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Author: Fulga, Ion Cosma × Start date: 2020 × DDC: 530 × Publisher: College Park, MD : APS ×

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Now showing 1 - 3 of 3
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    Magnetic warping in topological insulators
    (College Park, MD : APS, 2022) Naselli, Gabriele; Moghaddam, Ali G.; Di Napoli, Solange; Vildosola, Verónica; Fulga, Ion Cosma; van den Brink, Jeroen; Facio, Jorge I.
    We analyze the electronic structure of topological surface states in the family of magnetic topological insulators MnBi2nTe3n+1. We show that, at natural-cleavage surfaces, the Dirac cone warping changes its symmetry from hexagonal to trigonal at the magnetic ordering temperature. In particular, an energy splitting develops between the surface states of the same band index but opposite surface momenta upon formation of the long-range magnetic order. As a consequence, measurements of such energy splittings constitute a simple protocol to detect the magnetic ordering via the surface electronic structure, alternative to the detection of the surface magnetic gap. Interestingly, while the latter signals a nonzero surface magnetization, the trigonal warping predicted here is, in addition, sensitive to the direction of the surface magnetic flux. Our results may be particularly useful when the Dirac point is buried in the projection of the bulk states, caused by certain terminations of the crystal or in hole-doped systems, since in both situations the surface magnetic gap itself is not accessible in photoemission experiments.
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    Anomalous 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.
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    Chirality flip of Weyl nodes and its manifestation in strained MoTe2
    (College Park, MD : APS, 2021) Könye, Viktor; Bouhon, Adrien; Fulga, Ion Cosma; Slager, Robert-Jan; van den Brink, Jeroen; Facio, Jorge I.
    Due to their topological charge, or chirality, the Weyl cones present in topological semimetals are considered robust against arbitrary perturbations. One well-understood exception to this robustness is the pairwise creation or annihilation of Weyl cones, which involves the overlap of two oppositely charged nodes in energy and momentum. Here we show that their topological charge can in fact change sign, in a process that involves the merging of not two, but three Weyl nodes. This is facilitated by the presence of rotation and time-reversal symmetries, which constrain the relative positions of Weyl cones in momentum space. We analyze the chirality flip process, showing that transport properties distinguish it from the conventional, double Weyl merging. Moreover, we predict that the chirality flip occurs in MoTe$_2$, where experimentally accessible strain leads to the merging of three Weyl cones close to the Fermi level. Our work sets the stage to further investigate and observe such chirality flipping processes in different topological materials.