Interatomic spin-orbit coupling in atomic orbital-based tight-binding models

Abstract: Interatomic hopping mediated by spin-orbit coupling (SOC) entangles spin, orbital and sublattice degrees of freedom of electrons, leading to the emergence of intriguing phenomena such as novel topological insulators and exotic spin-dependent transport including chirality-induced spin selectivity (CISS). Despite these effects, a comprehensive microscopic formalism to describe the spin-dependent hopping remains insufficiently established. In this study, we systematically investigate SOC hopping by analytically deriving the hopping integrals within a two-center approximation based on atomic orbitals. Introducing independent parameters, or extended Slater-Koster symbols, that characterize SOC hopping, we explicitly determine the form of the hopping for $s$, $p$ and $d$ orbitals in the arbitrary hopping directions. Our formalism is then implemented in tight-binding models on several lattices. Furthermore, we examine the effect of SOC on band dispersion by employing a multipole decomposition for the SOC Hamiltonian, providing a fundamental understanding of SOC-induced phenomena. In particular, we derive an explicit expression for the SOC Hamiltonian that causes unique spin splitting in chiral systems by considering a triangular helical chain. Most importantly, the obtained SOC Hamiltonian does not contain a term that has the symmetry of electric toroidal monopole $G_0$ but rather an electric toroidal quadrupole $G_u$, which is the origin of chirality in this case.