Anisotropy dependence of the c-axis phonon dispersion in the high-temperature superconductors

Abstract

A recent calculation for La2CuO4 using a realistic electronic band structure based on the local-density approximation (LDA) for the description of the electronic density response has predicted a low-lying plasmon branch along the c-axis coupling with the phonons of appropriate symmetry. However, the calculated frequency of the plasmon is still high enough to screen the optical phonons more perfectly as seen in the optical c-axis spectra which display the typical features of an ionic insulator, namely optical A2u modes almost unchanged from the insulator upon doping. On the other hand, this means that corresponding A2u discontinuities must show up in the phonon spectrum. However, this is in contrast with the current interpretation of the measured c-axis neutron data for a La1.9Sr0.1CuO4 crystal looking as is typical for a three-dimensional anisotropic metal in the adiabatic approximation, very similar to what we obtained in our LDA-based calculation. A possible solution for this “inconsistency” between optical and neutron results is presented by investigating systematically the dependence of the phonon dispersion along the c axis on the anisotropy of the system. Moreover, calculating the phonon-induced changes of the self-consistent potential an electron feels as a function of anisotropy we find that LDA-based calculations, which underestimate the anisotropy of optimally doped LaCuO, lead to a very strong coupling (and thus to a favorable situation for pairing) in the plasmonlike channel. On the other hand, in a more strongly anisotropic scenario the phononlike modes provide a favorable situation for pairing.

Source-reported materials — not catalogue approval

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