Effect of strong Coulomb correlations on electron-phonon interactions in the copper oxides: Implications for transport

摘要

The effects of strong Coulomb correlations on the electron-phonon interactions in the high temperature superconductors are investigated. Very strong Coulomb correlations, which are essential for creating the insulating state at half filling, lead to a suppression of charge fluctuations as the insulator is approached. This thereby significantly reduces the electron-phonon coupling. We present a self-consistent ''frozen phonon'' scheme which is based on a Coulomb renormalized band structure of the copper-oxygen plane as a method for calculating these effects and their influence on transport properties. The microscopic spectral function has a two-peak structure, each peak of which can be associated with primarily copper-like and oxygen-like motion. A high density of these low frequency copper-dominated modes is important, for it leads to a linear resistivity extending down to temperatures of the order of 50K. A Drude fit to low frequency ac conductivity measurements yields a decrease in the plasma frequency as the insulator is approached, as well as a relatively concentration x independent scattering time (tau). The magnitude of the latter provides an upper bound on the electron-phonon coupling constant, which is found to be rather weak in accord with our calculations. The behavior of the calculated plasma frequency is also consistent with experiment and its x dependence is associated with an increase in the effective mass, due to incipient localization, as the insulator is approached. On the other hand, the calculated x independence of the electron-phonon contribution to the resistivity is not compatible with the increase observed in the total resistivity slope as the insulator is approached. This suggests that the phonon background accounts for a significant fraction of the measured resistivity slope at high x, but this contribution is not sufficient to explain the behavior at smaller x.