Dc SQUID Research Articles

A Spontaneous Magnetic Moment in an Organic Radical: Synthesis and Characterization of Benzodioxepinyl-1,3,2-dithiazolyl.

The synthesis of the 1,3,2-dithiazolyl radical (1) derived from 3,4-dihydro-2H-1,5-benzodioxepine is described. Crystals of 1 were grown by vacuum sublimation and adopt the orthorhombic space group Pbca. DC SQUID magnetometry reveals Curie-Weiss behavior for T > 20 K (C = 0.376 emu K mol-1 and θ = +5.7 K) consistent with local ferromagnetic interactions. The presence of weaker antiferromagnetic interactions led to magnetic ordering as a canted antiferromagnet (TN = 3.8 K) with a small spontaneous moment (2.5 × 10-4 Nβ) and a small coercivity (ca. 8 Oe) at 2 K. Magnetic ordering was reflected in a field dependence of the magnetic susceptibility below 3.8 K and a peak in the low temperature heat capacity data. A canting angle of 0.08° was estimated from M vs H data.

Induction of orbital currents and Kapitza stabilization in superconducting circuits with Laguerre-Gaussian microwave beams

We investigate the effects of a Laguerre-Gaussian (LG) beam on the superconducting state. We show that the vortex angular momentum of a LG beam affects the superconducting state and induces currents. The induction of the current by microwave radiation is illustrated on a Josephson loop and SQUID devices. In particular, we establish that coupling a dc SQUID to the ac magnetic flux of a LG beam can stabilize the π phase in the SQUID. This can happen via developing a global or local minimum in the effective potential at π. In the latter case, this happens via the Kapitza mechanism. Published by the American Physical Society 2024

Linear cooling of a levitated micromagnetic cylinder by vibration

We report feedback cooling of translational and librational degrees of freedom of a levitated m icromagnet cylinder, utilizing a piezoelectric actuator to apply linear feedback to high-Q mechanical modes. The normal modes are measured with a superconducting pick-up coil coupled to a dc SQUID, and phase information is fed back to the piezoelectric actuator to feedback cool a center-of-mass mode to ∼7K, and a librational mode to 830±200mK. Q-factors of 1.0×107 are evaluated for the center-of-mass mode. We find that it is plausible to achieve ground state cooling of the center-of-mass mode by introducing vibration isolation, optimizing the geometry of the pick-up coil to focus on the specific mode of interest and utilizing a state-of-the-art SQUID for detection. Published by the American Physical Society 2024

Observation of critical scaling in spin glasses below Tc using thermoremanent magnetization

Time-dependent thermoremanent magnetization (TRM) studies have been instrumental in probing energy dynamics within the spin glass phase. In this paper, we review the evolution of the TRM experiment over the last half century and discuss some aspects related to how it has been used in the understanding of spin glasses. We also report on recent experiments using high-resolution DC SQUID magnetometry to probe the TRM at temperatures less than but near to the transition temperature Tc. These experiments have been performed as a function of waiting time, temperature, and five different magnetic fields. We find that as the transition temperature is approached from below, the characteristic time scale of TRM is suppressed up to several orders of magnitude in time. In the highest-temperature region, we find that the waiting time effect subsides, and a waiting time-independent crossover line is reached. We also find that increasing the magnetic field further suppresses the crossover line. Using a first-principles energy argument across the crossover line, we derive an equation that is an excellent fit to the crossover lines for all magnetic fields probed. The data show strong evidence for critical slowing down and an H = 0 Oe phase transition.

Superconducting Quantum Magnetometer Based on Flux Focusing Effect for High-Sensitivity Applications.

