S. Benacka

Hysteresis of transport critical currents in high-temperature Y1Ba2Cu3O7-x superconductors: bulk samples and thin films

The current-voltage characteristics and transport critical currents at T=4.2 K and 77.3 K in Y1Ba2Cu3O7-x bulk and thin film samples in magnetic fields up to 9 T have been measured. Large hysteresis of resistively measured critical currents was observed. This is caused by the magnetic field history as well as by the transport current history. An additional explanation of the observed hysteresis based on the different surface and volume pinning mechanisms is proposed.

Epitaxial YBa2Cu3O7 superconducting films, without twin planes on Y2O3/YSZ/Si

Abstract Epitaxial c -axis oriented YBa 2 Cu 3 O 7 superconducting thin films have been grown by off-axis sputtering on Si substrates covered with an Y 2 O 3 /YSZ buffer layer. The films show good superconducting properties and are crack-free for thickness as high as 120 nm. TEM observations reveal an epitaxial growth of YBCO on Y 2 O 3 with an angle of 45° between the [100] directions. Both layers display a microstructure which consists of slightly misoriented rectangular twin-free monocrystalline mosaic blocks.

Interface properties of YBa2Cu3O7-x/I/metal junctions with native and artificial tunnelling barriers

Tunnelling spectroscopy was used to study interface properties of YBa2Cu3O7-x (YBCO)/artificial barrier/normal-metal and YBCO/native barrier/normal-metal tunnel junctions. As artificial tunnelling barriers, YBCO + 2% Co and CeO2 thin films were used. The native tunnel barrier of YBCO was prepared by a surface treatment to create a YBCO layer with oxygen deficiency. Tunnelling characteristics of the junctions were obtained as a rule, but the corresponding differential characteristics indicate suppression of the pair potential in YBCO near the barrier. Peculiarities like a linear dependence of the differential conductance versus voltage, smeared gap-like structures, and increase of the differential conductance close to zero bias voltage for these junctions were observed. These phenomena are discussed within the model of complex structure creation in the tunnel junction as a result of YBCO degradation near the contact with the metal, as well as pronouncedly inelastic processes in the degraded YBCO.

Study of superconducting a-axis oriented YBa2Cu3O7−δ thin films deposited on Y2O3/YSZ/Si with PrBa2Cu3O7−δ seed layer

Abstract Epitaxial a -axis oriented YBa 2 Cu 3 O 7−δ (YBCO) superconducting thin films have been grown by off-axis magnetron sputtering on Y 2 O 3 /YSZ/Si substrates with PrBa 2 Cu 3 O 7−δ (PBCO) seed layer. The YBCO thin films were deposited immediately after the on-axis magnetron sputtering of PBCO. XRD analyses show that the a -axis volume fraction for 120 nm thick YBCO films varies with substrate temperature during PBCO deposition and its maximum value is higher than 98%. The TEM study shows the clear dependence between the character of the R - T dependence and the microstructure of our YBCO thin films, which varies with the change of the volume ratio of a -axis to c -maxis oriented YBCO.

Microwave absorption in YBa2Cu3O x granular thin films at the dc magnetic fields

Localization of microwave absorption in YBa2Cu3Ox superconducting thin films in the presence of very low dc magnetic field was studied. The granular YBa2Cu3Ox thin films of submicrometer thickness had random orientation of grains, were inserted into a microwave cavity for maximum rf magnetic field. The microwave absorption in low dc magnetic field at temperatures just below the critical temperature shows properties which may be ascribed to superconducting grains. At lower temperatures the hysteretic effects and flux-trapping occur in the intergrain regions of the film.

YBa2Cu3O x superconducting thin films prepared by low temperature vacuum codeposition

Y-Ba-Cu-O films were prepared by low temperature codeposition of three components. The Y and Cu contents were evaporated from metallic sources, while Ba was vacuum-evaporated from Ba, BaO and BaF2 sources in separate codeposition experiments. The lowest temperature at which superconducting YBa2Cu3Ox thin films (about 0.5µm thick) preparedin situ was near 500°C. This process enables preparation of superconducting films on various substrates (SrTiO3, MgO, Al2O3, Si) without a buffer layer. Zero-resistance critical temperature was as high as 88 K and the critical current density was 104 A/cm2 at 4.2 K. The morphology of the films was granular with disordered grain orientation, the average grain size being typically 0.5µm.

Resistive transition of YBaCuO thin film in a high magnetic field

The authors have measured the resistive transition broadening of granular polycrystalline YBa2Cu3Ox thin films as a function of the magnetic field in the temperature range below 90 K and magnetic fields of up to 20 T. The experimental results show that the broadening of resistive transition with increasing magnetic field strength is consistent in many respects with Tinkhams model based on the concept of giant flux creep.