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Les événements de juin 2017

séminaire

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

    • Jeudi 1er juin 11:00-12:30 - Corentin Morice - IphT, CEA Sacaly

      Evolution of spectral and transport quantities with doping in the SU(2) theory of cuprates

      Résumé : Recent transport experiments in the cuprate superconductors shed new light on the connection between the enigmatic pseudogap phase and the evolution of the electronic dispersion under doping. The latter is known to evolve from Fermi arcs measured by ARPES in the underdoped regime, to a large hole Fermi surface at high doping, as seen e.g. in quantum oscillation measurements. Combined Hall number and resistivity measurements at high magnetic field showed that the carrier density sharply changes from p to 1+p at the pseudogap critical doping p*, linking the opening of the pseudogap to a change in electronic dispersion.
      The SU(2) theory of cuprates shows that antiferromagnetic short range interactions cause the arising of both charge and superconducting orders, which are related by an SU(2) symmetry. The fluctuations associated with this symmetry form a pseudogap phase, which was shown to account for Raman, ARPES and strange metal experimental evidence. We derived the renormalized electronic propagator under the SU(2) dome, and calculated the spectral functions and transport quantities of the renormalized bands. We show that their evolution with doping matches both spectral and transport measurements.

      Lieu : salle 315, aîle sud LPS, bât 510

      Article

  • Séminaire des doctorants

    • Jeudi 15 juin 11:00-12:00 - Thomas SCHMIDT - Université du Luxembourg

      Helical gaps in interacting Rashba wires

      Résumé : A partially gapped spectrum due to the application of a magnetic field is one of the main probes of Rashba spin-orbit coupling in nanowires. Such a "helical gap" manifests itself in the linear conductance, as well as in dynamic response functions such as the spectral function, the structure factor, or the tunneling density of states. We will present results about the signature of the helical gap in these observables with a particular focus on the interplay between Rashba spin-orbit coupling and electron-electron interactions.
      In a quasi-one-dimensional wire, interactions can open a helical gap even without magnetic field. To show how the two types of helical gaps, caused by magnetic fields or interactions, can be distinguished in experiments, we will present the dynamic response functions calculated using bosonization, a renormalization group analysis, and the exact form factors of the emerging sine-Gordon model. For special interaction strengths, the results can be verified by refermionization.
      The helical gap typically occurs at low electron densities where the Coulomb energy dominates over the kinetic energy. To address this strongly correlated limit, we have also investigated Rashba wires using Wigner crystal theory. To interplay of Rashba spin-orbit coupling and a magnetic field then leads to an effective spiral magnetic field. We show that helical gap exists even in the limit of strong interactions but its dependence on electron density differs significantly from the weakly interacting case.

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

      Article

  • Séminaire des doctorants

    • Vendredi 23 juin 11:00-12:00 - Piers Coleman - Center for Materials Theory, Rutgers University NJ

      SmB6 : Skyrme insulator on the brink of superconductivity

      Résumé : SmB6, an ultra-narrow gap insulator discovered more than 50 years ago, poses a paradox. On the one hand, current theory and some experiments indicate that SmB6 is likely a topological insulator. On the other hand, this system has been long known to display a large linear specific heat[1] and recent dHvA measurements[2] suggest that it contains a Fermi surface which responds to the Lorentz force, but is insulating. Optical measurements also reveal that this insulator is an AC conductor, with a large ingap ac conductivity and also exhibits a thermal conductivity that is proportional to the applied field[3], a feature reminiscent of an unpinned vortex lattice. These paradoxical results, if true, suggest an very unusual kind of insulator. I’ll argue that the apparent presence of a Fermi surface of nominally neutral quasiparticles which nevertheless respond to a Lorentz force requires broken Gauge invariance, which normally would imply superconductivity. This leads us to propose the concept of a "Skyrme insulator" : a condensate with a Meissner stiffness which is nevertheless topologically unable to support the quantization of circulation.
      The Skyrme insulator theory allows us to understand the linear specific heat of SmB6 in terms of a neutral Majorana Fermi sea, and the theory predicts that in a screened environment at below fields of order a Gauss, SmB6 will develop a Meissner effect.
      REFERENCES :
      [1] K. Flachbart, M. Reiffers, and S. Janos, Journal of Less Common Metals 88, L11 (1982).
      [2] B. S. Tan, et al, Science 349, 287 (2015).
      [3] N. J. Laurita, et al, Phys. Rev. B 94, 165154 (2016).
      [4] S. Sebastian, APS March meeting , 1603.09681 (2016).
      [5 ]Onur Erten, Po-Yao Chang, Piers Coleman and Alexei Tsvelik, arXiv 1701.XXXX to be published (2017).

