Microwave Surface Resistance Research Articles

The dc magnetic field, temperature and film thickness dependence of microwave surface resistance of YBCO and DyBCO thin films

We investigated the dc magnetic field and temperature dependences of the microwave surface resistance (Rs) of YBa2Cu3Oy (YBCO) and DyBa2Cu3Oy (DyBCO) superconducting thin films. The YBCO and DyBCO thin films, each with a thickness of 300, 500, or 700nm, were deposited on MgO (100) substrates by the thermal co-evaporation method. The Rs was measured using the dielectric resonator method. A dc magnetic field of up to 5.0T was applied parallel to the c-axis of the superconducting thin films. The results showed that the Rs value had almost the same temperature dependence at various thicknesses in a zero-external field. The Rs of the YBCO and DyBCO thin films increased with the applied dc magnetic field. The DyBCO thin films showed weaker magnetic field dependence of Rs than the YBCO thin films. The Rs ratio (defined as Rs(5T)/Rs(0T)) linearly increased with the film thickness. These results show that pinning strength decreased with an increasing film thickness.

Effects of Meissner surface current on the microwave surface resistance of type-II superconductors

Influence of the Meissner current on the field-induced variations of the microwave (mw) surface resistance in type-II superconductors is demonstrated to be well reproduced by exploiting different critical state models. R s ( B) /R n versus B a curves have been obtained using Kim and exponential type J c ( B) considering Meissner current circulating at the surface of the superconducting specimen.

Fabrication of double-sided Tl2Ba2CaCu2O8 superconducting thin films on large-area sapphire substrates

High quality Tl2Ba2CaCu2O8 (Tl-2212) superconducting thin films were fabricated on 2 inch double-sidedCeO2 bufferedr-cut sapphire substrates. The metal cerium target was used as the sputtering source to deposit theCeO2 buffer layerby the RF magnetron reactive sputtering technique, and the inter-diffusion of chemical elements betweenthe CeO2 layer and the substrate was studied by XPS depth profiling. The crack-free Tl-2212 films withdense crystal structure and smooth morphology were homogeneous over the whole area of thefilm. The XRD measurement confirmed the purity of the Tl-2212 phase and the texture withc axis perpendicularto the surface of the CeO2 buffered sapphire substrate. The electrical and magnetic measurementson the Tl-2212 films showed that the transition temperatureTc was around 106 K, and the critical current densitiesJc (77 K, 0 T) for bothside films were 2.9 ± 0.1 MA cm − 2 and 1.6 ± 0.2 MA cm − 2, respectively. The microwave surface resistanceRs on one side filmwas about 390 µΩ andon the other side 450 µΩ at 10 GHz and 77 K.

Fabrication and Properties of Double-Side Tl2Ba2CaCu2O8 Thin Film on CeO2 Buffered Sapphire Substrate

The double-side Tl2 Ba2 CaCu2O8 (Tl-2212) superconducting thin films were fabricated on CeO2 buffered sapphire substrates. The reactive magnetron sputtering technique was used to grow CeO2 buffer thin films on sapphire substrates. Making use of the metal cerium as a sputtering source, the depositing rate is much higher compared with the CeO2 target. The Tl-2212 thin films on CeO2 buffered sapphire substrates were fabricated by a dc magnetron sputtering and post-annealing process. The x-ray diffraction indicates that the thin film is pure Tl-2212 phase with the c-axis perpendicular to the substrate surfaces, and epitaxially grown on the CeO2 buffered sapphire. The critical transition temperature Tc is around 106K, the critical current density Jc is around 3.5MA/cm2 at 77K, and the microwave surface resistance Rs at 77K and 10 GHz of the film is as low as 390 μΩ.

A novel method for the measurement of frequency-character of surface resistance of HTS thin film

A novel method for microwave surface resistance measurement of high temperature superconductor thin film at multi-frequency by multimode of sapphire rod is introduced. This method has the advantages of high sensitivity and large dynamic range of the measurement. By using this method, the frequency dependent microwave surface resistance of a single piece of high temperature superconductor thin film can be characterized within one temperature cycle. This method is capable of reducing the amount of work in surface resistance measurement and providing supports to the research and the industrialization of high temperature superconductor thin film.

Fluxon dynamics in Li–Al codoped [formula omitted] by microwave surface resistance measurements

The magnetic-field-induced variations of the microwave surface resistance, R s , have been investigated in ceramic Mg 1− x (LiAl) x B 2, with x in the range 0.1 – 0.4 . The measurements have been performed on increasing and decreasing the DC magnetic field, H 0 , at fixed temperatures. At low temperatures, we have observed a magnetic hysteresis in the R s ( H 0 ) curves in all the investigated samples. On increasing the temperature, the range of H 0 in which the hysteretic behavior is visible shrinks; however, in the sample with x = 0.1 it is present up to temperatures close to T c . We show that the field dependence of R s can be quantitatively justified taking into account the critical-state effects on the fluxon lattice only in the sample with x = 0.4 . On the contrary, in the samples with x < 0.4 the hysteresis exhibits an unusual shape, similar to that observed in others two-gap MgB 2 samples, which cannot be justified in the framework of the critical-state models.

