Local Lattice Fluctuations and the Incoherent ARPES Background

Abstract

We consider high-temperature superconductors to be composed of molecular units that can undergo local charge fluctuations, alternating between quasi-free electrons (uncoupled to the lattice) and bipolaronic electron pairs (strongly coupled to local lattice deformations). On the basis of a generalized Boson-Fermion model, in which the internal degrees of freedom of the bosonic bipolarons are explicitly taken account, we show that the single-particle spectrum in the normal state displays a pseudogap together with a broad incoherent background arising from phonon shake-off effects, compatible with the features seen by ARPES. The behavior of pair distribution function, measuring the intracluster deformations, is characterized by a change-over from a single-peak to a double-peak structure as the temperature is increased, representing a hallmark of the lattice fluctuation origin of this incoherent background.