High Temperature Superconductor Josephson Junctions Research Articles

Terahertz Radiations and Switching Phenomena of Intrinsic Josephson Junctions in High-Temperature Superconductors: Josephson Phase Dynamics in Long- and Short-Ranged Interactions

Terahertz Radiations and Switching Phenomena of Intrinsic Josephson Junctions in High-Temperature Superconductors: Josephson Phase Dynamics in Long- and Short-Ranged Interactions

600-GHz high-temperature superconducting sub-harmonic mixer coupled using a double-Y-type slot integrated lens antenna

In this paper, we present a quasi-optically coupled 600-GHz high-temperature superconducting (HTS) sub-harmonic mixer for communication and sensing applications. The mixer features an innovative double-Y-type slot integrated lens antenna, which can efficiently couple the radio frequency (RF) and local oscillator (LO) signals with a small frequency ratio by exciting the half-wave and full-wave resonant current modes on the slot, respectively. Considering the low impedance characteristics of HTS Josephson junctions, a coplanar-waveguide stepped impedance transformer is utilized for minimizing the mismatching loss. A cascaded filter network is designed to prevent the high-frequency signal leakage at both bands while coupling the intermediate-frequency (IF) signal output efficiently. Based on this antenna design and an established HTS step-edge junction technology, a 600-GHz mixer prototype was designed, fabricated and measured, which was compared with the simulation results. The achieved conversion gain and noise temperature are the best performance specs as reported to date for HTS harmonic mixers at comparable frequencies and operating temperatures.

Fabrication of HTS Low-Noise Nanobridge Josephson Junction by Gallium FIB

High-temperature superconductor (HTS) Josephson junctions (JJs) are primarily based on artificial grain boundaries. Bi-crystal JJs are typically utilized in HTS superconducting quantum interference devices (SQUIDs). However, the performances of bi-crystal JJs depend significantly on the quality of the interface at the junction. In this study, we investigate a method for forming nanobridge JJs via gallium focused ion beam (FIB) irradiation. A 100-nm-thick 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-</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">δ</sub> was deposited on an MgO (100) substrate via pulsed laser deposition. An additional 20-nm-thick Au layer, which serves as a protection layer preventing ion damage during alignment, was deposited in situ. HTS microchannels with a width of 4 μm were defined via photolithography and argon ion milling. The HTS microchannel widths are decreased via FIB line scans to create nanobridges. The JJs are fabricated using two types of HTS thin films, in which one has a higher critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ) that the other. The fabrication process using the gallium FIB was optimized and nanobridges with a junction width of 80 nm were prepared. As a result, a better with higher J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> (5 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) HTS films allowed for making junctions exhibiting up to five Shapiro steps at 77 K.

Modulated Time Delays, Synchronized Josephson Junctions in High-Temperature Superconductors and Chaotic Terahertz Waves

We make the first report on chaos synchronization between Josephson junctions governed by a central junction, when the coupling time between the driving junction and driven junctions is modulated. The results are compared with the case of constant coupling time delays between the junctions. We establish that synchronization is quite robust to the modulation of the coupling time delays, including chaotically modulated delay times. Synchronized Josephson junctions can serve as a terahertz wave generators with enough power for practical applications in diverse scientific fields.

Dual Frequency Mixing Products and Terahertz Hilbert-Transform Spectroscopy With Josephson Cantilevers in a THz Microscope

