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- ItemExperimental Observation of Dirac Nodal Links in Centrosymmetric Semimetal TiB2(College Park, MD : American Physical Society, 2018) Liu, Z.; Lou, R.; Guo, P.; Wang, Q.; Sun, S.; Li, C.; Thirupathaiah, S.; Fedorov, A.; Shen, D.; Liu, K.; Lei, H.; Wang, S.The topological nodal-line semimetal state, serving as a fertile ground for various topological quantum phases, where a topological insulator, Dirac semimetal, or Weyl semimetal can be realized when the certain protecting symmetry is broken, has only been experimentally studied in very few materials. In contrast to discrete nodes, nodal lines with rich topological configurations can lead to more unusual transport phenomena. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, here, we provide compelling evidence of nodal-line fermions in centrosymmetric semimetal TiB2 with a negligible spin-orbit coupling effect. With the band crossings just below the Fermi energy, two groups of Dirac nodal rings are clearly observed without any interference from other bands, one surrounding the Brillouin zone (BZ) corner in the horizontal mirror plane σh and the other surrounding the BZ center in the vertical mirror plane σv. The linear dispersions forming Dirac nodal rings are as wide as 2 eV. We further observe that the two groups of nodal rings link together along the Γ-K direction, composing a nodal-link configuration. The simple electronic structure with Dirac nodal links mainly constituting the Fermi surfaces suggests TiB2 as a remarkable platform for studying and applying the novel physical properties related to nodal-line fermions.
- ItemPhotoemission of Bi2Se3 with circularly polarized light: Probe of spin polarization or means for spin manipulation?(College Park : American Institute of Physics Inc., 2014) Sánchez-Barriga, J.; Varykhalov, A.; Braun, J.; Xu, S.-Y.; Alidoust, N.; Kornilov, O.; Minár, J.; Hummer, K.; Springholz, G.; Bauer, G.; Schumann, R.; Yashina, L.V.; Ebert, H.; Hasan, M.Z.; Rader, O.Topological insulators are characterized by Dirac-cone surface states with electron spins locked perpendicular to their linear momenta. Recent theoretical and experimental work implied that this specific spin texture should enable control of photoelectron spins by circularly polarized light. However, these reports questioned the so far accepted interpretation of spin-resolved photoelectron spectroscopy.We solve this puzzle and show that vacuum ultraviolet photons (50-70 eV) with linear or circular polarization indeed probe the initial-state spin texture of Bi2Se3 while circularly polarized 6-eV low-energy photons flip the electron spins out of plane and reverse their spin polarization, with its sign determined by the light helicity. Our photoemission calculations, taking into account the interplay between the varying probing depth, dipole-selection rules, and spin-dependent scattering effects involving initial and final states, explain these findings and reveal proper conditions for light-induced spin manipulation. Our results pave the way for future applications of topological insulators in optospintronic devices.
- ItemPossible experimental realization of a basic Z 2 topological semimetal in GaGeTe(College Park, MD : American Institute of Physics, 2019) Haubold, E.; Fedorov, A.; Pielnhofer, F.; Rusinov, I.P.; Menshchikova, T.V.; Duppel, V.; Friedrich, D.; Weihrich, R.; Pfitzner, A.; Zeugner, A.; Isaeva, A.; Thirupathaiah, S.; Kushnirenko, Y.; Rienks, E.; Kim, T.; Chulkov, E.V.; Büchner, B.; Borisenko, S.We report experimental and theoretical evidence that GaGeTe is a basic Z2 topological semimetal with three types of charge carriers: bulk-originated electrons and holes as well as surface state electrons. This electronic situation is qualitatively similar to the classic 3D topological insulator Bi2Se3, but important differences account for an unprecedented transport scenario in GaGeTe. High-resolution angle-resolved photoemission spectroscopy combined with advanced band structure calculations show a small indirect energy gap caused by a peculiar band inversion at the T-point of the Brillouin zone in GaGeTe. An energy overlap of the valence and conduction bands brings both electron and holelike carriers to the Fermi level, while the momentum gap between the corresponding dispersions remains finite. We argue that peculiarities of the electronic spectrum of GaGeTe have a fundamental importance for the physics of topological matter and may boost the material's application potential.
