Superconducting Quantum Interference Device Research Articles

Intrinsic superconducting phase battery

Leveraging the quantization properties inherent in superconductors, we present the development of an intrinsic superconducting phase battery. This advancement is achieved by integrating a ferromagnetic π-phase Josephson junction (π-JJ) within a superconducting ring. The core innovation lies in harnessing the potential energy of the π-JJ to generate an intrinsic circulating current, thereby inducing a phase difference as the current goes through the geometric inductance. This mechanism allows for tuning the phase bias φ (0 < φ < π) through an arrangement of the geometric inductance in the battery. We integrate the intrinsic phase batteries into superconducting quantum interference devices, where we verified the effectiveness of the induced phase bias. The polarity of the phase battery is determined by the direction of the intrinsic circulating current, which can be initialized by an external magnetic field. The design methodology for precise intrinsic phase bias has been established. Our findings not only show the feasibility of generating an intrinsic and adjustable phase bias using established fabrication techniques but also open new avenues for enhancing the design, efficiency, and functionality of superconducting electronics, promising to accelerate advancements in digital and quantum computing technologies.

Strain- and Electron Doping-Induced In-Plane Spin Orientation at Room Temperature in Single-Layer CrTe2.

Ferromagnets with a Curie temperature surpassing room temperature (RT) are highly sought after for advancing planar spintronics. The ultrathin CrTe2 is proposed as a promising two-dimensional (2D) ferromagnet with a Curie temperature above 300 K. However, its single-layer film is highly susceptible to specific external perturbations, leading to variable magnetic features depending on the environment. The magnetic ordering of single-layer CrTe2 remains a topic of debate, and experimental confirmation of ferromagnetic order at RT is still pending. In our study, we utilized molecular beam epitaxy to create a single-layer 1T-CrTe2 on bilayer graphene, demonstrating ferromagnetism above 300 K with in-plane magnetization through superconducting quantum interference devices (SQUID) measurements. Our density functional theory (DFT) calculations suggest that the ferromagnetic properties stem from epitaxial strain, which increases the distance between adjacent Cr atoms within the layer by about 1.6% and enhances the Cr-Te-Cr angle by approximately 1.6°. Due to its interaction with the graphene substrate, the magnetic moment transitions from an out-of-plane to an in-plane orientation, while electronic doping exceeds 1.5 e/u.c. Combining DFT calculations with in situ scanning tunneling microscopy (STM) characterizations allowed us to determine the configuration of the CrTe2 single layer on graphene. This discovery presents the first experimental proof of ferromagnetic order in single-layer CrTe2 with a Curie temperature above RT, laying the groundwork for future applications of CrTe2 single-layer-based spintronic devices.

High-temperature superconductor quantum flux parametron for energy efficient logic

We report the fabrication and measurement of quantum flux parametron logic from high-transition-temperature Josephson junctions operating at 25 K, above the temperature of liquid helium. The circuits are written directly into the plane of a single-layer thin film of YBa2Cu3O7 using a focused helium ion beam. A single-cell quantum flux parametron was constructed and correct logic operation was verified by using an on-chip superconducting quantum interference device for readout. A three-cell shift register was also fabricated and tested using a three-phase clock cycle. We estimate the average bit energy to be about 1×10−20 J at 1 GHz.

Microwave quantum memcapacitor effect

Developing the field of neuromorphic quantum computing necessitates designing scalable quantum memory devices. Here, we propose a superconducting quantum memory device in the microwave regime, termed a microwave quantum memcapacitor. It comprises two linked resonators, the primary one is coupled to a Superconducting Quantum Interference Device, which allows for the modulation of the resonator properties through external magnetic flux. The auxiliary resonator, operated through weak measurements, provides feedback to the primary resonator, ensuring stable memory behavior. This device operates with a classical input in one cavity while reading the response in the other, serving as a fundamental building block toward arrays of microwave quantum memcapacitors. We observe that a bipartite setup can retain its memory behavior and gains entanglement and quantum correlations. Our findings pave the way for the experimental implementation of memcapacitive superconducting quantum devices and memory device arrays for neuromorphic quantum computing.

