Daniele Stradi1 Juan Manuel Marmolejo-Tejada2 Kapildeb Dolui3 Predrag Lazic4 Po-Hao Chang5 Soren Smidstrup1 Jess Wellendorff1 Petr Khomyakov1 Ulrik G. Vej-Hansen1 Maeng-Eun Lee1 Branislav Nikolic3 Kurt Stokbro1 Anders Blom6

1, Synopsys Quantumwise, Copenhagen, , Denmark
2, Universidad del Valle, Cali, , Colombia
3, University of Delaware, Newark, Delaware, United States
4, Rudjer Boskovic Institute, Zagreb, , Croatia
5, University of Nebraska–Lincoln, Lincoln, Nebraska, United States
6, Synopsys Inc., Mountain View, California, United States

The recently discovered heterostructures between topological-insulator (TI) and ferromagnetic metals (FM) are expected to pave the way for developing a plethora of novel technologically relevant spintronic effects due to the strong spin-orbit coupling (SOC) present at the TI/FM interface [1,2]. However, the lack of realistic models to describe the proximity effects on the electronic structure and on the spin-textures at the interface of the TI/FM heterostructure prevents their fundamental understanding.
In this talk I will discuss a novel approach which combines first-principles density functional theory (DFT) including non-collinear SOC effects and the non-equilibrium Green’s function (NEGF) technique. This DFT-NEGF method allows us to go beyond the traditional slab models used in DFT simulations and achieve an unprecedented insight into the electronic and spin properties of truly semi-infinite TI surfaces and TI/FM
interfaces [3,4].
We show how the proposed DFT-NEGF formalism provides an accurate description of the topologically protected surface states present at a single Bi2Se3(111) surface, which are free from spurious effects due to the interaction between surface states at the two surfaces of the slab [3]. Furthermore, we examine the band structure and the spin-texture at the interface between the Bi2Se3(111) surface and ferromagnetic cobalt, and show how the Rashba ferromagnetic model describes the spectral function on the surface of Bi2Se3(111) in contact with the cobalt film near the Fermi level, where circular and snowflake-like constant energy contours coexist which spin locks to momentum. The remnant of the Dirac cone is hybridized with evanescent wave functions injected by the metallic layer and pushed, due to charge transfer from the cobalt layer, tenths of eV below the Fermi level while hosting a distorted helical spin texture [4].
[1] Mellnik et al., Nature 511, 449 (2014)
[2] Fan et al. Nat. Mater., 13, 699 (2014)
[3] Smidstrup et al., Phys. Rev. B 96, 195309 (2017)
[4] Marmolejo-Tejada et al., Nano Lett. 2017, 17, 5626 (2017)