We have performed angle-resolved photoemission spectroscopy on HfSiS, which has been predicted to be a topological line-node semimetal with square Si lattice. We found a quasi-two-dimensional Fermi surface hosting bulk nodal lines, alongside the surface states at the Brillouin-zone corner exhibiting a sizable Rashba splitting and band-mass renormalization due to many-body interactions. Most notably, we discovered an unexpected Dirac-like dispersion extending one-dimensionally in k space -the Dirac-node arc -near the bulk node at the zone diagonal. These novel Dirac states reside on the surface and could be related to hybridizations of bulk states, but currently we have no explanation for its origin. This discovery poses an intriguing challenge to the theoretical understanding of topological line-node semimetals. In contrast to conventional semimetals with a finite band-overlap between valence band (VB) and conduction band (CB), topological semimetals are categorized by the band-contacting nature between VB and CB in the Brillouin zone (BZ); point-contact (Dirac/Weyl semimetals) or line-contact (line-node semimetals; LNSMs). The existence of three-dimensional (3D) Dirac semimetals was first confirmed by angle-resolved photoemission spectroscopy (ARPES) of Cd 3 As 2 [9, 10] and Na 3 Bi [11], where the VB and CB contact each other at the point (Dirac point) protected by rotational symmetry of the crystal [12,13]. Recent ARPES studies on noncentrosymmetric transition-metal monopnictides [14][15][16][17] have clarified pairs of bulk Dirac-cone bands and Fermi-arc SSs, supporting their Weyl-semimetallic nature [18,19]. While the existence of Weyl semimetals with point nodes has been confirmed experimentally, the experimental studies of LNSMs with line nodes are relatively scarce [20][21][22][23] despite many theoretical predictions [24][25][26][27][28][29][30].Recently, it was theoretically proposed by Xu et al. that ZrSiO with PbFCl-type crystal structure (space group P 4/nmm) and its isostructural family WHM (W = Zr, Hf, or La; H = Si, Ge, Sn, or Sb; M = O, S, Se and Te; see Fig. 1 . These studies demonstrated the realization of LNSM phase as well as an appearance of nearly-flat SSs around theX point, both were explained on the basis of band calculations.In this Letter, we report the ARPES results on HfSiS. In addition to the overall VB structure which is in support of the LNSM nature of HfSiS, we found new spectral features, such as a large Rashba splitting of SSs atX, a dispersion kink at ∼0.05 eV, and most importantly, unexpected Dirac-like SSs forming a "Dirac-node arc". This is a rare case in the research of topological materials that experiment finds novel SSs that were not predicted by theory.Figure 1(c) shows the ARPES-intensity plot in the VB region as a function of wave vector and binding energy (E B ) measured along theΓX cut at hν = 80 eV (see Supplemental Materials for details of sample preparation [33] and ARPES measurements). One can notice several dispersive bands; holelike bands atΓ (h) with the to...

Intrinsic magnetic topological insulators offer low disorder and large magnetic bandgaps for robust magnetic topological phases operating at higher temperatures. By controlling the layer thickness, emergent phenomena such as the quantum anomalous Hall (QAH) effect and axion insulator phases have been realized. These observations occur at temperatures significantly lower than the Néel temperature of bulk MnBi 2 Te 4 , and measurement of the magnetic energy gap at the Dirac point in ultra-thin MnBi 2 Te 4 has yet to be achieved. Critical to achieving the promise of this system is a direct measurement of the layer-dependent energy gap and verification of a temperature-dependent topological phase transition from large bandgap QAH insulator to a gapless TI paramagnetic phase. Here we utilize temperature-dependent angle-resolved photoemission spectroscopy to study epitaxial ultra-thin MnBi 2 Te 4 . We directly observe a layer-dependent crossover from a 2D ferromagnetic insulator with a bandgap greater than 780 meV in one septuple layer (1 SL) to a QAH insulator with a large energy gap (>70 meV) at 8 K in 3 and 5 SL MnBi 2 Te 4 . The QAH gap is confirmed to be magnetic in origin, as it becomes gapless with increasing temperature above 8 K.

One of the key challenges in condensedmatter physics is to establish a topological superconductor that hosts exotic Majorana fermions. Although various heterostructures consisting of conventional BCS (Bardeen−Cooper−Schrieffer) superconductors as well as doped topological insulators were intensively investigated, no conclusive evidence for Majorana fermions has been provided. This is mainly because of their very low superconducting transition temperatures (T c ) and small superconducting-gap magnitude. Here, we report a possible realization of topological superconductivity at very high temperatures in a hybrid of Bi(110) ultrathin film and copper oxide superconductor Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212). Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we found that three-bilayer-thick Bi(110) on Bi2212 exhibits a proximity-effect-induced s-wave energy gap as large as 7.5 meV which persists up to T c of Bi2212 (85 K). The small Fermi energy and strong spin−orbit coupling of Bi( 110), together with the large pairing gap and high T c , make this system a prime candidate for exploring stable Majorana fermions at very high temperatures.

Realization of topological superconductors (TSCs) hosting Majorana fermions is a central challenge in condensed-matter physics. One approach is to use the superconducting proximity effect (SPE) in heterostructures, where a topological insulator contacted with a superconductor hosts an effective p-wave pairing by the penetration of Cooper pairs across the interface. However, this approach suffers a difficulty in accessing the topological interface buried deep beneath the surface. Here, we propose an alternative approach to realize topological superconductivity without SPE. In a Pb(111) thin film grown on TlBiSe 2 , we discover that the Dirac-cone state of substrate TlBiSe 2 migrates to the top surface of Pb film and obtains an energy gap below the superconducting transition temperature of Pb. This suggests that a Bardeen-Cooper-Schrieffer superconductor is converted into a TSC by the topological proximity effect. Our discovery opens a route to manipulate topological superconducting properties of materials.

We have fabricated bismuth (Bi) ultrathin films on a charge-density-wave (CDW) compound 1T-TaS and elucidated electronic states by angle-resolved photoemission spectroscopy and first-principles band-structure calculations. We found that the Bi film on 1T-TaS undergoes a structural transition from (111) to (110) upon reducing the film thickness, accompanied by a drastic change in the energy band structure. We also revealed that while two-bilayer-thick Bi(110) film on Si(111) is characterized by a dispersive band touching the Fermi level ( E), the energy band of the same film on 1T-TaS exhibits holelike dispersion with a finite energy gap at E. We discuss the origin of such intriguing differences in terms of the CDW proximity effect.