high-Tc Superconducting Quantum Interference Device Research Articles

Variation of the performance of YBa[formula omitted]Cu[formula omitted]O[formula omitted] radio frequency superconducting quantum interference device with bicrystal grain boundary angle

High transition temperature (high-Tc) superconducting quantum interference devices (SQUIDs) based on bicrystal grain boundary junctions have been utilized in a wide range of applications. To explore the effect of the bicrystal grain boundary angle on the property of the high-Tc SQUID, we fabricated and characterized YBa2Cu3O7−δ (YBCO) radio frequency (rf) SQUIDs using [001]-tilt grain boundary junctions on SrTiO3 (STO) bicrystal substrates with three different misorientation angles of θ=18∘, 24°, and 30°. Unlike the device of θ=18∘ showing only the triangular waveform of the voltage–flux (V−Φ) characteristics, for device of θ=24∘ and 30°, the shape of the V−Φ curve can be tuned from approximately triangular into nearly rectangular by adjusting the working point of the device. It suggests that, as θ increases from 18° to 30°, the operation of the device may change from the hysteretic mode into the nonhysteretic mode. This is backed by electrical transport measurements of YBCO bicrystal junctions on separate STO substrates, which help to reveal that lying behind the indicated changeover of the SQUID operation mode is the exponential decrease of the junction critical current density Jc as θ increases. The white noise of the device, which decreases with increasing flux-to-voltage transfer coefficient ∂V/∂Φ, shows a large drop as θ increases to 24°. For both 24° and 30° devices, the white field noise at 77 K is about 90 fT/Hz1/2 at respective optimum working points. For device of 30° with nearly rectangular V−Φ characteristics showing the largest ∂V/∂Φ among the fabricated samples, the white noise region extends below 10 Hz and a field noise of 145 fT/Hz1/2 at 1 Hz is shown, demonstrating the potential of obtaining high-performance bicrystal YBCO rf SQUIDs with this grain boundary angle.

Nanoscale direct-write fabrication of superconducting devices for application in quantum technologies

In this Perspective article, we evaluate the current state of research on the use of focused electron and ion beams to directly fabricate nanoscale superconducting devices with application in quantum technologies. First, the article introduces the main superconducting devices and their fabrication by means of standard lithography techniques such as optical lithography and electron beam lithography. Then, focused ion beam patterning of superconductors through milling or irradiation is shown, as well as the growth of superconducting devices by means of focused electron and ion beam induced deposition. We suggest that the key benefits of these resist-free direct-growth techniques for quantum technologies include the ability to make electrical nanocontacts and circuit edit, fabrication of high-resolution superconducting resonators, creation of Josephson junctions and superconducting quantum interference device (SQUIDs) for on-tip sensors, patterning of high-Tc SQUIDs and other superconducting circuits, and the exploration of fluxtronics and topological superconductivity.

Preparation of high-quality factor dielectric resonators for low-noise YBa[formula omitted]Cu[formula omitted]O[formula omitted] radio frequency superconducting quantum interference devices

Tank circuit (resonator) is essential for the operation of the radio frequency (rf) superconducting quantum interference device (SQUID) and the quality factor of the tank circuit has significant impact on the sensitivity of the rf SQUID. For high transition temperature (high-Tc) rf SQUID, dielectric resonator is presently one of the most practical tank circuits. By growing YBa2Cu3O7−δ (YBCO) superconducting thin film on (100) SrTiO3 substrate with a pulsed-laser deposition technique and then patterning the YBCO thin film into flux concentrator with a washer structure, a series of dielectric resonators with unloaded quality factor Q0 ranging from 200 to 6000 has been prepared. Surface morphology and electrical transport measurements have been carried out to investigate the dependence of Q0 on the property of the YBCO thin film, which suggests that it is effective to boost the Q0 of the resonator through improvements in both the crystalline quality and the superconductivity of the thin film. By integrating the resonators with the same SQUID chip, a set of YBCO rf SQUIDs has been assembled to explore the influence of Q0 of the resonator on the performance of the rf SQUID. It is observed that the white noise of the device decreases substantially with the increase of Q0, in accordance with the theoretical suggestion. Moreover, as Q0 rises, the low frequency noise of the device is shown to reduce as well, revealing the importance of the high-Q0 resonator in depressing the noise of the high-Tc rf SQUID over the entire frequency range. With the resonator of Q0=6000, the YBCO rf SQUID at 77 K exhibits a field sensitivity of 50 fT/Hz1/2 in the white noise region, 58 fT/Hz1/2 and 110 fT/Hz1/2 at 10 Hz and 1 Hz respectively, suggesting its potential in applications such as magnetoencephalography in biomagnetism which have critical requirements on field sensitivity of the high-Tc SQUID especially at low frequencies.

