Topological insulators and semimetals as well as unconventional iron-based superconductors have attracted major recent attention in condensed matter physics. Previously, however, little overlap has been identified between these two vibrant fields, even though the principal combination of topological bands and superconductivity promises exotic unprecedented avenues of superconducting states and Majorana bound states (MBSs), the central building block for topological quantum computation. Along with progressing laser-based spin-resolved and angle-resolved photoemission spectroscopy (ARPES) towards high energy and momentum resolution, we have resolved topological insulator (TI) and topological Dirac semimetal (TDS) bands near the Fermi level (E F ) in the iron-based superconductors Li(Fe,Co)As and Fe(Te,Se), respectively. The TI and TDS bands can be individually tuned to locate close to E F by carrier doping, allowing to potentially access a plethora of different superconducting topological states in the same material. Our results reveal the generic coexistence of superconductivity and multiple topological states in iron-based superconductors, rendering these materials a promising platform for high-T c topological superconductivity.High-T c iron-based superconductors feature multiple bands near E F , which enhances the difficulty in understanding the details of unconventional pairing 1-3 . It, however, also allows for a wealth of, possibly topologically non-trivial, electronic bands, of which a recent example is the TI states discovered in the ironbased superconductor Fe(Te,Se) 4 , hinting at a promising direction to realize topological superconductivity and MBSs 5-9 . In view of Fe(Te,Se), a pressing subsequent question is to which extent this marks a generic phe-nomenon in different classes of iron-based high-T c superconductors. In this work, we find that the emergence of non-trivial topological bands near the Fermi level is indeed a common feature of various iron-based superconductors. Our first-principles calculations reveal that BaFe 2 As 2 , LiFeAs and Fe(Te,Se) all exhibit band inversions along k z . To confirm these calculations, the band structures of Li(Fe,Co)As and Fe(Te,Se) were investigated by laser-based high-resolution ARPES. Firstly, we observe that TI bands reminiscent of Fe(Te,Se) exist in Li(Fe,Co)As as well, supporting the generic existence of non-trivial topology in iron-based superconductors. Secondly and more interestingly, we predict and observe TDS bands in Li(Fe,Co)As and Fe(Te,Se), which we investigate via high-resolution ARPES, spin-resolved ARPES (SARPES), and magnetoresistance (MR) measurements. Finally, we discuss the phase diagram of these topological high-T c compounds as a function of Fermi level (doping). The combination of topological states and superconductivity may produce not only surface topological superconductivity deriving from the TI edge states, but also bulk topological superconductivity from the TDS bands.Normal insulator (NI), TI, and TDS constitute topologically disti...

Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi2Se4/Bi2Se3 heterostructure, which was fabricated by self-assembling a MnBi2Se4 layer on top of the Bi2Se3 surface as a result of the codeposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBi2Se4/Bi2Se3 heterostructure shows ferromagnetism up to room temperature and a clear Dirac cone gap opening of ∼100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number C = −1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future “topotronics” devices.

Magnetically doped topological insulators, possessing an energy gap created at the Dirac point through time-reversal-symmetry breaking, are predicted to exhibit exotic phenomena including the quantized anomalous Hall effect and a dissipationless transport, which facilitate the development of low-power-consumption devices using electron spins. Although several candidates of magnetically doped topological insulators were demonstrated to show long-range magnetic order, the realization of the quantized anomalous Hall effect is so far restricted to the Cr-doped (Sb,Bi)2Te3 system at extremely low temperature; however, the microscopic origin of its ferromagnetism is poorly understood. Here we present an element-resolved study for Cr-doped (Sb,Bi)2Te3 using X-ray magnetic circular dichroism to unambiguously show that the long-range magnetic order is mediated by the p-hole carriers of the host lattice, and the interaction between the Sb(Te) p and Cr d states is crucial. Our results are important for material engineering in realizing the quantized anomalous Hall effect at higher temperatures.

