Superconductivity

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 products of widely varying performance at different price-points, although simple availability of material in the lengths required for real-world devices presently remains a factor. With the wide range of proven applications of HTS spanning the full operating parameter space of temperature and magnetic field range, a targeted approach to the specification, selection and characterisation of the source material to be used in these large engineering projects becomes required, tailored to the particular application of interest. Here, we outline a number of aspects of contemporary HTS wire performance of relevance to device design that emerge upon detailed wire characterisation under targeted application conditions, and provide pointers towards those characteristics of wire performance that feed in to the informed selection of the most appropriate wire for a given application.

Nanoscale c-axis aligned one-dimensional artificial pinning centers (1D APCs) can provide strong correlated pinning and therefore reduce the magnetic field (H) orientation-dependence of the critical current density, J _c, an issue stemming from the layered structure of REBa₂Cu₃O_7-δ (RE-123, RE = rare earth including Y, Gd, Sm, etc.). The 1D APCs are self-assembled in RE-123 matrix driven by the strain field initiated from the 1D APC/RE-123 interface strained due to the lattice mismatch. A fundamental question arises as to how the microstructure and pinning efficiency of a 1D APC are affected by the 1D APC/YBCO interface? In order to shed light on this question, two model systems of 1D APCs of BaZrO₃ (BZO) and BaHfO₃ (BHO) of comparable lateral dimensions (5–6 nm) are selected in our recent studies. However, the interface lattice mismatch with RE-123 such as YBCO and the elastic properties of BZO and BHO differ subtly. The purpose of these studies is to quantify the impact of such differences on the microstructure (especially the areal concentration) and pinning efficiency of the 1D APCs in the BZO (BHO)/YBCO nanocomposite films with APC doping levels varied in the range of 2–6 vol.%. Intriguingly, we have found the impact is substantial. Specifically, the BZO/RE-123 interface is semi-coherent with a large number of dislocations consistent with prior reports, while that of the BHO/RE-123 remains coherent even at high BHO doping levels. This affects the microstructure and pining efficiency of BZO and BHO 1D APCs significantly. The BZO 1D APC concentration and hence matching field B∗ estimated from the TEM characterization increases linearly with the BZO doping, in contrast to a nonlinear trend peaked at 4 vol.% in the BHO case. In addition, a maximum pinning force density (F _pmax) at B//c ~182.0 GNm⁻³ at B _max > 9.0 T (instrument limit) and 65 K is obtained in BHO/YBCO nanocomposites, which is significantly higher than the F _pmax~73.0 GNm⁻³ at B _max = 5.0 T in its BZO/YBCO counterpart. Moreover, the B _max/B∗ ratio in both cases decreases monotonically with APC doping. However, it is up to 2.5–3.5 in the BHO/YBCO case in contrast to the maximum of 0.6–0.7 in the BZO/YBCO case. This result reveals the critical effect of APC/RE-123 interface on the microstructure and pinning efficiency of 1D APCs.

The transport of electrical currents in superconductors with much higher efficiency and without any dissipation is considered as the “energy superhighway.” After the discovery of YBa₂Cu₃O_7-δ (YBCO), a high temperature superconductor (HTS), the prospect of using superconducting materials in practical technological applications became very prominent. With much higher T _c (~ 92 K) than conventional low temperature superconductors (LTS), YBCO was considered very promising due to cheaper cooling requirements. The evolution of critical current density (J _c), however, took long time for the material to become useful in practical applications. This was achieved through continuous modification of the processing parameters, deposition of highly oriented thin films on single crystal and buffered metallic substrates and use of artificial pinning centers (APCs) for strong pinning of quantized magnetic vortices.

Pulsed laser deposition (PLD) technique is one of the most common and highly efficient techniques for depositing highly oriented YBCO thin films on single crystal and buffered metallic substrates. Using PLD technique, APCs are incorporated into YBCO thin films by many methods which include premixed target method, alternating target method and surface modified target method.

In this chapter, the use of surface modified target method to introduce different kinds of APCs into YBCO thin films is presented. These APCs are effective in improving the vortex pinning properties of YBCO thin films for different range of applied magnetic field and its orientation depending upon their geometry and density.