Scalp attached tangential magnetoencephalography using tunnel magneto-resistive sensors

Non-invasive human brain functional imaging with millisecond resolution can be achieved only with magnetoencephalography (MEG) and electroencephalography (EEG). MEG has better spatial resolution than EEG because signal distortion due to inhomogeneous head conductivity is negligible in MEG but serious in EEG. However, this advantage has been practically limited by the necessary setback distances between the sensors and scalp, because the Dewar vessel containing liquid helium for superconducting quantum interference devices (SQUIDs) requires a thick vacuum wall. Latest developments of high critical temperature (high-Tc) SQUIDs or optically pumped magnetometers have allowed closer placement of MEG sensors to the scalp. Here we introduce the use of tunnel magneto-resistive (TMR) sensors for scalp-attached MEG. Improvement of TMR sensitivity with magnetic flux concentrators enabled scalp-tangential MEG at 2.6 mm above the scalp, to target the largest signal component produced by the neural current below. In a healthy subject, our single-channel TMR-MEG system clearly demonstrated the N20m, the initial cortical component of the somatosensory evoked response after median nerve stimulation. Multisite measurement confirmed a spatially and temporally steep peak of N20m, immediately above the source at a latency around 20 ms, indicating a new approach to non-invasive functional brain imaging with millimeter and millisecond resolutions.

High-Tc rf SQUIDs with Large Flux Focusing

The high-Tc rf SQUID (superconducting quantum interference device) sensor, in which the rf SQUID was placed face to face with superconductor flux concentrator/dielectric resonator, was designed and fabricated. The SQUID sensor was made up of a SQUID washer (with an area of 9.6 mm2 and an inductance of 150 pH) and a flux focusing/dielectric resonator (with an area of 100 mm2 and an resonant frequency varied from 430 to 650 MHz). The author conducted a test to prove that with a white flux noise spectral density of SΦ 1/2=3×10-5Φ0/ Hz1/2 and field sensitivity of BN100 fT/Hz1/2, this flux focusing/dielectric resonator-type rf SQUID sensor can work stably where electromagnetic interference is smaller.

Investigation of Inkjet Printed Composites Based on Single-Walled Carbon Nanotubes With Superparamagnetic Nanoparticles

Porous-matrix composites comprising single-walled carbon nanotubes (SWCNTs) with bound magnetic nanoparticles (NPs) as filler are interesting multifunctional materials whose electrical and magnetic parameters can be used as hallmarks for identification of a genuine document or a bank note, for example. We have formulated stable functional ink based on commercially available product composed of SWCNTs (76 wt. %) and iron-based NPs (11 wt. %) and deposited them on four different thin sheet materials at a given dosage with the use of specialized inkjet printer. To study the magnetic properties, a high-Tc superconducting quantum interference device (HTS SQUID) was designed and fabricated. The printed samples were magnetized in the weak dc magnetic field (3.0 mT/μ0) and moved in the direction of applied field in close proximity to a SQUID sensor. It was found that peak-to-peak stray magnetic field of a magnetized sample (being proportional to its average magnetization) depends on the type of sheet material on which the ink was printed. The larger is the thickness of the printed layer comprising SWCNTs with magnetic NPs, the lower is the signal. Since iron-based NPs are superparamagnetic due to their small size (mean size ~ 4 nm), the difference in the magnetization may result from the difference in the contribution of exchange interaction between NPs. The performed calculations demonstrated that the decay length of electron wave function characteristic of the magnetization is about seven times larger than that of the hopping conductivity in the composites, thus suggesting the mechanism of indirect exchange interaction between NPs involving SWCNTs.

