Insights into the polaron physics of high temperature superconductors from optical and time-domain spectroscopy.

Polarons and percolation were mentioned as guiding concepts in the paper by Bednorz and Müller in their report on the discovery of superconductivity in Ba–La–Cu–O. Both of these concepts seem to have survived, but their role in high-temperature superconductivity is somewhat different than originally envisaged. Optical and time-resolved experiments have been extremely useful for understanding the roles and dynamical interplay of polaronic and itinerant charge carriers in the different states of matter that appear in the cuprate phase diagram. The present article outlines some of the connections between the original concepts, systematics and apparent dichotomies, and discusses them in the light of more recent experimental observations, leading to some important theoretical considerations on the mechanism for superconductivity. • The role of polarons in the mechanism for high T c superconductivity is presented. • Ultrafast quasiparticle dynamics and non-equilibrium superconductivity are discussed. • The coexistence of itinerant and localized states coincides with optimal T c. [ABSTRACT FROM AUTHOR]