R. Kutzner

Dielectric properties of rutile and its use in high temperature superconducting resonators

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.

Induced ferroelectricity in strained epitaxial SrTiO3 films on various substrates

The impact of strain on structure and ferroelectric properties of epitaxial SrTiO3 films on various substrate materials—substrates with larger (DyScO3) and smaller (NdGaO3 and CeO2/Al2O3) in-plane lattice constant, respectively—was analyzed. In all cases, (001)-oriented strained epitaxial SrTiO3 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 SrTiO3 films. The strain leads to a small tetragonal distortion of the SrTiO3 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 SrTiO3 that is exposed to in-plane tensile strain, i.e., SrTi...

Microwave losses and structural properties of large-area YBa/sub 2/Cu/sub 3/O/sub 7/ films on r-cut sapphire buffered with [001]/(111) oriented CeO/sub 2/

YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films were prepared on 2 inch in diameter (11_02) sapphire substrates buffered with CeO/sub 2/ layer of mixed [001]/(111) orientation. The thickness of the YBa/sub 2/Cu/sub 3/O/sub 7-x/ films was typically /spl sim/250 nm. The YBa/sub 2/Cu/sub 3/O/sub 7-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).10/sup 6/ A/cm/sup 2/ at 77 K and zero magnetic field. The low field microwave surface resistance (R/sub S/) of the YBa/sub 2/Cu/sub 3/O/sub 7-x/ films was measured at 18.7 GHz. Values of /spl sim/1.4 m/spl Omega/ were obtained at 77 K and <70 /spl mu//spl Omega/ below 20 K. Such low R/sub S/ values are comparable to the lowest reported values for thicker YBa/sub 2/Cu/sub 3/O/sub 7-x/ films grown epitaxially on structurally well-matched substrates, e.g. LaAlO/sub 3/. The elevation of the microwave power produced a weak increase of R/sub S/. No drastic changes in R/sub S/ occur up to the maximum magnetic field of /spl sim/35 Oe at 79 K and /spl sim/63 Oe at 50 K. The properties of the YBa/sub 2/Cu/sub 3/O/sub 7-x/ films do not degrade with time.

Growth of high‐quality YBa2Cu3O7−x films on CeO2 buffer of mixed (001)/(111) orientation on sapphire

High‐quality epitaxial YBa2Cu3O7−x (YBCO) thin films are grown on large area (1ub;‐21ubx02) sapphire (r‐cut) substrates buffered by CeO2 of mixed (001) and (111) orientation. It is shown that (001) and (111) CeO2 epitaxy on (1ub;‐21ubx02) sapphire is possible with further growth of well‐oriented (001) YBCO on both orientations of CeO2. In the case of (111) CeO2 one of the equivalent in‐plane trigonal directions, i.e., [10ub;‐21ubx], [ub;‐21ubx10], or [01ub;‐21ubx], is aligned along either [02ub;‐22ubx1], or [2ub;‐22ubx01] crystallographic axis of sapphire, which is indicated by the φ‐scan x‐ray diffraction pattern of the (115) CeO2 reflex. The resulting (001) YBCO film grown on the (001)/(111) CeO2 buffer reveals no disorientations in the a‐b plane other than conventional 90° twinning. The YBCO films exhibited very smooth surface and were free of screw dislocations. The high quality of the YBCO films is confirmed by the microwave surface resistance of Rs≤0.44 mΩ at 10 GHz and 77 K.

Reduction of the microwave surface resistance in YBCO thin films by microscopic defects

Abstract The impact of microscopic defects upon the microwave properties of high- T c superconductor (HTS) films is examined. YBa 2 Cu 3 O 7 films with different size and densities of Y 2 O 3 precipitates are grown on LaAlO 3 and sapphire by variation of the energy of the ions during sputter deposition. It is demonstrated, that the temperature dependence of the microwave surface resistance R s does not depend on the type of substrate material but on the density of the defects. Films grown at low ion energy (resulting in a low density of microscopic defects) show a characteristic shoulder in the R s ( T ) curve which shifts to higher temperature and decreases in size with increasing energy of the ions (i.e. increasing density of microscopic defects). Temperature dependence and reduction of the surface resistance with increasing density of defects are explained in terms of the two-fluid model with thermally excited quasiparticles characterised by a Drude-shaped conductivity spectrum. Values for the scattering rates can be derived from the measurements of the surface resistance, which agree with the classical Matthiesen rule. The impurity scattering rate increases with increasing defect density. Finally, the experimental data and the theoretical model demonstrate, that the surface resistance can be reduced by up to a factor of 2 over a wide temperature range. The reduction of the surface resistance is accompanied by an improvement of the mechanical properties of the HTS thin films which leads to an increased critical film thickness. Both properties, namely the increase of the critical thickness and the reduction of the microwave surface resistance, demonstrate the potential of microscopic defects for improvement of HTS films for applications.

