Pickup Loop Research Articles

Remote sensing of a levitated superconductor with a flux-tunable microwave cavity

We present a cavity-electromechanical system comprising a superconducting quantum interference device which is embedded in a microwave resonator and coupled via a pickup loop to a 6-μg magnetically levitated superconducting sphere. The motion of the sphere in the magnetic trap induces a frequency shift in the SQUID-cavity system. We use microwave spectroscopy to characterize the system, and we demonstrate that the electromechanical interaction is tunable. The measured displacement sensitivity of 10−7m/Hz defines a path towards ground-state cooling of levitated particles with Planck-scale masses at millikelvin environment temperatures. Published by the American Physical Society 2024

Design of a High-Tc SQUID Planer Gradiometer and Evaluation of Sensitivity Distribution

When using SQUID magnetic sensors to inspect metallic contaminants in magnetic matrices, such as battery components, it is advantageous to use a gradiometer rather than a magnetometer because a higher signal-to-noise ratio can be obtained. In this study, we reviewed the design of directly coupled SQUID gradiometers and optimized their pickup loops. Several gradiometers with pickup loops of different sizes were fabricated. To evaluate these gradiometers, we applied a small magnetic field produced by a magnetic field coil with a small diameter (effective diameter: 0.65 mm) to them. The sensitivity distribution of these gradiometers was evaluated using a special mechanism that allows for complete scanning over the device substrate. As a result, the magnitude of the peak value could be explained by an inductance imbalance between the pickup loop and the SQUID. Finally, we could find the optimal pickup coil design conditions. Furthermore, we could capture the sensitivity distribution of a small SQUID loop placed in the center of the device.

High-$T_{C}$ Superconducting Quantum Interference Device Implemented on a Pulsed Tube Cooler for 1 to 50 K Materials Characterization

Here we report the fabrication and electrical characterization of high-transition temperature SQUIDs directly written by helium ion beam irradiation for use in Quantum Design's pulsed tube cooler based physical property measurement system. Our device consists of a SQUID with a direct coupled 300 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m diameter flux focusing pickup loop. When DC biased above the critical current, we observe a modulation in the voltage with a peak-to-peak amplitude of 500 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> V, while having periodicity of 275 nT per flux quantum. The SQUID chip was mounted to the temperature-controlled system equipped with optically aligned piezo scanners which allow the accurate positioning of the sample less than 100 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m from the sensor. The system functions as a magnetic properties characterization tool which can operate over a wide temperature range (1 - 50K) without thermally isolating the SQUID from the sample.

Quality assessment of SUS316L fabricated by metal additive manufacturing with eddy current inspection

Guaranteeing the quality of additive manufacturing (AM) components after fabrication is very important. In this study, a discrete non-destructive evaluation (NDE) system that includes an eddy current testing (ECT) probe and a fluxgate sensor was developed. In conventional ECT, the defect detection depth is limited by the skin depth of the material under investigation and requires a high frequency of the order of MHz. However, in the NDE system, the high sensitivity of the fluxgate sensor and the use of an ECT coil make it possible to detect subsurface defects at a relatively low frequency. A double-excitation coil was used to excite eddy currents in the specimen at relatively low frequencies, whereas a planar differential coil was used as a pick-up loop connected to an input coil that was inductively coupled to the fluxgate sensor. Based on the developed NDE system, five stainless steel (SUS316L) specimens fabricated by powder bed fusion were inspected. A strong correlation was observed between the detected signals and internal porosity, which was confirmed using X-ray computed tomography images. Although this approach cannot detect the local existence of defects, it can be applied for quality assessment, such as density and distribution of porosity in metallic parts fabricated by AM.

Calibration method for planar SQUID gradiometers based on the magnetic gradient tensor components

The accuracy of tesla/volt calibration factors for superconducting quantum interference devices (SQUID) gradiometers is essential to take advantage of the magnetic gradient tensor (MGT), but the accuracy of the published calibration procedures ranges from tenths of percent to a few percent. In this paper, we propose an efficient calibration method for planar SQUID gradiometers based on MGT components. The factors affecting the calibration accuracy and the optimal inclination angle for calibration were investigated by analyzing the response characteristics of planar SQUID gradiometers to the MGT components. By superposition of square coils and gridding of pickup loops, numerical models of the response of planar gradiometers to different types of Maxwell coils were built to simulate theoretical calibration accuracy and calibration errors due to position, attitude and imbalance. We built a calibration setup to achieve high-precision simultaneous calibration of multi-channel planar SQUID gradiometers and investigated the effect of eddy current on the calibration accuracy. The results show that simulation and experimental results are in general agreement by taking into account various error factors. A calibration error of about 0.4% is achieved.

