Rashba spin–orbit interaction effect in twisted silicon nanotubes for chiral spintronics

Abstract

Using a relativistic symmetrized linear augmented cylindrical wave method, we investigated a formation of spin minigaps due to a torsion strain of the nonchiral hexagonal and gear-like armchair ( n, n) and zigzag ( n, 0) silicon nanotubes (SiNTs). In the absence of mechanical twisting, the hexagonal ( n, n) SiNTs have an inversion symmetry and metallic band structures with the spin-degenerate states at the Fermi region. The torsion deformation of hexagonal armchair SiNT breaks the inversion symmetry, transforming it into the chiral system with the right or left screw axis. Due to the combined effect of spin–orbit coupling and low-symmetry perturbation, the degeneration of levels is completely removed and the α and β type bands are formed. The torsion deformation works like a switch, the opposite twisting directions result in the opposite chirality of tubules and induce the opposite spin currents due to Rashba effect. Even in the ideal non-twisted gear-like SiNTs, there is no inversion symmetry. Such tubules have semiconductor band structures with the spin–orbit splittings of the valence and conduction bands. Twisting causes an increase in the antisymmetric component of the potential leading to a monotonic increase in the spin gaps.