Table of contents

The objective of the present study is to identify the equivalent stiffness components of superconducting windings. First, a 3D multi-material finite element (FE) model was established to investigate the influence of the winding structural behavior on the identified equivalent stiffnesses. Then, the ideas resulting from this FE analysis were applied to the experiment. Four in-plane stiffnesses of the double pancake were retrieved successfully with the virtual fields method using the actual strain fields obtained from stereo image correlation.

We report a theoretical study on the spin and charge transport in hybrid triplet Josephson junctions, of which the triplet pair potentials could have both different orbital symmetries and spin states. Based on a lattice model and a Hamiltonian method, we find that the spin/charge supercurrent is proportional to sin2φ with φ being the macroscopic superconducting phase, coming from the second-order Josephson effect, when the orbital symmetries of pair potentials in the two triplet superconductors are orthogonal to each other. A dissipationless transverse spin current is also found flowing at the interface of the junction and its polarization points along the cross-product of two d vectors, which arises from the combined effect of the orthogonal orbital symmetries and misalignment of d vectors. In a special hybrid junction, where the zero-energy states are absent at the interface of the junction, there is no net spin supercurrent flowing through the junction although two d vectors can be perpendicular to each other, whereas a mode-resolved spin supercurrent is flowing in the system instead.

We fabricated single- and three-core superconducting FeSe wires by a novel process based on a chemical transformation from hexagonal FeSe_{1 + d} (non-superconducting) to tetragonal FeSe (superconducting) via an optimal supply of Fe from the Fe sheath by annealing. This process enhanced the packing density of the core inside the sheath, owing to an expansion of lattice volume via a chemical transformation from high-density hexagonal FeSe_{1 + d} to low-density tetragonal FeSe. The obtained wire showed superconductivity below ∼ 10 K and the obtained transport critical current densities at 0 T and 4.2 K were 588  A  cm ^{− 2} for the three-core wire and 218 A  cm ^{− 2} for the single-core wire.

The spatial distribution and the magnetic field dependence of the critical current density J_c for a YBa₂Cu₃O_y (YBCO) thin film deposited on a SrTiO₃ (STO) bicrystal substrate have been investigated using the third harmonic voltage method. When a pick-up coil is mounted on the area without a crystal grain-boundary, the third harmonic voltage V₃ increases monotonically with increasing coil current I₀ over a certain threshold value. On the other hand, when the pick-up coil is mounted on the area with a crystal grain-boundary, the V₃–I₀ curve shows a strange behavior. However, the V₃–I₀ curves show a monotonic increase when a magnetic field over 0.07 T is applied to the YBCO thin film, because the shielding current hardly flows across the crystal grain-boundary in the large magnetic field. The magnetic field dependence of J_c is also measured using the four-probe method to clarify the strange behavior of the V₃–I₀ curve in the third harmonic voltage method.

Application of an external AC magnetic field parallel to superconducting tapes helps in eliminating the magnetization caused by the shielding current induced in the flat faces of the tapes. This method helps in realizing a magnet system with high-temperature superconducting tapes for magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) applications. The effectiveness of the proposed method is validated by numerical calculations carried out using the finite-element method and experiments performed using a commercially available superconducting tape. The field uniformity for a single-layer solenoid coil after the application of an AC field is also estimated by a theoretical consideration.

Geophysical exploration is getting more and more difficult—many of the easily explorable ore-bodies have been discovered and are already being exploited. Finding new mines requires new technologies and tools. Transient electromagnetics (TEM) is widely used in mineral exploration, but conventional sensors (especially induction coils) cannot fulfil the needs anymore: deep targets, very conductive targets or targets under conductive overburden are more easily (or sometimes only) detected using SQUIDs. In this paper we will focus on low temperature SQUID magnetometers. As the systems are applied worldwide it is necessary to strengthen them for all conceivable application scenarios. Here, we report on the latest development of these systems which are now routinely used in South Africa, Australia, Finland and Canada. This paper highlights the main features of the system and describes one example from mineral exploration.

