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.

A very sensitive dielectric resonator technique is employed to measure loss tangent tan δ and relative permittivity εr of lanthanum aluminate (LaAlO3) single crystals at 4–300 K and 4–12 GHz. A variety of single crystals grown by different techniques and purchased from different suppliers are considered. For T>150 K the loss tangent tan δ is almost sample independent with linear frequency dependence and monotonous temperature variation from 8×10−6 at 300 K to 2.5×10−6 at 150 K and 4.1 GHz. In this temperature range the experimental data are explained by a model based on lifetime broadened two-phonon difference processes. The loss tangent below 150 K is characterized by a peak in tan δ(T) at about 70 K. The height of this peak is frequency and strongly sample dependent. This leads to a variation of the loss tangent from 10−6 to 1.5×10−5 at 77 K and 8.6 GHz, the lowest values are generally achieved with Verneuil grown crystals and approach the intrinsic lower limit predicted by the phonon model. The peak is explained by defect dipole relaxation (local motions of ions). The activation energy of the relaxation process is determined from the measured data to be 31 meV. This low value indicates that the defect dipoles are associated with interstitials, possibly impurities in interstitial positions. Considering absorption due to phonons and due to defect dipole relaxation the loss tangent is calculated for a wide frequency range.

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Ab~truct-YBa~Cu~0~-, thin films were prepared on 2 inch in diameter (1102) sapphire substrates buffered with Ce02 layer of mixed (001)/(111) orientation. The thickness of the YBazCu307., films was typically -250 nm. The YBa2C~307-x thin films exhibited smooth surfaces (peak-to-valley roughness of less than 20 nm) free of cracks and outgrowths. The critical temperatures of these films were 87 -89 K, the critical current densities (2-3).106 Mcm2 at 77 K and zero magnetic field. The low field microwave surface resistance (Rs) of the YBa2C~307-n films was measured at 18.7 GHz. Values of -1.4 mR were obtained at 77 K and < 70 pR below 20 K. Such low Rs values are comparable to the lowest reported values for thicker YBa2Cu307., films grown epitaxially on structurally well-matched substrates, e.g. LaA103. The elevation of the microwave power produced a weak increase of R,. No drastic changes in Rs occur up to the maximum magnetic field of -35 Oe at 79 K and -63 Oe at 50 K. The properties of the YBa2Cu30,-x films do not degrade with time.

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