Superconductor Strip Research Articles

TLS noise reduction in microwave kinetic inductance detectors using a parallel plate capacitor with a three-layer dielectric

<p>Over the past two decades, microwave kinetic inductance detectors (MKIDs) have gained exceptional importance in millimeter and sub-millimeter astronomy. MKIDs consist of thin strip resonators capable of detecting changes in the surface impedance of superconductor strips, which result from variations in resonance circuit properties. The principal noise in MKIDs comprises excess frequency noise and two-level system noise. In this paper, we propose a technique to mitigate the effect of two-level system (TLS) noise in MKIDs using a parallel plate capacitor with three layers of high ε dielectrics. To achieve this, we employ three layers of Al<sub>2</sub>O<sub>3</sub>, HfO<sub>2</sub>, and TiO<sub>2</sub> with equal thickness between the capacitor plates. The experimental results demonstrate a nearly 30% reduction in TLS power spectral density.<strong></strong></p>

AC Loss Evaluation of Superconductor Strips Located Inside Iron Core Slots

AC Loss Evaluation of Superconductor Strips Located Inside Iron Core Slots

Advances in superconductor quantum and thermal detectors for analytical instruments

Analytical instruments or scientific instruments are indispensable for scientific research and industry. The analytical instruments require a detector that converts physical quantities to be measured (measurands) to electric signals. This Tutorial describes the basics of quantum and thermal detectors, the operation principles of superconductor detectors, and the ultimate performance of state-of-art analytical instruments with superconductivity. We still face fundamental issues, such as the classical Fano factor, the relation between energy gap and mean carrier creation energy, quasiparticle dynamics, and the intermediate state in the middle of superconducting transition; and engineering issues, such as the small sensitive area and the spatially nonuniform response. Nevertheless, enormous efforts have matured superconductor detectors, which enables us to solve the inherent problems of conventional analytical instruments. As an example of the analytical results, we describe x-ray spectroscopy and mass spectrometry at our institute by using three detector types: superconductor tunnel junction, transition edge sensor, and superconductor strip. Microwave kinetic inductance and metallic magnetic calorimetric types are also described. The analytical results may contribute to a wide range of fields, such as dentistry, molecular biology, energy-saving society, planetary science, and prebiotic organic molecules in space.

High-frequency diode effect in superconducting Nb3Sn microbridges

The superconducting diode effect has recently been reported in a variety of systems and different symmetry-breaking mechanisms have been examined. However, the frequency range of these potentially important devices still remains obscure. We investigated superconducting microbridges of ${\mathrm{Nb}}_{3}\mathrm{Sn}$ in out-of-plane magnetic fields; optimum magnetic fields of $\ensuremath{\sim}$10 mT generate $\ensuremath{\sim}10%$ diode efficiency, while higher fields of $\ensuremath{\sim}$15--20 mT quench the effect. The diode changes its polarity with magnetic field reversal. We documented superconductive diode rectification at frequencies up to 100 kHz, the highest reported as of today. Interestingly, the bridge resistance during diode operation reaches a value that is a factor of two smaller than in its normal state, which is compatible with the vortex-caused mechanism of resistivity. This is confirmed by finite-element modeling based on time-dependent Ginzburg-Landau equations. To explain experimental findings, no assumption of lattice thermal inequilibrium has been required. Dissimilar edges of the superconductor strip can be responsible for the inversion symmetry breaking by the vortex penetration barrier; visual evidence of this opportunity was revealed by scanning electron microscopy. Estimates are in favor of a much higher (GHz) range of frequencies for this type of diode.

