Homotopy Theory of Strong and Weak Topological Insulators

Abstract

We use homotopy theory to extend the notion of strong and weak topological insulators to the non-stable regime (low numbers of occupied/empty energy bands). We show that for strong topological insulators in d spatial dimensions to be "truly d-dimensional", i.e. not realizable by stacking lower-dimensional insulators, a more restrictive definition of "strong" is required. However, this does not exclude weak topological insulators from being "truly d-dimensional", which we demonstrate by an example. Additionally, we prove some useful technical results, including the homotopy theoretic derivation of the factorization of invariants over the torus into invariants over spheres in the stable regime, as well as the rigorous justification of replacing $T^d$ by $S^d$ and $T^{d_k}\times S^{d_x}$ by $S^{d_k+d_x}$ as is common in the current literature.

arXiv comment11 pages, 3 figures

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• Symmetry Protected Topological Hopf Insulator and its Generalizations
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• Floquet Hopf Insulators
• ${\mathbb{Z}}_2$ invariants of topological insulators as geometric obstructions
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• Neutrino zeromodes on electroweak strings in light of topological insulators
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