Electron Correlation Effects and High-Order Harmonic Spectrum of Perovskite BiFeO₃

We present the results of our analysis of the role of electron-electron correlations in the high-order harmonic generation (HHG) spectrum generated in BiFeO₃. The results were obtained by using the combined dynamical mean-field theory and time-dependent density-functional theory (DMFT-TDDFT) approach. Namely, the electronic spectral properties of the system were analyzed by using DFT+DMFT that takes into account the effects of strong electron correlations. Then, the TDDFT exchange-correlation kernel was calculated from the DMFT charge susceptibility and was used in the TDDFT analysis of the response of the perturbed system, with a focus on the HHG. We found that correlation (in addition to memory) effects increase the number of harmonics and the inter-band dynamics contribute more to the correlation-induced HHG compared to the intra-band one. Furthermore, correlation effects suppress emission when the pulse is directed along certain crystal axes, which can be explained by a longer free path of electrons and hence weaker dynamical correlations. The results may help to shed light on the role of correlation effects in the optical properties of strongly correlated materials like BiFeO₃, one of the most important perovskites.