Stuart C. Wimbush

• MPhys PhD
• Principal Magnets Engineer at UK Industrial Fusion Solutions Ltd

Increasingly ambitious space missions rely upon the advancement of propulsion systems, and innovations in electric propulsion represent a vital step toward potential solutions. Electric propulsion research has indicated improved thruster performance at higher magnetic fields, but real-world implementations have been encumbered by available magnet t...

One of the major impediments to the industrial take-up of high-temperature superconductors is the paucity of comprehensive, reliable and relevant performance data on commercially available wires. To address this, the Robinson Research Institute is making available its internal database of wire performance data, acquired on our in-house 1 kA critica...

The Spherical Tokamak for Energy Production (STEP) requires high-field magnet designs and has therefore adopted the REBCO-based high-temperature superconductor (HTS) as its current carrier. The HTS enables the toroidal field (TF) coils to be remountable, which unlocks STEP’s vertical maintenance approach; however, remountable joints, approximately...

The fusion power density produced in a tokamak is proportional to its magnetic field strength to the fourth power. Second-generation high temperature superconductor (2G HTS) wires demonstrate remarkable engineering current density (averaged over the full wire), JE, at very high magnetic fields, driving progress in fusion and other applications. The...

Coated conductors formed from the high-temperature superconducting (HTS) material REBCO (REBa2Cu3O7−δ) enable energy-efficient and high-power-density delivery of electricity, making them key materials for clean energy generation, conversion, transmission and storage. Widespread application of HTS coated conductor wires requires operation at high te...

AC losses in the high temperature superconducting (HTS) toroidal field (TF) magnets of the Spherical Tokamak for Energy Production (STEP) tokamak are analysed, focusing on the transient electromagnetic response of the centre column to the charging and discharging of the central solenoid (CS) and poloidal field (PF) magnets during a plasma initiatio...

Particle irradiation offers a route to incorporating additional flux pinning centres in high-temperature superconducting wires with minimal disruption to the pre-existing defect landscape, thereby further enhancing the critical current in a controllable fashion. This work is a comprehensive study of the fluence-dependence of proton irradiation usin...

Increasingly ambitious space missions rely upon the advancement of propulsion systems, and innovations in electric propulsion represent a vital step toward potential solutions. Electric propulsion research has indicated improved thruster performance at higher magnetic fields, but real-world implementations have been encumbered by available magnet t...

As space development and exploration progress, the need for high-performance propulsion systems becomes increasingly crucial. Electric propulsion utilizing a strong magnetic field to enhance thrust performance emerges as a promising candidate. In this study, we propose a kilowatt-class plasma thruster incorporating a high-temperature superconductin...

Magnets using high-temperature superconducting (HTS) tapes have been proposed for use in several space-borne applications. In particular they are well suited for use in applied-field magnetoplasmadynamic (MPD) thrusters for satellites as they can achieve a high magnetic field intensity in a suitable volume and mass range and can be cooled by commer...

The UKAEA's Spherical Tokamak for Energy Production (STEP) programme aims to demonstrate the ability of low aspect ratio tokamaks to generate net electricity from deuterium-tritium (DT) fusion. As STEP have selected REBCO coated conductor (CC) as the current carrier in most magnet systems, understanding how REBCO CC responds to the energetic partic...

Interplanetary transport of payloads of unprecedented mass, as envisaged beyond the lunar gateway, will require thrusters with high specific impulse as well as high thrust. To achieve this, innovations in propulsion are critical. Many classes of electric thruster utilise a magnetic applied field module to accelerate charged particles. Magnetoplasma...

Heavy-ion irradiation of solids produces damage tracks with radii typically of the order of 1 nm, depending on the ion species and energy. In cuprate superconductors this is close to the coherence length, which makes these defects highly effective flux pinning centers. Varying the ion-beam energy allows tuning of the dimensionality of the defects c...

Particle irradiation using light ions and heavy ions is found to be an effective method to introduce flux-pinning centers into REBCO films and coated conductors. The degree of enhanced critical current at various conditions depends upon the size, morphology, and orientation of ion tracks. Proton irradiation to the optimised fluence results in great...