Vortex dynamics induced by scanning SQUID susceptometry

We measured the local magnetic response of a niobium thin film by applying a millitesla-scale AC magnetic field using a micron-scale field coil and detecting the response with a micron-scale pickup loop in a scanning superconducting quantum interference device (SQUID) susceptometry measurement. Near the film's critical temperature, we observed a step-like nonlinear and dissipative magnetic response due to the dynamics of a small number of vortex-antivortex pairs induced in the film by the local applied AC field. We modeled the dynamics of the measurement using a combined two-dimensional London-Maxwell and time-dependent Ginzburg-Landau approach, allowing us to construct a detailed real-space picture of the vortex motion causing the observed dissipative response. This work pushes scanning SQUID susceptometry of two-dimensional superconductors beyond the regime of linear response and lays the foundation for microscopic studies of vortex dynamics and pinning in superconducting devices and more exotic materials systems.

Electro-optic Bdot probe measurement of magnetic fluctuations in plasma.

We propose a combined use of a Pockels electro-optic sensor with a pickup loop coil (Bdot probe) for the measurement of magnetic fluctuations in plasmas. In this method, induced fluctuating voltage on the coil loop is converted into an optical signal by a compact electro-optic sensor in the vicinity of the measurement point and is transferred across optical fiber that is unaffected by electric noise or capacitive load issues. Compared with conventional Bdot probes, the electro-optic Bdot probe (1) is electrically isolated and free from noise pickup caused by the metallic transmission line and (2) can be operated at a higher-frequency range because of the smaller capacitance of the operation circuit, both of which are suitable for many plasma experiments. Conversely, the sensitivity of the current electro-optic Bdot probe arrangement is still significantly lower than that of conventional Bdot probes. A preliminary measurement result with the electro-optic Bdot probe showed the detection of a magnetic fluctuation signal around the cyclotron frequency range in the RT-1 magnetospheric plasma experiment.

Mode structure measurements of ion cyclotron emission and sub-cyclotron modes on DIII-D

We report mode structure measurements of coherent ion cyclotron emission (ICE) and sub-cyclotron modes on DIII-D. Through a dedicated experiment, we aimed to characterize a variety of modes via the upgraded ICE diagnostic in both L- and H-mode plasmas. In the L-mode plasmas, autopower spectrum peaks at harmonics of the ion cyclotron frequency f ci were observed and are localized in the core of the plasma. Sub-cyclotron modes (f ∼ 0.5f ci) were also observed in L-mode plasmas when the toroidal magnetic field strength was lowered from B T = 2.17 T. In H-mode plasmas, many ICE harmonics localized to the plasma edge were observed, with some exceeding the nominal ICE diagnostic bandwidth of f ∈ [0, 100] MHz. Polarization estimates made using an orthogonal pair of vertical and horizontal magnetic pickup loops on the outer wall of the machine indicate that ICE and sub-cyclotron modes have compressional polarization at the plasma edge, the latter being consistent with simulation efforts for comparable DIII-D plasmas. For all modes, the same harmonics are observed on both the centerpost and outer wall loops, indicating that ICE is poloidally extended. Finally, toroidal mode numbers were calculated using three outer wall loops for both core ICE and sub-cyclotron modes in L-mode plasmas. The sub-cyclotron case served as a benchmark for our calculation method, with measured numbers of n ∈ [−24, −18] roughly agreeing with heuristic estimates of n ∈ [−20, −13]. Core ICE mode numbers were measured to be n ∈ [−10, 5], which is comparable to measurements made on other machines.

Novel Search for High-Frequency Gravitational Waves with Low-Mass Axion Haloscopes.

