M. Chacón

Plasma characterization of a high-pressure d(-sputtering system used for the in situ preparation of high-Tc superconducting thin films

Abstract A characterization of the high-pressure dc-Sputtering system, used for the “in situ” preparation of high-quality high-T c superconducting thin films, was performed by plasma electrical measurements using a single Langmuir probe. A correlation between plasma characteristics and thin film properties (transition temperature, T c , transition width, ΔT c and residual resistivity ratio RRR) is presented. The parameters of the plasma were obtained from measurements at pressures ranging from 2.0 to 4.0 mbar with unheated and heated substrates (around 750°C) during the “in situ” preparation of YBa 2 Cu 3 O 7− x thin films. We found that anode heating contributes to the formation of positive ions and reduces the formation of negative ions, which minimizes the re-sputtering effects during film growth. Optimal superconducting properties of the films were obtained at an oxygen pressure of 3.25 mbar; at this pressure the plasma has the largest density of positive ions and the lowest density of both electron and negative ions.

New solid ionic conductor based on poly(ethylene oxide) and sodium trifluoroacetate

Solid polymer electrolytes consisting of poly(ethylene oxide) (PEO) and sodium trifluoroacetate (CF 3 COONa) with various salt mass fractions (x ≤ 0.435) have been prepared by the solvent casting method. Temperature and concentration dependent impedance studies of the cells M electrolyte | M (M stands for stainless steel electrodes), as well as the conductivity behavior of the electrolyte suggest the existence of a complex in the blend. This is confirmed by differential scanning calorimetry (DSC) that shows a high melting (400 K) phase with a EO/Na ratio corresponding roughly to a 4:1 (x = 0.435) complex. The DSC measurements also indicate that the blends are thermally stable up to approximately 480 K. The conductivity (σ) of the electrolyte is very sensitive to temperature (T) and salt mass fraction (x) in the blend: samples with low salt concentration show a linear dependence of In a with (1/T) below 333 K (the euthetic melting temperature of PEO + complex), with an activation energy of 0.1 ± 0.05 eV, and an almost flat variation above this temperature. The sample with concentration x = 0.306 shows only one activated process in the whole temperature range (T < 400 K). Typically, blends with the highest salt concentrations exhibit an anhydrous conductivity of 10 -4.5 S cm -1 at 303 K and 10 -3.5 S cm -1 at 313 K. The conductivity results are discussed in terms of the high mobility of the Na + ions in the amorphous phase of the complex.

Superconducting properties of HoBa2Cu3O7/SrTiO3/HoBa2Cu3O7 heterostructures

Abstract We have produced HoBa 2 Cu 3 O 7 - x /SrTiO 3 /HoBa 2 Cu 3 O 7 - x heterostructures by a high pressure oxygen dc/rf sputtering technique on (001) SrTiO 3 substrates. The deposition conditions for HoBa 2 Cu 3 O 7 - x and SrTiO 3 are completely compatible. Cross-sectional transmission electron microscopy observations indicate abrupt HoBa 2 Cu 3 O 7 - x /SrTiO 3 interfaces with a 4.5 nm thick intermediate layer and virtually no disruption of the layered structure at the interface. X-ray diffraction measurements show that the HoBa 2 Cu 3 O 7 films grow with a well pronounced orientation (001) parallel to (001) SrTiO 3 . Both the top and the bottom HoBa 2 Cu 3 O 7 layers are superconductors above 90 K. These results indicate the potential for the growth of excellent SIS structures based on this heteroepitaxial system.

