The physical properties of semiconductors such as silicon and gallium arsenide are defined in terms of their band gap energy—the separation between the valence and conduction bands. Interestingly, novel materials called ‘gapless semiconductors’ exist in which the conduction and valence band edges touch, and energy is not required to move electrons from the valence to conduction band.

Fig. 1: One of the proposed four possible band structure configurations with spin gapless features described in this paper. There is a gap between the spin-up conduction band and the spin-down valence band, while the energy gap is zero between the spin-up valence band and the spin-down conduction band. The Fermi level can be shifted up and down (as indicated in the dashed lines) by external gate voltages. Cobalt doped PdPbO2 is predicted to have the spin gapless feature as shown in this figure.

Such gapless materials— such as HgCdTe, HgZnSe and graphene—exhibit unique properties including extremely high electron mobility and sensitivity to external stimuli such as magnetic and electric fields or pressure.

Inspired by the novel properties of the gapless semiconductors, in this paper, Xiaolin Wang of the University of Wollongong in Australia1, has proposed ‘spin gapless semiconductors’ in which both electrons and holes can be fully spin polarized and the density of charge carriers and their spin polarization directions can be easily manipulated by small external excitations.

“My motivation for this work was to design a new type of semiconductors with band structures that are totally different than existing materials for semiconductor and metallic spintronic applications,” says Wang.

Spin gapless semiconductors (SGS) have unique properties including, requiring only an extremely small amount of energy to excite electrons from the valence to the conduction band; the excited charge carriers (both electrons and holes) can be 100% spin polarized simultaneously and can be separated using Hall effect; and fully spin polarized electrons or holes with tunable densities can be easily manipulated by external voltages.

Wang used first-principles electronic band structure calculations to examine the possibility of using PbPdO2— an oxide gapless semiconductor—to produce spin gapless semiconductor structures. His calculations showed that pure PbPdO2 is a nonmagnetic gapless semiconductor—the first oxide-based gapless semiconductor. Furthermore, spin was introduced into the system by incorporation of cobalt (Co) into the lattice to replace Pd.

“The spin gapless or nearly gapless, band structures proposed in this paper are expected to exist in other types of new and existing materials,” says Wang.

The properties of SGS are extremely sensitive to external stimuli, which could lead to novel physics and improved spintronic, magnetic, optical, mechanical and chemical sensor devices.