Transport properties of curved mesoscopic superconducting strips are investigated in the framework of the time-dependent Ginzburg–Landau formalism. The geometries of the superconducting strips considered here are either a section of a cylindrical shell or a full cylindrical shell in which the magnetic field is applied perpendicular to the axis. The cylindrical section can exhibit considerably asymmetric transport properties, making it potentially interesting as a sub-micrometer scale superconducting current rectifier. The full cylindrical surface exhibits well developed dissipative branches in the voltage–current curves, that can be accounted for by kinematic vortex–antivortex phase slip lines. Such kinds of phase slip lines cause voltage oscillations in a frequency range higher than the one associated with the familiar flux flow regime.

Oxypnictides of types CeOFeAs_{1 − x}Sb_x, CeO_0.9F_0.1FeAs_{1 − x}Sb_x and CeO_0.8F_0.2FeAs_{1 − x}Sb_x have been synthesized in order to understand the effective change in electron doping by the twin effects of F ⁻ incorporation and the variation in Fe–As bonding by substitution of a larger ion (Sb) at the As site. All the compounds crystallize in a tetragonal ZrCuSiAs-type structure (space group: P4/nmm). Both the lattice parameters (a and c) increase with the increase in antimony substitution due to the larger ionic size of antimony. CeOFeAs_{1 − x}Sb_x (x = 0.1, 0.2 and 0.3) compounds were found to be semimetallic, like the parent CeOFeAs. Substitution of Sb in CeOFeAs_{1 − x}Sb_x results in a shift of the temperature of the resistance anomaly corresponding to the structural phase transition. CeO_0.9F_0.1FeAs_{1 − x}Sb_x ('x' = 0, 0.05, 0.1 and 0.15) was superconducting with a maximum transmission temperature (T_c) of 43.17 K for 'x' = 0.1, which is higher than the antimony-free CeO_0.9F_0.1FeAs (T_c = 38 K). Further increase in antimony (x > 0.15) results in the suppression of superconductivity. Similarly, Sb doping in optimally electron doped CeO_0.8F_0.2FeAs superconductor leads to the lowering of the superconducting transition temperature ('x' = 0.1) and further increase in antimony substitution ('x' > 0.1) leads to a loss of superconductivity. CeO_0.8F_0.2FeAs_0.9Sb_0.1 shows a very high value of the upper critical field H_c2(0) (137 T) as compared to Sb-free CeO_0.8F_0.2FeAs. In this paper, we discuss the first example among electron-doped oxypnictide superconductors where the T_c increases as a result of a decrease in chemical pressure (an increase in lattice parameters).

We have investigated the influence of the superconducting layer thickness, d, on the structural and transport properties of Co-doped BaFe₂As₂ films deposited on Fe buffered MgO substrates by pulsed laser deposition. The superconducting transition temperature and the texture quality of Co-doped BaFe₂As₂ films improve with increasing d due to a gradual relief of the tensile strain. For d ≥ 90 nm an additional 110 textured component of Co-doped BaFe₂As₂ was observed, which leads to an upward shift in the angle-dependent critical current density at . These results indicate that the grain boundaries created by the 110 textured component may contribute to the c-axis pinning.

YBCO films grown by the trifluoroacetate (TFA) method with increasing number of BaZrO₃ (BZO) nanoparticles have been measured by in-field angular transport measurements to investigate changes in the pinning landscape. The isotropic and anisotropic contributions to the critical current density, J_c(H), with the magnetic field applied in and orientation have been determined, allowing us to characterize the population of isotropic and correlated defects along the c axis and ab planes. First, the influence of the YBCO oxygenation process on the formation of different sorts of anisotropic defects in standard films is demonstrated. Next, we show that the addition of non-coherent BZO nanoparticles to the YBCO matrix produces an expansion of the single-vortex pinning regime toward higher fields, due to the presence of isotropic pinning centers. Moreover, by increasing the amount of isotropic defects in the BZO nanocomposites it is possible to extend the region dominated by strong isotropic pinning centers to large magnetic fields and thus enhance the irreversibility line.

