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Les événements de juin 2018

séminaire

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

    • Lundi 18 juin 11:00-12:00 - Ryuichi Shindou - ICQM iPeking University

      Theories of transport scaling in disordered semimetals and topological spin-nematic excitonic insulators in graphite under high magnetic field

      Résumé : In the first part of my talk, I will talk about transport scaling theories in disordered Weyl semimetal [1,2]. In electronic band structure of solid state material, two band touching points with linear dispersion (called as `Weyl node’) appear in pair in the momentum space. When they annihilate with each other, the system undergoes a quantum phase transition from Weyl semimetal (WSM) phase to a band insulator (BI) phase. The continuous phase transition is recently discovered in solid state materials [3]. The phase transition is described by a critical theory with a `magnetic dipole’ like object in the momentum space. I will argue that the critical theory hosts a new disorder-driven quantum multicritical phenomena. Based on the renormalization group argument, we clarify transport scaling properties around the Weyl node around the quantum multicritical point as well as the direct phase line between BI and WSM phases [1,2].
      In the second part of my talk, I will argue that three-dimensional topological excitonic insulator is realized in graphite under high magnetic field [4,5]. Graphite under high magnetic field exhibits consecutive metal-insulator (MI) transitions as well as re-entrant insulator-metal (IM) transition at low temperature. A part of the experiment was discovered more than 30 years ago, while the identities of the low-temperature insulating phases are still unclear by now. We identify these enigmatic insulator phases with excitonic insulator phases, where electron and hole pocket(s) form spin-triplet excitonic pairings. We show that the re-entrant IM transition in the graphite experiment can be naturally explained by an enhanced quantum spin fluctuation in the presence of smaller electron and hole pocket(s). We further argue that the odd-parity spin-triplet excitonic pairing reconstructs chiral surface Fermi arc state of electron and that of hole into a 2+1 massless surface Dirac fermion (topological excitonic insulator) [4,5].
      [1] https://arxiv.org/abs/1803.09051, under review [2] https://arxiv.org/abs/1710.00572, selected as PRB editors’ suggestion [3] Tian Liang, et.al., Science Advances, 3, e1602510 (2017) [4] https://arxiv.org/abs/1802.10253, under review [5] in preparation

      Lieu : Grand Amphi RDC @ LPS, bât 510

      Article

  • Séminaire des doctorants

    • Mardi 19 juin 11:00-12:00 - Andrei Bernevig - Princeton

      Topological Quantum Chemistry And Higher Order Topological Insulators

      Résumé : The past decade has seen tremendous success in predicting and experimentally discovering distinct classes of topological insulators (TIs) and semimetals. We review the field and we propose an electronic band theory that highlights the link between topology and local chemical bonding, and combines this with the conventional band theory of electrons. Topological Quantum Chemistry is a description of the universal global properties of all possible band structures and materials, comprised of a graph theoretical description of momentum space and a dual group theoretical description in real space. We classify the possible band structures for all 230 crystal symmetry groups that arise from local atomic orbitals, and show which are topologically nontrivial. We show how our topological band theory sheds new light on known TIs, and demonstrate the power of our method to predict a plethora of new TIs and on the newly discovered Higher-Order Topological Insulators in Bi.

      Lieu : Salle de Réunion Parc Club Orsay (commune FAST-LPS) - Bât. 23-25, Salle 215, 1er étage

      Article

  • Séminaire des doctorants

    • Jeudi 28 juin 11:00-12:30 - Keyan Bennaceur - Université de Sherbrooke

      Competing charge density waves probed by non-linear transport and noise in the second and third Landau levels

      Résumé : Spontaneous charge ordering is one of the many intriguing quantum phenomena occurring in a 2D electron gas (2DEG) under a magnetic field. In the first (N=0) Landau level (LL), the short range attractive part of Coulomb interaction is known to be responsible for the condensation of quasiparticles into an incompressible Laughlin liquid hosting a fractional quantum Hall effect (FQHE) [1] and carrying fractional charges. In higher LLs (N > 2), however, the situation is markedly different as the combination of short-range attractive with long- range repulsive Coulomb interaction leads to electronic phases forming charge density waves (CDWs) [2,3]. In this work, CDWs were investigated in the second and third Landau levels using non-linear electronic transport as well as noise measurements in a Corbino geometry. Unlike traditional geometries such as Hall or Van der Paw geometries, the Corbino geometry offers a direct access to the bulk conductivity without any contributions from edge channels [4]. The concomitant mapping of the conductivity versus bias voltage and magnetic field revealed different types of CDWs, such as pinned CDW phases and/or co-existing liquid crystal-like stripe phases. At particular filling fractions both in the second and third Landau levels, narrow band noise reveals The sliding transport characteristics under DC bias voltage. This is the typical signature of liquid-crystal like CDWs. of pinned CDWs is revealed by narrow band noise at particular filling fractions in both the second and third Landau levels. In particular, Tthese results shed new light on the elusive re-entrant quantum Hall states in the second Landau level Landau level, and they show that these are most likely liquid-crystal like CDWs that can coexist with the FQHE [5].
      [1] R. B. Laughlin, Phys. Rev. Lett. 50, 1395 (1983).
      [2] A. A. Koulakov, M. M. Fogler, and B. I. Shklovskii, Phys. Rev. Lett. 76, 499 (1996). [3] M. P. Lilly, K. B. Cooper, J. P. Eisenstein, L. N. Pfeiffer and K.W. West, Phys. Rev. Lett. 82, 394 (1999).
      [4] B. A. Schmidt, K. Bennaceur, S. Bilodeau, G. Gervais, L. N. Pfeiffer, and K. W. West, Solid State Commun. 217, 1 (2015). [5] K. Bennaceur, C. Lupien, B. Reulet, G. Gervais, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 120, 136801 (2018).

      Lieu : Grand Amphi RDC @ LPS, bât 510

      Article

  • Séminaire des doctorants

    • Vendredi 29 juin 14:00-15:00 - Y. Avishai - Ben Gurion University, Beer Sheva, Israel

      Phase Aharonov-Casher dans les systèmes mésoscopiques

      Résumé : La phase Aharonov-Casher joue un rôle important dans les systèmes mésoscopiques dans lesquels le couplage spin-orbite est pertinent. Dans cette présentation, nous examinons la dépendance des observables physiques pertinentes sur la phase Aharonov-Casher (telles que la conductance g et la polarisation électronique P). Premièrement, nous suggérons une expression de la phase qui est manifestement invariante de jauge. Ensuite, nous considérons un problème de diffusion dans lequel la phase dépend d’au moins deux paramètres x et y. Notre résultat principal est que la conductance g dépend des deux paramètres uniquement à travers la phase Aharonov-Casher, alors que la polarisation P dépend de chaque paramètre séparément

      Lieu : Grand Amphi RDC @ LPS, bât 510

      Article

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