Spin-orbit coupling in organic spintronics1

I will talk about spin-orbit coupling (SOC) in π-conjugated organicmaterials and its effects on spin characteristics including the spin-relaxation time, spin-diffusion length, and g factor [1]. While π electrons are responsible for low-energy electrical and optical processes in π-conjugated organic solids, σ electrons must be explicitly included to properly describe the SOC. The SOC mixes upand down-spin states and, in the context of spintronics, can be quantified by an admixture parameter in the electron and hole polaron states in π-conjugated organics. Molecular geometry fluctuations such as ring torsion, which are common in soft organic materials and may depend on sample preparation, are found to have a strong effect on the spin mixing. The SOC-induced spin mixing leads to spin flips as polarons hop from one molecule to another, giving rise to spin relaxation and diffusion. The spin-relaxation rate is found to be proportional to the carrier hopping rate. The spin-diffusion length depends on the spin mixing and hopping distance but is insensitive to the carrier mobility. The SOC influences the g factor of the polaron state and makes it deviate from the free-electron value. The SOC strengths in common organics are quantified based on first-principles calculations and their values in tris-(8-hydroxyquinoline) aluminum (Alq3) and in copper phthalocyanine (CuPc) are particularly strong, due to the orthogonal arrangement of the three ligands in the former and Cu 3d orbitals in the latter. The theory quantitatively explains the recent measured spin-diffusion lengths in Alq3 from muon spin rotation and in CuPc from spin-polarized two-photon photoemission.