## Events - Daily

**April 30, 2013**>>

Tuesday,
April 30 |
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High Energy Theory Seminar - Tolya Dymarsky, Cambridge - On the four-point correlation function of the stress-energy tensor in a CFT We discuss to what extent the full set of symmetries, i.e. conformal symmetry and general diffeomorphisms constraint the form of the four-point correlation functions of stress-energy tensors and conserved currents in a general conformal field theory. The conformal symmetry alone expresses the correlation function of four stress-energy tensors in terms of over six hundred scalar functions of the conformal cross-ratios. Imposing conservation restricts the number of the unconstrained degrees of freedom to 29 scalar function. Similarly, four-point function of the conserved currents depends on just 7 unconstrained scalar functions. The relatively small number of the surviving degrees of freedom raises hopes of formulating conformal bootstrap for these correlates in a self-contained practical way. PCTS Seminar Room · 2:00 p.m.– 3:30 p.m. |

Special Condensed Matter Seminar, Kai Sun, U of Michigan at Ann Arbor, Kondo insulators: insulator, metal or topological insulator? In the last few decades, various puzzles have emerged in the study of strongly-correlated materials. In a family of strongly-correlated insulators, known as Kondo insulators, one such long-standing puzzle has remained open for over 40 years. In particular, it has been found that some Kondo insulators display strange electrical transport that cannot be understood if one assumes that it is governed by the three-dimensional bulk. In this talk, I show that some Kondo insulators have the right ingredients to be topological insulators, which we called topological Kondo insulators. For a topological Kondo insulator, the low-temperature transport is dominated by the 2D surface rather than the 3D bulk, because the bulk of this material is an insulator while its surface is a topologically-protected 2D metal. This theoretical picture offers a natural explanation for the long-standing puzzle mentioned above. I will also discuss some recent experiments, which indicate that SmB$_6$ (a Kondo insulator) is indeed a bulk insulator with a conducting surface. PCTS Seminar Room · 3:30 p.m.– 5:00 p.m. |

Math Physics Seminar, Hugo Duminil, U of Geneva, Parafermionic observables in planar Potts models and Self Avoiding Walks In this talk, we will discuss the role of parafermionic observables in the study of several planar statistical physics models. These objects have been introduced recently by Smirnov and Cardy and have been instrumental in Smirnov's proof of conformal invariance of the Ising model. We will explain how they can be combined with combinatorial and probabilistic arguments to compute the connective constant for self-avoiding walks (the n=0 loop O(n)-model) on the hexagonal lattice, and to provide information on the critical phase of the Fortuin-Kasteleyn percolation (a graphical representation of Potts models). As an application of their use for FK percolation, we will show the absence of spontaneous magnetization for the critical planar Potts models with 2, 3 and 4 colors, thus proving part of the conjecture asserting that the planar Potts models undergo a discontinuous phase transition if and only if the number of colors is greater than 4. Jadwin A06 · 4:30 p.m.– 6:00 p.m. |