A.A. Valenzuela

Microwave properties of YBa2Cu3O7−x thin films studied with coplanar transmission line resonators

The microwave properties of epitaxial YBa2Cu3O7−x thin films with critical temperatures Tc up to 91.5 K and critical current densities Jc up to 6×106 A/cm2 at 77 K have been investigated between 5 and 20 GHz using coplanar transmission line resonators. Information about the surface resistance Rs and the magnetic penetration depth λL has been obtained from temperature‐ and power‐dependent measurements of the quality factors and resonance frequencies of the resonators. The lowest surface resistance was 105 μΩ at 77 K and 38 μΩ at 9 K measured at 6.2 GHz, respectively. The dependence of Rs on the surface magnetic field Brf showed that with decreasing film quality the films exhibit a granular behavior. At high critical current densities Jc≳4×106 A/cm2, the intrinsic properties of the material show up. The magnetic penetration depth was in the range between 160 and 270 nm. Both Rs and λL turned out to be sensitive to the film quality represented by the transition width ΔTc and the critical current density Jc a...

DC-magnetron sputtering of YBa2Cu3O7−δ thin films for microwave applications

Abstract High-quality YBa 2 Cu 3 O 7−δ thin films with optimized surface smoothens suitable for multilayer technology have been deposited by DC-magnetron sputtering. The films not only show high critical temperatures ( T c ) up to 90 K and high critical current densities ( j c ) up to 4.2×10 6 A/cm 2 at 77 K, but also a smooth surface with only one outgrowth of a size of more than 200 nm within an area of 150×150 μm 2 . The T c and j c values are slightly lower than our best values for rough films with optimized conditions for T c and j c , where we reach up to 6×10 6 A/cm 2 at 77 K and a T c offset up to 91.5 K on SrTiO 3 and La AlO 3 . Additionally, the surface resistances ( R s ) of our films were measured using coplanar resonators. We determined an R s of 120 μΩ at 77 K and 60 μΩ at 9 K and at 6.2 GHz for our best film. The penetration depth was evaluated to be between 160 and 200 nm for this film. A dependence of R s on the surface magnetic field showed a higher crystal quality of our films with increasing j c measured with a remanent field technique.

A coplanar transmission line high-T/sub c/ superconductive oscillator at 6.5 GHz on a single substrate

The design and construction of a planar, low-noise cryogenic oscillator operating at 6.5 GHz are presented. The oscillator has been built as a hybrid superconductive microwave integrated circuit (SMIC) on a single 10*10 mm LaAlO/sub 3/ substrate. Single-sided, coplanar line structures are used throughout the circuit with YBa/sub 2/Cu/sub 3/O/sub 7- delta / as conductor material. The oscillator was constructed around a GaAs MESFET as the active device. The complete oscillator is cooled by immersion in liquid nitrogen. An output power of 4.9 dBm was obtained. Single-sided noise power at 10 kHz offset from carrier was -90 dBc/Hz.<<ETX>>

Narrow-band YBCO superconducting parallel-coupled coplanar waveguide band-pass filters at 10 GHz

High-T/sub c/-superconducting coplanar-waveguide (CPW) three-pole, four-pole, and five-pole bandpass filters fabricated from in situ sputter-deposited YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin-films on LaAlO/sub 3/ substrates with dimensions 10 mm*25 mm are presented. The design is based on the evaluation of CPW test chips integrating different resonators and transmission line structures. The measured data were fed into a CAD (computer-aided design) program using general transmission line elements instead of CPW elements. Both YBCO and gold versions were constructed and mounted in gold-plated brass test fixtures. At liquid nitrogen temperature (77 K), the filters were characterized by center frequencies at 10 GHz and bandwidths smaller than 1.3%. Total insertion loss and out-of-band rejection values of the complete packaged devices were better than 2.2 dB and 30 dB, respectively.<<ETX>>

Planar transmission line resonators from YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films and epitaxial SIS multilayers

The authors have grown c-axis-oriented YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films and epitaxial YBCO/NdAlO/sub 3//YBCO trilayers by sputtering. From the YBCO films, coplanar half-wavelength transmission line resonators were patterned. At 77 K the unloaded quality factors of coplanar stripline resonators were up to 2570 at 5.6 GHz. The attenuation constant of such a line was evaluated to be 0.6 dB/m at 77 K and at 5.6 GHz. Coplanar waveguide resonators yielded quality factors up to 1200 at 77 K and at 6.2 GHz resulting in an attenuation constant of 1.8 dB/m. From these measurements the surface resistance values R/sub S/ of the films were determined. The lowest R/sub S/ was 105 mu Omega at 77 K and at 6.2 GHz. Trilayer structures with a dielectric as thick as 800 nm were patterned into microstrip resonators. At 50 K the authors measured a quality factor of 85 at 4.7 GHz, resulting in an attenuation of 26 dB/m and a surface resistance of 340 mu Omega . For thick dielectrics the upper YBCO film already contains significant portions of polycrystalline material.<<ETX>>

