Various challenges in realizing spin-gapless semiconductivity in Ti₂CoSi

Spin-gapless semiconductors are recently discovered class of materials that behave as an insulator for one spin channel and as a zero-gap semiconductor for the opposite spin. Here, we show from first-principle calculations that one such material Ti₂CoSi predicted to exhibit spin-gapless semiconductivity has an energetically close non-spin-polarized phase. In particular, we show that the regular Heusler phase of this material is non-magnetic, while the inverted Heusler phase is nearly spin-gapless semiconducting, with a very small energy difference of about 0.1 eV per 16-atom cell, in favor of the regular Heusler structure. Moreover, we also show that a 100% spin polarization in inverted Heusler phase is detrimentally affected by the emergence of surface states in thin-film geometry. These results need to be taken into account for realistic implementations of this and similar materials in nano-device applications, which rely on highly spin-polarized current in thin-film geometry.