Energy gaps and optical properties of ferromagnetic semiconductors CdCr2S4 and CdCr2Se4: Improved density-functional-theory study

We use a modified Becke-Johnson (mBJ) exchange plus local density approximation correlation functional within the density functional theory (DFT) [semi-local, orbital-independent, producing accurate semiconductor gaps, see Tran and Blaha, Phys. Rev. Lett., 102 (2009) 226401] to investigate the electronic structures of CdCr2S4 and CdCr2Se4 as two ferromagnetic semiconductors. Our results show that mBJ makes the empty Cr d-eg character bands higher by about 1.1 eV for CdCr2S4 and 0.7 eV for CdCr2Se4 for the majority-spin channel with respect to the generalized gradient approximation (GGA). For the minority-spin channel, mBJ raises the Cr d-eg character bands by about 0.8 eV for CdCr2S4 and CdCr2Se4. The mBJ gap is enhanced by about 104% for CdCr2S4 and 170% for CdCr2Se4 compared to GGA results, and is consistent with experiment in both of the cases. Our mBJ dielectric functions and the zero frequency refractive index n(0) for CdCr2S4 and CdCr2Se4 are also consistent with experimental results available. The improved DFT results can be understood in terms of mBJ-enhanced spin exchange splitting and the reduced band width of Cr d character bands. (C) 2012 Elsevier B.V. All rights reserved.