High-Tc superconducting detector for highly-sensitive microwave magnetometry

We have fabricated arrays of High-Tc Superconducting Quantum Interference Devices (SQUIDs) with randomly distributed loop sizes as sensitive detectors for Radio Frequency (RF) waves. These subwavelength size devices known as Superconducting Quantum Interference Filters (SQIFs) detect the magnetic component of the electromagnetic field. We used a scalable ion irradiation technique to pattern the circuits and engineer the Josephson junctions needed to make SQUIDs. Here, we report on a 300 SQUID series array with the loop area ranging from 6 to 60 μm2, folded in a meander line covering a 3.5 mm × 120 μm substrate area, made out of a 150 nm thick YBa2Cu3O7 film. Operating at a temperature of T = 66 K in an unshielded magnetic environment under low DC bias current (I = 60 μA) and a DC magnetic field (B = 3 μT), this SQIF can detect a magnetic field of a few picoteslas at a frequency of 1.125 GHz, which corresponds to a sensitivity of a few hundreds of fT/Hz and shows a linear response over 7 decades in RF power. This work is a promising approach for the realization of low dissipative subwavelength gigahertz magnetometers.

Measurement of Magnetic Nanoparticles by Small HTS SQUID Array

We propose a method to investigate the brain activity of a small animal by measuring blood flow with magnetic nanoparticles (MNPs) using a high-Tc Superconducting Quantum Interference Device (SQUID) array. For the preliminary study, we fabricated a three channel high-Tc SQUID array and demonstrated the detection of a small volume of water diluted MNPs in motion. Two detection methods were applied; one was dc field measurement and the other was dc with ac field measurement. In the latter case, a second harmonic response of the nonlinear M-H characteristics of MNPs was detected. In the dc magnetic field measurement, it was demonstrated that the signal from each SQUID magnetometer was dependent on its position and the movement of the MNP sample. The detectable minimum iron content was compared between the dc and dc with ac field measurement. As a result, it was found that the minimal detectable iron content by ac with dc measurement was 12.2 ng with an SNR of 2.3 and was 1/2 times smaller than that detected by the DC field measurement.

Volume-amplified magnetic bioassay integrated with microfluidic sample handling and high-Tc SQUID magnetic readout.

A bioassay based on a high-Tc superconducting quantum interference device (SQUID) reading out functionalized magnetic nanoparticles (fMNPs) in a prototype microfluidic platform is presented. The target molecule recognition is based on volume amplification using padlock-probe-ligation followed by rolling circle amplification (RCA). The MNPs are functionalized with single-stranded oligonucleotides, which give a specific binding of the MNPs to the large RCA coil product, resulting in a large change in the amplitude of the imaginary part of the ac magnetic susceptibility. The RCA products from amplification of synthetic Vibrio cholera target DNA were investigated using our SQUID ac susceptibility system in microfluidic channel with an equivalent sample volume of 3 μl. From extrapolation of the linear dependence of the SQUID signal versus concentration of the RCA coils, it is found that the projected limit of detection for our system is about 1.0 × 105 RCA coils (0.2 × 10−18 mol), which is equivalent to 66 fM in the 3 μl sample volume. This ultra-high magnetic sensitivity and integration with microfluidic sample handling are critical steps towards magnetic bioassays for rapid detection of DNA and RNA targets at the point of care.