Defect Evolution in Y<sub>0.5</sub>Gd<sub>0.5</sub>Ba<sub>2</sub>Cu<sub>3</sub>O<sub>7-</sub><sub>δ</sub> Layer by H Ion Irradiation

In order to further improve the superconducting current carrying capacity of REBCO coated conductor under strong magnetic field, ion irradiation is used to generate the pinning center of introduced magnetic flux in the REBCO coated conductor. In this paper, the H-ion irradiation of REBCO second generation high temperature superconductor strip was carried out by using the 320kV high charge state ion synthesis research platform. DB-SPBA combined with Raman spectroscopy was used to measure the change of microstructure in YBCO samples irradiated by H+ions within the range of 5.0×10<sup>14</sup>~1.0×10<sup>16</sup>. The positron annihilation parameters in YBCO before and after irradiation were analyzed. It is found that after 100 keV H+ion irradiation, a large number of defects including vacancy, vacancy group or dislocation group are produced in the superconducting layer. The larger the irradiation dose, the more vacancy type defects are produced, the more complex the defect types are, and the annihilation mechanism of positrons in the defects changes. Raman spectroscopy results show that with the increase of H+ion irradiation dose, the oxygen atoms in the coating rearrange, the plane spacing increases, the orthogonal phase structure of the coating is destroyed, and the degree of order decreases. The defects produced by such ion irradiation lay a foundation for the introduction of flux pinning centers. Further research can be carried out in combination with X-ray diffractometer, transmission electron microscope, superconductivity and other testing methods to provide theoretical and practical reference for the optimization of material properties.

Non-reciprocity of vortex-limited critical current in conventional superconducting micro-bridges

Non-reciprocity in the critical current has been observed in a variety of superconducting systems and has been called the superconducting diode effect. The origin underlying the effect depends on the symmetry breaking mechanisms at play. We investigate superconducting micro-bridges of NbN and also NbN/magnetic insulator (MI) hybrids. We observe a large diode efficiency of ≈30% when an out-of-plane magnetic field as small as 25 mT is applied. In both NbN and NbN/MI hybrid, we find that the diode effect vanishes when the magnetic field is parallel to the sample plane. Our observations are consistent with the critical current being determined by the vortex surface barrier. Unequal barriers on the two edges of the superconductor strip result in the diode effect. Furthermore, the rectification is observed up to 10 K, which makes the device potential for diode based applications over a larger temperature range than before.

AC loss modeling of stacked HTS strips with economic analysis

In order to fabricate high-current superconducting conductors for engineering applications, the configuration of stacked high-temperature superconductor (HTS) strips is one of the most popular choices. AC losses in HTS stacks have been vastly studied, but the economic study towards the AC losses with respect to the capital investment and operating costs of HTS stacks is still missing. First, the modelling strategy using the finite element method (FEM) based on the H-formulation is explained, and the basic AC loss simulation of a single strip is verified by the Norris analytical methods. The HTS stack (16 strips) is used as an example, and the corresponding current distribution and magnetic field are analyzed. The relationship between the AC loss and current-carrying capability is studied for multiple stacks with different numbers of strips. Economic analysis is performed with regard to the relationship between the AC losses and investment/operating costs of multiple stacked HTS strips. In this article, the numerical modeling together with the economic analysis of stacked HTS strips bring new visions, which can be beneficial for the future designs of high-current superconducting conductors and cables.

Enhanced topological protection in planar quasi-one-dimensional channels with periodically modulated width

We study one dimensional (1D) and quasi-1D periodic structures as possible platforms for the emergence of Majorana bound states with enhanced robustness against disorder and system inhomogeneity. First, using a simple 1D model, we analytically derive the effective parameters characterizing the minibands generated by the periodic potential. We show that, for strong enough periodic potentials, the higher energy minibands hosting Majorana bound states have significant advantages compared to their counterparts in uniform systems, including increased topological gaps, enhanced robustness against disorder, and enlarged parameter space regions consistent with the presence of topological superconductivity. We identify the problem of engineering a strong enough periodic potential as a key roadblock to realizing efficient periodic 1D structures. To address this challenge, we propose an efficient implementation of the periodic potential based on quasi-1D channels realized in 2D semiconductor heterostructures proximity coupled to superconductor strips of periodically modulated width. Our numerical study of the modulated channel device shows excellent agreement with the simple 1D model, reveals a topological phase diagram that is quite insensitive to the details of the confining potential associated with screening by the superconductor, and demonstrates that engineering patterned 2D structures represents a powerful and versatile approach to realizing robust Majorana bound states.