Superconducting and structural properties of epitaxial Bi2Sr2CaCu2O8−δ/Bi2Sr2YCu2O8−δ/Bi2Sr2CaCu2O8−δ heterostructures

Abstract We have prepared Bi 2 Sr 2 CaCu 2 O 8−δ /Bi 2 Sr 2 YCu 2 O 8−δ /Bi 2 Sr 2 CaCu 2 O 8−δ trilayer structures, using a “in situ” DC-sputtering process at high oxygen pressures on (001) SrTiO 3 substrates. Bi 2 Sr 2 CaCu 2 O 8−δ films were used for superconducting electrodes and semiconductor-like Bi 2 Sr 2 YCu 2 O 8−δ films with thickness down 5 nm were used as barrier layers. Both materials have the same crystalline structures and allowed the epitaxial growth of the heterstructure as shown by X-ray, RBS measurements and cross section TEM analysis. The superconducting properties were examined by resistance, AC-susceptibility and current-voltage characteristics as function of the temperature. For junctions with areas of 120×120μm 2 defined by photolitography a RSJ behavior was found at temperatures below T c .

Ionic Conductivity in the Polymer Electrolytes PEO/CF3COONa

In the present work, the electrical and thermal characterization of polymer electrolytes based on PEO/CF3COONa are reported, which turn out to be good ionic conductors near room temperature (of the order 10—4 Ω—1 cm—1 for high salt concentrations). The variation of conductivity with temperature (plotted as ln σ versus 1/T) and salt concentration suggests a complex formation. This is confirmed by differential scanning calorimetry (DSC), which also indicates that the blends are thermally stable up to approximately 480 K. The high conductivity and the single-phase behavior of the blends are explained in terms of the plastification effect of the organic salt on the PEO.

Grain boundary junctions with Ag-doped YBa2Cu3O7−x epitaxial thin films

Abstract Using a dc sputtering method at high oxygen pressures we have deposited epitaxial Ag-doped YBa 2 Cu 3 O 7−x thin films on SrTiO 3 bicrystals with missorientation angle of 36.8°. A 15 % Ag-doped YBa 2 Cu 3 O 7−x sintered target was used to sputter the films. Critical current of 4–5 10 6 A/cm 2 at 77 K were measured in doped films. When compared with 1 × 10 6 A/cm 2 for undoped films, indicated an improvement of the superconducting properties by Ag doping. 200–300 nm thick Ag-doped films were patterned across the bicrystal line to form 5–20 μm-wide junctions. Current-voltage characteristics of the grain boundary junctions at temperatures from 10 K to T C showed a resistively shunted junction (RSJ) behavior with I C R N values of 160–170 μV at 77 K which are higher than the measured values for undoped junctions.

Superconducting and structural characteristics of YBa2Cu3O7−x thin films on si(001) substrates with YSZ buffer layers

Abstract We have prepared high quality c-axis oriented superconducting YBa2Cu3O7−x thin films on Si(001) substrates covered with YSZ = (ZrO2 + 10 at.% Y2O3) layers by an in situ high pressure (3–5 mbar) oxygen d.c./r.f. sputtering process at substrate temperatures of about 750°C. The films were characterized by X-ray diffraction, transmission electron microscopy (TEM) and electron scanning (SEM) techniques. A superconducting transition temperature Tc(R− 0) = 89 K was determined by resistivity and susceptibility measurements. Cross sectional TEM analysis showed a sharp interface between YSZ and Si; however an inter diffusion zone was observed between YSZ and YBa2Cu3O7−x layers.

Superconducting properties of epitaxial YBa2Cu3O7/PrBa2Cu3O7/YBa2Cu3O7 heterostructures

Abstract Heteroepitaxial thin films of superconducting YBa 2 Cu 3 O 7 and semiconducting PrBa 2 Cu 3 O 7 with SIS structure were produced by dc-sputtering at high oxygen pressures (3–4 mbar) on SrTiO 3 (001) substrates using a set of MgO masks. The superconducting properties of YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 /YBa 2 Cu 3 O 7 trilayers were examined by resistivity, susceptibility and current-voltage characteristics. PrBa 2 Cu 3 O 7 interlayer thicknesses ranged between 10 and 100 nm. The superconducting transition temperatures exceed 82 K for very thin PrBa 2 Cu 3 O 7 barriers. A SNS-like behaviour was found. The epitaxial growth with no chemical reaction at the interfaces is very likely to be responsible for the excellent superconducting properties found in this multilayer system.