Abstract
Quasi-one-dimensional (1D) materials provide a superior platform for characterizing and tuning topological phases for two reasons: i) existence for multiple cleavable surfaces that enables better experimental identification of topological classification, and ii) stronger response to perturbations such as strain for tuning topological phases compared to higher dimensional crystal structures. In this paper, we present experimental evidence for a room-temperature topological phase transition in the quasi-1D material Bi$_4$I$_4$, mediated via a first order structural transition between two distinct stacking orders of the weakly-coupled chains. Using high resolution angle-resolved photoemission spectroscopy on the two natural cleavable surfaces, we identify the high temperature $\beta$ phase to be the first weak topological insulator with gapless Dirac cones on the (100) surface and no Dirac crossing on the (001) surface, while in the low temperature $\alpha$ phase, the topological surface state on the (100) surface opens a gap, consistent with a recent theoretical prediction of a higher-order topological insulator beyond the scope of the established topological materials databases that hosts gapless hinge states. Our results not only identify a rare topological phase transition between first-order and second-order topological insulators but also establish a novel quasi-1D material platform for exploring unprecedented physics.
Highlights
Topological phases and associated phase transitions in quantum materials have garnered tremendous interest in recent years since the discovery of the two-dimensional (2D) quantum spin Hall effect [1,2]
Benefiting from the relatively large cleavable (100) and (001) surfaces free of domains, here we unambiguously show that both phases have similar quasi-1D Fermi surfaces (FS) on the (100) side surface, and that the distinction between them is the subtle band doubling in the α phase, which was not resolved in previous studies
Topological properties of a variety of quantum materials have been verified by a combination of angleresolved photoemission spectroscopy (ARPES) and firstprinciples calculations during the past decades [10,11, 35,36,37,38,39,40,41,42]
Summary
Topological phases and associated phase transitions in quantum materials have garnered tremendous interest in recent years since the discovery of the two-dimensional (2D) quantum spin Hall effect [1,2]. The experimental identification between a strong and a weak TIs depends critically on the measurement of topological nontrivial surface states on multiple surfaces of a material, commonly probed via the technique of angleresolved photoemission spectroscopy (ARPES). The challenge of such experimental efforts is that in natural materials usually only a single preferred cleavage surface is experimentally accessible for ARPES. To resolve the aforementioned controversies, we systematically carried out electrical transport, x-ray diffraction, and ARPES measurements in combination with theoretical calculations to clarify the topological classification of α-Bi4I4 and β-Bi4I4 as tuned by the natural structural transition via temperature
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