Induced Superconductivity in Hybrid Au/YBa2Cu3O7-x Electrodes on Vicinal Substrates

Superconducting electrodes are an integral part of hybrid Josephson junctions used in many applications including quantum technologies. We report on the fabrication and characterization of superconducting hybrid Au/YBa2Cu3O7-x (YBCO) electrodes on vicinal substrates. In these structures, superconducting CuO2-planes face the gold film, resulting in a higher value and smaller variation of the induced energy gap compared to the conventional Au/YBCO electrodes based on films with the c-axis normal to the substrate surface. Using scanning tunneling microscopy, we observe an energy gap of about 10-17 meV at the surface of the 15- nm-thick gold layer deposited in situ atop the YBCO film. To study the origin of this gap, we fabricate nanoconstrictions from the Au/YBCO heterostructure and measure their electrical transport characteristics. The conductance of the nanoconstrictions shows a series of dips due to multiple Andreev reflections in YBCO and gold providing clear evidence of the superconducting nature of the gap in gold. We consider the Au/YBCO electrodes to be a versatile platform for hybrid Josephson devices with a high operating temperature

YBa2_{2}Cu3_{3}O7−x_{7-x} bicrystal Josephson junctions with high Ic_{c}Rn_{n}-products and wide-ranging resistances for THz applications

Josephson junctions with high characteristic voltages IcRn and resistances Rn ranging from subohm values to several hundred ohms are required for various terahertz (THz) applications. Due to optimization of technology, [100]-tilt YBa2Cu3O7-x bicrystal Josephson junctions with an average IcRn (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

Intrinsic AC Shunting in [100]-Tilt Bicrystal YBa2Cu3O 7−x_{7 - x} Josephson Junctions

[100]-tilt YBa2Cu3O7-x bicrystal Josephson junctions are good candidates to realize high values of characteristic voltages IcRn, which are expected due to large energy gaps in high-Tc materials. Here, we report on detailed electrical characterization of [100]-tilt YBa2Cu3O7-x bicrystal Josephson junctions, which have the maximal IcRn-values up to 9.4 mV, for resistances Rn 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 Re. 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 YBa2Cu3O7-x electrodes

High-Tc Josephson Junctions as Quasiclassical THz Detectors

[100]-tilt YBa 2 Cu 3 O 7-x bicrystal Josephson junctions (JJs) with resistances R n = (50-500) Ω and characteristic voltages I c R n (50 K) of 2.5 mV have been fabricated and their potential as classical terahertz detectors has been evaluated. The dc I-V curves of the JJs at temperatures T above 40 K have been found free of hysteresis and demonstrated high nonlinearity with the second derivatives d 2 V/dI 2 up to 10 7 -10 8 Ω/A at T = 50 K. The response ΔV(I) at the current I corresponding to the maximum of d 2 V/dI 2 has been observed in a power range of more than four orders of magnitude, and ΔV is directly proportional to the incident power P (square-law detection) at low power, then ΔV ~ P 1/2 (linear detection) at higher power. The responses ΔV(V) at low voltages V to external radiation with the frequencies from 0.05 to 3 THz have been experimentally found to be proportional to d 2 V/dI 2 (classical detection) at low frequencies and to dV/dI (Josephson detection) at high frequencies with a crossover near the low-frequency limit f l = (0.9 ± 0.3) THz of the ac Josephson effect in our JJs. Numerical simulations of such quasiclassical detector with R n = 300 Ω at T = 50 K predict the NEP-values of 5·10 -15 W/Hz 1/2 at the frequencies up to 1 THz

Testing of Josephson Spectrometer with Waveguide Coupling

One of the challenges in public security is the quick and reliable identification ofthreat liquids in bottles, when vapour analysis is not possible. Recently, we demonstrated that itis possible to rapidly identify liquids by EM measurements of their dielectric functions in thesub-THz range with a high-Tc Josephson spectrometer. Following this approach, we havedeveloped a Josephson spectrometer with a new radiation coupling system, based on dielectricwaveguides. In this paper, we present the results of spectroscopic measurements on liquidsamples of various purities including 30% H2O2/H2O, performed using our Josephsonspectrometer with waveguide coupling. Also, the signal and noise characteristics of a classicalJosephson detector used in our liquid identifier were numerically simulated and the powerdynamic range was estimated for a wide spread of junction parameters

In Situ Hilbert-Transform Spectral Analysis of Pulsed Terahertz Radiation of Quantum Cascade Lasers by High- T c Josephson Junctions

We have studied the applicability of high- Tc Josephson junctions (JJs) and Hilbert-transform spectral analysis (HTSA) for characterization of pulsed terahertz (THz) radiation. We have reconsidered the extension of HTSA into the THz range, including an impact of such emerging factors as nonequilibrium voltage fluctuations and dc Joule heating in high- Tc JJs, and found an unexpected weak temperature dependence of Josephson linewidth δf and a spectral dependence δf(f) with a minimum. Due to this analysis, we have chosen YBa 2 Cu 3 O 7-x bicrystal JJs for experimental study. The JJs have analyzed pulsed THz radiation from quantum cascade lasers (QCLs) located in the same cryogenic environment at 50 K. The spectra recovered by the JJ with Rn = 43 Ω consist of a line with a central frequency around 2.2 THz, which have a symmetrical form and the linewidth of 90 GHz close to the intrinsic Josephson linewidth of this JJ. The spectra measured by the JJ with Rn = 7 Ω and the intrinsic Josephson linewidth of 16 GHz consist of several lines at the frequencies fi around 2.2 THz, which are due to the excitation of different resonant modes in the active volume of the QCL. The difference between the fitted central frequencies f 2 - f 1 of the strong lines and the ratio of their intensities are in agreement with the spectrum of the same QCL measured by Fourier spectroscopy. The developed technique paves the way for detailed and rapid characterization of pulsed THz sources in the frequency domain