Effect of the insulator-metal transition on the phonon anomalies inLa2CuO4

Abstract

We investigate the experimentally observed anomalous softening of certain high-frequency oxygen bondstretching modes during the insulator-metal transition via the underdoped phase in La 2 CuO 4 . The calculations show that the screening processes producing the softening behavior are well described in terms of charge fluctuations on the outer shells of the ions composing the crystal. In order to discriminate between the charge response in the insulator and the metal suitable models are used being consistent with rigorous sum rules for the density response in the long-wavelength limit of the various phases. In the case of the underdoped phase a response sum rule halfway between that of an insulator and a metal is proposed expressing a loss in the partial density of states for the Cu orbitals at the Fermi level.

Oxygen phonon branches in overdopedLa1.7Sr0.3Cu3O4

“…Consequently, this issue has been addressed in several theoretical papers. Falter and co-workers start from the electronic band structure calculated within the local density approximation (LDA) [10,11,12,13,14]. The failure of LDA theory to reproduce the insulating ground-state of the undoped parent compounds is remedied by suitably imposing the long-wavelength limit of the electronic polarizability [14].…”

Microscopic modeling of phonon dynamics and charge response in metallicBaBiO3

“…In our calculations for the metallic phase of the cuprate HTSC's we have found 3,5,6 that neglecting the q-dependence in the polarizability and taking into account the diagonal on-site parameters Π 0 0 κ κ alone already provides a good approximation for the calculation of the phonon dispersion, in particular for the anomalous generic OBSM in these materials. This means that the parameters Π 0 0 κ κ are approximated by the corresponding PDOS.…”

“…27,28. In this context, our modeling of the charge response has shown that the delocalized component of the electronic state, i. e., in this case Cu4s, is not required to describe the phonon anomalies in these "phases" of the cuprates 2,3,6,20 . Polaronic effects also occur in the small non-adiabatic region around the c-axis of the cuprat-HTSC's where an insulatorlike charge response and phonon-plasmon mixing is found 20,29 .…”

“…However, such a diagonal approximation does not hold for the insulating state of a material. Due to the charge gap in the spectrum off-site elements Π 0 a κ κ ′ necessarily must occur and interfere in such a way that the following compressibility sum rules for the kinetic single particle part of the charge response are satisfied 1,3,6 .…”

“…Common to both materials, however, is the strong component of ionic binding favoring long-ranged Coulomb interactions and a related strong nonlocal electron-phonon coupling in terms of localized charge fluctuations (CF's) of the ionic orbitals. This type of coupling has been shown in the cuprate-HTSC's to lead to the generic phonon anomalies of the high-frequency oxygen bond-stretching modes (OBSM) 1,2,3,4,5 . Recently, giant phonon anomalies for similar OBSM (oxygen breathing modes) also have been found by inelastic neutron scattering measurements in superconducting Ba 0.6 K 0.4 BiO 3 7,8,9 .…”