Direct Current Superconducting Quantum Interference Research Articles

Coexistence of Ferromagnetism and Superconductivity at KTaO3 Heterointerfaces.

The coexistence of superconductivity and ferromagnetism is a long-standing issue in superconductivity due to the antagonistic nature of these two ordered states. Experimentally identifying and characterizing novel heterointerface superconductors that coexist with magnetism presents significant challenges. Here, we report the observation of two-dimensional long-range ferromagnetic order in a KTaO3 heterointerface superconductor, showing the coexistence of superconductivity and ferromagnetism. Remarkably, our direct current superconducting quantum interference device measurements reveal an in-plane magnetization hysteresis loop persisting above room temperature. Moreover, first-principles calculations and X-ray magnetic circular dichroism measurements provide decisive insights into the origin of the observed robust ferromagnetism, attributing it to oxygen vacancies that localize electrons in nearby Ta 5d states. Our findings suggest KTaO3 heterointerfaces as time-reversal symmetry breaking superconductors, injecting fresh momentum into the exploration of the intricate interplay between superconductivity and magnetism enhanced by the strong spin-orbit coupling inherent to the heavy Ta in 5d orbitals.

Frequency-phase-locking mechanism inside DC SQUIDs and the analytical expression of current-voltage characteristics

Direct-current superconducting quantum interference devices (dc SQUIDs) are ultra-sensitive flux-to-voltage convertors widely applied for biomagnetism and geophysics; they are a kind of magnetic-field-effect transistors (MFETs) with flux-modulated current-voltage characteristics. Compared to semiconductor FETs, dc SQUIDs still lack analytical expressions to interpret their inner formation mechanisms of the current-voltage characteristics. This article presents a frequency-phase-locking (FPL) model to derive the analytical expression of dc SQUIDs and reveal how the current-voltage characteristics are shaped by the circuit parameters between two Josephson junctions. The application of the analytical expression in the calculations of current-voltage characteristics is demonstrated; the results are compared with the numerical simulations. It is shown that a dc-SQUID is an FPL system inside and works as a current-modified nonlinear resistor in readout circuits; its current-voltage characteristics are the projections of three impedances of the network between Josephson currents. Those understandings enable electronic engineers to evaluate the design of dc-SQUID circuits directly through three network impedances extracted from the layout.

Chiral antiferromagnetic Josephson junctions as spin-triplet supercurrent spin valves and d.c. SQUIDs

Spin-triplet supercurrent spin valves are of practical importance for the realization of superconducting spintronic logic circuits. In ferromagnetic Josephson junctions, the magnetic-field-controlled non-collinearity between the spin-mixer and spin-rotator magnetizations switches the spin-polarized triplet supercurrents on and off. Here we report an antiferromagnetic equivalent of such spin-triplet supercurrent spin valves in chiral antiferromagnetic Josephson junctions as well as a direct-current superconducting quantum interference device. We employ the topological chiral antiferromagnet Mn3Ge, in which the Berry curvature of the band structure produces fictitious magnetic fields, and the non-collinear atomic-scale spin arrangement accommodates triplet Cooper pairing over long distances (>150 nm). We theoretically verify the observed supercurrent spin-valve behaviours under a small magnetic field of <2 mT for current-biased junctions and the direct-current superconducting quantum interference device functionality. Our calculations reproduce the observed hysteretic field interference of the Josephson critical current and link these to the magnetic-field-modulated antiferromagnetic texture that alters the Berry curvature. Our work employs band topology to control the pairing amplitude of spin-triplet Cooper pairs in a single chiral antiferromagnet.

