high-Tcsuperconducting Films Research Articles

Microwave surface resistance in nanostructured high-Tc superconductor films

The impact of artificially created defects nanostructure, formed by implanted dielectric nanoparticles or irradiation defects, on microwave properties of high-Tc superconductor films is analyzed in the framework of phenomenological theory for microwave response of type-II superconductors. We have calculated the surface resistance for such a kind of nanostructured type-II superconductor film and investigated conditions for the emergence of nonlinear response caused by the entrance of microwave-induced vortices in the film's interior through its edges. The obtained results indicate that artificial defect nanostructure in the film's interior formed by point-like or columnar structural defects can significantly improve its microwave characteristics in both the Meissner and mixed states and also increase the threshold for the onset of nonlinear response.

Increasing the Transition Temperature of High-TC Superconductor Thin Films by Organic Linking of Gold Nanoparticles

The transition temperature, TC, of two types of optimally doped cuprate high-TC superconductor films, YBa2Cu3O7-δ and La1.85Sr0.15CuO4, is found to increase upon linking Au nanoparticles to their surface via organic molecules. At the same time, and quite surprisingly, the critical current is reduced. We attribute these results to screening of the Coulomb interactions and smearing the pinning potential landscape. The transition temperature was increased also when the Au NPs were not chemically linked to the surface, consistent with the screening scenario. The linking molecules, however, enable selective adsorption of the gold nanoparticles and their separation from the surface, thus eliminating the conventional proximity effect.

Microwave Response of Nanostructured High-Tc Superconductor Thin Films

A model for the microwave response of a nanostructured high-Tc superconductor (HTS) film, with implanted nanoparticles and nanorods of a dielectric material or point-like and columnar irradiation defects with a nano-sized cross-section is developed. In this case, the microwave surface resistance Rs(T,H,ω) is calculated both for the Meissner and mixed states of a superconductor film in an applied dc magnetic field. The obtained results indicate that the implantation of dielectric nanoparticles or point-like radiation defects can significantly improve superconductor characteristics at microwave frequencies. Namely, these nano-sized structural defects can decrease the surface resistance in the Meissner state and eliminate the oscillations of Abrikosov vortices and the related microwave energy losses, thus decreasing the contribution of Abrikosov vortices to the Rs value in the mixed state of a HTS film.

Magnetoresistance due to the Plane Current Effect through the Gap Layer Between the High-Tc Superconductor Film and the Giant Magnetoresistance-Spin Valve Multilayer

Magnetoresistance due to the Plane Current Effect through the Gap Layer Between the High-Tc Superconductor Film and the Giant Magnetoresistance-Spin Valve Multilayer

New Type of Microwave High-$T_{c}$ Superconductor Microstrip Resonator and Its Application Prospects

A new type of planar high-Tc superconductor (HTS) resonator, proposed by the authors, is described. Its feature is excitement with whispering gallery modes (WGM) unlike the known microstrip resonators excited with the lower modes. The resonant structure of two-planar disks and two microstrip lines was simulated and made on the basis of a DyBa 2 Cu 3 O 7-δ superconductor epitaxial film of 600-nm thickness using photolithography technique. The film was deposited on MgO single crystal substrate of thickness h = 0.5 mm. Microwave response in a form of Q-factor and resonant frequency depending on temperature was measured in Kand Ka-bands. The obtained results show higher values of Q-factor of WGM microstrip HTS resonators in comparison with the known HTS microstrip resonators with lower modes. In the authors' opinion, these resonators open prospects for development of new planar microwave devices and techniques. The response of HTS disk resonators obtained earlier and about six months later at approximately the same conditions showed the lowering Q-factor, which may be explained by the degradation of HTS film properties and requires further study.

Peculiarities in magnetron sputtering of YBCO epitaxial films for applications in superconductor electronics devices

We consider the main factors determining the growth of YBa2Cu3O7–δ high-Tc superconductor films during magnetron sputtering in the planar axial geometry. Special attention is paid to the increase of the growth rate of the films suitable for superconductor electronics devices. Magnetron sputtering is used for obtaining YBa2Cu3O7–δ films with high structural and electrophysical characteristics for a growth rate up to 200 nm/h, which were used in constructing microwave disk resonators and long Josephson junctions on bicrystal substrates. The unloaded Q factor of cavities exceeds 80000 at a frequency of 7.1 GHz at a temperature of 77 K, which corresponds to the best results in this field. Josephson junction of length 50–350 μm are characterized by critical current density jc = 12–33 kA/cm2 at T = 77 K and jc = 93–230 kA/cm2 at T = 6 K in zero magnetic field. The characteristic voltage IcRn is 0.8–1.96 mV.

Rapid Solid-State Synthesis of Oxides by Means of Irradiation with Light

A novel kind of synthesis for oxide materials, especially complex ones, has been developed. The process involves the intense irradiation of the powder mixture of the starting oxides by light at elevated temperature. The spectral range tested extends from infrared to ultraviolet with sufficient intensity for the solid-state reaction between the reagents to take place. The resulting reaction speed exceeds the conventional thermal one by up to two orders of magnitude. This method should be applicable for the synthesis of epitaxial thin films of high-TC superconductors used in RABITS or IBAD technologies, high-Q filters, and magnetic oxide films for magnetoelectronics.

