Bicrystal Josephson Junctions Research Articles

Vector substrate-based Josephson junctions

We present a way to fabricate bicrystal Josephson junctions of high-Tc cuprate superconductors that are not grown on bulk bicrystalline substrates. Based on vector substrate technology, this approach makes use of a few tens-of-nanometer-thick bicrystalline membranes transferred onto conventional substrates. We demonstrate 24° [001]-tilt YBa2Cu3O7−x Josephson junctions fabricated on sapphire single crystals by utilizing 10-nm-thick bicrystalline SrTiO3 membranes. This technique allows one to manufacture bicrystalline Josephson junctions of high-Tc superconductors on a large variety of bulk substrate materials, providing distinctive degrees of freedom in designing the junctions and their electronic properties. Furthermore, it offers the capability to replace the fabrication of bulk bicrystalline substrates with thin-film growth methods.

Multi-source excited travelling-wave bowtie antenna based on a meander series of YBCO bicrystal Josephson junctions

Series superconducting Josephson junctions (JJs), as terahertz (THz) mixers, have attracted increasing attentions due to their advantages in solving the saturation problem of superconducting mixers, achieving higher mixing harmonics and higher sensitivity in THz band. However, the normal-state resistances of the series JJs are so low that there exists an impedance mismatch between the coupled antenna and the JJs. In this paper, a meander embedding travelling-wave bowtie antenna is proposed for a series of bicrystal JJs. With this antenna, not only the problem of impedance mismatch can be solved, but also more series JJs can be inserted into the antenna. Five and even seven series JJs in the meander match the proposed antenna well. Furthermore, combined current distribution, far-field radiation patterns and parameter study are investigated in the numerical simulation to analyze this antenna.

Terahertz Absorption Spectroscopy in Bicrystal Josephson Junctions Formed from Mutually Tilted c-Axes YBa2Cu3O7−x Films

Spectral analysis of terahertz (THz) and sub-THz emission from quantum cascade lasers has been recently demonstrated using conventional YBa2Cu3O7−x bicrystal Josephson junctions made from c-axes thin films. Josephson frequencies of alternative bicrystal junctions made from YBa2Cu3O7−x films with mutually tilted c-axes extend further into the THz range. However, these THz oscillations can weaken due to new absorption channels in the junction environment. Here, using Josephson admittance spectroscopy, THz losses in YBa2Cu3O7−x bicrystal junctions with mutually tilted c-axes are studied. Absorption maximizes at a reproducible set of THz frequencies close to those of collective modes in bulk YBa2Cu3O7−x recovered by Fourier spectroscopy. Annealing junctions in atomic oxygen reduces the losses at frequencies of 2.7 and 3.6 THz, while the losses increase at frequencies of 2.3 and 4.6 THz. Thus, as a THz spectrum analyzer, YBa2Cu3O7−x bicrystal junctions require post-fabrication correction of the oxygen content. In addition, the fine structure of the absorption spectrum appears at frequencies near 4.6 THz. Significant absorption near 2.3 THz may be due to effects associated with the second Josephson harmonic or second-order nonlinearity of the susceptibility in YBa2Cu3O7−x. This work paves the way towards probing collective modes in high-Tc materials in situ using the Josephson oscillations.

Влияние контактного сопротивления интерфейса YBCO|Au на транспортные и СВЧ-свойства массивов джозефсоновских контактов из высокотемпературных сверхпроводников

The influence of the contact resistance of the interface of the YBCO/Au structure on the transport and microwave properties of arrays of high-temperature bicrystal Josephson junctions embedded in a coplanar transmission line has been studied. The current-voltage characteristics of Josephson structures fabricated using various technologies are studied: in-situ and ex-situ with annealing in an oxygen atmosphere. The results obtained can be used to create a quantum ac voltage generator based on high-temperature Josephson junctions.

Influence of the contact resistance of the YBCO/Au interface on the transport and microwave properties of arrays of high-temperature Josephson junctions

The influence of the contact resistance of the interface of the YBCO/Au structure on the transport and microwave properties of arrays of high-temperature bicrystal Josephson junctions embedded in a coplanar transmission line has been studied. The current-voltage characteristics of Josephson structures fabricated using various technologies are studied: in situ and ex situ with annealing in an oxygen atmosphere. The results obtained can be used to create a quantum ac voltage generator based on high-temperature Josephson junctions. Keywords: high-temperature superconductors, Josephson junctions, microwave, contact resistance.

