Ramp-edge Josephson Junctions Research Articles

Design and Fabrication of HTS-SQUID Gradiometer With Imbalance of 10–4 Using a Gradiometric SQUID Structure

We have succeeded in fabricating a high-temperature superconducting (HTS) thin-film gradiometer with small imbalance by applying an HTS multilayer process, including crossovers, superconductive contacts, and ramp-edge Josephson junctions. The gradiometer consists of a series pickup coil and a gradiometric superconducting quantum interference device (SQUID) structure. Since the intrinsic imbalance of the series pickup coil is found to be small, a highly balanced gradiometer can be obtained by selecting the gradiometric SQUID inductor with imbalance that cancels the imbalance of the pickup coil out. The fabricated gradiometer showed the imbalance better than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">–4</sup> by applying a homogeneous magnetic field using a large circular coil in an outdoor experiment.

Observation of local magnetic distribution in ramp-edge Josephson junctions

The observation of magnetic images for ramp-edge Josephson junctions (JJ) by scanning SQUID microscopy is reported. The dependence of the trapped vortex at a JJ on the external magnetic field was investigated, and it was found to be consistent with the modulation of the maximum Josephson current, showing that the vortex was not continuously changing, but that the change was quantized. A novel vortex trapped at the junction under a relatively large field is observable, which increases in quantized units as the external field is increased. Three different types of ramp-edge junctions were also investigated, showing that the difference in the magnetic penetration around the junctions reflects the junction property. These results provide us easy and useful information and an evaluation method of the junctions and the superconductivity of the films with no lead line.

Highly Sensitive Liquid-Phase Detection of Biological Targets With Magnetic Markers and High &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;$T_{\rm c}$&lt;/tex-math&gt;&lt;/inline-formula&gt; SQUID

We developed a highly sensitive HTS SQUID system for liquid-phase detection of biological targets using magnetic markers. The SQUID consists of a ramp-edge Josephson junction made using an RE123-based multilayer process, and it showed a flux noise of 7.5 μΦ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> in the white noise region. The 1/f noise was 14 μΦ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> at f = 1 Hz when the SQUID was operated in AC bias mode. Using the SQUID system, we detected a biological target called biotin. In the experiment, biotins were fixed to a large polymer bead 3.3 μm in diameter. Streptavidin-coated magnetic markers were used for the detection. The bound and unbound (free) markers were magnetically distinguished using the difference in their Brownian relaxation time, i.e., without using a washing process to separate them. The minimum detectable number of biotins was as low as 2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> , corresponding to a sensitivity of 9.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-19</sup> mol/ml. This result indicates that the developed method is highly sensitive. We also show the effect of the excitation field on the signals from free and bound markers for quantitative evaluation of the immunoassay.

Visualization of ion transportation in an electrolyte using an HTS-SQUID gradiometer

Direct detection of magnetic fields generated by the ion currents in electrolytic cells was performed and the ion currents were mapped during a cyclic voltammetry experiment. A high-Tc superconducting quantum interference device (SQUID) gradiometer with ramp-edge Josephson junctions was used as the detector and thus the first derivative of the magnetic field along the electrolytic cell surface was obtained. The electric potential vs. the reference electrode was measured between −0.3V and 0.8V and the hysteresis signals caused by the redox reactions of the electrolyte were observed. The vector component of the ion current during cyclic voltammetry was mapped along the electrolytic cell. As a demonstration of visualization of ion transportation, the magnetic signal was also measured at various positions above the electrolytic cell surface.

Fabrication of SQUID chip for large planer gradiometer using HTS coated conductor

We have fabricated a large high-Tc superconducting (HTS) SQUID gradiometer with a baseline length of 130mm for geophysical application. It consists of an HTS-SQUID gradiometer chip with an integrated input coil and a large pickup coil made of an HTS coated conductor (CC). In this work, fabrication of the chip containing ramp-edge Josephson junctions and multilayer structures is reported. An MgO(100) substrate with a size of 15×15mm2 was used. The half area of the substrate was occupied by gold pads for connection with an HTS-CC. A directly-coupled thin-film gradiometer with a baseline length of 7mm was prepared in the remained half area. SmBCO and La-doped ErBCO having Tc’s around 90K were used for base- and counter-electrodes, respectively. A 5.5-turn input coil made of La-doped ErBCO was formed on the thin-film pickup coil made of SmBCO. The fabricated gradiometer chip was operated by supplying electrical current through the input coil, but modulation corresponding to only three flux quanta was observed in the V–Φ curve. We found that the critical current density of the thin-film pickup coil was insufficient. We speculated that the rather wide pickup coil was not fully oxygenated. After longer annealing treatment, the number of modulation peaks in the V–Φ curve was significantly increased.