- ItemTopological origin of edge states in two-dimensional inversion-symmetric insulators and semimetals(Bristol : IOP Publ., 2016-11-28) Miert, Guido van; Ortix, Carmine; Smith, Cristiane MoraisSymmetries play an essential role in identifying and characterizing topological states of matter. Here, we classify topologically two-dimensional (2D) insulators and semimetals with vanishing spin-orbit coupling using time-reversal (T) and inversion (I) symmetry. This allows us to link the presence of edge states in I and T symmetric 2D insulators, which are topologically trivial according to the Altland-Zirnbauer table, to a ℤ2 topological invariant. This invariant is directly related to the quantization of the Zak phase. It also predicts the generic presence of edge states in Dirac semimetals, in the absence of chiral symmetry. We then apply our findings to bilayer black phosphorus and show the occurrence of a gate-induced topological phase transition, where the ℤ2 invariant changes.
- ItemTopological Electronic Structure and Intrinsic Magnetization in MnBi4Te7: A Bi2Te3 Derivative with a Periodic Mn Sublattice(College Park, MD : American Physical Society, 2019) Vidal, R.C.; Zeugner, A.; Facio, J.I.; Ray, R.; Haghighi, M.H.; Wolter, A.U.B.; Corredor, Bohorquez, L.T.; Caglieris, F.; Moser, S.; Figgemeier, T.; Peixoto, T.R.F.; Vasili, H.B.; Valvidares, M.; Jung, S.; Cacho, C.; Alfonsov, A.; Mehlawat, K.; Kataev, V.; Hess, C.; Richter, M.; Büchner, B.; Van Den Brink, J.; Ruck, M.; Reinert, F.; Bentmann, H.; Isaeva, A.Combinations of nontrivial band topology and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topological magnetoelectric effect. Following theoretical advances, material candidates are emerging. Yet, so far a compound that combines a band-inverted electronic structure with an intrinsic net magnetization remains unrealized. MnBi2Te4 has been established as the first antiferromagnetic topological insulator and constitutes the progenitor of a modular (Bi2Te3)n(MnBi2Te4) series. Here, for n=1, we confirm a nonstoichiometric composition proximate to MnBi4Te7. We establish an antiferromagnetic state below 13 K followed by a state with a net magnetization and ferromagnetic-like hysteresis below 5 K. Angle-resolved photoemission experiments and density-functional calculations reveal a topologically nontrivial surface state on the MnBi4Te7(0001) surface, analogous to the nonmagnetic parent compound Bi2Te3. Our results establish MnBi4Te7 as the first band-inverted compound with intrinsic net magnetization providing a versatile platform for the realization of magnetic topological states of matter.
- ItemAnomalous levitation and annihilation in Floquet topological insulators(College Park, MD : APS, 2020) Liu, Hui; Fulga, Ion Cosma; Asbóth, János K.Anderson localization in two-dimensional topological insulators takes place via the so-called levitation and pair annihilation process. As disorder is increased, extended bulk states carrying opposite topological invariants move towards each other in energy, reducing the size of the topological gap, eventually meeting and localizing. This results in a topologically trivial Anderson insulator. Here, we introduce the anomalous levitation and pair annihilation, a process unique to periodically driven, or Floquet, systems. Due to the periodicity of the quasienergy spectrum, we find it is possible for the topological gap to increase as a function of disorder strength. Thus, after all bulk states have localized, the system remains topologically nontrivial, forming an anomalous Floquet-Anderson insulator (AFAI) phase. We show a concrete example for this process, adding disorder via on-site potential “kicks” to a Chern insulator model. By changing the period between kicks, we can tune which type of (conventional or anomalous) levitation and annihilation occurs in the system. We expect our results to be applicable to generic Floquet topological systems and to provide an accessible way to realize AFAIs experimentally, without the need for multistep driving schemes.
