  We use a density-functional-theory (DFT) approach with a modified
Becke-Johnson exchange plus local density approximation (LDA) correlation
potential (mBJLDA) [semi-local, orbital-independent, producing accurate
semiconductor gaps. see F. Tran and P. Blaha, Phys. Rev. Lett. 102, 226401
(2009)] to investigate the electronic structures of zincblende transition-metal
(TM) pnictides and chalcogenides akin to semiconductors. Our results show that
this potential does not yield visible changes in wide TM d-t_{2g} bands near
the Fermi level, but makes the occupied minority-spin p-bands lower by
0.25~0.35 eV and the empty (or nearly empty) minority-spin e_g bands across the
Fermi level higher by 0.33~0.73 eV. Consequently, mBJLDA, having no
atom-dependent parameters, makes zincblende MnAs become a truly half-metallic
(HM) ferromagnet with a HM gap (the key parameter) 0.318eV, being consistent
with experiment. For zincblende MnSb, CrAs, CrSb, CrSe, or CrTe, the HM gap is
enhanced by 19~56% compared to LDA and generalized gradient approximation
results. The improved HM ferromagnetism can be understood in terms of the
mBJLDA-enhanced spin exchange splitting.
