Superconductivity in the A15 Structure

TL;DR

This paper reviews the history, physical properties, and significance of A15 structure superconductors, highlighting their record Tc for 32 years and discussing recent discoveries like Cs3C60 with higher Tc.

Contribution

It provides a comprehensive overview of A15 superconductors, their development, physical understanding, and recent advancements, emphasizing their enduring technological relevance.

Findings

A15 superconductors held the record for highest Tc for 32 years.

Discovery of Tc=38 K in Cs3C60 at high pressure expands the A15 superconductor family.

Despite newer materials, A15 superconductors remain crucial for high-current applications.

Abstract

The cubic A15 structure metals, with over 60 distinct member compounds, held the crown of highest Tc superconductor starting in 1954 with the discovery of Tc=18 K in Nb3Sn. Tc increased over the next 20 years until the discovery in 1973 of Tc = 22.3 K (optimized to approximately 23 K a year later) in sputtered films of Nb3Ge. Attempts were made to produce - via explosive compression - higher (theorized to be 31-35 K) transition temperatures in not stable at ambient conditions A15 Nb3Si. However, the effort to continue the march to higher Tc values in A15 Nb3Si only resulted in a defect-suppressed Tc of 19 K by 1981. Focus in superconductivity research partially shifted with the advent of heavy Fermion superconductors (CeCu2Si2, UBe13, and UPt3 discovered in 1979, 1983 and 1984 respectively) and further shifted away from A15 superconductors with the discovery of the perovskite structure cuprate superconductors in 1986 with Tc=35 K. However, the A15 superconductors, and specifically doped Nb3Sn, are still the material of choice today for most applications where high critical currents (e. g. magnets with dc persistent fields up to 21 T) are required. Thus, this article discusses superconductivity, and the important physical properties and theories for the understanding thereof, in the A15 superconductors which held the record Tc for the longest time (32 years) of any known class of superconductor since the discovery of Tc=4.2 K in Hg in 1911. The discovery in 2008 of Tc=38 K at 7 kbar in A15 Cs3C60 (properly a member of the fullerene superconductor class), which is an insulator at 1 atm pressure and otherwise also atypical of the A15 class of superconductors, will be briefly discussed.