Universality and stability of the edge states of chiral-symmetric topological semimetals and surface states of the Luttinger semimetalUniversality and stability of the edge states of chiral-symmetric
le vendredi 13 décembre 2019 à 11h00
Séminaire théorie
Personne à contacter : Serge Florens ()
Lieu : Amphithéâtre, maison des Magistères
Résumé : We theoretically demonstrate that the chiral structure of the nodes of nodal semimetals is responsible for the existence and universal local properties of the edge states in the vicinity of the nodes. We perform a general analysis of the edge states for an isolated node of a 2D semimetal, protected by chiral symmetry and characterized by the topological winding number N. We derive the asymptotic chiral-symmetric boundary conditions and find that there are N+1 universal discrete classes of them. The class determines the numbers of flat-band edge states on either side off the node in the 1D spectrum and the winding number N gives the total number of edge states. We then show that the edge states of chiral nodal semimetals are robust: they persist in a finite-size stability region of parameters of chiral-asymmetric terms. This significantly extends the notion of 2D and 3D topological nodal semimetals. We demonstrate that the Luttinger model with a quadratic-node for j=3/2 electrons (Luttinger semimetal) is a 3D topological semimetal in this new sense and predict that alpha-Sn, HgTe, possibly Pr2Ir2O7, and many other semimetals described by it are topological and exhibit surface state.
Reference: M. Kharitonov, J.-B. Mayer, and E. M. Hankiewicz, Phys. Rev. Lett. 119, 266402 (2017).
The pursuit of fractionalized excitations in Kitaev Materials
le jeudi 19 décembre 2019 à 11h00
Séminaire théorie
Personne à contacter : Serge Florens ()
Lieu : Amphithéâtre, maison des Magistères
Résumé : Quantum spin liquids (QSLs) are long-range entangled states of matter with emergent gauge fields and fractionalized excitations. In my talk I will focus on how to search for fractionalized excitations in Kitaev materials, which believe to harbor a variety of QSLs. These Kitaev QSLs exhibit two types of fractionalized quasiparticle excitations - itinerant Majorana fermions and gapped Z2 fluxes. In recent years, a remarkable theoretical and experimental progress has been achieved in understanding that these fractionalized quasiparticles and, in particular, Majorana fermions can be effectively probed by conventional spectroscopic techniques such as inelastic neutron scattering, Raman scattering with visible light, and resonant inelastic X-ray scattering. Another promising direction is to look for experimental signatures of fractionalized quasiparticles in phonon spectra.
Nagaoka ferromagnetism observed in a quantum dot plaquette
le mardi 28 janvier 2020 à 14h00
Séminaire nano-électronique quantique
Personne à contacter : Robert Whitney ()
Lieu : Salle Rémy Lemaire K223, Institut Néel
Résumé : The analytical tractability of Nagaoka ferromagnetism makes it a convenient model to explore the capabilities of quantum simulators of collective electron interactions. However, the small ground-to-excited state energy compared to electron interactions, as well as the difficulty of measuring magnetization in few particle devices, have made the Nagaoka model experimentally unattainable. Here we present experimental signatures of the ferromagnetic ground state, predicted for 3 electrons in a 4 site square plaquette, engineered using electrostatically defined quantum dots. We test the robustness of the Nagaoka condition under different scenarios of lattice topology, device homogeneity and magnetic flux through the plaquette. This long-sought demonstration of Nagaoka ferromagnetism establishes quantum dot systems as prime candidates for quantum simulators of magnetic phenomena driven by electron-electron interactions.
James L. Crowley (Grenoble Institut Polytechnique, Univ. Grenoble-Alps, Chair on Intelligent Collaborative Systems, M)
Artificial Intelligence: A Rupture Technology for Scientific Research
le vendredi 31 janvier 2020 à 11h00
Séminaire théorie
Personne à contacter :
Lieu : Amphithéâtre, maison des Magistères
Résumé : The Turing test defines intelligence as human-level performance at interaction. After more than 50 years of research, Machine Learning has provided an enabling technology for constructing intelligent systems with abilities at or beyond human level for interaction with people, with systems, and with the world. This technology creates a fundamental rupture in the way we build systems, and the kind of systems that we can build. It can also provide a new approach for modelling and understanding natural phenomena.
In this talk I will review of recent progress in Machine Learning, and examine how these technologies change the kind of systems that we can build. Starting with a summary of the multi-layer perceptron and back propagation, I will describe how massive computing power combined with planetary scale data and the world wide web have created the rupture technology known as deep learning. I will discuss common architectures, and review recent advances such as Generative Adversarial Networks, Natural Language Processing and Cognitive Computing. I will describe how these technologies can be used to build systems for collaborative assistance for scientific discovery and conclude with a discussion of open problems concerning explainable, verifiable, and trustworthy artificial intelligence.
Laboratoire de Physique et Modélisation des Milieux Condensés
