Yuri Divin

Spectral range of the ac Josephson effect in [001]-tilt YBa/sub 2/Cu/sub 3/O/sub 7-x/ bicrystal junctions

We have studied the ac Josephson effect in YBa/sub 2/Cu/sub 3/O/sub 7-x/ bicrystal junction by monitoring the voltage dependence of the dc current response /spl Delta/I(V) induced by low-power monochromatic radiation with the frequencies f/sub i/. The intensity of odd-symmetric resonances in the function /spl Delta/I(V) near the voltages V/sub i/=hf/sub i//2e is proportional to the amplitude of Josephson oscillations at the frequency f/sub i/. The resonance intensities have been mapped as a function of the Josephson frequency f/sub i/ in the range from 5 GHz to 5 THz, the junction resistance R/sub n/ in the range from 0.4 to 80 Ohm and junction characteristic voltages I/sub c/R/sub n/ in the range from 5 /spl mu/V to 1.8 mV. The central frequency of the range was scaled with the I/sub c/R/sub n/-product of the junction and it could be shifted from GHz-range to the THz-range by a temperature decrease. The spectral range was limited by an enhanced noise broadening of the Josephson linewidth at low frequencies and by increased dc Joule heating at high frequencies.

Terahertz Hilbert Spectroscopy by High-Tc Josephson Junctions

Hilbert spectroscopy is based on the frequency-selective detection of weak electromagnetic radiation by Josephson junctions described by the resistively shunted junction (RSJ) model. The YBa2Cu3O7-x grain-boundary Josephson junctions fabricated on twin-free NdGaO3 bicrystals were found to be close to the RSJ model. Laboratory prototypes of general-purpose Hilbert spectrometers were developed and characterized. The spectral bandwidth of Hilbert spectroscopy for any junction temperature between 30 and 85 K is as large as one frequency decade. The middle frequency of this bandwidth scaled with the characteristic frequency f c = (2e/h)I c R n of the Josephson junction and a total bandwidth of almost two orders, from 50 GHz to 4 THz, was covered by one Josephson junction at two temperatures. A spectral resolution δf/f of the order of 10-3 was realized in the terahertz range. The dynamic range of intensities of radiation which can be measured by Hilbert spectroscopy is close to five orders. With the help of Hilbert spectroscopy we measured the following emission spectra: Lorentz spectra of Josephson oscillations, spectra of high-harmonic content in commercial millimeter-wave oscillators, spectra of terahertz radiation from optically-pumped gas lasers and spectra of transition radiation from relativistic electron bunches at DESY (Hamburg).

Epitaxial YBa2Cu3O7-x thin films with tilted c-axis orientation

Abstract The microstructure and electrical transport properties of the YBa 2 Cu 3 O 7tx thin films deposited on NdGaO 3 substrates with tilted (0≤α≤45°) [110] orientation were studied. The electrical transport along the tilt (perpendicular to the rotation axis of the tilt) for low angles might be mainly attributed to the contribution of the c-axis component. The transport in the thin films with large α is effected by a network of 90° grain boundaries resulting from the domain-antidomain structure of the films. At 100 K the intrinsic resistivity ϱ c along the c-axis was determined to be about 50 mΩ·cm and an anisotropy ϱ c /ϱ ab up to ∼600 was observed.

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.

Terahertz Characterization of External Resonant Systems by High-

High-Tc Josephson technology looks promising for THz applications. One of the remaining problems, important both for detection and emission of THz radiation, is an optimum coupling of high-Tc junctions with environment. Recently, we have demonstrated a possibility to characterize electromagnetic systems integrated with high-Tc junctions using dc I-V curves of these junctions. Here, we report in details on modification of the dc I-V curves of YBa2Cu3O7-x bicrystal junctions, related to interaction of Josephson oscillations with terahertz resonance antennas. Thin-film log-periodic antennas with various resonance structures were fabricated on the same substrate as Josephson junctions and were excited by Josephson oscillations. Frequency-dependent admittances of the antennas were derived from fine log-periodic structures on the dc I-V curves of the junctions and compared with the results of numerical simulations. Data obtained can be used for optimization of coupling between the junctions and antennas.

T_{c}

We have studied a voltage dependence of the response /spl Delta/I(V) of high-T/sub c/ Josephson detector to millimeter-wave radiation as a function of power of incident radiation. YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain-boundary junctions with the resistances R/sub n/=0.5-1.5 Ohm and the I/sub c/R/sub n/-product in the range 0.16-0.25 mV at 80 K have been fabricated for this study. Gunn oscillator with the frequency f=86 GHz and a set of calibrated attenuators with total attenuation of 80 dB have been used for the measurements. The dynamic range, i.e. the range of power of electromagnetic radiation at which the response of the detector is directly proportional to radiation power, was found to be around 40 dB above the noise-equivalent power for frequency-selective response at voltages V near hf/2e and >45 dB for the broadband response at low voltages.

Josephson Junctions

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.

Dynamic range of frequency-selective response of high-T/sub c/ Josephson detector to millimeter-wave radiation

A spread of the critical currents in a series array of up to 100 YBa/sub 2/Cu/sub 3/O/sub 7-x/ bicrystal junctions has been studied by Laser Scanning Microscopy. The values of the critical current I/sub c/ of individual junctions in the array have been obtained by focusing a laser beam on each junction and measuring the current at which the maximum laser-induced voltage response /spl Lambda/V on the array has appeared. The distribution of critical currents in logarithmic scale was close to a Gaussian one. The I/sub 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.

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

The [100]-tilt high-Tc Josephson junctions are characterized by high characteristic voltages IcRn but their current range is limited by low critical currents Icf of the thin-film electrodes with tilted c-axes. The impact of chemical and thermal treatments of NdGaO3 substrates on morphology and IV curves of the YBa2Cu3O7−x thin films and the bicrystal thin-film junctions have been studied. It was found that IV curves of the films above a critical current Icf are described by a power-law dependence V = V0(I/Icf – 1)n, where Icf, V0 and n are dependent on the c-axis tilt of the film and treatment of the substrate. Critical current densities jc(78K) up to 1.4107 A/cm2 have been reached in 1.7°-tilt YBa2Cu3O7−x films perpendicular to the tilt. Bicrystal junctions have been fabricated with optimized YBa2Cu3O7−x thin films, and RSJ-like IV-curves in the current ranges from Ic up to 5 Ic with IcRn (78K)-values of 1 mV have been achieved.

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

Josephson admittance spectroscopy was applied for spectral analysis of broadband log-periodic sinuous antenna with modified log-periodic geometry, which provides pronounced resonances. A correspondence of experimentally observed spectral features to the results of numerical simulations was established.