The chiral edge state and Majorana fermion in hole-doped YBa2Cu3O7−δ mesoscopic strip

Edge states of nontrivial topology are investigated by diagonalizing the tight-binding Hubbard model Hamiltonian for the copper-oxide superconductor YBCO strip in the presence of the Rashba spin-orbit interaction and the Zeeman field. It is found that chiral edge states may develop under appropriate spin-orbit coupling and the exchange field strengths. By defining the quasi-particle creation (annihilation) operators in terms of the obtained particle and hole functions, the zero-energy chiral edge states are proved to be the Majorana zero-energy modes on the opposite edges of the superconductor strip. Manipulations on the Majorana modes are promising by applying an external magnetic field normal to the strip plane. It is also showed that the chiral edge state and the Majorana fermions are more feasible in the underdoped samples. Moreover, domains distinguishing the d-wave pairings from the s-wave pairings are found, implying the coexistence of the d-wave and s-wave gaps for the underdoped YBCO samples.

Topological π Junctions from Crossed Andreev Reflection in the Quantum Hall Regime

We consider a two-dimensional electron gas (2DEG) in the quantum Hall regime in the presence of a Zeeman field, with the Fermi level tuned to a filling factor of ν=1. We show that, in the presence of spin-orbit coupling, contacting the 2DEG with a narrow strip of an s-wave superconductor produces a topological superconducting gap along the contact as a result of crossed Andreev reflection (CAR) processes across the strip. The sign of the topological gap, controlled by the CAR amplitude, depends periodically on the Fermi wavelength and strip width and can be externally tuned. An interface between two halves of a long strip with topological gaps of opposite sign implements a robust π junction, hosting a pair of Majorana zero modes that do not split despite their overlap. We show that such a configuration can be exploited to perform protected non-Abelian tunnel-braid operations without any fine tuning.

AC Susceptibility of a High-Temperature Superconductor Strip: Field-Dependent Power-Law Flux-Creep Model

The field-amplitude-dependent ac susceptibility $\chi (H_m)$ and the voltage-current curve $V(I)$ of rectangular samples cut from the same high-temperature superconductor strip are measured at 77 K. The high- $H_m$ part of $\chi (H_m)$ and the high- $I$ part of $V(I)$ can be consistently well simulated by a flux-creep model of power-law dependence of the electric field on a Kim-type magnetic-field-dependent sheet current density. The low- $H_m$ and low- $I$ departure from our experiment results from edge damage due to cutting.

AC susceptibility as a characterization tool for coated conductor tapes

The measurement and analysis of magnetic AC susceptibility is a useful tool in the study of superconductor (SC) materials. Exposure of a sample to a magnetic field changing in time generates loops of electrical currents that are detectable in a contactless way with the help of a suitable pick-up system. In this paper the applicability of this technique in the characterization and quality control of coated conductor (CC) tapes is evaluated. First we recollect the essential results of the analytical theory derived for thin SC strips and their extrapolation to strips with finite thickness. From the analytical expressions one can see how the properties of CC tape that are important for application in electric power devices, namely its critical current and AC loss, can be deduced from AC susceptibility data in straightforward way. The main focus of our study is to investigate the influence that various cases of non-uniformities in SC layer exhibit on the magnetic properties examined in an AC regime. Numerical computations were used to explore the consequences of lateral variation in the critical current density. Predictions derived for some model cases were compared with experimental findings. A dedicated experiment was also carried out to demonstrate that a transverse scratch that would be detrimental for DC transport could sneak unobserved through the AC magnetic experiment on a long sample. Our study shows that the analysis of both parts of the complex magnetic susceptibility in place of a mere AC loss determination in a common AC magnetization experiment is worth the additional effort.