Property Calculations of a Superconducting Coplanar Waveguide Resonator

We present calculations of the properties of the first coplanar waveguide transmission line resonators patterned from high-Tc superconducting films at 8.8 and 6.5 GHz [1, 2]. Based on field calculations of the corresponding coplanar waveguides with infinite conductivity, performed by a partial wave synthesis (mode-matching method) [3], we obtain the superconductor losses subsequently using Londons theory. Due to the high conductivity of the superconductive material both the electric and magnetic fields within the superconductor may be assumed to be parallel to the surface planes. For the small conductor thickness the tangential electrical field on each surface plane depends on the tangential magnetic fields on both surface planes. This relation is modelled by the surface impedance matrix Zs. The substrate losses are treated in the field calculations and are also taken into account. The calculated overall Q values are compared with the measured values and it has to be pointed out, that the superconducting thin films are due to the available material datas of the YBaCuO superconductor and the used substrate materials of a very good quality, because they reach nearly their upper limit given by the known material datas.

YBCO superconducting microwave components

Abstract The design and performance of coplanar high-Tc superconducting microwave components which have been fabricated from YBCO thin-films on LaAlO3 substrates are presented. The devices are as follows: 1. (i) a resonator at 6.5 GHz consisting of a short-circuited halfwave transmission line; 2. (ii) a 6.5 GHz low-noise single-substrate oscillator on a substrate with dimensions 10 × 10 mm2; 3. (iii) narrow-band three-pole and five-pole parallel-coupled band-pass filters at 10 GHz; 4. (iv) bent (meander line) interconnection lines on a substrate with dimensions 20 × 20 mm2.

High temperature superconducting coplanar and epitaxially grown microstrip transmission lines studied by a half-wavelength resonator technique

Abstract Coplanar YBa 2 Cu 3 O 7− x (YBCO) thin film and epitaxial YBCO/NdAlO 3 /YBCO microstrip transmission lines were investigated using planar half wavelength resonators. The results were compared to theoretical calculations. Coplanar transmission lines in coplanar waveguide and coplanar stripline geometries exhibited unloaded quality factors up to 2600 and attenuation constants down to 0.6 dB/m at 77 K and at frequencies of 5.5 GHz. These results are two orders of magnitude below normal metal lines. Planar microstrip resonators which were epitaxially grown by combined d.c./r.f. sputtering showed attenuation constants down to 26 dB/m at 50 K and at 4.7 GHz. The results achieved with the microstrip lines require a further technological development in order to be improved while they are already significantly below normal metal based lines. These results are already promising with respect to a use as transmission lines in integrated circuits.

Surface resistance and magnetic penetration depth of structured YBa2Cu3O7-δ-thin films

Abstract We have determined the high-frequency properties of epitaxial YBa2Cu3O7−δ-thin films that are crucial for applications of this high-temperature superconducting material for passive microwave devices. From the quality factor and resonance frequency of integrated coplanar halfwave resonators the surface resistance and the magnetic penetration depth as a function of temperature and rf-power are deduced. Notably this quantities can be obtained for individual thin films and so their microwave performance can comprehensively be characterized. This planar resonator based method not only offers high sensitivity within a wide frequency range but also the ease of integrating resonators as a small-sized test structure with loss mechanisms that would also be present in actual passive devices. As the interpretation of the measured data needs full knowledge of the electric and magnetic field distribution in the cross-section of the resonator line a theoretical field wave analysis is performed employing a “partial wave synthesis” (mode-matching method). Any ambiguity in the determination of the temperature dependence of the magnetic penetration depth is removed by the simultanuous evaluation of pairs of resonator structures of different width. This novel approach yields the magnetic penetration depth as a function of temperature without assuming a specific model. Knowledge of the absolute value of the penetrationg depth also allows the determination of the surface resistance with great accuracy as it is shown in comparison with other methods. Power dependent measurements finally give information about weak-links in the epitaxial films possibly associated with grain- and twin boundaries.

Testchip for high temperature superconductor passive devices

A testchip for fully characterizing high-temperature-superconducting (HTS) thin-film properties relevant to planar passive microwave device applications is presented. The chip integrates coplanar resonators and transmission lines along with structures for process monitoring. Measurements of the quality factor of coplanar resonators as a function of temperature and input power are reported. For the coupling of the resonators to the input signals, microwave probes with 40-GHz bandwidth have been used within the cryo-environment. Quality values obtained at 5 GHz and 77 K are superior to that of an equivalent copper resonator by a factor of about 40.<<ETX>>