Unraveling quantum states of charge and spin in topological spintronics

August 11, 2017 - In the recent Nano Letters article, Prof. Nikolic, postdoctral fellow Dr. Kapildeb Dolui, visiting graduate student Juan Manuel Marmolejo-Tejada from Colombia and DPA alumni Dr. Po-Hao Chang, together with collaborators from Denmark and Croatia, have developed new computational approach offering a unique microscopic insight into quantum states of charge and spin at  topological-insulator/ferromagnetic-metal (TI/FM) interfaces. The control of recently observed spintronic effects in heterostructures with strong interfacial spin-orit coupling, such as TI/FM ones, is thwarted by the lack of understanding of band structure and spin textures around their interfaces. The articles demonstrates how to combine density functional theory with Green's function techniques to obtain the spectral function at any plane passing through atoms of TI and FM or normal-metal (NM) layers comprising the interface. Such spectral functions and spin textures are typically probled by spin- and angle-resolved photoemission spectroscopy, which has played an important role in the earily discoveries of topological materials, but they cannot be used to probe interfaces buried more than few nanometers below the surface.

Instead of naively assumed Dirac cone gapped by the proximity exchange field spectral function, the article shows that the Rashba ferromagnetic model describes the spectral function on the surface of Bi2Se3 in contact with Co near the Fermi level, where circular and snowflake-like constant energy contours coexist around which spin locks to momentum. The remnant of the Dirac cone is hybridized with evanescent wave functions from metallic layers and pushed, due to charge transfer from FM or NM layers, few tenths of eV below the Fermi level for both Bi2Se3/Co and Bi2Se3/Cu interfaces while hosting distorted helical spin texture wounding around a single circle.

These features explain recent observation of sensitivity of spin-to-charge conversion signal at TI/Cu interface to tuning of $E_F^0$. Crucially for spin-orbit torque in TI/FM heterostructures, few monolayers of Co adjacent to Bi2Se3 host spectral functions very different from the bulk metal, as well as in-plane spin textures (despite Co magnetization being out of plane) due to proximity spin-orbit coupling in Co induced by Bi2Se3. The article also predicts that the so-called tunneling anisotropic magnetoresistance in  Cu/Bi2Se3/Co vertical heterostructure can serve as a sensitive probe of the type of spin texture residing at the Fermi level.

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Nano Letters published by American Chemical Society reports on fundamental research in all branches of the theory and practice of nanoscience and nanotechnology, providing rapid disclosure of the key elements of a study, publishing preliminary, experimental, and theoretical results on the physical, chemical, and biological phenomena, along with processes and applications of structures within the nanoscale range.  Out of 69 journals in Nanoscience and Nanotechnology, Nano Letters is #2 in citations and impact factor (12.712 in 2016).