## Events - Weekly

**November 2012 >>**

Sunday,
November 11 |
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Monday,
November 12 |

Condensed Matter Seminar - Nuh Gedik, Massachusetts Institute of Technology, " Ultrafast Probing of Dirac Fermions in Topological Insulators" The three-dimensional topological insulator (TI) is a new quantum phase of matter that exhibits quantum-Hall-like properties, even in the absence of an external magnetic field. Charge carriers on the surface of a TI behave like a two-dimensional gas of massless helical Dirac fermions for which the spin is ideally locked perpendicular to the momentum. In this talk, I will discuss recent experiments in which we used ultrafast laser pulses to directly visualize spin texture of topological surface electrons and their coupling to the bulk electrons. In order to obtain the spin texture, we performed novel time of flight based angle-resolved photoemission spectroscopy (ARPES) using circularly polarized laser pulses. The difference between the spectra obtained with right and left circularly polarized laser pulses enable simultaneous mapping of all three components of spin over the entire Dirac cone of a TI. We find that an idealized description of helical Dirac fermions only applies within a small energy window about the Dirac point, beyond which strong textural deformations occur. In order to visualize the coupling of topological surface electrons to other excitations, we recorded the temporal evolution of ARPES spectra in response to photoexcitation by a separate ultrashort laser pulse. This yields a movie of evolution of electronic band structure with femtosecond time resolution. These dynamics reveal strong phonon-assisted surface-bulk coupling at high lattice temperature and total suppression of inelastic scattering between the surface and the bulk at low lattice temperature. PCTS Seminar Room · 1:15 p.m.– 2:30 p.m. |

Tuesday,
November 13 |

Math Phys Seminar: Sylvia Serfaty (Université Pierre et Marie Curie - Paris 6 and Courant Institute) 'Towards crystallization in Coulomb systems' We are interested in the statistical mechanics of (classical) two-dimensional Coulomb gases and one-dimensional log gases in a confining potential. We connect the Hamiltonian to the "renormalized energy", a way to compute the total Coulomb interaction of an infinite jellium, and whose minimum is expected to be achieved by the triangular lattice in 2D, and is achieved by the lattice Z in 1D. We apply this to the study of the finite temperature situation. Results include computations of the next order term in the partition function, equidistribution of charges, and concentration to the minimizers of the renormalized energy as the temperature tends to zero. This is based on joint works, mostly with Etienne Sandier. Jadwin A06 · 3:30 p.m.– 5:00 p.m. |

Wednesday,
November 14 |

Thursday,
November 15 |

Friday,
November 16 |

High Energy Theory Seminar - Alfred Shapere - University of Kentucky - "Time Crystals" I consider a class of simple classical dynamical systems which exhibit motion in their lowest-energy states. Their Lagrangians have wrong-sign kinetic terms and their Hamiltonians are multivalued functions of momentum, yet they are amenable to quantization. Their field theoretical generalizations may have applications in condensed matter physics and cosmology. PCTS Seminar Room · 1:30 p.m.– 2:30 p.m. |

Saturday,
November 17 |