Pseudogap and Amplitude Fluctuations in High Temperature Superconductors

Amplitude fluctuations of the pairing field are responsible together with phase fluctuations for the pseudogap phenomena in high temperature superconductors. Here we present the more detailed theory of the amplitude and phase fluctuations approach in the framework of a fermionic pairing model. New experimental comparisons are presented for the specific heat of the curprate LSCO confirming the generality of this phenomenological approach. The strong decrease of amplitude fluctuations near optimal doping induces the illusion of a "quantum critical point", which in fact does not exist since the pseudogap energy scale is always different from zero even in the overdoped regime.