Harmonics distribution of iron oxide nanoparticles solutions under diamagnetic background

The static and dynamic magnetizations of low concentrated multi-core iron oxide nanoparticles solutions were investigated by a specially developed high-Tc Superconducting Quantum Interference Device (SQUID) magnetometer. The size distribution of iron oxide cores was determined from static magnetization curves concerning different concentrations. The simulated harmonics distribution was compared to the experimental results. Effect of the diamagnetic background from carrier liquid to harmonics distribution was investigated with respect to different intensity and position of peaks in the magnetic moment distribution using a numerical simulation. It was found that the diamagnetic background from carrier liquid of iron oxide nanoparticles affected the harmonics distribution as their concentration decreased and depending on their magnetic moment distribution. The first harmonic component was susceptible to the diamagnetic contribution of carrier liquid when the concentration was lower than 24 μg/ml. The second and third harmonics were affected when the peak position of magnetic moment distribution was smaller than m = 10−19 Am2 and the concentration was 10 ng/ml. A highly sensitive detection up to sub-nanogram of iron oxide nanoparticles in solutions can be achieved by utilizing second and third harmonic components.

Superconducting Quantum Interferometers for Nondestructive Evaluation.

We review stationary and mobile systems that are used for the nondestructive evaluation of room temperature objects and are based on superconducting quantum interference devices (SQUIDs). The systems are optimized for samples whose dimensions are between 10 micrometers and several meters. Stray magnetic fields from small samples (10 µm–10 cm) are studied using a SQUID microscope equipped with a magnetic flux antenna, which is fed through the walls of liquid nitrogen cryostat and a hole in the SQUID’s pick-up loop and returned sidewards from the SQUID back to the sample. The SQUID microscope does not disturb the magnetization of the sample during image recording due to the decoupling of the magnetic flux antenna from the modulation and feedback coil. For larger samples, we use a hand-held mobile liquid nitrogen minicryostat with a first order planar gradiometric SQUID sensor. Low-Tc DC SQUID systems that are designed for NDE measurements of bio-objects are able to operate with sufficient resolution in a magnetically unshielded environment. High-Tc DC SQUID magnetometers that are operated in a magnetic shield demonstrate a magnetic field resolution of ~4 fT/√Hz at 77 K. This sensitivity is improved to ~2 fT/√Hz at 77 K by using a soft magnetic flux antenna.

Improved coupling of nanowire-based high-Tc SQUID magnetometers—simulations and experiments

Superconducting quantum interference devices (SQUIDs) based on high critical-temperature superconducting nanowire junctions were designed, fabricated, and characterized in terms of their potential as magnetometers for magnetoencephalography (MEG). In these devices, the high kinetic inductance of junctions and the thin film thickness (50 nm) pose special challenges in optimizing the field coupling. The high kinetic inductance also brings difficulties in reaching a low SQUID noise. To explore the technique for achieving a high field sensitivity, single-layer devices with a directly connected pickup loop and flip-chip devices with an inductively coupled flux transformer using a two-level coupling approach were fabricated and tested. Two-level coupling is an approach designed for flip-chip nanowire-based SQUIDs, in which a washer type SQUID pickup loop is introduced as an intermediate coupling level between the SQUID loop and the flux transformer input coil. The inductances and effective areas of all these devices were simulated. We found that at T = 77 K, flip-chip devices with the two-level coupling approach (coupling coefficient of 0.37) provided the best effective area of 0.46 mm2 among all the tested devices. With a flux noise level of 55 0 , the field sensitivity level was 240 fT . This sensitivity is not yet adequate for MEG applications but it is the best level ever reached for nanowire-based high-Tc SQUID magnetometers.

