Microscopic modeling of phonon dynamics and charge response inNdCuO

Abstract

A description of phonon dynamics and charge response of the $n$-doped high-temperature superconductor (HTSC) $\mathrm{NdCuO}$ is presented based upon a microscopic modeling of the electronic density response. This is accomplished starting from the insulating state via the underdoped strange metallic to the more conventional metallic state by consecutive orbital selective incompressibility-compressibility transitions in terms of strict sum rules for the charge response. The approach proposed in this work for the $n$-doped HTSC's modifies the modeling recently applied to the $p$-doped compounds and expresses an electron-hole asymmetry introduced by doping. A qualitative physical picture consistent with our modeling of the electronic state in the cuprates is given in which a sufficiently broad set of orbital degrees of freedom, i.e., $\mathrm{Cu}\phantom{\rule{0.2em}{0ex}}3d∕4s$ and $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$, is essential. Within the framework of linear response theory we calculate full phonon dispersion curves in the different phases. In particular, the strongly doping dependent anomalous high-frequency oxygen bond-stretching modes (OBSM) found experimentally in the $p$-doped HTSC's and also recently for $n$-doped ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4}$ are investigated and compared with experimental results from inelastic neutron scattering and inelastic x-ray scattering, respectively. We calculate an anticrossing scenario for the OBSM in $n$-doped $\mathrm{NdCuO}$ which is absent in the case of $p$-doped $\mathrm{LaCuO}$ and relate it to the different crystal structure. Phonon-induced electronic charge redistributions of the anomalous OBSM due to nonlocal electron-phonon interaction effects of charge-fluctuation type giving reason to dynamic stripes are also studied. Finally, calculations of a characteristic rearrangement of the phonon density of states across the insulator-metal transition are presented and a comparison with experimental results has been accomplished.