBa2Cu3O7−x (YBCO) films on this substrate show sharp ac susceptibility transitions indicating good homogeneity and ab plane orientation.

Measurement of microwave surface resistance of patterned superconducting thin films

A simple and versatile measurement technique has been demonstrated for characterization of thin film superconductors and the substrates on which they are deposited. Unlike other state of the art measurements that characterize only the top layer of unpatterned thin films in end-plate replacement or perturbation-type cavity arrangements, we test the sample under conditions very close to those of actual microwave circuit applications. Measurements on a laser deposited, thin-film YBCO superconductor, patterned in the form of a meander line on a LaAlO3 substrate, show a microwave surface resistance one order of magnitude lower than that of copper at 79 K and 11 GHz. At 1.4 GHz and 79 K, an improvement by over two orders of magnitude is observed. This is the first demonstration of superior microwave performance of a comb filter structure using a HTS thin film.

30-Way Radial Power Combiner for Miniature GaAs FET Power Amplifiers

A compact radial divider/combiner structure is described that combines 30 miniature GaAs FET power amplifiers. A power output of 26 W has been achieved at 11.3 GHz with a 0.5-dB bandwidth of 600 MHz. The combiner is small, lightweight, low loss and offers graceful degradation.

Buffer layers for high-Tc thin films on sapphire

Buffer layers of various oxides including CeO2 and yttrium-stabilized zirconia (YSZ) have been deposited onR-plane sapphire. The orientation and crystallinity of the layers were optimized to promote epitaxial growth of YBa2Cu3O7−δ (YBCO) thin films. An ion beam channeling minimum yield of ∼3% was obtained in the CeO2 layer on sapphire, indicating excellent crystallinity of the buffer layer. Among the buffer materials used, CeO2 was found to be the best one for YBCO thin films onR-plane sapphire. HighTc andJc were obtained in YBCO thin films on sapphire with buffer layers. Surface resistances of the YBCO films were ∼4 mω at 77 K and 25 GHz.

Performance of a narrow band microwave filter implemented in thin-film YBa2Cu3O7-δ with ohmic contacts

Using in situ deposited thin‐film Y1Ba2Cu3O7−δ on LaAlO3 substrates, we have developed a pseudo‐interdigital three‐pole stripline filter with a minimum passband loss under 3.5 dB at 9.5 GHz and 77 K. The passband is very narrow, having a bandwidth of only 50 MHz (0.5%). The filter occupies a volume of less than 0.1 cm3. An equivalent copper filter produced a loss of 18.5 and 28 dB at 77 and 300 K, respectively. The input and output coupling is achieved by ex situ sputtered Au thin‐film contacts defined on the Y1Ba2Cu3O7−δ by a lift‐off technology and ultrasonically bonded to hermetic K connectors. Some of the major concerns in the realization of narrow band filters such as air gaps between high dielectric constant substrates and uniformity of the substrate’s dielectric constant are addressed.

Off‐axis sputter deposition of YBa2Cu3O7 thin films for microwave applications

Thin films of superconducting YBa2Cu3O7 were grown in situ by off‐axis sputter deposition for microwave device fabrication. These ∼1 cm2 films, which are reproducible, exhibit midpoint Tc’s of 89–90.5 K as measured by ac susceptibility, ion channeling yields of 4.7%–6%, and c‐axis rocking‐curve half‐widths of 0.5°, even with a rich microstructure as seen by scanning electron microscopy. Two films were photodefined into miniature X‐band microwave bandpass filters. These narrow‐band filters (0.5% bandwidth) exhibited 4.4‐and 4.5‐dB insertion losses at 77 K and 9.25 GHz, with little temperature dependence below 80 K.

Superconducting narrow band pass filters for advanced multiplexers

X-band microstrip filters with a 50-MHz-wide passband that provide 1.3-dB insertion loss at 77 K using copper ground planes have been built. Recent advances in double side coating of lanthanum aluminate substrates with YBa/sub 2/Cu/sub 3/O/sub 7- delta / will lower this figure to 0.6 dB. Circuit designs were evaluated for intermodulation distortion effects. An improvement over previous designs has increased the third-order output intercept point to +29 dBm. Methods of nondestructive dielectric and conductor loss testing were used to evaluate materials throughout the filter development.<<ETX>>

Superconducting nonreciprocal devices for microwave systems

The feasibility of applying high-temperature superconductor (HTS) technology to nonreciprocal microwave devices has been demonstrated for the first time. Such devices in the form of isolators and circulators are used widely to achieve stability, reliability, and reproducibility in microwave circuit performance. The experimental X-band device, a three-port stripline circulator consisting of a thin sapphire substrate coated with a YBCO film and sandwiched between two ferrite disks, showed low insertion loss (0.25 dB) and very high isolation (>30 dB) at an operating temperature of 77 K. This technology permits the integration of many HTS microwave components together with nonreciprocal devices on a common substrate.<<ETX>>

High temperature superconducting components for microwave systems

Abstract Starting with an overview of the various requirements placed on substrates for superconducting microwave applications, the current state of the art of high temperature superconducting thin films on different substrate types is discussed. Recent results for several superconducting microwave components that could form important building blocks for advanced microwave systems are presented, including performance data on experimental devices such as narrow band-pass and band-stop filters, superconducting circulators and miniature, low-frequency spiral resonators. Some examples of system types that can most likely benefit from high temperature superconductivity are identified.