Josephson cantilevers, based on high-temperature superconducting yttrium barium copper oxide Josephson junctions, can be employed for the measurement of frequency and power of microwave and terahertz radiation. Each Josephson cantilever carries at least one Josephson junction with a dedicated antenna structure for better coupling. In the THz microscope, such cantilevers are employed for spatially resolved measurements of radiation distributions over either active emitting or passive samples that are irradiated by an external terahertz source. The power distribution can be reconstructed from Shapiro steps in the current-voltage characteristic of the cantilever's Josephson junction by methods including Hilbert spectroscopy. We show that simulations with simultaneous excitation with more than a single frequency result in the appearance of Shapiro steps at sum and difference frequencies. The amplitudes of these steps approximately follow Bessel functions, as with the Shapiro steps from single frequency excitation. This is confirmed by a setup in the THz microscope, where the Josephson cantilever is simultaneously subjected to 40 GHz radiation of variable power and 693 or 762 GHz radiation from a far-infrared laser system. Josephson mixing spectra show peaks at sum and difference frequencies following Bessel curves with increasing power, as it was predicted for the according Shapiro steps. These products should be avoided for a reconstruction of the original signals by Hilbert spectroscopy.

A Study of the HTS Josephson Junction Formed by a Ga Focused Ion Beam

High Temperature Superconductor Josephson Junctions (HTS-JJs) are based on artificial grain boundaries. However, the layout of HTS Superconducting QUantum Interference Devices (SQUIDs) on a bi-crystal substrate was restricted. Therefore, we considered to form low noise nano-bridge JJs by Ga Focused Ion Beam (Ga-FIB) irradiation. An 100 nm thick and 4 μm wide YBa2Cu3O7-δ (YBCO) micro-channels protected by a 20 nm thick Au protection layer were irradiated by line scans of Ga-FIB (Acceleration voltage: 40 kV, Beam diameter: ∼30 nm) and they were became normal conductivity except for the restricted region. In those nano-bridges, W N = 500 nm and fluence Φ = 2.0×1015 ions/cm2 of the nano-bridge was I C = 170 μA and we confirmed Shapiro steps by microwave irradiation of f RF = 2 GHz.

IV-characteristics and power of emission from stacks of long Josephson junctions with Gaussian spread of critical currents

The creation of sources of coherent emission in the terahertz region of frequencies from stacks of intrinsic Josephson junctions in high-temperature superconductors becomes the recently important problem of applied nanoscience. We investigated numerically IV-characteristics of stacks of three and four long Josephson junctions and power of emission from edges of the stack at different small Gaussian standard deviations of critical currents. We found the split zero-field steps in IV-characteristics. Positions of zero-field steps in the IV-curve can be described by a simple analytical expression in the theory of electrical properties of stacks of Josephson junctions placed in the external magnetic field. Maxima of coherent emission appear at voltages corresponding to the frequencies of the resonance of the in-phase collective modes of the whole stack with modes of geometrical resonances. We found maximal tolerant standard deviations of critical currents at which this resonance exists (from 1 to 10% of averaged critical currents of junctions).

340 GHz Double-Sideband Mixer Based on Antenna-Coupled High-Temperature Superconducting Josephson Junction

Wireless communication and sensing are moving from microwave, millimeter-wave into the terahertz (THz) frequency regime to meet the fast growing demand of ultrahigh data-rate communications and super resolution imaging. Faced with severe atmospheric absorption attenuation and the lack of power efficient transmitting source at the higher band, ultrasensitive and cost-effective receiver frontend technology is required for advanced THz wireless systems. To date, the most sensitive heterodyne mixers, the key components of frontend receiver systems, are based on low-temperature superconducting materials that operate at liquid helium (4.2 K) temperature range, requiring expensive and bulky cryogenic cooling systems thus hindering them from commercial applications such as wireless communications and sensing. In this article, we present a 340 GHz double-sideband fundamental mixer based on thin-film antenna-coupled high-temperature superconducting (HTS) Josephson junction that operates at a much higher temperature range attainable with smaller and cheaper cryocoolers. Based on our innovative work in terms of advanced device circuit and on-chip antenna designs, accurate parametric simulation analyses, and Josephson junction parameter optimizations, the reported mixer exhibits a measured noise temperature of 470 and 780 K at operating temperatures of 20 and 40 K respectively at 340 GHz, a performance significantly higher than any HTS THz mixers reported to date.

