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arXiv · 2509.19825

$\mathbb{Z}_2$ topological invariant in three-dimensional PT- and PC-symmetric class CI band structures

Ken Shiozaki
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Abstract

We construct a previously missing $\mathbb{Z}_2$ topological invariant for three-dimensional band structures in symmetry class CI defined by parity-time (PT) and parity-particle-hole (PC) symmetries. PT symmetry allows one to define a real Berry connection and, based on the $\eta$-invariant, a spin-Chern--Simons (spin-CS) action. We show that PC symmetry quantizes the spin-CS action to $\{0,2\pi\}$ with $4\pi$ periodicity, thereby yielding a well-defined $\mathbb{Z}_2$ invariant. This invariant is additive under direct sums of isolated band structures, reduces to a known $\mathbb{Z}_2$ index when a global Takagi factorization exists, and in general depends on the choice of spin structure. Finally, we demonstrate lattice models in which this newly introduced $\mathbb{Z}_2$ invariant distinguishes topological phases that cannot be detected by the previously known topological indices.

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Ken Shiozaki. 2025-09-24. $\mathbb{Z}_2$ topological invariant in three-dimensional PT- and PC-symmetric class CI band structures. https://doi.org/10.21468/scipostphyscore.9.2.029

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