Feedback solutions for low crosstalk in dense arrays of high-Tc SQUIDs for on-scalp MEG

Magnetoencephalography (MEG) systems based on a dense array of high critical temperature (high-Tc) superconducting quantum interference devices (SQUIDs) can theoretically outperform a state-of-the-art MEG system. On the way towards building such a multichannel system, we evaluate feedback methods suitable for use in dense high-Tc SQUID arrays where the sensors are in very close proximity to the head (on-scalp MEG). We test on-chip superconducting coils and direct injection of the feedback current into the SQUID loop as alternatives to the wire-wound copper coils commonly used in single-channel high-Tc SQUID-based MEG systems. For the evaluation, we have performed coupling, noise, and crosstalk measurements. We conclude that direct injection is the optimal solution for dense on-scalp MEG as it gives crosstalk below 0.5% even between SQUIDs whose pickup loops are within 0.8 mm of one another. Further, this solution provides sufficient flux coupling without adding additional noise. Finally, it does not compromise the standoff distance, which is important for on-scalp MEG.

Operation of a high-TC SQUID gradiometer with a two-stage MEMS-based Joule–Thomson micro-cooler

Practical applications of high-TC superconducting quantum interference devices (SQUIDs) require cheap, simple in operation, and cryogen-free cooling. Mechanical cryo-coolers are generally not suitable for operation with SQUIDs due to their inherent magnetic and vibrational noise. In this work, we utilized a commercial Joule–Thomson microfluidic two-stage cooling system with base temperature of 75 K. We achieved successful operation of a bicrystal high-TC SQUID gradiometer in shielded magnetic environment. The micro-cooler head contains neither moving nor magnetic parts, and thus does not affect magnetic flux noise of the SQUID even at low frequencies. Our results demonstrate that such a microfluidic cooling system is a promising technology for cooling of high-TC SQUIDs in practical applications such as magnetic bioassays.

Harmonic Analysis for Finding the Optimum Working Point of High-Tc RF SQUID

High-Tc radio-frequency (RF) superconducting quantum interference devices (SQUIDs) are well understood and have been adequately expounded in theory, but in practice, it is not so easy to find the optimum working points for their low-noise operations. Starting from the basic equations describing the RF SQUID in nonhysteretic mode, a polynomial expression for its flux-to-voltage characteristic was deduced. For simplicity, the expansion was truncated to the first three harmonics in the flux quantum. The optimum working point is determined as the location of the maximum gradient of its transfer function. The analysis was compared with experimental flux-to-voltage characteristics of the RF SQUIDs with a step-edge junction, measured at 77 K for different settings of the readout electronics. Six typical kinds of the measured flux-to-voltage curves were fitted and analyzed with respect to their harmonics in the magnetic flux. The findings can be of referential value for automatically setting the working points of high-Tc RF SQUIDs in the practical applications.

Effect of diamagnetic contribution of water on harmonics distribution in a dilute solution of iron oxide nanoparticles measured using high-Tc SQUID magnetometer

The magnetization curve of iron oxide nanoparticles in low-concentration solutions was investigated by a highly sensitive high-Tc superconducting quantum interference device (SQUID) magnetometer. The diamagnetic contribution of water that was used as the carrier liquid was observed in the measured magnetization curves in the high magnetic field region over 100mT. The effect of the diamagnetic contribution of water on the generation of harmonics during the application of AC and DC magnetic fields was simulated on the basis of measured magnetization curves. Although the diamagnetic effect depends on concentration, a linear relation was observed between the detected harmonics and concentration in the simulated and measured results. The simulation results suggested that improvement could be expected in harmonics generation because of the diamagnetic effect when the iron concentration was lower than 72μg/ml. The use of second harmonics with an appropriate bias of the DC magnetic field could be utilized for realization of a fast and highly sensitive detection of magnetic nanoparticles in a low-concentration solution.