To explain a reported Cooper-pair mass anomaly in niobium it has been predicted that rotating superconductors or superfluids might produce large non-classical frame-dragging fields. Anomalous gyroscope signals close to the measurement resolution in the proximity of rotating superconductors or liquid helium have also been reported while trying to investigate this theoretical concept. Based on lessons from various setups, we succeeded in building an experimental facility that allowed us to rotate a niobium superconductor, liquid helium, superfluid helium and low temperature matter with high accelerations at high speed exceeding all previous efforts. A military-grade SRS-1000 gyroscope at close proximity in different locations was used to measure any anomalous frame-dragging-like fields. No such anomalies were found within three times the noise level of our setup (± 5 × 10 ^{− 8} rad s ^{− 1}). Measurements with an electric motor at speeds up to 5000 rpm enabled us to set low boundaries for any coupling or frame-dragging-like effect outside of a rotating niobium superconductor or liquid helium to 4 × 10 ^{− 11} and for superfluids to 3 × 10 ^{− 10}. Due to the high speeds used, these results are up to two orders of magnitude below any previous result.

A series of polycrystalline SmFeAs_{1 − x}O_x bulks was prepared to systematically investigate the influence of sample density on flux pinning properties. Different sample densities were achieved by controlling the pelletizing pressure. The superconducting volume fraction, the critical current densities J_cm and the flux pinning force densities F_p were estimated from the magnetization measurements. Experimental results show that: (1) the superconducting volume fraction increases with the increasing of sample density; (2) the J_cm values have a similar trend except for the sample with very high density due to different connectivity and pinning mechanisms, moreover, the J_cm(B) curve develops a peak effect at approximately the same field at which the high density sample shows a kink; (3) the F_p(B) curve of the high density sample shows a low-field peak and a high-field peak at several temperatures, which can be explained by improved intergranular current, while only one peak can be observed in F_p(B) of the low density samples. Based on the scaling behaviour of flux pinning force densities, the main intragranular pinning is normal point pinning.

The partial insulation winding was examined to ameliorate the slow charge–discharge shown by coils wound without insulation. Single pancake coils of GdBCO coated conductor were wound without insulation, with kapton tape every five turns, and with the full use of kapton tape. They were characterized by charge–discharge, sudden discharge, and over-current testing. The improved charging and discharging and high thermal and electrical stabilities of the partially insulated coil demonstrate its potential for use in HTS power applications.

We have studied the high-field properties of carbon-doped MgB₂ thin films prepared by hybrid physical–chemical vapor deposition (HPCVD). Carbon doping was accomplished by adding carbon-containing gas, such as bis(methylcyclopentadienyl)magnesium and trimethylboron, into the hydrogen carrier gas during the deposition. In both cases, T_c drops slowly and residual resistivity increases considerably with carbon doping. Both the a and c lattice constants increase with carbon content in the films, a behavior different from that of bulk carbon-doped MgB₂ samples. The films heavily doped with trimethylboron show very high parallel H_c2 over 70 T at low temperatures and a large temperature derivative near T_c. These behaviors are found to depend on the unique microstructure of the films, which consists of MgB₂ layers a few-nanometers thick separated by non-superconducting MgB₂C₂ layers. This leads to an increase in the parallel H_c2 by the geometrical effect, which is in addition to the significant enhancement of H_c2 due to changes in the scattering rates within and between the two bands present in films doped using both carbon sources. The high H_c2 and high-field J_c(H) values observed in this work are very promising for the application of MgB₂ in high magnetic fields.

Accurate inductance calculations are critical for the design of both digital and analogue superconductive integrated circuits, and three-dimensional calculations are gaining importance with the advent of inductive biasing, inductive coupling and sky plane shielding for RSFQ cells. InductEx, an extraction programme based on the three-dimensional calculation software FastHenry, was proposed earlier. InductEx uses segmentation techniques designed to accurately model the geometries of superconductive integrated circuit structures. Inductance extraction for complex multi-terminal three-dimensional structures from current distributions calculated by FastHenry is discussed. Results for both a reflection plane modelling an infinite ground plane and a finite segmented ground plane that allows inductive elements to extend over holes in the ground plane are shown. Several SQUIDs were designed for and fabricated with IPHT's 1 kA cm ^{− 2} RSFQ1D niobium process. These SQUIDs implement a number of loop structures that span different layers, include vias, inductively coupled control lines and ground plane holes. We measured the loop inductance of these SQUIDs and show how the results are used to calibrate the layer parameters in InductEx and verify the extraction accuracy. We also show that, with proper modelling, FastHenry can be fast enough to be used for the extraction of typical RSFQ cell inductances.

The PDF file provided contains web links to all articles in this volume.