The long-standing issue of critical current density (J _c) degradation with thickness in Second-Generation High-Temperature Superconductors (2G-HTS) and progress in understanding and overcoming this hurdle is reviewed. Since the early days of 2G-HTS development, it has been realized that J _c strongly decreases with film thickness, thereby preventing arbitrary increase in conductor critical current, I _c, by growing thicker films, irrespective of the deposition technique used. In addition, saturation in I _c at thickness of approximately 1.5–2 μm has been reported, implying a formation of “dead layer” that carries no current above this thickness. The phenomenon has strong implications in limiting the development of 2G-HTS both in terms of improvement in performance and decreasing cost in terms of cost/performance ratio.

The chapter reviews the progress in overcoming the thickness dependence issue starting from earliest reports on this phenomenon, and ending with recent very promising results demonstrating growth of thick films (>4 μm) with no texture degradation and record high critical current levels, both at self-field, 77 K, and under applied magnetic fields at various temperatures of interest. Different approaches, from physical models of degradation in I _c due to increasing self-field with thickness, material deposition issues such as increase in fraction of a-axis or randomly oriented grains and decreased J _c-thickness dependence in films grown with Artificial Pinning Centers (APC) are addressed. Finally, recent breakthroughs, such as reaching 1400–1500 A/cm-width at 77 K, self-field, and achieving Engineering Current Density (J _e) of over 5 kA/mm² at 4.2 K, 14 T, five times higher than Nb3Sn, in thick 2G-HTS films are reviewed.

Several articles are already published on how to synthesize the nanocrystals with desired properties, but a lack of acknowledge exist on how to use or incorporate these nanocrystals in the specific applications such as the formation of superconducting nanocomposite film. In the quest for the commercial breakthrough of coated conductors in power applications, we started to study and understand the missing link between the nanocrystal surface chemistry and the final nanocomposites performance. We successfully deposited the nanocomposite film via several chemical solution deposition techniques and optimized the growth process of epitaxial YBa₂Cu₃O_7−δ and at the same time understood what the influence of the preformed nanocrystals and the growth of pinning-active nanocrystals in the YBa₂Cu₃O_7−δ matrix.

Vortex activation energy in the AC magnetic response of superconductors takes unexpectedly high values around the DC irreversibility line, especially at low DC magnetic fields. It is shown that this is the result of a non-diffusive, thermally activated vortex hopping at short-time scales, where the hopping length overcomes the distance between the pinning centers. Vortex pinning remains significant even above the DC irreversibility line (in the “pinned” vortex liquid domain), owing to the incomplete smearing of the vortex pinning potential by thermal vortex fluctuations for a short pinning time.

Defects are ubiquitous in materials. In high-temperature superconductors (HTS), certain defects play an important role; by pinning quantized vortices in the presence of magnetic field, they enable dissipationless transport of high current densities. Therefore, determining the atomic structure of defects as well as understanding how they behave and interact is critical to control the physical properties of HTS. This chapter presents an in-depth look into the complex microstructure of YBa₂Cu₃O_7−x, a paradigmatic HTS, at different length scales using aberration-corrected scanning transmission electron microscopy (STEM). Furthermore, a synergistic combination of aberration-corrected STEM imaging, electron energy loss spectroscopy, X-ray magnetic circular dichroism, and density-functional-theory calculations have recently revealed point defects, such as individual vacancies and complex vacancy clusters, which affect the host crystal structure on a single unit-cell level. One such defect consisting of a complex of copper and oxygen vacancies is also shown to induce dilute ferromagnetism in YBCO HTS, which opens a playground to study the interaction between the two highly antagonistic phenomena by atomic-scale control over these defects.

The discovery of an iron-based high critical-temperature superconductor, fluorine-doped 1111-type LaFeAsO, in 2008 globally re-emerged extensive study on new superconductor materials. The rapid research progress for past decade in the science and technology has resulted in the accumulation of a vast amount of knowledge on materials, paring mechanisms, physical/chemical properties, and future applications. This chapter reviews entire progress in the technical status of materials, thin films, and devices such as Josephson junctions and coated conductors of iron-based superconductors.