Complexes of Parallel Grain Boundaries in YBa2Cu3O7 Film for Josephson Junction Chains and Arrays

Substrates consisting of five parallel planar grain boundaries (GBs), oriented perpendicular to the surface and spaced by several micrometers, have been grown by solid-phase bonding of fianite crystals in order to obtain large complexes of closely spaced bicrystal Josephson junctions based on high-temperature superconductor (HTSC) films. The substrates had a crystallographic orientation (100) and dimensions of 1.0 × 10.0 × 10.0 mm3. All GBs were of symmetrical type, and their lattices were rotated by 12° in opposite directions around the axis perpendicular to the surface. The distance between GBs in a packet complex was 15 µm, and this value was maintained with an error of no more than 1 µm along their entire length of 10 mm. The deviation of the shape of all five GBs from an ideal geometric plane was within 1 µm along their entire length. Films of HTSC YBa2Cu3O7 with inheritance of GBs were grown on these substrates. Structures of two types were formed on the films using lithography. It is shown that the use of these fianite substrates in HTSC-based devices of cryogenic electronics increases significantly the density of bicrystal Josephson junctions; reduces several times the total length of superconducting lines connecting these junctions; and, correspondingly, decreases significantly the inductive resistance of these circuits and improves their electrical characteristics in comparison with the structures having similar functions but based on one or two parallel GBs.

Tl2Ba2CaCu2O8 bicrystal Josephson junctions operated close to 100 K

We have fabricated Tl2Ba2CaCu2O8 (Tl-2212) bicrystal Josephson junctions (BJJs) on LaAlO3 substrates and operated them at bath temperatures close to 100 K. The current-voltage characteristics of BJJs reveal that the critical current Ic has a power-law relationship with temperature. In addition, the normal resistance Rn has a universal value independent of the bath temperature. The performance of BJJs exhibits a perfect agreement with the Ambegaokar-Halperin model close to 100 K. Under microwave irradiation, Shapiro steps were clearly observed, confirming the genuine Josephson coupling through the grain boundary. We demonstrate that the maximum operating temperature of the proposed junction is up to 100 K, which is fairly close to the Tc of the specimen. The results of Tl-2212 bicrystal Josephson junctions may open a new perspective in further studies on low-cost and portable superconducting devices.

Bowtie loaded meander antenna for a high-temperature superconducting terahertz detector and its characterization by the Josephson effect.

In a quasi-optical system, the high temperature superconducting terahertz detector often suffers from a fundamental problem of low coupling efficiency with the terahertz signal, especially for the detector based on YBa2Cu3O7-δ (YBCO) bicrystal Josephson junction (JJ) due to a small normal-state resistance. Here, we developed a bowtie loaded meander antenna to enhance coupling efficiency. Differing from the conventional characterization confining on vector network analyzers, we applied three methods to evaluate the antenna, including the measurements of the maximal size of the first order Shapiro step under per incident power, the coupling efficiency between the antenna and the junction, and voltage responsivity. Furthermore, with simulation analysis, we propose that the inductive reactance of the YBCO bicrystal JJ is around 60 ohms under terahertz irradiation at 210 GHz, thus, the reactance is comparable as that of the antenna.

Series YBCO grain boundary Josephson junctions as a terahertz harmonic mixer

Josephson devices have demonstrated the capability to work as harmonic mixers at terahertz frequencies. Low temperature devices, in particular Nb–Nb point contacts have proven particularly successful. However, practical applications of these devices have been limited by the need of utilizing liquid helium. To overcome this limitation we have investigated the use of high-Tc devices. In this paper, we report mixings up to the 154th harmonic at zero-bias in a series of YBa2Cu3O7−δ (YBCO) grain boundary Josephson junctions. We have integrated a meander series of three bicrystal YBCO Josephson junctions with a log-periodic antenna. When properly operated, this configuration allows to reach a harmonic number much higher than what possible with a single junction. This shows that the mixing performance benefits from the synchronous operation which also results in an improvement of dynamic range. We believe that the integration of a higher number of phase-locked junctions may further improve the mixer’s performance. Future studies should investigate the effect of higher number of junctions on the mixer performances, as well as the effect of the meander length.

