Bicrystal Josephson Junctions Research Articles

Characteristics of Off-Chip Millimeter-Wave Radiation from Serial Josephson Junction Arrays

We investigate the self-emissions from serial high-temperature superconductor bicrystal Josephson junction arrays embedded in a quasi-optical resonator. A bicrystal substrate is used as a dielectric resonator antenna, which increases the coupling strength between the junction array and the electromagnetic (EM) wave. Both three-dimension (3D) electromagnetic simulations and experiments are performed. Strong off-chip radiations are measured from the junction array at 78 GHz and 78 K. The proposed method and the experimental results are important for millimeter wave applications in junction arrays.

Evolution of Electrical and Electrodynamic Properties of ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-{\rm x}}$ Bicrystal Josephson Junctions With Oxygen Loading

The I-V curves and dynamic resistances Rd (V) of [001]-tilt YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-x</sub> 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 xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">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 YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-x</sub> antennas, integrated with the junctions. Fine structures on the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IV</i> -curves, related to antenna resonances, were modified with oxygen content due to better antenna-junction coupling and lower electromagnetic losses in YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-x</sub> film.

Characteristics of an HTS-SQUID gradiometer with ramp-edge Josephson junctions and its application on robot-based 3D-mobile compact SQUID NDE system

In this paper, we investigated detailed behavior of novel HTS-dc-SQUID gradiometers with ramp-edge Josephson junctions (JJs) in both an ac magnetic field and a dc magnetic field. In the both fields, the novel gradiometers shows the superior performance to the conventional YBa2Cu3O7−x (YBCO) HTS-dc-SQUID gradiometer and a bare HTS-dc-SQUID ring with bicrystal JJs concerning durability against entry and hopping of flux vortices, probably due to their differential pickup coils without a grain boundary and multilayer structure of the ramp-edge JJs. A robot-based compact HTS-SQUID NDE system utilizing the novel gradiometer was reviewed, and detectability of the system to detect non-through cracks in a carbon fiber reinforced plastic (CFRP)/Al double-layer structure was demonstrated. A new excitation coil in which the supplied currents flowed in the orthogonal directions was applied to detect cracks that oriented vertical and parallel to the baseline of the gradiometer.

Josephson admittance spectroscopy application for frequency analysis of broadband THz antennas

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.

A method for self-radiation of Josephson junction arrays

We proposed a novel approach to self-radiation of Josephson junction arrays. BicrystalJosephson junction arrays fabricated on a bicrystal substrate were integrated with ahalf-wavelength dipole array and embedded in a Fabry–Perot resonator. Considered as adielectric resonator, the substrate could be stimulated to resonate in an appropriate modeat the same frequency as the Fabry–Perot resonator, which realized a coupling betweenJosephson junctions as well as a coupling between the junctions and microwave structuresby means of a resonant interaction between junctions and resonators. Both simulations andexperiments were performed. A YBCO bicrystal Josephson junction array with 166junctions was measured at liquid nitrogen temperature. The maximal self-radiation powerof 10 pW was detected at 75.2 GHz, which was in agreement with the simulation results.

Extraction of the spectral information of terahertz signals using superconducting Josephson junction

Using YBa2Cu3O7−δ/MgO bicrystal Josephson junction, we have developed a setup for spectrum analyses of terahertz signals, up to 2.5 THz, with a frequency resolution of 3 GHz. The junction works at liquid nitrogen temperature when the signal frequencies are low while for signals at 1.6 THz and 2.5 THz the junction is cooled to 6 K. Digital signal processing (DSP) chips are made use of to build a data acquisition and processing system so that it can be compact, fast, and intelligent. With the completion of the relevant hardwares and softwares, the measurements are now automated.

Millimeter-wavelength radiation from arrays of discrete high-temperature superconductorJosephson junctions

Josephson junction arrays could be competitive candidates for radiation sources in thefrequency range from 0.5 to 1.5 THz. This is the range where compact solid-statecontinuous wave sources are most lacking. We investigated the millimeter-wave radiationfrom arrays of high-temperature superconductor bicrystal Josephson junctions embedded ina quasi-optical resonator. The novel approach of utilizing the intrinsic resonance modes ofthe substrate to improve the impedance coupling between the series array of discreteJosephson junctions and the quasi-optical resonator was investigated by experiments andelectromagnetic field simulations. Radiation from the arrays containing up to 536 junctionswas detected at about 80 GHz and liquid nitrogen temperature using a low-noisereceiver. The detected maximum radiation power of the arrays was about 50 pW andthe peak voltage agreed exactly with the Josephson current–voltage relation.