Fabrication and properties of large area Tl2Ba2CaCu2O8 superconducting thin film on sapphire substrate

High quality large area Tl2Ba2CaCu2O8(Tl-2212)superconducting thin films were fabricated on CeO2 buffered two-side sapphire substrates. Using metallic cerium target as the sputtering source, CeO2 buffer film with c-axis orientation was deposited by radio frequency reactive magnetron sputtering, and the influence of preparation conditions on the structure and surface morphology of the CeO2 layer was studied. The Tl-2212 superconducting thin film was fabricated on CeO2 buffered sapphire substrate by direct current magnetron sputtering and post annealing. Scanning electron microscope showed that the film has a compact microstructure with uniform flat surface. The X-ray diffraction indicated that the film was pure Tl-2212 phase with c-axis perpendicular to the substrate surface, and epitaxially grown on the CeO2 buffered sapphire. The superconducting film exhibited excellent uniform electric properties. The critical transition temperature Tc was around 105 K, the critical current density Jc(77 K, 0 T) was around (1.2±0.1) MA/cm2 and (1.25±0.1) MA/cm2, respectively, and the microwave surface resistance Rs (77 K, 10 GHz ) of the film was as low as 390 μΩ.

Batch production of large-area double-sidedYBa2Cu3O7−δ thin films by DC magnetron sputtering

Using a novel DC magnetron sputtering system, high qualityYBa2Cu3O7−δ (YBCO) thin films were deposited on both sides of twelve pieces of 3-inchLaAlO3(00l) single crystal substrates in one batch. This sputtering system has a rotatable disc sample holder withthe LaAlO3 substrates being positioned in the middle of upper and lower YBCO targets. These twosputter guns are arranged to simultaneously deposit YBCO thin films on both sides of thesubstrate. The rotatable sample holder is an important approach to large scale depositionof high quality YBCO thin films. In this manner, the sputtered species can diffuse wellaround the substrates during one rotation cycle. Furthermore, the anode is installed in theinner of rotatable disc sample holder to avoid negative ion bombardment. Thus, a gooduniformity over the whole area for all the films has been realized. The full-width athalf-maximum (FWHM) values of the (005) rocking curves typically range from0.49° to0.56°. The superconducting critical temperatures fall in the range of89–90 K and the critical current densities are between 3.75 and4.0 MA cm−2 at 77 K. The microwavesurface resistance, RS (77 K, 10 GHz), of the YBCO thin films range typically from 0.56 to0.71 mΩ, which make these films suitable for microwave applications.

Field-induced suppression of theπ-band superconductivity and magnetic hysteresis in the microwave surface resistance ofMgB2 at temperaturesnear Tc

We report on the magnetic-field-induced variations of the microwave surface resistance,Rs, ina polycrystalline MgB2 sample, at different values of temperature. We have detected a magnetic hysteresis inRs, which exhibits an unexpected plateau on decreasing the DC magneticfield below a certain value. In particular, at temperatures nearTc the hysteresis manifests itself only through the presence of the plateau.Although we do not quantitatively justify the anomalous shape of the magnetichysteresis, we show that the results obtained in the reversible region of theRs(H) curve can be quite well accounted for by supposing that, in this range of magnetic field, theπ gap is almost suppressed by the applied field and, consequently, all theπ-band charge carriers are quasiparticles. On this hypothesis, we have calculatedRs(H) supposing that fluxons assume a conventional (single core) structure and theflux dynamics can be described in the framework of conventional models.From the fitting of the experimental results, we determine the values ofHc2π(T) at temperaturesnear Tc. In our opinion, the most important result of our investigation is that, at least at temperatures nearTc, the value of the applied field that separates the reversible and irreversible regions of theRs(H) curvesis just Hc2π(T); a qualitative discussion of the possible reasons for this finding is given.

Reduction in the field-dependent microwave surface resistance in YBa2Cu3O7−δ with submicrometric BaZrO3 inclusions as a function of BaZrO3 concentration

In order to study the vortex pinning determined by artificially introduced pinning centers in the small-vortex displacement regime, we measured the microwave surface impedance at 47.7 GHz in the mixed state of YBa2Cu3O7−δ thin films, where submicrometric BaZrO3 particles have been incorporated. As a function of the BaZrO3 content, we observed that the absolute losses slightly decrease up to a BaZrO3 content of 5%, and then increase. We found that the magnetic-field-induced losses behave differently in that they are not monotonic with increasing BaZrO3 concentration. At small concentration (2.5%) the field-induced losses increase, but large reduction in the losses themselves, by factors up to 3, is observed upon further increasing the BaZrO3 concentration in the target up to 7%. Using measurements of both surface resistance and surface reactance, we estimate vortex pinning-related parameters. We found that BaZrO3 inclusions introduce deep and steep pinning wells. In particular, the minimum height of the energy barrier for single vortices is raised. At larger BaZrO3 content (5% and 7%) the phenomenon is at its maximum, but it is unclear whether it shows a saturation or not, leaving room for further improvements.

