Abstract
Recent progress in the field of topological states of matter1,2 has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs; refs 3,4,5,6), followed by closely related ternary compounds7,8,9,10,11,12,13,14,15,16 and predictions of several weak TIs (refs 17,18,19). However, both the conceptual richness of Z2 classification of TIs as well as their structural and compositional diversity are far from being fully exploited. Here, a new Z2 topological insulator is theoretically predicted and experimentally confirmed in the β-phase of quasi-one-dimensional bismuth iodide Bi4I4. The electronic structure of β-Bi4I4, characterized by Z2 invariants (1;110), is in proximity of both the weak TI phase (0;001) and the trivial insulator phase (0;000). Our angle-resolved photoemission spectroscopy measurements performed on the (001) surface reveal a highly anisotropic band-crossing feature located at the 
point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.
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Acknowledgements
We thank J. H. Dil, M. Ruck, M. Richter and K. Koepernik for fruitful discussions, H. Lee for discussions regarding the computational methodology, B. Kim for support during the beamtime on Merlin, M. Münch, K. Zechel and A. Weiz for assistance with synthesis and SEM/EDX measurements. We are grateful to E. Schmid for ultramicrotomy, to U. Kaiser and C. T. Koch for providing beam time for the TEM characterization. G.A. and O.V.Y. acknowledge support by the Swiss NSF (grant No. PP00P2_133552), ERC project ‘TopoMat’ (grant No. 306504) and NCCR-MARVEL. A.I. acknowledges the Priority Program 1666 ‘Topological Insulators’ of the Deutsche Forschungsgemeinschaft (DFG, grant No. IS 250/1-1). L.M. acknowledges support by the Swiss NSF (grant No. PA00P21-36420). The Advanced Light Source and the laser-based ARPES measurements, part of the Ultrafast Materials Program at Lawrence Berkeley National Laboratory, are supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract No. DE-AC02-05CH11231. W.V.d.B. acknowledges the Carl-Zeiss Foundation. Electronic structure calculations have been performed at the Swiss National Supercomputing Centre (CSCS) under project s515.
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O.V.Y. initiated and directed this research project; G.A. performed first-principles calculations; A.I. pointed out, synthesized and characterized the material; L.M. conducted the ARPES measurements, together with R.M. and W.Z. for the laser-based experiments; L.M., J.C.J. and M.G. analysed the ARPES data; A.P. and L.F. carried out the transport measurements; T.G.F. and A.N.K. performed and optimized sample preparation; W.V.d.B. assisted in the TEM experiments; Y.K., K.S.K., J.D.D., A.L., E.R. and A.B. assisted in the ARPES measurements and data analysis. All authors contributed to discussions and manuscript revision.
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Autès, G., Isaeva, A., Moreschini, L. et al. A novel quasi-one-dimensional topological insulator in bismuth iodide β-Bi4I4. Nature Mater 15, 154–158 (2016). https://doi.org/10.1038/nmat4488
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DOI: https://doi.org/10.1038/nmat4488
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