Stray Coupling Research Articles

Algorithm for Resonator Parameter Extraction From Symmetrical and Asymmetrical Transmission Responses

We describe an algorithm capable of extracting the unloaded quality factor and the resonant frequency of microwave resonators from vector S-parameters. Both symmetrical (Lorentzian) and asymmetrical (Fano) transmission responses are supported. The algorithm performs an adaptive outlier removal to discard measurement points affected by noise or distortion. It removes the effects caused by imperfections in the device (such as modes with close resonance frequencies or stray coupling between the resonator ports) or the experimental setup (such as lack of isolation or dispersion in the test set and cables). We present an extensive assessment of the algorithm performance based on a numerical perturbation analysis and the evaluation of S-parameter fitting results obtained from network analyzer measurements and resonator equivalent circuits. Our results suggest that uncertainty is mainly caused by factors that distort the frequency dependence of the S-parameters, such as cabling and coupling networks, and is highly dependent on the device measured. Our perturbation analysis shows improved results with respect to those of previous publications. Our source code is written in Python using open-source packages and is publicly available under a freeware license.

Evaluation of flux trapping moat position on AQFP cell performance

With recent advances in InductEx and the TetraHenry numerical engine, it is now possible to analyze the coupling of trapped flux in moats to superconductor circuit structures. Here we show the analysis of moat position in the layout of an adiabatic quantum-flux-parametron (AQFP) buffer cell on the coupling to critical inductors such as the load inductor. This inductor is often carefully laid out to balance current flow and cancel out stray coupling between clock and dc current excitation lines. We investigate the effect of trapped flux that couples asymmetrically to the circuit—either due to the moat position and shape or due to only one of a symmetrical duo of moats being filled with a trapped fluxon—on circuit operating margin and performance. We conclude with suggestions on moat placement to reduce the chances of circuit failure when fluxons are trapped in the moats.

Synthesizing multi-phonon quantum superposition states using flux-mediated three-body interactions with superconducting qubits

Massive mechanical resonators operating at the quantum scale can enable a large variety of applications in quantum technologies as well as fundamental tests of quantum theory. Of crucial importance in that direction is both their integrability into state-of-the-art quantum platforms as well as the ability to prepare them in generic quantum states using well-controlled high-fidelity operations. Here, we propose a scheme for controlling a radio-frequency mechanical resonator at the quantum scale using two superconducting transmon qubits that can be integrated on the same chip. Specifically, we consider two qubits coupled via a capacitor in parallel to a superconducting quantum interference device (SQUID), which has a suspended mechanical beam embedded in one of its arms. Following a theoretical analysis of the quantum system, we find that this configuration, in combination with an in-plane magnetic field, can give rise to a tuneable three-body interaction in the single-photon strong-coupling regime, while enabling suppression of the stray qubit-qubit coupling. Using state-of-the-art parameters and qubit operations at single-excitation levels, we numerically demonstrate the possibility of ground-state cooling as well as high-fidelity preparation of mechanical quantum states and qubit-phonon entanglement, i.e. states having negative Wigner functions and obeying non-classical correlations. Our work significantly extends the quantum control toolbox of radio-frequency mechanical resonators and may serve as a promising architecture for integrating such mechanical elements with transmon-based quantum processors.

The influence of diameter on the magnetic saturation in [formula omitted]/MgO [0 0 1] multilayered islands

The saturation field of circular islands, consisting of [Fe84Cu16/MgO]9Fe84Cu16 multilayers, increases with decreasing diameter of the islands. When the diameter of the islands is below 450 nm the field induced changes are dominated by a coherent rotation of the moment of the Fe84Cu16 layers. For diameters of 2 μm and larger, a signature of domain nucleation and evolution is observed. The changes in the saturation field with diameter of the islands are ascribed to the interplay between interlayer exchange coupling, stray field coupling at the edges and the crystalline anisotropy of the Fe84Cu16 layers.

Internal flux dynamics and stray field-mediated coupling in the AC susceptibility response of Nb/Ni thin-film bilayers

The in-phase ac magnetic susceptibility (X′) of Nb/Ni bilayers was measured using a small in-plane magnetic field (the drive field). The sample’s response changes considerably in the presence of perpendicular stray fields that arise during magnetization reversal of the Ni film. The interplay between stray fields and flux lines inside the superconductor is associated to an enhancement of vortex entrance and exit, suggesting an effective magnetic coupling of the ferromagnetic and superconducting components. As a DC field probes the different configurations of the Ni layer throughout its hysteresis loop, the X′ signal traces a curve with irreversible features that are analogous to those of well-known magnetoresistance measurements. A modification of the measuring temperature provides access to various kinds of superconductor/ferromagnet interaction.