Depinning frequency in a heavily neutron-irradiated MgB2 sample

The magnetic-field-induced variations of the microwave surface resistance have been investigated in a heavily neutron-irradiated MgB2 sample, in which the irradiation has caused the merging of the two gaps into a single value. The experimental results have been analyzed in the framework of the Coffey and Clem model. By fitting the experimental data, we have determined the field dependence of the depinning frequency, ω0, at different values of the temperature. Although the pinning is not particularly effective, the value of ω0 obtained at low temperatures is considerably higher than that observed in conventional low-temperature superconductors.

A study on stress affecting the microwave surface resistance of YBCO high temperature superconducting thin films

We calculate stress in YBa 2Cu 3O 7− x /MgO (YBCO/MgO) and YBa 2Cu 3O 7− x /LaAlO 3 (YBCO/LAO) by XRD of the sample, σ 1 = −1.2 GPa and σ 2 = −1.4 GPa, respectively, which shows that the stress in YBCO/LAO is stronger than that in YBCO/MgO. In addition, microwave response of the two pieces of thin films is also investigated by microstrip resonator technique. Surface resistance and penetration depth of the films are obtained by analyzing S 21 resonant curves of the microstrip resonator, the penetration depth λ 0 = 280 nm for YBCO/MgO and λ 0 = 265 nm for YBCO/LAO, and surface resistance R s = 0.376 mΩ for YBCO/MgO and R s = 2.660 mΩ for YBCO/LAO at 78 K, 10 GHz. The results show stronger stress in YBCO/LAO which lead to a larger microwave surface resistance than YBCO/MgO's. Some rational explanations are also analyzed and discussed in the paper.

Growth and characterization of 2 inch double-sidedTl2Ba2CaCu2O8 thinfilms on LaAlO3 substrates

Superconducting Tl2Ba2CaCu2O8 thin films (Tl-2212) have been reproducibly fabricated on both sides of 2 inchLaAlO3(001) substrates by using a so-called two-step process. A simple and flexible horizontalarrangement inside the crucible during thallination has been proposed, toensure a good convection of thallous oxide vapor around the film surface, andthus a high film quality on both sides. A good uniformity over the wholearea of the films has been realized. The films are strongly textured, with thec-axis perpendicular to the substrate surface, and they show the fourfold symmetry of the tetragonalcrystal structure. The full width at half maximum of the (002) peak for both films on the best sampleis 0.43° and 0.55° respectively. The films exhibit flat terraces with particles and shallowholes, but free of microcracks. The superconducting critical temperaturesfall in the range 105–108.5 K. The critical current densities are between1.0 × 106 and 4.8 × 106 A cm−2 at 77 K. The microwave surface resistances at the central area of the films are around500 μΩ at 77 K and 10 GHz. These films are suitable for microwave applications.

Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field

We report on the microwave surface resistance of two polycrystalline Mg11 B2 samples; one consists of pristine material, the other has been irradiated at very high neutron fluence. It has already been reported that in the strongly irradiated sample the two gaps merge into a single value. The mw surface resistance has been measured in the linear regime as a function of the temperature and the DC magnetic field, at increasing and decreasing fields. The results obtained in the strongly irradiated sample are quite well justified in the framework of a generalized Coffey and Clem model, in which we take into account the field distribution inside the sample due to the critical state. The results obtained in the pristine sample show several anomalies, especially at low temperatures, which cannot be justified in the framework of standard models for the fluxon dynamics. Only at temperatures near Tc and for magnetic fields greater than 0.5Hc2(T) the experimental data can quantitatively be accounted for by the Coffey and Clem model, provided that the upper-critical-field anisotropy is taken into due account.

