Higher-Spin Witten Effect and Two-Dimensional Fracton Phases

TL;DR

This paper explores the role of generalized theta terms in higher-spin U(1) gauge theories, revealing a higher-spin Witten effect, boundary fracton phenomena, and potential for new 2D fracton phases.

Contribution

It introduces higher-spin theta terms that bind electric and magnetic charges, derives quantization conditions, and uncovers boundary fracton Chern-Simons theories in 3D and 2D.

Findings

Higher-spin theta terms induce charge binding analogous to the Witten effect.

Time-reversal symmetry restricts theta to discrete values.

Boundary theories include fracton excitations coupled to tensor Chern-Simons fields.

Abstract

We study the role of "$\theta$ terms" in the action for three-dimensional $U(1)$ symmetric tensor gauge theories, describing quantum phases of matter hosting gapless higher-spin gauge modes and gapped subdimensional particle excitations, such as fractons. In Maxwell theory, the $\theta$ term is a total derivative which has no effect on the gapless photon, but has two important, closely related consequences: attaching electric charge to magnetic monopoles (the Witten effect) and leading to a Chern-Simons theory on the boundary. We will find that a similar story holds in the higher-spin $U(1)$ gauge theories. These theories admit generalized $\theta$ terms which have no effect on the gapless gauge mode, but which bind together electric and magnetic charges (both of which are generally subdimensional) in specific combinations, in a higher-spin manifestation of the Witten effect. We derive the corresponding Witten quantization condition. We find that, as in Maxwell theory, imposing time-reversal invariance restricts $\theta$ to certain discrete values. We also find that these new $\theta$ terms imply a non-trivial boundary structure. The boundaries host fracton excitations coupled to a tensor $U(1)$ gauge field with a Chern-Simons-like action, in both chiral and non-chiral varieties. These boundary theories open a door to the study of $U(1)$ fracton phases described by tensor Chern-Simons theories, not only on boundaries of three-dimensional systems, but also in strictly two spatial dimensions. We explicitly work through three examples of bulk and boundary theories, the principles of which can be readily extended to arbitrary higher-spin theories.