Irina I. Gundareva

Josephson admittance spectroscopy of log-periodic antenna at the submillimeter wavelength range

Application of Josephson admittance spectroscopy for the spectral analysis of a broad-band log-periodic antenna was demonstrated at the frequency range from 50 to 700 GHz. A real part of the antenna admittance was reconstructed as a function of the frequency, when the antenna was connected to a Josephson junction. Small-scale features were observed on this dependence. They demonstrated a periodicity in the logarithmic frequency scale and were related to the antenna resonances.

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}

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.

Josephson Junctions

The I-V curves and dynamic resistances Rd (V) of [001]-tilt YBa₂Cu₃O_7-x bicrystal Josephson junctions have been experimentally studied at various levels of oxygen loading. Oxygen content was modified by annealing in ozone atmosphere, in vacuum or in air. Deviations of the <i>IV</i> -curves from those predicted by the RSJC model in the form of additional nonlinearities at the middle and high voltages were found to decrease after enhancement of oxygen content in junctions. Electrodynamic properties of Josephson junctions were analyzed using log-periodic YBa₂Cu₃O_7-x antennas, integrated with the junctions. Fine structures on the <i>IV</i>-curves, related to antenna resonances, were modified with oxygen content due to better antenna-junction coupling and lower electromagnetic losses in YBa₂Cu₃O_7-x film.

Optimization of electrical and structural parameters of YBa2Cu3O7−x thin-film bicrystal Josephson junctions with chemical and thermal treatments of substrates

Application of Josephson admittance spectroscopy for the spectral analysis of a broad-band log-periodic superconducting antenna was demonstrated at the frequency range from 50 to 700 GHz. The [001]-tilt YBa2Cu3O7−x bicrystal Josephson junctions, integrated with sinuous log-periodic YBa2Cu3O7−x antennas, were fabricated on NdGaO3 bicrystal substrates. A real part of the antenna admittance ReY(f) as a function of the frequency f was reconstructed from the modification of the dc current-voltage characteristic of the junction, induced by the antenna. Resonance features were observed in the recovered ReY(f)-spectra with a periodicity in the logarithmic frequency scale, corresponding to log-periodic geometry of the antenna. The ReY(f)-spectra, recovered by Josephson spectroscopy, were compared with the ReY(f)-spectra, obtained by CAD simulation, and both spectra were shown to be similar in their main features. A value of 23 was obtained for an effective permittivity of the NdGaO3 bicrystal substrates by fitting simulated data to those obtained from Josephson spectroscopy.

Evolution of Electrical and Electrodynamic Properties of

[100]-tilt YBa₂Cu₃O_7-x bicrystal Josephson junctions are good candidates to realize high values of characteristic voltages I_cR_n, which are expected due to large energy gaps in high-T_c materials. Here, we report on detailed electrical characterization of [100]-tilt YBa₂Cu₃O_7-x bicrystal Josephson junctions, which have the maximal I_cR_n-values up to 9.4 mV, for resistances R_n in the range from 0.9 to 400Ω. The I-V curves have been fitted well to those of the RSJ model with ac-coupled resistive shunts R_e. The first derivatives dV/dI versus V have demonstrated a reproducible set of resonances of two types. Numerical simulations of the dV/dI versus V for Josephson junctions coupled with external THz RLC-resonators have been made and both types of resonances have been fitted. A form of the I-V curves and resonances in the first derivatives dV/dI versus V have been attributed to intrinsic broadband and frequency-selective losses, due to interaction of Josephson oscillations with transverse and longitudinal IR-active phonons in YBa₂Cu₃O_7-x electrodes.

{\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-{\rm x}}

Josephson junctions with high characteristic voltages I_cR_n and resistances R_n ranging from subohm values to several hundred ohms are required for various terahertz (THz) applications. Due to optimization of technology, [100]-tilt YBa₂Cu₃O_7-x bicrystal Josephson junctions with an average I_cR_n (5 K) value of 7 mV at an Rn-range from 0.4 to 400 Ω have been fabricated on (320) bicrystal NdGaO₃ substrates. Classical and frequencyselective detection of THz radiation have been demonstrated with the fabricated Josephson junctions. The main characteristics of Josephson oscillators, spectrum analyzers, admittance spectrometers, and classical detectors have been evaluated within this range of junction parameters, and promising results have been obtained.

Bicrystal Josephson Junctions With Oxygen Loading

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.

Josephson admittance spectroscopy application for frequency analysis of broadband THz antennas

High-T<inf>c</inf> Josephson technology looks very promising for terahertz applications. Here, we report on frequency analysis of broadband THz antenna by Josephson admittance spectroscopy at the frequency range of 50–700 GHz. The [001]-tilt YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7−x</inf> bicrystal junction was used for log-periodic antenna characterization. A frequency-dependent real part of antenna admittance ReY(f) was derived from the measured IV -curves and a log-periodic frequency scaling in ReY(f), corresponding to antenna geometry, was found.