The major breakthroughs in the understanding of topological materials over the past decade were all triggered by the discovery of the Z 2 topological insulator (TI). In three dimensions (3D), the TI is classified as either "strong" or "weak" [1, 2], and experimental confirmations of the strong topological insulator (STI) rapidly followed the theoretical predictions [3][4][5]. In contrast, the weak topological insulator has so far eluded experimental verification, since the topological surface states emerge only on particular side surfaces which are typically undetectable in real 3D crystals [6][7][8][9][10]. Here we provide experimental evidence for the WTI state in a bismuth iodide, β-Bi4I4. Significantly, the crystal has naturally cleavable top and side planes both stacked via van-der-Waals forces, which have long been desirable for the experimental realization of the WTI state [11, 12]. As a definitive signature of it, we find quasi-1D Dirac TSS at the side-surface (100) while the top-surface (001) is topologically dark. Furthermore, a crystal transition from the β-to α-phase drives a topological phase transition from a nontrivial WTI to the trivial insulator around room temperature. This topological phase, viewed as quantum spin Hall (QSH) insulators stacked threedimensionally [13, 14], and excellent functionality with on/off switching will lay a foundation for new technology benefiting from highly directional spin-currents with large density protected against backscattering.The quasi-1D compounds α-Bi 4 I 4 and β-Bi 4 I 4 share similar crystal structures, formed from arrangements of Bi 4 I 4 chains within the space group C2/m (No. 12) [15]. They differ only in their stacking sequences along the c-axis as shown in Figs. 1a and 1b. The unit cell of the β-phase consists of a single Bi 4 I 4 block, while the α-phase has a different stacking of double Bi 4 I 4 blocks along the c axis, leading to a larger cell. Despite the small difference between these two crystal structures, distinct transport properties are obtained: the α-phase exhibits a typical semiconductor-like resistivity, whereas the β-phase, in contrast, presents conductive behavior (Fig. 1c). A crystal phase transition with a hysteresis is observed in the resistivity around room temperature while the temperature is slowly swept at a rate of 3 K/min (Fig. 1d). However, since the high temperature phase (the β-phase) can be pinned by quenching crystals (Method, Supplementary information), both phases can be equally investigated at low temperatures.
The van der Waals (vdW) materials with low dimensions have been extensively studied as a platform to generate exotic quantum properties [1][2][3][4][5][6]. Advancing this view, a great deal of attention is currently paid to topological quantum materials with vdW structures, which give new concepts in designing the functionality of materials. Here, we present the first experimental realization of a higher-order topological insulator by investigating a quasi-one-dimensional (quasi-1D) bismuth bromide Bi 4 Br 4 [7][8][9][10][11] built from a vdW stacking of quantum spin Hall insulators (QSHI) [12] with angle-resolved photoemission spectroscopy (ARPES). The quasi-1D bismuth halides can select various topological phases by different stacking procedures of vdW chains, offering a fascinating playground for engineering topologically non-trivial edge-states toward future spintronics applications.The Z 2 weak topological insulator (WTI) phases have been confirmed in the materials with stacked QSHI layers, where the side-surface becomes topologically non-trivial by accumulating helical edge states of QSHI layers [13,14]. Similarly, higher-order topological insulators (HOTIs) are expected to be built from stacking QSHIs, which, however, accumulate the 1D edge-states to develop 1D helical hinge-states in a 3D crystal [15,16]. Such HOTI phases have been theoretically predicted recently in materials previously regarded as trivial insulators under the Z 2 criterion by extending the topological classification to the Z 4 topological index [17][18][19][20][21][22]. To date, only one material has been experimentally confirmed to be in the higher-order topological phase, which is bulk bismuth [23]. However, bulk bismuth is a semimetal, which cannot become insulating even by carrier doping. Materials science is, therefore, awaiting the first experimental realization of a HOTI, which enables one to explore various quantum phenomena including spin currents around hinges and quantized conductance under the external fields.A quasi-1D bismuth bromide, Bi 4 Br 4 , with a bilayer structure of chains (Fig. 1b) is theoretically predicted to be a topological crystalline insulator of Z 2,2,2,4 = {0, 0, 0, 2}, protected by the C 2 -rotation symmetry [10,11,[19][20][21]. This state should develop 2D topological surface states in the cross-section (010) of the chains [24,25]. Significantly, theory also categorizes this system as a HOTI, and expects that 1D helical hinge-states emerge between the top-surface (001) and the side-surface (100) of a crystal due to the second-order bulk-boundary correspondence [10,11]. Nevertheless, the topological phase of Bi 4 Br 4 has
The chiral crystal is characterized by a lack of mirror symmetry and an inversion center, resulting in the inequivalent right-and left-handed structures. In the noncentrosymmetric crystal structure, the spin and momentum of electrons are locked in the reciprocal space with the help of the spin-orbit interaction. To reveal the spin textures of chiral crystals, here we investigate the spin and electronic structure in p-type semiconductor elemental tellurium with a chiral crystal structure by using spin-and angle-resolved photoemission spectroscopy. Our data demonstrate that the highest valence band crossing the Fermi level has a spin component parallel to the electron momentum around the BZ corners. Significantly, we have also confirmed that the spin polarization is reversed in the crystal with the opposite chirality. The results indicate that the spin textures of the right-and left-handed chiral crystals are hedgehog-like, leading to unconventional magnetoelectric effects and nonreciprocal phenomena.
Recent progress in understanding the electronic band topology and emergent topological properties encourage us to reconsider the band structure of well-known materials including elemental substances. Controlling such a band topology by external field is of particular interest from both fundamental and technological view point. Here we report the pressure-induced topological phase transition from a semiconductor to a Weyl semimetal in elemental tellurium probed by transport measurements. Pressure variation of the periods of Shubnikov-de Haas oscillations, as well as oscillations phases, shows an anomaly around the pressure theoretically predicted for topological phase transition. This behavior can be well understood by the pressure-induced band deformation and resultant band crossing effect. Moreover, effective cyclotron mass is reduced toward the critical pressure, potentially reflecting the emergence of massless linear dispersion. The present result paves the way for studying the electronic band topology in well-known compounds and topological phase transition by the external field. 3 Significance Topological Weyl semimetal, which is a gapless semimetallic phase protected by symmetry, generally appears by band gap closing in noncentrosymmetric semiconductors. So far, there has been only a limited number of reports of such a topological phase transition so that many basic aspects have been remained unexplored.Here we report the topological phase transition under pressure in elemental tellurium with noncentrosymmetric chiral crystal structure. Our result represents that elemental tellurium is one of the model systems for the topological phase transition from a semiconductor to a Weyl semimetal, opening the avenue for studying the topological phase transition by the external field and resultant topological properties.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.