- ItemBulk-boundary-defect correspondence at disclinations in rotation-symmetric topological insulators and superconductors(Amsterdam : SciPost Foundation, 2021) Geier, Max; Fulga, Ion Cosma; Lau, AlexanderWe study a link between the ground-state topology and the topology of the lattice via the presence of anomalous states at disclinations -- topological lattice defects that violate a rotation symmetry only locally. We first show the existence of anomalous disclination states, such as Majorana zero-modes or helical electronic states, in second-order topological phases by means of Volterra processes. Using the framework of topological crystals to construct d-dimensional crystalline topological phases with rotation and translation symmetry, we then identify all contributions to (d−2)-dimensional anomalous disclination states from weak and first-order topological phases. We perform this procedure for all Cartan symmetry classes of topological insulators and superconductors in two and three dimensions and determine whether the correspondence between bulk topology, boundary signatures, and disclination anomaly is unique.
- Item2D layered transport properties from topological insulator Bi2Se3 single crystals and micro flakes(London : Nature Publishing Group, 2016) Chiatti, Olivio; Riha, Christian; Lawrenz, Dominic; Busch, Marco; Dusari, Srujana; Sánchez-Barriga, Jaime; Mogilatenko, Anna; Yashina, Lada V.; Valencia, Sergio; Ünal, Akin A.; Rader, Oliver; Fischer, Saskia F.Low-field magnetotransport measurements of topological insulators such as Bi2Se3 are important for revealing the nature of topological surface states by quantum corrections to the conductivity, such as weak-antilocalization. Recently, a rich variety of high-field magnetotransport properties in the regime of high electron densities (∼1019 cm−3) were reported, which can be related to additional two-dimensional layered conductivity, hampering the identification of the topological surface states. Here, we report that quantum corrections to the electronic conduction are dominated by the surface states for a semiconducting case, which can be analyzed by the Hikami-Larkin-Nagaoka model for two coupled surfaces in the case of strong spin-orbit interaction. However, in the metallic-like case this analysis fails and additional two-dimensional contributions need to be accounted for. Shubnikov-de Haas oscillations and quantized Hall resistance prove as strong indications for the two-dimensional layered metallic behavior. Temperature-dependent magnetotransport properties of high-quality Bi2Se3 single crystalline exfoliated macro and micro flakes are combined with high resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy, confirming the structure and stoichiometry. Angle-resolved photoemission spectroscopy proves a single-Dirac-cone surface state and a well-defined bulk band gap in topological insulating state. Spatially resolved core-level photoelectron microscopy demonstrates the surface stability.
- ItemCoincident-site lattice matching during van der Waals epitaxy(London : Nature Publishing Group, 2015) Boschker, Jos E.; Galves, Lauren A.; Flissikowski, Timur; Lopes, Joao Marcelo J.; Kiemer, Alexandra K.; Riechert, Henning; Calarco, RaffaellaVan der Waals (vdW) epitaxy is an attractive method for the fabrication of vdW heterostructures. Here Sb2Te3 films grown on three different kind of graphene substrates (monolayer epitaxial graphene, quasi freestanding bilayer graphene and the SiC (6√3 × 6√3)R30° buffer layer) are used to study the vdW epitaxy between two 2-dimensionally (2D) bonded materials. It is shown that the Sb2Te3 /graphene interface is stable and that coincidence lattices are formed between the epilayers and substrate that depend on the size of the surface unit cell. This demonstrates that there is a significant, although relatively weak, interfacial interaction between the two materials. Lattice matching is thus relevant for vdW epitaxy with two 2D bonded materials and a fundamental design parameter for vdW heterostructures.