Š. Chromik

High Tc Y-Ba-Cu-O thin films on Si substrates

A low temperature process of vacuum codeposition at 550 °C was used to prepare superconducting Y‐Ba‐Cu‐O thin films on Si substrates without a buffer layer. A zero resistance critical temperature Tce as high as 81.5 K was reached without further high‐temperature post‐annealing treatment. The surface morphology of films has a granular or porous character dependent on substrate temperature. The atomic emission spectroscopy shows the diffusion of Si from the substrate into the films at the level of several atomic percent.

Studies of resistance switching effects in metal/YBa2Cu3O7−x interface junctions

Current-voltage characteristics of planar junctions formed by an epitaxial c-axis oriented YBa2Cu3O7-x thin film micro-bridge and Ag counter-electrode were measured in the temperature range from 4.2 K to 300 K. A hysteretic behavior related to switching of the junction resistance from a high-resistive to a low-resistive state and vice-versa was observed and analyzed in terms of the maximal current bias and temperature dependence. The same effects were observed on a sub-micrometer scale YBa2Cu3O7-x thin film - PtIr point contact junctions using Scanning Tunneling Microscope. These phenomena are discussed within a diffusion model, describing an oxygen vacancy drift in YBa2Cu3O7-x films in the nano-scale vicinity of the junction interface under applied electrical fields.

Thin YBCO films prepared by low-temperature spray pyrolysis

Thin YBCO films were prepared from aerosol by a low-temperature deposition process consisting of two steps-the deposition at atmospheric pressure and firing in vacuum degraded by partial pressure of oxygen-both at temperatures not exceeding 600 degrees C. A stoichiometric 1-2-3 aqueous nitrate solution of Y, Ba and Cu constituents was used as a source of aerosol. MgO and Al2O3 substrates were heated to 140 to 160 degrees C during deposition. The obtained films were 1 to 10 mu m thick with Tc>80 K and Jc=103 to 104 A cm-2. However, detailed TG, DTG and DTA studies performed by the authors and others showed that thermal decomposition of the nitrates used only starts at temperatures higher than 200 degrees C. The use of low substrate temperatures during deposition (140 to 160 degrees C in this case) is apparently the reason for rather low Jc values. Optimization of the preparation process is suggested, which could lead to higher Jc values at processing temperatures not exceeding 600 to 700 degrees C. Such temperatures are still interesting for substrates with elevated diffusivity into prepared films.

High /ital T//sub /ital c// Y-Ba-Cu-O thin films on Si substrates

A low temperature process of vacuum codeposition at 550 °C was used to prepare superconducting Y‐Ba‐Cu‐O thin films on Si substrates without a buffer layer. A zero resistance critical temperature Tce as high as 81.5 K was reached without further high‐temperature post‐annealing treatment. The surface morphology of films has a granular or porous character dependent on substrate temperature. The atomic emission spectroscopy shows the diffusion of Si from the substrate into the films at the level of several atomic percent.

DC and microwave properties of YBa2Cu3Ox thin films in a weak magnetic field

Abstract Measurements of the transport critical current and the modulated microwave absorption in low magnetic fields up to 15 mT are presented for granular crystallographically disordered as well as for c -axis oriented YBa 2 Cu 3 O x thin films at 4.2 and 77 K. Both the transport critical current and the microwave losses are believed to be determined by the weak links existing in both types of samples. The intergranular critical state and the influence of the intragranular magnetization are introduced in order to account for the magnetic hysteresis effects. The experimental results are fitted by a theoretical model, and the differences between disordered and oriented thin films are discussed.

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.

High Tc Y-Ba-Cu-O thin films prepared by in situ low-temperature codeposition of Y, BaF2, and Cu on α-Al2O3 substrates

Thin Y‐Ba‐CU‐O films were prepared in‐situ by low‐temperature codeposition on Al2O3 substrates. The temperatures during preparation did not exceed 600 °C. As a source of Ba the BaF2 was used. The zero resistance Tc values were higher than 84 K, Jc =4×104 A/cm2 at 4.2 K/OT. The Auger electron spectroscopy analysis has shown almost a homogeneous distribution of the film components throughout the film thickness. X‐ray diffraction revealed the presence of unoriented 1‐2‐3 phase besides BaF2 and CuO and, one undefined phase, which might be oxyfluoride compound.

Effect of crystallographic anisotropy on the resistance switching phenomenon in perovskites

Resistance switching effects in metal/perovskite contacts based on epitaxial c-axis oriented YBa2Cu3O6+c (YBCO) thin films with different crystallographic orientation have been studied. Three types of Ag/YBCO junctions with the contact restricted to (i) c-axis direction, (ii) ab-plane direction, and (iii) both were designed and fabricated, and their current-voltage characteristics have been measured. The type (i) junctions exhibited conventional bipolar resistance switching behavior, whereas in other two types the low-resistance state was unsteady and their resistance quickly relaxed to the initial high-resistance state. Physical mechanism based on the oxygen diffusion scenario, explaining such behavior, is discussed.

Highly oriented Y1Ba2Cu3O7 thin films prepared in situ by vacuum co-evaporation

The thin films YBa2Cu3O7 have been prepared on single crystal (100)-SrTiO3 and (200)-MgO substrates by vacuum co-evaporation of Y, BaF2 and Cu. The substrate temperature and total film thickness have been 650-700°C and 150 nm, respectively. The X-ray diffraction, scanning electron microscopy and low-temperature electrical measurements show that prepared films are highly c-axis oriented perpendicular to the substrate, the onset critical temperature Ton ≈ 90 K, zero-resistance critical temperature is TCO = 85 K, and critical current density jC ≈ 3x105 A/cm2 at 4.2 K and zero magnetic field.

Magnetic field in the vicinity of YBCO thin film strip and strip with filamentary structure

Planar YBCO thin film strip was divided into parallel filaments to reduce its hysteresis losses. Magnetic field due to magnetization currents induced by external magnetic field variation was measured in the vicinity of the samples using a Hall Probe. From these measurements the critical current density and hysteresis losses can be deduced. The distance between the Hall Probe and the sample plays an important role for correct loss evaluation from the experimental data. The results are compared with the existing theories describing the behavior of Type II superconducting strips in perpendicular magnetic fields.