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Publication# A novel quasi-one-dimensional topological insulator in bismuth iodide beta-Bi4I4

László Forró, Marco Grioni, Oleg Yazyev, Gabriel Albert Autes, Luca Moreschini, Andrea Pisoni, Jens Christian Johannsen, Wentao Zhang

2016

Journal paper

2016

Journal paper

Abstract

Recent progress in the field of topological states of matter(1,2) has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs; refs 3-6), followed by closely related ternary compounds(7-16) and predictions of several weak TIs (refs 17-19). However, both the conceptual richness of Z(2) classification of TIs as well as their structural and compositional diversity are far from being fully exploited. Here, a new Z(2) topological insulator is theoretically predicted and experimentally confirmed in the beta-phase of quasi-one-dimensional bismuth iodide Bi4I4. The electronic structure of beta-Bi4I4, characterized by Z(2) 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) over bar 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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A topological insulator is a material whose interior behaves as an electrical insulator while its surface behaves as an electrical conductor, meaning that electrons can only move along the surface of the material. A topological insulator is an insulator for the same reason a "trivial" (ordinary) insulator is: there exists an energy gap between the valence and conduction bands of the material. But in a topological insulator, these bands are, in an informal sense, "twisted", relative to a trivial insulator.

Topological order

In physics, topological order is a kind of order in the zero-temperature phase of matter (also known as quantum matter). Macroscopically, topological order is defined and described by robust ground state degeneracy and quantized non-Abelian geometric phases of degenerate ground states. Microscopically, topological orders correspond to patterns of long-range quantum entanglement. States with different topological orders (or different patterns of long range entanglements) cannot change into each other without a phase transition.

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