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Les événements de février 2019

séminaire

<p>séminaire</p>
  • Séminaire des doctorants

    • Jeudi 14 février 11:00-12:00 - Bruno Uchoa - University of Oklahoma

      3D quantum Hall effects in Dirac loops

      Résumé : Nodal-line semimetals describe a class of 3D systems where the low energy bands have Dirac quasiparticles along a closed line in momentum space (Dirac loop) rather than a discrete set of Dirac points [1]. One conceptually interesting family of lattice models that describes this physics is the family of the hyperhoneycomb lattices, which form 3D structures of crossed 1D chains where all sites have the same planar trigonal connectivity of graphene. In this talk, I will show that Coulomb interactions could drive the system to show an anomalous quantum Hall effect (AQHE) [2], with current loops that produce zero net flux in the 3D unit cell. I will show that in the AQH state, because of lack of inversion symmetry, the Haldane mass has nodes along the nodal line, giving rise to pairs of Weyl points connected by Fermi arcs. I will discuss the anomalous Hall conductivity of this system and the connections with strain field deformations encoded in the elastic Hall viscosity [3].
      [1] K. Mullen, B. Uchoa, D. Glatzhofer, Phys. Rev. Lett. 115, 026403 (2015).
      [2] S. W. Kim, K. Seo, B. Uchoa, Phys. Rev. B 97, 201101(R) (2018).
      [3] S. W. Kim, B. Uchoa, arXiv:1901.00574 (2019).

      Lieu : salle 315, aîle sud LPS, bât 510

      Article

  • Séminaire des doctorants

    • Jeudi 7 février 11:00-12:00 - Sergey Krishtopenko - Laboratoire Charles Coulomb, Université Montpellier 2

      Topological states and fermion zoo in 3D and 2D systems based on HgCdTe and III-V semiconductors

      Résumé : Fermions in high energy physics are classified as Dirac, Majorana or Weyl particles according to different solutions of Schrödinger equation. However, being free of the stringent Lorentz symmetry, low-energy excitations in condensed matter systems significantly extend the fermion zoo, known from the Standard model.
      In the first part of my talk, I focus on so-called Kane fermions [1], known for zinc-blend crystals. I will show that bulk crystals hosting Kane fermions with negative-rest mass have multiple branches of topological surface states [2], in contrast to conventional topological insulators. Then, I briefly present our experimental results on temperature-driven topological phase transition and observation of massless Dirac fermions in HgTe quantum well (QW) [3,4]. Finally, I show that tunnel-coupling between two HgTe QWs yields very reach phase diagram of the system [5].
      In the second part of my talk I will present multi-layer 2D system based on III-V semiconductors, which offers a flexible platform for realization of high-temperature Quantum Spin Hall Insulator (QSHI), massless Dirac fermions and analog of bilayer graphene analog [6]. The first experimental results from our group will be shown as well [7,8]. If I have enough time at the end of the talk, I will briefly discuss possibility of realization of stable 2D Weyl fermions and the 2nd order QSHI on the basis of this conventional 2D system.
      [1] F. Teppe, M. Marcinkiewicz, S.S. Krishtopenko et al., Nat. Commun 7, 12576 (2016).
      [2] S. S. Krishtopenko, M. Antezza, F. Teppe (to be published).
      [3] S. S. Krishtopenko et al. Phys. Rev. B 94, 245402 (2016).
      [4] A. M. Kadykov, S. S. Krishtopenko, B. Jouault et al., Phys. Rev. Lett. 120, 086401 (2018).
      [5] S. S. Krishtopenko, W. Knap, F. Teppe. Sci. Reports 6, 30755 (2016).
      [6] S. S. Krishtopenko, F. Teppe, Science Advances 4, eaap7529 (2018).
      [7] S. S. Krishtopenko et al. Phys. Rev. B 97, 245419 (2018).
      [8] S. S. Krishtopenko et al. (to be published).

      Lieu : LPS, bât 510, Moyen Amphi

      Article

  • Séminaire des doctorants

    • Jeudi 14 février 14:00-15:00 - Anaelle Legros - Service de Physique de l’Etat Condensé, CEA - Université de Sherbrooke, Canada

      New insight into the pseudogap phase of cuprate superconductors thanks to transport measurements

      Résumé : Cuprates, discovered more than thirty years ago, are peculiar superconductors that posess the highest critical temperatures known nowadays and display a complex temperature—doping phase diagram. One of the many phases in this diagram is the pseudogap phase, characterized by a partial drop in the density of states and whose nature remains unclear to date. Understanding the origin of the pseudogap in cuprate superconductors is essential to elucidate the cause of high critical temperatures in these materials. A way to investigate this peculiar state is to look for signatures of a phase transition in doping, at very low temperatures (in the T → 0 limit). In this presentation, we will show how transport measurements can be used to probe the critical doping p* where the pseudogap phase ends at T = 0. I will reveal some recent information provided by these measurements during my PhD. In addition, I will also present new results regarding the crystal growth of the mercury-based cuprate HgBa2CuO4+δ, a model cuprate synthesized only in a few research groups.

      Lieu : LPS, bât 510, Moyen Amphi

      Article

  • Séminaire des doctorants

    • Jeudi 21 février 11:00-12:00 - Björn Trauzettel - University of Würzburg

      Z4 parafermions in weakly interacting superconducting constrictions at the helical edge of quantum spin Hall insulators

      Résumé : Parafermions are generalizations of Majorana fermions that may appear in interacting topological systems. They are known to be powerful building blocks of topological quantum computers. Existing proposals for realizations of parafermions typically rely on strong electronic correlations which are hard to achieve in the laboratory. We identify a novel physical system in which parafermions generically develop. It is based on a quantum constriction formed by the helical edge states of a quantum spin Hall insulator in vicinity to an ordinary s-wave superconductor. Interestingly, our analysis suggests that Z4 parafermions are emerging bound states in this setup even in the weakly interacting regime. Furthermore, we identify conditions under which parafermions and Majorana fermions coexist.

      Lieu : LPS, bât 510, Moyen Amphi

      Article

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