Evidence for a higher-order topological insulator in a three-dimensional material built from van der Waals stacking of bismuth-halide chains

Abstract

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

Higher-order topological insulator in cubic semiconductor quantum wells

“…The 3D HOTIs are gapped in the bulk and on all surfaces, but they have one-dimensional (1D) gapless modes along “hinges”, where two surfaces meet. These hinge states were experimentally observed in bismuth 16 , Bi Br 17 and WTe crystals 18 , 19 and theoretically predicted for strained SnTe 12 , transition metal dichalcogenides 20 and antiperovskites 21 .…”

High-throughput Investigations of Topological and Nodal Superconductors

“…The recently established method of symmetry indicators (SIs) [22] and topological quantum chemistry [23] enables efficient diagnosis of topological phases by examining irreducible representations (irreps) of space groups. These theories underlie recent large-scale topological material discoveries based on first-principles calculations [24][25][26][27], which have indeed assisted experimentalists to find new topological materials [28]. Very recently, the method of SIs has been extended to superconductors [29][30][31][32][33][34].…”

Topological Phase Transitions and Evolution of Boundary States Induced by Zeeman Fields in Second-Order Topological Insulators

“…Thus far, the experimental studies on higher-order topological insulators and semimetals have also been mainly done in metamaterial systems owing to their great advantages in controllability in design and measurements [58]. In comparison, in spite of the accumulation in quantum material candidates for higher-order topological insulator and semimetals [59][60][61][62][63][64][65][66][67][68][69][70][71][72], related experiments on quantum materials remain relatively rare due to the much higher complexity in material growth and detecting the expected signatures of higher-order topology [73][74][75][76][77].…”