Recent Developments of High-Tc Electronic Devices with Multilayer Structures and Ramp-Edge Josephson Junctions

Recent developments of electronic devices containing Josephson junctions (JJ) with high-Tc superconductors (HTS) are reported. In particular, the fabrication process and the properties of superconducting quantum interference devices (SQUIDs) with a multilayer structure and ramp-edge-type JJs are described. The JJs were fabricated by re-crystallization of an artificially deposited Cu-poor precursory layer. The formation mechanism of the junction barrier is discussed. We have fabricated various types of gradiometers and magnetometers. They have been actually utilized for several application systems, such as a non-destructive evaluation (NDE) system for deep-lying defects in a metallic plate and a reel-to-reel testing system for striated HTS-coated conductors.

Noise properties of HTS-SQUID gradiometer in an elevated magnetic field

Abstract Stability of a high-temperature superconducting quantum interference device (HTS-SQUID) gradiometer with ramp-edge Josephson junctions against application of magnetic fields was investigated. The gradiometer with a SQUID inductor having a 5-μm-wide line and pickup coils of 0.5x1.5 mm for one side was used. Noise spectra were measured for the gradiometer in zero field after cooled in dc magnetic fields up to 300 μT. no increase in noise level was observed until 95 T, being higher than the earth field. Noise spectra in elevated magnetic fields up to 320 μT were also measured for the gradiometer cooled in zero field. No appreciable increase in noise was observed until 150 μT. By decreasing the applied field from 320 μT to zero, the gradiometer recovered the original low noise level. These results indicated that the examined gradiometer had a good durability against magnetic fields and could be used without a magnetic shield for various applications.

High-Temperature Superconducting Gradiometer Coupled With Large Pickup Coil Made of GdBCO Coated Conductor

We have fabricated a large high-temperature superconducting gradiometer for geophysical application such as a Transient Electro-Magnetic (TEM) method. The seamless gradiometric pickup coil was made of a commercially available GdBCO coated conductor formed on a Hastelloy tape. Two pickup loops with a diameter of 100 mm and an integrated input coil on a SQUID chip were connected with low resistance contacts. The input coil was magnetically coupled with a directly-coupled gradiometer which was made by a RE123-based multi-layer process including two superconducting layers with crossover, contact structure and ramp edge Josephson junctions. The baseline length defined as the distance of two pickup loops is about 130 mm. The performances of the gradiometer such as flux noise, gradiometer balance and frequency dependence of sensitivity were investigated.

Fabrication of Low-Noise HTS-SQUID Gradiometers and Magnetometers With Ramp-Edge Josephson Junctions

We have studied the fabrication process of HTS-SQUIDs with ramp-edge Josephson junctions (JJs). SmBa2Cu3Oy (SmBCO) and Er0.95La0.1Ba1.95Cu3Oy (L1ErBCO) having Tc's around 90 K were used for base- and counter-electrodes, respectively. SrSnO3 (SSO) was used for insulating layers. A black-colored Pr1.4Ba1.6Cu2.6Ga0.4Oy (P4G4) layer was first grown on MgO substrate via a thin BaZrO3 buffer layer. It was expected to work as a temperature homogenizer over the whole substrate area during deposition of the upper layers. We fabricated planar gradiometers having pickup coils with sizes of 0.5 × 1.0, 1.0 × 1.0 and 3.0 × 3.0 mm2. Their baselines are 1.0, 1.0 and 3.0 mm, respectively. The gradiometers exhibited flux noise at 77 K as low as 3.3 ~ 4.5 μΦ0/Hz1/2 at 1 kHz. Directly and inductively coupled magnetometers having a 13.5 mm square pickup coil were also fabricated. The latter has an integrated 20-turn input coil. They showed field noise levels of 55 and 10 fT/Hz1/2 at 1 kHz, respectively.

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.

