The major breakthroughs in understanding of topological materials over the past decade were all triggered by the discovery of the Z2-type topological insulator—a type of material that is insulating in its interior but allows electron flow on its surface. In three dimensions, a topological insulator is classified as either ‘strong’ or ‘weak’1,2, and experimental confirmations of the strong topological insulator rapidly followed theoretical predictions3,4,5. By contrast, the weak topological insulator (WTI) has so far eluded experimental verification, because the topological surface states emerge only on particular side surfaces, which are typically undetectable in real three-dimensional crystals6,7,8,9,10. Here we provide experimental evidence for the WTI state in a bismuth iodide, β-Bi4I4. Notably, the crystal has naturally cleavable top and side planes—stacked via van der Waals forces—which have long been desirable for the experimental realization of the WTI state11,12. As a definitive signature of this state, we find a quasi-one-dimensional Dirac topological surface state at the side surface (the (100) plane), while the top surface (the (001) plane) is topologically dark with an absence of topological surface states. We also find that a crystal transition from the β-phase to the α-phase drives a topological phase transition from a nontrivial WTI to a normal insulator at roughly room temperature. The weak topological phase—viewed as quantum spin Hall insulators stacked three-dimensionally13,14—will lay a foundation for technology that benefits from highly directional, dense spin currents that are protected against backscattering.
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The data that support the findings of this study are available from the corresponding authors on reasonable request.
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We thank Y. Okada, D. Hamane, M. Lippmaa, Y. Yoshida, T. Miyamachi, T. Hattori, Y. Hasegawa and F. Komori for scanning electron microscope (SEM)/atomic force microscope (AFM)/scanning tunnelling microscope (STM) characterization of the Bi4I4 surface and for fruitful discussions; and Y. Ishida for supporting analysis of data. We thank the Diamond Light Source for access to beamline I05 under proposals SI15095, SI16161 and SI17816, contributing to the results presented here. We also thank the Elettra Light Source for access to the spectromicroscopy beamline. The GISAXS experiments were performed under the approval of Public Finance–Public Accountability Collective (PF–PAC) no. 2016G548. The work done at the Tokyo Institute of Technology was supported by a JST Core Research for Evolutional Science and Technology (CREST) project (JPMJCR16F2) and a Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (B) (JP16H03847). R.N. acknowledges support from JSPS under KAKENHI grant JP18J21892, and by JSPS through the Program for Leading Graduate Schools (Advanced Leading Course for Photon Science). This work was also supported by the ‘Topological Materials Science’ (JP16H00979) KAKENHI on Innovative Areas from JSPS, and by JSPS KAKENHI grants JP16H06013, JP17K14319, JP16H02209, JP16K13829, and JP18H01165. R.A. acknowledges support from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, under KAKENHI grant JP16H06345.
Nature thanks J. Sanchez-Bárriga and the other anonymous reviewer(s) for their contribution to the peer review of this work.