Topological Classification of Insulators: III. Non-interacting Spectrally-Gapped Systems in All Dimensions

Abstract: We study non-interacting electrons in disordered materials which exhibit a spectral gap, in each of the ten Altland-Zirnbauer symmetry classes, in all space dimensions. We define an appropriate space of Hamiltonians and a topology on it so that the so-called strong topological invariants become complete invariants yielding the Kitaev periodic table, but now derived as the set of path-connected components of the space of Hamiltonians, rather than as $K$-theory groups. We thus confirm the conjecture regarding a one-to-one correspondence between topological phases of gapped non-interacting systems and the respective Abelian groups ${0},\mathbb{Z},2\mathbb{Z},\mathbb{Z}_2$ in the spectral gap regime. We do this by providing the appropriate notion of locality, as well as a novel, so-called bulk non-triviality, which together reproduce the Kitaev table. Once the natural definitions are identified, the main technical achievement is lifting $K$-theory calculations to $π_0$ of unitaries and projections.