Different shapes of spin textures as a journey through the Brillouin zone

Crystallographic point group symmetry (CPGS) such as polar and nonpolar crystal classes have long been known to classify compounds that have spin-orbit-induced spin splitting. While taking a journey through the Brillouin zone (BZ) from one k-point to another for a fixed CPGS, it is expected that the wave vector point group symmetry (WPGS) can change, and consequently, a qualitative change in the texture of the spin polarization can occur [the expectation value of spin operator (S)over-right-arrow(nk0) in Bloch state u(n, k) and the wave vector k(0)]. However, the nature of the spin texture (ST) change is generally unsuspected. In this paper, we determine a full classification of the linear-in-k ST patterns based on the polarity and chirality reflected in the WPGS at k(0). The spin-polarization vector (S)over-right-arrow(nk0) controlling the ST is bound to be parallel to the rotation axis and perpendicular to the mirror planes, and hence, symmetry operation types in WPGSs impose symmetry restriction to the ST. For instance, the ST is always parallel to the wave vector k in nonpolar chiral WPGSs since they contain only rotational symmetries. Some consequences of the ST classification based on the symmetry operations in the WPGS include the observation of ST patterns that are unexpected according to the symmetry of the crystal. For example, it is usually established that spin-momentum locking effect (spin vector always perpendicular to the wave vector) requires the crystal inversion symmetry breaking by an asymmetric electric potential. However, we find that polar WPGS can have this effect even in compounds without electric dipoles or external electric fields. We use the determined relation between WPGS and ST as a design principle to select compounds with multiple STs near band edges at different k valleys. Based on high-throughput calculations for 1481 compounds, we find 37 previously fabricated materials with different STs near band edges. The ST classification as well as the predicted compounds with multiple STs can be a platform for potential application for spin-valleytronics and the control of the ST by accessing different valleys.

Automated summary

This paper provides a full classification of linear-in-k spin texture patterns based on the relationship between crystallographic point group symmetry and wave vector point group symmetry. Surprising observations, such as the presence of spin-momentum locking effect in polar compounds without external electric fields, were made. The findings suggest that compounds with multiple spin textures near band edges could have potential applications in spin-valleytronics through the control of spin texture in different valleys.