## Events - Weekly

**February 2013 >>**

Sunday,
February 24 |
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Monday,
February 25 |

Christina Marchetti (Syracuse U.) "Phase separation and jamming of dense active matter" Recent experiments on confluent layers of epithelial cells and vibrated granular media have motivated interest in the behavior of active systems at high density, where the interplay of steric repulsion and activity can yield active glassy or jammed states. In this talk I will discuss the behavior of dense collections of self-propelled particles in two dimensions. Two specific results will be highlighted: (i) the suppression of motility due to steric repulsion yields macroscopic phase separation in the absence of any aligning or attractive interaction; and (ii) confinement yields jammed active states where the dynamics is dominated by spatio-temporal heterogeneities reminiscent of glassy systems. Joseph Henry Room · 12:00 p.m.– 1:00 p.m.Condensed Matter Seminar - Juerg Froehlich, ETH Zurich & IAS, Princeton - Gauge Theory and Topological Phases of Matter We study a general class of systems of condensed matter including electron liquids and atom gases. Thanks to the U(1)- and SU(2)- gauge invariance of the quantum theory of such systems, one may study their response to coupling the electric current to a vector potential and the spin current to an SU(2)- gauge field. Response equations are found by determining their effective actions (or free energies) as functionals of the U(1)-vector potential and SU(2)-gauge field (and of the metric of the sample). The physical interpretation of these fields is explained. Using only general principles – gauge invariance, cluster properties of connected (current) Green functions in systems with a mobility gap, power counting – the form of the effective actions is determined. This leads to a “gauge theory of phases of matter” complementary to the well-known Landau theory. Applications to systems exhibiting the quantum Hall and the spin Hall effect, to 3D topological insulators, and to the primordial plasma in the universe are discussed. PCTS Seminar Room · 1:15 p.m.– 2:30 p.m.High Energy Theory Seminar - IAS - Piljin Yi, Korea Institute for Advanced Study - “Wall-Crossing & Quiver Invariants” Bloomberg Lecture Hall - Institute for Advanced Study · 2:30 p.m.– 4:00 p.m. |

Tuesday,
February 26 |

Cancelled: Math Physics Seminar - Jakob Yngvason, University of Vienna - Interacting Bosons in a Random Potential We study the effects of random scatterers on the ground state of the one-dimensional Lieb-Liniger model of interacting bosons in the Gross-Pitaevskii regime. We prove that Bose Einstein condensation survives even a strong random potential, but the character of the wave function of the condensate depends in an essential way on the interplay between randomness and the strength of the two-body interaction. This is joint work with Robert Seiringer and Valentin Zagrebnov. Jadwin A06 · 4:30 p.m.– 6:00 p.m. |

Wednesday,
February 27 |

High Energy Theory Seminar - Bertrand Duplantier, Institute for Theoretical Physics, Saclay - Schramm-Loewner Evolution and Liouville Quantum Gravity Liouville quantum gravity in two dimensions is described by the ``random Riemannian manifold'' obtained by changing the Lebesgue measure in the plane by a random conformal factor, the exponential of the Gaussian free field. This ``random surface" is believed to be the continuum scaling limit of certain discretized random surfaces that can be studied with combinatorics and random matrix theory. When boundary arcs of a Liouville quantum gravity random surface are conformally welded to each other (in a boundary quantum-length-preserving way) the resulting interface is a random curve described by the Schramm-Loewner evolution (SLE). This allows to develop a theory of quantum fractal measures, consistent with the Knizhnik-Polyakov-Zamolochikov (KPZ) relation, and to analyze their evolution under conformal welding maps related to SLE. As an application, one can construct quantum length and boundary intersection measures on the SLE curve itself. (Joint work with Scott Sheffield, MIT- Math) PCTS Seminar Room · 3:30 p.m.– 4:30 p.m. |

Thursday,
February 28 |

Hamilton Colloquium Series - Abhay Pasupathy, Columbia University, "What drives electronic nematicity in the iron-based superconductors?" The iron arsenides are a recently discovered class of unconventional superconducting materials. This class of materials consists of various families (cryptically called 111, 122, 1111, etc.) each of which has a "parent" compound. These parent compounds (e.g. NaFeAs) are typically not superconducting, but display a spin-density wave phase at low temperature. Superconductivity emerges in these materials when the chemical composition of the parent is tuned. We believe that electronic interactions are responsible for both the magnetic and superconducting states, and identifying the details of these interactions is a key experimental goal. Along these lines, recent experiments have shown that the electronic structure of these materials spontaneously breaks the underlying lattice symmetry at high temperature, forming an electronic nematic state. I will present recent scanning tunneling microscopy experiments probing the nature of this nematic state at the atomic scale, and discuss its relationship to superconductivity in these materials. Jadwin A10 · 4:30 p.m.– 5:30 p.m. |

Friday,
March 1 |

High Energy Theory Seminar - Enrico Pajer, Princeton University - The Effective Field Theory of Large Scale Structures Abstract: An analytical understanding of large-scale matter inhomogeneities is an important cornerstone of our cosmological model an helps us interpreting current and future data. The standard approach, namely Eulerian perturbation theory, is unsatisfactory for at least three reasons: there is no clear expansion parameter since the density contrast is not small everywhere; it does not consistently account for deviations at large scales from a perfect pressureless fluid induced by short-scale non-linearities; for generic initial conditions, loop corrections are UV divergent, making predictions cutoff dependent and hence unphysical. I will present the systematic construction of an Effective Field Theory of Large Scale Structures and show that it successfully addresses all three issues. The idea is to smooth the density and velocity fields on a scale larger than the non-linear scale. The resulting smoothed fields are then small everywhere and provide a well-defined small parameter for perturbation theory. Smoothing amounts to integrating out the short scales, whose non-linear dynamics is hard to describe analytically. Their effects on the large scales are then determined by the symmetries of the problems. They introduce additional terms in the fluid equations such as an effective pressure, dissipation and stochastic noise. These terms have exactly the right scale dependence to cancel all divergences at one loop, and this should hold at all loops. I will present a clean example of the renormalization of thetheory in an Einstein deSitter universe with self-similar initial conditions and discuss the relative importance of loop and effective corrections. PCTS Seminar Room · 1:30 p.m.– 3:00 p.m. |

Saturday,
March 2 |