High Tc Superconductor Thin Films Research Articles

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.

YBa<inline-formula> <tex-math notation="LaTeX">$_2$</tex-math> </inline-formula>Cu<inline-formula> <tex-math notation="LaTeX">$_3$</tex-math> </inline-formula>O<inline-formula> <tex-math notation="LaTeX">$_{7-\delta }$</tex-math> </inline-formula>-CeO<inline-formula> <tex-math notation="LaTeX">$_2$</tex-math> </inline-formula>-YBa<inline-formula> <tex-math notation="LaTeX">$_2$</tex-math> </inline-formula>Cu<inline-formula> <tex-math notation="LaTeX">$_3$</tex-math> </inline-formula>O<inline-formula> <tex-math notation="LaTeX">$_{7-\delta }$</tex-math> </inline-formula> Multilayers Grown by Reactive Co-Evaporation on Sapphire Wafers

High-T C superconductor thin film heterostructures were deposited using reactive co-evaporation for dual layer electronic applications. The epitaxial structure consisted of 35-nm YBa 2 Cu 3 O 7-δ (YBCO), 75-nm CeO 2 , 150-nm YBCO, and 20-nm CeO 2 on r-plane sapphire wafers. The critical temperature was measured to be 83.6 K and 84.8 K for the bottom and top YBCO layers, respectively. Atomic force microscopy reveals smooth surfaces with RMS roughness of the top YBCO layer to be 4.7 nm. The CeO 2 insulating layer exhibited hopping conduction that freezes out at low temperature, making these structures suitable for electrical circuits with isolated ground planes.

Electrical transport effects in YBCO/LSMO bilayer junctions

The electrical transport effects in interfaces of high TC superconducting and ferromagnetic thin film bilayer structures (YBa2Cu3Ox/La0.67Sr0.33MnO3 junctions with areas of ∼0.2 × 10−4 cm2 and ∼1.21 × 10−4 cm2) were investigated in view of the importance of such structures for oxide electronics and spintronics applications. In some structures the junction resistance Rj, measured by a standard four probe method at T < TC, can be significantly lower (even a few orders) than the real interface resistance Rif because of which a wrong interpretation of the last may occur. We show how to evaluate the right interface resistance and the real Rif was determined from the measurement of RJ for the above two junctions. An unordinary effect - a “change” of the sign of the I-V characteristics in dependence on the current I0 flowing across the bilayer junction was observed in our experiments. We explained the essence of the effect and quantitatively described the effect using a model, proposed in the paper. The conditions for observation of a “negative” value of RJ at T > TC were discussed as well.

Time-Correlated Vortex Tunneling in Layered Superconductors

The nucleation and dynamics of Josephson and Abrikosov vortices determine the critical currents of layered high-Tc superconducting (HTS) thin films, grain boundaries, and coated conductors, so understanding their mechanisms is of crucial importance. Here, we treat pair creation of Josephson and Abrikosov vortices in layered superconductors as a secondary Josephson effect. Each full vortex is viewed as a composite fluid of micro-vortices, such as pancake vortices, which tunnel coherently via a tunneling matrix element. We introduce a two-terminal magnetic (Weber) blockade effect that blocks tunneling when the applied current is below a threshold value. We simulate vortex tunneling as a dynamic, time-correlated process when the current is above threshold. The model shows nearly precise agreement with voltage-current (V-I) characteristics of HTS cuprate grain boundary junctions, which become more concave rounded as temperature decreases, and also explains the piecewise linear V-I behavior observed in iron-pnictide bicrystal junctions and other HTS devices. When applied to either Abrikosov or Josephson pair creation, the model explains a plateau seen in plots of critical current vs. thickness of HTS-coated conductors. The observed correlation between theory and experiment strongly supports the proposed quantum picture of vortex nucleation and dynamics in layered superconductors.

From high-Tc superconductors to highly correlated Mott insulators—25 years of pulsed laser deposition of functional oxides in Leipzig

Pulsed laser deposition (PLD) in Leipzig started in 1989 with Bi2Sr2Ca1Cu2O8 high-Tc superconducting thin films grown in a laser-ionization mass spectrometer. Here, we briefly review 25 years of development of advanced PLD processes in Leipzig and their application to high-Tc superconducting, wide-bandgap semiconducting and multiferroic thin films, nanostructures and composites. The first two decades were devoted to large-area and double-sided YBa2Cu3O7−δ thin films and hetero- and homoepitaxial ZnO-based films and nanowires, respectively. Based on that, transparent, oxide-based electronic devices are processed with epitaxial n-ZnO:Mg, Ga2O3, In2O3, or TiO2 films. Amorphous oxide films of p-ZnCo2O4 and p-NiO provide p-type counterparts in highly rectifying pn-junction devices and are an environmentally friendly alternative. Magnetoelectric multiferroic composites, and highly correlated iridate thin films are other current hot research topics. PLD appears as one of the most flexible growth techniques for functional oxides on research and demonstrator level.

