Angle-resolved photoemission spectroscopy and optical renormalizations: Phonons or spin fluctuations

Improved resolution in both energy and momentum in angular-resolved photoemission spectroscopy (ARPES) data has led to the establishment of a definite energy scale in the dressed quasiparticle dispersion relations. The observed structure around 80 meV has been taken as evidence for coupling to phonons and has refocused the debate about the mechanism of superconductivity in the cuprates. Here we address the relative merits of phonon as opposed to spin-fluctuation mechanisms. Both possibilities are consistent with ARPES. On the other hand, when the considerations are extended to infrared optical data, a spin-fluctuation mechanism provides a more natural interpretation of the combined sets of data in Bi2Sr2CaCu2O8+delta.