High Temperature Superconductor Circuits Research Articles

Very large group delay in VHF band using coupled high temperature superconducting resonators

Storing a very high frequency (VHF) band (30–300 MHz) electromagnetic wave has many potential applications, such as phase modulation, buffering, and radio frequency memory. It can be effectively achieved by applying coupled resonator-based electromagnetically induced transparency (EIT) due to its slow light effect. However, the wavelength in the VHF band is too long to design resonators, and the group delay is limited by the high resistive loss of metal. The practical application of EIT in the VHF band is still a big challenge. In this work, we propose and experimentally demonstrate EIT response in a high-temperature superconducting (HTS) microwave circuit with coupled-resonator-induced transparency. The chip size of the HTS circuit is only 34 mm × 20 mm with a very low transparency frequency of 198.55 MHz. In addition, we implement very large group delay higher than 12.3 μs and 16.2 μs with working temperatures of 65 K and 50 K separately, which is much longer than the previous reported works on slow wave. The fabricated circuit is planar with working temperature about 65 K, and thus can be easily integrated into other microwave devices under the cryogenic conditions provided by a commercial portable Stirling cryocooler. Our proposed method paves a way for studying EIT in the microwave region due to the high quality factor of the HTS resonator, which has great potential use for radio-frequency memory in the future.

A 36 GHz HTS MMIC Josephson Mixer—Simulation and Measurement

Modeling, simulation, and measurement of a compact 36 GHz high-temperature superconducting (HTS) monolithic Josephson junction mixer are presented in this paper. A full HTS microwave monolithic integrated circuit (MMIC) simulation was carried out for the circuit combining HTS passive devices and the Josephson junction. Optimal impedance matching and bias conditions were investigated, and the circuit layout was designed accordingly. The HTS circuit has a compact dimension of 5 × 4 × 0.3 mm3, including filters, resonators, and impedance matching circuits. The HTS MMIC mixer was fabricated and packaged with an LNA to realize a receiver front end with a total dimension of 28 × 25 × 15 mm3. Measurement result showed an overall conversion gain around 35 dB, with local oscillator driving power around −45 dBm at operating temperature of 40 K.

Through-Wall Excitation of a Magnet Coil by an External-Rotor HTS Flux Pump

High-temperature superconducting (HTS) magnet systems conventionally require normal-conducting current leads, which connect between the HTS circuit and an external power supply located at room temperature. These current leads form a thermal bridge across the cryostat wall, and they represent the dominant heat load for many magnet applications. The use of a superconducting flux pump device is an alternative approach to exciting a magnet coil, which can eradicate this parasitic heat load, as such devices do not require direct physical connection to the HTS circuit. However, earlier proposed flux pump designs have required power-dissipating active components to be located within the cryogenic envelope, thus imposing their own parasitic heat load. Here, we report the successful demonstration of a mechanically rotating HTS flux pump, which operates entirely outside of the cryogenic envelope. This prototype device projects flux across a cryostat wall, leading to the injection of a direct current into a thermally isolated closed HTS circuit. This is achieved through the implementation of a flux-concentrating magnetic circuit employing ferromagnetic yoke pieces, which enables flux penetration of the HTS circuit at large flux gaps. We have demonstrated the injection of direct currents of > 30 A into a closed HTS circuit while operating this device across a cryostat wall.

A compact high temperature superconducting bandpass filter for integration with a Josephson mixer

A compact eight-pole high temperature superconducting (HTS) filter is designed for the RF signal input of an HTS Josephson mixer. The filter has 18.2% fractional bandwidth centered at 11 GHz. A stepped-impedance hairpin resonator is employed to minimize the filter size. The obtained filter size is 10 mm × 2.79 mm, which is very compact. The monolithic HTS Josephson frequency down-converter that incorporated this new filter demonstrated the highest conversion efficiency to date for an HTS Josephson mixer. Both the filter and the on-chip HTS circuit module were fabricated on a 0.5 mm thick MgO wafer with a single-sided YBCO film. The design, simulation, and experimental characterization of the HTS filter are presented in this paper. The early measurement results of the integrated HTS down-converter incorporated this filter are also described.

