Oral:Oxidation and reduction processes of CaCoO₂ – CaCoO_2.5 thin films investigated by operando optical spectroscopy and density function theory calculations

Recently, new calcium cobaltate thin films, i.e., CaCoO_x, have been found to exhibit strong lattice distortions indicative of intriguing orbital and spin structures [1], which might result in a drastic change in their electronic and magnetic properties. It has attracted attention since it offers a way of tuning materials’ functional properties by oxidation and reduction processes. However, the oxidation and reduction processes of this system are not fully understood.

In this presentation, we will discuss our experimental studies on the oxidation and reduction processes of CaCoO_x (x = 2.0 or 2.5) using operando optical spectroscopic ellipsometry alongside density function theory (DFT) calculations. We have successfully observed drastic changes in the real-time optical spectra while the thin film samples undergo oxidation and reduction. Spectral analyses and DFT calculations have provided the key to understanding the electronic structures of the samples and revealed that as oxidation levels vary, there are critical electronic changes, including shifts in band gaps and distinct altermagnetic band splitting. A clear difference was observed between the dynamics of oxidation and reduction processes. Moreover, the oxidation process was affected by SrTiO₃ capping-layer thicknesses and oxygen partial pressure, indicating that the oxidation process is dominated by oxygen diffusion through the capping layer. Our finding provides valuable insight for understanding this system.









[1] W. J. Kim et al., Nature, 615, 237 (2023)