Superconducting Quantum Interference Device Array Research Articles

Investigation of Arrays of Two-dimensional High-T C SQUIDs for Optimization of Electrical Properties.

In this work, we investigate two-dimensional arrays of High-T C superconducting quantum interference devices (SQUIDs) for optimization of their electrical transport characteristics. Specifically, we look at devices with different electrode configurations in between the series segments to gain insight into how the array spacing, in the direction of the bias current, affects the voltage magnetic field characteristics. Our results suggest that for spacing dimensions greater than the penetration depth interactions are minimal. Furthermore, comparisons of voltage field characteristics reveal higher modulation voltages and narrower peaks with as the numbers of SQUIDs in the parallel direction increases from 1 to 6. For larger numbers of SQUIDs in parallel greater than 6 little change is observed. These results suggest a pathway to SQUID array scaling for very large numbers of SQUIDs within in a small area.

Understanding the Saturation Power of Josephson Parametric Amplifiers Made from SQUID Arrays

We report on the implementation and detailed modelling of a Josephson Parametric Amplifier (JPA) made from an array of eighty Superconducting QUantum Interference Devices (SQUIDs), forming a non-linear quarter-wave resonator. This device was fabricated using a very simple single step fabrication process. It shows a large bandwidth (45 MHz), an operating frequency tunable between 5.9 GHz and 6.8 GHz and a large input saturation power (-117 dBm) when biased to obtain 20 dB of gain. Despite the length of the SQUID array being comparable to the wavelength, we present a model based on an effective non-linear LC series resonator that quantitatively describes these figures of merit without fitting parameters. Our work illustrates the advantage of using array-based JPA since a single-SQUID device showing the same bandwidth and resonant frequency would display a saturation power 15 dB lower.

Evaluation of Josephson Junction Parameter Dispersion Effects in Arrays of HTS SQUIDs

We report on the experimental evaluation of dispersion effects on superconducting quantum interference device (SQUID) arrays made with high-temperature superconductor technology. The dispersion of Josephson junction parameters not only results in different operational temperature ranges but also in a flux shift of the voltage versus flux response of the SQUID. Another contribution to the phase shift originates from unbalanced inductances of each branch of the SQUID loop. Measurements show that both contributions can be discriminated. Optimal operation of SQUID arrays is obtained in principle only when these self-induced fluxes are negligible. Degradation of the response is more important when the involved currents are larger, i.e., at lower temperature. We compensated these phase shifts for an array of a few SQUIDs connected in series by direct current injection in one of the branches of each SQUID, thereby recovering the characteristic “antipeak” response. This demonstrates that in the lower temperature range of SQUID operation, phase shifts are mainly responsible for the degradation of the SQUID array response.

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.

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.

Imaging Coherent Response of Superconducting Metasurface

We study microwave response of the individual metaatoms of a superconducting metasurface formed by a two-dimensional array of superconducting quantum interference devices (SQUIDs). In our experiment, RF currents in the metasurface are directly imaged by using the laser scanning microscopy (LSM) technique. We tested a sample with 21 × 21 SQUID array in a waveguide cavity designed to achieve a uniform microwave distribution over the entire array. The demonstrated tunability of 2D SQUID metasurface resonance frequency by external magnetic field is about 56%, covering 8-12.5-GHz range. The obtained LSM images of the RF current distributions over the SQUID array confirm a high degree of coherence of the entire metasurface. The SQUID-based metasurfaces combine low losses and frequency tunability and can be useful for designing compact cryogenic RF systems.

Development of a Two-Dimensional Micro-SQUID Array for Investigation of Magnetization Spatial Distribution

We developed a two-dimensional array of superconducting quantum interference devices (SQUIDs) for investigation of fine spatial distribution of magnetization in superconducting Sr\(_{2}\)RuO\(_{4}\). Micrometer-sized SQUIDs based on homogeneously formed Al/AlO\(_{x}\)/Al tunnel-type Josephson junctions were fabricated using shadow evaporation technique. Unnecessary electrodes formed by the shadow evaporation were removed by inductively coupled plasma reactive ion etching, in order to realize a dense array of SQUIDs. We measured the magnetic modulation of the maximum Josephson current of each SQUID in the array and evaluated the interaction among the SQUIDs.

