Standard model of electromagnetism and chirality in crystals

Abstract

We present a general, quantitative theory of electromagnetism and chirality in crystals. Symmetry is its guiding principle, enabling us to consider macroscopic multipole densities without reference to specific microscopic configurations. We use a formal analogy between space inversion $i$ and time inversion $\theta$ to identify two complementary, comprehensive classifications of crystals, based on five categories of electric and magnetic multipole order---called polarizations---and five categories of chirality. The five categories of polarizations (parapolar, electropolar, magnetopolar, antimagnetopolar, and multipolar) embody the ways in which multipole order can be realized in solids, thus expanding the familiar notion of electric dipolarization in ferroelectrics and magnetization in ferromagnets to higher-rank multipole densities characterizing, e.g., altermagnets and alterelectrics. Group theory also permits the absolute, gauge-invariant quantification of these multipole densities. The five categories of chirality (parachiral, electrochiral, magnetochiral, antimagnetochiral, and multichiral) extend the notion of enantiomorphism---conventionally associated with the lack of spatial mirror symmetries---to include all possibilities for creating non-superposable images by applying the inversions $i$, $\theta$, and $i\theta$. In particular, multichiral systems lack all inversion symmetries and thus have four different enantiomorphs. Each category of chirality arises from particular superpositions of electric and magnetic multipole densities. Our theory has great predictive power by relating electric and magnetic multipolar order with, e.g., distinctive features in the crystal structure, the electronic band structure, and property tensors of quantum materials. These findings are critical for a large community working on fundamental and applied aspects of novel quantum materials.