R. Wördenweber

Preparation of regular arrays of antidots in YBa2Cu3O7 thin films and observation of vortex lattice matching effects

Optimized sputtered YBa2Cu3O7 (YBCO) thin films on CeO2-buffered sapphire substrates are patterned into square lattices of submicrometer holes (antidots) with diameters of 250–450 nm and lattice parameters of d=500–1000 nm without deterioration of superconducting properties. In the mixed state, matching effects between the Abrikosov vortex lattice and the artificial antidot lattice are observed. These effects are in the form of peaks or cusps in the critical current density recorded as a function of magnetic induction at integers n and specific rationals k/l of the matching field Bm=(Φ0/d2). The experimental results are discussed in the context of existing theories. The existence of multiquanta vortices confined by the holes in YBCO films are considered.

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.

Large-area epitaxial CeO2 buffer layers on sapphire substrates for the growth of high quality YBa2Cu3O7 films

Abstract It is demonstrated that sapphire with a CeO2 buffer layer can be exploited as an excellent substrate material for growth of large-area YBa2Cu3O7 films. Smooth, epitaxial and crack-free CeO2 buffer layers 130 nm thick were deposited homogeneously on 2 in r-plane sapphire substrates by fully computer-controlled on-axis magnetron sputtering. YBa2Cu3O7 was grown successively on the buffer layer. X-ray diffraction experiments revealed the epitaxial orientations: YBa 2 Cu 3 O 7 (001)//CeO 2 (001)//Al 2 O 3 (1 1 02) , with YBa2Cu3O7[110] and [1 1 0]//CeO 2 [100]// Al 2 O 3 [11 2 0] . A postdeposition heat treatment improved the crystalline quality and surface morphology of the CeO2 buffer layers, resulting in a channelling minimum yield of less than 2.5% and surface roughness less than 3 nm. No cracks could be detected by scanning electron or atomic force microscopy. The microwave surface resistances of YBa2Cu3O7 films grown on optimized CeO2 buffer layers are comparable with those reported for YBa2Cu3O7 on LaAlO3 wafers.

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...

Structural perfection of (001) CeO2 thin films on (11_02) sapphire

Large-area (001) oriented epitaxial CeO2 films with extremely high crystalline perfection characterized by x-ray diffraction rocking curves of the (002) CeO2 reflection with a full width at half maximum (FWHM) Δω⩽0.013° and thickness dependent oscillations in the Bragg-Brentano x-ray diffraction spectra were deposited via rf-magnetron sputtering on (11_02) sapphire. Pole figure measurements of the space symmetry confirmed that the examined sharp reflections belong to CeO2 and no other phases like CeAlO3 are present. The improvement of the crystalline quality was obtained by optimization of the high-pressure sputter deposition process and the use of large-area substrates. The [100] CeO2 axis was slightly tilted with respect to the [11_02] sapphire axis by 0.185°. Subsequently sputter-deposited high-Tc YBa2Cu3O7−x thin films revealed structural properties characterized by FWHM <0.06° of the (005) θ-2θ peaks and by FWHM of the (005) peaks rocking curves of Δω=0.3°.

Design of tunable ferroelectric filters with a constant fractional band width

The main principle of design of tunable ferroelectric filters with a constant fractional band width in the tuning range are discussed. Lumped ferroelectric capacitors are used as the tunable components of the microstrip filter. The limitation of the filter performance is determined by the commutation quality factor of the ferroelectric varactor. The influence of the loss factor of the ferroelectric capacitor is analyzed. The 3-pole 1% fractional band pass filter with a 3 band width tuning range without a degradation of the characteristics is designed.

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.

Low dielectric loss BaNd2Ti5O14 thick films prepared by an electrophoretic deposition technique

BaNd2Ti5O14 (BNT) films 12–52μm thick were fabricated on platinum metallic foils by electrophoretic deposition (EPD). 52-μm-thick BNT film exhibits a dielectric constant and a loss tangent of 107 and 0.0006 (Q of 1600) at 1MHz, respectively. Variation in permittivity is less than 0.02% at a bias voltage ±8kV∕cm. Change of film permittivity with temperature (30–120°C) is below +58.5ppm∕°C, pointing to a good thermal stability. Preliminary microwave measurements indicate that the losses are not significantly increased up to the gigahertz regime. EPD derived BNT thick films on metallic foils are attractive candidates for microwave communication devices.