Operation of Toggle Flip-Flop Circuits Up to 500 GHz Based on Vertically-Stacked High-Temperature Superconductor Josephson Junctions

We report a developed single-flux-quantum (SFQ) circuit fabrication process for vertically-stacked interface-treated Josephson junctions by using high-temperature superconductor (HTS) and also report the 500 GHz operation of a toggle flip-flop (TFF). As the first trial of the circuit operation, we fabricated TFF circuits, because TFF is used as a benchmark circuit to indicate potential of the circuit operating speed. To develop the circuit fabrication process, we utilized the counter YBCO layer as a wiring layer. We obtained resistively-shunted-junction behavior of single junction fabricated by new process. In addition, we observed current-voltage characteristics at the input port and the output port of the TFF, and confirmed that the input voltage coincided with twice the output voltage up to 1.1 mV at 4.2 K, which corresponds to the correct operation up to 500 GHz. This frequency is the highest value in HTS circuits ever observed, which suggests the high potential of the vertically-stacked JJs for the circuit operation.

Fabrication and operation of HTS SFQ logic circuits

High-temperature superconducting (HTS) single flux quantum logic circuits such as an inverter and an OR gate were designed. Circuit parameters were optimized for the HTS circuits to be successfully operated. The test circuits were fabricated by using the ramp-edge junction technology, and their logic operations were examined. The inverter was successfully operated at temperatures from 4.2K to 33K, and finite operating margins were observed in this temperature range. We also confirmed correct operation of the OR gate with dc supply current margins at temperatures from 40K to 50K.

Suppression of oxygen loss from junctions in fabrication process of HTS circuits

We improved the fabrication technology of high-temperature superconducting (HTS) single-flux-quantum (SFQ) circuits by suppression of oxygen loss. Among a lot of difficulties in the HTS circuit fabrication process, loss of oxygen is one of the most significant problems. To understand the oxygen going in and out at each step of the fabrication process, we utilized a room-temperature probing system and a low-temperature probing system. We found that some of the steps after the fabrication of junctions, such as deposition of an insulating layer, ashing of photo-resist, and RF sputtering of an Au wiring layer, cause loss of oxygen from the junctions. We constructed the peripheral fabrication process by combining the annealing process in oxygen and the process with oxygen loss, and obtained controlled Ic, sufficiently large IcRn, and Tc of the junction.

Utilization of process TEG for fabrication of HTS circuits

We improved the fabrication process of high-temperature superconducting (HTS) sampler circuits with multilayer structures by utilizing a test elements group (TEG). Among a lot of difficulties in the HTS circuit fabrication process, loss of oxygen is one of the most significant problems. Since the film transition temperature (Tc) has a strong relationship with the resistance at room temperature, we fabricated a test pattern on the same wafer of the circuits and measured the resistance at room temperature by using a prober to estimate the Tc of each layer. By introducing the measurement of the normal resistance after each process, we found better process conditions without a Tc drop. Moreover, we constructed a low-temperature probing system, in which we can measure the junction TEG. The system enabled feedback of the fabrication condition soon after the junction process. The utilization of the process TEG contributed to reproducible fabrication of HTS circuits and that is a promising advance of the HTS circuit technology.

Observation of 45 GHz current waveforms using HTS sampler

We succeeded in observing high-frequency current waveforms up to 45GHz using a high-temperature superconducting (HTS) sampler. In this experiment, we used a sampler circuit with a superconducting pickup coil, which magnetically detects current signals flowing through a micro-strip line on a printed board placed outside the cryochamber. This type of measurement enables non-contact current-waveform observation that seems useful for analyses of EMI, defects in LSI, etc. Computer simulation reveals that one of our latest versions of HTS sampler circuits having Josephson transmission lines with optimized biases as buffers has a potential of sampling high-frequency signals with a bandwidth above 100GHz. To realize the circuit parameters required in the simulations, we developed an HTS circuit fabrication process employing a lower ground plane structure with SrSnO3 insulating layers. We consider that improvement of the circuit fabrication process and optimization of the pickup coil lead to much higher signal frequency observable by the sampler.

