Yu. Ya. Divin

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.

Frequency-Selective Josephson Detector: Power Dynamic Range at Subterahertz Frequencies

The power dynamic range of an YBa2Cu3O7−x frequency-selective Josephson detector for the Hilbert-transform spectroscopy was experimentally studied in the subterahertz frequency range. At a temperature of 80 K, the dynamic range of 2×105 was achieved at a noise equivalent power of 8×10−15 W/Hz1/2.

Terahertz spectroscopy based on high-Tc Josephson junctions

A review of the authors’ studies on the spectroscopy employing the ac Josephson effect in high-temperature superconducting junctions is presented. It is demonstrated that Hilbert spectroscopy and admittance spectroscopy can be successfully implemented in the terahertz frequency range with the use of high-temperature superconductor bicrystal Josephson junctions.

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.

MILLIMETER-WAVE HILBERT-TRANSFORM SPECTRUM ANALYZER BASED ON JOSEPHSON JUNCTION.

ConclusionsA prototype of the Hilbert-transform spectrum analyzer based on the Josephson junctions is described and first spectral measurements are demonstrated in millimeter-wave band. The measurements in the whole millimeter-wave band may be carried out by this spectrum analyzer with resolution as low as 2 GHz and during a time interval as low as a few milliseconds. According to this set of parameters, this Josephson-effect spectrum analyzer may be used for microwave diagnostics of the thermonuclear plasma.

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.

Extension of the frequency range of Josephson impedance spectroscopy

The results of the spectral analysis of a high-temperature superconducting (HTSC) log-periodic antenna integrated with a HTSC Josephson junction (JJ) and operating, in this case, as a test resonant system are presented. Current-voltage characteristics of the JJ and dependences of its differential resistance on the voltage across the JJ are measured at a temperature of 5 K by varying the JJ critical current by an external magnetic field in order to extend the operating frequency range of the spectral analysis. The low-frequency boundary of the spectral analysis, which is determined by temperature fluctuations, is lowered to 64 GHz. The equivalent circuit of an integrated device consisting of the JJ and the log-periodic antenna is analyzed. The real part of the antenna admittance is reconstructed from experimental data in the frequency range 65–1200 GHz. Experimental results are compared with the results of numerical calculation.

Josephson effects in YBa2Cu3O7−x grain-boundary junctions on (001) NdGaO3 bicrystal substrates

Abstract Grain-boundary YBa2Cu3O7−x Josephson junctions were fabricated on (001) NdGaO3 bicrystal substrates with a misorientation angle of 2×18.4°. Both DC and AC Josephson effects were studied in these high-Tc junctions in the range from liquid-helium up to liquid-nitrogen temperatures. The values of characteristic voltages were up to 0.34 mV at a temperature of 10 K. The I-V curves of the junctions and their response to low-intensity millimeter-wave radiation were shown to be described by the RSJ model with thermal fluctuations. Values of responsivity up to 106 V/W and a noise equivalent power as low as 10−14W/ Hz 1 2 were obtained for these junctions operating both in wideband and selective detection modes at a frequency of 78 GHz. The applicability of YBa2Cu3O7−x Josephson junctions made on NdGaO3 bicrystal substrates for detection and spectral analysis of millimeter-wave and far-infrared radiation is emphasized.

Signal and noise characteristics of a terahertz frequency-selective YBa2Cu3O7−x Josephson detector

The performance characteristics of a frequency-selective Josephson detector based on a symmetrical [001]-tilt YBa2Cu3O7−x bicrystal junction have been studied in the terahertz frequency range. At an external radiation frequency of 0.692 THz and a detector temperature of 55 K, the volt-watt responsivity reaches (7 ± 2) × 104 V/W, which corresponds to theoretical estimates calculated with allowance for thermal fluctuations in the junction. The noise equivalent power (2.9 ± 0.9) × 10–13 W/Hz1/2 and the power dynamic range D = 47 ± 3 dB of the detector are determined by the excess 1/f type noise in the junction. It is shown that NEP values up to 5 × 10−15 W/Hz1/2 and D above 60 dB can be reached with high-frequency modulators or pulsed sources of terahertz radiation.

Josephson spectroscopy for local diagnostics of planar resonator systems in the millimeter wave band

The results of studies of interaction between a bicrystalline YBa2Cu3O7–x Josephson junction and external planar resonators are presented. The square resonators of the millimeter wave band are fabricated with splits on separate substrates and were applied to the Josephson junction. Specific features related to excitation of resonance modes corresponding to one-half of the wavelength fitting in the average physical length of the resonator are observed in the voltage dependences of the differential resistance of the Josephson junction. Resonance frequencies (48.7 and 74.9 GHz) and Q factors (14 and 25) are obtained for two resonators with different dimensions. Thus, the possibility to determine the parameters of external passive components of microwave microelectronics from static electrical characteristics of the Josephson junction is demonstrated.