The microwave properties of single crystalline TiO2 (rutile) were investigated. At a frequency of 7.5 GHz the loss tangent tan δ was found to increase from 1.4×10−7 at 4 K to 4×10−6 at 70 K for electric fields parallel to the crystallographic a,b plane. The high permittivity of 105 and the small tanδ in combination with the low microwave losses of high temperature superconductors (HTS) were utilized to construct a miniaturized X-band resonator with a high quality factor Q. An assembly of two YBa2Cu3O7 films of 8 mm in diameter separated by a rutile cylinder of 2 mm height provides a TE011 resonance at 9.7 GHz with Qs ranging from 6×105 at 10 K to 105 at 70 K. Frequency scaling of the losses in rutile and in the HTS films indicates Qs in excess of 106 at 1.8 GHz using YBa2Cu3O7 films of two inches in diameter. Such resonators are considered to be key elements for high-power filters in mobile communications.

Epitaxial thin films of YBa2C!u307 have been prepared on SrTiOs and LaAlOs substrates by a high-pressure planar dc-sputtering technique. By covering the substrate heater with a frame of polycrystalline YBazCu30~ -substantial improvements of the YBa2Cu307 film properties were achieved. These are characterized by dc-resistivity values p(T) of less than 50 @I cm at 100 K and ~(300 K)/p( 100 K) values of up to 3.9. Significant deviations from the usual linear p(T) behavior were found. Critical temperatures above 90 K, resistive transition widths down to 0.3 K, and critical current densities of about 5 x lo6 A/cm2 at 77 K confirm the high quality of the films. As indicated by Rutherford backscattering and high-resolution transmission electron microscopy the films exhibit a microstructure characterized by a reduced density of lattice defects. However, lattice-coherent precipitates with a diameter of about 5-10 nm were observed. As an outstanding feature the films exhibit, besides the initial steep falloff at T, a further gradual decrease of the microwave surface resistance at 87 GHz below 50 K by at least one order of magnitude. These results are very promising for millimeter-wave applications of epitaxial YBaz.Cu307 thin films.

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We have measured the microwave properties of high quality YBCO thin films using a copper cavity (87GHz) and a dielectric resonator (19GHz) for unpatterned films as well as a coplanar resonator (5, 8, 10,15 and 16GHz) for patterned films. For the patterned films the surface resistance (R& and non-linear effects were studied as a function of temperature below T, . For practical applications, the assessment of non-linear effects in the surface resistance are of great importance.The best films showed R, approximately constant up to microwave peak current densities of the order of the dc critical current density. The measurements were performed on samples fabricated by a number of different techniques.

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