P.M. Shadrin

Properties of YBa/sub 2/Cu/sub 3/O/sub 7/ thin films deposited on substrates and bicrystals with vicinal offcut and realization of high I/sub c/R/sub n/ junctions

The surface morphology, microstructure and transport properties of epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ and PrBa/sub 2/Cu/sub 3/O/sub 7/ thin films and heterostructures deposited on slightly vicinal substrates of SrTiO/sub 3/ by high pressure oxygen sputtering were studied. The vicinal angles of the substrates and bicrystals were less then 13/spl deg/. Depending on the tilt angle of the substrate a transition from spiral or island to step-flow growth leading to an improvement of the surface roughness was observed. Atomic force and transmission electron microscopy were used for these investigations. Furthermore, electrical and structural properties of YBa/sub 2/Cu/sub 3/O/sub 7/ thin films on vicinal offcut SrTiO/sub 3/ bicrystals with different grain boundary types were studied. This included junctions with a 2/spl times/12/spl deg/ tilt or twist of the YBa/sub 2/Cu/sub 3/O/sub 7/ c-axis across the grain boundary. In comparison to conventional [001]-tilt grain boundaries bicrystal Josephson junctions [100]-tilt grain boundaries showed high I/sub c/R/sub n/-products of up to 1.2 meV at 77 K and up to 8 meV at 4.2 K. IV-curve instabilities, probably of magnetic origin due to flux flow in the electrodes, often could be observed for junctions biased with high current densities.

Submicrometer electrical imaging of grain boundaries in high-Tc thin-film junctions by laser scanning microscopy

Abstract High-resolution spatially resolved study of electrical inhomogeneities in high-Tc thin-film junctions on bicrystal substrates has been carried out. A laser beam has been focused into a submicrometer spot on the surface of the sample and induced an increase of its local temperature. Due to bolometric or thermo-electric effects in the heated region, the change of a voltage, ΔV, across the junction has been developed and it has been measured as a function of the beam position (x,y). The ΔV(x,y) images of YBa2Cu3O7−x grain-boundary junctions made from c-axis and tilted c-axis thin films on bicrystal substrates have been obtained. In spite of the difference in the symmetry of the bolometric and thermo-electric ΔV(x,y) images across the grain boundary, the correlation between these two images along the grain-boundary has been shown. Due to an odd symmetry of thermo-electric images ΔV(x,y) across the grain boundary, it is possible to locate the position of a grain boundary in high-Tc junctions with an improved resolution as low as 0.1 μm. Within this accuracy, the deviations of a grain boundary in thin films from a bicrystal plane in the substrate have been demonstrated in grain-boundary junctions made from c-axis YBa2Cu3O7−x films and the absence of this faceting has been shown for grain-boundary junctions made from tilted c-axis films.

Laser probing of high-T/sub c/ superconducting thin films

The spatial inhomogeneity of the electrical characteristics of high-T/sub c/ thin films was studied by optical laser probing. It was shown that the characteristic length L of inhomogeneous current distribution in high-T/sub c/ films may be much larger than the grain size a; for example, in polycrystalline YBa/sub 2/Cu/sub 3/O/sub 7-x/ film with critical current densities j/sub c/(4.2 K) approximately 10/sup 3/-10/sup 4/ A/cm/sup 2/ and grain size a approximately 1 mu m, the characteristic length L was equal to approximately 15 mu m at T>T/sub c/ and approximately 100 mu m at T<or approximately=T/sub c/. The large values of L are related to large-scale percolation in the system of intergrain Josephson junctions with very wide distribution of critical currents I/sub ci/ and normal-state resistances R/sub ni/.

Hilbert spectroscopy from gigahertz to terahertz frequencies by high-Tc Josephson junctions

Abstract Hilbert spectroscopy is based on a frequency-selective detection of electromagnetic radiation by a Josephson junction described by the resistively shunted junction (RSJ) model. The YBa 2 Cu 3 O 7− x grain-boundary junctions fabricated on NdGaO 3 bicrystals were found to be close to the RSJ model. General-purpose Hilbert spectrometers were developed and characterized using voltage-biased low-resistance junctions. The spectral bandwidth of Hilbert spectroscopy was shown to be of several frequency decades for any junction temperature between 48 and 85 K. A total bandwidth from 6 GHz to 2.5 THz has been covered using one junction at different temperatures. The spectral resolving power f /δ f of Hilbert spectroscopy was found to be of three orders in the terahertz range. The dynamic range of intensities of electromagnetic radiation in Hilbert spectroscopy was shown to be of five orders.

