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Les événements de octobre 2019

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  • Séminaire des doctorants

    • Mercredi 9 octobre 22:15-23:15 - Manohar Kumar - LPA ENS

      Microwave photons emitted by fractionally charged quasiparticles

      Résumé : In a partially filled Landau Level, the electron-electron interactions, for example Coulomb interactions, become dominant at high magnetic fields. It leads to manifestation of the many-body states like fractional quantum Hall (FQH) states where quasiparticle excitations carry fractional charges (e*=e/m, m = 3,5...) ; implying that many electrons acting in concert can create a quasiparticle with a charge smaller than that of an electron[1]. This apparent paradox has been well studied for last three decades using zero-frequency current noise[2,3]. However, unambiguous detection of the fractional charge of the quasiparticles is still missing ; which is extremely susceptible to intricate dynamics of the bulk-edge correspondence[4]. Due to this dynamical feature, the emission and absorption of microwave by these quasiparticles will carries the imprints of fractional excitations. This require a measurement of current fluctuations at very high frequency[5] Here we developed the emission microwave spectroscopy for the FQH states by measuring non-equilibrium current fluctuations at frequency ν (hν > [kB*T, e*VB]). The microwave photons emitted by FQH states are independent of bias voltage (e*VB < hν) ; but for e*VB > hν, the current fluctuations cross over to the regular voltage-dependent shot noise. This crossover voltage carries the characteristics of the quasiparticle excitations namely their charge and statistics. Additionally, we use the non-equilibrium fluctuation dissipation relation [6,7] to connect the high and low frequency shot noise measurement, hence unambiguously determining the charge of the quasi-particle excitations which is found to be e*=e/3 for the case of n = 4/3 and 2/3. [8]
      This measurement can be extended towards the study of non-classicality of these emitted photons in an ac-driven system and can probe the anyonic statistics of FQH systems, which are not yet experimentally demonstrated.
      [1] R. B. Laughlin, Anomalous Quantum Hall Effect : An Incompressible Quantum Fluid with Fractionally Charged Excitations, Phys. Rev. Lett. 50, 1395 (1983).
      [2] R. de Picciotto, et al. Direct observation of a fractional charge, Nature 389, 162 (1997).
      [3] L. Saminadayar, D.C. Glattli, Y. Jin, and B. Etienne, Observation of the e/3 fractionally charged Laughlin quasiparticles, Phys. Rev. Lett. 79, 2526 (1997).
      [4] M. Dolev, Y. Gross, Y. C. Chung, M. Heiblum, V. Umansky, and D. Mahalu, Dependence of the tunneling quasiparticle charge determined via shot noise measurements on the tunneling barrier and energetics, Phys. Rev. B 81, 161303(R) (2010).
      [5] R. J. Schoelkopf, P. J. Burke, A. A. Kozhevnikov, D. E. Prober, and M. J. Rooks, Frequency Dependence of Shot Noise in a Diffusive Mesoscopic Conductor, Phys. Rev. Lett. 78, 3370 (1997).
      [6] I Safi and P. Joyez, Time-dependent theory of nonlinear response and current fluctuations, Phys. Rev. B. 84, 205129 (2011).
      [7] I. Safi, Time-dependent transport in arbitrary extended driven tunnel junctions, arxiv:1401.5950 ; see also B. Roussel, P. Degiovanni and I. Safi, Perturbative fluctuation dissipation relation for nonequilibrium finite-frequency noise in quantum circuits, Phys. Rev. B 93, 045102 (2016).
      [8] R. Bisognin, H. Bartolomei, M. Kumar, I. Safi2, J.-M. Berroir, E. Bocquillon , B. Plaçais , A. Cavanna 3, U. Gennser, Y. Jin and G. Fève, Microwave photons emitted by fractionally charged quasiparticles, Nature Communications 10, 1708 (2019).
      Also see : M. Kapfer, P. Roulleau, M. Santin, I. Farrer, D. A. Ritchie and D. C. Glattli, A Josephson relation for fractionally charged anyons, Science, 363, 846, (2019).

