Finite-size effects and quantum phonon fluctuations in the optical absorption edgeof dimerized chains

Abstract

We study effects of quantum phonons on the optical conductivity \ensuremath{\sigma}(\ensuremath{\omega}) for finite dimerized chains by using two numerical methods: exact numerical diagonalization and by factorizing the electron-phonon wave function (AA, adiabatic ansatz). For small chains, we compare the results of these methods and find a criterion for a valid AA. AA turns out to be: (a) a substantial improvement with respect to the adiabatic approximation usually employed in condensed-matter physics, and (b) much less affected by computer storage limitations than exact diagonalization. We use AA and get \ensuremath{\sigma}(\ensuremath{\omega}) reliably for sufficiently large chains, which could be realized experimentally. The optical gap as well as the \ensuremath{\sigma}(\ensuremath{\omega}) curve near the absorption edge possess a surprisingly strong dependence on the chain size; this could be relevant for experiments on mesoscopic systems.