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

We report a strong thickness dependence of the complex frequency-dependent optical dielectric function $ϵ̃(ω)$ over a spectral range from 1.24 to 5 eV in epitaxial CaMnO₃(001) thin films on SrTiO₃(001), LaAlO₃(001), and SrLaAlO₄(001). A doubling of the peak value of the imaginary part of $ϵ̃(ω)$ and spectral shifts of 0.5 eV for a given magnitude of absorption are observed. On the basis of experimental analyses and first-principles density functional theory calculations, contributions from both surface states and epitaxial strain to the optical dielectric function of CaMnO₃ are seen. 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 coexist and influence $ϵ̃(ω)$⁠.