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

      Notes de dernières minutes : Work done in collaboration with Onur Erten, Po-Yao Chang and Alexei Tsvelik, supported by US Department of Energy Grants FG02-99ER45790 and DE-AC02-98CH10886

      Article

  • Séminaire des doctorants

    • Mercredi 28 juin 11:00-12:00 - Debasis Sadhukhan - Harish-Chandra Research Institute, Allahabad, India

      Effects of disorder on nonclassicality in many-body systems

      Résumé : Quantum correlations have been shown to be useful resources for a variety of quantum information and computational tasks. Many-body systems constitute a natural ground for the physical realization of such quantum information protocols. However, pure quantum systems are very rare in nature due to decoherence, thermal fluctuations, impurities, etc. Thus, disordered systems form one of the center stages of studies in many-body physics and often lead to counterintuitive yet appealing physical phenomena such as many-body localization, disorder-induced order, etc. From a quantum information theoretic perspective, I will discuss several interesting phenomena in the realm of quenched disorder which include enhancement of quantum correlation length, overcoming no-go theorems, and enhanced disorder-induced order of entanglement under thermal fluctuations. In a similar spirit, inhomogeneity also leads to generation of new phases in the system. Introduction of an alternating chemical potential in an 1D Fermi gas on a lattice develops a new phase compared to the same system with uniform chemical potential which can be characterized by quantum correlations present in the system. I will also talk about the trends of quantum correlations in such system, under closed and open dynamics. Finally, I will show that bipartite entanglement can be frozen over time with a proper choice of the many-body substrate, which is in contact with the environment via a repetitive interaction.

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

      Notes de dernières minutes : En collaboration avec les séminaires de la matière quantique

      Article

  • Séminaire des doctorants

    • Jeudi 22 juin 11:00-12:30 - Johannes Knolle - University of Cambridge

      The anomalous de Haas-van Alphen effect and Excitons in a topological Kondo insulator

      Résumé : The de Haas-van Alphen effect, describing oscillations of the magnetization as a function of magnetic field, is commonly assumed to be a definite sign for the presence of a Fermi surface (FS) in a metal. Here we show that, in contrast to this canonical situation, there can be quantum oscillations even for insulators of certain types. We draw connections to recent experiments on the topological Kondo insulator (TKI) candidate SmB6. In addition, we show that TKIs are susceptible to the formation of bound electron-hole pairs. These excitons can account for long-standing thermodynamic and recent transport anomalies in SmB6 which is crucial for the identification of bulk topological signatures.

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

      Article

événement important

<p>événement important</p>
  • Séminaire des doctorants

    • Mercredi 28 juin 11:00-12:00 - Debasis Sadhukhan - Harish-Chandra Research Institute, Allahabad, India

      Effects of disorder on nonclassicality in many-body systems

      Résumé : Quantum correlations have been shown to be useful resources for a variety of quantum information and computational tasks. Many-body systems constitute a natural ground for the physical realization of such quantum information protocols. However, pure quantum systems are very rare in nature due to decoherence, thermal fluctuations, impurities, etc. Thus, disordered systems form one of the center stages of studies in many-body physics and often lead to counterintuitive yet appealing physical phenomena such as many-body localization, disorder-induced order, etc. From a quantum information theoretic perspective, I will discuss several interesting phenomena in the realm of quenched disorder which include enhancement of quantum correlation length, overcoming no-go theorems, and enhanced disorder-induced order of entanglement under thermal fluctuations. In a similar spirit, inhomogeneity also leads to generation of new phases in the system. Introduction of an alternating chemical potential in an 1D Fermi gas on a lattice develops a new phase compared to the same system with uniform chemical potential which can be characterized by quantum correlations present in the system. I will also talk about the trends of quantum correlations in such system, under closed and open dynamics. Finally, I will show that bipartite entanglement can be frozen over time with a proper choice of the many-body substrate, which is in contact with the environment via a repetitive interaction.

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

      Notes de dernières minutes : En collaboration avec les séminaires de la matière quantique

      Article

  • Séminaire des doctorants

    • Jeudi 29 juin 11:00-12:00 - Dirk Morr - University of Illinois at Chicago

      Design of Majorana Edge States in Topological Superconductors

      Résumé : The experimental observation of Majorana bound states in topological superconductors represents a major breakthrough in realizing their applications in quantum computation. Recent proposals focusing on the creation of Majorana states on the edges of 2D magnetic (Shiba) islands have raised the question of whether Majorana states can be designed at the interface between complex magnetic structures and s-wave superconductors.
      In this talk, I will demonstrate that it is possible to design Majorana modes by engineering the spatial and magnetic structure of Shiba islands located on the surface of s-wave superconductors. I show that by investigating the relation between local transport properties – as reflected in the spatial flow of charge – and global transport properties, one can identify characteristic transport properties of Majorana modes, such as a quantized conductance that is proportional to the Chern number. In addition, I demonstrate that topological low-energy edge modes carry a supercurrent whose chirality reflects the sign of the Chern number. Finally, I show that topological superconductors exhibit an uncon-ventional spatial structure of superconducting triplet correlations which can be both time reversal (TR) breaking and TR preserving within the same system.

      Lieu : Amphi Moyen, RdC LPS, bât 510

      Notes de dernières minutes : Séminaire large publique

      Article

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