Energy gaps and optical properties of ferromagnetic semiconductors CdCr2S4CdCr2S4 and CdCr2Se4CdCr2Se4: 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 CdCr2S4CdCr2S4 and CdCr2Se4CdCr2Se4 as two ferromagnetic semiconductors. Our results show that mBJ makes the empty Cr d-eg character bands higher by about 1.1 eV for CdCr2S4CdCr2S4 and 0.7 eV for CdCr2Se4CdCr2Se4 for the majority-spin channel with respect to the generalized gradient approximation (GGA). For the minority-spin channel, mBJ raises the Cr d character bands by about 0.8 eV for CdCr2S4CdCr2S4 and CdCr2Se4CdCr2Se4. The mBJ gap is enhanced by about 104% for CdCr2S4CdCr2S4 and 170% for CdCr2Se4CdCr2Se4 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 CdCr2S4CdCr2S4 and CdCr2Se4CdCr2Se4 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.