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.

Using ion irradiation to make high-Tc Josephson junctions

In this article we describe the effect of ion irradiation on high-Tc superconductor thin film and its interest for the fabrication of Josephson junctions. In particular, we show that these alternative techniques allow to go beyond most of the limitations encountered in standard junction fabrication methods, both in the case of fundamental and technological purposes. Two different geometries are presented: a planar one using a single high-Tc film and a mesa one defined in a trilayer structure.

Alternating Over-Current Characteristics of HTSC Tape Using DFT

The quench characteristics of high-TC superconducting (HTSC) tape for alternating over-current applications have been investigated at various amplitudes of the over-current and its quench developments have been analysed by applying discrete Fourier transform (DFT) for the voltage and the current waveforms of HTSC tape. Generally, the HTSC tape shows a broad superconducting and normal transition region above its critical current due to the metal sheath unlike HTSC thin film or HTSC bulk. Moreover, the voltage-current characteristics of HTSC tape are complicated because the quench and the recovery between the superconducting state and the normal state are repeated. In this paper, the alternating over-current experiments for the HTSC tape were performed to analyse its quench developments. The fundamental frequency components for the current and the voltage waveforms of the HTSC tape were abstracted by applying DFT. From those fundamental frequency components, the numerical formula for HTSC tape's resistance has been derived. By applying its numerical formula into circuit equation, the quench developments of HTSC tape dependent on alternating over-current have been estimated and the agreement with the experimental results has been found

Magneto-optical imaging of magnetic flux patterns in superconducting films with antidots

We present the results of experiments on visualization of magnetic flux distribution and its dynamics in high-temperature superconductors with artificial defects. High-Tc superconductor thin films were equipped with a special arrangement of antidots in order to separate the streams of magnetic flux moving in (or out of) the sample. A possibility to alter the direction and depth of flux penetration is clearly demonstrated by means of magneto-optical imaging. The resolution was sufficient for observation of flux in particular antidots, which allows more detailed dynamic analysis of such systems.

Role of twins in peak effect phenomenon observed at microwave frequencies in high Tc superconductor thin films

Measurements of microwave surface resistance, R s, at subcritical currents as a function of temperature with varying dc magnetic field upto 0.8 T have shown peak effect (PE) in epitaxial DyBa 2Cu 3O 7− δ (DBCO) and YBa 2Cu 3O 7− δ (YBCO) thin films grown by pulsed laser deposition on 〈1 0 0〉 LaAlO 3 substrates. Microwave measurements were performed on microstrip resonators as test vehicles. Occurrence of a peak in R s in dc magnetic field is governed by the nature and concentration of defects. Evidence shows that thinner films with a higher ratio of areal density of extended defects, n e (such as twin boundaries), to the areal density of point defects, n p, show PE at the measurement frequencies 4.88 and 9.55 GHz; whereas, thicker films (⩾3000 Å) with a smaller n e/ n p ratio do not show PE. 2500 Å thick YBCO film shows a double peak structure at 9.55 GHz, thereby suggesting two sets of twin boundaries in this film having different κ p values. Measurements carried out on low-twinned LaAlO 3 substrates show that 2400 Å thick DBCO film does not exhibit the PE phenomenon at 4.88 GHz upto to an applied field of 0.8 T; this indicates that twins propagated from the LaAlO 3 substrates are responsible for the occurrence of PE at microwave frequencies. Oxygen ion irradiation (90 MeV, 3 × 10 13 ions/cm 2) of 2500 Å DBCO film has been found to shift the peak to lower temperature at 4.88 GHz, but significantly suppress the peak at 9.55 GHz. Depinning frequency, ω p vs. T plot obtained for the 2400 Å DBCO film shows a peak due to the peak in its R s vs. T plots.

Optimum thickness for laser induced thermoelectric voltage effect in high Tc superconductor and CMR thin film

Based on the formula of the time dependence of laser-induced voltage in high Tc superconductor(HTSC) and collosal magneto-resistance (CMR) thin films, we have successfully deduced the function of the optimum thickness for producing largest laser-induced thermoelectric voltage. It is indicated that a direct proportional relationship between laser-induced thermoelectric voltage and the reciprocal of thickness does not correctly describe the situation. In fact, an optimum thickness must exist. This function provides the main factor of affecting the design for fast-time-response device. We have discussed the parameter of thin film influencing the optimum thickness, and compared it with experimental result.

Ludwig Genzel

Ludwig Genzel

Measurement method of microwave surface resistance of high Tc superconductive thin films

A measurement system for characterization of microwave surface resistance of HTS thin films is presented. In this system, a high Q factor sapphire resonator probe with TE 011+ δ mode was developed to measure the surface resistance R S of a single piece of HTS thin film at 12 GHz and 77 K. The size of HTS films can be equal to about 2 in. The method has advantages of nondestructivity, single sample, high Q value, high sensitivity, convenient experiment setup, small volume, and flexibility in operation.