The guidance of kinematic vortices in a mesoscopic superconducting strip with artificial defects

Within the time-dependent Ginzburg–Landau (TDGL) theory, we theoretically investigated the dynamic properties of vortex–antivortex (V–Av) pairs in a current-carrying superconductor strip with one tilted slit or two transversely/longtitudinally arranged slits in the presence of a weak magnetic field. The effect of the rotation angle on the resistive state in the sample with one tilted slit was considered. The location of phase slippage in the sample can be predetermined by the rotation angle of the tilted slit. We found the coalescence of the phase-slip lines (PSLs) during a periodic multiharmonic voltage oscillation. This leads to two stages of voltage oscillation at zero applied field and three stages at weak magnetic field since the coalescence of left and right PSLs are staggered in the latter case. Moreover, the influence of relative position and size of the two slits on the transport properties of the sample are also studied. The dynamic behavior of V–Av pairs depends on not only the slit length but also their horizontal or vertical interval distance between the two slits. Since the geometry of sample has an important influence on the distribution of supercurrent across the sample, we demonstrate the possibility to efficiently guide the kinematic vortices by adjusting the relevant parameters.

Biomolecular ion detection using high-temperature superconducting MgB2 strips

Superconducting strip ion detectors (SSIDs) are promising for realization of ideal ion detection with 100% efficiency and nanosecond-scale time response in time-of-flight mass spectrometry. We have detected single biomolecular ions in the keV range using a 10-nm-thick and 250-nm-wide strip of a high temperature superconductor, magnesium diboride (MgB2), at temperatures of up to 13 K. The output pulse shape is explained remarkably well using circuit simulations and time-dependent Ginzburg-Landau simulations coupled with a heat diffusion equation. The simulations show that the hot spot model is applicable to the proposed MgB2-SSIDs and the normal region expansion is completed within 16 ps, which corresponds to a maximum length of 1010 nm.

Fracture analysis of a transversely isotropic high temperature superconductor strip based on real fundamental solutions

Real fundamental solution for fracture problem of transversely isotropic high temperature superconductor (HTS) strip is obtained. The superconductor E–J constitutive law is characterized by the Bean model where the critical current density is independent of the flux density. Fracture analysis is performed by the methods of singular integral equations which are solved numerically by Gauss–Lobatto–Chybeshev (GSL) collocation method. To guarantee a satisfactory accuracy, the convergence behavior of the kernel function is investigated. Numerical results of fracture parameters are obtained and the effects of the geometric characteristics, applied magnetic field and critical current density on the stress intensity factors (SIF) are discussed.

Hysteretic ac loss of a superconductor strip subject to an oscillating transverse magnetic field: Geometrical and electromagnetic effects

Numerical simulations of geometrical and electromagnetic effects on the distributions of the magnetic induction, the electric field, the current density, the power loss density, and the hysteretic ac loss of a type-II superconductor strip exposed to an oscillating transverse magnetic field are performed by resorting to the quasistatic approximation of a vector potential approach. The underlying definition of the superconducting constituent makes use of a generalized “smoothed” Bean model of the critical state, which includes the field dependence of the induced current as well. Based on the Jacobian-free Newton-Krylov approach and the backward Euler scheme, the numerical analysis at hand is tailored to the problem of a variable width/thickness aspect ratio of the superconductor strip. Assigning representative materials characteristics and conditions of the applied magnetic field, the main findings include: (i) at high amplitudes of the applied magnetic field, variations of the magnetic induction, the induced electric field, the induced current density, and the power loss density across the thickness of the strip die away as the latter quantity abates; (ii) at low and moderate amplitudes of the applied magnetic field, the hysteretic ac loss abates rapidly, as the aspect ratio of the strip augments, the field dependence of the induced current merely playing an insignificant part thereby; conversely, whereas the geometrical effect controlled by the aspect ratio of the strip is minute at high amplitudes of the applied magnetic field, a reduction of the hysteretic ac loss occurs due to Kim's extended Ansatz for the critical state.