We investigate changes to the pinning landscape in ion-irradiated coated conductors through comparisons of critical current and n-values as a function of field, field angle and temperature, where n is the power law exponent in the current density-electric field relation, ${\bm{E\ }} = {{\bm{E}}}_{\bm{c}}\ {( {{\bm{J}}/{{\bm{J}}}_{\bm{c}}} )}^{\bm{...

Recent advances in commercial miniaturised cryocoolers and high-temperature superconductors (HTS) have revived the discussion of using HTS electromagnets to enhance the thrust and efficiency of electric thrusters for space applications. An HTS applied-field magnetoplasmadynamic (AF-MPD) thruster is currently being developed. While the thruster is...

Superconductor magnets generate high magnetic fields which can increase the performance of electric propulsion devices, but many such thruster operational regimes remain unexplored. In this work, a superconductor magnet and electrostatic thruster were integrated on a thrust measurement stand. Thruster performance was characterised at applied fields...

Normal-incidence irradiation by 100 MeV Ag ions is used to improve flux pinning in previously optimised commercial REBCO tapes from the American Superconductor Corporation. We observe distinct critical-current anisotropy enhancements below and above 40 K. Above 40 K a strong c-axis peak appears in the angular dependence of the critical current, as...

We have used silver-ion irradiation and proton irradiation to produce point-like and spherical defects in REBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w...

This study investigates crystal orientation evolution at microscopic level, and changes in the superconducting properties at macroscopic level of rare-earth-barium-copper-oxide (REBCO) superconductor tape under severe deformation. At the microscopic level, in-situ electron backscatter diffraction is used for real-time observation of the crystal ori...

The field-angle dependence of the critical current in a REBa2Cu3O7 coated conductor has been measured over a fine range of temperatures from 15 K to 80 K. The particular sample is demonstrated to have a very low fraction of extrinsic planar defects by its near-isotropic critical current at 77 K. At a representative magnetic field of 3 T we are able...

Applied-field magnetoplasmadynamic thrusters (AFMPDTs) have been shown to perform at high efficiencies when the magnetic field is supplied by a high temperature superconducting magnet, which can generate higher (>1 T) magnetic fields than conventional copper electromag-nets. However, mounting such strong magnets onboard a spacecraft can lead to iss...

The magnetic-field anisotropy of the critical current is an extrinsic property of superconducting wires that is of greatest relevance to the design of high temperature superconducting (HTS) devices. It is also a highly useful diagnostic tool to understand the dominant flux-pinning mechanisms active in different temperature and field regimes. REBa <...

Ion irradiation of REBCO films and coated conductors, in which the ions pass completely through the REBCO film, produces damage tracks which form near-ideal flux-pinning defects. The radius and aspect ratio of the tracks depends on the mass and energy of the incident ions. We have investigated the effect of Ag ion irradiation, at different incident...

A detailed methodology is presented for modelling the electromagnetic characteristics of HTS coils using angle-dependent critical current data obtained from experimental measurements of real wire samples. The results of such an analysis are contrasted with those obtained using more prevalent approaches such as a global minimum critical current meth...

A 6.5 MVA/25 kV high temperature superconducting (HTS) transformer for the Chinese Fuxing high-speed train has been proposed to replace the oil-based transformers while achieving higher efficiency, lighter weight, and minimized volume. The high targeted efficiency (> 99%) makes AC loss reduction a vital issue. HTS coated conductors generally exhibi...

High-temperature superconductor (HTS) based devices have the potential to be useful technologies for space applications, allowing very high current transfer and magnetic field generation. However, HTS technology requires cryogenic temperatures (<90 K) to operate, and it is not well understood if and how this can be achieved for HTS devices integrat...

High-temperature superconductivity (HTS) has the potential to be a useful technology for space applications, allowing for high current densities and magnetic field generation in compact devices. However, HTS requires cryogenic temperatures and it is not well understood how this can best be achieved in a space environment. Using a modelling approach...

We have previously reported the development of a cryogen-free critical current characterization system able to measure field-angle dependences of voltage-current characteristics of short-length superconducting tapes to temperatures of 25 K, fields up to 8 T and currents up to 875 A. We have now extended the existing system and built a parallel syst...

