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The difference between strong and weak topological insulators is rooted in the difference between homotopy classes of maps from a sphere as opposed to maps from a torus. I will give a simple proof that the former constitute a subset of the latter, which I use to give a definition of "strong". Another aspect I will address is the possibility of stacking lower dimensional systems into higher dimensions. I will give two examples that demonstrate that not all weak topological insulators come from stacking and - vice versa - that stacking insulators can give more topological phases than one might guess from considering only the lower dimension.

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Ricardo Kennedy, University of Cologne
Seminarraum Theoretische Physik
Contact: Thomas Quella