Switching characteristics for thin films of high-Tc superconductors driven by optical and current pulses with varied lengths

This work proposes a method for lowering the energy of an optical impulse required to switch superconducting key elements, based on driving such a components with the use of current and optical radiation impulses with varied lengths. The investigation was conducted with the aid of numerical modelling of the superconducting switching element response for a structure composed of a thin YBa2Cu3O7-x superconducting film deposited on a sapphire substrate (Al2O3) with a buffer layer composed of strontium titanate (SrTiO3). It was proven that thanks to the driving with control impulses of varied lengths, it is possible to switch such elements with 1.6–3.5 times lower energy of the optical impulse and the current slightly higher than the critical current, as compared with the switching mechanism employing impulses with the same length.

Substrate Contribution to the Surface Impedance of HTS Films on Si

We study the effects of semiconductor substrates on the surface impedance of high-T c Superconductor (HTS) YBa2Cu3O7−δ (YBCO) films. The characteristic impedance of silicon (Si) (for different doping levels and for different charge carrier scattering times) is evaluated. In particular, the most relevant features of Si electrodynamics are highlighted by the introduction of suitable normalized quantities. The effective surface impedance of the YBCO films on Si substrates is then calculated and discussed for different temperatures and frequencies in the microwave range, comparing the obtained results to their limiting expressions for bulk and thin-film HTS. Our analysis shows how the widely used thin-film approximation for the surface impedance can fail, critically highlighting the conditions it requires to be correctly used. We show that substrate contributions can heavily influence the overall response.

Temporal Variations in Densities of Y, Ba, and Cu Atoms in Magnetron Sputtering Plasmas with YBaCuO Target

In this study, we used optical emission spectroscopy (OES) for examining the temporal variations in the densities of Y, Ba, and Cu atoms sputtered from a YBaCuO target in a magnetron plasma source. It was found that the densities of Y and Cu changed with the discharge period. In a low-pressure, low-power discharge, the temporal variations in the Y and Cu densities were small, whereas they became significant at a high pressure and a high power. Ten minutes after the initiation of the discharge, Y density decreased to 8% of the initial value at an rf power of 70 W and a gas pressure of 300 mTorr. On the other hand, the optical emission intensities from Ar and Ba were roughly constant during the same discharge period. The roughly constant Ar intensity indicates that electron density and electron temperature were almost constant. The discovery of temporal variations in the Y and Cu densities is important for optimizing the discharge parameters for the deposition of high-quality, high-critical-temperature (high-Tc) superconductor films.

Nanodimensional effects in the structure of epitaxial YBa2Cu3O7−y films

We have studied the structure of YBa2Cu3O7−y (YBCO) high-T c superconductor films obtained by deposition for various times under otherwise identical conditions. Thin YBCO films prepared by the nonaxial laser shadow deposition technique are epitaxial and have microstrains below 1×10−3. The main factor determining variations of the unit cell parameter c in the films is the difference of the crystal block size 〈D〉 in the direction normal to the film surface, which reflects the nanodimensional effects known in oxides.

Far-infrared thermal spectroscopy of low-Tc and high-Tc superconductor films

Temperature dependence of far-infrared transmission of NbN thin films deposited on MgO and Si substrates was measured at several frequencies from 0.4 to 4.3 THz. Activated exponential increase of relative penetration depth at low temperatures and a peak in transmission near Tc were observed for frequencies below the optical gap. On the other hand, the transmission measured at frequencies above the gap exhibits only flat, almost linear temperature dependence. This behaviour is consistent with the BCS theory of superconductivity. Similar measurements were performed also on YBa2Cu3O7−δ thin films deposited on MgO and sapphire substrates. Low-temperature variation of transmission indicates the d-wave symmetry. The peak below Tc predicted by the BCS theory is not observed. Flat temperature dependence of transmission at higher frequencies shows that the photon energy was sufficient for excitation over the optical gap. The s-wave BCS theory is adequate for NbN films but not for the YBa2Cu3O7−δ materials. Using the theoretical BCS model we show that the transmission peak is not correlated to the coherence peak in real conductivity at given frequency and other parameters relevant to experiment.

Low-dose radiation effects in thin films of high-temperature superconducting YBa2Cu3O7−x irradiated by 1-MeV electrons

Radiation effects are investigated in thin epitaxial films of the high-Tc superconductor (HTSC) YBa2Cu3O7−x irradiated by low doses of 1-MeV electrons. The maximum radiation dose (4×1016 electrons/cm2) is chosen from the condition that the defects formed as a result of electron–nucleus collisions cause a negligible lowering of the critical temperature of YBa2Cu3O7−x. Under this condition the main source of radiation effects in HTSC films can be processes involving excitation of the electronic subsystem of YBa2Cu3O7−x. When YBa2Cu3O7−x films are irradiated by doses of (1–4)×1016 electrons/cm2 their critical temperature Tc is observed to increase (in contrast to published reports of a decrease in Tc at irradiation doses greater than 1018 electrons/cm2) and then, after the irradiation has stopped, to relax over time to its original value. These effects are similar to those observed in the photoexcitation of the electronic subsystem of YBa2Cu3O7−x (photostimulated superconductivity). A decrease of the critical current density in the irradiated YBa2Cu3O7−x films is also observed, which is due to radiation-stimulated changes of the transmissivity to supercurrent of the dislocation walls in low-angle interblock boundaries.