Arrays of nano-high-transition temperature superconductor quantum interference devices

We report the fabrication and testing of arrays of nanoscale superconducting quantum interference devices (SQUIDs) directly written into a thin film of the high-transition temperature superconductor YBa2Cu-3O7−δ with a focused helium ion beam. We compare three array configurations with 400 nm by 400 nm nanoSQUIDs connected in series and parallel and a two-dimensional (2D) combination of both. Our electrical transport measurements show that series arrays of three nanoSQUIDs exhibit modulation voltages greater than 1 mV and that combining the devices in parallel greatly enhances the slope of the voltage–magnetic field characteristic. A 2D array with 3 SQUIDS in series and 7 in parallel exhibited a transfer function of 5.51 mV/mT.

The SQUID Controller Unit for the LiteBIRD Space Mission: Description, Functional Tests and Early Performance Assessment

AbstractLiteBIRD is a satellite mission to be launched by JAXA in the early 2030s. It will measure the Cosmic Microwave Background (CMB) primordial B-modes with an unprecedented sensitivity. Microwave radiation will be detected by Transition Edge Sensors (TESs) arrays multiplexed in frequency domain and read by Superconducting QUantum Interference Devices (SQUIDs). The LiteBIRD SQUID Controller Unit (SCU), based on the heritage of the successful design used for the ground-based SPT3G experiment, presents some novel elements that make it suitable for a space-borne application. We compare our first breadboard model with the ground-based, Off-The-Shelf Components (COTS) version, by driving the same SQUID Array Amplifier (SAA) at 4 K, measuring relevant quantities such as noise, gain and bandwidth. We demonstrate that the noise added by our first prototype (including a switching part for redundancy purposes) never exceeds the noise added by the COTS-based electronics board, representing our benchmark. We also present the first noise estimates with the SAA cooled below 1 K, going closer to the conditions expected for LiteBIRD operation.

Study of Molecular Dimer Morphology Based on Organic Spin Centers: Nitronyl Nitroxide Radicals.

In this work, in order to investigate the short-range interactions between molecules, the spin-magnetic unit nitronyl nitroxide (NN) was introduced to synthesize self-assembly single radical molecules with hydrogen bond donors and acceptors. The structures and magnetic properties were extensively investigated and characterized by UV-Vis absorption spectroscopy, electron paramagnetic resonance (EPR), and superconducting quantum interference devices (SQUIDs). Interestingly, it was observed that the single molecules can form two different dimers (ring-closed dimer and "L"-type dimer) in different solvents, due to hydrogen bonding, when using EPR to track the molecular spin interactions. Both dimers exhibit ferromagnetic properties (for ring-closed dimer, J/kB = 0.18 K and ΔES-T = 0.0071 kcal/mol; for "L"-type dimer, the values were J/kB = 9.26 K and ΔES-T = 0.037 kcal/mol). In addition, the morphologies of the fibers formed by the two dimers were characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM).

Qualification of Microwave SQUID Multiplexer Chips for Simons Observatory

AbstractThe Simons Observatory is a cosmic microwave background experiment stationed atop Cerro Toco, at an elevation of 5200 ms in Chile’s Atacama Desert. The receivers of the Observatory will contain more than 60,000 transition edge sensor bolometers. In order to read out this large detector count in a scalable manner, we utilize a microwave superconducting quantum interference device (SQUID) multiplexing scheme where each detector is inductively coupled to an rf SQUID, which in turn is inductively coupled to a GHz resonator. More than 2000 SQUIDs and resonators are fabricated on a single 76.2-mm-diameter silicon wafer. To qualify wafers before integration, we cryogenically screen $$\sim$$ ∼ 10% of the devices on each wafer by use of a standard set of measurements. From these data, we report parameter value trends in 47 wafers that were fabricated in the past two years. We show good control in key parameters such as frequency placement, internal quality factor, and response to applied flux. We demonstrate a wafer acceptance yield of 86%.

SQUID magnetoneurography: an old-fashioned yet new tool for noninvasive functional imaging of spinal cords and peripheral nerves.

