Modified Becke-Johnson potential investigation of half-metallic Heusler compounds

We have investigated the electronic structures of various potentially half-metallic Heusler compounds with the Tran-Blaha modified Becke-Johnson+local density approximation (TB-mBJLDA) potential within the density functional theory. The half-metallic gaps are considerably enhanced with respect to values from the Perdew-Burke-Ernzerhof (PBE) functional. In particular the unoccupied densities of states are modified by the mBJLDA potential, and agreement with experiment is considerably worse than for PBE results. The agreement of the densities of states can be improved by reducing the Tran-Blaha parameter $c$. However, ground-state properties such as the hyperfine fields are more accurately described by the PBE functional than by the mBJLDA. Despite its success for ionic and covalent semiconductors and insulators, we conclude that the mBJLDA is not a suitable approximation for half-metallic Heusler compounds.

Figure 1

Density of states plots for selected Heusler and inverse Heusler compounds. Shaded blue areas correspond to PBE calculations; solid red lines represent mBJLDA calculations. Spin majority states are positive, minority states are negative. The energy scale is given with respect to the Fermi energy.

Figure 2

Top: Dependence of the half-metallic and semiconducting gaps on the mBJ model parameter $c$. Co${}_2$FeSi (open symbols) is not a half-metal for the plotted range of $c$. Bottom: DOS plots for Co${}_2$MnSi with the PBE approximation and the BJLDA ($c=1$).

Figure 3

Experimental Co $L$${}_3$ absorption spectra of Co${}_2$TiSn and Co${}_2$MnSi (bottom row) and corresponding $d$ DOSs with PBE approximation and mBJLDA. Data taken from Refs. 43 and 44.