## Events - Weekly

Sunday,
May 21 |
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Monday,
May 22 |

Condensed Matter Seminar, Piers Coleman, Rutgers, "SmB6: Strange insulator at the brink of superconductivity?" SmB6, an ultra-narrow gap insulator discovered more than 50 years ago, poses a paradox. On the one hand, current theory and some experiments indicate that SmB6 is likely a topological insulator. On the other hand, this system has been long known to display a large linear specific heat[1] and recent dHvA measurements[2] suggest that it contains a Fermi surface which responds to the Lorentz force, but is insulating. Optical measurements also reveal that this insulator is an AC conductor, with a large ingap ac conductivity and also exhibits a thermal conductivity that is proportional to the applied field[3], a feature reminiscent of an unpinned vortex lattice. These paradoxical results, if true, suggest an very unusual kind of insulator. I'll argue that the apparent presence of a Fermi surface of nominally neutral quasiparticles which nevertheless respond to a Lorentz force requires broken Gauge invariance, which normally would imply superconductivity. This leads us to propose the concept of a "Skyrme insulator": a condensate with a Meissner stiffness which is nevertheless topologically unable to support the quantization of circulation. The Skyrme insulator theory allow us to understand the linear specific heat of SmB$_{6}$ in terms of a neutral Majorana Fermi sea, and the theory predicts that in a screened environment at below fields of order a Gauss, SmB$_{6}$ will develop a Meissner effect. [*]Work done in collaboration with Onur Erten, Po-Yao Chang and Alexei Tsvelik, supported by US Department of Energy Grants FG02-99ER45790 and DE-AC02-98CH10886. [1]K. Flachbart, M. Reiffers, and S. Janos, Journal of Less Common Metals 88, L11 (1982). [2]B. S. Tan, et al, Science 349, 287 (2015). [3]N. J. Laurita, et al, Phys. Rev. B 94, 165154 (2016). [4]S. Sebastian, APS March meeting , 1603.09681 (2016). [5]Onur Erten, Po-Yao Chang, Piers Coleman and Alexei Tsvelik, arXiv 1701.06582 (2017). PCTS Seminar Room · 1:15 p.m.– 2:30 p.m.HET Seminar | Sunil Mukhi, Tata Institute for Fundamental Research | “Entanglement, Replicas and Thetas” Bloomberg Lecture Hall - Institute for Advanced Study · 2:30 p.m.– 3:30 p.m. |

Tuesday,
May 23 |

Wednesday,
May 24 |

Thursday,
May 25 |

Friday,
May 26 |

HET Seminar | Kiril Hristov, Bulgarian Academy of Sciences, Sofia | "Black hole microstates in AdS_4 and AdS_5 from supersymmetric partition functions This talk addresses the problem of counting the microstates of supersymmetric asymptotically AdS black holes in terms of a holographically dual field theory. We focus on a particular BPS class of AdS_2 x S^2 near-horizon geometry in N=2 gauged supergravity relevant both for AdS_4 and AdS_5 black holes. We consider in the large N limit the topologically twisted index of the ABJM theory and the Casimir energy of N=4 SYM and show that they correctly reproduce the Bekenstein-Hawking entropy of the AdS_4 and AdS_5 black holes, respectively. An extremization of the partition functions with respect to a set of chemical potentials is required. We interpret it as the selection of the exact R-symmetry of the superconformal quantum mechanics describing the horizon of the black holes. PCTS Seminar Room · 1:45 p.m.– 2:45 p.m. |

Saturday,
May 27 |