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Events on Tuesday, March 1st, 2022

R. G. Herb Condensed Matter Seminar
Unraveling the Bulk and Surface Theories of Helical Higher-Order Topological Insulators
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Ben Wieder, MIT
Abstract: Solid-state materials including bismuth, MoTe2, and BiBr have been predicted to be higher-order topological insulators (HOTIs). In theoretical HOTI models, the 3D bulk and 2D surfaces are gapped, and odd numbers of 1D gapless topological modes appear bound to the hinges of finite-sized 3D samples, providing an indicator of the bulk HOTI phase in the presence of global crystal symmetries. However, the boundaries of real material samples lack the global symmetries of HOTI models, and there exist topologically trivial models with extrinsic hinge states. In HOTIs with chiral hinge states, the bulk topology has been shown to be characterized by a nontrivial axion angle, and hence chiral HOTIs can in principle be characterized experimentally through the framework of axion electrodynamics, rather than higher-order topology. For helical HOTIs, however, the bulk axion angle is trivial, and the only experimental signatures proposed to date rely on global symmetry arguments and hinge-state measurements. It is hence desirable to identify unambiguous bulk and surface signatures of helical HOTI phases analogous to - but distinct from - the axionic magnetoelectric effects present in 3D topological insulators (TIs) and chiral HOTIs. In this talk, I will present numerical and theoretical analysis of helical HOTIs demonstrating the existence of quantized bulk topological signatures beyond the axion angle, placing helical HOTIs on the same physical footing as well-understood 3D TIs and magnetic axion insulators.​
Host: Robert McDermott
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Network in Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) Seminar
Neutrinoless double beta decay in effective field theory
Time: 2:00 pm
Place: virtual -
Speaker: Wouter Dekens, UCSD
Abstract: Neutrinoless double beta decay (NLDBD) is the most sensitive probe of lepton-number violation. Its discovery would be a clear signal of physics beyond the Standard Model, confirm the Majorana nature of neutrinos, and provide insight into scenarios of baryogenesis through leptogenesis. Whenever lepton-number violation arises at a scale well above the electroweak scale, it can be described by effective interactions in an effective-field theory framework. In this talk, I will outline the steps necessary to assess the impact of these interactions on NLDBD half lives, paying special attention to the matching of the effective interactions onto Chiral Effective Theory at low energies. In addition, I will discuss how this framework can be extended to include the effects of light sterile neutrinos and give an overview of the resulting constraints on the lepton-number violating interactions.
Host: Baha Balantekin
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