Basic scientific questions and tantalizingly revolutionary applications have been intertwined throughout the 100-year history of superconductivity. Within two years of his discovery of superconductivity in 1911, H. Kamerlingh Onnes imagined high-field applications for superconducting wires, only to have his hopes dashed by limitations of upper critical field and critical current density. Over the next 98 years, a scientific tango would play out repeatedly between (1) discovering and understanding new superconductors, often with higher transition temperature values and (2) improving these materials’ upper critical field and critical current values while keeping manufacturing costs down. In this article, we take stock of where the field currently stands, with mature, developing, and recently discovered superconductors, and try to give a sense of where it may be going.

In this article, we review the reasons why high-temperature cuprate superconductors are inadequate for electric power applications, above liquid nitrogen temperatures, and examine the underlying causes. The most important reason is their low superconducting Cooper pair density, which for thermodynamic reasons reduces the theoretical maximum critical current density. We also discuss how low pair density (and high anisotropy) increase thermodynamic phase fluctuations of the macroscopic quantum pair wave function, which in turn leads to a limitation on the transition temperature itself. Finally, we discuss how, in highly correlated superconductors, there may be a conflict between the conditions necessary to achieve high transition temperatures in the face of phase fluctuations and the conditions necessary to produce strong pairing interactions.

Electric power grid applications impose many requirements on high-temperature superconductor (HTS) materials. In addition to a high superconductor transition temperature, these include all the parameters enabling a cost-effective, robust, and high-performance wire: high current-carrying capability in relevant ranges of field and temperature, flexibility and mechanical strength in a wire form, electrical and chemical stability, low ac loss, high wire uniformity, and low wire manufacturing cost with high reproducibility and yield. This daunting list explains why it has taken so long to bring HTS wires to where they are today—starting to be used in commercial power projects. The benefits of these wires are very significant: high efficiency and power density in an accessible temperature range, enabling high-capacity and easily installed cables, compact and powerful rotating machinery, and unique current-limiting functionality. However, the job is not done. Improved wire properties and reduced manufacturing costs of existing materials will further broaden the impact of this technology. Meanwhile the search for new materials—and for room-temperature superconductors—must continue, with more attention to thermal fluctuations, flux creep, and reduced anisotropy, which are critical to their application potential.

The history of superconductivity in MgB2 has been short, but intense. Ten years after its discovery, the two-gap mechanism of superconductivity in MgB2 has been mastered to a considerable extent while developing its superconducting properties in wires that meet the technical and economic requirements of industrial applications. The hope for dry superconductivity (i.e., without any liquid cryogen) using this simple and low-cost material has been recently fulfilled, with current commercial availability of MgB2-based dry MRI machines. We expect that scientific progress in understanding and developing MgB2 conductors will continue, strengthening the base for further deployment of MgB2 in applications. This article presents the main scientific and technical highlights of MgB2, describing its two-gap superconductivity, progress in improving its superconducting properties, the advances toward making MgB2 a fully recognized practical superconductor, and its prospects for ongoing and upcoming applications.

The solid state provides a richly varied fabric for intertwining chemical bonding, electronic structure, and magnetism. The discovery of superconductivity in iron pnictides and chalcogenides has revealed new aspects of this interplay, especially involving magnetism and superconductivity. Moreover, it has challenged prior thinking about high-temperature superconductivity by providing a set of materials that differ in many crucial aspects from the previously known cuprate superconductors. Here we review some of what is known about the superconductivity and its interplay with magnetism, chemistry, and electronic structure in Fe-based superconductors.

We present a brief review of the phase diagrams of the transition metal, electron-doped BaFe2As2 systems and a comparison between them. This article also reviews the phase diagrams of hole- and isoelectronic-doped BaFe2As2, as well as BaFe2As2 under pressure. Empirical rules on the conditions necessary to induce superconductivity in this material are outlined. Evidence for multiple Lifshitz transitions in Co-doped BaFe2As2 and possible connections to superconductivity are also discussed.

This article gives a brief comparative overview of the upper critical fields Hc2(T)—the magnetic fields above which superconductivity disappears—of MgB2- and Fe-based superconductors. We discuss manifestations of multiband superconductivity, Pauli pair-breaking, and pairing symmetry in the shapes of Hc2(T) curves, and the ways of tuning Hc2(T) by doping and by impurity scattering. We show that the effective route to high Hc2 is by disordering in MgB2 and by doping-assisted tuning of the Fermi surface in Fe-based superconductors. These effects allow extremely high Hc2 values in both material classes that well exceed those found in Nb-based superconductors, opening up new opportunities for high-field applications.

Graphene has been known for a long time, but only recently has its potential for electronics been recognized. Its history is recalled starting from early graphene studies. A critical insight in June 2001 brought to light that graphene could be used for electronics. This was followed by a series of proposals and measurements cumulating in a comprehensive patent for graphene-based electronics filed in 2003. The Georgia Institute of Technology (GIT) graphene electronics research project group selected epitaxial graphene as the most viable route for graphene-based electronics, as described in their 2004 paper on transport and structural measurements of epitaxial graphene. Subsequently, the field developed rapidly, and multilayer graphene was discovered at GIT. This material consists of many graphene layers, but it is not graphite; in contrast to graphite, the layers are rotated with respect to each other, causing electronic decoupling so that each layer has the electronic structure of graphene. Currently, the field has developed to the point where epitaxial graphene-based electronics may be realized in the not too distant future.