The SuperCDMS SNOLAB Detector Tower

The SuperCDMS collaboration is moving forward with the design and construction of SuperCDMS SNOLAB, where the initial deployment will include $$\sim $$ 30 kg of Ge and $$\sim $$ 5 kg of Si detectors. Here, we will discuss the associated cryogenic cold hardware required for the detector readout. The phonon signals will be read out with superconducting quantum interference device arrays and the ionization signals will use high electron mobility transistor amplifiers operating at 4 K. A number of design challenges exist regarding the required wiring complex impedance, noise pickup, vibration, and thermal isolation. Our progress to date will be presented.

Parasitic effects in superconducting quantum interference device-based radiation comb generators

We study several parasitic effects on the implementation of a Josephson radiation comb generator based on a dc superconducting quantum interference device (SQUID) driven by an external magnetic field. This system can be used as a radiation generator similarly to what is done in optics and metrology, and allows one to generate up to several hundreds of harmonics of the driving frequency. First we take into account how the assumption of a finite loop geometrical inductance and junction capacitance in each SQUID may alter the operation of the devices. Then, we estimate the effect of imperfections in the fabrication of an array of SQUIDs, which is an unavoidable source of errors in practical situations. We show that the role of the junction capacitance is, in general, negligible, whereas the geometrical inductance has a beneficial effect on the performance of the device. The errors on the areas and junction resistance asymmetries may deteriorate the performance, but their effect can be limited to a large extent by a suitable choice of fabrication parameters.

Flux-coherent series SQUID array magnetometers operating above 77 K with superior white flux noise than single-SQUIDs at 4.2 K

A very promising direction to improve the sensitivity of magnetometers based on superconducting quantum interference devices (SQUIDs) is to build a series-array of N non-interacting SQUIDs operating flux-coherently, because in this case their voltage modulation depth, ΔV, linearly scales with N whereas the white flux noise SΦ1/2 decreases as 1/N1/2. Here, we report the realization of both these improvements in an advanced layout of very large SQUID arrays made of YBa2Cu3O7. Specially designed with large area narrow flux focusers for increased field sensitivity and improved flux-coherency, our arrays have extremely low values for SΦ1/2 between (0.25 and 0.44) μΦ0/Hz1/2 for temperatures in the range (77–83) K. In this respect, they outperform niobium/aluminium trilayer technology-based single-SQUIDs operating at 4.2 K. Moreover, with values for ΔV and transimpedance in the range of (10–17) mV and (0.3–2.5) kΩ, respectively, a direct connection to a low-noise room temperature amplifier is allowed, while matching for such readout is simplified and the available bandwidth is greatly increased. These landmark performances suggest such series SQUID arrays are ideal candidates to replace single-SQUIDs operating at 4.2 K in many applications.

Detection of Far-Field Radio-Frequency Signals by Niobium Superconducting Quantum Interference Device Arrays

The capability of an all-niobium superconducting quantum interference device (SQUID) array to operate as an electrically small antenna capable of detecting radio frequency from distant sources was demonstrated. The measurements were performed in three different arrays, with each array consisting of 2400 identical cells. The intrinsic broadband characteristics of the device were confirmed by the flat frequency response of the ratio of the output and input voltages (the S21 parameter) of the array between 300 kHz and 1 GHz. Setting the device to its optimal operating parameters allowed the detection of signals from local frequency-modulation stations.

Effects of Spread in Critical Currents for Series- and Parallel-Coupled Arrays of SQUIDs and Bi-SQUIDs

High-temperature-superconductor (HTS) superconducting quantum interference devices (SQUIDs) have a much greater variance in the values of the critical currents than their low-temperature-superconductor counterparts. To explore the effects of this increased variance, we add a noise term into the critical current of each junction. We perform numerical simulations of previously derived phase equations from series- and parallel-coupled arrays of SQUIDs, and series-and parallel-coupled arrays of bi-SQUIDs. These results support ongoing work to design an electrically small magnetic field antenna and low-noise amplifier with high bandwidth using HTS SQUIDs.