Millimeter wave irradiation characteristics of high temperature superconductor bicrystal Josephson junction embedded in a Fabry-Perot resonator

The millimeter wave irradiation characteristics of a Tl-2212 high-temperature superconducting thin film bicrystal Josephson junction embedded in a hemispherical Fabry-Perot resonator was studied. The coupling characteristics of the bicrystal Josephson junction with the Fabry-Perot resonator have been investigated experimently, showing that it was greatly affected by the factors such as the location of the sample in the open resonator, the angle between them, the pattern of the junction and the distance between the two reflecting mirrors of the resonator. The optimized coupling between Josephson junction and applied microwave was achieved by refined adjustment of the factors. In this case, 9 orders distinct Shapiro steps were observed. Taking advantage of the Fabry-Perot resonator technique, the coupling problem between high-temperature superconducting thin film Josephson junction and applied millimeter wave was well solved, which is of significance for the research on radiation and detection of millimeter wave and terahertz wave of high-temperature superconducting Josephson junction.

Simulations of impedance matching and phase locking of Josephson junction arrays embedded in a Fabry-Perot resonator

We proposed a method for implementing impedance matching and phase locking of Josephson junction arrays based on Hes previous report on the coupling mechanism of a millimeter wave radiation to bicrystal Josephson junction arrays embedded in a Fabry-Perot resonator，and relevant electromagnetic simulations and numerical calculations were performed. A bicrystal Josephson junction array was fabricated on a YSZ bicrystal substrate，and embedded in a Fabry-Perot resonator. Impedance matching was implemented by integrating an optimized half-wavelength dipole array fed in series with the junction array，and numerical calculations showed that the radiation efficiency of a single junction reached 94%. Through making good use of the radiation characteristics of the antenna array and optimizing the model，the FP resonator and the substrate can be driven to resonate in proper modes respectively，which benefits the high frequency electromagnetic coupling between the junction array and the resonators. The numerical simulations proved that this method is effective for realizing self-voltage locking and mutual-phase locking of junction arrays. The method proposed in this paper and calculation results are valuable for guiding the applications of Josephson junction arrays in THz signal generators and voltage standards.

Millimeter wavelength coherent emission from high temperature superconducting bicrystal Josephson junction array

The experimental results of mm-wave coherent emission from a high temperature superconducting bicrystal Josephson junction array embedded in a Fabry-Perot resonator are reported in this paper.The mm-wave coherent emission from the series junction array results from the interaction of the junctions with the substrate serving as the dielectric resonator as well as the Fabry-Perot resonator.The central frequency of emission of a Josephson junction array containing 166 bicrystal junctions was 75.84 GHz,and the detected emission power was about 10 pW at 77 K.

Test and analysis of bicrystal Josephson junction mixer&amp;rsquo;s conversion efficiency

The research work on the conversion efficiency of high-temperature superconductivity bicrystal Josephson junction mixer is reported. Using ion etching, the junction integrated with antenna is prepared. Experiment on harmonic mixing at 3 mm waveband is carried out. The relationship between conversion efficiency and the parameters of local oscillation power, microwave signal power, harmonic order, bias electric current, etc., is tested and analyzed.

Robot-Based NDE System Using 3D-Mobile HTS-SQUID

A novel HTS-SQUID gradiometer with ramp-edge Josephson junctions (JJs) composed of SmBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> base-electrode and La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> Er <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.95</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.95</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> counter-electrode was developed. The robustness of the gradiometer in AC field perpendicular to the device plane was investigated. In an AC field of less than 2.8 mT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub> at 100 Hz, flux-trapping, jumping and increase in noise did not happen in the gradiometer, while a traditional HTS-SQUID gradiometer with YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-x</sub> thin film and bicrystal JJs with the same dimension as the novel gradiometer could not be operated properly in an AC field of about several tens nT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub> . A robot-based NDE system utilizing the novel HTS-SQUID gradiometer was developed. The system could move the gradiometer three-dimensionally (3D) in an unshielded environment without magnetic shielding or compensation technique. Eddy-current-based detection of a through hole in magnetized stainless steel board was demonstrated by the robot-based HTS-SQUID NDE system.

Higher Order HTSC Gradiometer for Measurements in Unshielded Environment

We present a new higher order gradiometer layout for bicrystal Josephson junctions based on high-temperature superconducting thin films. Our single layer gradiometers are suitable for the simultaneous measurement of magnetic field gradients with different spatial orientations. The sophisticated technology for bicrystal Josephson junctions is combined with custom substrates with two crossing 30 degree grain boundaries. We present the technological challenges focusing on the low ohmic contact resistance using gold nanoparticles embedded in the thin film. The sensor performance is characterized using SQUID electronics. Its main parameters such as critical current and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ICRN</i> -product are compared with parameters of first order gradiometers on single 30 degree bicrystal substrates. Special emphasis is put on the optimization of the noise properties in unshielded environment.

Signal performance of DC-SQUIDs with respect to YBCO thin film deposition rate

The signal performances of YBa 2Cu 3O 7− δ (YBCO) direct current superconducting quantum interference devices (DC-SQUIDs) have been investigated as a function of the thin film structure affected by the growth process. YBCO thin films of 200 nm thicknesses were deposited by DC magnetron sputtering using different deposition rates between 1.0 nm/min and 2.0 nm/min onto 24° bicrystal SrTiO 3 (STO) substrates. The thin film samples were subsequently analyzed by XRD and AFM in order to determine their crystalline structures and surface morphologies respectively. The 67 pH directly coupled DC-SQUIDs with 4 μm-wide bicrystal Josephson junctions were fabricated, and characterized with respect to their device performances. The variations in the critical current ( I c), the voltage modulation depth (Δ V) and the noise performance of DC-SQUIDs were reported. The SQUIDs having relatively low deposition rate of 1.0 nm/min was observed to have larger voltage modulation depth as well as higher critical current than that of the samples having larger rate of 2.0 nm/min. The better noise performances were observed as the film deposition rate decreases. The results were associated with the thin film structure and the SQUID characteristics.