Single-incision laparoscopic surgery increases the risk of unintentional thermal injury from the monopolar "Bovie" instrument in comparison with traditional laparoscopy.

Single-incision laparoscopic surgery (SILS) places multiple instruments in close, parallel proximity, an orientation that may have implications in the production of stray current from the monopolar "Bovie" instrument. The purpose of this study was to compare the energy transferred during SILS compared to traditional four-port laparoscopic surgery (TRD). In a laparoscopic simulator, instruments were inserted via SILS or TRD setup. The monopolar generator delivered energy to a laparoscopic L-hook instrument for 5-s activations on 30-Watts coag mode. The primary outcome (stray current) was quantified by measuring the heat of liver tissue held adjacent to the non-electrically active 10-mm telescope tip and Maryland grasper in both the SILS and TRD setups. To control for the potential confounder of stray energy coupling via wires outside the surgical field, the camera cord and active electrode wires were oriented parallel or completely separated. SILS and TRD setups create similar amounts of stray current as measured by increased tissue temperature at the non-electrically active telescope tip (41±12 vs. 39±10°C; p=0.71). Stray current was greater in SILS compared to TRD at the tip of the non-electrically active Maryland forceps (38±9 vs. 20±10°C; p<0.01). Separation of the active electrode and camera cords did not change the amount of stray energy in the SILS orientation for either telescope (39±10°C bundled vs. 36±10°C separated; p=0.40) or grasper (38±9°C bundled vs. 34±11°C separated; p=0.19) but did in the TRD orientation (41±12 bundled vs. 24±10 separated; p<0.01). When SILS was compared to TRD with the cords separated, SILS increased stray energy at both the telescope tip and grasper tip (36±10 vs. 24±10°C; p<0.01 and 34±11 vs. 17±8°C; p<0.01). SILS increases stray energy transfer nearly twice as much as TRD with the use of the monopolar instrument. Strategies to mitigate the amount of stray energy in the TRD setup such as separation of the active electrode and camera cords are not effective in the SILS setup. These practical findings should enhance surgeons using the SILS approach of increased stray energy that could result in injury.

Non-contact measurement of lightning and switching transient overvoltage based on capacitive coupling and pockels effects

Measurement of lightning and switching transient overvoltage in power system is significant to the research that guides the protection of power system. A new type of sensor for non-contact measurement of lightning and switching transient overvoltage in power system is designed in this study. This sensor is composed of two parts, namely, voltage induction and optoelectronic conversion units. Given the existing stray coupling capacitor C1 between the transmission line or bus bar and a metal plate placed under it, a voltage induction unit is designed by utilizing the C1 and a capacitor with fixed value C2 connected in series, then the high-voltage non-contact induction on a transmission line or bus bar is achieved. The optoelectronic conversion unit is designed based on the Pockels effect of the LiNbO3 crystal. The voltage signal induced by the voltage induction unit can be converted to an optical signal by the optoelectronic conversion unit for long-distance transmission through optical fibers without power supply at the monitoring site. Laboratory and field tests revealed that the new developed sensor possesses good linear region, high precision, and wide frequency band of the response to lightning and switching transient overvoltage in power system. The sensor can also be applied to the non-contact measurement of lightning and switching transient overvoltage in power system with different voltage levels by conveniently adjusting the distance between the sensor and the line.

Spin torque-induced magnetization dynamics in giant magnetoresistance devices with Heusler alloy layers

We studied the spin torque-induced magnetization dynamics of full-Heusler Co2(Fe,Mn)Si (CFMS) layers in current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) devices, and obtained the phase diagrams of spin torque-induced phenomena such as magnetization switching and spin torque oscillation. For a device with a 3 nm-thick CFMS free layer and a 20 nm-thick CFMS fixed layer, spin torque oscillation was induced in the free layer magnetization. As the free layer thickness was increased from 3 to 7 nm, on the other hand, the magnetization dynamics were remarkably changed. A 7 nm-thick CFMS free layer showed no clear spin torque oscillation even though it showed the reversible magnetization switching. On the contrary, the magnetization in the 20 nm-thick CFMS fixed layer oscillated at a certain condition. This spin torque-induced instability in the fixed layer magnetization is attributable to the increase in the free layer thickness, the structure of the fixed layer having the extended film, and the stray field coupling between the free and the fixed layers.