Large-Area YBa2Cu3O7 − x Films on Sapphire with Excellent Microwave Power Handling Capability

Crack-free thick YBa2Cu3O7 − x films are prepared on CeO2 buffered r-cut sapphire (2 inch in diameter) with thickness up to 700 nm, smooth surfaces (“peak-to-valley” roughness <10 nm), high critical currents (JC > 2 MA/cm2 at 77 K and 0 T), and low microwave surface resistances (Rs(77K) ≍.4mω and Rs(4.2K) ≍.110μω at 19.15GHz) comparable to the best values reported in the literature for YBCO films on structurally better matched substrates. These thick YBCO films were able to handle high microwave power corresponding to magnetic field amplitudes (BHF) up to 54, 37, and 17.4 mT at 4.2, 50, and 77 K, respectively, which for the lower temperatures were limited by the available power of the 25-W HF amplifier. The high-power performance, which to our knowledge belongs to the best reported so far for unpatterned YBa2Cu3O7 − x films, was achieved without any degradation of the samples despite frequent thermal cycling.

Current limiting properties of superconducting YBa/sub 2/Cu/sub 3/O/sub 7/ films on various substrates

The current limiting properties of textured and epitaxial YBa₂Cu₃0₇ films with different thickness on various substrates (i.e. r-cut sapphire, polycrystalline Al₂O₃ and Y-stabilized ZrO₂) are examined. The correlation between structural, morphological, superconducting properties and the evailution of current and electrical field during dc and ac current pulses are determined. The best results are obtained in epitaxial YBa₂Cu₃O₇ films on large-area CeO₂/sapphire with a thin Au-top layer. They are characterized by T_c(ind)=88-90K, ΔT_c(ind)=O.5K, J_c(77K,res.)=2-3.7MA/cm² and FWHM(005) = 0.2°. During voltage pulse measurements the highest values for the dissipated power P_max/A=2.6MW/m², voltage U_max/l= 10.9 V/cm, and R_max/R_o=2.33 and the highest mean temperature of the film of T_max=650-700K could be obtained. The maximum current of (2-6)I_c was reached after ∼4μsec and within lmsec the current was reduced to values below I_{c</sub. both being an indicatiion for the fast reaction time and homogeneity of the films. The best values for YBa₂Cu₃O₇ on polycrystalline Al₂O₃ and YSZ are T_c,off(ind)=87K, ΔT_c=2K, J_c(77K)= 0.2MA/cm<sup>2} and FWHM(005)=0.9° yielding P_max/A=O.7MW/m₂, U_max/l=6.5 V/cm, R_max/R_o=1.4 (T_max≈500K) and reaction times of ∼10msec.

Highly stabilized sputter deposition of YBa2Cu3O7

A method of stabilizing and homogenizing the plasma during on‐axis high‐pressure dc sputter deposition of YBa2Cu3O7 is introduced, which leads to improved reproducibility and uniformity of the deposition process. The plasma is monitored and controlled via optical emission spectroscopy. Strong changes of the plasma intensity and intensity profile above the target are observed in the standard sputter process, which lead to unstable and inhomogeneous deposition of high Tc material. These changes are counteracted by adequate variation of the composition of the process gas (Ar/O2). The intensity of the plasma flicker is thus reduced by a factor of 4–5, long term drifts in the plasma intensity, and arcing of the plasma are eliminated.

Stress generated modifications of epitaxial ferroelectric SrTiO3 films on sapphire

The effect of lattice-mismatch induced stress upon the crystallographic structure, strain, strain relaxation, and the generation of different types of defects in heteroepitaxially grown SrTiO3 films on CeO2 buffered sapphire is examined. Depending on the thickness of the SrTiO3 layer, characteristic changes in the structural perfection of the layers, their crystallographic orientation with respect to the substrate system, and their strain is observed. For thin films misfit dislocations partially compensate the stress in the SrTiO3 layer, whereas cracks develop in thicker SrTiO3 films. The cracks are orientated along two predominant crystallographic orientations of the sapphire. The structural modifications and the formation of misfit defects and cracks are explained in a model based on lattice misfit induced stress, on the one hand, and energy considerations taking into account the stress release due to crack formation and the energy necessary for the formation of new surfaces at the crack, on the other h...

Pattern induced phase transition of vortex motion in high-Tc films

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.