Simulation and measurement of millimeter-wave radiation from Josephson junction array**Project supported by the National Natural Science Foundation of China (Grant No. 51002081), the Fundamental Research Funds for the Central Universities, China, the China Manned Space Advance Research Program, China (Grant No. 030201), and the Research Program of Application Foundation and Advanced Technology of Tianjin, China (Grant No. 15JCQNJC01300).

We report the circuit simulations and experiments of millimeter-wave radiation from a high temperature superconducting (HTS) bicrystal Josephson junction (BJJ) array. To study the effects of junction characteristic parameters on radiation properties, new radiation circuit models are proposed in this paper. The series resistively and capacitively shunted junction (RCSJ) models are packaged into a Josephson junction array (JJA) model in the simulation. The current-voltage characteristics (IVCs) curve and radiation peaks are simulated and analyzed by circuit models, which are also observed from the experiment at liquid nitrogen temperature. The experimental radiation linewidth and power are in good agreement with simulated results. The presented circuit models clearly demonstrate that the inconsistency of the JJA will cause a broad linewidth and a low detected power. The junction radiation properties are also investigated at the optimal situation by circuit simulation. The results further confirm that the consistent JJA characteristic parameters can successfully narrow the radiation linewidth and increase the power of junction radiation.

High sensitivity high Tc superconducting Josephson junction antenna for 200 GHz detection

ABSTRACTA high sensitivity rounded bow-tie antenna was optimized based on electromagnetic simulations. The bicrystal Josephson junction detector with a thin layer of YBCO was located exactly at the bicrystal misorientation point at the center of the antenna. The Au antenna was fabricated on an (100) MgO bicrystal substrate. In the presence of 200 GHz RF radiation, a noise equivalent power (NEP) of around 6×10−12 W Hz−1/2 was measured, with a voltage sensitivity, , of 30 dB with the detector at 60 K. Even though no coupling lens was used, these results are as good as those reported in systems in which the impinging radiation was focused with lenses over the detectors. Thus, important simplification of the system was obtained without loss of sensitivity.

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

Abstract—[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.

YBa2Cu3O7-x bicrystal Josephson junctions with high IcRn-products and wide-ranging resistances for THz applications

Josephson junctions with high characteristic voltages $I_{c}R_{n}$ 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 YBa2Cu3O7-x bicrystal Josephson junctions with an average $I_{c}R_{n}$ (5 K) value of 7 mV at an $R_{n}$ -range from 0.4 to 400 $\Omega$ have been fabricated on (320) bicrystal NdGaO3 substrates. Classical and frequency-selective 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.

Millimeter wave inducing subharmonic steps in the Tl2Ba2CaCu2O8 thin film bicrystal Josephson junction

The bicrystal Josephson junction (BJJ) was fabricated by patterning microbridge into Tl2Ba2CaCu2O8 (Tl-2212) thin film grown epitaxially on the bicrystal SrTiO3 (STO) substrate. The millimeter wave responses of BJJ were researched by experiment and numerical simulation. Shapiro steps and subharmonic steps were both observed in the current–voltage (I–V) curve at the liquid nitrogen temperature. In the resistive–capacitive–inductive shunted junction (RCLSJ) model, both of the Shapiro steps and subharmonic steps were reproduced with varying capacitances and inductances. The result of simulation has a good agreement with the experiment. The relative large capacitance and inductance correspond to distinct subharmonic steps.

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

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.

Dual flux-to-voltage response of YBa2Cu3O7−δ asymmetric parallel arrays of Josephson junctions

We fabricated a parallel array of 440 YBa2Cu3O7−δ bicrystal grain boundary Josephson junctions having an inductive asymmetric loop configuration within the array. Families of current–voltage characteristics (IVCs) have been measured in the temperature range (4.7–92) K for various values of a magnetic flux applied via a control current Ictrl. For both positive and negative current biases, I current-driven chains of magnetic vortices are propagating along the array producing flux-flow current resonances on the IVCs. However, at 77 K and above, due to the system’s inductive asymmetry the flux flow is suppressed (enhanced) for negative (positive) I. Consequently, the system shows a dual flux-to-voltage response. For negative I it operates like a flux-interferometer having a rather sinusoidal V (Ictrl) response. In contrast, for positive I the device’s response V (Ictrl) remains periodic but highly non-sinusoidal due to the interplay between multiple flux-flow modes. Below 60 K such a dual behaviour is far less pronounced as a result of flux-flow modes being suppressed due to a decrease of the dissipation coefficient with temperature.

