P. Lahl

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.

Nonlinear microwave properties of ferroelectric thin films

The nonlinear microwave properties of ferroelectric SrTiO3 thin films are characterized via complex analysis of the intermodulation distortion (IMD) signals up to high microwave power. The measurements reveal an unusual dependence of the IMD signals on the input power, which indicates the presence of two different nonlinear properties being responsible for the generation of IMD, i.e., the nonlinear permittivity and conductivity at low and high rf power levels, respectively. The IMD signal strength cannot be explained in terms of the classical description based on a Taylor expansion of the nonlinear parameter. In contrast, simulations of the frequency spectra using more appropriate descriptions of two nonlinear parameters yield an excellent agreement between theory and experiment, and, thus, demonstrate that the IMD experiments together with the simulation might provide further insight into the mechanism of nonlinear behavior of these materials.

Probing microwave properties of high-Tc films via small dc magnetic fields

It is demonstrated that the combination of vortex matter and rf measurements yields new insight into the microwave properties of superconducting thin-film devices, both in small magnetic fields and zero field. The comparison of field-cooled and different types of field-sweep experiments on coplanar high-Tc thin-film resonators shows that the microwave properties strongly depend on magnetization and vortex distribution in the superconducting film as well. Thus, using vortices as a kind of local probe for the microwave properties leads to a consistent explanation of the microwave power handling in nonzero and zero magnetic fields. In a model that is derived from the experiments, the nonlinear microwave behavior is explained by the limitation of the total current density in the device considering contributions of the rf field and the screening of the magnetic field and vortices to the current. The limiting current value seems to be related to the dc critical current of the superconductor.

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.

Correlation of power handling capability and intermodulation distortion in YBa2Cu3O7−δ thin films

The nonlinearity of the microwave properties of coplanar thin-film YBa2Cu3O7−δ resonators are examined by measurements of the degradation of the quality factor QL and the increase of the two-tone third-order intermodulation distortion signal with increasing microwave power. A linear correlation between the data of the characteristic microwave powers, which are obtained for the degradation of QL and the intermodulation signal, is experimentally observed and explained in terms of a theoretical model based solely on well-known expressions for the nonlinear surface resistance. Due to these experimental observations and the theoretical model, we conclude, that the degradation of the resonator Q factor and the generation of intermodulation distortion are determined by the same physical mechanism and that thermal effects can most likely be excluded.

Microwave properties of MgB/sub 2/ thin films

The microwave surface impedance Z/sub s/=R/sub s/+j/spl omega//spl mu//sub 0//spl lambda/ of MgB/sub 2/ thin films was measured via advanced dielectric resonator (DR) techniques. First, the temperature dependence of the penetration depth /spl lambda/ measured with a sapphire puck at 17.9 GHz can be well fitted from 5 K close to T/sub c/ by the standard BCS integral expression assuming the reduced energy gap /spl Delta/(0)/kT/sub c/ to be as low as 1.0-1.1 assuming /spl lambda/(0)=100--110 nm. These results clearly indicate the s-wave nature of the order parameter. Similar good fits were achieved by an anisotropic one gap and an isotropic two-gap model. Second, the temperature dependence of surface resistance R/sub s/, as measured with a rutile puck, indicates an exponential behavior below about T/sub c//2 with a reduced energy gap being consistent with the one determined from the /spl lambda/ data. The R/sub s/ value at 4.2 K was found to be as low as 19 /spl mu//spl Omega/ at 7.2 GHz, which is comparable with that of a high-quality high temperature thin films of YBa/sub 2/Cu/sub 3/O/sub 7/. A higher-order mode at 17.9 GHz was employed to investigate the frequency f dependence of R/sub s//spl alpha/f/sup n(T)/. Our results revealed an decrease of n with increasing temperature ranging from n=2 below 8 K to n=1 close to T/sub c/. Finally, the microwave power handling of MgB/sub 2/ films was deduced and compared with values for YBa/sub 2/Cu/sub 3/O/sub 7/ films. We found that the power handling of MgB/sub 2/ is comparable or even better than that of YBa/sub 2/Cu/sub 3/O/sub 7/ films for temperature below 30 K.

Nonlinear microwave properties of HTS thin film coplanar devices

It is demonstrated that the combination of vortex matter and rf measurements yields new insight into the microwave properties of superconducting thin film devices, both in small magnetic fields and zero field. The comparison of field-cooled and different types of field sweep experiments on coplanar high-T/sub c/ thin film resonators shows that the microwave properties strongly depend on magnetization and vortex distribution in the superconducting film as well as on the way, the magnetic field is approached. Thus, using vortices as a kind of local probe for the microwave properties leads to a consistent explanation of the microwave power handling in nonzero and zero magnetic field. In a model that is derived from the experiments, the nonlinear microwave behavior is explained by the limitation of the total current density in the device considering contributions of the rf field and the screening of the magnetic field and vortices to the current. The limiting current value seems to be related to the dc critical current of the superconductor.

Magnetic flux in active and passive superconducting devices

In this work we demonstrate (i) that on one hand active and passive superconducting thin-film devices can be used as very sensitive tools for detecting motion and penetration of vortices in superconducting material and (ii) that on the other hand the analysis of the local distribution of vortices in the devices can be used for optimization of device properties. Two candidates are discussed, i.e. the noise reduction in SQUIDs by means of strategically positioned antidots and the examination of the non-linear microwave behavior of coplanar superconducting resonators by means of flux and vortex penetration.

Improvement of the microwave properties of Y-Ba-Cu-O films with artificial defects

In this paper, the potential of defects for optimizing the microwave properties of YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) thin films is demonstrated. On one hand, microscopic Y/sub 2/O/sub 3/ precipitates, which can be created in YBCO thin films by modification of the deposition process, serve as ideal scattering centres for quasiparticles and, thus, lead to a considerable reduction of the microwave surface resistance R/sub s/. The modification R/sub s/(T) can be explained in terms of the two-fluid model. Data for the quasiparticle scattering rate can be obtained from the measurements. On the other hand, the impact of artificial defects, so called antidots, upon the microwave properties is analyzed. R/sub s/ measurements demonstrate that the ion beam etching creates a -20 nm broad damaged area at the edge of the antidots. First measurements of the power handling capability of YBCO thin film resonators indicate that the magnetic contribution to the nonlinear behavior can be reduced by antidots. The implementation of antidots, which have been proven to be an ideal and easy tool to improve active YBCO thin film devices, might be of use for microwave applications as well.

Power handling capability and intermodulation distortion of high-Tc microwave resonators

Abstract The nonlinearity of the microwave properties of coplanar thin-film YBa2Cu3O7−δ resonators are examined by measurements of the degradation of the quality factor QL and the increase of the two-tone third-order intermodulation distortion signal with increasing microwave power. A linear correlation between the data of the characteristic microwave powers, which are obtained for the degradation of QL and the intermodulation signal, is experimentally observed and explained in terms of a theoretical model. Due to the experimental observation and the theoretical model, we conclude, that (i) the degradation of the resonator Q factor and the generation of intermodulation distortion are determined by the same physical mechanism and that (ii) thermal effects can most likely be excluded.