V. N. Gubankov

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.

Signal and Noise Characteristics of Terahertz Frequency-Selective and Broadband High-

Josephson detectors based on YBa2Cu3O7-x [100]-tilt bicrystal junctions were fabricated and their frequency-selective and broadband modes of operation were studied at terahertz frequencies. The resistances of the [100]-tilt junctions were in the range of 1-200 Ohm and their characteristic voltages were up to 8 mV at 4.2 K. Values of the responsivity up to at radiation frequency of 0.7 THz were demonstrated for the low-resistance frequency-selective detectors at a temperature of 55 K. The responsivity values were found to be in accordance with the predictions of the RSJ model, when only thermal fluctuations are considered. But, the values of noise equivalent power (NEP) and power dynamic range measured in the same experiment at a modulation frequency of 2 kHz were found to be determined by 1/f noise of the junctions and equal to 3ldr10-13 W/Hz1/2 and 5ldr104, correspondingly. It was shown that the values of NEP of 5ldr10-15 W/Hz1/2 and power dynamic range of 106 could be reached in this mode if high-frequency modulation or pulsed radiation were used. Broadband classical detection in high-resistance [100]-tilt junctions was experimentally found to reach the terahertz range. It follows from numerical simulations that broadband detection by the [100]-tilt YBCO junction with the resistance of 300 Ohm might be characterized by NEP-values down to 3ldr10-15 W/Hz1/2 and a bandwidth of 1.5 THz. Josephson detectors based on YBCO [100]-tilt bicrystal junction are promising for various terahertz applications such as real-time spectral analysis of continuous or pulsed radiation sources and terahertz imaging for medical or security screening.

T_{c}

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.

Josephson Detectors

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 spectroscopy based on high-Tc Josephson junctions

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

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.

Extension of the frequency range of Josephson impedance spectroscopy

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.

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

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

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.

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

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.