Strong reduction of the field-dependent microwave surface resistance in YBCO with BaZrO3 inclusions

We present measurements of the magnetic field dependent microwave surface resistance in laser-ablated YBa2Cu3O7-δ films on SrTiO3 substrates. BaZrO3 crystallites were included in the films using composite targets containing BaZrO3 inclusions with mean grain size smaller than 1 μm. X-ray diffraction showed single epitaxial relationship between BaZrO3 and YBa2Cu3O7-δ. The effective surface resistance was measured at 47.7 GHz for 60<T<90 K and 0<μ0H<0.8 T. The magnetic field had a very different effect on pristine YBa2Cu3O7-δ and YBa2Cu3O7-δ/BaZrO3, while for μ0H=0 only a reduction of Tc in the YBa2Cu3O7-δ/BaZrO3 film was observed, consistent with dc measurements. At low enough T, in moderate fields YBa2Cu3O7-δ/BaZrO3 exhibited an intrinsic thin film resistance lower than the pure film. The results clearly indicate that BaZrO3 inclusions determine a strong reduction of the field-dependent surface resistance. From the analysis of the data in the framework of simple models for the microwave surface impedance in the mixed state we argue that BaZrO3 inclusions determine very steep pinning potentials.

Anomalous magnetic hysteresis in the microwave surface resistance of MgB2 superconductor

We report experimental results of the field-induced variations of the microwave surface resistance in samples of MgB2, produced by different methods. By sweeping the DC magnetic field at increasing and decreasing values, we have detected a magnetic hysteresis that can be ascribed to the different magnetic induction, due to the critical state of the fluxon lattice. The hysteresis observed in the bulk samples has an unusual shape, which cannot be justified in the framework of the critical-state models.

Strain effect on microwave surface resistance of YBa2Cu3O7-δ/LaAlO3 and Tl2Ba2CaCu2O8/LaAlO3 high temperature superconducting thin films

YBa2Cu3O7-δ (YBCO) thin films were prepared by thermal coevaporation on (LaAlO3) LAO substrates,and Tl2Ba2CaCu2O8 (TBCCO) thin films were synthesized by magnetron sputtering method on LAO substrates. We obtained the strain from the results of XRD of the films,which had the values of ΔCY=48483×10-3 and ΔCT=85272×10-5. We found that the strain in YBCO was larger than that in TBCCO. In addition, the curve of surface resistance of the films as a funttion of temperature was obtained by coplanar resonator technique, which showed the resistance of YBCO was larger than that of TBCCO at the same reduced temperature. In the paper, we also analyzed the strain effect on the surface resistance of the films.

A study on the microwave responses of YBCO and TBCCO thin films by coplanar resonator technique

YBa2Cu3O7(YBCO) thin films have been prepared by thermal coevaporation on LaAlO3(LAO) substrates, and Tl2Ba2CaCu2O8(TBCCO) thin films are synthesized by magnetron sputtering method on LAO substrates. The transition temperature Tc is90 K for YBCO/LAO and 104 K for TBCCO/LAO. Microwave responses of the films are studied systematically by coplanar resonator technique. Energy gaps of the films obtained are Δ0 = 1.04kBTc for YBCO films and Δ0 = 0.84kBTc for TBCCO films by analysing the temperature dependence of resonant frequencies of coplanar resonator. Penetration depth at 0 K λ0 = 198 nm for YBCO films and λ0 = 200 nm for TBCCO films could also be obtained by using the weak coupling theory and two fluid theory. Results of penetration depth and energy gap confirm the weak coupling properties of the films. In addition, microwave surface resistances Rs of YBCO/LAO and TBCCO/LAO are also investigated by analysing the quality factor and insert loss of the coplanar resonator. Surface resistance of TBCCO/LAO is less than that of YBCO/LAO, so that TBCCO/LAO films may have more potential applications.

THE EFFECT OF THE INTERACTION BETWEEN THE FILMS AND THE SUBSTRATES ON THE MICROWAVE PROPERTIES Of YBCO THIN FILMS

The microwave properties of YBa 2 Cu 3 O 7- x / LaAlO 3 (YBCO/LAO) and YBa 2 Cu 3 O 7- x / MgO (YBCO/MgO) thin films have been investigated by microstrip resonator technique. The penetration depth λ0(0 K ) and the microwave surface resistance Rs can be obtained by analyzing the experimental data of the temperature dependence of resonant frequency, unload quality factor of the microstrip resonator. The stress in the two thin films can be obtained from their XRD patterns. The result shows that the stronger stress in YBCO/LAO thing film makes it have larger microwave surface resistance at lower temperature.

Polycrystalline ${\rm MgB}_{2}$ Films on Flexible YSZ Substrates Grown by Hybrid Physical-Chemical Vapor Deposition

We report properties of polycrystalline MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films deposited on thin flexible yttium-stabilized zirconia (YSZ) substrates by hybrid physical-chemical vapor deposition. The MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films show a transition temperature of 38.9 K with a narrow superconducting transition (0.1 K). The self-field critical current density in the films reaches 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at low temperatures. These properties do not change after repeated bending over a radius of 10 mm. Low microwave surface resistance comparable to those obtained in epitaxial MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films on single-crystalline sapphire substrate was observed. The result shows promise of polycrystalline MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films for applications in superconducting digital circuits as well as in coated conductor wires.