Parallel array of YBa2Cu3O7−δ superconducting Josephson vortex-flow transistors with high current gains

We have developed a Josephson vortex-flow transistor based on a parallel array of 440 YBa2Cu3O7−δ bicrystal grain boundary Josephson junctions. The array's critical current Ic was measured as a function of the control current Ictrl through a control line that is inductively coupled to the array. The device has a highly asymmetric Ic(Ictrl) curve with several regions where a switching behaviour is observed characterized by a maximum current gain gmax = ∂Ic/∂Ictrl of 19 and a significant dynamic range of 20 μA at 77 K. In the range 4.7–92 K gmax versus temperature is non-monotonic with a maximum recorded at 77 K.

Enhancement of high-TC superconducting thin film devices by nanoscale polishing

The effects of mechanical nanoscale polishing on the superconducting parameters of YBa2Cu3O7−δ (YBCO) thin films and bi-crystal grain boundary Josephson junctions have been investigated. We prepared samples with additional gold nanocrystallites in the YBCO film. As they are distributed throughout the whole YBCO film, they provide a low-resistance ohmic contact even if parts of the film are removed. Polishing was performed either before or after the patterning and did not change the properties of the grain boundary. However, nanopolishing reduces the film roughness in a significant way, which makes it an indispensable tool for the preparation of integrated superconducting circuits. We also succeeded in tuning the IC and RN of the Josephson junctions of direct current superconducting quantum interference devices (dc-SQUIDs) by systematically reducing the film thickness, which opens up new possibilities in the application of magnetic field sensors.

Trimming of High-Tc Bicrystal Josephson Junctions by Ozone and Vacuum Annealing

Abstract Specific values of characteristic voltages IcRn and normal-state resistances Rn of high- Tc Josephson junctions (JJ) are often required for their THz applications in detection and spectroscopy. Recently, evolution of these electrical parameters of [001]-tilt YBa2Cu3O7-x bicrystal JJ after low-temperature ozone annealing has been studied. Here, we present the effect of ozone and vacuum annealing on the IV -curves of [100]-tilt YBa2Cu3O7-x bicrystal JJ. Annealing temperature Ta was varied from 100 °C to 225 °C. It was demonstrated, that annealing in ozone-oxygen mixture and in vacuum has an opposite effect on JJ parameters, and the changes were reversible at Ta up to 145 °C. Ozone annealing at Ta =100 °C resulted in up to 1.5 times decrease of Rn . After annealing in vacuum at higher Ta , a transition temperature Tc of JJ electrodes was reduced from 90 K to 52K, and significant, multifold changes of IcRn and Rn were observed. Further annealing in ozone at Ta up to 225 °C restored Tc and Rn almost to their initial values, but IcRn values couldn’t be increased above 20% of the initial value. The obtained results can be used for trimming of junction parameters after main fabrication procedure.

Importance of grain-boundary Josephson junctions in the electron-doped infinite-layer cuprate superconductor Sr1−xLaxCuO2

Grain boundary bicrystal Josephson junctions of the electron-doped infinite-layer superconductor Sr$_{1-x}$La$_x$CuO$_2$ ($x = 0.15$) were grown by pulsed laser deposition. BaTiO$_3$-buffered $24\,^\circ$ [001]-tilt symmetric SrTiO$_3$ bicrystals were used as substrates. We examined both Cooper pair (CP) and quasiparticle (QP) tunneling by electric transport measurements at temperatures down to 4.2\,K. CP tunneling revealed an extraordinary high critical current density for electron-doped cuprates of $j_c > 10^3\,$A/cm$^2$ at 4.2\,K. Thermally activated phase slippage was observed as dissipative mechanism close to the transition temperature. Out-of-plane magnetic fields $H$ revealed a remarkably regular Fraunhofer-like $j_c(H)$ pattern as well as Fiske and flux flow resonances, both yielding a Swihart velocity of $3.1\cdot10^6\,$m/s. Furthermore, we examined the superconducting gap by means of QP tunneling spectroscopy. The gap was found to be V-shaped with an extrapolated zero temperature energy gap $\Delta_0 \approx 2.4\,$meV. No zero bias conductance peak was observed.