Gravitational waves (GWs) generate oscillating electromagnetic effects in the vicinity of external electric and magnetic fields. We discuss this phenomenon with a particular focus on reinterpreting the results of axion haloscopes based on lumped-element detectors, which probe GWs in the 100kHz-100MHz range. Measurements from ABRACADABRA and SHAFT already place bounds on GWs, although the present strain sensitivity is weak. However, we demonstrate that the sensitivity scaling with the volume of such instruments is significant-faster than for axions-and so rapid progress will be made in the future. With no modifications, DMRadio-m^{3} will have a GW strain sensitivity of h∼10^{-20} at 200MHz. A simple modification of the pickup loop used to readout the induced magnetic flux can parametrically enhance the GW sensitivity, particularly at lower frequencies.

Search for dark matter with an LC circuit

The focus on the dark matter search has expanded to include low-mass particles such as axions or axionlike particles, and novel theoretical schemes extending the phenomenological landscape, within QCD and beyond, also garnered additional interest in recent decades. Assuming dark matter is composed of axions, in the presence of a solenoidal magnetic field, they induce a displacement current that gives rise to a toroidal magnetic field. The weakly interacting slender particle detection with an $LC$ circuit (WISPLC) is a precision direct detection experiment that will search for light dark matter candidates such as axionlike particles in parts of the parameter space previously unexplored. We present two detection schemes of the signal in a pickup loop capturing the flux of this toroidal magnetic field. WISPLC operates in a broadband and a resonant scheme where an $LC$ circuit is used to enhance the signal with an expected $Q$ factor $\ensuremath{\sim}{10}^{4}$. Taking into account the irreducible flux noise of the detector, we estimate the sensitivity of the experiment in the axion mass range between ${10}^{\ensuremath{-}11}$ and ${10}^{\ensuremath{-}6}\text{ }\text{ }\mathrm{eV}$ to reach a detectable axion-photon coupling of ${g}_{a\ensuremath{\gamma}\ensuremath{\gamma}}\ensuremath{\approx}{10}^{\ensuremath{-}15}\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$, making it possible to probe mass ranges corresponding to ultralight axions motivated by string theory. The WISPLC experiment is fully funded and currently in the construction phase.

Determining the temperature-dependent London penetration depth in HTS thin films and its effect on SQUID performance

The optimum design of high-sensitivity Superconducting Quantum Interference Devices (SQUIDs) and other devices based on thin high-temperature superconductor (HTS) films requires accurate inductance modeling. This needs the London penetration depth λ to be well defined, not only at 77 K, but also for any operating temperature, given the increasingly widespread use of miniature low-noise single-stage cryocoolers. Temperature significantly affects all inductances in any active superconducting device, and cooling below 77 K can greatly improve device performance; however, accurate data for the temperature dependence of inductance and λ(T) for HTS devices are largely missing in the literature. We report here inductance measurements on a set of 20 different thin-film YBa2Cu3O7−x SQUIDs at 77 K with thickness t = 220 or 113 nm. By combining experimental data and inductance modeling, we find an average penetration depth λ(77)=391 nm at 77 K, which was independent of t. Using the same methods, we derive an empirical expression for λ(T) for a further three SQUIDs measured on a cryocooler from 50 to 79 K. Our measured value of λ(77) and our inductance extraction procedures were then used to estimate the inductances and the effective areas of directly coupled SQUID magnetometers with large washer-style pickup loops. The latter agrees better than 7% with experimentally measured values, validating our measured value of λ(77) and our inductance extraction methods.

Towards analytical calculation of the magnetic flux measured by magnetometers

This paper reviews magnetic flux signal calculations through pick-up loops using vector spherical harmonic expansion under the quasi-static approximation, and presents a near-analytical method of evaluating the flux through arbitrary parametrizable pick-up loops for each expansion degree. This is done by simplifying the surface flux integral (2D) into a line integral (1D). For the special case of tangential circular sensors, we present a fully analytical recursion calculation. We then compare commonly-used cubature approximations to our (near-)analytical forms, and show that cubature approximations suffer from increasing errors for higher spatial frequency components. This suggests the need for more accurate evaluations for increasingly sensitive sensors that are being developed, and our (near-)analytical forms themselves are a solution to this problem.