Smoothing of YBa2Cu3O7−δ films by ion cluster beam bombardment

Smoothing high-temperature superconductor (HTS) surfaces, especially HTS thin-film surfaces, is crucial for HTS thin-film device processing. In this letter, we describe a method to planarize the surface of a YBa2Cu3O7−δ HTS film down to a smoothness with a standard deviation of 1 nm or better. The method includes first smoothing the HTS surface by ion cluster beam bombardment, followed by annealing in oxygen ambient to regrow the damaged surface layer. Additional YBCO layers can be grown epitaxially on the treated surface, even without removing the top surface layer, which contained some residual damage after annealing. This method can be integrated into HTS circuit fabrication as a key step of planarization.

High-T/sub c/ SNS Josephson junctions: moving beyond adolescence

Attempts to develop high temperature superconductor (HTS) microelectronics have been limited by the immature nature of HTS circuit technology. Much of the present effort worldwide is based on superconductor-normal-superconductor (SNS) Josephson junctions (JJ's). Only within the past three years have SNS junctions been fabricated which can be interpreted with conventional proximity effect theory. We discuss the history and present status of HTS SNS devices and the prospects for producing a mature technology for practical applications.

A computationally efficient approach to microwave circuit modeling of complex high temperature superconductor circuits

A computationally efficient approach to modeling complex high temperature superconductor (HTS) microwave circuits is described which combines two-dimensional electromagnetic solutions of microwave transmission lines, incorporating a phenomenological description of HTS behavior, with conventional commercially available microwave-circuit computer-aided-design software. Internal inductance effects are known to have a substantial impact on the design and implementation of narrowband HTS circuits. Phase velocities and characteristic impedances can be well modeled using a phenomenological description of HTS superconductivity along with a generalized approximate boundary condition and a two dimensional electro-magnetic solution to the waveguiding structure. Resultant parameterized design equations can then be incorporated into standard commercially available microwave-circuit computer-aided-design software. The test case examined in detail was the superconducting channelized receiver delivered by the Naval Research Laboratory to Phase II of the High Temperature Superconducting Space Experiment. Excellent agreement was obtained between measurements made on the demultiplexer and this HTS microwave circuit modeling approach.

A magnetically coupled SQUID for the digital readout of integrated HTS circuits

A magnetically coupled SQUID (superconducting quantum interference device) for the digital readout of integrated HTS (high-temperature superconductor) circuits has been developed and a multilayer integration process for HTS circuits was evaluated. We employed ramp edge junctions in this process; the SQUID's control line was formed using the counter layer of the junctions. Three ex situ epitaxial layers were grown. Ion beam etchings were used for the base electrode and insulation layer without substrate rotation. Clear voltage modulation for the SQUID was observed up to 65 K. The critical current of the wide control line was 1.2 mA at 65 K.

Monolithic HTS microwave phase shifter and other devices

We describe a monolithic high-temperature superconductor (HTS) phase shifter based on the distributed Josephson inductance (DJI) design integrated monolithically into a 10-GHz microstrip line. This microwave circuit incorporates >1000 HTS rf SQUIDS. Recent data demonstrate the performance of this broadband HTS circuit. We observed phase shifts greater than 150° in resonant structures, and 20° in broadband circuits. The nonlinear inductance of the superconducting transmission line can be used for other novel applications, including parametric amplification. A comparison of the DJI circuit to a series array of Josephson elements (used for pulse sharpening) will contrast these two new and exciting nonlinear transmission line circuits.

High-T/sub c/ superconducting monolithic phase shifter

A high temperature superconducting (HTS) X-band phase shifter using a distributed Josephson inductance (DJI) approach was designed and fabricated. Phase swings of over 60 degrees were measured at 65 K and below, with measurable phase shifts at temperatures above 77 K. High quality HTS films and superconducting quantum interference devices (SQUIDs) were deposited by laser ablation. A total of 40 HTS step edge SQUIDs were successfully integrated into a monolithic HTS circuit to produce a phase shifter in a resonant configuration. The magnitude of the Josephson inductance is calculated and a lumped element model is compared to measurements.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Monolithic high-T/sub c/ superconducting phase shifter at 10 GHz

A monolithic high-temperature-superconductor (HTS) phase shifter integrated into a 10-GHz microstrip line is described. This is the first demonstration of a nonresonant HTS circuit based on a distributed Josephson inductance approach. Phase shifts greater than 150 degrees in resonant structures and 20 degrees in broadband circuits were observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>