Quantum interference effects in 1D parallel high-Tc SQUID arrays with finite inductance

The quantum interference effects of one-dimensional (1D) parallel arrays of high-temperature superconducting (HTS) SQUIDs were investigated experimentally and theoretically via the voltage–magnetic field responses for 4–81 Josephson junctions. The sensitivity of the arrays generally decreased as the number of junctions (and SQUIDs) in parallel increased, contrary to the predictions of models in the low (zero) inductance limit. A full theoretical description was developed to describe 1D parallel HTS SQUID arrays with finite inductances in an applied magnetic field, by extending the model for a single DC SQUID to multiple loops in parallel and including the flux generated by currents circulating through all loops in the array. Calculations were extended from SQUID arrays with equal loop areas to arrays with a distribution of loop areas, otherwise known as superconducting quantum interference filters. The model uses parameters relevant to HTS arrays, including typical variations (up to 30%) in HTS Josephson junction parameters, such as critical current and normal resistance. The effect of the location of the current biasing leads was also explored through the calculations. This model shows good agreement with experimentally measured 1D arrays of different lengths and highlights the importance of the geometry of the current biasing leads to the arrays when optimizing the array response.

Superior impedance matching of THz antennas with high temperature superconducting Josephson junctions

A bow-tie antenna with a step-edge Josephson junction as the detection element, located at the center of the antenna, was investigated. The performance of these detection systems is highly sensitive to impedance mismatch between the antenna and the Josephson junction. Impedance matching optimization was performed using advance microwave simulations. Based on simulations results a bow-tie antenna system was implemented and analyzed. Following our experimental results, in the presence of a 200 GHz RF signal, and further simulations, we developed an improved structure where the junction is placed between the ends of two matching strips rather than in center of the antenna. This configuration was analyzed for two antenna structures: bow-tie and planar wire dipole. We found that the dimensions of the matching strips affect both detection magnitude and frequency of return loss dip, and that reflection is extremely sensitive to the junction length. The reflectivity obtained for an antenna without a matching unit was around −14 dB while for an antenna with a matching unit return loss up to −35 dB was derived. Simulation results also showed a high directivity of the antenna, with high radiation gain and low return loss. By varying antenna and junction dimensions, we derived an optimal design in the range of 200–280 GHz.

Voltage Standard Based on Dry-Cooled High-Temperature Superconductor Josephson Junctions

A commercially available compact Josephson voltage standard N4-21 based on high-temperature superconductor Josephson junctions with dry nitrogen temperature cooling is presented. The cryocooler unit of the voltage standard including the cryogenic system and the high-temperature superconductivity Josephson array chip is described. The measured relative standard uncertainties between output voltages of N4-21 and the primary state voltage standard of Russia are only a few parts in 108. Such low uncertainties are sufficient for the application of voltage standard N4-21 in the highest-level metrology. The implementation of the link between Josephson junctions and Zener diodes largely eliminates the uncertainty contributions associated with Zeners nonlinear drift, environmental effects, and the impact due to shipping.

The N4-21 Voltage Standard Based on Josephson Junctions of High-Temperature Superconductors

The principles for building the multivalued N4-21 voltage standard based on the precision properties of Zener diodes and the quantum mechanical properties of high-temperature superconductor Josephson junctions operating at 77 K are discussed. The metrological characteristics of the new voltage standard are presented. The relative uncertainty in the output voltages of the standard at 1 and 10 V is found to be less than 5·10–8.

HTS step-edge Josephson junction terahertz harmonic mixer

A high-temperature superconducting (HTS) terahertz (THz) frequency down-converter or mixer based on a thin-film ring-slot antenna coupled YBa2Cu3O7–x (YBCO)/MgO step-edge Josephson junction is reported. The frequency down-conversion was achieved using higher order harmonics of an applied lower frequency (19–40 GHz) local oscillator signal in the Josephson junction mixing with a THz signal of over 600 GHz, producing a 1–3 GHz intermediate frequency signal. Up to 31st order of harmonic mixing was obtained and the mixer operated stably at temperatures up to 77 K. The design details of the antenna, HTS Josephson junction mixer, the matching and isolation circuits, and the DC and RF performance evaluation are described in this paper.