Dynamic characteristics of Tl-2212 bicrystal Josephson junctions on SrTiO3 substrates and the effect of noise on it

Tl-2212 bicrystal Josephson junctions on SrTiO3 substrates have been fabricated. The I-V characteristic of a Josephson junction under microwave radiation were measured，and microwave induced steps were observed in accordance with the Josephson frequency-voltage relationship. The RCSJ model including noise of bicrystal Josephson junction was constructed through numerical simulation，and the results of simulation were in good agreement with the experimental data. Using the RCSJ model，we investigated the effect of noise on the dynamic characteristics of Josephson junction through simulation，and explained the phenomenon of reduced height and increased breadth of the minima of microwave induced steps. We proposed that the effective noise temperature should be the sum of operational temperature and equivalent temperature of the external noise.

Terahertz spectroscopy based on high-T c Josephson junctions

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.

Effective-barrier-thickness fluctuation and critical-current spread of grain-boundary Josephson junctions

The phenomenological formulas for the critical-current spread of the bicrystal grain-boundary Josephson junctions are proposed. By considering the intrinsically shunted-junction model and the gamma distribution, the fluctuation of effective-barrier thickness from 0.01to0.1nm corresponds to the 1−σ critical-current spread from 14% to 106%. With a fixed scale parameter, the critical-current spread is inversely proportional to the square root of the bridge width. When the bridge width is fixed, the spread is inversely proportional to the square root of the mean critical-current density. These predictions are qualitatively in agreement with the experimental results reported to date.

Features of growth of non-cubic bicrystals from the melt (example of leucosapphire and neodymium gallate)

Neodymium gallate (NdGaO3) and sapphire (Al2O3) are most suitable materials to produce bicrystal substrates for high-temperature superconducting (HTSC) films' microwaves Josephson junctions. Perfect bicrystal border is important for the devices, and it can be obtained in bicrystals grown directly from the melt. Unfortunately both materials have non-cubic symmetry and anisotropic thermophysical properties what complicates the bicrystal growth process. The experimental results of NdGaO3 bicrystals growth by Czochralski method using compound bicrystal seeds with different symmetries in the plane perpendicular to the growth axis are presented. The results proved to be very unstable and depended critically upon growth conditions and on bicrystal border orientation. The bicrystals with misorientation angle 180° which correspond to the natural (110) twins are grown. When bicrystal angles are within 22-28° range which is optimum for bicrystal Josephson junctions it is very difficult to grow the long bicrystals: the bicrystal border goes out of the crystal quickly. It is supposed that in the last case the symmetry of compound bicrystal seed in the plane perpendicular to the growth direction can differ from the heat field symmetry in the growth system. Requirements for growth of anisotropic crystals by Czochralski method are formulated. It is shown that the requirements of seed and heat field symmetry correspondence can be satisfied more easily in Stepanov's growth method (EFG). The sapphire bicrystal ribbons suitable for manufacturing of high quality substrates for HTSC film Josephson junctions using Stepanov's method are grown.

Meandering bicrystal Josephson junction arrays in a hemispherical Fabry–Perotresonator

We have explored the coupling mechanism of a millimeter wave radiationto bicrystal Josephson junction arrays embedded in a Fabry–Perot resonator.In order to optimize the system, three-dimensional simulations and relativeexperiments were performed. By analyzing the results of computer simulations andexperiments, we found that the layout of the array and the distance between thearray and the plane mirror should minimize the effects on the electrical fieldE distributionin the fundamental mode of the Fabry–Perot resonator without the sample. In this case, the uniformityof the E field in the bicrystal arrays was demonstrated together with an improvement in thecoupling efficiency of the mm wave irradiation to high-temperature superconductor arraysof Josephson junctions.

Optimization of Bicrystal Josephson Junctions and Arrays in a Fabry-Perot Resonator

We report on the investigation of the coupling mechanism of a resonant millimeter wave electromagnetic field with bicrystal Josephson junction arrays embedded in a hemispherical Fabry-Perot resonator. We have found that our high-temperature superconductor array can be modeled as a thin film grid polarizer. In agreement with this model a strong dependence of the coupling of the Josephson junctions on the polarization of the electric field in the resonator was measured. Numerical field simulations confirm this model. According to this model, the dimension and interval of the grids and the dimension of the Josephson junctions are optimized by simulations and experiments. After optimized, a maximum Josephson voltage of 28 mV for an array of 182 bicrystal junctions at a temperature of 75 K and a frequency of 74.39 GHz was achieved for the first time. Our coupling method has a strong potential to be used up to terahertz frequencies.