Enhancement of spin polarization by chaos in graphene quantum dot systems

When graphene is placed on a substrate of heavy metal, the Rashba spin-orbit interaction of substantial strength can occur. In an open system such as a quantum dot, the interaction can induce spin polarization. Would classical dynamics have any effect on the spin polarization? Here we consider the quantum-dot setting, where the Rashba interaction is confined within the central scattering region whose geometrical shape can be chosen to yield distinct types of dynamics, e.g., regular or chaotic, in the classical limit. We find that as compared with regular or mixed dynamics, chaos can lead to significantly smooth fluctuation patterns of the spin polarization in its variation with the Fermi energy. Strikingly, in the experimentally feasible range of the Rashba interaction strength, the average polarization for a chaotic dot can be markedly larger than that for a regular or mixed dot. From the semiclassical viewpoint, a key quantity that determines the average spin polarization is the angle distribution of the outgoing electrons at the interface between regions with and without the Rashba interaction, respectively. Classical chaos generates a different distribution which, in turn, leads to higher average spin polarization. There was little previous work on the interplay between classical chaos and electron spin, and the phenomenon of chaos-enhanced spin polarization uncovered here can be exploited for spintronics applications.