Recent breakthrough in search for the analogs of fundamental particles in condensed matter systems lead to experimental realizations of 3D Dirac and Weyl semimetals [1][2][3][4][5]6]. Weyl state can be hosted either by non-centrosymmetric or magnetic materials and can be of the first or the second type [7][8][9][10]. Several non-centrosymmetric materials have been proposed to be type-II Weyl semimetals, but in all of them the Fermi arcs between projections of multiple Weyl points either have not been observed directly or they were hardly distinguishable from the trivial surface states which significantly hinders the practical application of these materials [11][12][13][14][15][16][17][18][19][20][21]. Here we present experimental evidence for type-II non-centrosymmetric Weyl state in TaIrTe 4 where it has been predicted theoretically [22]. We find direct correspondence between ARPES spectra and calculated electronic structure both in the bulk and the surface and clearly observe the exotic surface states which support the quasi-1D Fermi arcs connecting only four Weyl points. Remarkably, these electronic states are spin-polarized in the direction along the arcs, thus highlighting TaIrTe 4 as a novel material with promising application potential.Weyl semimetals are compounds hosting Weyl fermions, which have a fundamental importance. These fermions are quasiparticles described by the Weyl equation [23] thus being a condensedmatter realization of the elusive Weyl particles, long sought in high-energy physics. The exotic behavior is attributed to the electronic states emerging due to crossings of non-degenerate bands in a single point of k-space, the Weyl point, which can be realized in the materials where ei-1 arXiv:1609.09549v1 [cond-mat.mes-hall]

Topological insulators (TIs) are a new quantum state of matter. Their surfaces and interfaces act as a topological boundary to generate massless Dirac fermions with spin-helical textures. Investigation of fermion dynamics near the Dirac point (DP) is crucial for the future development of spintronic devices incorporating topological insulators. However, research so far has been unsatisfactory because of a substantial overlap with the bulk valence band and a lack of a completely unoccupied DP. Here, we explore the surface Dirac fermion dynamics in the TI Sb2Te3 by time- and angle-resolved photoemission spectroscopy (TrARPES). Sb2Te3 has an in-gap DP located completely above the Fermi energy (EF). The excited electrons in the upper Dirac cone stay longer than those below the DP to form an inverted population. This was attributed to a reduced density of states (DOS) near the DP.

Weyl semimetals are characterized by the presence of massless band dispersion in momentum space. When a Weyl semimetal meets magnetism, large anomalous transport properties emerge as a consequence of its topological nature. Here, using in−situ spin- and angle-resolved photoelectron spectroscopy combined with ab initio calculations, we visualize the spin-polarized Weyl cone and flat-band surface states of ferromagnetic Co2MnGa films with full remanent magnetization. We demonstrate that the anomalous Hall and Nernst conductivities systematically grow when the magnetization-induced massive Weyl cone at a Lifshitz quantum critical point approaches the Fermi energy, until a high anomalous Nernst thermopower of  ~6.2 μVK−1 is realized at room temperature. Given this topological quantum state and full remanent magnetization, Co2MnGa films are promising for realizing high efficiency heat flux and magnetic field sensing devices operable at room temperature and zero-field.

A synergistic effect between strong electron correlation and spin–orbit interaction has been theoretically predicted to realize new topological states of quantum matter on Kondo insulators (KIs), so-called topological Kondo insulators (TKIs). One TKI candidate has been experimentally observed on the KI SmB6(001), and the origin of the surface states (SS) and the topological order of SmB6 has been actively discussed. Here, we show a metallic SS on the clean surface of another TKI candidate YbB12(001) using angle-resolved photoelectron spectroscopy. The SS shows temperature-dependent reconstruction corresponding to the Kondo effect observed for bulk states. Despite the low-temperature insulating bulk, the reconstructed SS with c–f hybridization is metallic, forming a closed Fermi contour surrounding on the surface Brillouin zone and agreeing with the theoretically expected behaviour for SS on TKIs. These results demonstrate the temperature-dependent holistic reconstruction of two-dimensional states localized on KIs surface driven by the Kondo effect.

Conventional Rashba spin polarization is caused by the combination of strong spin–orbit interaction and spatial inversion asymmetry. However, Rashba–Dresselhaus-type spin-split states are predicted in the centrosymmetric LaOBiS2 system by recent theory, which stem from the local inversion asymmetry of active BiS2 layer. By performing high-resolution spin- and angle-resolved photoemission spectroscopy, we have investigated the electronic band structure and spin texture of superconductor LaO0.55F0.45BiS2. Here we present direct spectroscopic evidence for the local spin polarization of both the valence band and the conduction band. In particular, the coexistence of Rashba-like and Dresselhaus-like spin textures has been observed in the conduction band. The finding is of key importance for fabrication of proposed dual-gated spin-field effect transistor. Moreover, the spin-split band leads to a spin–momentum locking Fermi surface from which superconductivity emerges. Our demonstration not only expands the scope of spintronic materials but also enhances the understanding of spin–orbit interaction-related superconductivity.