Antiferromagnetic topological insulator with selectively gapped Dirac cones.

Antiferromagnetic (AF) topological materials offer a fertile ground to explore a variety of quantum phenomena such as axion magnetoelectric dynamics and chiral Majorana fermions. To realize such intriguing states, it is essential to establish a direct link between electronic states and topology in the AF phase, whereas this has been challenging because of the lack of a suitable materials platform. Here we report the experimental realization of the AF topological-insulator phase in NdBi.

Dirac semimetal phase and switching of band inversion in XMgBi (X = Ba and Sr).

Topological Dirac semimetals (TDSs) offer an excellent opportunity to realize outstanding physical properties distinct from those of topological insulators. Since TDSs verified so far have their own problems such as high reactivity in the atmosphere and difficulty in controlling topological phases via chemical substitution, it is highly desirable to find a new material platform of TDSs. By angle-resolved photoemission spectroscopy combined with first-principles band-structure calculations, we show that ternary compound BaMgBi is a TDS with a simple Dirac-band crossing around the Brillouin-zone center protected by the C symmetry of crystal.

Nanomosaic of Topological Dirac States on the Surface of PbBiSe Observed by Nano-ARPES.

We have performed scanning angle-resolved photoemission spectroscopy with a nanometer-sized beam spot (nano-ARPES) on the cleaved surface of PbBiSe, which is a member of the (PbSe)(BiSe) homologous series (PSBS) with m = 4 consisting of alternate stacking of the topologically trivial insulator PbSe bilayer and four quintuple layers (QLs) of the topological insulator BiSe. This allows us to visualize a mosaic of topological Dirac states at a nanometer scale coming from the variable thickness of the BiSe nanoislands (1-3 QLs) that remain on top of the PbSe layer after cleaving the PSBS crystal, because the local band structure of topological origin changes drastically with the thickness of the BiSe nanoislands. A comparison of the local band structure with that in ultrathin BiSe films on Si(111) gives us further insights into the nature of the observed topological states.

Observation of Chiral Fermions with a Large Topological Charge and Associated Fermi-Arc Surface States in CoSi.

Topological semimetals materialize a new state of quantum matter where massless fermions protected by a specific crystal symmetry host exotic quantum phenomena. Distinct from well-known Dirac and Weyl fermions, structurally chiral topological semimetals are predicted to host new types of massless fermions characterized by a large topological charge, whereas such exotic fermions are yet to be experimentally established. Here, by using angle-resolved photoemission spectroscopy, we experimentally demonstrate that a transition-metal silicide CoSi hosts two types of chiral topological fermions, a spin-1 chiral fermion and a double Weyl fermion, in the center and corner of the bulk Brillouin zone, respectively.

Fermi-level tuning of the Dirac surface state in (Bi Sb )Se thin films.

We report on the electronic states and the transport properties of three-dimensional topological insulator (Bi Sb )Se ternary alloy thin films grown on an isostructural BiSe buffer layer on InP substrates. By angle-resolved photoemission spectroscopy, we clearly detected Dirac surface states with a large bulk band gap of 0.2-0.