High-temperature superconductor Josephson junctions for voltage standards

This paper presents the results of investigations of the high-frequency properties of multi-junction Josephson circuits based on high-temperature superconductors and the specific features of their use in voltage standards.

Detection of Coherent Terahertz Radiation from a High-Temperature Superconductor Josephson Junction by a Semiconductor Quantum-Dot Detector

We examine the application of Josephson radiation emitters to spectral calibration of single-photon-resolving detectors. Josephson junctions are patterned in a YBCO film on a bicrystal sapphire substrate and are voltage controlled to generate radiation in the frequency range of 0.05-1 THz. The detector used in this work consists of a gate-defined quantum-dot photon-to-charge transducer coupled to a single-electron transistor. Both the emitter and the detector are equipped with a matching on-chip wide-band antenna. The combination of a tuneable emitter and detector allows us to determine the efficacy of the YBCO emitter and also to analyze the elementary processes involved in the detection.

A cryogen-free HTS Josephson junction detector for terahertz imaging

A cryogen-free terahertz (THz) imaging system based on a high-temperature superconducting (HTS) Josephson junction detector is reported. The detector was made of a YBa2Cu3O7−x step-edge Josephson junction and integrated into an on-chip thin-film antenna. The HTS Josephson detector was cooled via a commercial mechanical cryocooler; an important step towards cryogen-free THz instrumentation, which is critical for industrial acceptance. In addition, it is shown that operating the detector in a cryocooler provides improved flexibility for optimizing the detector parameters and performance due to the ability to adjust the temperature compared to liquid nitrogen cooling methods. The dc and ac characteristics, the detector responsivity and the noise-equivalent power of the detector, and resulting image quality were studied as the function of operating temperatures.

HTS ion damage Josephson junction technology for SQUID arrays

The high temperature superconducting (HTS) Josephson Junction (JJ) ion damage technology we are developing is well suited for making large SQUID arrays. We have studied arrays of similar SQUIDs together with large SQIFs (Superconducting Quantum Interference Filter) with 2000 SQUIDs of different loop areas. Magnetic field sensitivity has been measured in both types of devices as a function of bias current and temperature. The effects of the barrier thickness (from 20 to 80 nm) and JJ length (2 or 5 μm) on characteristics have been investigated.

Critical current variation in YBa2Cu3O7−x step-edge junction arrays on MgO substrates

The dc transport properties of high-temperature superconducting Josephson junction arrays were investigated. Each array had the same design of 50 junctions in series, with each junction formed from a Y Ba2Cu3O7−x (YBCO) grain boundary grown on an engineered MgO step-edge substrate. The variation of critical current was analysed and modelled. Comparisons were made on the average and one sigma standard deviation, σ, of Ic values for arrays fabricated both on the same chip and a different chip. For arrays containing 2 μm wide junctions, the typical values of σ were 20–35%. Increasing the junction width reduces σ to 8–15% in arrays with 4 μm wide junctions, qualitatively consistent with the prediction using a model of multiple channels across the width of the junction.

Towards large scale HTS Josephson detector arrays for THz imaging

We present the design and implementation of a high-temperature superconducting (HTS) Josephson junction detector array for terahertz (THz) imaging. The array device is made of ten YBa2Cu3Ox−7 (YBCO) step-edge junctions coupled to gold thin-film ring-slot antennas on a MgO substrate. The design and characterization of the detector array in response to a 0.6 THz signal are presented. The development of multi-channel biasing and read-out electronics and the system integration with a commercial cryocooler are also described.

Extension of the frequency range of Josephson impedance spectroscopy

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.