Fluxon mobility in an array of asymmetric superconducting quantum interference devices.

Fluxon dynamics in the dc-biased array of asymmetric three-junction superconducting quantum interference devices (SQUIDs) is investigated. The array of SQUIDs is described by the discrete double sine-Gordon equation. It appears that this equation possesses a finite set of velocities at which the fluxon propagates with the constant shape and without radiation. The signatures of these velocities appear on the respective current-voltage characteristics of the array as inaccessible voltage intervals (gaps). The critical depinning current has a clear minimum as a function of the asymmetry parameter (the ratio of the critical currents of the left and right junctions of the SQUID), which coincides with the minimum of the Peierls-Nabarro potential.

High-gain weakly nonlinear flux-modulated Josephson parametric amplifier using a SQUID array

We have developed and measured a high-gain quantum-limited microwave parametric amplifier based on a superconducting lumped LC resonator with the inductor L including an array of 8 superconducting quantum interference devices (SQUIDs). This amplifier is parametrically pumped by modulating the flux threading the SQUIDs at twice the resonator frequency. Around 5 GHz, a maximum gain of 31 dB, a product amplitude-gain x bandwidth above 60 MHz, and a 1 dB compression point of -123 dBm at 20 dB gain are obtained in the non-degenerate mode of operation. Phase sensitive amplification-deamplification is also measured in the degenerate mode and yields a maximum gain of 37 dB. The compression point obtained is 18 dB above what would be obtained with a single SQUID of the same inductance, due to the smaller nonlinearity of the SQUID array.

Fabrication of Arrays of Nano-Superconducting Quantum Interference Devices Using a Double-Angle Processing Approach

Arrays of superconducting quantum interference devices (SQUIDs) from high-temperature superconductors such as YBCO have been shown to significantly improve the signal-to-noise ratio when compared to single SQUIDs from high- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> material. This is based on the finding that the voltage response of a current biased array increases by the number of SQUID loops <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> , while the noise only increases as <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> . It is therefore desirable to fabricate array structures with as many SQUIDs placed in series as possible. We present a fabrication technique for SQUID arrays based on ion-damage Josephson Junctions (JJ) that allows for creating virtually any SQUID-array design on any thin-film superconducting materials such as YBCO or MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . The fabrication employs electron beam lithography (EBL) to write the SQUID-array, electrical contact pads and JJ fine lines in a single lithography step. It is performed on a thick trilayer structure that allows for high-aspect ratio features. The superior resolution of EBL makes SQUID loop diameters down to a few hundred nanometers possible. Subsequent high-energy ion-implantation creates the JJ. Using low-energy argon ion milling at an angle to the sample shadows the fine lines used for creation of the JJ and transfers the EBL-defined SQUID-array and electrical contact pads into the thin-film superconductor.

Simulation of Series Arrays of Superconducting Quantum Interference Devices

The voltage as a function of applied magnetic field (V-B) was calculated for arrays of superconducting quantum interference devices (SQUIDs) connected in series. Comparisons were made between arrays of equal area SQUIDs and superconducting quantum interference filters (SQIFs). The areas for the SQIFs were varied exponentially, so that the V-B had a sharp minimum at zero field. We used equations for the dc SQUID based on resistively shunted junctions, with typical parameters for YBa2Cu3O7 - δ ion damage Josephson junctions. The maximum transfer coefficient of the central minimum VB = (∂V/∂B)max of the SQIF decreases as the area range increases. We find that the equal area array is more robust to the effects of non-uniform junction critical currents than the SQIF, for the junction parameters and SQUID area distributions chosen. Furthermore, we find that slight variations (~5%) to the area due to fabrication irregularities have little effect on the central minimum of V-B for either device.

