  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.
