Nonlocal topological electromagnetic phases of matter

Abstract

In 2+1D, topological electromagnetic phases are defined as atomic-scale media which host photonic monopoles in the bulk band structure and respect bosonic symmetries. Additionally, they support topologically protected spin-1 edge states, which are fundamentally different than spin-1/2 and pseudo-spin-1/2 edge states arising in fermionic and pseudo-fermionic systems. The striking feature of the edge state is that all electric and magnetic field components vanish at the boundary. This surprising open boundary solution of Maxwell's equations, dubbed the quantum gyroelectric effect [Phys. Rev. A 98, 023842 (2018)], only occurs in the presence of temporal as well as spatial dispersion (nonlocality) and is the supersymmetric partner of the topological Dirac edge state where the spinor wave function completely vanishes at the boundary. In this paper, we generalize these topological electromagnetic phases beyond the continuum approximation to the exact lattice field theory of a periodic atomic crystal. To accomplish this, we put forth the concept of microscopic (nonlocal) photonic band structure of solids, analogous to the traditional theory of electronic band structure. Our definition of topological invariants and topological phases uses optical Bloch modes and can be applied to naturally occurring crystalline materials. For the photon propagating within a crystal, our theory shows that besides the Chern invariant $\mathfrak{C}\in\mathbb{Z}$, there are also symmetry-protected topological (SPT) invariants $\nu\in\mathbb{Z}_N$ which are related to the cyclic point group $C_N$ of the crystal $\nu=\mathfrak{C}\mod N$. Due to the rotational symmetries of light $\mathcal{R}(2\pi)=+1$, these SPT phases are manifestly bosonic and behave very differently from their fermionic counterparts $\mathcal{R}(2\pi)=-1$ encountered in conventional condensed matter systems.