Vortex arrangement in an ultrathin superconducting bilayer disc

A direct current superconducting quantum interference device (SQUID) fixed on a recently developed superconducting bilayer disc is effective for studying fractional magnetic quantum and fractional vortices. However, such devices cannot magnetically image fractional vortices. To overcome this limitation, we compared the bilayer SQUID signal with and without a pinhole to experimentally investigate the vortex arrangement. We measured the amount of flux passing through the SQUID ring via the SQUID signal phase shift, which reflected the vortex locations. We measured the voltages of a series of 100 SQUIDs in which the bilayer discs were present under each SQUID. When we applied a constant bias current of 5.5 μA and swept the external magnetic field, a voltage appeared when the amount of magnetic flux passing through the SQUID ring became close to half of the flux quantum. A sharp voltage peak was obtained for the bilayers with a pinhole, whereas these peaks were broader for the bilayers without a pinhole. We evaluated the amount of the flux originating from the vortices trapped in the bilayer discs and estimated the vortex locations from the peak position. When the amount of flux originating from the trapped vortices increased, the bilayer discs without a pinhole exhibited reduced amounts of flux originating from the trapped vortices by approximately 15% compared to the bilayer discs with a pinhole. When no pinhole was present, the vortex locations varied inside the SQUID ring. The results indicated that the vortices were located at the same position for all 100 bilayer discs with a pinhole; therefore, we concluded that the pinholes must trap the vortices.

Generation of microwave photon perfect W states of three coupled superconducting resonators

We propose an efficient method for the generation of perfect W states on three microwave superconducting resonators, of which the two nearest neighbors are coupled by a symmetric direct current superconducting quantum interference device (dc-SQUID). With suitable external magnetic fluxes applied to the dc-SQUID symmetry loops, on-chip tunable interactions between neighboring resonators can be realized, and different perfect W states can be deterministically created on-demand in one step. Numerical simulations show that high-fidelity target states can be generated and our scheme is robust against imperfect parameter tuning and environment-induced decoherence. The present work may have potential applications for implementing quantum computation and quantum information processing based on microwave photons.

Atomically Precise Platinum Carbonyl Nanoclusters: Synthesis, Total Structure, and Electrochemical Investigation of [Pt27(CO)31]4- Displaying a Defective Structure.

The molecular Pt nanocluster [Pt27(CO)31]4– (14–) was obtained by thermal decomposition of [Pt15(CO)30]2– in tetrahydrofuran under a H2 atmosphere. The reaction of 14– with increasing amounts of HBF4·Et2O afforded the previously reported [Pt26(CO)32]2– (32–) and [Pt26(CO)32]− (3–). The new nanocluster 14– was characterized by IR and UV–visible spectroscopy, single-crystal X-ray diffraction, direct-current superconducting quantum interference device magnetometry, cyclic voltammetry, IR spectroelectrochemistry (IR SEC), and electrochemical impedance spectroscopy. The cluster displays a cubic-close-packed Pt27 framework generated by the overlapping of four ABCA layers, composed of 3, 7, 11, and 6 atoms, respectively, that encapsulates a fully interstitial Pt4 tetrahedron. One Pt atom is missing within layer 3, and this defect (vacancy) generates local deformations within layers 2 and 3. These local deformations tend to repair the defect (missing atom) and increase the number of Pt–Pt bonding contacts, minimizing the total energy. The cluster 14– is perfectly diamagnetic and displays a rich electrochemical behavior. Indeed, six different oxidation states have been characterized by IR SEC, unraveling the series of 1n– (n = 3–8) isostructural nanoclusters. Computational studies have been carried out to further support the interpretation of the experimental data.

Rotation-driven transition into coexistent Josephson modes in an atomtronic dc superconducting quantum interference device

By means of a two-mode model, we show that transitions to different arrays of coexistent regimes in the phase space can be attained by rotating a double-well system, which consists of a toroidal condensate with two diametrically placed barriers. Such a configuration corresponds to the atomtronic counterpart of the well-known direct-current superconducting quantum interference device. Due to the phase gradient experimented by the on-site localized functions when the system is subject to rotation, a phase difference appears on each junction in order to satisfy the quantization of the velocity field around the torus. We demonstrate that such a phase can produce a significant change on the relative values of different types of hopping parameters. In particular, we show that within a determined rotation frequency interval, a hopping parameter, usually disregarded in nonrotating systems, turns out to rule the dynamics. At the limits of such a frequency interval, bifurcations of the stationary points occur, which substantially change the phase space portrait that describes the orbits of the macroscopic canonical conjugate variables. We analyze the emerging dynamics that combines the $0$ and $\pi$ Josephson modes, and evaluate the small-oscillation time-periods of such orbits at the frequency range where each mode survives. All the findings predicted by the model are confirmed by Gross-Pitaevskii simulations.

