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Register a new userThe emergence of topologically protected surface states in epitaxial Bi(111) thin films
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Quantum transport measurements including the Altshuler-Aronov-Spivak (AAS) and Aharonov-Bohm (AB) effects, universal conductance fluctuations (UCF), and weak anti-localization (WAL) have been carried out on epitaxial Bi thin films ($10-70$ bilayers) on Si(111). The results show that while the film interior is insulating all six surfaces of the Bi thin films are robustly metallic. We propose that these properties are the manifestation of a novel phenomenon, namely, a topologically trivial bulk system can become topologically non-trivial when it is made into a thin film. We stress that whats observed here is entirely different from the predicted 2D topological insulating state in a single bilayer Bi where only the four side surfaces should possess topologically protected gapless states.
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In minimally twisted bilayer graphene, a moir{e} pattern consisting of AB and BA stacking regions separated by domain walls forms. These domain walls are predicted to support counterpropogating topologically protected helical (TPH) edge states when the AB and BA regions are gapped. We fabricate designer moir{e} crystals with wavelengths longer than 50 nm and demonstrate the emergence of TPH states on the domain wall network by scanning tunneling spectroscopy measurements. We observe a double-line profile of the TPH states on the domain walls, only occurring when the AB and BA regions are gapped. Our results demonstrate a practical and flexible method for TPH state network construction.
We have performed a systematic investigation of the formation of topologically protected boundary states (TPBS) in topological/normal insulators (TI/NI) heterostructures. Using a recently developed scheme to construct {it ab-initio} tight-binding Hamiltonian matrices from density functional theory (DFT) calculations, we studied systems of realistic size with high accuracy and control over the relevant parameters such as TI and NI band alignment, NI gap and spin-orbit coupling strength. Our findings point to the existence of an NI critical thickness for the emergence of TPBS and to the importance of the band alignment between the TI and NI for the appearance of the TPBS. We chose Bi$_{2}$Se$_{3}$ as a prototypical case where the topological/normal insulator behavior is modeled by regions with/without spin-orbit coupling. Finally, we validate our approach comparing our model with fully relativistic DFT calculations for TI/NI heterostructures of Bi$_{2}$Se$_{3}$/Sb$_{2}$Se$_{3}$.
Using high-resolution angle-resolved photoemission spectroscopy, the electronic structure near the Fermi level and the topological property of the Bi(111) films grown on the Bi$_2$Te$_3$(111) substrate were studied. Very different from the bulk Bi, we found another surface band near the $bar{M}$ point besides the two well-known surface bands on the Bi(111) surface. With this new surface band, the bulk valence band and the bulk conduction band of Bi can be connected by the surface states. Our band mapping revealed odd number of Fermi crossings of the surface bands, which provided a direct experimental signature that Bi(111) thin films of a certain thickness on the Bi$_2$Te$_3$(111) substrate can be topologically nontrivial in three dimension.
The growth and characterization of epitaxial Co3O4(111) films grown by oxygen plasma-assisted molecular beam epitaxy on single crystalline a-Al2O3(0001) is reported. The Co3O4(111) grows single crystalline with the epitaxial relation Co3O4(111)[-12-1]||a-Al2O3(0001)[10-10], as determined from in situ electron diffraction. Film stoichiometry is confirmed by x-ray photoelectron spectroscopy, while ex situ x-ray diffraction measurements show that the Co3O4 films are fully relaxed. Post-growth annealing induces significant modifications in the film morphology, including a sharper Co3O4/a-Al2O3 interface and improved surface crystallinity, as shown by x-ray reflectometry, atomic force microscopy and electron diffraction measurements. Despite being polar, the surface of both as-grown and annealed Co3O4(111) films are (1 * 1), which can be explained in terms of inversion in the surface spinel structure.
Epitaxial thin films of CuMnAs have recently attracted attention due to their potential to host relativistic antiferromagnetic spintronics and exotic topological physics. Here we report on the structural and electronic properties of a tetragonal CuMnAs thin film studied using scanning tunneling microscopy (STM) and density functional theory (DFT). STM reveals a surface terminated by As atoms, with the expected semi-metallic behavior. An unexpected zigzag step edge surface reconstruction is observed with emerging electronic states below the Fermi energy. DFT calculations indicate that the step edge reconstruction can be attributed to an As deficiency that results in changes in the density of states of the remaining As atoms at the step edge. This understanding of the surface structure and step edges on the CuMnAs thin film will enable in-depth studies of its topological properties and magnetism.
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