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Les événements de mai 2018


  • Séminaire des doctorants

    • Jeudi 24 mai 11:00-12:00 - Bastien Loret - MPQ, Paris-Diderot

      Superconductivity and pseudogap in cuprates : a tumultuous relationship

      Résumé : Although high T c superconductivity has been discovered more than 29 years,Automatic word wrap
      the mechanism of superconductivity is not yet understood and raises fundamentalAutomatic word wrap
      questions [1]. My talk will be focus on the link betwen superconductivity and theAutomatic word wrap
      pseudogap, its origin remains mysterious. I will report our recent progress to grow single crystals of HgBa2Ca2Cu3O8 (Hg-1223) cuprate with a new self-ux growth technique. The crystals thus synthesized are submilimetric, slightly underdoped and show high surface quality for spectroscopic measurements. Magnetometry and X-Ray diffraction have been performed. We show for the first time that with adequate heat treatment, we can largely underdope the Hg-1223 compound and control its doping level [2]. Our raman spectroscopy study of a slightly underdoped single crystal of Hg-1223 allowed us to observe that the superconducting pair-breaking peak is associated with a dip on its higher-energy side, disappearing together at T c . This result reveals a key aspect of the unconventional pairing mechanism : spectral weight lost in the dip is transferred to the pair-breaking peak at lower energies. This conclusion is supported by cellular dynamical mean-field theory on the Hubbard model, which is able to reproduce all the main features of the Raman response and explain the peak-dip behavior in terms of a nontrivial relationship between the superconducting and the pseudogap [3]. We confirmed this result on others compounds revealing his universality. We tracked this structure with doping on Bi2Sr2CaCu2O8 [4]. The pseudogap survives in the overdoped region and then disappears above p 0.225, which corresponds to the normal state disappearance of the pseudogap in coincidence with a Lifshitz transition. This suggests a vertical ending line of the pseudogap in the T-p phase diagram.

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


  • Séminaire des doctorants

    • Jeudi 17 mai 11:00-12:00 - Vardan Kaladzhyan - KTH Royal Institute of Technology, Stockholm

      Conductance of Weyl semimetal nanowires

      Résumé : We study longitudinal magnetotransport in Weyl semimetal nanowires. We show that depending on radii of nanowires and magnetic field amplitudes there exist two different regimes. In the strong magnetic field regime (magnetic length much smaller than the radius of the nanowire), Landau level spectrum resembles that of a bulk Weyl semimetal, and thus we obtain a conductance plateau for low chemical potentials due to the presence of the 0th chiral Landau level. On the contrary, in the weak magnetic field regime (magnetic length much larger than the radius of the nanowire), the 0th LL is not chiral any more, and therefore the conductance plateau at low chemical potentials disappears. We argue that both regimes are relevant for the ongoing experiments.

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


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