J. Hudner

Conductance noise and percolation in YBa2Cu3O7 thin films

Abstract Experimental results of the conductivity noise in the superconducting transition region of YBCO thin films prepared by co-evaporation are presented. In the case of ex situ fabricated samples, Cooper-pair number fluctuations (induced by electron trapping) have been identified in the high-temperature part of the transition. Classical percolation noise was found in the low-temperature part of the transition. In the case of samples made by the in situ method, the noise is smaller by several orders of magnitude in the upper part of the transition. This indicates a much more ordered microstructure in these samples. Electron mobility fluctuations, shunted by the conductance of Cooper-pairs, were identified in this temperature range. In the low-temperature part of the transition, a new type of fluctuation has been discovered; the fluctuation of the volume fraction of the superconducting phase. This implies new scaling exponents very different from the exponents of classical percolation models. This effect is a consequence of intergrain critical current fluctuations and can be caused by, for instance, magnetic flux motion, defect motion or trapping of electrons in the barriers between grains. Moreover, in the in situ fabricated samples, a dimensional crossover, 3D→2D, of the percolating network has been observed. From this effect, the length scale of the microscopic disorder can be estimated.

Growth and characterization of yttrium oxide thin layers on silicon deposited by yttrium evaporation in atomic oxygen

Polycrystalline yttrium oxide films have been grown cinder high vacuum conditions by evaporation of yttrium in an atomic oxygen plasma. Well-oxidized uniform layers in the thickness interval 30-170 ...

Scaling spontaneous fluctuations in high-Tc superconductor films☆

Abstract Spontaneous conductivity fluctuations in high-Tc superconducting films on sapphire and SrTiO3 substrates have been measured through the superconducting transition. It is found that the normalized spectrum for the sapphire film is two orders of magnitude larger than for the SrTiO3 film, due to a stronger disorder. At sufficiently low temperatures, the normalized spectrum scales with the conductivity, which proves the existence of a percolation superconducting network.

Spontaneous conductivity fluctuations in YBaCuO thin films: Scaling of fluctuations, experimental evidence of percolation at the superconducting transition

Abstract Spontaneous conductivity fluctuations in high- T c superconducting films on sapphire and SrTiO 3 substrates have been measured through the superconducting transition. It is found that the normalized spectrum for the sapphire substrate film is two orders of magnitude larger than for the SrTiO 3 substrate film, due to a stronger disorder. At sufficiently low temperatures, in the superconducting transition region, the normalized spectrum scales with the conductivity. This is experimental evidence of a temperature controlled percolating superconducting network in high- T c superconductors.

Coevaporated thin films of Y-Ba-Cu-O utilizing quadrupole mass-spectrometer rate control

Abstract A study of coevaporation of Y, BaF 2 and Cu using a quadrupole mass-spectrometer as an evaporation rate control has been performed. Actual rate values have been determined by RBS analysis and compared with mass-spectrometer settings for different source combinations and partial pressures of oxygen during deposition. It is shown that Y-Ba-Cu films of precise composition can be codeposited by using a mass-spectrometer controlled coevaporation process. The method has been utilized for a deposition on seven different substrates (LiNbO 3 , SrTiO 3 , LaAlO 3 , CaNdAlO 4 , LaGaO 3 , MgO and Al 2 O 3 ) simultaneously. After a post-heat treatment in flowing wet oxygen the films have been analysed regarding resistivity and critical current density.

In-Situ Preparation of Y-Ba-Cu-O Thin Films Using Mass-Spectrometer Rate Control and Atomic Oxygen

INSITU PREPARATION OF Y-BA-CU-O THIN-FILMS USING MASS-SPECTROMETER RATE CONTROL AND ATOMIC OXYGEN

YBaCuO thin films on sapphire substrates

Abstract Thin films of YBaCuO have been fabricated by co-evaporation of Y, Cu, and BaF 2 -powder in an oxygen atmosphere. The effect of various post-deposition annealing procedures are reported. Results from resistivity and critical current measurements as well as SIMS and Auger analysis are presented.

Studies of YBa2Cu3O7−x formation in coevaporated YBaF2CuO thin films on sapphire substrates

Abstract Degradation mechanisms when films of YBa2Cu3O7−x (YBCO) were formed on sapphire substrates have been studied. The films were synthesized by post-deposition annealings in a wet oxygen atmosphere of coevaporated layers of YBaF2CuO. X-ray diffraction revealed the existence of impurity phases in the annealed samples, the most significant being BaAl2O4, Y2BaCuO5, CuO and Y2O3. Redistribution of the constituents as a function of time was examined by secondary ion mass spectrometry and Rutherford backscattering spectrometry, giving evidence of a Ba redistribution against the interfacial region and an agglomeration of Cu within the film. While zero-resistivity temperatures of 1 μm thick films typically were between 80 and 88 K, magnetic characterization indicated weak coupling between the superconducting grains, resulting in low transport critical currents ⩽ 103 A cm−2 at 77 K. The film properties of YBCO/sapphire were compared with those of YBCO processed in parallel on SrTiO3 or LaAlO3.

Properties of Yttrium Oxide Thin Films on Silicon (100) Prepared by Evaporation of Yttrium in Atomic Oxygen

Thin layers of Y 2 O 3 have been prepared on silicon (100) by an activated reactive evaporation process involving evaporation of metal Y in an atomic oxygen plasma. The presence of the oxygen plasma was found to be crucial for the formation of homogeneous Y 2 O 3 films on Si. The formation of Y 2 O 3 films on Si (100) at different substrate temperatures was investigated. X-ray diffraction analysis showed that Y 2 O 3 films formed between 300 °C and 650 °C were (111) textured while Y 2 O 3 prepared at lower substrate temperatures (80 °C) exhibited mixed orientations. Rutherford backscattering spectrometry indicated that films were stoichiometric. No pronounced channeling was observed in films grown at 350 °C, suggesting polycrystalline film structures. Atomic force microscopy revealed very smooth surface morphologies with average surface roughness

Magnetic measurements on coevaporated YBa2Cu3O7 thin films

Abstract The field an time dependence of the magnetic susceptibility of coevaporated YBa 2 Cu 3 O 7 thin films on SrTiO 3 and sapphire substrates have been investigated. Different relaxation and field dependent behaviour for the two films is reported.