Surface- and strain-tuning of the optical dielectric function in epitaxially grown CaMnO3

TL;DR

This study investigates how the optical dielectric function of epitaxial CaMnO3 thin films varies with thickness and strain, revealing significant surface and strain effects that alter optical properties.

Contribution

It provides a detailed analysis combining experimental measurements and first-principles calculations to understand thickness-dependent dielectric behavior in CaMnO3 films.

Findings

Dielectric function doubles in peak value with thickness

Spectral shifts of 0.5 eV observed in optical response

Surface effects dominate in ultrathin, strain-coherent films

Abstract

We report a strong thickness dependence of the complex frequency-dependent optical dielectric function in epitaxial CaMnO3(001) thin films on SrTiO3(001), LaAlO3(001), and SrLaAlO4(001) substrates. A doubling of the peak value of the imaginary part of the dielectric function and spectral shifts of 0.5 eV for a given magnitude of absorption are observed. On the basis of the experimental data and first-principles density functional theory calculations of the dielectric function, its evolution with thickness from 4 to 63 nm has several regimes. In the thinnest, strain-coherent films, the response is characterized by a significant contribution from the free surface that dominates strain effects. However, at intermediate and larger thicknesses approaching the bulk-like film, strain coherence and partial strain relaxation persist and in influence the dielectric function.