Noise Performance Comparison Between Current- and Voltage-Bias Readout Schemes for DC SQUID

There are two bias schemes, namely current-bias and voltage-bias, for the directly coupled readout circuit of direct current superconducting quantum interference device (dc SQUID). In order to find out the difference between two bias schemes, we present the theoretical analyses on their equivalent circuits, working point locations, and noise performances. The theoretical analysis and experimental validation prove that the voltage-bias scheme achieves a larger transfer coefficient and exhibits a better noise performance than the current-bias scheme in the reading of conventional dc SQUID.

Solitonic black holes induced by magnetic solitons in a dc-SQUID array transmission line coupled with a magnetic chain

We investigate analog black holes generated in an array of direct current superconducting quantum interference devices (dc SQUIDs) coupled in parallel with a one-dimensional chain of single-domain nanomagnets. Magnetic solitons in the chain provide magnetic fields perpendicular to the SQUID array. This leads to the spatially varying velocities of electromagnetic waves through the nonlinear inductance in the array required for creating effective event horizons, resulting in analog black holes. We derive the Hawking temperature in this analog black hole based on the tunneling mechanism for Hawking radiation. The formula reflecting the soliton properties shows that Hawking radiation is observable using the current state-of-the-art technologies.

Flux ramp modulation based MHz frequency-division dc-SQUID multiplexer

We present a MHz frequency-division direct-current superconducting quantum interference device (dc-SQUID) multiplexer that is based on flux ramp modulation and a series array of N identical current-sensing dc-SQUIDs with tightly coupled input coil. By running a periodic, sawtooth-shaped current signal through additional modulation coils being tightly but non-uniformly coupled to the individual SQUIDs, the voltage drop across the array changes according to the sum of the flux-to-voltage characteristics of the individual SQUIDs within each cycle of the modulation signal. In this mode of operation, an input signal injected in the input coil of one of the SQUIDs and being quasi-static within a time frame adds a constant flux offset and leads to a phase shift of the associated SQUID characteristics. The latter is proportional to the input signal and can be inferred by channelizing and down-converting the sampled array output voltage. Using a prototype multiplexer as well as custom readout electronics, we demonstrate the simultaneous readout of four signal sources with MHz bandwidth per channel.

Phase-tunable thermoelectricity in a Josephson junction

Superconducting tunnel junctions constitute the units of superconducting quantum circuits and are massively used both for quantum sensing and quantum computation. In previous works, we predicted the existence of a nonlinear thermoelectric effect in a electron-hole symmetric system, namely, a thermally biased tunnel junction between two different superconductors, where the Josephson effect is suppressed. In this paper we investigate the impact of the phase-coherent contributions on the thermoelectric effect, by tuning the size of the Josephson coupling changing the flux of a direct-current Superconducting Quantum Interference Device (dc-SQUID). For a suppressed Josephson coupling, the system generates a finite average thermoelectric signal, combined to an oscillation due to the standard ac Josephson phenomenology. At large Josephson couplings, the thermoelectricity induces an oscillatory behaviour with zero average value of the current/voltage with an amplitude and a frequency associated to the Josephson coupling strength, and ultimately tuned by the dc-SQUID magnetic flux. In conclusion, we demonstrate to be able to control the dynamics of the spontaneous breaking of the electron-hole symmetry. Furthermore, we compute how the flux applied to the dc-SQUID and the lumped elements of the circuit determine the frequency of the thermoelectric signal across the structure, and we envision a frequency modulation application.

