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

Unconventional features of relativistic Dirac/Weyl quasi-particles in topological materials are most evidently manifested in the 2D quantum Hall effect (QHE), whose variety is further enriched by their spin and/or valley polarization. Although its extension to three dimensions has been long-sought and inspired theoretical proposals, material candidates have been lacking. Here we have discovered valley-contrasting spin-polarized Dirac fermions in a multilayer form in bulk antiferromagnet BaMnSb 2 , where the out-of-plane Zeeman-type spin splitting is induced by the in-plane inversion symmetry breaking and spin-orbit coupling (SOC) in the distorted Sb square net. Furthermore, we have observed well-defined quantized Hall plateaus together with vanishing interlayer conductivity at low temperatures as a hallmark of the half-integer QHE in a bulk form.The Hall conductance of each layer is found to be nearly quantized to 2(N+1/2)e 2 /h with N being the Landau index, which is consistent with two spin-polarized Dirac valleys protected by the strong spin-valley coupling. arXiv:2001.08683v1 [cond-mat.str-el] 23 Jan 2020 Researches of topological materials have currently been one of the central topics of the condensed matter physics. Their topologically non-trivial electronic structure leads to the relativistic quasiparticles, Dirac/Weyl fermions, whose most prominent feature is seen in QHE in 2D systems[1], such as the relativistic QHE in graphene[2, 3] and topological insulator films[4]. The half-integer quantization of the Hall plateaus and the zero-energy Landau level forming at the charge neutral Dirac point were experimentally clarified, which are associated with the Berry phase of Dirac fermions and hence have no analog in conventional 2D systems. More recently, the variety of QHE in topological materials has been further expanded by utilizing the spin and/or valley polarization in the system[5-19K21851, JP19H05173) and the Asahi Glass Foundation. The synchrotron radiation experiments were performed at the BL25SU of SPring-8 with the approval of the Japan Synchrotron

In cuprate superconductors with high critical transition temperature (Tc), light hole-doping to the parent compound, which is an antiferromagnetic Mott insulator, has been predicted to lead to the formation of small Fermi pockets. These pockets, however, have not been observed. Here, we investigate the electronic structure of the five-layered Ba2Ca4Cu5O10(F,O)2, which has inner copper oxide (CuO2) planes with extremely low disorder, and find small Fermi pockets centered at (π/2, π/2) of the Brillouin zone by angle-resolved photoemission spectroscopy and quantum oscillation measurements. The d-wave superconducting gap opens along the pocket, revealing the coexistence between superconductivity and antiferromagnetic ordering in the same CuO2 sheet. These data further indicate that superconductivity can occur without contribution from the antinodal region around (π, 0), which is shared by other competing excitations.

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A large anomalous Hall effect (AHE) has been observed in ferromagnetic Fe3Sn2 with breathing kagome bilayers. To understand the underlying mechanism for this, we investigate the electronic structure of Fe3Sn2 by angle-resolved photoemission spectroscopy (ARPES). In particular, we use both of vacuum ultraviolet and soft X-ray, which allow surface-and bulk-sensitive measurements respectively, and distinguish bulk states from surface contributions unrelated to the AHE. In contrast to surface-sensitive ARPES with vacuum ultraviolet light, our bulk-sensitive ARPES with soft X-ray reveals three-dimensional band dispersions with a periodicity of the rhombohedral unit cell in the bulk. Our data show a good consistency with a theoretical prediction based on density functional theory, suggesting a possibility that Fe3Sn2 is a magnetic Weyl semimetal. arXiv:2001.08925v1 [cond-mat.mtrl-sci]