Capacitive driven-right-leg circuit design

Fil: Haberman, Marcelo Alejandro. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto de Investigaciones en Electronica, Control y Procesamiento de Senales. Universidad Nacional de La Plata. Instituto de Investigaciones en Electronica, Control y Procesamiento de Senales; Argentina

Effects of spacer thickness on perpendicular anisotropy L1-FePt/TiN/L1-FePt pseudo spin valves

Pseudo spin valves (PSVs) with the structure MgO substrate/L10-Fe50Pt50/TiN/L10-Fe50Pt50 were fabricated with varying TiN spacer thickness from 3 to 7 nm. The giant magnetoresistance (GMR) reached a maximum before diminishing with increasing TiN spacer thickness. The initial enhancement of the GMR was attributed to the reduction in interlayer coupling between the L10-FePt layers. However, a decline in GMR sets in when the current shunting effects negated the enhancement brought about by the improved decoupling. Magnetostatic coupling was the primary source of interlayer coupling in the PSVs. The dependence of interlayer coupling on the remanent state of the hard L10-FePt was also examined based on the magnitude and direction of shift in the center of the minor hysteresis loop. While magnetostatic coupling was present in fully saturated hard L10-FePt, dipolar stray field coupling contributed more significantly to the interlayer coupling strength in partially saturated hard L10-FePt. The stray field coupling strength depended on both the thickness of the spacer and the density of the reversed domains in the hard L10-FePt.

Composite pulses for robust universal control of singlet–triplet qubits

Precise qubit manipulation is fundamental to quantum computing, yet experimental systems generally have stray coupling between the qubit and the environment, which hinders the necessary high-precision control. Here, we report the first theoretical progress in correcting an important class of errors stemming from fluctuations in the magnetic field gradient, in the context of the singlet-triplet spin qubit in a semiconductor double quantum dot. These errors are not amenable to correction via control techniques developed in other contexts, as here the experimenter has precise control only over the rotation rate about the z axis of the Bloch sphere, and this rate is furthermore restricted to be positive and bounded. Despite these strong constraints, we construct simple electrical pulse sequences that, for small gradients, carry out z axis rotations while cancelling errors up to the sixth order in gradient fluctuations, and for large gradients, carry out arbitrary rotations while cancelling the leading order error.

Electromagnetic Design of Feedhorn-Coupled Transition-Edge Sensors for Cosmic Microwave Background Polarimetry

Observations of the cosmic microwave background (CMB) provide a powerful tool for probing the evolution of the early universe. Specifically, precision measurement of the polarization of the CMB enables a direct test for cosmic inflation. A key technological element on the path to the measurement of this faint signal is the capability to produce large format arrays of background-limited detectors. We describe the electromagnetic design of feedhorn-coupled, TES-based sensors. Each linear orthogonal polarization from the feedhorn is coupled to a superconducting microstrip line via a symmetric planar orthomode transducer (OMT). The symmetric OMT design allows for highly-symmetric beams with low cross-polarization over a wide bandwidth. In addition, this architecture enables a single microstrip filter to define the passband for each polarization. Care has been taken in the design to eliminate stray coupling paths to the absorbers. These detectors will be fielded in the Cosmology Large Angular Scale Surveyor (CLASS).

Estimation of stray coupling capacitances in biopotential measurements

Biopotential measurements are very sensitive to electromagnetic interference (EMI) from power-lines. Interference conditions are mainly imposed by electric-field coupling, whose effects can be described by coupling capacitances. The main of them are the patient-to-ground and the patient-to-power-line capacitances, usually denoted as C(B) and C(P), respectively. A technique to estimate these elements and experimental data obtained in different environmental conditions are presented. It was found that C(B) ranges from hundreds of pF to nF, and C(P) from hundredths of pF to few pF. The presented technique also lets it know the small amplifier-to-ground and amplifier-to-power-line capacitances. The knowledge of all these capacitances allows estimating the EMI conditions that biopotential amplifiers can be subject to, thus, resulting useful data for specifying their design requirements and constraints in real working conditions.