Analysis of LC-Resonance in DC SQUID Using Dynamic System Model

Direct current superconducting quantum interference device (dc SQUID) can be regarded as a hybrid system with two nonlinear Josephson currents driving a linear network consists of conventional resistors (R), inductors (L), and capacitors (C). There must be LC resonances inside the SQUID loop with influences on its static current-voltage characteristics. To study the working principle of the LC resonance, we build an equivalent dynamic system model transformed directly from the circuit equations of dc SQUID. With dynamic system analyses in both dc and alternating current (ac) domains, we derive the analytical expressions of the resonance point with only the RLC circuit parameters. The analytical derivation proves that the LC resonance inside a symmetrical dc SQUID with resonance frequency ω = 1/√LC results in the flux modulation suppression with I-V curves concentrated on one resonance point. Based on those mathematical expressions, the resonance point is easily indentified from the measured I-V curves, and can be utilized for practical SQUID parameters characterization.

Control of the transition frequency of a superconducting flux qubit by longitudinal coupling to the photon number degree of freedom in a resonator

We control transition frequency of a superconducting flux qubit coupled to a frequency-tunable resonator comprising a direct current superconducting quantum interference device (dc-SQUID) by microwave driving. The dc-SQUID mediates the coupling between microwave photons in the resonator and a flux qubit. The polarity of the frequency shift depends on the sign of the flux bias for the qubit and can be both positive and negative. The absolute value of the frequency shift becomes larger by increasing the photon number in the resonator. These behaviors are reproduced by a model considering the magnetic interaction between the flux qubit and dc-SQUID. The tuning range of the transition frequency of the flux qubit reaches $\approx$ 1.9 GHz, which is much larger than the ac Stark/Lamb shift observed in the dispersive regime using typical circuit quantum electrodynamics devices.

Electron spin resonance with up to 20 spin sensitivity measured using a superconducting flux qubit

We report on electron spin resonance spectroscopy measurements using a superconducting flux qubit with a sensing volume of 6 fl. The qubit is read out using a frequency-tunable Josephson bifurcation amplifier, which leads to an inferred measurement sensitivity of about 20 spins in a 1 s measurement. This sensitivity represents an order of magnitude improvement when compared to flux-qubit schemes using a direct current-superconducting quantum interference device switching readout. Furthermore, noise spectroscopy reveals that the sensitivity is limited by flicker (1/f) flux noise.

Analogue Soliton with Variable Mass in Super-Conducting Quantum Interference Devices**Supported by the National Natural Science Foundation of China under Grant Nos. 11875025, 11475061, and 11675052, and the CAS Key Laboratory for Research in Galaxies and Cosmology, Chinese Academy of Sciences under Grant No. 18010203.

It is difficult to investigate the behavior of solitons in realistic inhomogeneity media in experiment due to inhomogeneity of the media and noise from the unwanted coupling. We propose to use a waveguide-like transmission line which is based on direct-current super-conducting quantum interference devices to simulate behavior of solitons because we find that the behavior of the node flux in this transmission is similar to that of solitons with variable mass.

Ultrasensitive Magnetic Field Sensors for Biomedical Applications.

The development of magnetic field sensors for biomedical applications primarily focuses on equivalent magnetic noise reduction or overall design improvement in order to make them smaller and cheaper while keeping the required values of a limit of detection. One of the cutting-edge topics today is the use of magnetic field sensors for applications such as magnetocardiography, magnetotomography, magnetomyography, magnetoneurography, or their application in point-of-care devices. This introductory review focuses on modern magnetic field sensors suitable for biomedicine applications from a physical point of view and provides an overview of recent studies in this field. Types of magnetic field sensors include direct current superconducting quantum interference devices, search coil, fluxgate, magnetoelectric, giant magneto-impedance, anisotropic/giant/tunneling magnetoresistance, optically pumped, cavity optomechanical, Hall effect, magnetoelastic, spin wave interferometry, and those based on the behavior of nitrogen-vacancy centers in the atomic lattice of diamond.

