Exploration of new superconductors and functional materials, and fabrication of superconducting tapes and wires of iron pnictides

Abstract

This review shows the highlights of a 4-year-long research project supported by the Japanese Government to explore new superconducting materials and relevant functional materials. The project found several tens of new superconductors by examining ∼1000 materials, each of which was chosen by Japanese experts with a background in solid state chemistry. This review summarizes the major achievements of the project in newly found superconducting materials, and the fabrication wires and tapes of iron-based superconductors; it incorporates a list of ∼700 unsuccessful materials examined for superconductivity in the project. In addition, described are new functional materials and functionalities discovered during the project.

Endohedral gallide cluster superconductors and superconductivity in ReGa 5

“…A sharp, deep pseudogap is seen about 0.25 eV below the Fermi level, associated with 22 e-per RhGa 4.5 . Similarly to what we observed for Mo@Ga 10 clusters from molecular orbitals calculations, we find that the Rh-centered Ga 9 cluster is more stable than the empty Ga 9 cluster due to the splitting in energy of degenerate orbitals at the Fermi level (Fig. 2, Middle Left).…”

“…The perovskites are a well-known example of this in metal oxides, and in intermetallic compounds, the "122" ThCr 2 Si 2 structure type is a good example (8)(9)(10). It is the discovery of these new structural families of superconductors that often leads, sometimes slowly or sometimes quickly, to advances in new superconducting materials.…”

“…(E) Adding more Ga atoms to T-Ga systems forms other Ga-rich compounds, such as the superconductor Mo 8 Ga 41 . In this compound, Mo@Ga 10 clusters are found (23). empirical relationship between electron counting and superconducting transition temperature in these compounds; we find that as the number of electrons per formula unit decreases, the critical temperature for superconductivity first increases and then decreases.…”

“…To investigate the electronic factors behind the vertex-sharing Mo@Ga 10 cluster architecture in Mo 8 Ga 41 , we begin with the electronic structure of the hypothetical model compound Mo 8 Ga 40 , which is based on removing the Ga that is in the interstitial region between the Mo@Ga 10 clusters in Mo 8 Ga 41 . Hypothetical Mo 8 Ga 40 (i.e., MoGa 5 ) made only of the endohedral clusters sharing corner Ga, was then subject to complete structural optimization using Vienne Ab Initio Simulation Package (VASP) (27).…”

Optimal boson energy for superconductivity in the Holstein model

“…The multi-orbital nature and strong electron correlation intrinsically complicate the problem, as there can be several competing phases [12,13]. For this reason, searches of unconventional high temperature superconductors are mainly based on "perturbing the existing superconductors" (via doping, applying pressure, interfacing with other materials ... etc) and "exhausting all possible compounds" [14]. The searches of conventional high temperature superconductors, on the other hand, are essentially guided by the BCS theory, or by the more realistic Eliashberg model [15][16][17][18][19][20][21], which is different from BCS theory in its explicit inclusion of the phonon (or the Boson in general) degrees of freedom.…”