  Spin gapless semiconductors (SGSs) are a new class of zero gap materials
which have a fully spin polarised electrons and holes. They bridge zero gap
materials and half-metals. The band structures of the SGSs can have two types
of energy dispersions: Dirac linear dispersion and parabolic dispersion. The
Dirac type SGSs exhibit fully spin polarized Dirac cones, and offer a platform
for massless and fully spin polarized spintronics as well as dissipationless
edge state via quantum anomalous Hall effect. Due to its fascinating spin and
charge states, they hold great potential application in spintronics. There have
been tremendous efforts worldwide on searching for suitable candidates of SGSs.
In particularly, there is an increasing interest in searching for Dirac type
SGSs. In the past decade, a large number of Dirac or parabolic type SGSs have
been predicted by density functional theory and some of parabolic SGSs have
been experimentally demonstrated. The SGSs hold great potential for high speed
and low-energy consumption spintronics, electronics and optoelectronics. Here,
we review both Dirac and parabolic types of SGSs in different materials systems
and outline the concepts of SGSs, novel spin and charge states, and potential
applications of SGSs in next generation spintronic devices.