Magnetic background field-tolerant SQIF-based current sensors

We report on the development of magnetic background field-tolerant superconducting quantum interference filter (SQIF) based on low-capacitance sub-micrometer sized cross-type Josephson junctions either as current sensing amplifiers—even on chip—for advanced superconducting quantum interference device (SQUID) readout circuits or as magnetic field sensor in flux transformer configuration especially for geophysical measurement systems. Their very small parasitic magnetic sensitive areas enable them to operate in the Earth magnetic field and allow for magnetically unshielded cool-down. The careful consideration of magnetic sensitive areas inside each SQUID in the SQIF in all three dimensions result in developed SQIFs with 28 SQUIDs in series exhibiting large voltage swing and transfer function of more than 2 mV and , respectively. We report on the electrical parameters and field stability as well as on the noise performance of the devices under investigation. SQIFs and devices with additional on-chip flux-transformers show input referred current noise levels of and , respectively. We furthermore demonstrate their potential as SQIF-based magnetometers using them in addition with thin-film pickup loops resulting in a white magnetic field noise of . The implemented SQIFs are thus highly compatible with state-of-the-art single SQUID-devices offering beneficial features such as a unique working point, making them excellent suited for implementation of e.g. geophysical instruments.

Performance optimization of the nano-sized pick-up loop of a dc-SQUID

A flux transformer, consisting of a superconducting primary loop (pick-up loop) in series with a superconducting secondary loop on which measurement is done, is considered to optimize the approach and sensitivity of the Superconducting QUantum Interference Device (SQUID). Performance of such a pick-up loop placed above a magnetic particle is investigated using the numerical Ginzburg-Landau (GL) simulations. By solving 3D GL equations, static properties of the device such as the distribution of Cooper-pair density and the screening current in the secondary coil have been investigated as a function of the dimensions of the primary loop. Dynamic properties, such as current-voltage characteristics and flux-dependent critical current of the device have also been addressed, all leading to conclusion that smaller size pick-up loop is the first requirement for its optimal sensitivity.

The magnetoelectric effect due to a semispherical capacitor surrounded by a spherical topologically insulating shell

We calculate the magnetic field produced by a capacitor formed by two semispherical perfectly conducting plates of radius a, subjected to a potential difference and surrounded by a spherical shell of a topologically insulating material having internal radius r1 and external radius r2. Fixing r2 = 1 μm and considering the case where the shell touches the metallic plates, i. e. when r1 = a, we find that the maximum magnetic field occurs at the external surface of the shell when r1≲ 0.75 μm. For these cases we examine the effect in the two remaining variables: the thickness of the shell and axial angular dependence of the magnetic field. For r1 = 0.5 μm we find a highly isotropic angular distribution with an average field of magnitude G. The angular anisotropy increases with r1, yielding for r1 = 0.62 μm and for r1 = 0.75 μm. These magnitudes fall within the sensitivities of magnetometers based upon nitrogen-vacancy centers in diamond, as well as of devices using scanning SQUID magnetometry. In the latter case we obtain fluxes in the range of (6 − 10) × 10−10 G cm2 for a pickup loop of radius 10 μm centered at the axial symmetry axis and located parallel to the equatorial plane at distances ranging from 2 to 6 μm from the center of the capacitor.

Dc SQUID Design with Femtotesla Sensitivity for Quantum-Ready Readouts

Among some of the current uses of the DC Superconducting QUantum Interference Devices (SQUIDs) are qubit-readouts and sensors for probing properties of quantum materials. We present a rather unique gradiometric niobium SQUID design with state-of-the-art sensitivity in the femto-Tesla range which can be easily tuned to specific readout requirements. The sensor is a next generation of the fractional SQUIDs with tightly optimized input coil and a combination of all measures known for restraining parasitic resonances and other detrimental effects. Our design combines the practical usefulness of well-defined pickup loops for superior imaging kernel and tunable-probing applications with the fractionalization approach to reduce undesired inductances. In addition, our modeling predicts small dimensions for these planar sensors. These features make them of high relevance for material studies and for detection of magnetic fields in small volumes, e.g. as part of a cryogenic scanning quantum imaging apparatus for efficient diagnostics and quantum device readouts. This manuscript will benefit scientists and engineers working on quantum computing technologies by clarifying potential general misconceptions about DC SQUID optimization alongside the introduction of the novel flexible compact DC SQUID design.

