Polaron Dynamics and Bipolarons in Cuprates

Abstract

The Frohlich electron–phonon interaction in cuprates, manganites, and other charge-transfer ionic Mott insulators is much stronger than any magnetic interaction. The polaron shift due to the Frohlich interaction, which is about 1 eV, suggests that carriers in those systems are small (bi)polarons at all temperatures and doping levels. We show both analytically and numerically that (bi)polarons exist in the itinerant Bloch states at temperatures below the characteristic phonon frequency no matter which values the parameters of the system take. The small Frohlich polaron has spectral features compatible with the single-particle tunneling and photoemission in cuprates. Whereas the band energy dispersion of intersite bipolarons is responsible for the d-wave symmetry of the condensate wave-function, the single-particle excitation spectrum is s-like in agreement with the tunneling data. Two different energy scales in Giaver tunneling and Andreev reflection experiments in cuprates can be understood in the framework of the bipolaron theory as well.