Do pairs exist above<i>T_c</i>?

Abstract

Theories describing superconductivity in high-Tc oxides and in doped fullerenes can be placed in four categories: Fermi-liquid (FL), BCS-like schemes and non-FL theories with electro-phonon or electronic mechanisms of pairing. The main difficulties are analysed in the light of some experimental observations paying special attention to those in which pairs exist or are claimed to be observed above Tc.