An asymmetric dependence of the critical current on the direction of an applied magnetic field in HTS coated conductors has a non-trivial influence on the AC loss of coil windings. We report the modelled influence of real conductor critical current asymmetry on the AC loss characteristics of a 1 MVA HTS transformer design previously demonstrated by...

Traction transformers are critical components of Chinese high-speed-trains. We are currently building a single–phase 6.5 MVA superconducting traction transformer which can achieve targets of less than 3 tons of transformer system weight, better than 99% efficiency, and 43% short-circuit impedance. The proposed transformer consists of four single-ph...

High-speed superconducting motors and generators stand to benefit from superconductor magnetic levitation bearings if their stiffness characteristics can be improved. Here we investigate a novel thrust bearing geometry, comprising a conical frustum shaped permanent magnet and matching superconducting toroid and puck assembly, aimed at producing hig...

Dynamic resistance is observed in type-II superconductors carrying a DC transport current while simultaneously exposed to an alternating magnetic field. The appearance of a non-zero resistance is attributed to the interaction between the transport current and moving fluxons. This effect is relevant to many superconductor components such as high-tem...

HTS wire cost is a critical factor for successful commercialization of HTS traction transformer technology. Wire cost might be minimized without significantly increasing AC loss by introducing a hybrid winding structure: the end-part of the windings is wound with high-cost high- I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="ht...

As applications of high-temperature superconductors (HTS) begin to move beyond the demonstrator stage to prototypes of production devices, the need to develop robust design methodologies tailored to efficiency and reliability emerges. Questions of economics also begin to play a role with multiple established manufacturers of HTS wire offering produ...

High-speed superconducting motors and generators stand to benefit from superconductor magnetic levitation bearings if their stiffness characteristics can be improved. Here we investigate a novel thrust bearing geometry, comprising a conical frustum shaped permanent magnet and matching superconducting toroid and puck assembly, aimed at producing hig...

High-speed superconducting motors and generators stand to benefit from superconductor magnetic levitation bearings if their stiffness characteristics can be improved. Here we investigate a novel thrust bearing geometry, comprising a conical frustum shaped permanent magnet and matching superconducting toroid and puck assembly, aimed at producing hig...

The possibility of p-wave pairing in superconductors has been proposed more than five decades ago, but has not yet been convincingly demonstrated. One difficulty is that some p-wave states are thermodynamically indistinguishable from s-wave, while others are very similar to d-wave states. Here we studied the self-field critical current of NdFeAs(O,...

Critical current and AC loss in coil windings are two important factors for various HTS applications. Many coated conductors exhibit asymmetry in the variation of the critical current with magnetic field angle. This asymmetry results in different coil critical current values depending on the orientation of the conductors in the coil windings. We re...

We report on the design and operation of a transport critical-current measurement option for superconductors based on the widely used Physical Property Measurement System from Quantum Design. The system is capable of supplying transport currents up to 30 A while maintaining a sample temperature of 2.0 +/- 0.1 K, and currents up to 200 A at higher s...

The possibility of p-wave pairing in superconductors has been proposed more than five decades ago, but has not yet been convincingly demonstrated. One difficulty is that some p-wave states are thermodynamically indistinguishable from s-wave, while others are very similar to d-wave states. Here we studied the self-field critical current of NdFeAs(O,...

A sharp dip in critical current has been observed in the field dependence of the critical current $I$ $_c$ of commercial AMSC second-generation high-temperature superconducting tapes on Ni-5%W substrates, when the field is applied parallel to the plane of the tape. The dip occurs centered at a non-zero field for one field polarity only – the pol...

Recently, we showed that the self-field transport critical current, Ic(sf), of a superconducting wire can be defined in a more fundamental way than the conventional (and arbitrary) electric field criterion, Ec = 1 microV/cm. We defined Ic(sf) as the threshold current, Ic,B, at which the perpendicular component of the local magnetic flux density, me...

Recently, we showed that the self-field transport critical current, Ic(sf), of a superconducting wire can be defined in a more fundamental way than the conventional (and arbitrary) electric field criterion, Ec = 1 μV/cm. We defined Ic(sf) as the threshold current, Ic,B, at which the perpendicular component of the local magnetic flux density, B⊥, me...