Toward the magnetoresistive detection of single magnetic nanoparticles: New strategies for particle detection by adjustment of sensor shape

In this work, different approaches in order to enhance the sensitivity of tunnel magnetoresistive sensors are discussed by means of finite element simulations. Several sensor layouts consisting of a free CoFeB sensing layer and a pinned bottom electrode are investigated. A decrease in the detection threshold is predicted by introducing magnetic areas at the sensor boundaries which can be easily switched due to a combined interaction of exchange contribution and stray field coupling of the layers.

Interlayer magnetostatic coupling induced Co layer coercivity enhancement and exchange bias in [Pd/Co]/Cu/Co spin valves

Interlayer stray field coupling in hybrid perpendicular/Cu/in-plane magnetic anisotropy [Pd/Co]/Cu/Co single spin valves and [Pd/Co]/Cu/Co/Cu/[Co/Pd] dual spin valves was investigated. Enhancements of Co layer coercivity were observed after ac or dc demagnetization of the samples. This effect originates from the pinning of Co domain walls by the in-plane stray field from Co/Pd stripe domains, and the enhancements are more efficient as the Co/Pd domain size decreases. Moreover, the magnetostatic interaction between the net in-plane Co/Pd remanent moments and the Co moments leads to exchange bias in the Co layer of the dual spin valve after the dc demagnetization.

The effect of tilted edges on the shape anisotropy and stray field coupling of uniformly magnetized rectangular elements

The influence of tilted edges on the magnetostatic properties of uniformly magnetized thin rectangular elements is studied. To calculate the magnetostatic energy, the Poisson equation is solved. The shape of the magnetic element is approximated by horizontally assembled thin cuboids and the solutions of Rhodes and Rowlands [Proc. Leeds Phil. Soc. 6, 191 (1954)] are utilized. A second approach is the straightforward integration of the Poisson equation taking into account the trapezoidal shape of the side faces due to the tilted edges. For an adequate number of cuboids, both methods agree very well. It is found that the shape anisotropy of a single magnetic element with tilted edges is reduced compared to that of an ideal cuboid. For a two element system the shape anisotropy competes with the magnetostatic interaction favoring a magnetization orientation parallel to the connecting line of the elements. If the elements are oriented in-line with their short axes, the easy magnetization axis switches at a critical distance between the elements. This distance increases when the elements have tilted edges.

Modeling and Optimization of Compact Microwave Bandpass Filters

This paper presents the modeling and optimization of compact microwave bandpass filters whose compactness leads to complex and strong stray coupling paths, thereby making the identification of a simple and sparse coupling topology difficult and even impossible. The strong coupling coefficients needed for a broadband response can also cause an ambiguity in identifying the spatial extent of local resonances. An equivalent circuit, which is extracted directly from Maxwell's equations, is used in optimizing these filters. The filter is represented by its global resonances instead of individual resonators. The extraction of the parameters of the equivalent circuit is carried out in the physical frequency and not in the normalized frequency in order to preserve the physicality of the equivalent circuit, especially for asymmetric responses. The technique is successfully applied to the optimization of second-order suspended stripline bandpass filters with one transmission zero either below or above the passband, as well as fourth-order filters with three transmission zeros. A fourth-order filter with three transmission zeros is fabricated and measured.

Straightforward field calculations for uniaxial hardmagnetic prisms: stray field distributions and dipolar coupling in regular arrays

Elements of hard- or moderately hardmagnetic materials may exhibit remanent states with a stable uniform magnetization distribution due to the high magnetocrystalline anisotropy energy. In this work, we present analytical calculations of magnetic field distributions arising from appropriate prismatic magnets and apply the results to the special case of hardmagnetic cuboids. The influence of the aspect ratio on the stray field distribution and its range is analysed. Based on these calculations a simple quasi-phase diagram for the emergence of dipolar coupling between individual elements with perpendicular easy axis in regular arrays is derived.