We are engaged in the development and clinical application of a neural magnetic field measurement system that utilizes biomagnetic measurements to observe the activity of the spinal cord and peripheral nerves. Unlike conventional surface potential measurements, biomagnetic measurements are not affected by the conductivity distribution within the body, making them less influenced by the anatomical structure of body tissues. Consequently, functional testing using biomagnetic measurements can achieve higher spatial resolution compared to surface potential measurements. The neural magnetic field measurement, referred to as magnetoneurography, takes advantage of these benefits to enable functional testing of the spinal cord and peripheral nerves, while maintaining high spatial resolution and noninvasiveness. Our magnetoneurograph system is based on superconducting quantum interference devices (SQUIDs) similar to the conventional biomagnetic measurement systems. Various design considerations have been incorporated into the SQUID sensor array structure and signal processing software to make it suitable for detecting neural signal propagation along spinal cord and peripheral nerve. The technical validation of this system began in 1999 with a 3-channel SQUID system. Over the course of more than 20 years, we have continued technological development through medical-engineering collaboration, and in the latest prototype released in 2020, neural function imaging of the spinal cord and peripheral nerves, which could also be applied for the diagnosis of neurological disorders, has become possible. This paper provides an overview of the technical aspects of the magnetoneurograph system, covering the measurement hardware and software perspectives for providing diagnostic information, and its applications. Additionally, we discuss the integration with a helium recondensing system, which is a key factor in reducing running costs and achieving practicality in hospitals.

Magnetic Flux Sensor Based on Spiking Neurons with Josephson Junctions.

Josephson junctions (JJs) are superconductor-based devices used to build highly sensitive magnetic flux sensors called superconducting quantum interference devices (SQUIDs). These sensors may vary in design, being the radio frequency (RF) SQUID, direct current (DC) SQUID, and hybrid, such as D-SQUID. In addition, recently many of JJ's applications were found in spiking models of neurons exhibiting nearly biological behavior. In this study, we propose and investigate a new circuit model of a sensory neuron based on DC SQUID as part of the circuit. The dependence of the dynamics of the designed model on the external magnetic flux is demonstrated. The design of the circuit and derivation of the corresponding differential equations that describe the dynamics of the system are given. Numerical simulation is used for experimental evaluation. The experimental results confirm the applicability and good performance of the proposed magnetic-flux-sensitive neuron concept: the considered device can encode the magnetic flux in the form of neuronal dynamics with the linear section. Furthermore, some complex behavior was discovered in the model, namely the intermittent chaotic spiking and plateau bursting. The proposed design can be efficiently applied to developing the interfaces between circuitry and spiking neural networks. However, it should be noted that the proposed neuron design shares the main limitation of all the superconductor-based technologies, i.e., the need for a cryogenic and shielding system.

Three Oxidation States of Cobalt(I/II/III) Complexes by Thiaporphyrin.

Reaction of tetraphenyl-21-thiaporphyrin (HSTPP) with cobalt salt yields a pentacoordinated high-spin 3/2 [CoIICl(STTP)] (1). Through ion exchange, a roughly square-planar-geometry low-spin 1/2 CoIISTTP(BArF24) (2) complex was isolated. These two paramagnetic precursors were examined by single X-ray diffraction, nuclear magnetic resonance, electron paramagnetic resonance, superconducting quantum interference device, and density functional theory calculations. These two allowed the development of one electron reduction and oxidation to give [CoI(STTP)] (3), [CoIII(STTP)Cl(CH3CN)](BF4) (4), and [CoIII(STTP)Cl2] (5). The products of the chemical redox reactions were isolated and fully characterized. In addition, the reactivity of [CoIICl(STTP)] (1) was examined by azide (N3), cyanate (OCN), and thiocyanate (SCN) and featured a preferential N-coordination to the cobalt metal.

NbN-based half-flux-quantum element for integration with superconducting qubits

Half-flux-quantum (HFQ) logic is a superconductor logic family comprising conventional Josephson junctions (0-JJs) and π Josephson junctions (π-JJs). The energy scale of HFQ logic can be flexibly reduced by phase shifts owing to π-JJs; thus, HFQ circuits are a promising building block for qubit interface circuits. In this study, we demonstrate an NbN-based HFQ circuit element compatible with NbN-based superconducting qubits for all-NbN monolithic integration of qubits and HFQ circuits. The use of NbN is beneficial for both qubits and HFQ circuits owing to its high-quality junctions and high kinetic inductance. First, we developed a prototype of an NbN-based 0-JJ/π-JJ hybrid fabrication process for designing HFQ circuits. We evaluated 0-JJs and π-JJs fabricated through the hybrid process, which demonstrated that the 0-JJs had a small deviation with regard to critical current density and the π-JJs had a sufficiently high critical current density to work as π phase shifters. Furthermore, we fabricated an HFQ superconducting quantum interference device, which is one of the most fundamental elements in HFQ circuits, and observed clear HFQ-period modulation in the magnetic flux dependence of the maximum current at 4.2 K.