Peak-effect, a new phenomenon observed at microwave frequencies in high Tc superconductor thin films

Measurements of microwave surface resistance in magnetic fields (parallel to c-axis), ranging between 0.2 and 0.9 T, of high quality epitaxial DyBa 2Cu 3O 7− y and YBa 2Cu 3O 7− y superconducting thin films show a pronounced peak near the superconducting transition temperature. It is more pronounced at a frequency of 9.55 GHz as compared to 4.88 GHz. The exact nature of the peak is somewhat sample-dependent thereby indicating that the nature and distribution of defects in the films or the pinning sites determine the nature of the peaks; however, it follows the general trend that as the magnetic field is increased the peak shifts towards the lower temperature and is absent when the field is zero. The temperature and field dependence of the peak suggests that this peak could be associated with the order–disorder transition of the flux-line lattice. The peak-effect shows strong frequency dependence close to the depinning frequency of the flux line lattice. We explain our data in terms of the temperature dependence of the Labusch parameter within the collective pinning scenario in type II superconductors.

高温超電導薄膜の精密な表面抵抗測定のための低損失マイクロ波共振器の構造

We have constructed a microwave resonator module using High Tc superconducting (HTS) thin films and a dielectric sapphire rod to measure microwave surface resistances of the HTS thin films. In this method the surface resistance is calculated from the measured unloaded Q values of the resonator. For precise measurements of surface resistance, therefore, the microwave loss not generated by HTS thin films must be minimized. By reducing the parasitic Q factors of the resonator module, we obtained Q values of as high as 1.2×107 at 17.3K and 12GHz. From the high Q values measured in this improved resonator, the surface resistance of the YBa2Cu3Oy superconducting thin films on MgO substrates was calculated and found to be 0.064 mohm at 20.2K and 12GHz.

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.

Terahertz radiation imaging of supercurrent distribution in vortex-penetrated YBa2Cu3O7−δ thin film strips

We have developed a supercurrent distribution imaging system for high Tc superconductive thin films and demonstrated the visualization of the supercurrent distribution in the vortex-penetrated YBa2Cu3O7−δ thin film strips. The terahertz (THz) radiation and detection system with a scanning femtosecond laser was employed to visualize the distribution. The imaging system utilizes the principle that the femtosecond optical pulses excite THz radiation into the free space by optical supercurrent modulation, and the radiation amplitude is proportional to the local supercurrent density at the optically excited area. Prior to the observation of the supercurrent distribution, we studied optical excitation effects on the vortices trapped in the strips, calibration of the current density from the THz radiation amplitude, temperature dependence of the THz radiation properties, etc. The laser power dependence of the THz radiation in the remanent state revealed that the excitation with powers larger than the relatively weak finite value (about 10 mW in the present case) strongly affects the vortices trapped in the films. We attributed this behavior to the optically excited depinning effect. We derived a calibration function from the THz radiation images into the supercurrent density distributions by observing the bias-current dependence of the THz radiation, and applied it for the diagnosis of the distributions in the vortex-penetrated strips. The THz radiation images were successfully transferred into the supercurrent density distributions with quantitative agreement. The minimum magnetic flux resolution at the optically excited area was roughly estimated to be 3 φ0 where φ0 is a single flux quantum. The measurement of the laser beam profile indicated that the spatial resolution of the THz radiation images is limited by the laser beam diameter: 25 μm in our case. The observed distributions revealed that the vortices easily penetrate into the strip under an external magnetic field BEX of 0.9 mT, and the persistent supercurrent exists only near the strip edges in the remanent state after removal of the field. The calculations of the convolution between the observed laser pattern and the trial functions suggested that the supercurrent distribution width in the remanent state after removal of the field of 0.9 mT is estimated to be less than 1 μm. The temperature dependence of the supercurrent distributions revealed that, below 60 K, the thermal activation produces no significant effects on the penetrated vortices at BEX=0.9 mT, whereas, the vortices in the remanent state after removal of the field of 15 mT were strongly affected by the thermal activation. The decreasing rate of the supercurrent density at the edge with increasing temperature was larger than that inside the strip. This suggested that the vortices trapped near the edges exhibit rather different behavior from the ones that penetrated into the inner part of the strip.

Evaluation of motion of a high- Tc superconducting thin film in microgravity experiments

Motion of a high- T c superconducting (HTS) thin film was observed in microgravity experiments with a free fall drop shaft facility. The field-cooled thin film was suspended under a permanent magnet in the free fall experiments. When the magnet was rotated in microgravity, the thin film moved to a stable position where centrifugal force was balanced to restoring force caused by pinning of superconductor. The motion of the HTS thin film is numerically evaluated by using the frozen field model, which is based on an assumption that fluxoids are fixed at pinning points on the surface of the superconductor. The numerical evaluation is useful to explain the experimental results.