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

      Article

  • Séminaire des doctorants

    • Jeudi 10 octobre 11:00-12:30 - Cristiane Morais Smith - Institute for Theoretical Physics, Utrecht University

      Atom-by-atom engineering of electronic states of matter

      Résumé : Feynman’s original idea of using one quantum system that can be manipulated at will to simulate the behavior of another more complex one has flourished during the last decades in the field of cold atoms. More recently, this concept started to be developed in nanophotonics and in condensed matter. In this talk, I will discuss a few recent experiments, in which 2D electron lattices were engineered on the nanoscale. The first is the Lieb lattice [1,2], and the second is a Sierpinski gasket [3], which has dimension D = 1.58. The realization of fractal lattices opens up the path to electronics in fractional dimensions. Finally, I will show how to realize topological states of matter using the same procedure. We investigate the robustness of the zero modes in a breathing Kagome lattice, which is the first experimental realization of a designed electronic higher-order topological insulator [4]. Then, we investigate the importance of the sample termination in determining the existence of topological edge modes in crystalline topological insulators. We focus on the breathing Kekule lattice, with two different types of termination [5]. In all cases, we observe an excellent agreement between the theoretical predictions and the experimental results.
      [1] M.R. Slot, T.S. Gardenier, P.H. Jacobse, G.C.P. van Miert, S.N. Kempkes, S.J.M. Zevenhuizen, C. Morais Smith, D. Vanmaekelbergh, and I. Swart, “Experimental realisation and characterisation of an electronic Lieb lattice”, Nature Physics 13, 672 (2017).
      [2] M. R. Slot et al., “p-band engineering in artificial electronic lattices”, Phys. Rev. X 9, 011009 (2019).
      [3] S.N. Kempkes, M.R. Slot, S.E. Freeney, S.J.M. Zevenhuizen, D. Vanmaekelbergh, I. Swart, and C. Morais Smith, “Design and characterization of electronic fractals”, Nature Physics 15, 127(2019).
      [4] S.N. Kempkes, M. R. Slot, J. J. van den Broeke, P. Capiod, W. A. Benalcazar, D. Vanmaekelbergh, D. Bercioux, I. Swart, and C. Morais Smith “Robust zero-energy modes 
in an electronic higher-order topological insulator : the dimerized Kagome lattice”, ArXiv : 1905.06053, to appear in Nature Materials (2019).
      [5] S. E. Freeney, J. J. van den Broeke, A. J. J. Harsveld van der Veen, I. Swart, and C. Morais Smith, “Edge dependent topology in Kekulé lattices”, submitted (2019).

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

      Article

  • Séminaire des doctorants

    • Jeudi 17 octobre 11:00-12:00 - Marco Marciani - Laboratoire de Physique, ENS de Lyon

      Optical topological chiral modes flowing between non-topological materials

      Résumé : The most remarkable feature of so-called "topological crystals" is the presence of states flowing at their edge that are robust against disorder. A beautiful mathematical theory allows to predict the properties of such states directly from the topological invariants (e.g. the Chern numbers) of the bulk bands. Given the great success of this theory in terms of theoretical impact and technological advance, in recent years much effort has been put to make the extension from the field of electronics to other fields[1] and from crystals to various non-crystaline systems such as quasi-crystals and amorphous materials. In this talk I will show how to deal with continuous systems governed by linear Maxwell’s equations[2]. Even though band Chern numbers cannot be defined and optical materials are non-topological, we discover that interface Chern numbers can always be defined by means of the theory of spectral flows[3]. These invariants correctly describe chiral modes as we verified numerically on interfaces between different gyrotropic materials.
      [1] S. Raghu and F. D. M. Haldane, Phys. Rev. A78, 033834 (2008).
      [2] M. G. Silveirinha, Phys. Rev. B92, 125153 (2015).
      [3] M. Marciani and P. Delplace, arXiv:1906.09057 (2019).

      Lieu : LPS, bât 510, Moyen Amphi

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