Spin gapless semiconductors are a novel class of materials with unique electrical band structure for different spin channels in such a way that there is no band gap (i.e., 'gapless') for one spin channel while there is a finite gap in another spin channel. In a spin-gapless semiconductor, conduction and valence band edges touch, so that no threshold energy is required to move electrons from occupied (valence) states to empty (conduction) states. This gives spin-gapless semiconductors unique properties: namely that their band structures are extremely sensitive to external influences (e.g., pressure or magnetic field). Because very little energy is needed to excite electrons in an SGS, charge concentrations are very easily ‘tuneable’. For example, this can be done by introducing a new element (doping) or by application of a magnetic or electric field (gating). A new type of SGS identified in 2017, known as Dirac-type linear spin-gapless semiconductors, has linear dispersion and is considered an ideal platform for massless and dissipationless

spintronics Spintronics (a portmanteau meaning spin transport electronics), also known as spin electronics, is the study of the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-sta ...

because spin-orbital coupling opens a gap for the spin fully polarized conduction and valence band, and as a result, the interior of the sample becomes an insulator, however, an electrical current can flow without resistance at the sample edge. This effect, the

quantum anomalous Hall effect Quantum anomalous Hall effect (QAHE) is the "quantum" version of the anomalous Hall effect. While the anomalous Hall effect requires a combination of magnetic polarization and spin-orbit coupling to generate a finite Hall voltage even in the abse ...

has only previously been realised in magnetically doped topological insulators. As well as Dirac/linear SGSs, the other major category of SGS are parabolic spin gapless semiconductors. Electron mobility in such materials is two to four orders of magnitude higher than in classical semiconductors. SGSs are topologically non-trivial.

Prediction and discovery

The spin gapless semiconductor was first proposed as a new

concept and a new class of candidate spintronic materials in 2008 in a paper by

Xiaolin Wang Xiaolin Wang is a Chinese-Australian scientist recognised for his work in advanced materials synthesis and characterisation and spintronics. He is director of the Institute for Superconducting and Electronic Materials, University of Wollongong. Wan ...

of the

University of Wollongong The University of Wollongong (abbreviated as UOW) is an Australian public research university located in the coastal city of Wollongong, New South Wales, approximately 80 kilometres south of Sydney. As of 2017, the university had an enrolment of ...

in Australia.

Properties and applications

The dependence of bandgap on spin direction leads to high carrier-spin-polarization, and offers promising spin-controlled electronic and magnetic properties for spintronics applications. The spin gapless semiconductor is a promising candidate material for

because its charged particles can be fully spin-polarised, so that spin can be controlled via only a small applied external energy.

References

Condensed matter physics Semiconductors Spintronics {{CMP-stub