The discovery of iron-based superconductors (FeSCs) was a surprise for the condensed matter community and became the second family of high temperature superconductors. With their attractions of very high upper critical fields and small electromagnetic anisotropy, a lot of research works have been done over past decade in accumulation of a vast amount of knowledge on materials, properties, mechanism, and applications. In this chapter, we have reviewed the current progress based on the technical applications of iron-based superconductors in terms of a future potential candidate. The basic characteristics of superconductors are summarized and define key concepts towards enhancing applied parameters such as transition temperature (T _c), upper critical field (H _c2), irreversibility field (H ^*), and critical current density (J _c).

Grain boundaries (GBs) as two-dimensional defects strongly influence and determine the current carrying capabilities of superconductors, may it be by providing efficient pinning centres or by constituting weak links. In this chapter, GB issues of polycrystalline bulks and wires as well as polycrystalline and bicrystal thin films of Fe-based superconductors are discussed for the most-studied, application-relevant systems: LnFeAs(O,F) (Ln: lanthanoid), P-, Co-, and Ni-containing AeFe₂As₂, (Ae: alkali earth), and Fe(Se,Te) in comparison to the cuprate high-T _c superconductors.

Dense samples with starting composition (MgB₂)_0.99(Te_x(HoO_1.5)_y)_0.01, x/y = 0/0, 0.73/027, 0.57/043, 0.4/0.6, 0.31/0.69, and 0.25/0.75 were processed by spark plasma sintering. Material is a bi-composite with a systematic microstructural change between “clean” and “dirty” MgB₂ regions. The maximum self-field critical current density J _c0 and volume pinning force F _{p, max} are obtained for x/y = 0.4/0.66 = 0.67 where “clean” regions separate as isolated islands in a “dirty” matrix. The maximum value of F _{p, max} is high (9.2 GN/m³ at 5 K and 4.4 GN/m³ at 20 K). In the samples with x/y > 0.67 variation of the pinning-force-related parameters is small, while a decreasing x/y < 0.67 promotes a strong variation towards grain boundary pinning. In the curves of the pinning force vs. magnetic field, a shoulder occurs for a decreasing x/y (x/y < 0.67). Two pinning-behavior regimes can be defined, namely, below and above ∼25 K. When x/y decreases, variation of the midpoint critical temperature T _{c, midpoint} and of the irreversibility field H _irr is relatively small: T _{c, midpoint} decreases with less than 0.7 K and H _irr for the co-added samples is similar or slightly higher than for pristine one. Results enable material design with controlled properties.

The transition metal oxide interface has attracted extensive attention due to its unique, strong correlation properties. In particular, LaAlO₃/SrTiO₃(LAO/STO) interface has a high mobility of two-dimensional electron gas (2DEG) and specific physical phenomena such as superconductivity, ferromagnetism, and the coexistence of superconductivity and ferromagnetism are observed. Other transition metal oxide interface systems also show high mobility and similar novel quantum phenomena. Superconducting 2DEG at the LAO/STO interface offers an appealing platform for quantum phase transition from a superconductor to a weakly localized metal. Although the research on the transition metal oxide interface is extensive, there are still many outstanding unsolved issues, such as the origin of 2DEG, the superconducting pairing mechanism of 2DEG, the origin of ferromagnetism, and the coexistence of superconductivity and ferromagnetism. Considering these aspects, the 2DEG of the transition metal oxide interface is of high interest for basic research and potential applications. This chapter presents a brief state-of-the-art view of the research results on superconductivity in 2DEG based on literature and our findings.

Within this current scientific era, there is an increased demand for enhanced global health care delivery; whereas, cost escalation remains to be a significant problem to be tackled. The effective design and integration of superconducting magnetic technologies proved to obtain cost benefits along with long-term reliability to perform superior imaging capabilities and non-invasive diagnosis of heart and brain. This article aims to provide an overview of superconducting magnetic applications and its revolutionary role within the medical field. It begins with a brief history of the development of magnets and then follows a brief, striking review of numerous superconducting applications within the medical field.