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The transport critical current, Ic, is usually defined in terms of a threshold electric field criterion, Ec, with the convention Ec = 1 μV/cm chosen somewhat arbitrarily to provide “reasonably small” electric power dissipation in practical devices. Thus Ic is not fundamentally determined. However, recently it has been shown that the self-field crit...

We carried out electromagnetic field analyses on the shielding-current-induced field (SCIF) of a cosine-theta dipole magnet, which is wound with coated conductors with finite electric field (E)–current density (J) characteristics and must ramp up its magnetic field. E–J characteristics, which depend on the field amplitude and orientation, were form...

We have been seeking effective ways to reduce ac losses in REBCO coil assemblies. The ac loss characteristics of assemblies of REBCO double-pancake coils (DPCs) are influenced by the magnetic interactions between coils as well as the electric field ( ${\boldsymbol E}$ )-current density ( ${\boldsymbol J}$ ) characteristics of REBCO coated conductor...

The effect of stacking faults (SF) on flux pinning and critical current (Ic) in rare earth based coated conductors was investigated. The SF density in YBa2Cu3O7−δ (YBCO) films with and without Dy addition, produced by metal organic deposition, was modified by altering the oxygenation temperature. A detailed microstructural analysis of the coated co...

We grow BaHfO3 (BHO) nanorods in REBa2Cu3O7−x (REBCO, RE: Gd or Y) thin films on metal tapes coated with the inclined substrate deposited (ISD)-MgO template by both electron beam physical vapour deposition and pulsed laser deposition. In both cases the nanorods are inclined by an angle of 21°–29° with respect to the sample surface normal as a conse...

Heavy-ion irradiation of high-temperature superconducting thin films has long been known to generate damage tracks of amorphized material that are of close-to-ideal dimension to effectively contribute to pinning of magnetic flux lines and thereby enhance the in-field critical current. At the same time, though, the presence of these tracks reduces t...

The dependence of the critical current (Ic) of an RBCO superconductor on the angle of a magnetic field applied within its basal plane (coplanar with the transport current) is commonly held to follow a variable Lorentz force behavior. Ic varies trigonometrically from some maximum value determined by flux-line cutting or other effects in the force-fr...

In high-temperature superconducting coated conductors it is well known that engineered flux pinning defects and the deposition techniques themselves often lead to strong anisotropic variations in the critical current that are not necessarily aligned with the coordinate system of the wire. Nevertheless, it is common in the literature and in practice...

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We examine the temperature, field and field angle dependence of the critical current of a SmBa2Cu3Oy coated conductor produced by reactive co-evaporation. A transmission electron microscopy based microstructural analysis shows the film contains extended c-axis defects, stacking faults, and two different species of inclined defects. By applying a ma...

New Zealand has one of the longest-running and most consistently funded (relative to GDP) programmes in high temperature superconductor (HTS) development and application worldwide. As a consequence, it has a sustained breadth of involvement in HTS technology development stretching from the materials discovery right through to burgeoning commercial...

The mechanism of formation of the superconductor Bi2Sr2CaCu2O8+x (Bi-2212) has been an open question since its discovery in 1988. By controlling crystal growth through the use of biopolymers as multivalent cation chelating agents, it is demonstrated through X-ray diffraction and thermogravimetric analysis, that it is the formation of a mixed metal...

We report the effects of heavy-ion irradiation on critical currents in (Y,Dy)Ba2Cu3O7-δ coated conductors made by a commercial metal organic deposition process. Irradiation with 185 MeV Au ions is investigated and contrasted with previous results on 74 MeV Ag ions. The change in critical current (Ic) is reported as a function of temperature (30-77...

We present highly detailed temperature, magnetic field and field angle dependent measurements of the critical current of a state-of-the-art commercial first-generation Bi-2223 high temperature superconductor wire (Sumitomo new type H high current density DI-BSCCO). By scaling the applied field values at different angles using the Blatter electron m...

Modulation of the transport properties of a superconducting La1.85Sr0.15CuO4-based ionic-liquid gated transistor has been achieved. For an applied gate bias Vg ≥ 2 V, the characteristic sheet resistivity vs. temperature (Rs–T) curves exhibit a fully reversible foot feature below the superconducting transition temperature (Tc). In contrast to the be...

