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Les événements de November 2019

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  • Séminaire des doctorants

    • Thursday 7 November 11:00-12:00 - Nathan Andrei - Rutgers, The State University of NJ

      Quantum Impurity in Interacting Environments

      Résumé : Quantum impurities, when coupled to interacting systems, show a variety of striking phenomena. I will first discuss a quantum impurity – a local scattering center or a quantum dot - coupled to a Luttinger liquid wire, and in the second part of my talk I will discuss a Kondo impurity coupled to a superconducting wire.
      In the first part I shall show how the interaction in Luttinger liquid affects the properties of the impurity. I will present exact solutions of the various systems by means of the Off Diagonal Bethe Ansatz approach. I will derive the spectra of the models for all coupling constant regimes and will calculate the impurity free energy and discuss in detail the high and low energy behavior of the system for both repulsive and attractive interactions. Among other things I will show that the competition between the tunneling and the backscattering leads to non-Fermi liquid exponents in the specific heat and capacitance of the dot.
      I shall then turn to a Kondo impurity coupled to a 1-d superconductor. Here we consider a magnetic impurity coupled to a quantum wire with pairing interactions, which dynamically generate a mass gap. Solving the system exactly we find that it exhibits both screened and unscreened phases for an antiferromagnetic impurity. We determine the ground state density of states and magnetization in both phases as well as the excitations. In contrast to the well-studied case of magnetic impurities in mean-field superconductors, we find that there are no intragap bound states in the spectrum. The phase transition is not associated to a level crossing but to quantum fluctuations.

      Lieu : LPS, bât 510, Moyen Amphi

      Article

  • Séminaire des doctorants

    • Thursday 28 November 11:00-12:00 - Maciej Maska - Department of Theoretical Physics, University of Silesia, Poland

      Temperature-driven crossovers and transitions in a coupled boson-fermion system

      Résumé : Motivated by strongly correlated and frustrated systems, we propose a coupled bose-fermi model in two dimensions that describes the dynamics of pairs of opposite spin fermions scattering from localized bosons. Tracing out one of the degrees of freedom, either the bosons or fermions, generates temperature-dependent long range effective interactions between the entities that we investigate using Monte Carlo techniques. The behavior of bosons is dominated by vortex-antivortex unbinding, with effective inter-boson interactions beyond the nearest neighbor Josephson coupling of phases. Remarkably in the fermion sector we observe a temperature-induced BCS-BEC crossover followed by phase transitions to Anderson and Bose insulator phases, even without quenched disorder.
      Tunneling and angle resolved photoemission spectroscopy on bose-fermi mixtures in cold atomic systems and superconducting islands on graphene are some of the promising experimental platforms to test our predictions.

      Lieu : LPS, bât 510, salle 208a

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