Leggett modes in a Dirac semimetal

AbstractExperiments have shown that several materials, including MgB2, iron-based superconductors and monolayer NbSe2, are multiband superconductors. Superconducting pairing in multiple bands can give rise to phenomena not available in a single band, including Leggett modes. A Leggett mode is the collective periodic oscillation of the relative phase between the phases of the superconducting condensates formed in the different bands. The experimental observation of Leggett modes is challenging because multiband superconductors are rare and because these modes describe charge fluctuations between bands and therefore are hard to probe directly. Also, the excitation energy of a Leggett mode is often larger than the superconducting gaps, and therefore they are strongly overdamped via relaxation processes into the quasiparticle continuum. Here, we show that Leggett modes and their frequency can be detected in a.c. driven superconducting quantum interference devices. We then use the results to analyse the measurements of such a quantum device, one based on a Dirac semimetal Cd3As2, in which superconductivity is induced by proximity to superconducting Al. These results show the theoretically predicted signatures of Leggett modes, and therefore we conclude that a Leggett mode is present in the two-band superconducting state of Cd3As2.

Experimental Techniques for the Characterization of Magnetic Thin Films

This review article describes the various experimental techniques, which are required for the characterization of magnetic thin films. Characterization is an essential process to understand the properties of magnetic thin films. By knowing the properties of these materials, one can utilize them in various technological applications as per requirements. There are different characterization tools to study the different properties of nanostructured materials. Some important experimental techniques employed to characterize the magnetic thin films are: X-Ray Diffraction, X-Ray reflectivity, Field emission scanning electron microscopy, Energy dispersive spectrometer, Transmission electron microscopy, Superconducting quantum interference device, Raman spectroscopy and X-ray absorption spectroscopy etc. These techniques will develop a better understanding of the structural, morphological, and magnetic properties of the materials to the scientific community. The discussion made in the present review article would be extremely helpful to the researchers who are doing research in the various branches of science and engineering.

Detection of biomagnetic signals from induced pluripotent stem cell-derived cardiomyocytes using deep learning with simulation data

The detection of spontaneous magnetic signals can be used for the non-invasive electrophysiological evaluation of induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs). We report that deep learning with a dataset that combines magnetic signals estimated using numerical simulation and actual noise data is effective in the detection of weak biomagnetic signals. To verify the feasibility of this method, we measured artificially generated magnetic signals that mimic cellular magnetic fields using a superconducting quantum interference device and attempted peak detection using a long short-term memory network. We correctly detected 80.0% of the peaks and the method achieved superior detection performance compared with conventional methods. Next, we attempted peak detection for magnetic signals measured from mouse iPS-CMs. The number of detected peaks was consistent with the spontaneous beats counted using microscopic observation and the average peak waveform achieved good similarity with the prediction. We also observed the synchronization of peak positions between simultaneously measured field potentials and magnetic signals. Furthermore, the magnetic measurements of cell samples treated with isoproterenol showed potential for the detection of chronotropic effects. These results suggest that the proposed method is effective and has potential application in the safety assessment of regenerative medicine and drug screening.

Magnetic field-to-voltage coefficient evaluation of axial SQUID gradiometer in unshielded environment

Due to the existence of earth magnetic field, it is difficult to detect weak magnetic field from some parts of the body like heart, muscle, nerve and so on. In addition to the requirement for high-sensitivity magnetic field sensors, it is also essential to take certain measures to suppress noise. The axial gradiometer based on low-Tc Superconducting QUantum Interference Device (SQUID) can achieve a noise suppression ratio of 30 dB through the spatial gradient difference of the magnetic field, but the calibration of the magnetic field-to-voltage coefficient (B-V coefficient) generally requires a magnetically shielded room (MSR), which is complex and costly. With finite element simulation and practical experiments, a calibration method for B-V coefficient of the axial hardware gradiometer without shielding was presented in this paper. That is, a larger calibration field was generated for coefficient evaluation in an unshielded environment. At the same time, when the test signal was far away, the spatial gradient difference was used to improve the authenticity of the evaluation results. Through simulation and experimental verification, for the same performance and configuration of the gradiometer, the calibration result was 1.6 nT/V with 0.1 nT/V difference from that in a shielding room. The results showed that the unshielded evaluation method proposed in this paper was practical and could be used to evaluate the B-V coefficient of gradiometer.

