On the role of strain- and doping-induced disorder in epitaxial CaCuO2 films: Lattice and spin dynamics in light scattering response

The infinite layer structure of CaCuO2 (CCO) represents a benchmark system for exploring the physics of CuO2 planes, which play a fundamental role in superconductive behavior at the high critical temperature of cuprates. In this work, we carried out an extensive investigation of the Raman spectrum of epitaxial films of CCO by varying the degree and origin of disorder in the crystal lattice. A stoichiometric film synthesized on a [LaAlO3]0.3[Sr2AlTaO6]0.7 substrate served as a reference to account for the nearly ideal crystal behavior, while defective samples were obtained either by intercalating oxygen in excess in the lattice or by growing the CCO film on a La2CuO4 substrate with large lattice mismatch. In the low-frequency region of the Raman spectrum, a strong correlation was observed between the degree of disorder of the film and the enhancement of the phononic activity at resonant excitation energies near the charge transfer gap. Conversely, the magnon-pair band, visible in the spectrum at excitation energies well above the charge transfer gap, displayed only subtle differences among the three samples. A systematic analysis as a function of the in-plane polarization angle identified two contributions to the magnon-pair light scattering mechanism: a dominant one with B1g representation and a secondary one with A1g representation, which appeared to be enhanced in the strained film grown on the La2CuO4 substrate. Overall, our work provides new insights into the response of CCO epitaxial films to strain- and doping-induced disorders. The results indicate a greater sensitivity of lattice dynamics to defects compared with spin dynamics in light scattering processes, suggesting a decoupling between lattice- and spin-related degrees of freedom.