Development of microwave kinetic inductance detectors and their readout system for LiteBIRD

Primordial gravitational waves generated by inflation have produced an odd-parity pattern B-mode in the cosmic microwave background (CMB) polarization. LiteBIRD (Light satellite for the studies of B-mode polarization and Inflation from cosmic background Radiation Detection) aims at detecting this B-mode polarization precisely. It requires about 2000 detectors capable of detecting a frequency range from 50GHz to 250GHz with ultra low noise. Superconducting detectors are suitable for this requirement. We have fabricated and tested microwave kinetic inductance detectors (MKIDs) and developed a new readout system.We have designed antenna-coupled MKIDs. Quasi-particles are created by incident radiation and are detected as a change of the surface impedance of a superconductor strip. This change of the surface impedance is translated into the change of the resonant frequency of a microwave signal transmitted through the resonator.We also have developed a new readout system for MKIDs. The newly developed readout system is not only able to read out the amplitude and the phase data with the homodyne detection for multi-channels, but also provides a unique feature of tracking the resonant frequency of the target resonator. This mechanism enables us to detect signals with a large dynamic range. We report on the recent R&D status of the developing MKIDs and on the read-out system for LiteBIRD.

Thermo-electromagnetic properties of a magnetically shielded superconductor strip: theoretical foundations and numerical simulations

Numerical simulations of thermo-electromagnetic properties of a thin type-II superconductor strip surrounded by open cavity soft-magnetic shields and exposed to an oscillating transverse magnetic field are performed by resorting to the quasistatic approximation of a vector potential approach in conjunction with the classical description of conduction of heat. The underlying definition of the superconducting constituent makes use of an extended ‘smoothed’ Bean model of the critical state, which includes the field and temperature dependence of the induced supercurrent as well. The delineation of the magnetic shields exploits the reversible-paramagnet approximation in the Langevin form, as appropriate for magnetizations with narrow Z-type loops, and considers induced eddy currents too. The coolant is envisaged as acting like a bath that instantly takes away surplus heat. Based on the Jacobian-free Newton–Krylov approach and the backward Euler scheme, the numerical analysis at hand is tailored to the problem of a high width/thickness aspect ratio of the superconductor strip. Assigning representative materials characteristics and conditions of the applied magnetic field, the main findings for a practically relevant magnet configuration include: (i) an overall rise of the maximum temperature of the superconductor strip tending to saturation in a superconducting thermo-electromagnetic steady state above the operating temperature, magnetic shielding lending increased stability and smoothing the temperature profile along the width of the superconductor strip; (ii) a washing out of the profile of the magnetic induction and a lowering of its strength, a relaxation of the profile of the supercurrent density and an increase of its strength, a tightening of the power loss density and a reduction of its strength, all inside the superconductor strip. The hysteretic ac loss suffered by the superconductor strip is seen to be cut back or, at most, to converge on that of an unshielded strip, thermo-electromagnetic coupling merely playing an insignificant part thereby.

Analysis of Nonlinearities in Superconducting Microstrip Straight Bends; FDTD Method in Comparison with Nonlinear Circuit Modeling

This paper presents the prediction of nonlineari- ties in the superconducting microstrip straight bends in mi- crowave frequencies based on two different methods; FDTD simulation as a numerical approach, and nonlinear circuit modeling as an analytical method. In the FDTD method, the superconducting microstrip structures are simulated with London's equations. In the simulation, the penetration depth and normal conducting coefficient are considered as func- tions of current density of superconductor. To simulate the thin strip of superconductor, a non-uniform mesh has been used. For the nonlinear circuit modeling, we use distributed RLGC parameters for superconducting microstrip transmis- sion lines. These parameters are considered as functions of the current distribution. This yields an equivalent nonlinear circuit model for bends. The final equivalent nonlinear cir- cuit is analyzed using the harmonic balance (HB) method. Different straight bend structures have been considered and the two methods' results are compared.

Efficiency of quasiparticle evacuation in superconducting devices

We have studied the diffusion of excess quasiparticles in a current-biased superconductor strip in proximity to a metallic trap junction. In particular, we have measured accurately the superconductor temperature at a near-gap injection voltage. By analyzing our data quantitatively, we provide a full description of the spatial distribution of excess quasiparticles in the superconductor. We show that a metallic trap junction contributes significantly to the evacuation of excess quasiparticles.