We will present our recent combined experimental and theoretical results on the band structure engineering in 3D topological insulator (3D TI) bilayers  and superlattices  grown by molecular beam epitaxy (MBE) on Si(111). These studies show how new topologies emerge in complex structures, as compared to the routine Fermi level control by alloying [3, 4]. Our results provide a starting point in search for novel topological phases.
MBE growth of Sb2Te3 and Bi2Te3 leads to the p-type and n-type material respectively, due to the low formation energy of charged vacancies and antisites. We have succeeded in growing high quality heterostructures of Sb2Te3 grown on Bi2Te3 as confirmed by atomic resolution transmission electron microscopy images. The heterostructures form a vertical p-n junction where the Fermi level position at the surface can be controlled by the thicknesses of the two layers, which has been confirmed by photoemission .
Bi-Te compounds can be grown at various stoichiometries, which at the atomic level are combinations of Bi2Te3 quintuple layers and Bi bilayers. The Bi1Te1 stoichiometry results from combining two Bi2Te3 quintuple layers with one Bi bilayer in the unit cell. In such superlattice new dual topological properties emerge. According to our theoretical predictions the material is simultaneously a topological crystalline insulator (TCI) and a weak topological insulator (WTI), and our photoemission results demonstrate the existence of TCI crossings away from the Brillouin zone center . This opens up the possibility of controlling the topological protection on different surfaces selectively by breaking respective (mirror or time-reversal) symmetries.
Encouraged by these results we propose future research directions, which include preparation of heterostructures based on ferromagnetic insulators, and search for 2D materials which exhibit non-trivial topologies. In particular we are optimizing EuO thin films as ferromagnetic insulator substrates which may enable realization of QAHE by deposition of monolayers of heavy metals. With high-resolution photoemission we are searching for band crossings in ferromagnets which locally exhibit non-zero Berry curvature, as these contribute to the intrinsic contribution to AHE, with one example being demonstration of spin-orbit gaps in Fe(001) thin film . With novel photoemission microscope we demonstrate how the band structure of single flakes of 2D materials can be probed , which enables microscopic band structure mapping of 2D materials where non-trivial phases have been predicted.
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