Spin-momentum locked interface states in Bi2Se3-Ni bilayer from first-principles*

Topological insulators (TIs) exhibit one of new quantum states of
matter with topologically robust Dirac surface states. Spin-momentum
locking and a possibility of inducing topological superconductivity
by doping suggest new avenues for their applications for spintronics
and quantum information. Bi2Se3 family compounds were extensively
studied due to a large bulk band gap and a single Dirac cone
at a surface. Recent experiments showed that a strong spin-transfer
torque can be induced in a thin Bi2Se3 film grown on ferromagnetic
permalloy, despite strong hybridization between the TI film and
the ferromagnetic metal. Motivated by this, we investigate the
interface effect on topological surface states in a thin Bi2Se3(111)
film underneath a ferromagnetic metal Ni film with in-plane magnetization,
by using density-functional theory (DFT). Our DFT calculations
reveal interesting interface states arising from the TI film
near the Fermi level with almost spin-momentum locking, in contrast
to previous calculations. Our DFT results are in good agreement with
effective model calculations.