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Les événements de septembre 2019

séminaire

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

    • Jeudi 12 septembre 11:00-12:00 - Alberto Crepaldi - EPFL

      Electronic properties and electronic dynamics in topological semimetals

      Résumé : Since the discovery of topological insulators, the field of topological materials has attracted increasing attention in the condensed matter community [1]. On the one hand, the high mobility and the spin polarization of the states at the Fermi level make those compounds promising for applications. On the other hand, the low-energy electronic properties of those materials realize exotic quasiparticles, which were only postulated in high-energy physics (Dirac and Weyl Fermions) [2, 3]. Angle-resolved photoelectron spectroscopy (ARPES) has proven to be a powerful tool to directly resolve the signature of these different topological phases. In this talk I will provide a summary of my research activity of the last years. I will introduce the concept of topological semimetals, focusing on the protection and stability of the 3D Dirac semimetal phase [4-6]. By exploiting the capability of time-resolved ARPES to study the band structure upon excitation of electrons in the unoccupied conduction band, I will show you that the electron dynamics is sensitive to the formation of Weyl points [7], and that the material band dispersion can be modified by varying the electronic screening [8].
      [1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010)
      [2] N. P. Armitage et al., Rev. of Mod. Phys. 90, 015001 (2018)
      [3] B. Bradlyn et al., Science 353, 6299 (2016)
      [4] G. Manzoni et al., PRL 117, 237601 (2016)
      [5] S. Roth et al., PRB 97, 165439 (2018)
      [6] A. Crepaldi et al., Phys. Rev. B 95, 041408(R) (2017)
      [7] A. Crepaldi et al., Phys. Rev. B 96, 241408(R) (2017)
      [8] G. Gatti et al., in preparation.

      Lieu : Moyen Amphi RDC @ LPS, bât 510

      Article

  • Séminaire des doctorants

    • Vendredi 13 septembre 11:00-12:20 - Philip Phillips - University of Illinois Urbana-Champaign

      Detecting Anomalous Dimensions in the Strange Metal Phase of the Cuprates

      Résumé : We all learned that conserved quantities such as the current in a metal cannot acquire an anomalous dimension in any theory that respects charge conservation.
      A recent theory of the strange metal of the cuprates has reached the conclusion that all of the properties of this phase can be understood if the current does in fact acquire an anomalous dimension. I will show how this seemingly contradictory prediction can be understood and also show that a finger print of such an anomaly is the Aharanov-Bohm flux through a strange metal ring. In the presence of an anomalous dimension, the AB phase deviates strikingly from the standard result and offers a precise diagnostic as to what is strange about the strange metal. I will also construct a Virasoro algebra for such anomalous currents and show that they correspond to a new class of non-local yet conformal theories.

      Lieu : Moyen Amphi RDC @ LPS, bât 510

      Article

  • Séminaire des doctorants

    • Mardi 10 septembre 11:00-12:00 - Doru Sticlet - National Institute for Research and Development of Isotopic and Molecular Technologies, Romania

      Topological superconductivity from ferromagnetic impurity chains on NbSe2

      Résumé : Recent experiments have resolved Yu-Shiba-Rusinov bound states induced by magnetic impurities (atoms or molecules) on NbSe2 superconductors. In our work, we show that it is possible to generate Majorana bound states localized at the ends of chains of ferromagnetic impurities on top of NbSe2. The robustness of topological phases is analyzed as a function of coupling strength between substrate and impurity, the orientation of the impurity chains on the superconductor, and the impurity spin direction.
      Reference :
      D. Sticlet and C. Morari, Phys. Rev. B 100, 075420 (2019)

      Lieu : salle 208a, aîle sud LPS, bât 510

      Article

  • Séminaire des doctorants

    • Jeudi 26 septembre 11:00-12:00 - Denis Golez - Center for Computational Physics, Flatiron Institute, New York

      Energy conversion in photo-excited charge transfer insulators

      Résumé : Charge excitations across electronic band gaps are a key ingredient for transport in optoelectronics and light-harvesting applications. I will start with a comparison of the photo-doped state in the Mott and charge-transfer insulator. The latter is described within the three-band Emery model as relevant for copper oxides. We will employ a non-equilibrium extension of dynamical mean-field theory taking into account changes in the screening environment (GW+EDMFT)[1]. In contrast to Mott insulators, a strong renormalization of the charge-transfer gap and a substantial broadening of bands is present in charge-transfer insulators[2]. The inclusion of dynamical screening leads to an ultra-fast conversion of excess kinetic energy into plasmonic excitations. The comparison with different experimental pump-probe techniques, like time-resolved ARPES and optical conductivity, shows qualitative agreement and exemplifies that dynamical correlations are essential for a proper description of the photo-doped state.
      I will extend the theoretical description to nickel oxides and compare the dynamics after photo-doping with time-resolved photo-emission spectroscopy [3]. Short time dynamics reveals the importance of Hund physics, photo-induced in-gap states, and antiferromagnetism. The conversion of energy between Hund and magnetic degrees of freedom leads to long-lived coherent THz oscillations whose frequency corresponds to superexchange coupling.
      [1] DG, L.Boehnke, H.U.R. Strand, M.Eckstein, P.Werner, PRL 118, 246402 (2018)
      [2] DG, L.Boehnke, M.Eckstein, P.Werner, PRB 100, 041111 (2019)
      [3] K.Gillmeister, DG, C.Chiang, N.Bittner, P.Werner, Y.Pavlyukh, J. Berakdar, and W.Widdra, arXiv:1909.00828 (2019).

      Lieu : Moyen Amphi RDC @ LPS, bât 510

      Article

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