Electronic structure and optical properties of layered perovskites Sr2 M O4 (M=Ti, V, Cr, Mn, and Co): An ab initio study

A series of layered perovskites Sr2 M O4 (M=Ti, V, Cr, and Mn) is studied by ab initio calculations within generalized gradient approximation (GGA) and 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 Sr2 Ti O4 is always a nonmagnetic band insulator. For Sr2 Mn O4, both GGA and GGA+U calculations show that the insulating AFM-II state has the lowest total energy among all the considered configurations. For M=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 GGA+U. The results indicate that U is overestimated in Sr2 V O4 and Sr2 Cr O4. To make up such a deficiency of GGA+U, the contributions from proper changes in the ligand field, acting cooperatively with U, are discussed and shown to be efficient in Sr2 Cr O4.