Ramp-edge Josephson junctions made of Pr2−xCexCuO4±δ electrodes and barriers

We present the fabrication steps and the properties of ramp-edge Josephson junctions using electron-doped high Tc cuprate Pr2-xCexCuO4 (PCCO) for the electrodes and the barrier. The superconducting properties of these PCCO junctions are similar to those fabricated from the hole-doped high temperature superconductors. For superconducting electrodes with x=0.13, 0.15 or 0.17 and a nonsuperconducting barrier with x=0.05 and thicknesses as large as 35 nm, we observe large critical current densities, periodic oscillations of critical current Ic in a magnetic field and Shapiro steps. The temperature dependence of the maximum critical current reveals the junction are of superconducting-normal-superconducting type.

Robot-arm-based NDE of hydrogen fuel tank using a 3D-movable HTS-SQUID gradiometer

In this study, we constructed a robot-arm-based SQUID-NDE system utilizing a novel HTS-SQUID gradiometer with ramp-edge Josephson junctions for hydrogen fuel tank inspection, and developed an automatic 3D-scanning program. A cylindrical hydrogen fuel tank with a double-layer structure, in which a 3mm-thick Al liner was reinforced by a 3mm-thick carbon-fibre reinforced plastic (CFRP) cover, was prepared. The tank had a 10-mm-long through crack in the Al liner made by pressure cycle test. To inspect the tank using the SQUID-NDE system, we adopted a low-frequency eddy current technique that enables to excite deep part in the Al liner. By applying an excitation field of 7.5 μT at 0.4 to 10 kHz to the tank from a double-D coil, the tank was scanned by the system while moving the HTS-SQUID gradiometer along curved surface of the tank in magnetically unshielded environment. Magnetic responses from the deep-lying crack in the tank were successfully detected by the system.

Improved reproducible fabrication process of HTS-SQUIDs with ramp-edge Josephson junctions and multilayer structures

We have studied the fabrication process for high-Tc superconducting quantum interference devices (HTS-SQUIDs) containing ramp-edge-type Josephson junctions and multilayer superconducting wiring. 5-Channel gradiometer arrays for non-destructive evaluation of striated coated conductors were fabricated. Our previous process contained some problems which often brought about property spreads of the gradiometers and/or degradation in superconducting electrodes. We attempted to improve homogeneity of substrate temperature during barrier preparation. The thickness of a black-colored Pr1.4Ba1.6Cu2.6Ga0.4Oy embedded at the lowest in the multilayer was increased. The substrate on which bare ramp-surfaces were arranged was heated from the rear side by thermal radiation. A sapphire plate as a temperature homogenizer was inserted between the substrate and the heater. At the final procedure of the process, careful oxygenation containing surface cleaning by dry etching prior to heat-treatment was carried out. By employing them, five HTS-SQUIDs exhibiting the peak-to-peak modulation voltage over 50μV were successfully fabricated within 6×6-mm2 area in a chip. Their noise levels at 1kHz and 77K were as low as 4μΦ0/Hz1/2.

Development of Integrated HTS SQUIDs With a Multilayer Structure and Ramp-Edge Josephson Junctions

We have fabricated high-temperature superconductor (HTS) dc superconducting quantum interference devices (SQUIDs) with a multilayer structure and ramp-edge Josephson junctions. For stable operation at 77 K, SmBa2Cu3Oy (SmBCO) and La0.1-Er0.95Ba1.95Cu3Oy (La-ErBCO) films with a T c above 90 K were employed as materials for the base-electrode and counter-electrode layers, respectively. Junctions were fabricated by employing a Cu-poor La-ErBCO layer as a precursor of the barrier. The fabricated parallel gradiometers with a baseline of 1-3 mm exhibited the field-induced modulation voltage of 20-50 muV at 77 K, and a clear modulation up to 86.8 K. Noise measurements at 77 K revealed a white flux noise of 4.5-10 muPhi0/Hz1/2 at 1 kHz. The magnetometer with an integrated 20-turn flux transformer and the effective area of about 1.8 mm2 exhibited a field noise of 25 fT/Hz1/2 or less at 1 kHz.

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.

Tuning the current-voltage characteristics of Josephson junctions by strong microwave fields

We observe some unusual resonance-like structures on the current-voltage characteristics (CVC) of thin film ramp-edge Josephson junctions between YBCO and Nb when applying strong microwave fields (MW). The unusual resonances have their voltage position on the CVC dependent on the MW power rather than the MW frequency, while their amplitude is largely unaffected by an increase in the MW power. Such behaviour indicates an intriguing Josephson dynamics associated with the switching from a parametric excitation regime induced by the magnetic field of the MW via oscillations of the Josephson critical current to an ac-current excitation regime triggered by the electric field of the MW. We propose a model, which describes the experimentally observed features on the CVC in terms of MW-induced multiple switching between running and locked solutions of sine-Gordon equation. Such tuning of the CVC via MW could be used to optimize the output of THz emitters.