Imaging of electrical inhomogeneities in YBa2Cu3O7−x thin-film structures by room-temperature laser scanning microscopy

Abstract Imaging of local electrical inhomogeneities in YBa 2 Cu 3 O 7− x thin films and grain-boundary junctions was demonstrated at room temperature. The spatial distributions of the voltage response of the samples to the focused laser beam were measured with and without bias current through the samples. High-contrast electrical images consisting of 512 x 512 points with 8 bit signal resolution and a spatial resolution of up to 1 μm were obtained for all YBa 2 Cu 3 O 7− x samples under study. The zero-bias response was shown to reveal the grain-boundary regions in the samples and might be explained by thermoelectric effects. The response proportional to the current bias was shown to be due to the bolometric effect in the material itself and this response can be used to study the inhomogeneities in grain composition.

Spread of critical currents in thin-film YBa/sub 2/Cu/sub 3/O/sub 7-x/ bicrystal junctions and faceting of grain boundary

The statistical distributions of critical currents in series arrays of YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain-boundary junctions have been studied by low-temperature laser scanning microscopy. A set of arrays has been fabricated on [110] NdGaO/sub 3/ bicrystal substrates with misorientation angles from 2 /spl times/ 10/spl deg/ up to 2 /spl times/ 26/spl deg/ and patterned to the widths from 1.7 up to 5 micrometers. The critical current values of the individual junctions in the array have been obtained by focusing a laser beam on each junction and measuring the bias current at which the maximum laser-induced voltage response has appeared on the array. The measured critical current distributions have been demonstrated to be close to a log-normal Gauss function. A spread of this distribution has been found to increase with a bicrystal angle. YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain-boundary topography has been studied by atomic force microscopy. From results of these measurements we suppose that the maximum values of critical current density might be assigned to the symmetrical facets of grain boundary.

YBa2Cu3O7−x thin-film Josephson junctions on 2 × 12° bicrystal (110) NdGaO3 substrates

Abstract Grain-boundary YBa2Cu3O7−x Josephson junctions with normal-state resistances from 0.1 to 10 Ω were fabricated on (110) NdGaO3 bicrystal substrates with a misorientation angle of 2 × 12°. Electrical imaging of the junctions in the temperature range 80 K–300 K was performed by a technique based on laser scanning microscopy. The average values of the electrical transport parameters of the fabricated junctions were shown to be comparable to those of reference junctions made on (100) SrTiO3 bicrystals. Values of the Josephson linewidth as low as 1 GHz at 77 K have been obtained from the response of the junctions to low-intensity 94 GHz electromagnetic radiation. When high-intensity 94 GHz radiation was applied to the junctions, current steps appeared in the I–V curves at voltages Vn = n hf/2 e up to 6.5 mV. These results demonstrate the applicability of YBa2Cu3O7−x grain-boundary junctions made on NdGaO3 bicrystal substrates for millimeter- and submillimeter-wave detection.

Local characterization of Y-Ba-Cu-O thin films

A high-resolution spatially resolved study of electrical inhomogeneities in high-T/sub c/ thin films by Laser Scanning Microscopy has been carried out. A laser beam was focused onto a submicrometer spot on the surface of the thin film microbridge resulting in an increase in its local temperature. Bolometric or thermo-electric effects in the heated region result in a voltage change across the sample that has been measured as a function of the beam position. These two electrical images have been compared with optical images formed by subtracting two conventional images taken with polarized light. The correlation between the three images strongly suggest that interfaces between grains with different twin boundary orientations are more resistive than interfaces between grains with the same twin boundary orientation.

Local characterization of HTS thin films by laser scanning microscopy

Abstract A high-resolution spatially resolved study of electrical inhomogeneties in high-T c thin films by Laser Scanning Microscopy has been carried out. A laser beam was focused onto a submicrometer spot on the surface of a thin film microbridge that induced an increase in its local temperature. Bolometric or thermo-electric effects in the heated region resulted in a voltage change, δV, across the sample that has been measured as a function of the beam position (x,y). Direct measurements of the spatial distribution of the critical current density in the film has been performed from the δV(x,y) images of HTS microbridges. A system of weak links in the film is observed that may be attributed to the growth island boundaries. A spatial redistribution of the critical current as a function of temperature is observed in this system. It has been demonstrated that for some conditions, the electrical characteristics of the entire microbridge can be determined by only one weak link. The position of the weak links could be observed at room temperature using the thermo-electric response.

Spread of critical currents in thin-film YBa2Cu3O7−x bicrystal junctions and faceting of grain boundary

Abstract A spread of the critical currents in a series array of 100 YBa 2 Cu 3 O 7− x bicrystal junctions has been studied by laser scanning microscopy. The values of the critical current I c of individual junctions in the array have been obtained by focusing a laser beam on each junction and measuring the current through the array at which the maximum laser-induced voltage response has appeared on the array. The distribution of critical currents in logarithmic scale was close to a Gaussian one. The I c -spread has been found to increase with the increase of misorientation angle of bicrystal substrate and the decrease of the width of the junctions in the array.