Bound pair states beyond the condensate for Fermi systems below T_c: the pseudogap as a necessary condition

TL;DR

This paper proves that bound fermion pair states with nonzero momentum exist below T_c, implying the pseudogap phenomenon persists in the superconducting phase due to these bound states, beyond the condensate.

Contribution

It demonstrates, in a model-independent way, that bound pair states are present below T_c, explaining the pseudogap as a consequence of these states rather than solely the condensate.

Findings

Bound states of fermion pairs exist beyond the condensate below T_c.

The pseudogap persists below T_c due to these bound states.

The result is model-independent and relates to high-T_c superconductors.

Abstract

As is known, the 1/q^2 theorem of Bogoliubov asserts that the mean density of the fermion pair states with the total momentum q obeys the inequality n_q > C/q^2 (q \to 0) in the case of the Fermi system taken at nonzero temperature and in the superconducting state provided the interaction term of its Hamiltonian is locally gauge invariant. With the principle of correlation weakening it is proved in this paper that the reason for the mentioned singular behaviour of n_q is the presence of the bound states of particle pairs with nonzero total momenta. Thus, below the temperature of the superconducting phase transition there always exist the bound states of the fermion couples beyond the pair condensate. If the pseudogap observed in the normal phase of the high-T_c superconductors is stipulated by the presence of the electron bound pairs, then the derived result suggests, in a model-independent manner, that the pseudogap survives below T_c.