This chapter shows that the main ingredient of the superconducting state is that electron pairs form a macroscopic matter wave. The well-defined phase of the matter wave leads to the ideal diamagnetism at not too large fields and to the vortex state in type-II superconductors. The chapter introduces important length scales like the London penetrati...

We report on a study of electric field-induced doping of the highly conductive ruthenate SrRuO3 using an ionic liquid as the gate dielectric in a field-effect transistor configuration. Two distinct carrier transport regimes are identified for increasing positive gate voltage in thin (10 nm) films grown heteroepitaxially on SrTiO3 substrates. For V...

A cryogenic electrical transport measurement system is described that is particularly designed to meet the requirements for routine and effective characterization of commercial second generation high-temperature superconducting (HTS) wires in the form of coated conductors based on YBa2 Cu 3O7. Specific design parameters include a base temperature o...

One of the central themes in cuprate superconductivity is the evolution of the electronic properties with the charge carrier concentration, p, expressed as the number of holes per Cu ion in the CuO2 planes. In YBa2Cu3O7−δ (YBCO), p is well known to vary with oxygen content (δ) and with substitution of Ca2+ for Y3+. In this Letter, we show that p al...

We have developed a cryogen-free critical-current (Ic) measuring system comprising a conduction-cooled 8 T HTS magnet and convection-cooled sample, both cooled by commercial cryocoolers. The sample can be rotated and transport currents of up to 800 A delivered with less than 0.5 K temperature rise during the Ic measurement. The system is automated...

We provide an alternative explanation for the results of Matsui et al. [J. Appl. Phys. 114, 233911 (2013)] by considering the effects of disorder on flux pinning.

We describe and develop an example of the successful application of a broadly applicable biomimetic growth process to the crystal growth of the complex inorganic quaternary high-temperature superconducting ceramic oxide YBa2Cu3O7−δ. By combining effective biopolymer mediation with established synthetic single crystal growth processes, we leverage a...

A new method of synthesizing the high temperature superconducting ceramic YBa2Cu3O7 with high aspect ratio platelike grains was studied and the superconducting connectivity between the grains by using Bean model was also investigated. The results indicate that there is a naturally arising superconducting connectivity between adjacent platelike grai...

The effect of different salt additions on the granular microstructure of samples of the high temperature superconductor YBa2Cu3O7δ (YBCO) synthesized by a biomimetic method has been investigated. Attempting biotemplating using salts with the rock-salt structure, most such salts were found to be chemically compatible with the YBCO biosynthesis, but...

The effect of different oxygen annealing treatments on the structural and electrical properties of samples of the high temperature superconductor YBa2Cu3O7δ (YBCO) synthesized by a biomimetic method has been studied. By oxygen annealing, the oxygen deficiency resulting from the synthesis in air can be adequately compensated. A two-stage annealing p...

The variation in transition temperature Tc with oxygen deficiency or calcium substitution in Y1-xCaxBa2Cu3O7-δ (YBCO) is a well-known manifestation of the generic hole-doping phase diagram governing the superconducting cuprates. Less well understood is the role that microstructural defects can play in determining hole doping. We have investigated t...

Analyzing critical currents from an information theoretic or statistical point of view allows one to identify distinct populations of microstates contributing to the critical current under particular conditions of temperature and applied field. We show how this knowledge can be correlated with the known microstructure of a sample to identify how di...

The biomimetic method of achieving dense platelike grain growth of the high temperature superconducting ceramic YBa2Cu3O7 (YBCO) has been studied. By using various organic matrices (dextran and chitosan) having different molecular structures, the final size and arrangement of the platelike YBCO grains exhibit significant differences, possible reaso...

In this article, we report a facile, one-pot route to phase-pure Fe3C nanoparticles (mean diameter = 20 nm) that show a remarkably high saturation magnetization (~130 emu/g), higher than iron oxide (Fe3O4) and comparable to that of bulk Fe3C (~140 emu/g). A readily available biopolymer (gelatin) is used as a matrix to disperse an aqueous iron aceta...

This work represents the first study into the addition of iridium into the sol-gel synthesis of the high temperature superconductor YBa2Cu3O7-δ (Y123). Through a biopolymer-mediated synthetic approach, the homogeneous nature of the precursor sol and the preferred nucleation and growth of Y123 phases allow for a high yield of superconducting nanopar...

