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.

The impact of strain on structure and ferroelectric properties of epitaxial SrTiO 3 films on various substrate materials-substrates with larger ͑DyScO 3 ͒ and smaller ͑NdGaO 3 and CeO 2 /Al 2 O 3 ͒ in-plane lattice constant, respectively-was analyzed. In all cases, ͑001͒-oriented strained epitaxial SrTiO 3 was obtained. It is demonstrated that the mismatch of the lattices or, alternatively, the mismatch of the thermal expansion coefficients of films and substrate, imposes biaxial strain on the SrTiO 3 films. The strain leads to a small tetragonal distortion of the SrTiO 3 lattice and has a large impact on the ferroelectric properties of the films. With decreasing film thickness and at low temperatures the permittivity deviates from the "classical" Curie-Weiss behavior. Furthermore, strain-induced ferroelectricity is observed, which agrees with theoretical predictions. For electric fields parallel to the film, surface-induced ferroelectricity is observed for SrTiO 3 that is exposed to in-plane tensile strain, i.e., SrTiO 3 on DyScO 3 and sapphire. Transition temperatures of T o Ϸ 210 K and T o Ϸ 325 K are obtained for SrTiO 3 on CeO 2 /Al 2 O 3 and DyScO 3 , respectively.

We report on combined dc and microwave electronic measurements of magnetic flux transport in micron and submicron-patterned high-T c films. In a given temperature regime below the superconducting transition temperature T c , the current-driven flux transport is restricted to flux motion guided by the submicron patterns. Via frequency-dependent measurements of the forward transmission coefficient S 21 it is demonstrated that the mechanism of the guided flux transport depends on the microwave frequency and the geometrical size of the superconducting structures. At low frequencies, flux is transported via Abrikosov vortices leading to additional microwave losses. Above a geometrically defined frequency, a different mechanism seems to be responsible for flux transport that does not contribute to the microwave losses and most likely represents a phase-slip type mechanism. The limiting vortex velocity obtained from the frequency dependence of the microwave properties agrees with the Larking-Ovchinnikov critical vortex velocity that is determined via dc pulse measurements. In spite of the change of mechanism, guidance of flux persists in these nanopatterns up to high frequencies of several GHz.

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A micropattern induced transition in the mechanism of vortex motion and vortex mobility is demonstrated for high-Tc films. The competition between the anomalous Hall effect and the guidance of vortices by rows of microholes (antidots) leads to a sudden change in the direction of vortex motion that is accompanied by a change of the critical current density and microwave losses. The latter demonstrates the difference in vortex mobility in the different phases of vortex motion in between and within the rows of antidots.