Magnetic Properties of Poly(trimethylene terephthalate‐block‐Poly(tetramethylene oxide) Copolymer Nanocomposites Reinforced by Graphene Oxide–Fe3O4 Hybrid Nanoparticles

Thermoplastic elastomeric nanocomposites based on poly(trimethylene terephthalate‐block‐poly(tetramethylene oxide) copolymer (PTT‐PTMO) and graphene oxide (GO)–Fe3O4 nanoparticles hybrid are prepared by in situ polymerization. Superparamagnetic GO–Fe3O4 hybrid nanoparticles before introducing to elastomeric matrix are characterized by X‐ray photoelectron spectroscopy (XPS), X‐ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The effect of loading (0.3 and 0.5 wt%) of GO–Fe3O4 nanoparticle hybrid on the phase structure, tensile, and magnetic properties of synthesized nanocomposites is investigated. The phase structure of nanocomposites is evaluated by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). Dispersion of GO–Fe3O4 nanoparticles in elastomeric matrix is evaluated by transmission electron microscopy (TEM). Magnetic properties of GO–Fe3O4 nanoparticle hybrid and nanocomposites with their content are characterized using two different techniques: direct current superconducting quantum interference device (dc SQUID) magnetization measurements as a function of temperature (from 2 to 300 K) and external magnetic field and ferromagnetic resonance (FMR) at microwave frequency.

Two-terminal wide range SQUID amplifier with hysteretic quasi linear transfer characteristics

To implement wide range measurement using direct current Superconducting Quantum Interference Device (dc SQUID), we presented a simple two-terminal SQUID amplifier consists of a shunt resistor and a conventional SQUID in series with feedback coil. Only two terminals are led out with coaxial cable for high speed and low heat loss. Periodically repeated quasi linear flux-to-voltage transfer characteristics are achieved by internal flux feedback. Wide range flux input is modulated by the SQUID amplifier into voltage signal with rapid transient edges. The reconstructed output is the synthesis of measured voltage and counts of flux-quanta counting. Working principles of linearized SQUID amplifier were simulated and experimentally verified. Wide range measurement with error within ±0.1Φ0 was demonstrated in the experiment.

SQUID Gradiometer Module for Fetal Magnetocardiography Measurements Inside a Thin Magnetically Shielded Room

Gradiometer module plays a significant role in fetal magnetocardiography (fMCG) system and its performance determines the signal qualities directly. In order to obtain the best signal-to-noise ratio (SNR) of the fMCG signals in a thin magnetically shielded room (tMSR), the baseline of the axial gradiometers need to be optimized. In this paper, direct current superconducting quantum interference devices which is composed of weakly damped Josephson Junctions with βc > 3 and has a flux-to-voltage transfer coefficient of 500 μ V/Φ0 is employed for our gradiometers. Superconducting connectivity technique is utilized to connect input coil and pickup coil. First-order gradiometers with different baselines are thus developed. After the evaluation of the magnetic flux density and gradient flux density inside the tMSR, measurement of fMCG signals are taken with the gradiometers mentioned earlier. The simulated fetal QRS peak signals about 2.5 pT could easily be identified in the realtime gradiometer module outputs with an SNR larger than 11 dB.

Fabrication and measurement of Nb-based SQUID magnetometer

We have developed low-noise direct current-superconducting quantum interference devices (DC-SQUIDs) based on Nb/Al-AlO x /Nb Josephson junctions on thermally oxidized silicon substrate. The SQUID fabrication was realized with a simple 5-mask-level process, in which one single insulting layer (SiO 2 ) was used not only to insulate different metal layers, but also to protect the junction shunt resistor from oxidation or corrosion. In our SQUID design, a washer-type loop with an inductance of 140 pH and a coupled flux transformer with a multi-turn spiral input coil connecting to a pickup coil of 8.5 × 8.5 mm 2 were used, thus leading to a SQUID magnetometer with magnetic field sensitivity of 0.7 nT/Φ 0 . We measured the voltage-flux characteristics and flux noise in superconducting niobium shielding at 4.2 K. The measured white flux noise in a flux-locked loop was 5 µΦ 0 /√Hz, corresponding to magnetic filed noise of 3.5 fT/√Hz.