ar X iv : c on d - m at / 9 70 22 06 v 1 2 2 Fe b 19 97 1 Cooper pairs as low - energy excitations in the normal state

A number of studies of high-Tc superconductors show that cuprate superconductors have a short coherence length, comparable to the average distance between doped holes. While this observation casts doubt on the applicability of the BCS model, the limit of local pairs is not a good description of the cuprates either, as the existence of a Fermi surface demonstrates. A more appropriate model of a superconductor with medium-size Cooper pairs should feature both fermionic and bosonic degrees of freedom at low energies.