Infrared response of ordered polarons in layered perovskites

Abstract

We report on the infrared absorption spectra of three oxides where charged superlattices have been recently observed in diffraction experiments. In La 1.67 Sr 0.33 NiO 4 , polaron localization is found to suppress the low-energy conductivity through the opening of a gap and to split the E 2u -A 2u vibrational manifold of the oxygen octahedra. Similar effects are detected in Sr 1.5 La 0.5 MnO 4 and in La 2 NiO 4+y , with peculiar differences related to the type of charge ordering. 71.38.+i, 72.10Di, 78.20.Dj Typeset using REVT E X 1

Optical conductivity and charge ordering inNaxCoO2

“…[23,24] This shows that charge ordering in Na 0.5 CoO 2 affects smaller energy regions around E F . However, the strong depression of σ(ω) below 150 cm −1 is consistent with the presence of a gap over the entire Fermi surface.…”

The small-polaron crossover: Comparison between exact results and vertex correction approximation

“…We show that the crossover occurs when both strong coupling (λ > 1) and multiphonon (α 2 > 1) conditions are fulfilled leading to different relevant coupling constants (λ) in adiabatic and (α 2 ) anti adiabatic region of the parameters space. We also show that the self-consistent calculations obtained by including the first electron-phonon vertex correction give accurate results in a sizeable region of the phase diagram well separated from the polaronic crossover.Recent optical measurements of the insulating parent compounds of the high-temperature superconductors [1] show the presence of polaronic carriers, and evidence for strong electronphonon (el-ph) coupling effects has been given also for the colossal magnetoresistance manganites [2] and Nickel compounds [3]. These findings underline the necessity of a clear theoretical description of electron-phonon coupled system and more specifically of the constraints for the existence of the small polaron ground state.…”

Optical absorption and single-particle excitations in the two-dimensional Holsteint−Jmodel

“…Here the anomalous broad optical absorption detected for the La 2−x Sr x [Cu, Ni]O 4 compounds in several (normal-state) infrared experiments has attracted a lot of attention. In particular for the nickelate system, where the existence of self-trapped charge carriers is well established, the mid-infrared absorption band has been interpreted successfully in terms of photon-assisted hopping of small-size lattice polarons [16,17]. On the other hand, from a comparison of the optical absorption of several perovskites (La 1.67 Sr 0.33 NiO 4 , Sr 1.5 La 0.5 MnO 4 , and Nd 1.96 Ce 0.04 CuO 4 ), it has been argued that in the low-doping cuprates the lattice polarons appear to be rather large [18,19].…”