A superconducting quantum magnetometer for high-sensitivity applications has been developed by exploiting the flux focusing of the superconducting loop. Unlike conventional dc SQUID magnetometers that use a superconducting flux transformer or a multiloop design, in this case, a very simple design has been employed. It consists of a bare dc SQUID with a large washer-shaped superconducting ring in order to guarantee a magnetic field sensitivity BΦ less than one nT/Φ0. The degradation of the characteristics of the device due to an inevitable high value of the inductance parameter βL was successfully compensated by damping the inductance of the dc SQUID. The size of the magnetometer, coinciding with that of the washer, is 5 × 5 mm2 and the spectral density of the magnetic field noise is 8 fT/√Hz with a low frequency noise knee of two Hz. The excellent performance of this simple magnetometer makes it usable for all high-sensitivity applications including magnetoencephalography.

Design dependence of noise in Nb-based dc SQUIDs

Noise in superconducting quantum interference devices (dc SQUIDs) with different design, based on Nb/AlOx/Nb technology, has been systematically measured at temperature T = 4.2 K. We demonstrate that for all our devices the power spectral density of the white noise (above a frequency of about 1 kHz) is determined by the shunt resistors, reaching about 5 times Planck’s constant. The low-frequency noise level was measured for washer- and multi-loop-type SQUIDs with systematically varied effective area, SQUID loop perimeter, and inductance. We show that at 1 Hz the noise spectral density is approximately 40 times higher than the white noise level and its scaling with washer width and effective area is negligible. These results are incompatible with several models of the flicker noise source. Possible origins of this noise are discussed.

Syntropic spin alignment at the interface between ferromagnetic and superconducting nitrides.

The magnetic correlations at the superconductor/ferromagnet (S/F) interfaces play a crucial role in realizing dissipation-less spin-based logic and memory technologies, such as triplet-supercurrent spin-valves and 'π' Josephson junctions. Here we report the observation of an induced large magnetic moment at high-quality nitride S/F interfaces. Using polarized neutron reflectometry and DC SQUID measurements, we quantitatively determined the magnetization profile of the S/F bilayer and confirmed that the induced magnetic moment in the adjacent superconductor only exists below T C. Interestingly, the direction of the induced moment in the superconductors was unexpectedly parallel to that in the ferromagnet, which contrasts with earlier findings in S/F heterostructures based on metals or oxides. First-principles calculations verified that the unusual interfacial spin texture observed in our study was caused by the Heisenberg direct exchange coupling with constant J∼4.28 meV through d-orbital overlapping and severe charge transfer across the interfaces. Our work establishes an incisive experimental probe for understanding the magnetic proximity behavior at S/F interfaces and provides a prototype epitaxial 'building block' for superconducting spintronics.

Measurements of DC SQUID Damping Effects on Superconducting Resonant Circuits

We present experimental measurements of SQUID-induced damping effects on strongly coupled, lumped-element resonators in the 500kHz-1 MHz frequency range. While dc SQUIDs are commonly used for sensitive readout of electromagnetic signals such as in lumped-element axion dark matter searches, coupling a dc SQUID to a resonant circuit modifies the circuit's resonance frequency, quality factor, noise, and impedance. These parameters have a direct impact on the science reach of axion dark matter detectors, making it important to understand SQUID damping contributions. We describe a helium dip probe and two-stage SQUID readout scheme used to probe SQUID damping effects. Measurements from this setup inform the design and operation of both future test platforms as well as next-generation dark-matter experiments.

Influence of Frustration Effects on the Critical Current of DC SQUID

In this paper, we conducted the calculation of the critical current of DC SQUID based on the Josephson junction on a multi-band superconductor with frustration effect. It is shown that the critical current of DC SQUID on the frustrated multi-band superconductor with a small geometrical inductance of the loop is determined by the supercurrent amplitude in different channels and by the external magnetic field. In the case of a DC SQUID with high inductance, frustration effects can be ignored.

Bi-SQUID Versus dc SQUID in Flux-Driven Traveling-Wave Parametric Amplifier

Characteristics of artificial waveguide lines composed of finite-size artificial cells are analyzed and discussed with relation to designing traveling-wave parametric amplifiers. From this point of view, the flux-driven amplifier suggested recently can be considered as the most successful amplifier design. To increase dynamic range of the amplifier, dc SQUID cells of the artificial line in use can be substituted for bi-SQUID cells. The 1 dB compression points of the amplifier gain are calculated and compared for both types of cells.