High- <inline-formula> <tex-math notation="TeX">$T_{\rm c}$</tex-math></inline-formula> SQUID vs. Low- <inline-formula> <tex-math notation="TeX">$T_{\rm c}$</tex-math></inline-formula> SQUID-Based Recordings on a Head Phantom: Benchmarking for Magnetoencephalography

We explore the potential that high critical-temperature (high-Tc) superconducting quantum interference device (SQUID) technology has for magnetic recordings of brain activity, i.e., magnetoencephalography (MEG). To this end, we performed a series of benchmarking experiments to directly compare recordings with a commercial (low-Tc SQUID-based) 306-channel MEG system (Elekta Neuromag TRIUX, courtesy of NatMEG) and a single channel high-Tc SQUID system. The source on which we recorded is a head phantom including 32 artificial current dipoles housed inside a half-spherical shell (courtesy Elekta Oy) for calibrating MEG systems. The high-Tc SQUID magnetometer consisted of a single layer YBa2Cu3O7-x (YBCO) film on a 10 mm × 10 mm bicrystal substrate with a magnetic field sensitivity of ~40 fT/Hz down to 10 Hz. We recorded serial activations of eight tangential current dipoles located at different depths from the surface of the head phantom. Results indicate that our individual high-Tc SQUID demonstrated signal-to-noise ratios (SNRs) about 7-14 times lower than that of similarly-positioned low-Tc SQUIDs in a commercial MEG system. Only considering single-channel SNR, high-Tc SQUIDs with resolution better than 18 fT/Hz would be required to outperform the low-Tc system for shallow dipole sources. This work demonstrates a proof of principle study for future multichannel high-Tc MEG system development.

Development of Metallic Contaminant Detection System Using RF High-Tc SQUIDs for Food Inspection

In this study, we developed a practical magnetic metallic contaminant detector using three high-Tc RF superconducting quantum interference devices (SQUIDs) for food inspection. Finding small metallic contaminants is important for food safety. When contamination occurs, the manufacturer of the product suffers a great loss to recall the tainted products. Therefore we developed a practical food contaminant detection system based on high-Tc RF SQUIDs. The system was covered with waterproof stainless steel plates, and the outer dimensions of the system were 2450 mm (W) × 714 mm (D) × 1555 mm (H). An acceptable inspected object size was 150 mm (W) × 100 mm (H), which is sufficiently large for practical food inspection. A digital filtering technique has been newly introduced to reduce noise. As a result, the signal-to-noise ratio (SNR) was dramatically improved, and we were able to robustly detect a steel ball as small as 0.3 mm in diameter.

Magnetic Coupling between SQUID and Probe of STM-SQUID

An STM-SQUID microscope has been developed by combining a scanning tunneling microscope (STM) and a high-Tc superconducting quantum interference device (SQUID) to simultaneously observe the surface morphology and the local magnetic field. In our microscope, a permalloy probe with high permeability, whose tip is electrochemically polished, is placed between the rf-SQUID and the sample. The probe plays a flux guide role of transferring the local magnetic field of the sample to the SQUID. The probe tip could be approached as close to the sample surface as possible within a few nm by an STM feedback control. The SQUID detects the magnetic field from the end of probe. It is also important to bring the probe end close to the SQUID. In this paper, we have investigated the magnetic coupling between the SQUID and the probe in the STM-SQUID microscope setup by decreasing the gap distance between the SQUID and the end of probe from 4.5mm to 0.1mm. The magnetic signal detected by the SQUID was increased as the distance between the SQUID and the probe was decreased. As the result, the magnetic images of a nickel thin film sample became clearer with decreasing the distance.

Ultra-Sensitive Contaminant Detection System Using High-Tc SQUID

We report the fabrication of magnetic metallic contaminant detectors using multiple high-Tc superconducting quantum interference devices (SQUIDs) for food. Finding small metallic contaminants is important for food safety. People may ingest contaminants that are accidentally mixed in with the food. Hence, a detection method of small contaminants in food is required. Given the lower detection limit for practical X-ray usage is in the order of 1 mm, a detection system using a SQUID is a more powerful tool for sensitive inspections. We design and set up a three-channel high-Tc rf-SQUID inspection system for food. We report the successful detection of a small φ0.3-mm steel particle for the food detection system.