A novel superconducting CPW slow‐wave bandpass filter

AbstractA novel slow‐wave half‐wavelength coplanar waveguide (CPW) resonator using high‐Tc superconducting (HTS) films has been studied and used in a new filter topology. The slow‐wave effect is achieved by the capacitively loaded structure with a symmetrical interdigital line loaded on both sides of the central line. The velocity reduction (Vp/c0) of the resonator is about 0.050. A three‐pole Butterworth bandpass filter has been designed and fabricated with the use of the resonator. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 34: 255–259, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10431

Modeling thermal transient process in HTS microstrip line with non-homogeneous current density distribution

Thermal destruction of superconducting state in microwave high- T c superconducting (HTS) devices considered as a thermal transient process caused by Joule heating of HTS films has been investigated. A new approach to modeling time dependent temperature distribution has been suggested and realized in a microstrip HTS resonator taking into account non-homogeneous current distribution along the resonator. The results have been discussed. Thermal destruction of superconducting state in microstrip HTS line at microwaves has been investigated. Time dependent temperature distribution has been modeled in the line using the non-linear model of the surface resistance dependent on current density. Modeling has been done for HTS films of various quality taking into account non-homogeneous current density distribution in the line cross-section. The results have been compared and discussed.

Application of a one-port dielectric resonator for measurement of the intermodulation in high temperature superconducting films

A technique for characterization of intermodulation performance of unpatterned high TC superconducting (HTS) films, based on the application of the resonator with a single coupling probe is proposed. It allows us to sufficiently increase the sensitivity in comparison with the conventional method based on the resonator with two coupling probes. The measurements of the power at mixed frequency as a function of input power are performed for epitaxial HTS films of various surface resistance and thickness. From the experimental data an intermodulation coefficient α is determined, which provides the film intermodulation properties independently on the measuring system.

Symmetrical high-T c superconducting bicrystal Josephson junctions: Dependence of the electrical properties on the misorientation angle

The electrical properties of junctions at symmetrical bicrystal boundaries in high-T c superconducting films are studied as functions of the misorientation angle in the range 8°–45°. The junctions are prepared by growing YBa2Cu3O7 (YBCO) epitaxial films on Y-ZrO2 (YSZ) bicrystal substrates. The proportional relationship between the characteristic voltages and the normal conductivities of junctions is derived from the dependences of the critical current and the normal resistance on the misorientation angle. The results are interpreted within the model of a superconductor-dielectric with defect levels in the band gap of the superconductor. The deviations from the proportional relationship are explained by the junction inhomogeneity. The thickness of the effective dielectric layer in the bicrystal junction and the Bohr radius of electrons on the defects are estimated.

Optical control of magnetic flux quanta in YBCO thin film loops by selected femtosecond laser pulses

We report on an optical magnetic flux control in high-Tc superconductive loops and films using a femtosecond (fs) laser. The YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films are patterned into strips with two rectangular parallel holes, with ordered arrays of antidots, or without any holes. The fs optical pulses are focused onto the strips while biasing the current. After removal of the bias current, the supercurrent density distributions are visualized by terahertz radiation imaging. The images indicate that the flux and anti-flux quanta are optically generated in the strips, and the number of the flux quanta between 15 and 2000 flux quantum is controlled by both the bias current and laser power. We then put forward the system to control the number of the fs pulses to generate the flux. The results reveal that even single shot fs optical pulse can generate the flux quanta in the strips.

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

The impact of microscopic defects upon the microwave properties of high- T c superconductor (HTS) films is examined. YBa 2Cu 3O 7 films with different size and densities of Y 2O 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.

Femtosecond spectroscopy of relaxation processes in metals and high-T c superconductors

Spectral dependences of charge carrier relaxation rates, γe-e(ħω) and γ e-ph(ħω), were observed in Au and Cu films and YBa2Cu3O7-δ high-Tc superconductor films. The relaxation rates decreased substantially in the spectral region corresponding to interband transitions to the Fermi level region (ħωAu = 2.45 eV, ħωCu = 2.15 eV, and ħ ω1 = 1.89 eV and ħω2 = 2.08 eV for YBa2Cu3O7-δ). This relaxation deceleration opens up possibilities for developing a new method, based on the spectral dependences of relaxation rates, for the determination of the Fermi level position and the parameters of electron-electron and electron-phonon interactions on the one hand and for studying deviations from the Fermi-liquid behavior in strongly correlated electronic systems. The linear γe-e (ħω) ∝ |ħω− EF| spectral dependence was observed for a YBa2Cu3O7-δ film near ħω2 = 2.08 eV, which may be evidence of a non-Fermi-liquid behavior of the electronic subsystem.