Electronic structures and optical properties of layered perovskites Sr_2MO_4 (M=Ti, V, Cr, and Mn): An ab initio study
Abstract
A series of layered perovskites Sr
2
M
O
4
(
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 Sr
2
TiO
4
is always a nonmagnetic band insulator. For Sr
2
MnO
4
, 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 Sr
2
VO
4
and Sr
2
CrO
4
. 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 Sr
2
CrO
4
.