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.

SUPERCONDUCTIVITY IN THE QUATERNARY INTERMETALLIC COMPOUND YNI2-XTXB2C

We report here our observance of superconductivity in compounds derived from the quaternary intermetallic superconductor YNi2B2C by partial substitutions for Ni. Other 3d transition metals T Î {Ti, V, Cr, Mn, Fe, Co, Cu, Zn} were used in replacement fractions x = 0.05 and x = 0.10. AC magnetic susceptibility cAC showed all except T = Zn had the characteristic diamagnetic response seen in superconductors. DC SQUID magnetometry on several samples confirmed type II superconducting behavior. X-ray diffraction showed a crystal structure similar to the parent compound for all but Zn. Some of the x = 0.10 compounds appeared to have minor secondary phases present despite annealing.

Активный сверхпроводящий терагерцовый детектор

The concept of an active superconducting terahertz detector for array applications is based on the combination of an RFTES bolometer and a microwave preamplifier based on a DC SQUID within the common integrated circuit providing the maximum, theoretically possible, signal transmission from the sensor to the amplifier. The problems associated with the design and positioning of the amplifier, that restrict the functionality and sensitivity of the ultra-low temperature detector, are considered. For the first time, a method for connecting a SQUID amplifier to an RFTES bolometer using the principle of partial loads of a resonator has been proposed and analyzed. The presented electromagnetic model of the active detector is suitable for optimization of RFTES, MKID and other detectors using high-Q superconducting planar resonators.

Hybrid Molecular Magnets with Lanthanide- and Countercation-Mediated Interfacial Electron Transfer between Phthalocyanine and Polyoxovanadate.

A series of {V12}-nuclearity polyoxovanadate cages covalently functionalized with one or sandwiched by two phthalocyaninato (Pc) lanthanide (Ln) moieties via V-O-Ln bonds were prepared and fully characterized for paramagnetic Ln = SmIII-ErIII and diamagnetic Ln = LuIII, including YIII. The LnPc-functionalized {V12O32} cages with fully oxidized vanadium centers in the ground state were isolated as (nBu4N)3[HV12O32Cl(LnPc)] and (nBu4N)2[HV12O32Cl(LnPc)2] compounds. As corroborated by a combined experimental (EPR, DC and AC SQUID, laser photolysis transient absorption spectroscopy, and electrochemistry) and computational (DFT, MD, and model Hamiltonian approach) methods, the compounds feature intra- and intermolecular electron transfer that is responsible for a partial reduction at V(3d) centers from VV to VIV in the solid state and at high sample concentrations. The effects are generally Ln dependent and are clearly demonstrated for the (nBu4N)3[HV12O32Cl(LnPc)] representative with Ln = LuIII or DyIII. Intramolecular charge transfer takes place for Ln = LuIII and occurs from a Pc ligand via the Ln center to the {V12O32} core of the same molecule, whereas for Ln = DyIII, only intermolecular charge transfer is allowed, which is realized from Pc in one molecule to the {V12O32} core of another molecule usually via the nBu4N+ countercation. For all Ln but DyIII, two of these phenomena may be present in different proportions. Besides, it is demonstrated that (nBu4N)3[HV12O32Cl(DyPc)] is a field-induced single molecule magnet with a maximal relaxation time of the order 10-3 s. The obtained results open up the way to further exploration and fine-tuning of these three modular molecular nanocomposites regarding tailoring and control of their Ln-dependent charge-separated states (induced by intramolecular transfer) and relaxation dynamics as well as of electron hopping between molecules. This should enable us to realize ultra-sensitive polyoxometalate powered quasi-superconductors, sensors, and data storage/processing materials for quantum technologies and neuromorphic computing.