Study of Low-Frequency Noise Performance of Nanobridge-Based SQUIDs in External Magnetic Fields

We report on the low-frequency noise performance of niobium dc superconducting quantum interference devices (SQUIDs), which contain nanobridges fabricated by focused ion beam lithography as the active Josephson elements. The devices have feature sizes down to 70 nm. We have measured devices of different loop sizes in two readout configurations: nano-scale loop sizes in small signal mode using a series SQUID array as a low-temperature pre-amplifier, and larger micron-scale loop sizes in a conventional flux-locked loop. We investigate the different contributions to the low frequency noise and report on electrical measurements made in applied magnetic fields of up to 0.5 T (in-plane) and 0.1 T (perpendicular to the plane) at operating temperatures around 7 K. We compare the measurements with the existing theories of noise in a dc SQUID.

ASSA13-03-14 Drug-Induced QT Prolongation in Guinea Pig is Automatically Analysed by Micro-Magnetocardiography System with Superconducting Quantum Interference Device: Comparison with ECG

Objectives Magnetocardiogram (MCG) with superconducting quantum interference device can measure cardiac electrical activity and has non-contact characteristics, and is not interfered with lung filed. We newly developed and tested the validity of micro MCG for small animals using superconducting quantum interference device (SQUID) on a single silicon chip. Methods Ten male Hartley guinea pigs (250g) were anaesthetised with pentobarbital. Platinum needle electrodes were attached to animals. Excessive quinidine (60mg/kg) was administered intraperitoneally. MCG and ECG were recorded simultaneously at baseline, 3, 5, 7, 10, 15 min after injection. QT interval was automatically analysed by Power Lab and QTc was calculated by Bazett formula. Results In ginea pigs, QTc was prolonged from 284 ± 10 to 299 ± 11msec in ECG (QTc-ECG). QTc was prolonged from 280 ± 16msec to 295 ± 10msec in MCG (QTc-MCG). QTc-MCG correlated well with QTc-ECG at baseline (r 2 = 0.944, p 2 = 0.780, p = 0.0007). Conclusions Newly developed micro MCG for small animals with a single chip SQUID array enabled us to measure drug-induced QT prolongation by milli-seconds. This system may facilitate the high throughput screening test and also basic scientific research of electromagnetic properties of the small animal heart.

Relativistic dynamics of domain wall in one-dimensional SQUID array

We investigate the dynamics of a domain wall in a one-dimensional array of superconducting quantum interference device (SQUID) composed of three conventional Josephson junctions and a π-junction. The domain wall is formed between two domains with oppositely circulating current through the SQUID loop. It is shown that the SQUIDs in this array can be approximately described by a double sine-Gordon (DSG) model which obeys Einstein's special theory of relativity. We conduct numerical simulations of a discrete DSG equation and show that the domain wall propagates solitonically through the SQUID array and exhibits quasi-relativistic behavior, such as the Lorentz contraction and the relativistic time dilation, which agrees reasonably well with the predictions from a relativistic equation of motion of a particle, whose rest mass is extremely small compared to that of a single electron.

Comparison of measurements and simulations of series-parallel incommensurate area superconducting quantum interference device arrays fabricated from YBa2Cu3O7− δ ion damage Josephson junctions

We have fabricated series-parallel (two-dimensional) arrays of incommensurate superconducting quantum interference devices (SQUIDs) using YBa2Cu3O7−δ thin film ion damage Josephson junctions. The arrays initially consisted of a grid of Josephson junctions with 28 junctions in parallel and 565 junctions in series, for a total of 15 255 SQUIDs. The 28 junctions in the parallel direction were sequentially decreased by removing them with photolithography and ion milling to allow comparisons of voltage–magnetic field (V–B) characteristics for different parallel dimensions and area distributions. Comparisons of measurements for these different configurations reveal that the maximum voltage modulation with magnetic field is significantly reduced by both the self inductances of the SQUIDs and the mutual inductances between them. Based on these results, we develop a computer simulation model from first principles which simultaneously solves the differential equations of the junctions in the array while considering the effects of self inductance, mutual inductance, and non-uniformity of junction critical currents. We find that our model can accurately predict V–B for all of the array geometries studied. A second experiment is performed where we use photolithography and ion milling to split another 28 × 565 junction array into 6 decoupled arrays to further investigate mutual interactions between adjacent SQUIDs. This work conclusively shows that the magnetic fields generated by self currents in an incommensurate array severely reduce its performance by reducing the maximum obtainable modulation voltage.