Inductively Coupled RF Coil for Imaging a 40 µm-Thick Histology Sample in a Clinical MRI Scanner

A histology coil is a U-shaped radio-frequency (RF) coil that is designed to provide a high signal-to-noise ratio (SNR) in magnetic resonance imaging (MRI) of microscopically thin samples. In this work, we demonstrate an inductively coupled histology coil that can be easily integrated with a human MRI scanner, providing a simple method to conduct novel, microscopic imaging experiments in a clinical scanner. All experiments were conducted in a 3T whole-body MRI scanner. The coil consisted of a U-shaped copper foil with tuning capacitors shunting the ends of the “U”. This design maximizes the B1-field efficiency and the homogeneity in the coil’s cavity. For inductive coupling, the histology coil was connected to an untuned single-turn pick-up loop via a coaxial cable. A standard 3T receive-only surface coil, connected to the scanner’s standard receive circuitry, was then inductively coupled to the pick-up loop. Separately, the RF pulse transmitted from the MRI body coil inductively drove the histology coil. In gel phantom imaging, we confirmed a higher (threefold) SNR for the histology coil than for the scanner’s standard surface coil, as well as high B1 homogeneity for the histology coil. Gradient echo images of a 40-µm-thick rat brain slice, at an in-plane resolution of 219 × 219 µm2, could be obtained with SNR > 10 (in the cortex) in about 2 hours. Our work demonstrates the feasibility of imaging microscopically thin tissue slices in a clinical MRI scanner using an inductively coupled histology coil. The method can be applied to multi-modality and multi-orientation imaging of ex-vivo and engineered tissue samples.

Development of scanning SQUID microscope system and its applications on geological samples: A case study on marine ferromanganese crust

We present developments and applications of a high resolution scanning superconducting quantum interference device (SQUID) microscope for imaging magnetic field of geological samples at room temperature. A directly coupled low-temperature SQUID with a 200 μm × 200 μm pickup loop was mounted on a sapphire rod and separated from room temperature by a sapphire window. The environmental noise of the SQUID was successfully reduced by subtracting the signal of an additional reference SQUID placed inside a cryostat. The resulting system noise level was estimated to be about 50 pT. A geological thin section could be placed on a non-magnetic sample holder with an XYZ stage for scanning in an area of 100 mm × 100 mm. The minimum achievable distance from the SQUID to the sample is measured as ∼200 µm. An application of the SSM to a marine ferromanganese crust successfully provided beautiful stripe patterns in the magnetic images. The patterns could be correlated to the history of geomagnetic field reversals. The boundaries of the magnetic polarity domains were useful guides for the estimation of the deposition age by correlation with the standard geomagnetic polarity timescale. The established age model gave an average growth rate of ∼2.7 mm/Ma, which is consistent with that obtained by radiometric dating using 10Be (∼2.6 mm/Ma).

Measurement of Magnetic Particles by Hexagonal Pseudo Seven-Channel HTS SQUID Array

We developed a high-Tc superconducting (HTS) three-channel Superconducting Quantum Interference Device (SQUID) array and applied it to the measurement of a flow of magnetic nano-particles (MNPs).Three magnetometers were positioned in-line with a separation of 2.3 mm. The pick-up loop consisted of a 1 mm × 1 mm square loop; the size of a SQUID washer slit was 5 μm × 60 μm. Using this array, we arranged a hexagonal pseudo seven-channel SQUID array. The signal from the water-diluted MNPs, Resovist, in motion was measured by the array in a DC bias magnetic field and an AC modulation magnetic field. The dependence of the SQUID signal on the angle of the MNP flow to the baseline of the array was measured and discussed. The results suggest that the signal peak values are highly dependent on the flow path angle and that most of the flow path can be identified by the shape of the waveforms. This method is a promising candidate for brain activity investigation of small animals.

Properties of grooved Dayem bridge based YBa2Cu3O7−δ superconducting quantum interference devices and magnetometers

The transport properties of a YBa2Cu3O7−δ superconducting quantum interference device (SQUID) based on grooved Dayem bridge weak links are studied as a function of temperature: at high temperatures (60 K&amp;lt;T&amp;lt;Tc=89 K), the weak links show properties similar to superconductor–normal conductor–superconductor junctions, while at temperatures below 60 K, the weak links behave like short Dayem bridges. Using these devices, we have fabricated SQUID magnetometers with galvanically coupled in-plane pickup loops: at T = 77 K, magnetic field white noise levels as low as 63 fT/Hz have been achieved.