Electronic structure and optical properties of layered perovskites Sr2MO4 (M=Ti, V, Cr, and Mn) : An ab initio study

A series of layered perovskites ${\mathrm{Sr}}_{2}M{\mathrm{O}}_{4}$ ($M=\mathrm{Ti}$, V, Cr, and Mn) is studied by ab initio calculations within generalized gradient approximation (GGA) and $\mathrm{GGA}+U$ schemes. The total energies in different magnetic configurations, including the nonmagnetic, ferromagnetic, the layered antiferromagnetic with alternating ferromagnetic plane, and the staggered in-plane antiferromagnetic (AFM-II) order, are calculated. It is found that ${\mathrm{Sr}}_{2}\mathrm{Ti}{\mathrm{O}}_{4}$ is always a nonmagnetic band insulator. For ${\mathrm{Sr}}_{2}\mathrm{Mn}{\mathrm{O}}_{4}$, both GGA and $\mathrm{GGA}+U$ calculations show that the insulating AFM-II state has the lowest total energy among all the considered configurations. For $M=\mathrm{V}$ and Cr, the GGA is not enough to give out the insulating AFM-II states and including the on-site electron-electron correlation effect $U$ is necessary and efficient. The AFM-II state will have the lowest total energy in both cases when $U$ is larger than a critical value. Further, the optical conductivity spectra are calculated and compared with the experimental measurements to show how well the ground state is described within the GGA or $\mathrm{GGA}+U$. The results indicate that $U$ is overestimated in ${\mathrm{Sr}}_{2}\mathrm{V}{\mathrm{O}}_{4}$ and ${\mathrm{Sr}}_{2}\mathrm{Cr}{\mathrm{O}}_{4}$. To make up such a deficiency of $\mathrm{GGA}+U$, the contributions from proper changes in the ligand field, acting cooperatively with $U$, are discussed and shown to be efficient in ${\mathrm{Sr}}_{2}\mathrm{Cr}{\mathrm{O}}_{4}$.