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PUBLISHED ONLINE: 14 DECEMBER 2015 | DOI: http://dx.doi.org/10.1038/nmat4488
Web End =10.1038/NMAT4488
A novel quasi-one-dimensional topological insulator in bismuth iodide -Bi4I4
Gabriel Auts1,2, Anna Isaeva3, Luca Moreschini4, Jens C. Johannsen5, Andrea Pisoni5, Ryo Mori6,7, Wentao Zhang6,8, Taisia G. Filatova9, Alexey N. Kuznetsov9, Lszl Forr5, Wouter Van den Broek10, Yeongkwan Kim4,11, Keun Su Kim12,13, Alessandra Lanzara6,8, Jonathan D. Denlinger4, Eli Rotenberg4, Aaron Bostwick4, Marco Grioni5 and Oleg V. Yazyev1,2*
Recent progress in the eld of topological states of matter1,2
has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs; refs 36), followed by closely related ternary compounds716
and predictions of several weak TIs (refs 1719). However, both the conceptual richness of Z2 classication 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 conrmed in the -phase of quasi-one-dimensional bismuth iodide Bi4I4. The
electronic structure of -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 M point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.
The -phase of the quasi-one-dimensional (quasi-1D) bismuth iodide Bi4I4 crystallizes in a monoclinic base-centred lattice (space group C12/m1 (No. 12), a = 14.386 , b = 4.430 , c = 10.493
and = 107.9 ), with the structure shown in Fig. 1a (ref. 20).
-Bi4I4 belongs to a family of bismuth-rich iodides, which also includes -Bi4I4, Bi14I4, Bi16I4 and Bi18I4, all composed of one-dimensional molecular fragments held together by weaker non-covalent interactions21. Two modifications of the Bi4I4 crystal structure, - and -, arise from the diering stacking of molecular chains, whose structure and composition remain unchanged. The 1D building blocks of -Bi4I4, aligned along the b axis, can be viewed as narrow nanoribbons of a bismuth bilayer (four Bi atoms in width) terminated by iodine atoms (Fig. 1b). The crystal structure of -Bi4I4
features two types of non-equivalent bismuth atoms: peripheral BiA atoms saturated by covalent bonds to four iodine atoms and inner BiB...