Visualizing thickness-dependent magnetic textures in few-layer Cr2Ge2Te6

Magnetic ordering in two-dimensional (2D) materials has recently emerged as a promising platform for data storage, computing, and sensing. To advance these developments, it is vital to gain a detailed understanding of how the magnetic order evolves on the nanometer-scale as a function of the number of atomic layers and applied magnetic field. Here, we image few-layer Cr2Ge2Te6 using a combined scanning superconducting quantum interference device and atomic force microscopy probe. Maps of the material’s stray magnetic field as a function of applied magnetic field reveal its magnetization per layer as well as the thickness-dependent magnetic texture. Using a micromagnetic model, we correlate measured stray-field patterns with the underlying magnetization configurations, including labyrinth domains and skyrmionic bubbles. Comparison between real-space images and simulations demonstrates that the layer dependence of the material’s magnetic texture is a result of the thickness-dependent balance between crystalline and shape anisotropy. These findings represent an important step towards 2D spintronic devices with engineered spin configurations and controlled dependence on external magnetic fields.

Quantum interference in a high-transition-temperature superconductor based on nanoslits on SrTiO3 substrate

The Josephson junction, a key component of superconducting quantum circuits, is much less mature, especially for the high-transition (high-Tc) temperature cuprate superconductors. Herein, we report on the realization of high-Tc Josephson junction based on nanoslits on a SrTiO3 substrate. We demonstrate the ability to manipulate the current–voltage characteristics of YBa2Cu3O7−x bridges continuously from superconducting current-dominated to Josephson junction behavior by changing the irradiation parameters of the focused Ga+ ion beam on a single-crystal SrTiO3 substrate. The periodic critical current that depends on the magnetic flux coupled into the superconducting quantum interference devices was observed, which exactly reflects the effects of quantum tunneling and flux quantization. To some extent, this weak link within the framework of the semiconductor manufacturing process shows the potential to provide a cost-effective, highly efficient, and reliable pathway for scaling up quantum mechanical superconducting circuits, which is promising for the fabrication process.

Stable π-Extended Thio[7]helicene-Based Diradical with Predominant Through-Space Spin-Spin Coupling.

In this work, a novel π-extended thio[7]helicene scaffold was synthesized, where the α-position of the thiophene unit could be functionalized with bulky phenoxy radicals after considerable synthetic attempts. This open-shell helical diradical, ET7H-R, possesses high stability in the air, nontrivial π conjugation, persistent chirality, and a high diradical character (y0 of 0.998). The key feature is a predominant through-space spin-spin coupling (TSC) between two radicals at the helical terminals. Variable-temperature continuous-wave electron spin resonance (cw-ESR) and superconducting quantum interference device (SQUID) magnetometry in the solid state reveal a singlet ground state with a nearly degenerate triplet state of ET7H-R. These results highlight the significance of a stable helical diradicaloid as a promising platform for investigating intramolecular TSCs.

Heteropentanuclear {Ru(II)Cu(II)4} Kuratowski Complexes Assembled from a Ruthenium(II) Precursor Complex to Study Competing Exchange Interactions in M(II)(ta)2 Networks [ta(-) = 1,2,3-Triazolate

We report a directed two-step synthesis toward pentanuclear Kuratowski complexes. First, six 5,6-dimethylbenzo[1,2,3]triazole ligands (Me2btaH) are coordinated to a single Ru(II) ion, providing a topologically ideal template for the addition of further metal ions. The synthesis and crystal structures of [RuCu4X4(Me2bta)6] [X = acetylacetonate (acac) and tris(3,5-dimethyl-1-pyrazolyl)borate (Tp*)] are described. Both represent new members of the family of so-called Kuratowski (K3,3) complexes. The coordination units feature triazolato-bridged metal-centered {MM4} tetrahedra, which are known for frustrated magnetic interactions in both complexes and metal-organic frameworks. The novel Ru(II)-centered complexes were synthesized in order to investigate the influence of the presence or absence of a paramagnetic central metal ion in the Kuratowski complex. Superconducting quantum interference device and electron spin resonance measurements demonstrate that small deviations in bond lengths and valence angles can lead to the formation of pairs of magnetic exchange-coupled Cu(II) ions. Which Cu(II) ions pair up can be predicted in Jahn-Teller active compounds by the overlap of the respective orbitals. These data are compared with those gleaned for M(II)(ta)2 (ta = 1,2,3-triazolate) lattices, in which structurally similar {MM4} tetrahedra constitute the secondary building units.