The unexpected and therefore really amazing discovery of J. G. Bednorz
and K. A. R32 Miiller, that certain oxide compounds enter a
superconducting state at temperatures above 30 K pushed research on
superconductivity into the limelight of science in general in a way that
seemed reserved for a while for high-energy or particle physics only.
The common interest was then even more aroused when subsequent work
rather quickly established that in the same class of compounds (oxides),
critical temperatures of superconductivity above R36 the boiling point
of nitrogen could be achieved . It might therefore be expected, that
this entire review would solely deal with superconductivity at high
temperatures, i. e., above the boiling point of hydrogen. From my point
of view, however, any unexpected occurrence of superconductivity is a
challenge to scientists interested either in the physics of this
phenomenon or in its materials-science aspects. In this respect, the
last ten years have been quite revolutionary in the sense that on
various occasions, superconductivity was discovered in materials whose
physical properties were not obviously favourable for adopting this
ground state. This period started with the observation that homogeneous
coexistence of superconductivity and magnetic order in the same material
was possible. Later it was found that electrons whose effective mass was
tremendously enhanced by magnetic interactions, may also form a
superconducting state, namely in materials that were subsequently
identified as heavy- electron superconductors.