Influence of Pinned-Layer Dispersion on Magnetic Tunnel Junction Switching Distributions

The influence of pinning field dispersion (PFD) on the coercivity (Hc) and offset field (Ho) of IrMn/NiFeCo/AlOx/NiFeCo magnetic tunnel junctions (MTJ) has been studied by repeated in situ measurement of the resistance as a function of magnetic field, R(H), hysteresis loops of the MTJs after resetting the IrMn/NiFeCo pinned layer. Magneto-thermal magnetic random access memory, MT-MRAM, cells were used to perform this task. Here, the pinned layer is reset by pulsing a current through the tunnel barrier of the MT-MRAM MTJ in the presence of an external magnetic field. This permits rapid field cooling of the pinned layer through the blocking temperature of the IrMn. Repeated measurements of an R(H) loop after a pinned layer reset cycle show only minor variation. R(H) loops measured for different pinned layer reset cycles however, can show large changes in Hc , Ho, and R(H) loop shape. The effect is believed to be due to the random magnetostatic field distribution resulting from the frozen-in PFD of the IrMn/NiFeCo pinning layer. In order to test this conclusion, the evolution of the R(H) loop is studied as a function of reset pulse amplitude. Because of the intrinsic distribution of blocking temperatures of the IrMn layer, PFD should decrease as reset pulse amplitude increases. Results show that Hc increases, while the R(H) loops become more square as the reset pulse amplitude is increased. Ho shows a more complex dependence, which is a competition between ferromagnetic magnetic-roughness-induced Neel coupling and anti-ferromagnetic stray field coupling. A micromagnetic model for studying the dependence of the R(H) loops on the alignment of the IrMn crystallites was developed, and simulated results are in good agreement with the measurements. This work provides a simple explanation for large switching field distributions that can result in arrays of seemingly identical MRAM bits

Reciprocity and gyrotropism in magnetic resonance transduction

We give formulas for transduction in magnetic resonance---i.e., the appearance of an emf due to Larmor precession of spins---based upon the modified Lorentz reciprocity principle for gyrotropic (also called ``nonreciprocal'') media, i.e., in which a susceptibility tensor is carried to its transpose by reversal of an external static field [cf., R. F. Harrington and A. T. Villeneuve IRE Trans. Microwave Theory and Technique MTT6, 308 (1958)]. Prior applications of reciprocity to magnetic resonance, despite much success, have ignored the gyrotropism which necessarily arises due to nuclear and/or unpaired electronic spins. For detection with linearly polarized fields, oscillating at the Larmor frequency, the emf is written in terms of a volume integral containing a product of two factors which we define as the antenna patterns, i.e., $({H}_{1x}\ifmmode\pm\else\textpm\fi{}i{H}_{1y})$, where, e.g., for a single transceive antenna, the $H$'s are just the spatially dependent oscillatory magnetic field strengths, per the application of some reference current at the antenna terminals, with the negative sign obtaining for transmission, and the positive for reception. Similar expressions hold for separate transmit and receive antennas; expressions are also given for circular polarization of the fields. We then exhibit a receive-only array antenna of two elements for magnetic resonance imaging of protons, which, due an intensity artifact arising from stray reactive coupling of the elements, produces, despite its own bilateral symmetry, asymmetric proton NMR images of a symmetric cylindrical phantom containing aqueous saline solution [J. Tropp and T. Schirmer, J. Magn. Reson. 151, 146 (2001)]. Modification of this two-port antenna, to function in transmit-receive mode, allows us to demonstrate highly nonreciprocal behavior: that is, to record images (of cylindrical test phantoms containing aqueous saline solution) whose appearance dramatically changes, when the roles of transmission and reception are swapped between the two antenna ports---giving in one instance a signal intensity pattern whose form resembles an umbrella (i.e., with a central column of moderate intensity surmounted by a bright canopy), and in the other, a distorted oval with slight concavities at its horizontal extremes, whose outline suggests that of a cat's eye. The relation between image patterns and drive scheme can be shown to reverse if the static polarizing field is reversed. Electromagnetic and circuit calculations, together with the modified reciprocity principle, allow us to reproduce these pattern changes in numerical simulations, closely and convincingly. Although the imaging experiments are performed at a static field of $3.0\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, and consequently a Larmor frequency of $128\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, the nonreciprocal effects are not related to the shortness of the wavelength in aqueous medium, but appear equally in simulations based in either the quasistatic or full electromagnetic regimes. Finally, we show that although antenna patterns for transmission and reception are swapped with reversal of the polarizing field, meaning that the receive pattern equals the transmit pattern with the field reversed, this in no way invalidates the familiar rotating wave model of spin dynamics in magnetic resonance.