Controlling Josephson dynamics by strong microwave fields

We observe several sharp changes in the slope of the current-voltage characteristics (CVCs) of thin-film ramp-edge Josephson junctions between YBa2Cu3O7−delta and Nb when applying strong microwave fields. Such behavior indicates an intriguing Josephson dynamics associated with the switching from a parametric excitation regime induced by the magnetic field of the microwave via oscillations of the Josephson critical current to an ac-current-excitation regime triggered by the electric field of the microwave. We propose a model, which describes the observed features on the CVC in terms of microwave-induced multiple switching between running and locked solutions of sine-Gordon equation.

Preparation of multilayer films for integrated high- Tc SQUIDs with ramp-edge Josephson junctions

We proposed a novel multilayer structure having a new combination of oxides for integrated high- T c SQUID with ramp-edge Josephson junctions. La 0.1Er 0.95Ba 1.95Cu 3O y (L1ErBCO) and SmBa 2Cu 3O y (SmBCO) were used as counter- and base-electrodes, respectively. An SrSnO 3 (SSO) layer was deposited as an insulating layer. Prior to deposition of SmBCO, Pr 1.4Ba 1.6Cu 2.6Ga 0.4O y (P4G4) and SSO were deposited on MgO (100) substrate in order to improve film quality of L1ErBCO/SSO/SmBCO layers. The black-colored P4G4 layer was expected to work as a temperature homogenizer over a whole substrate area during deposition of the upper layers. All the layers except L1ErBCO were deposited by an off-axis magnetron sputtering. An L1ErBCO layer was deposited by a pulsed laser deposition method. A thin Cu-poor L1ErBCO layer was initially deposited to form an adequate barrier on the ramp-edge of SmBCO. Gradiometer arrays having multilayered feedback coils and pickup loops were made in a chip and their proper operation at 77 K was confirmed. The present multilayer structure is promising for application to SQUIDs with more complicated designs.

Advances in High-Tc Single Flux Quantum Device Technologies

We have developed the fabrication process, the circuit design technology, and the cryopackaging technology for high-T c single flux quantum (SFQ) devices with the aim of application to an analog-to-digital (A/D) converter circuit for future wireless communication and a sampler system for high-speed measurements. Reproducibility of fabricating rampedge Josephson junctions with I c R n products above 1 mV at 40 K and small I c spreads on a superconducting groundplane was much improved by employing smooth multilayer structures and optimizing the junction fabrication process. The separated base-electrode layout (SBL) method that suppresses the J c spread for interface-modified junctions in circuits was developed. This method enabled low-frequency logic operations of various elementary SFQ circuits with relatively wide bias current margins and operation of a toggle-flip-flop (T-FF) above 200 GHz at 40 K. Operation of a 1:2 demultiplexer, one of main elements of a hybrid-type Σ-△ A/D converter circuit, was also demonstrated. We developed a sampler system in which a sampler circuit with a potential bandwidth over 100 GHz was cooled by a compact stirling cooler, and waveform observation experiments confirmed the actual system bandwidth well over 50 GHz.

Preparation of non-superconducting Pr1.4Ba1.6Cu2.6Ga0.4Oy film for HTS devices

Deposition of Pr1.4Ba1.6Cu2.6Ga0.4Oy (P4G4) thin films as non-superconducting layers in HTS electronic devices having multilayer structures with YBCO-type superconducting layers was studied. Since P4G4 has similarity in crystal structure with YBCO, it may be suitable for constructing multilayer structures containing YBCO. It is also expected that the black-colored P4G4 similar to YBCO can work as a temperature homogenizer for a chip during deposition of the upper layers. c-axis-oriented P4G4 films with flat surfaces were successfully deposited by an off-axis RF magnetron sputtering method. It was found that the insertion of a BaZrO3 buffer layer was effective to fabricate high-quality P4G4 films. As an example of the application of the P4G4, we tried fabricating ramp-edge Josephson junctions with an interface-engineered barrier on a P4G4 layer. Junctions exhibiting resistively shunted-junction-type current–voltage characteristics were successfully obtained.