A novel method of achieving microscopic morphological control during the bulk synthesis of the high temperature superconducting ceramic YBa2Cu3O7 (YBCO) has been studied. By incorporating appropriate amounts of the additives dextran (a biopolymer) and NaCl (a high melting point ionic salt) into the synthesis protocol, it is proven possible to engin...

We present a biochemical synthesis protocol for the efficient production of strongly anisotropic, plate-like crystallites of YBa2Cu3O7–δ (YBCO) of typical dimension 6 um x 6 um x 1 um. The hierarchical structure of these crystallites, revealed by transmission electron microscopy, is responsible for an unprecedented bulk intragranular critical curre...

We apply the vortex path model of critical currents to a comprehensive analysis of contemporary data on defect-engineered superconductors, showing that it provides a consistent and detailed interpretation of the experimental data for a diverse range of materials. We address the question of whether electron mass anisotropy plays a role of any conseq...

This study represents a key evolution in the synthesis of the superconductor MgB2, as it is the first to demonstrate that templated synthesis can be carried out successfully without a sealed reaction vessel. This is possible owing to the strong chelating ability of the biopolymer dextran, whose morphological complexity effectively cocoons the react...

A straightforward and reproducible method has been developed for creating < 100 nm wide, 400–600 nm long c-axis columnar defects in fluorine-free MOD YBCO films (∼700 nm thick) using ion milling through free-standing nanoporous anodic aluminum oxide membranes positioned in contact with the films. At 77.3 K, a moderate Jc increase of ∼50% was achiev...

Templated syntheses of YBCO phases have become ever more popular, owing to their fine control over phase purity and/or crystal morphology. Biological materials have recently been successfully used as templates owing to their ease of use. Here we show that the high temperature superconducting material YBa2Cu3O7−δ can be synthesized successfully usin...

Self-assembled, segmented nanorods of c-axis-aligned Ba2(Y/Gd)(Nb/Ta)O6 as well as randomly distributed nanoparticles of (Y/Gd)2O3 and (Y/Gd)Ba2Cu4O8 were grown into YBa2Cu3O7−δ (YBCO) thin films by pulsed-laser deposition. The complex pinning landscape proves to be extremely effective, particularly at higher fields where the segmented vortices yie...

Superconducting cuprate conductors present several challenges and research opportunities related to the areas of performance and cost. In order to meet these challenges, a combination of radical new approaches is required. This paper proposes several novel ideas and demonstrates early results in some of these areas.

A multilayer thin film comprising pure YBa2Cu3O7-δ (YBCO) interlayered with heavily YFeO3-doped YBCO has been fabricated on a vicinal-cut SrTiO3 substrate, thereby separating geometrically the contributions of the intrinsic flux pinning due to the YBCO ab planes and the pinning due to the magnetically-doped interlayers. It is consequently found tha...

A multilayer thin film comprising pure (YBCO) interlayered with heavily - doped YBCO has been fabricated on a vicinal-cut substrate, thereby separating geometrically the contributions of the intrinsic flux pinning due to the YBCO planes and the pinning due to the magnetically-doped interlayers. It is consequently found that the magnetic planes prov...

Crystallization of the high temperature superconductor YBa2Cu3O7-δ (Y123) in a plate-like morphology has been achieved through the use of the short-chained oligosaccharides. This is the first time that biotemplates with such a high degree of monodispersity have been used to synthesize superconducting oxides, with control over crystallization being...

The use of biopolymer templates to control crystallization of superconducting phases is a recent phenomenon which is generating a lot of interest both from the superconductor community and more widely in materials chemistry circles. The high temperature superconducting material LuBa2Cu3O7-δ has been synthesized successfully using the biopolymer dex...

An attempt has been made to prepare a YBa2Cu3O7-d (YBCO) thin film doped with ferromagnetic CoFe2O4. Transmission electron microscopy of the resultant samples shows, however, that Y(Fe,Co)O3 forms as a nanoparticulate dispersion throughout the film in preference to CoFe2O4, leaving the YBCO yttrium deficient. As a consequence, the superconducting p...