Frequency Adjustable Resonator as a Tunable Coupler for Xmon Qubits

We propose a scheme of tunable coupler based on a frequency adjustable resonator for scalable quantum integrated circuits. One side of the T-shape quarter-wave resonator is short to the ground through a DC SQUID, which is used to tune the frequency of the resonator coupler. In the other side with two open ends, either end capacitively couples to a different Xmon qubit, which determines the effective coupling between the two qubits. The fundamental mode of the tunable resonator dominates the interaction with the qubits and functions as a tunable coupler to switch off/on the qubit–qubit interaction. The required external magnetic flux for the tunable coupler’s switching off frequency is affected by the asymmetry degree of the DC SQUID, which can help to choose a better noise environment (of the qubits) induced by the resonator coupler. The T-shape tunable resonator can help to reduce the direct interaction between two qubits, which should be an important way to suppress the ZZ crosstalk and realize high-fidelity quantum gates. The DC SQUID is several millimeters away from the Xmon qubits, which in principle creates less flux noises compared with the transmon-type coupler.

Flux-Pumped Josephson Travelling-Wave Parametric Amplifiers Based on Bi-SQUID Cells

We have proposed a new design of the Josephson traveling-wave parametric amplifier (JTWPA) capable of providing much higher spurious-free dynamic range (SFDR). For this purpose, we substitute dc SQUIDs for bi-SQUIDs in the used artificial signal transmission line driven by external travelling-wave magnetic flux. The increase of SFDR follows from the increased linearity domain of the phase-current relation in bi-SQUID as compared to dc SQUID. Optimal parameters for bi-SQUIDs have been determined. The first version of JTWPA was modeled to be fabricated using niobium process.

Study of the Flux-to-Voltage Characteristics of DC SQUID With a Small Screening Parameter and a Large Stewart-McCumber Parameter

It is known that SQUID has three important parameters, namely, Stewart-McCumber parameter <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> , Screening parameter <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> and thermal noise parameter <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Γ</i> . They play a very important role in SQUID performance, especially <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> . Recently, researchers extended the SQUID operating area to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</i> c > 1 due to a large noise parameter <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Γ</i> in experiments. When <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> > 1, SQUID exhibits a large flux-voltage transfer coefficient <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∂V/∂</i> Φ, which is beneficial in reducing the noise contribution of the readout circuit <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">δ</i> Φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> . From the simulation results, we found that when <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> >1, SQUID produces a larger <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∂V/∂</i> Φ than <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> <1, and there was no hysteresis, at the same time, we also observed that <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∂V/∂</i> Φ increases with the decrease of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> . In order to further verify our findings, we selected two groups of SQUIDs for experiments ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> < 1, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> > 1), by changing the value of the shunt resistance on the Josephson junction to adjust the value of changing <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> , and then we observe the value of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∂V/∂</i> Φ. Finally, we find that when <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> < 1, the flux-to-voltage transfer coefficient <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∂V/∂</i> Φ of SQUID increases while <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> increasing. A large <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∂V/∂</i> Φ will help us to obtain an optimal SQUID measure system.

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.

Entanglement dynamics of a dc SQUID interacting with a single-mode radiation field

In this work, we study the coupling between a superconducting device as a dc SQUID, simulated from an artificial atom with two degrees of freedom, and a single-mode radiation field for the information transference process. We demonstrate that the population transfer among the energy levels of the artificial atom yields entanglement dynamics, which leads to the generation of a pair of photons. Moreover, we show the quantum information transference between the internal modes of the superconducting device, initially in a maximally entangled state, and the radiation field. The artificial atom absorbs the photon, and the radiation field modes become entangled as quantum coherence is transmitted from the superconducting device to the photons. These results strengthen the applicability of superconducting devices for the transference of quantum information, contributing to promising applications in emerging quantum technologies.

Return current of dc SQUID based on tunnel Josephson junctions with unconventional current-phase relation

We carried out the analysis of the return current of dc SQUID based on tunnel Josephson junction with unconventional current-phase relation. We analyzed two cases of current-phase relation with additional terms to the first harmonic sin φ: a case of the second harmonic sin 2φ and the case of the fractional term sin (φ/2). It is shown that the changing of the return current of dc SQUID on junctions with unconventional current-phase relation is determined by the amplitude of the second term in current-phase relation, geometrical inductance, and external magnetic field.