engleski

Charge-charge correlation functions in the Emery three-band model

The influence of the long-range Coulomb forces on the charge-charge correlation functions has been examined in the Emery three-band model. The $U_d = 0$ limit and the mean-field approximation of the $U_d \rightarrow \infty$ limit have been studied. The intraband and interband contributions to the dynamically screened correlation functions are found both for the intercell (monopole) and intracell (quadrupole) charge fluctuations. It appears that the interband monopole processes are responsible for the optical interband transitions. For strong local correlations ($U_d \rightarrow \infty$), the threshold energy of these processes is found to be only slightly dependent on the bare hybridization parameter $t^0_{;pd}; / \Delta^0_{;pd};$. The value of the threshold energy is comparable with the bare first-neighbor overlap energy $t^0_{;pd};$. As expected from experimental observations and previous static, symmetry-based theoretical considerations, the oxygen-oxygen charge correlation function is not screened in the tetragonal lattices, in contrast to the oxygen-copper ($pd$) charge correlation function. The intraband coupling of the Raman-active phonons to the $pd$ intracell charge fluctuations becomes thus substantially screened, but does not vanish, at variance with the predictions of the static-screening models. It is also found that the mean-field approximation of the $U_d \rightarrow \infty$ case can explain the measured magnitude of the plasma frequency, as well as its dependence on doping, but only in the overdoped high-$T_c$ superconductors.

superconductivity ; optical properties

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